SENSORS, ACTUATORS AND SENSOR NETWORKS

Sensors, Actuators and Sensor Networks and Real-Time and Distributed Systems - Fundamental
principles and applications of sensors, actuators.
Smart sensors and micro sensor/micro actuator array devices.
Introduction to signal processing and sensor/actuator networks, deployment and architecture,
wireless communication, multiple access control layer, data gathering, routing and querying,
collaborating signal processing - Time dependent systems, clock synchronization, real-time
communication protocols, specification of requirements, task scheduling. Validation of timelines,
real-time configuration management. Middleware architecture for distributed real-time and secure
services.

Sensors, Fundamental principles and applications of Sensors

Sensors

The sensor is the bond of a sensor network node. Examples of sensors include temperature
sensors, accelerometers, infrared detectors, proximity sensors, and motion detectors.
Figure 3, shows the sensors used in a self-driving (autonomous) car.

Figure 3: Sensors used in a self-driving car

Fundamental principles of sensors

There are two types of sensing measuring principles available,

Static measurement: measures deflection induced by applied force or surface stress.

Dynamic measurement: measures change in resonant frequency.
Static measurement
For sensor systems where analyte binding induces surface stress, flexible cantilevers
functionalized on one side are used to transduce that stress into a measurable deflection. The
operating principles and physics behind deflection-based MEMS and NEMS cantilever
sensors will be explored.
Principles
Change of stress in a thin film on the surface of a cantilever is illustrated in Fig. 1. On the
left, the cantilever bends downward and expands until the stress created in the cantilever
balances the expanding film on top. The stress on the top side of the supporting cantilever is
tensile, because it is pulled by the film. In the case of a contracting film, the cantilever bends
upwards, creating compressive stress on the top side.

Fig 1 Expansion and contraction.

Dynamic measurement
Principles
Most of the dynamic sensing methods are based on measurement of resonant frequencies.
When a cantilever is initially actuated, it will oscillate at a particular frequency, known as its
natural frequency. When analyte molecules are adsorbed to the cantilever, the additional mass
induces a change in the resonant frequency. Here, we remind the readers of
the responsivity of a mechanically resonating system:

Rsp=−f/2meff = Δf/Δm
where f and meff are the resonant frequency and the effective mass of the system, respectively,
and Δm and Δf are changes in the mass and the frequency, respectively. Sensitivity is defined
as the minimum input level required to produce an output that overcomes the threshold,
which is typically chosen just above the noise level.

Applications of Sensors
Sensors find usage in various industries like Automotive, Manufacturing, Aviation, Marine,
Medical, Telecom, Chemical, and Computer Hardware. Let’s examine some of the
applications of sensors in these Industries.

1. Automotive
Here are some of the automotive applications of sensors given below:

 Braking and Traction control: Antilock Braking System (ABS) Sensors connected to
the wheel, measures the speed of the wheel and braking pressure and keeps sending
them to ABS controlling When the driver applies the sudden brake, ABS system, with
breaking pressure and speed data received from the sensors, releases the braking
pressure to avoid skidding/locking of wheels. It is one of the critical safety aspects of
vehicles.
 Air Bags – Anti Cushion Restraint System (ACRS): Crush sensors and accelerometers
placed in the vehicle measures the force and sends it to During accidents on sensing
the force exceeds the limit, ACRS will activate the Airbag and save the life of
passengers.
 Avoiding Collisions: Proximity sensors in the front, back, and sides of the vehicle
forewarn the driver of a possible Infrared, Video assistance, Ultrasonic technologies
assist drivers while parking their vehicles.
  Comfort and Convenience: Many sensors provide inputs and warnings to drivers on
Vehicle Speed, Engine Speed, Fuel level, Tire pressure, Door/deck, light bulbs for
driving comfort and convenience.
 Engine Data: Sensors provides so much data on Engine performance, such as Ignition,
b. Combustion, c. Exhaust gas oxygen, d. Fuel mix, e. Exhaust gas recycling, f.
Transmission control etc.,
 Other Applications

2. Manufacturing
Here are some of the manufacturing applications of sensors given below:

 Predictive maintenance of the machinery, Assembly equipment using the data

collected from sensors in the machines.
 Optimal utilization of Machines by continuously monitoring the performances and
effectively rejigging the operations with the data collected from sensors.
 Fine-tuning the Quality systems and enhance the quality standards using the data
collected from sensors. Design notifications and alerts in case of a deterioration of
quality and process standards.
 Agility in reacting to market demands.

3. Aviation
Sensors deployed in the aviation industry measures the data during navigation of aircraft,
monitoring various systems, and controlling instruments. These data are utilized inefficient
flight operations, improve aircraft performance and design improvements.

Some of the instrumentation sensors are tachometers, gauges to measure engine pressure and
oil& fuel quantity, Altimeters, airspeed meters, etc. Sensors help measure the testing of the
ground conditions, vibration and environment factors and provide useful inputs to the pilot to
manage the general operation and emergency conditions.

4. Medical & Healthcare

Signals generated by Sensors in Medical equipment, surgical instruments and devices are
used for diagnosis, treatment and control functions by Doctors.

Some of the applications are:

1. Blood pressure monitoring (self).

2. Continuous glucose monitoring by Individuals.
3. Automatic measurement of vitals of the patient and sending it to the patient’s doctor.
4. More home care facilities and ambulatory treatments.
5. Automatic detection of visitors spreading the disease to patients in hospitals.
6. Decentralized laboratories.
7. Robotics in Operation Theater.
5. Marine
Sensors in ship measures fuel tank levels, liquid cargo levels, tank pressure/temperature.
Pitch, roll, speed and other vessel moments are also measured and monitored with sensors’
help. There are a lot of sensors in Engines measuring typical attributes of internal combustion
parameters.

Actuators, Fundamental principles and applications of actuators

Actuators

An actuator is a machine component that is used for moving and controlling a system or
mechanism. To perform its operation, An actuator needs a control signal and a power source.
They are widely used in valves, gates, conveyors, automatic control systems, etc.1

Actuators are an integral part of instrumentation systems that power the machines that exist
today. These devices make it possible for the machines to interact with their surroundings the
way they do. In this article, we take a closer look at the omnipresent technology.

An actuator is a machine, or rather a part of a machine used to convert externally available

energy into motion based on the control signals. [1] Much like how hands and legs enable
humans to move around and perform actions, actuators let machines perform various
mechanical movements. The topic for discussion for this article is actuators. We will explain
what is an actuator, how actuators work, and what are the different types of actuators used in
industrial and domestic applications.

From the perspective of systems engineering, functions of any engineering product can be
classified into three distinct categories; the collection of input, processing and producing an
output.

For electromechanical systems, the input is detected and measured by a device called a
sensor. The task of a sensor is to sample the signals available to it and convert them into a
form understandable by the system. The system then processes the information and decides
how to respond. But how exactly does a system respond?

The answer is, with the help of an Actuator. Typically, an actuator consists of:
  Energy source: Energy sources provide actuators with the ability to do work.
Actuators draw electrical or mechanical energy from external sources for carrying out
their operation. The energy available to the actuator can be regulated or unregulated
depending on the system that it is a part of.
 Power converter: If the energy source attached to the actuators is unregulated, it
requires some additional apparatus to regulate it and convert it into a form suitable for
the actuation action. Hydraulic valves or solid-state power electronic converters are
examples of converters used in industrial actuators.

 Controller: In addition to enabling the operation of the power converter, a control

unit is responsible for generating actuating signals. In some systems, it provides the
user with an interface to provide inputs or check the system’s status.
 Load: The mechanical system attached to the actuator that uses the motion of the
actuator is called the load. Characteristics like Force/Torque and Speed are carefully
tuned before interfacing an actuator with the load.
Applications for Actuators

1. Material handling. This is a universal need for every type of manufacturing operation.
2. Robotics. The automotive industry and any number of others are now using robotics
to improve production quality and accuracy and control production costs. Electric
linear actuators meet the sophisticated needs of robotics. They can control and repeat
extremely precise movements, control rate of acceleration and deceleration, and
 control the amount of force applied. And they can combine all these movements on
multiple axes simultaneously.
3. Food and beverage manufacturing. Cleanliness is critical in these industries, and
electric linear actuators are both clean and quiet. In addition, food and beverage,
medical device, semiconductor, and some other applications also require stringent
washdown protocols. Electric actuators are corrosion-resistant and have a smooth
design that offers few crevices where bacteria or dirt might accumulate.
4. Window automation. Manufacturing facilities and other large-scale indoor operations
are constructed with heavy-duty ventilation systems, but in some cases, natural
ventilation is also desirable, especially to help control indoor temperature. Electric
linear actuators make it easy to remotely open and close heavy and/or high windows.
5. Agricultural machinery. Although heavy equipment and attachments are often
powered with hydraulics, machines that directly contact food or which require
finessed movements can be fitted with electrical actuators instead. Examples include
combines that thresh and convey grains, spreaders with adjustable nozzles, and even
tractors.
6. Solar panel operation. For optimal operation, solar panels must tilt to directly face the
sun as it moves across the sky. Electric actuators enable commercial installations and
utilities to efficiently and consistently control large solar farms.
7. Cutting equipment. For example, factories that manufacture carpet or printing
facilities may use electric actuators to raise and lower cutting blades. Again,
cleanliness can be important in these environments even if it isn’t a food safety issue.
8. Valve operation. Many types of processing plants incorporate valves to control the
flow of raw materials and finished products throughout the facility.
9. Non-Industrial Applications. We’re talking about how electric linear actuators are
used in industrial applications, but they are also used increasingly in residential or
office settings where hydraulics and pneumatics are not an option. They are tidy,
clean, and simple. Electric actuators now offer easy remote operation of windows and
window coverings, for example, strictly as a convenience feature or to assist disabled
individuals.

Sensor Networks:

A sensor network comprises a group of small, powered devices, and a wireless or wired
networked infrastructure. They record conditions in any number of environments including
industrial facilities, farms, and hospitals. The sensor network connects to the internet or
computer networks to transfer data for analysis and use.
Sensor network nodes cooperatively sense and control the environment. They enable
interaction between persons or computers and the surrounding environment.
Figure 1, illustrates the widespread use of sensors connected via multiple sensor networks.
 Figure 1: Smart City - Sensors and Sensor Networks

Wired vs. Wireless Sensor Networks

Sensor networks can be wired or wireless. Wired sensor networks use ethernet cables to
connect sensors. Wireless sensor networks (WSNs) use technologies such as Bluetooth,
cellular, wifi or near field communication (NFC) to connect sensors.
WSNs are easier to deploy and maintain and offer better flexibility of devices. With the rapid
development of sensors and wireless technologies, WSNs have become a key technology of
the IoT. WSNs don't need the physical network infrastructure to be modified.

Operation of a Sensor Network

Sensor networks typically include sensor nodes, actuator nodes, gateways, and clients. Sensor
nodes group inside the sensor field and form networks of different topologies. The following
process describes how sensor networks operate:

 A sensor node monitors the data collected by the sensor and transmits this to other
sensor nodes.
 During the transmission process, data may be handled by multiple nodes as it reaches
a gateway node.
 The data is then transferred to the management node.
 The management node is managed by the user and determines the monitoring required
and collects the monitored data.

Figure 2, shows the components of a sensor network.

Figure 2: Sensor network components

Sensor Nodes
There are many nodes in a sensor network. These nodes are the detection stations. There is a
sensor/transducer, microcontroller, transceiver, and power source:

 A sensor senses the physical condition, and if there is any change, it generates
electrical signals.
 The signals go to the microcontroller for processing.
 A central processor sends commands to the transceiver and data is transmitted to a
computer.

A real-time system is any information processing system which has to respond to externally
generated input stimuli within a finite and specified period – the correctness depends not only
on the logical result but also the time it was delivered
Distributed System
Distributed system a system in which components are distributed across multiple locations
and computer-network.
Today’s information systems are no longer monoUdl.ic, m.alnf.rame computer-based system
. Instead, they are built on some combination of networks to form distributed systems. A
distributed system is one in which the components of an information system are distributed
to multiple locations in a computer network. Accordingly, the processing workload required
to support these components Is also distributed across multiple computers on the network.
The opposite of distributed systems are centralized systems. In centralized systems, a
central, multi user computer (usually a mainframe) hosts all components of an information
system. The users interact with this host computer via terminals (or, today, a PC emulating a
terminal), but virtually all of the actual processing and work Is done on the host computer.
Distributed systems are Inherently more complicated and more difficult to implement than
centralized solutions. So why is the trend toward distributed systems?

 Modem businesses are already distributed, and, thus, they need distributed system
solutions.
  Distributed computing moves information and services closer ro the customers that
need them.
 Distributed computing consolidates the incredible power resulting from the
proliferation of personal computers across an enterprise (and society in general).
Many of these personal computers are only used to a fraction of their processing
potential when used as stand-alone PCs.
 In general, distributed system solutions are more user.frlendly because they use the
PC as the user interface processor.

 Personal computers and network servers are much Jess expensive than main-frames.
(But admittedly)', the total cost of ownership is at least as expensive once the
networking complexities are added ln.)

There is a price to be paid for distributed systems. Network data traffic can cause congestion
that actually slows performance. Data security and Integrity can also be more easily
compromised In a distributed solution. Still there Is no arguing the trend toward distributed
systems architecture. While many centralized, legacy applications still exist, they are
gradually being transformed Into distributed Information systems. Conceptually, any
information system application can be mapped to the layers:

 The presentation layer is the actual user interface- the presentation of inputs and
outputs ro the user.
 The presentation logic layer is any processing that must be done to generate
the presentation. Examples include editing input data and formatting output data.
 The application logic layer Includes all the logic and processing required to support
the actual business application and rules. Examples include credit checking,
calculations, data analysis.
 The data manipulation layer includes all the commands and logic required to store and
retrieve data to and from the database.
 The data layer is the actual stored data in a database.

Smart sensors and micro sensor/micro actuator array devices.

Smart sensors

A smart sensor is a device that takes input from the physical environment and uses built-in
compute resources to perform predefined functions upon detection of specific input and then
process data before passing it on.

Smart sensors enable more accurate and automated collection of environmental data with less
erroneous noise amongst the accurately recorded information. These devices are used for
monitoring and control mechanisms in a wide variety of environments including smart grids,
battlefield reconnaissance, exploration and many science applications.

The smart sensor is also a crucial and integral element in the internet of things ( IoT), the
increasingly prevalent environment in which almost anything imaginable can be outfitted
with a unique identifier and the ability to transmit data over the internet or a similar network.
One implementation of smart sensors is as components of a wireless sensor and actuator
network (WSAN) whose nodes can number in the thousands, each of which is connected with
one or more other sensors and sensor hubs, as well as individual actuators.

Compute resources are typically provided by low-power mobile microprocessors. At a

minimum, a smart sensor is made of a sensor, a microprocessor and communication
technology of some kind. The compute resources must be an integral part of the physical
design -- a sensor that just sends its data along for remote processing isn't considered a smart
sensor.

A smart sensor has three components: a sensor that captures data, a microprocessor that
computes on the output of the sensor via programming and communications capabilities.

A smart sensor might also include several other components besides the primary sensor.
These components can include transducers, amplifiers, excitation control, analog filters and
compensation. A smart sensor also incorporates software-defined elements that provide
functions such as data conversion, digital processing and communication to external devices.
How do smart sensors work?
A smart sensor ties a raw base sensor to integrated computing resources that enable the
sensor's input to be processed.

The base sensor is the component that provides the sensing capability. It might be designed to
sense heat, light or pressure. Often, the base sensor will produce an analog signal that must be
processed before it can be used. This is where an intelligent sensor's integrated technology
comes into play. The onboard microprocessor filters out signal noise and converts the
sensor's signal into a usable, digital format.

Smart sensors also contain integrated communications capabilities that enable them to be
connected to a private network or to the internet. This enables communication to external
devices.

What are smart sensors used for?

There are countless use cases for smart sensors. They are very commonly used in industrial
environments and are the driving force behind Industry 4.0.

Factories often use smart temperature sensors to make sure machines aren't overheating, and
vibration sensors to make sure machines aren't at risk of vibrating loose. Smart sensors also
enable process control, such as monitoring a process, like manufacturing an item, and making
any adjustments that might be required to meet quality or production goals. This was once a
manual process, but smart sensors can be used to automate process control.

Smart sensors also play a key role in modern security systems. Thermal imaging sensors can
be used to detect an intruder's body heat. Similarly, devices such as smart locks, motion
sensors, and window and door sensors are commonly connected to a common network. This
enables the security sensors to work together to paint a comprehensive picture of the current
security status.

Micro sensors - Introduction

Presently, there is a trend to make sensors smaller and smaller. Initial stages show an
evolution from a single sensor element to an intelligent sensor system with extremely small
dimensions by MST. The so-called smart (or integrated) sensing devices can be developed by
integrating sensor components with those for signal processing. This integration also
decreases the noise that is often created by the transmission of signals to an external data
processing unit. Thus it will be possible to measure and evaluate for a certain task all
interesting parameters at one place and at one time. An important step toward the further
development of micro sensors is the conception and design of intelligent electronic signal
processors. This will lead to advanced distributed sensor systems in which noisy sensor
signals, resulting from cross-talk or insufficient selectivity, can be successfully evaluated.
The signal processing system of humans is very advanced; sensor signals are received over
the nervous system and transferred to the brain which reliably evaluates them by a natural
parallel computing system
Force and Pressure Micro sensors

Due to their simple construction and wide applicability, mechanical sensors play the
most important part in MST. Pressure micro sensors were the first ones developed and used
by industry. Miniaturized pressure sensors must be inexpensive and have a high resolution,
accuracy, linearity and stability. Presently, silicon-based pressure sensors are most often
used; they can easily be integrated with their signal processing electronics on one chip. Their
advantages include low production costs, high sensitivity and low hysteresis.

Pressure is most often measured via a thin membrane which deflects when pressure is
applied. Either the deflection of the membrane or its change in resonance frequency is
measured; both of these values are proportional to the pressure applied. These mechanical
changes are transformed into electric signals. Membranes can be manufactured by bulk

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micromachining of a (100) silicon substrate, whereby the membrane is produced with one of
the etch stop techniques. Pressure sensors usually employ capacitive or piezo resistive
measuring principles.

Fig. 1. Piezo resistive pressure sensor

Fig.1. shows the design of a piezo resistive pressure sensor. The piezo resistors are
integrated in the membrane; they change their resistance proportionally to the applied
pressure. The resistance change indicates how far the membrane is deflected and is measured
with a Wheatstone bridge. The deflection value is proportional to the pressure.

Capacitive pressure sensor

Fig.2. Capacitive pressure sensors

Capacitive sensors make use of the change of the capacitance between two metal
plates. The membrane deflects when pressure is applied, which causes the distance between
the two electrodes to be changed. Through this the capacitance increases or decreases. From
the amount of membrane deflection the capacitance change is measured and the pressure
value can be calculated.

A capacitive sensor is shown in Fig.2. The electrodes are made up of a planar comb structure.
Here, the applied force is exerted parallel to the sensor surface. In force sensors which use
membranes, the force is usually applied perpendicular to the sensor surface. Here,

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nonlinearity and cross-sensitivity may cause problems. In the device described here, the
sensor element mainly consists of two parts: first, a movable elastic structure which
transforms a force into a displacement, and second, a transformation unit consisting of the
electrodes which transform the displacement into a measurable change of capacitance.
Capacitive force sensor made of silicon.

The displacement is restored by an elastic suspension beam. The capacitors consist of

two electrically insulated thin electrodes with a very narrow gap between them
(approximately 10um). They are placed on both sides of the sensor chip, making the
capacitance on one side increase and decrease on the other side. By the separate measurement
of the capacitance changes on both sides a high linearity and sensitivity is obtained. The
sensor unit is made by anisotropically etching (110) silicon and then fastening it to a pyrex
substrate through anodic bonding. The prototype of the capacitive micro sensor had a nominal
capacitance of1 pF. Measurements in this range can easily be handled by commercially
available microelectronic measuring devices. It was possible to measure very small forces
with a resolution of 20 nm (0.01-10 N). The same structure can be used as a positioning unit
for nanorobots.

A force sensing resistor

A new measurement principle was realized by using a so-called force sensing resistor.
The device is fundamentally different from capacitive, piezo resistive and resonant sensors,
since here the resistance is inversely proportional to the pressure. The sensor consists of a
polymer foil to which planar electrodes are fastened, on top of this a semiconductor polymer
film is placed, Fig. 6.10. If a voltage is applied to the electrodes and there is no force, the
resistance is at least 1 MOhm. When a force is applied, the resistance decreases due to current
that flows across the shunting polymer foil.

Fig. 3. A force sensing resistor

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 The dynamic range of the sensor can be influenced by producing a finer electrode
structure. This, however, is accompanied by increasing production costs due to a lower yield
rate. The sensitivity can be increased by varying the foil thickness. The device can be
operated at temperatures of up to 400°Cand is very durable, e.g. over 10 million repeated
measurements were made with a 5% deviation. The measurement range is between 10 g and
10 kg. A major disadvantage is the hysteresis, which appears during pressure changes.
Despite of this fact, the device can be usefully employed for many dynamic measuring
applications. It is inexpensive, compact, robust and resistant to external influences.

Ultrasound distance sensors

Ultrasound distance sensors are well suited as position sensors for micro robots, since
they do not depend on the optical properties of the object being detected and they are robust
and can obtain reproducible results. Ultrasound distance sensors use the pulse-echo principle.
Here a pulse sequence is emitted with the help of an ultrasound transducer which is usually
made from piezo ceramics. The signals reflected by objects as echos are received by a sensor
and evaluated using the propagation time of the sound signal.

Since the transducer needs some time for recovery after transmission; a "blind spot"
appears when the detector is too close to an object, which means that an object might not be
detected. The results obtained with a new concept of an ultrasound micro transformer were
reported in (M2S293].
Fig. 4.Ultrasound distance micro sensor

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 In this device, two identical independent ultrasound membranes were integrated next
to each other on a silicon substrate; one served as a transmitter and the other one as a
receiver. The schematic design of a single sensor membrane and the measurement principle
are shown.

The transmitter membrane is brought to resonance electro thermally with integrated

heating resistors. The acoustic pressure response is then detected by piezo resistors, integrated
in the form of a Wheatstone bridge in the receiver membrane. The sensitivity of this
prototype was about 3 uV/mPa at abridge voltage of 5V.

Capacitive rotational speed sensor

In many technical systems like navigation and landing gear controllers, compact and
inexpensive angular speed sensors are required. Conventional sensors using piezoelectric
resonators or optical glass fibers are very sensitive, but are usually expensive. The following
described silicon sensor was produced using a batch fabrication method [Hash94]. The
operating principle of the resonating sensor is presented in Fig.5.

Fig.5.Operating principle of a rotational speed micro sensor

A 200umthick tuning fork arrangement made of (110) silicon is used as the resonator.
It is positioned by two torsion bars which also serve as electrical terminals. When the
resonator is introduced into a magnetic field and alternating current is applied it starts to
oscillate due to Lorenz forces. If the sensor rotates at omega degrees about its longitudinal
axis, the Coriolis forces induce a rotational movement in the opposite direction about this
axis; this movement is proportional to the swing angle omega. The amplitude of the swing is

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detected by the capacitance change between the fork prongs (movable electrodes)and the
fixed detection electrodes, not shown in Figure 6.17. The latter are integrated into a glass
casing consisting of two pyrex glass layers, each250umthick. A sensor prototype with a base
area of 2 cm x 2 cm was built; it had a sensitivity of 0.5 mVsec/deg at an exciting frequency
of 470Hz

Acceleration Micro sensors

Cantilever principle

Miniaturized acceleration sensors will mostly find their place in the automotive
industry. They are also of interest to the air and space industries and for many other
applications. Acceleration micro sensors will help to improve the comfort, safety and driving
quality of automobiles. However, in order for them to become a product of general interest,
their production costs must be drastically lowered. As with pressure (Fig. 6 and Fig. 7),
acceleration is usually detected by piezo resistive or capacitive methods. Mostly an elastic
cantilever is used to which a mass is attached. When the sensor is accelerated the mass
displaces the cantilever and the displacement is picked up by a sensor. Such a sensor is shown
in Fig. 6. It uses the capacitive measuring method to record deflection. From the deflection
the acceleration can be calculated.

Fig.6. Capacitive measurement of accelerations

Piezo resistive principle

To effectively measure acceleration with this principle, piezo resistors are placed at
points of the cantilever where the largest deformation takes place. The stability and accuracy
of the sensor improves with increasing number of piezo elements. If a mass moves due to
acceleration, it deforms the piezo resistors, thereby changing their resistance, in fig.7. The
acceleration is determined from the resistance change.

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 Fig.7 Piezo resistive principle for acceleration measurement

By increasing the movable mass the sensitivity of the sensor will be improved. The
mass's center of gravity should be as close to the end of the cantilever as possible. Piezo
resistive acceleration micro sensors are usually produced using the silicon technology. This
allows the microelectronic processing unit to be integrated onto the sensor chip, making the
system compact and robust.

Capacitive cantilever micro sensor

An acceleration sensor produced by the surface micromachining technique was

described in [Fricke93]. A sketch of this sensor is shown in Figure .The sensor consists of
one or more cantilevers acting as one electrode; they are suspended freely over an opposite
electrode and a contact strip. There is only a small gap between the cantilever and the
electrode to maximize the electrostatic forces and to keep the mechanical stresses as small as
possible.

Fig. 8. Capacitive cantilever acceleration micro sensor

 As opposed to conventional capacitive sensors, a so-called threshold voltage is
applied to offset the forces caused by the acceleration; it will give an indication of the current
acceleration. With this device, a saw tooth voltage is applied in defined steps across the

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cantilever and the electrode, which gradually increases the electrostatic force acting on the
cantilever. When the critical voltage is reached, the system becomes unstable and the
cantilever bends towards the contacts and finally touches them. The voltage falls to zero, and
the saw tooth voltage is applied again. The actual value of the threshold voltage to be applied
depends on the magnitude of the acceleration.

Chemical Sensors

Chemical sensors detect the presence or concentration of a chemical substance in a

solution. They may be used for qualitative and quantitative measurements. In medical
diagnostics, nutritional science, environmental protection and the automobile industry, many
different chemical quantities are to be measured.

About 60% of all chemical sensors are gas sensors. The rest is used to detect
substances and concentrations in liquids. An important application potential of chemical
sensors is in environmental protection, medical applications and process engineering. Many
industrial countries will soon be adopting very strict environmental standards and laws that
will rapidly increase the demand for gas and liquid sensors. Present research is concentrated
on the integration of these sensors in measurement systems.

The potentiometer principle in connection with field effect transistors (FET), acoustic
sensors using the change of mass principle and optical sensors are most often applied. Many
gas and liquid sensors are based on these principles and have similar structures. It is usually
very important for chemical sensors to have a low cross-sensitivity, i.e. the measured values
are not influenced significantly by other substances in the solution being analyzed. For
measuring chemical substances, a sensitive layer or a specific area of the sensor is used to
contact the chemical substance.

Fig.9. General Structure of chemical sensors

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 During measurements, a chemical reaction occurs on this sensitive layer/area and a
transducer, of which the physical, optical, acoustical or dielectric properties are changed,
transforms the recorded phenomenon into an electric signal. This signal is then amplified and
evaluated by a microelectronic component. The general structure of a chemical sensor system
is shown in Fig.9.

Field effect transistor sensor principle

Ion-sensitive field effect transistors are used to measure the concentration of ions of various
elements such as hydrogen, sodium, potassium or calcium. The structure of an ion sensitive
FET and its measurement principle can be seen in Figure.

Fig.10. Chemical sensor

Initially, when no chemical substance is in contact with the ion-sensitive layer

deposited on the gate area of the transistor, the gate potential Vas is equal to Vref· When the
substance to be measured contacts the ion-sensitive layer, the gate potential Vas changes; this
voltage change is caused by the ions in the chemical substance. Thereby, the current los
between the source and drain changes as well. The gate potential Vas is then corrected by
adjusting the voltage Vrer until the original transistor current los flows again; the voltage Vos
is held constant. The value delta V ref is proportional to the ion concentration of the analyzed
substance to be measured, Fig. 10. The area of the sensitive layer can be as small as a few
2
um , allowing very small amounts of substance to be measured.
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Biosensors

However, in a biosensor the biologically sensitive elements such as enzymes,

receptors and antibodies are integrated with the sensor. The interaction between the protein
molecules of the bio element and the molecules of the substance causes a modulation of a
physical or chemical parameter. This modulation is converted into an electrical signal by a
suitable transducer. The signal represents the concentration to be measured. In many
molecular interactions, gases are either released or consumed, e.g. the change of oxygen can
be registered by a chemical 02 sensor. Very selective and sensitive measurements are possible
with biological receptors.

There are many applications for miniaturized biosensors. In biological and nutritional
research, these sensors are extremely important to analyze trace elements, especially when
toxic substances like heavy metals or allergens have to be found. Considering that there are
more than 5 million different inorganic and organic compounds known today, and that
100,000 different substances can be identified, it is getting clear that there is an enormous
need for such small, inexpensive and reliable biosensors.

Also in medicine, where a variety of substances are to be monitored during surgery or

during an in-situ investigation, an increasing number of small biosensors will be used to
record vital patient data for a correct and quick diagnosis. Biosensors are divided into two
groups. They are metabolism sensors and immuno-sensors. A metabolism sensor uses bio
sensitive enzymes as biocatalysts to detect molecules in a substance and to catalyze a
chemical reaction. The analyzed substance is chemically transformed and the course of a
reaction can be detected and evaluated by a chemical sensor indicating the concentration of
the substance in a solution.

This mechanism is illustrated in Figure with an example of an enzyme based

measurement of phosphate in waste water treatment. The enzyme nucleoside phosphorylase
(NP) is used to determine the phosphate content. This enzyme detects the phosphate and
triggers a chemical reaction when inosine is added. One product of this reaction is
hypoxanthine (HX). This substance then takes part in another chemical reaction and is
transformed into xanthine oxidase (XO) after consuming oxygen. The amount of oxygen
consumed can be registered with a chemical 0 2 sensor and the phosphate concentration can
be determined from this.

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 Fig.11 Phosphate measurement with metabolism sensor

To detect chemically inactive molecules in a substance, immuno-sensors are used;

their bio sensitive elements are antibodies. The detection method for an antigen molecule is
known as the lock and key principle. When it interacts with the analyzed substance,
immobilized antibody molecules ("lock") on the sensor surface bond with an antigen
molecule "key" in the substance. No other molecule can bond with these antibodies.

The bonding process can either be directly registered over a transducer or indirectly
through antigen markers, e.g. using molecules of another substance; depending on the type of
sensor. From this measurement the concentration can then be determined. The sensor in
Fig.12 detects the concentration of the antigens directly with an interferometric method. The
light intensity changes are here due to the bonding process. An attempt is being made to
integrate biosensors into Microsystems to take advantage of the many functions they offer.
The integration of biosensors with micro pumps and micro valves would make it possible to
manufacture very small measuring systems that only need small samples and can measure
quickly.

Fig. 12. Immuno sensing using an optical transducer

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 One difficulty encountered, however, is that a system integration of biosensors may
produce only short-lived sensors because the proteins are not very stable for a very long time.
Another problem to date is the immobilization of the proteins. Now, some practical
developments in the area of miniaturized biosensors will be introduced.

Temperature Sensors

Temperature sensors play an important role in different types of monitoring systems,

especially in process, medicine and environmental protection technologies. They are also
indispensable for controlling beatings or air conditioning systems and many household
appliances. A temperature value may also be a parameter of indirect measurements of other
parameters in gas or flow sensors or it may be used for error compensation of temperature-
dependent sensors and actuators. There is a wide range of conventional temperature sensors
available, like the thermo element, thermo resistor, thermo diode, etc. The next examples
describe the development of miniaturized temperature sensors.

Fiber optical thermometer

Several glass fiber thermometers were described in Fig.13 shows a simplified

temperature sensor. The sensor contains a light source, a glass fiber, serving both as
waveguide and temperature sensor, and a photodiode. The multi-modal glass fiber is made of
materials that have different temperature coefficients in the core and the mantle (quartz-
silicon system).The light is introduced by an LED into the glass fiber and is propagated
through the sensitive fiber area. When the temperature varies in the sensor surrounding, the
local index of refraction in the fiber changes, which results in an optical light attenuation.

Fig.13. Fiber optic thermometer

This, in turn, leads to a change in the intensity of light leaving the thermometer; the
change of light is measured by the photodiode. This measurement can be used to calculate the
temperature. A prototype of this sensor has been used for a long time. It can measure

108
temperatures of up to 90°C with an accuracy of 0.1 oc. The thermometer is insensitive to
electromagnetic noise, it costs about seven dollars.

Flow Sensors

There is a need for miniaturized sensors to measure very small liquid and gas flows,
since in many applications, like in medical instruments and automobiles, micro fluidic
components are becoming an indispensable part of a system. Most of these sensors operate on
the principle of thermal energy loss, which occurs when the heating element is located in a
flowing substance (thermal dilution). Also, transit time measurements of a trace element
injected into the flow can be used to determine a velocity. Another measurement principle
uses the forces or torques exerted on an object which is placed in the fluid flow.

Two-mode flow sensor

A flow sensor was shown which can be operated in two modes. A sketch of this
sensor is depicted in Fig14. In one mode, the sensor uses the elapsed time of the locally
heated flow medium. A 5 Hz signal was applied to the heater and the time was measured until
the temperature rise is recorded downstream by the sensor. In the second mode (thermal
dilution),the heater was supplied with constant energy and the temperature difference
between the upstream and downstream sensors was measured. The highest sensitivity
registered was in the range of 0.05-0.2 ml/min. One disadvantage of this method would be a
possible change in the liquid property due to the heat impulses.

Fig.14. Flow sensor

The device was made to measure both liquid and gas flows. It has two temperature
sensors, one in front of the heating element and one behind it. Here, the thermal energy loss
between the two temperature sensors was measured. All components of the flow sensor are
integrated into one silicon chip. The flow channel is 7 mm long, 1.3 mm wide and 350 um
high; it was made by the bulk micromachining technique. Both the inlet and outlet opening
have a diameter of 0.7 mm. The silicon membrane is 40um thick.

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Thermal flow sensor

A sensor which uses a thermal measurement principle for recording gas flows was
developed and shown. A schematic diagram of the sensor is shown in Fig.15. The sensor has
a circular silicon disc into which a heating resistor is implanted. The center of the chip is in
contact with the gas and serves as a heating element. There is a ring-like silicon dioxide layer
around the disc, which guarantees good thermal insulation between the hot and cold sensor
parts. The chip cover was made of a 3um thick polyimide film. The flow speed is determined
from the thermal interaction between the hot silicon and the gas. Two diodes are implanted
into the silicon chip using a CMOS process (not shown in the diagram). They are the
thermometers to measure the temperature difference of the gas at the heating element and at a
fixed point downstream.

Fig.15. Thermal flow sensor

Several prototypes with an overall dimension of 5 mm x 5 mm were produced. The

heating membranes had a diameter between 75um and 500um; the membrane and Si02 layer
thicknesses were 15um and 30um, respectively. The measured results agreed with the
theoretically calculated values up to a flow speed of 2.5 m/s. The sensor sensitivity was 10
cm/s. The development of new chemical, physical and biological sensors is a continuous
process. Only a small sample of existing sensors could be covered here. A detailed
description of current research in this area is given in[Gard94]. Numerous sensor principles
had been developed in a variety of forms. The manufacturing technology of semiconductors
and thin-films will probably play the largest role in the future to make the sensors and to
integrate them with the control circuits on one chip.

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Micro actuators:

Principles and Examples

Introduction

The MST applications suggest the use of new micro actuator systems which allow motions to
be realized with micrometer accuracy. Conventional motion concepts or manufacturing
methods are no longer able to fulfil the demands concerning miniaturization and all questions
connected with it. Microsystems, and in particular future micro robots, require the
development of new advanced actuators with very small dimensions, simple mechanical
construction and high reliability.

It is rather difficult to determine exactly what the name "micro actuator" implies. In the
literature, the term "micro actuators" is used for devices ranging in size from micrometers to
several decimeters, proving the typical classification difficulties of this young scientific field.
This book considers a micro actuator as a device of a few micrometers to a few centimetres in
size having a functional principle applicable in the micro world.

In addition to the miniaturization, mechanical micro devices having components such

as pumps, valves, robot grippers, linear and rotational positioning elements, simple cantilever
actuators and complex artificial muscle systems, must be functional to provide a micro
system with task-dependent capabilities. Micro pumps and -valves for treating liquids and
gasses at the micro level can be applied in medicine, where implantable, highly accurate
micro systems are needed for the dosing of medication or for chemical and biotechnological
analysis where minute volumes of liquid must be transported and measured. They can also be
applied for technical devices such as ink jet printers.

Micro actuators using the cantilever principle can be applied for various applications
to generate minute motions. In optics such micro actuators can serve as electronically
tuneable mirrors, in fluid dynamics as valves, and in micro robotics as grippers. Micro motors
also have a great potential for MST applications.

Actuation Principles

Functional properties of actuators are determined above all by the underlying

actuation principles. The forces and displacements that can actually be reached for a certain
component size depend on the scaling behaviour, actuator design, and technical limits.
Therefore, the total dimensions of an actuator determine which actuation principle is best

111
suited for a given application. The scaling behaviour of the forces is derived from the force
laws by introducing a scaling variable r, which describes the component size in any spatial
direction. In analogy, the scaling behaviour of work and power density can be determined.

Electrostatic Principles

Electrostatic actuators make use of Coulomb’s attraction force between oppositely

charged bodies. In the simplest case, two charged plates oriented in parallel are available.
Apart from the force component F z that acts vertically between the plates, lateral-plate offset
additionally causes a force F x acting in the lateral direction.

Magnetic Principles

The magnetic principles of force and movement generation can be classified in electro
dynamic, reluctance, and electromagnetic principles. Electro dynamic micro actuators make
use of the Lorentz force on a conductor passed by a current in a magnetic field. So far, such
actuators have hardly been noticed due to their difficult-to-achieve three-dimensional
geometry. Contrary to this, the reluctance principle is the most frequently applied magnetic
actuation principle in micro technology. Micro technical implementation requires an
acceptable effort, since only a single field source and no hard magnetic materials are needed.

Fluidic Principles

Fluidically driven micro pistons allow high testing forces and large displacements to
be reached in, e.g., micro tensile testing machines. The force-displacement characteristics
exhibit a constant behaviour. The principle of pressure-dependent membrane deflection is
applied in a number of micro valve versions. Other fluidic arrangements are based on fluid
dynamics principles or electro rheological and magneto rheological principles.

Inverse Piezo effect

Piezoelectric actuators make use of the coupling of mechanical deformation and

electric polarization in ferroelectric crystals, in crystals with a triad axis, or in certain
polymers. A major class of materials is made up of Pb(ZrxTi1_x)03 (PZT) crystals due to
their high coupling factors of about 0.7 and piezoelectric coefficients du of about 0.5 nm/V.

Shape Memory Effects

The term "shape memory" describes the unusual ability to remember shape, which
can be initiated in certain materials either thermally or mechanically. Even after heavy
deformation, materials with shape memory are able to recover a previously memorized shape.
This phenomenon was discovered in brass alloys as early as the late thirties. However, the
real importance of the effect has become obvious only since its discovery in a NiTi alloy,
where it is particularly pronounced.

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 The most important materials of commercial significance can be classified either as
metal alloys or as polymers. Furthermore, there are ceramics and biological systems in which
shape memory properties are observed as well. An example to be mentioned in this respect is
bacteriophages, which use a shape memory mechanism when entering host cells. SMAs are
currently the focus of interest, as they have proved to function in a number of applications
and show an unforeseeable potential for future applications.

In contrast to conventional structural materials, in which shape changes are made up

of elastic, plastic or thermal contributions, SMAs show three additional types of shape
changes that are associated with shape memory characteristics. These effects are illustrated in
Fig. by the example of a helical spring:

(a) One-way effect: After removal of the load F, the helical spring shows permanent
deformation. This seemingly plastic deformation recedes completely upon heating.

(b) Two-way effect: In addition to the one-way effect, there is also a defined shape change
upon cooling.

(c) Pseudo elasticity: Mechanical loading, F, expands the helical spring to a large extent.
When the load is removed, the spring still returns to its initial shape.

One-Way Effect

In the martensitic state (Fig.1, T<Mf), the material exhibits a very low elastic limit.
Elastic straining is followed by a pseudo plastic strain range in which the component can be
strained reversibly by up to several percent. Further strain behaviour is characterized by
plastic deformation up to fracture. After relaxation in the pseudo-plastic range, an apparent
deformation, CIW, is retained. By heating the deformed component above the austenite finish
temperature, Af , a complete shape recovery is possible. The maximum reversible strain, for
instance of NiTi single crystals in the direction, is 10.7%. As this effect only occurs upon
heating, i.e., only in one direction, it is referred to as a one-way effect. Renewed cooling and
heating no longer changes the shape. The original memorized shape is imprinted upon the
component prior to loading by heat treatment. If the shape recovery during reverse
transformation to austenite is hindered, high forces occur that can be used to perform work.
This is the basis of SMA actuators.

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 Fig.1 Stress- Strain – temperature characteristic for the one-way
effect Two-Way Effect

The two-way effect is associated with a shape change upon heating and cooling
without requiring any external load. This gives rise to a strain characteristic located within the
s-T plane, Fig. 2. The shape change can be repeated without renewed deformation. However,
in principle, the shape change attained is less pronounced than with the one-way effect. The
shape change upon cooling is achieved by imprinting ordered internal stress fields on the
material. The underlying mechanism is based on the formation of preferred martensite
variants . One special case of the two-way effect is the all-round effect caused by the
formation of ThNi4 precipitates with a preferred orientation.

Fig.2 Strain – temperature characteristic for the two-way effect

Pseudo elasticity

In the austenitic state (T>Ar), the material exhibits pseudo elastic behaviour, Fig. 3. In
contrast to previous effects, no temperature change is required in this case. The strain
characteristic is, therefore, located in the cr-s plane. Above the elastic limit, there is a plateau
in which a highly nonlinear deformation occurs up to a virtual yield limit, Spe . Above Spe ,
there is plastic deformation up to fracture. If the component is loaded only as far as Spe ,
unloading passes through the lower hysteresis loop with the strain disappearing completely.
In polycrystalline NiTi, the maximum reversible strain can be 7 to 8% , and in some Cu-
114
based SMAs, up to 18% . The plateau region is caused by stress-induced transformation of
austenite into martensite.

Fig.3 Stress- Strain characteristic for pseudo elasticity

For the stress-induced formation of martensite there is an upper temperature limit,

Md, above which competing irreversible processes, such as the formation of dislocations and
slipping, are favoured thermodynamically. The temperature window, Md>T>Ar, in which
pseudo elastic behaviour occurs can be set by various thermo mechanical processes [146].
Above Md, SMAs behave like conventional materials with elastic strain characteristics and
subsequent plasticity up to fracture.

Electrostatic Micro actuators

Electrostatic micro shutter

Electrostatic actuators using membranes will be presented first. In metrology and in

micro optics, so-called micro shutters have become of great interest. The principle of this
shutter is based on the electrostatic deflection of a movable electrode (micro shutter), made of
aluminium, gold or doped poly silicon. During an operation, the shutter moves against a fixed
silicon electrode (substrate) which was produced by anisotropic wet etching in (110) silicon.
This principle of such a shutter is shown.

Fig.4. Electrostatic micro shutter

Electrostatic micro pump

The electrostatic membrane principle is well-suited for designing micro pumps. Fig.5
shows a sketch of an electrostatic micro membrane pump. The device consisting of 4 silicon

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chips was produced by the bulk micromachining technique. The two upper chips form the
drive consisting of the membrane and the electrode; the latter is part of the outer frame. The
identical lower chips form the inlet and outlet valves. If a voltage is applied between the
membrane and the electrode, the membrane arches toward the electrode thereby generating a
partial vacuum .in the chamber. This causes the inlet valve to open and liquid to be drawn
into the pump chamber. By removing the voltage, the liquid is pushed through the outlet
valve. Since the drive unit and the pump chamber are separated, the liquid is not affected by
the electric field. This is important when the liquid contains ions, e.g. as in salt solutions or
medicines.

Fig.5. Electrostatic micro pump

Piezoelectric Micro actuators

Micro membrane pump

The micro membrane pump consists of two glass plates and a silicon disc which is
sandwiched between the plates, Fig.6.
 Fig.6. Micro membrane pump

The silicon disc is structured by etching and contains a pump chamber as well as
suction and discharge valves. The upper glass plate serves as a pressure-sensitive membrane.

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It can change the volume of the pump chamber with the help of a bonded piezo disc (the
actuator When a voltage is applied, the membrane buckles downward and the liquid is forced
out through the discharge valve. When the voltage is removed, the membrane returns to its
original position and the pump sucks in liquid through the suction valve.

Magnetostrictive Micro actuators

Various solid state actuators have been investigated in the past few years; one
example using piezo ceramic was described in the previous section. While piezo actuators
have served for a variety of applications, magnetostrictive actuators are still at the threshold
of industrial exploration.

The elastic wave motor

A very interesting development is the Elastic Wave Motor (EWM), which takes
advantage of the properties of Terfenol-D. Here, electric energy is directly transformed into a
continuous linear movement. A sketch of the function principle and a schematic design of the
motor having a movable Terfenol-D rod and external coils (stator) is shown in Figure. In this
device, the rod is placed into a guide tube, the end of which is attached to a rigid support.
Several short coils are placed along the outer surface of the tube to produce the magnetic
field. If the magnetic field is successively switched on and off from one end of the tube to the
other in the coils, the Terfenol-D rod moves within the tube in the opposite direction, as
shown in Figure. The speed, force and position of the rod are controlled by the magnetic
field. The design is successfully being used in the paper industry. It controls the paper
thickness during manufacture and piling by moving a blade across the entire paper width.

Fig.7. Magnetostrictive Micro actuators

Electromagnetic Micro actuators

Electromagnetically driven micro actuators are gaining in significance as manufacturers are

improving the three-dimensional production methods for a variety of materials. With
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electromagnetic actuators, electric energy is transformed into mechanical energy like
forces or torques.

Linear micromotor

Numerous research projects are concerned with the development of electromagnetic

linear actuators. Since almost all present efforts to design linear actuators are based on the
silicon technology, the available structures are limited to a height of about 20um, which
means that the forces that can be produced are very weak. There are few devices using planar
coils; a linear motor with a sliding rare earth magnet. The magnet slides in a channel between
two silicon chips which are attached to a glass substrate. The operating principle of this
motor is depicted in Fig. Planar coils located in the silicon chips are progressively energized
to generate the linear motion of the magnet. There are 8 pairs of planar coils, integrated in
parallel to the guiding channel of the chip, Fig. The coils opposite one another are driven
sequentially with a current of the same magnitude so that a travelling perpendicular magnetic
field (parallel to the magnetization of the permanent magnet) is produced. Thus, the magnet
is pulled along the channel in a synchronous manner by the moving magnetic field.

Fig.8.Linear micro motor

SMA-based Micro actuators

When shape memory alloys (SMAs) are deformed under a certain critical temperature
and then heated up to above this critical temperature, they will "remember" their original
form and assume it again. This effect can be used for generating motions or forces.
Characteristic for actuators that use SMA are their low complexity, light weight, small size
and large displacement; e.g. SMA components have been used for several years as active
pipe connectors. However, the potential use of these alloys in MST has just recently been
recognized.

The SMA effect was discovered in various copper alloys, in which a reversible,
thermal-mechanical transformation of the atomic structure of the metal takes place at certain
temperatures. When the temperature

118
is raised or lowered, the metallurgical structure of an SMA transforms from a martensitic
state (low temperatures) to the austenitic state (high temperatures), or vice versa.

Fig.9. Schematic representation of SMA effect

In Fig,9 the basic transformation mechanism is schematically shown. Starting from a

stable and rigid austenitic state, the SMA transforms into the martensitic state as the
temperature sinks under the critical temperature; thereby the shape of the SMA can be

deformed by up to 8% (as for NiTi-alloys [Menz93]). In the low temperature state, the SMA
keeps the desired deformed shape until it is exposed to a higher temperature. When it is
warmed up above a threshold temperature, the deformed martensite is transformed back to
austenite and the SMA takes on its original form (thermal shape memory). With this
property, large displacements can be obtained compared to other actuator principles.
 Fig.10. (a)Stress-Strain diagram and (b) the hysteresis cureve of an SMA

Micro endoscopes and catheters

Minimal-invasive surgery and new diagnostic techniques require the availability of a

new class of micro and miniature instruments, like endoscopes and catheters, which are

119
equipped with sensors and effectors. The present trend towards minimal invasive therapy
requires that precise catheter systems with active guidance will be available to enable the
surgeon to enter the various cavities of the human body or to direct them into a specific
branch of a blood vessel

Fig.11. Active endoscope using SMA

There is an electrical connector on each end of the SMA wires to which the electric
voltage can be applied. The wires contract when an electric current is applied to them,
causing a temperature increase. When the power is turned off, the wires take on their original
form after cooling. The direction of motion of the endoscope and its angle of the bend can be
controlled by selectively applying electric voltage to each of the three wires.

Hydraulic and Pneumatic Micro actuators

Flexible rubber micro actuators

A flexible micro actuator to be used by miniaturized robots was shown.The actuator is

driven by hydraulic or pneumatic pressure, can be bent in every direction and is designed for
use as robot hands or legs for various applications. The structure of this device is shown in
Fig12. It is made of rubber reinforced with nylon fibers and has three autonomous actuator
chambers. The internal pressure in every chamber can be controlled individually by flexible
hoses and valves leading to them. The device can be expanded along its longitudinal axis
when the pressure is increased equally in all three chambers. If the pressure is only increased
in one chamber, the device bends in the opposite direction.
Fig.12. Principle of the pneumatically driven flexible micro actuator

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Hydraulic piston micro actuator

An interesting hydraulic micro actuator system was shown.. The piston actuator and
its integrated calibration system is depicted in Figure. The actuator chamber with its inlet for
its operating fluid, (ex) water, was made by the LIGA process. The unit contains a force-
transmitting piston which can be moved along the side walls of the chamber by a fluid. The
device is covered by a glass plate (not shown in the figure). A stop groove is added to absorb
excessive adhesive which may ooze out when the glass cover plate is being fixed; this is
necessary to prevent the piston from sticking to the walls of the chamber.

Fig.13. Hydraulic piston micro actuator

Chemical Microactuators

Chemical actuators are based on different chemical processes taking place in fluid or gaseous
media. E.g. many chemical reactions produce gases which can be used to create a high
pressure in a chamber.

Polymer micro pump

The uni-directional microcapsule polymer pump is shown in Fig.14. It can be used as

a medicine dosing system implanted in a patient. The pump cylinder has a semi permeable
membrane on its inlet side, which only allows a substance to flow in the direction indicated; a
one-way valve is located on the outlet side.
 Fig.14. Polymer micro pump (a) Initial state (b) final state

121

The pump is separated by a thin film and filled with a pharmaceutical solution which
is on the left side of the chamber. On the right side of the chamber a highly concentrated
water-absorbing polyacrylamide gel is located.

The work cycle of the pump is as follows: the osmotic pressure difference across the
membrane takes water from the ambient solution, i.e. from the patient’s blood, and drives it
through the membrane into the right side of the pump chamber, causing the polymer gel to
swell. The pharmaceutical in the microcapsule is pushed out due to the volume increase of the
polymer gel. When the pump space is completely occupied by the gel, the medicine is fully
injected and the cycle is completed. The duration of the cycle depends on the concentration
difference between the polymer gel solution and the ambient solution.
Introduction to signal processing and sensor/actuator networks, deployment and
architecture, wireless communication, multiple access control layer, data gathering, routing
and querying,

Signal processing is an electrical engineering subfield that focuses on

analysing, modifying, and synthesizing signals such as sound, images, and
scientific measurements.

Sensors are sophisticated devices used for sensing the phenomena (temp.,
intensity of earthquake etc.) or detecting the targets (moving vehicles, individuals
etc.). So, there may be many types of sensors depending upon their applications,
e.g. seismic, passive infrared intrusion (PIR), magnetic sensing device, still video
cameras, acoustic microphone arrays etc. Sensor may be stationary or mobile. The
individual sensor can share the information with its neighbors and also with the
user via a processing unit (sink). Thus the individual sensor can be grouped to do
some particular task or multiple tasks. So, it is needed to realize a network of
sensors called sensor network. This sensor network may be used in military
application, health, security, surveillance etc.

(i) Networking of sensors:-

Sensors are deployed in the region called Sensor field , where we want to
sense or track or detect the target. So, each sensor must have capability for
sensing, as well as transmitting the data to their neighbors. This means, each
sensor can be activated; it can transmit data to its neighbors and also can be able
to communicate with the user through centre processing location (Sink) [1]. This
is illustrated in fig 1a.
 Fig 1a. Sensors deployed in sensor field

There may be other type of sensor network which includes less no of sensors (few
tens of sensors) for medical application or tracking an enemy vessel in territory.
But here we consider the network of a large number of micro sensors (hundreds
to several thousands)

(ii) Protocol Stack:

The protocol stack used by the sensor network and sink, is similar to that of
computer network [1].
Here each of the layers and protocol used in those layers are illustrated briefly.

Fig 1b.
Physical layer:

This is the ground layer of the network protocol stack. The task of this layer is
to select the frequency, generate the carrier frequency, detect the signal and modulate
it.
So, signal propagation effects, modulation schemes for effective communication, are
important things in this layer.
Binary or M-array modulation scheme can be used in this layer. Low power
spread spectrum modulation can also be used [1].

Data link layer:

This layer is above the physical layer and below the network layer. The main task
of this layer is multiplexing of data streams and to detect data frame. This layer also
establishes point to point or point to multipoint connection. The Medium access
Protocol (MAC) used in this layer must be different.
Sensor networks must be designed in such a way that power consumption is less.
Unlike cellular system, the power sources in sensor network can not be replaced or
recharged. So, MAC protocol used in this layer must be different from that case of
cellular system. Self Organization Medium Access Control for Sensor Network
(SMACS) protocol can be used in this layer.

Self Organization Medium Access Control for sensor Network is illustrated in

[2]. In this network, nodes in the sensor networks are assumed to have the capability
of turning its 'radio' on and off and also able to tune the carrier frequency. 'Channel
or Time division multiple access (TDMA) slot is assigned to a link just after its
existence is discovered. Here each link will operate on a different frequency to avoid
collision during transmission or receiving.
(a) In this protocol, when a node (say node 1) is 'waken up', it listen to the 'channel'
for a certain time. If it does not receive any response for that time interval, it sends one
message (invitation) by the end of its listening time.
(b) The other nodes which are activated in between, will receive that message. And
as soon as they receive it, they also send reply to that node. Then the node 1 will
receive those entire message if those 'reply' do not collide. After that, node 1 will select
one node (say node 2) whose reply is received first or depending on the highest signal
level of the replies it received.
(c) In third step, it will send another message to all the nodes informing which node
it selects. All the nodes, except the node which is selected, turn off their radio for
some time and after some time those nodes repeat the searching procedure.
So, finally one link between node 1 and node 2 is established. And they start
receiving and transmitting each other for two time slot (One for receiving and another
for transmitting). And these time slots will be repeated periodically every certain fixed
time interval (Tframe) called super frame. This Tframe is fixed for each node. When any
new node is found, super frame will be filled with another two time slots for that. The
main characteristic of this protocol is to use 'non synchronous' assignment in time slot.
After a link is established, a node knows when to transmit and when to receive.
Hence, it will turn off its radio in its idle state.
 Network layer:

The network layer of the sensor network is quite different from other networks.
So a special type of routing protocol discussed in [1] can be used here.
Energy efficient routes discussed in [1] consider the available power (PA) in the
nodes and the energy required ( ) to transmit data from one node to another.
Node S (Fig 2a) is the source and it sends data to the sink. There are 4 routes from
sink to source.

Fig 2a.

Route 1: Sink-A-B-S Total PA = 4

Route 2: Sink-E-S Total PA= 3
Route 3: Sink-C-D-S Total PA = 3
Route 4: Sink-C-A-B-S Total PA =
6

Maximum PA Route:
Route 4 has maximum PA. But it is nothing but the extended path of the Route 1.
So, it is not power efficient.

Minimum hop (MH) Route:

Here Route 2 has the minimum hop. So, route 1 will be selected when minimum
hop route is considered.

Maximum minimum PA node Route:

Route 2 has minimum maximum PA.
Minimum energy Route:
Route 1 is minimum energy route.
Data aggregation:

Sometimes it is effective to aggregate the data of an 'attribute' of a phenomena .

In the picture shown below, node 1 and node 2 are used to sense the same
phenomena and send the data to the node 4. Here, node 4 aggregates the data from
node 1 and 2 and transmits it to the next node, instead of sending the data collected
form node 1 and 2 separately.

Fig 2b.(Data aggregation)

Transport Layer.

It is above the network layer and below application layer. This layer is
responsible for communication with other networks through internet. TCP protocol
used for computer network may be applicable here. But UDP-type protocol will be
preferred during communication between sink and sensor nodes for limited memory of
sensor nodes.

Application layer:

Application layer is important when communication is needed between multiple

networks. Protocols used in this layer are Sensor management protocol, Task
Assignment and Data Advertisement Protocol (TADAP) and Sensor Query and
Data Dissemination etc [1].
Collaborative Signal Processing:

(i) What is collaborative signal processing?

By signal we understand 'something' that signifies some occurrence of events of

our interest. It may be deterministic in nature or may not be. But it conveys some
information. Processing means understanding that signal, or to modify (transformation,
selective retention) that signal in order to extract the information it carries. So, signal
processing means to process the signal to extract information that it carries.
Collaboration means co-operation or working together . Hence, collaborative signal
processing means to process the signals received by a group of elements. In case
of sensor network, these elements are sensors and the information is the location of the
target(s) .

(ii) Why is it needed in Sensor Network?

We know that Unity is strength . The importance of collaborative signal

processing lies in this line. In order to achieve a bigger goal, information must be
shared. In case of sensor network, main critical thing is power and the 'goal' is to
detect, identify and track any target. Again sensors are powered by fixed energy
sources which are supplied at the time of network forming. So, 'limited power' is key
factor here. Receiving, transmitting, and processing of data is to be done with that
limited power.
Each sensor must process the signal for certain time period. Again it may detect only
the local events. So, it must share the information with other. Hence, collaborative
Signal Processing among sensor-nodes is needed to fulfill the goal.

(iii) Sampling in space and time [3]:

The signal from each object is time varying and that signal must be sensed by the
sensors. Sensors are densely deployed in a region. This can be thought that nodes
sample the time varying signature field spatially. The total number of sensors in
the sensor field will depend upon the changing rate of the signature field . Again each
sensor will sample the signal (time sampling) at a rate depending upon the bandwidth.
Now, one may consider moving objects as moving peak in the spatial signature field
. So, they may think the entire space time region, consisting of a number of space
time cells, where the 'space time signature field is constant. The size of this cell
depends on the velocity of moving target.
 Fig 3. (taken from [3])

The figure (taken from [3]) above illustrates this. Here different shades indicate
the variation of the signature field.
Time dependent systems, clock synchronization

Distributed System is a collection of computers connected via the high speed communication network.
In the distributed system, the hardware and software components communicate and coordinate their
actions by message passing. Each node in distributed systems can share their resources with other
nodes. So, there is need of proper allocation of resources to preserve the state of resources and help
coordinate between the several processes. To resolve such conflicts, synchronization is used.
Synchronization in distributed systems is achieved via clocks.
The physical clocks are used to adjust the time of nodes.Each node in the system can share its local
time with other nodes in the system. The time is set based on UTC (Universal Time Coordination).
UTC is used as a reference time clock for the nodes in the system.
The clock synchronization can be achieved by 2 ways: External and Internal Clock Synchronization.
1. External clock synchronization is the one in which an external reference clock is present.
It is used as a reference and the nodes in the system can set and adjust their time
accordingly.
2. Internal clock synchronization is the one in which each node shares its time with other
nodes and all the nodes set and adjust their times accordingly.
There are 2 types of clock synchronization algorithms: Centralized and Distributed.
1. Centralized is the one in which a time server is used as a reference. The single time server
propagates its time to the nodes and all the nodes adjust the time accordingly. It is
dependent on single time server so if that node fails, the whole system will lose
synchronization. Examples of centralized are- Berkeley Algorithm, Passive Time Server,
Active Time Server etc.
2. Distributed is the one in which there is no centralized time server present. Instead the
nodes adjust their time by using their local time and then, taking the average of the
differences of time with other nodes. Distributed algorithms overcome the issue of
centralized algorithms like the scalability and single point failure. Examples of Distributed
algorithms are – Global Averaging Algorithm, Localized Averaging Algorithm, NTP
(Network time protocol) etc.

Real Time communication protocol

Real-time communication (RTC) is a category of software protocols and communication hardware

media that gives real-time guarantees, which is necessary to support real-time guarantees of real-time
computing.[1] Real-time communication protocols are dependent not only on the validity and integrity of
data transferred but also the timeliness of the transfer.
The following protocol involved in Real time communication protocols are
WebSocket
WebSocket is a web technology providing full-duplex communications channels over a single TCP
connection. The WebSocket API is being standardized by the W3C, and the WebSocket protocol has
been standardized by the IETF as RFC 6455.
XMPP (Jabber)
Extensible Messaging and Presence Protocol (XMPP) is a communications protocol for message-
oriented middleware based on XML (Extensible Markup Language).

WebRTC
WebRTC (Web Real-Time Communication) is an API definition being drafted by the World Wide Web
Consortium (W3C) to enable browser to browser applications for voice calling, video chat and P2P file
sharing without plugins.

The Bayeux Protocol

Bayeux is a protocol for transporting asynchronous messages (primarily over HTTP), with low latency
between a web server and a web client.

Server-Sent Events
Server-Sent Events (SSE) are a way for server to initiate data transfer to clients after the client connects.
It is used for streaming continuous or low latency messages to the client. The browser API is called
EventSource.

Wave Federation Protocol

The Wave Federation Protocol (formerly Google Wave Federation Protocol) is an open protocol,
extension of the Extensible Messaging and Presence Protocol (XMPP) that is used in Apache Wave. It is
designed for near real-time communication between the computer supported cooperative work wave
servers.

IRC
Internet Relay Chat (IRC) is a protocol for real-time Internet text messaging (chat) or synchronous
conferencing. It is mainly designed for group communication in discussion forums, called channels, but
also allows one-to-one communication via private message as well as chat and data transfer, including
file sharing.

Real-Time Publish-Subscribe (RTPS) Protocol

The Real-Time Publish-Subscribe (RTPS) protocol is designed for use with Internet Protocol (IP) one-
to-many Multicast and connectionless best-effort transports such as IP User Datagram Protocol (UDP).
It enables, among other things, best-effort and reliable publish-subscribe communications for real-time
applications using standard IP networks.

Task Scheduling

Task scheduling algorithms :

The process of deciding which task will utilize the cpu time is called task scheduling. The scheduling of
the task may be on the basis of their priorities. The priority assignment mechanism for the tasks can be
either static or dynamic. In the case of static priority assignment the priority of task is assigned as soon
as the task is created, thereafter the priorities cannot be changed. In dynamic assigning the priorities of
the task can be changed during the runtime.
The various scheduling algorithms followed are:
1. First in first out (FIFO): In this algorithm simply the task that came in first in the ready to run state
will go out first into the running state. The task that goes out of the running state goes to the ready to run
state. Fig 11.5.a shows the movement of the task in the ready to run and running state

Fig 11.5.a Advantage – it is very easy to implement Disadvantage – we cannot have any priority
mechanisms, in real time examples each task has a different priority and it is to be implemented, but
using FIFO we cannot implement priority based scheduling.
2. Round robin scheduling: In this case each task gets their turn after all the task are given their time
slots. Thus it can be said that it is a time slicing wherein the time for each time slot is same and is given
tasks one by one. The CPU time utilization for three tasks according to round robin is shown in fig
11.5.b. In this example, it is assumed that the time slots are 5 milliseconds each.
The switching of tasks occurs in the following case:

1. current task has completed its work before the completion of its time slot
2. current task has no work to be done
3. current task has completed the time slice allocated to it
● advantage – very easy to implement
● disadvantage – all the tasks are considered at same level
3. Round robin scheduling with priority :
 It is modified version of round robin scheduling mechanism
 In this case the tasks are given priorities based on their significance and deadlines. Thus task
with higher priority can interrupt the processor and utilize the CPU time
  If multiple tasks have same priority then round robin scheduling will be used for them. But
whenever a higher priority task occurs, it will be executed first. The CPU will suspend the task it
was executing and will execute higher priority task.
 For example, bar code scanner can use this scheduling algorithm. This method can be used in
soft real time systems.
4. Shortest job first (SJF) scheduling:
● In this case the task with the shortest execution time is executed first. This ensures less number of
tasks in the ready state. Thus it can be said that the priority is higher for a task with lesser execution time
and the priority of the task is lesser for the task with higher execution time.
● Disadvantage – if there were too many short execution time tasks, then the task with more execution
time will never be executed.
● Advantage – the implementation of this scheduling method is also simpler as just the execution time
of each of the tasks is ready to run state are to be compared to decide which task will be executed by the
processor.
5. Non preemptive scheduling:
● This scheduling mechanism can be implemented in any of the previously seen scheduling mechanisms
that have the concept of priority.
● As the name says in this case if a task (say task 1) is in running state and another task (say task 2) with
higher priority enters into the ready to run state, the earlier task i.e. task 1 continues with the execution
until its time slice and the higher priority task i.e. task 2 has to wait for its turn. The fig 11.5.c shows an
example of non-pre-emptive scheduling.
6. Preemptive scheduling:
● This scheduling can be implemented on any of the scheduling mechanisms having concept of priority.
● As the name says in this case if a task (say task 1) is in running state and another task (say task 2) with
higher priority enters into the ready to run state , the earlier task i.e. task 1 has to release the CPU and
the later task i.e. task 2 will get the execution.
● Thus the higher priority task will get the CPU as soon as it enters into the ready to run state, and this
higher priority task will enter to running state. The fig 11.5.d shows an example of preemptive
scheduling.
Validation of timeline, Real-time Configuration Management

Configuration Management is the process of maintaining systems, such as computer hardware and
software, in a desired state. Configuration Management (CM) is also a method of ensuring that systems
perform in a manner consistent with expectations over time.
Originally developed in the US military and now widely used in many different kinds of systems, CM
helps identify systems that need to be patched, updated, or reconfigured to conform to the desired state.
CM is often used with IT service management as defined by the IT Infrastructure Library (ITIL). CM is
often implemented with the use of configuration management tools such as those incorporated into
VMware vCenter.

Why is Configuration Management important?

Configuration Management helps prevent undocumented changes from working their way into the
environment. By doing so, CM can help prevent performance issues, system inconsistencies, or
compliance issues that can lead to regulatory fines and penalties. Over time, these undocumented
changes can lead to system downtime, instability, or failure.

Performing these tasks manually is too complex in large systems. Software configuration management
can involve hundreds or thousands of components for each application, and without proper
documentation IT organizations could easily lose track of which systems require attention, what steps
are necessary to remediate problems, what tasks should be prioritized and whether changes have been
validated and propagated throughout the system.

A Configuration management system allows the enterprise to define settings in a consistent manner,
then to build and maintain them according to the established baselines. A configuration management
plan should include a number of tools that:

 Enable classification and management of systems in groups

 Make centralized modifications to baseline configurations
 Push changes automatically to all affected systems to automate updates and patching
 Identify problem configurations that are underperforming or non-compliant
 Automate prioritization of actions needed to remediate issues•
 Apply remediation when needed.

Structure and Functionality of DOC Middleware

Just as networking protocol stacks can be decomposed into multiple layers, such as the physical, data-link,
network, transport, session, presentation, and application layers, so too can DOC middleware be decomposed into
multiple layers, such as those shown in Figure 1.

Figure 1. Layers of DOC Middleware and Surrounding Context

Below, we describe each of these middleware layers and outline some of the COTS technologies in each layer that
have matured and found widespread use in recent years.

Host infrastructure middleware encapsulates and enhances native OS communication and concurrency
mechanisms to create reusable network programming components, such as reactors, acceptor-connectors, monitor
objects, active objects, and component configurators [Sch00b, Sch01]. These components abstract away the
peculiarities of individual operating systems, and help eliminate many tedious, error-prone, and non-portable
aspects of developing and maintaining networked applications via low-level OS programming APIs, such as
Sockets or POSIX pthreads. Widely used examples of host infrastructure middleware include:
 • The Sun Java Virtual Machine (JVM) [JVM97], which provides a platform-independent way of executing
code by abstracting the differences between operating systems and CPU architectures. A JVM is responsible
for interpreting Java bytecode, and for translating the bytecode into an action or operating system call. It is the
JVM’s responsibility to encapsulate platform details within the portable bytecode interface, so that
applications are shielded from disparate operating systems and CPU architectures on which Java software
runs.

• .NET [NET01] is Microsoft's platform for XML Web services, which are designed to connect information,
devices, and people in a common, yet customizable way. The common language runtime (CLR) is the host
infrastructure middleware foundation upon which Microsoft’s .NET services are built. The Microsoft CLR is
similar to Sun’s JVM, i.e., it provides an execution environment that manages running code and simplifies
software development via automatic memory management mechanisms, cross-language integration,
interoperability with existing code and systems, simplified deployment, and a security system.

• The ADAPTIVE Communication Environment (ACE) [Sch01] is a highly portable toolkit written in C++ that
encapsulates native operating system (OS) network programming capabilities, such as connection
establishment, event demultiplexing, interprocess communication, (de)marshaling, static and dynamic
configuration of application components, concurrency, and synchronization. The primary difference between
ACE, JVMs, and the .NET CLR is that ACE is always a compiled interface, rather than an interpreted
bytecode interface, which removes another level of indirection and helps to optimize runtime performance.

Distribution middleware defines higher-level distributed programming models whose reusable APIs and
components automate and extend the native OS network programming capabilities encapsulated by host
infrastructure middleware. Distribution middleware enables clients to program distributed applications much like
stand-alone applications, i.e. , by invoking operations on target objects without hard-coding dependencies on their
location, programming language, OS platform, communication protocols and interconnects, and hardware. At the
heart of distribution middleware are request brokers, such as:

• The OMG's Common Object Request Broker Architecture (CORBA) [Omg00], which is an open standard for
distribution middleware that allows objects to interoperate across networks regardless of the language in
which they were written or the platform on which they are deployed . In 1998 the OMG adopted the Real-time
CORBA (RT-CORBA) specification [Sch00a], which extends CORBA with features that allow real-time
applications to reserve and manage CPU, memory, and networking resources.

• Sun's Java Remote Method Invocation (RMI) [Wol96], which is distribution middleware that enables
developers to create distributed Java-to-Java applications, in which the methods of remote Java objects can be
invoked from other JVMs, possibly on different hosts. RMI supports more sophisticated object interactions by
using object serialization to marshal and unmarshal parameters, as well as whole objects. This flexibility is
made possible by Java’s virtual machine architecture and is greatly simplified by using a single language..

• Microsoft's Distributed Component Object Model (DCOM) [Box97], which is distribution middleware that
enables software components to communicate over a network via remote component instantiation and method
invocations. Unlike CORBA and Java RMI, which run on many operating systems, DCOM is implemented
primarily on Windows platforms.
 • SOAP [SOAP01] is an emerging distribution middleware technology based on a lightweight and simple
XML-based protocol that allows applications to exchange structured and typed information on the Web.
SOAP is designed to enable automated Web services based on a shared and open Web infrastructure. SOAP
applications can be written in a wide range of programming languages, used in combination with a variety of
Internet protocols and formats (such as HTTP, SMTP, and MIME), and can support a wide range of
applications from messaging systems to RPC.

Common middleware services augment distribution middleware by defining higher-level domain-independent

services that allow application developers to concentrate on programming business logic, without the need to
write the “plumbing” code required to develop distributed applications by using lower-level middleware directly.
For example, application developers no longer need to write code that handles transactional behavior, security,
database connection pooling or threading, because common middleware service providers bundle these tasks into
reusable components. Whereas distribution middleware focuses largely on managing end-system resources in
support of an object-oriented distributed programming model, common middleware services focus on allocating,
scheduling, and coordinating various resources throughout a distributed system using a component programming
and scripting model. Developers can reuse these component services to manage global resources and perform
common distribution tasks that would otherwise be implemented in an ad hoc manner within each application.
The form and content of these services will continue to evolve as the requirements on the applications being
constructed expand. Examples of common middleware services include:
 • The OMG’s CORBA Common Object Services (CORBAservices) [Omg98b], which provide domain-
independent interfaces and capabilities that can be used by many DOC applications. The OMG
CORBAservices specifications define a wide variety of these services, including event notification, logging,
multimedia streaming, persistence, security, global time, real-time scheduling, fault tolerance, concurrency
control, and transactions.

• Sun’s Enterprise Java Beans (EJB) technology [Tho98], which allows developers to create n-tier distributed
systems by linking a number of pre-built software services—called “beans”—without having to write much
code from scratch. Since EJB is built on top of Java technology, EJB service components can only be
implemented using the Java language. The CORBA Component Model (CCM) [Omg99] defines a superset of
EJB capabilities that can be implemented using all the programming languages supported by CORBA.

• Microsoft’s .NET Web services [NET01], which complements the lower-level middleware .NET capabilities,
allows developers to package application logic into components that are accessed using standard higher-level
Internet protocols above the transport layer, such as HTTP. The .NET Web services combine aspects of
component-based development and Web technologies. Like components, .NET Web services provide black-
box functionality that can be described and reused without concern for how a service is implemented. Unlike
traditional component technologies, however, .NET Web services are not accessed using the object model–
specific protocols defined by DCOM, Java RMI, or CORBA. Instead, XML Web services are accessed using
Web protocols and data formats, such as the Hypertext Transfer Protocol (HTTP) and eXtensible Markup
Language (XML), respectively.

Domain-specific middleware services are tailored to the requirements of particular domains, such as telecom, e-
commerce, health care, process automation, or aerospace. Unlike the other three DOC middleware layers, which
provide broadly reusable “horizontal” mechanisms and services, domain-specific middleware services are
targeted at vertical markets. From a COTS perspective, domain- specific services are the least mature of the
middleware layers today. This immaturity is due partly to the historical lack of distribution middleware and
common middleware service standards, which are needed to provide a stable base upon which to create domain-
specific services. Since they embody knowledge of a domain, however, domain-specific middleware services
have the most potential to increase system quality and decrease the cycle- time and effort required to develop
particular types of networked applications. Examples of domain-specific middleware services include the
following:

• The OMG has convened a number of Domain Task Forces that concentrate on standardizing domain-specific
middleware services. These task forces vary from the Electronic Commerce Domain Task Force, whose
charter is to define and promote the specification of OMG distributed object technologies for the development
and use of Electronic Commerce and Electronic Market systems, to the Life Science Research Domain Task
Force, who do similar work in the area of Life Science, maturing the OMG specifications to improve the
quality and utility of software and information systems used in Life Sciences Research. There are also OMG
Domain Task Forces for the healthcare, telecom, command and control, and process automation domains.

• The Siemens Medical Engineering Group has developed syngo which is both an integrated collection of
domain-specific middleware services, as well as an open and dynamically extensible application server
platform for medical imaging tasks and applications, including ultrasound, mammography, radiography,
 flouroscopy, angiography, computer tomography, magnetic resonance, nuclear medicine, therapy systems,
cardiac systems, patient monitoring systems, life support systems, and imaging- and diagnostic-workstations.
The syngo middleware services allow healthcare facilities to integrate diagnostic imaging and other
radiological, cardiological and hospital services via a blackbox application template framework based on
advanced patterns for communication, concurrency, and configuration for both business logic and
presentation logic supporting a common look and feel throughout the medical domain.

• The Boeing Bold Stroke [Sha98, Doe99] architecture uses COTS hardware and middleware to produce a non-
proprietary, standards-based component architecture for military avionics mission computing capabilities,
such as navigation, display management, sensor management and situational awareness, data link
management, and weapons control. A driving objective of Bold Stroke was to support reusable product line
applications, leading to a highly configurable application component model and supporting middleware
services. Associated products ranging from single processor systems with O(10 5) lines of source code to
multi-processor systems with O(10 6) lines of code have shown dramatic affordability and schedule
improvements and have been flight tested successfully. The domain-specific middleware services in Bold
Stroke are layered upon common middleware services (the CORBA Event Service), distribution middleware
(Real-time CORBA), and host infrastructure middleware (ACE), and have been demonstrated to be highly
portable for different COTS operating systems (e.g. VxWorks), interconnects (e.g. VME), and processors
(e.g. PowerPC).
2.2 Benefits of DOC Middleware

Middleware in general–and DOC middleware in particular–provides essential capabilities for developing

distributed applications. In this section we summarize its improvements over traditional non-middleware oriented
approaches, using the challenges and opportunities described in Section 1 as a guide:

• Growing focus on integration rather than on programming – This visible shift in focus is perhaps the major
accomplishment of currently deployed middleware. Middleware originated because the problems relating to
integration and construction by composing parts were not being met by either

1.Applications, which at best were customized for a single use,

2.Networks, which were necessarily concerned with providing the communication layer, or

3.Host operating systems, which were focused primarily on a single, self-contained unit of resources .

In contrast, middleware has a fundamental integration focus, which stems from incorporating the perspectives
of both operating systems and programming model concepts into organizing and controlling the composition
of separately developed components across host boundaries. Every DOC middleware technology has within it
some type of request broker functionality that initiates and manages inter-component interactions.

Distribution middleware, such as CORBA, Java RMI, or SOAP, makes it easy and straightforward to connect
separate pieces of software together, largely independent of their location, connectivity mechanism, and
technology used to develop them. These capabilities allow DOC middleware to amortize software life-cycle
efforts by leveraging previous development expertise and reifying implementations of key patterns into more
encompassing reusable frameworks and components. As DOC middleware continues to mature and
incorporates additional needed services, next-generation applications will increasingly be assembled by
modeling, integrating, and scripting domain-specific and common service components, rather than by

1. Being programmed either entirely from scratch or

2. Requiring significant customization or augmentation to off-the-shelf component implementations.

• Demand for end-to-end QoS support, not just component QoS – This area represents the next great wave of
evolution for advanced DOC middleware. There is now widespread recognition that effective development of
large-scale distributed applications requires the use of COTS infrastructure and service components.
Moreover, the usability of the resulting products depends heavily on the properties of the whole as derived
from its parts. This type of environment requires visible, predictable, flexible, and integrated resource
management strategies within and between the pieces.

Despite the ease of connectivity provided by middleware, however, constructing integrated systems remains
hard since it requires significant customization of non-functional QoS properties, such as predictable
latency/jitter/throughput, scalability, dependability, and security. In their most useful forms, these properties
extend end-to-end and thus have elements applicable to

• The network substrate

• The platform operating systems and system services
• The programming system in which they are developed
• The applications themselves and
 • The middleware that integrates all these elements together.
Two basic premises underlying the push towards end-to-end QoS support mediated by middleware are that:
1.Different levels of service are possible and desirable under different conditions and costs and
2.The level of service in one property must be coordinated with and/or traded off against the level of service
in another to achieve the intended overall results.

To manage the increasingly stringent QoS demands of next-generation applications, middleware is becoming
more adaptive and reflective. Adaptive middleware [Loy01] is software whose functional and QoS-related
properties can be modified either:
• Statically, e.g., to reduce footprint, leverage capabilities that exist in specific platforms, enable functional
subsetting, and minimize hardware/software infrastructure dependencies or
• Dynamically, e.g., to optimize system responses to changing environments or requirements, such as
changing component interconnections, power levels, CPU/network bandwidth, latency/jitter; and
dependability needs.

In mission-critical systems, adaptive middleware must make such modifications dependably, i.e., while
meeting stringent end-to-end QoS requirements. Reflective middleware [Bla99] goes further to permit
automated examination of the capabilities it offers, and to permit automated adjustment to optimize those
capabilities. Reflective techniques make the internal organization of systems–as well as the mechanisms used
in their construction–both visible and manipulable for middleware and application programs to inspect and
modify at runtime. Thus, reflective middleware supports more
 advanced adaptive behavior and more dynamic strategies keyed to current circumstances, i.e., necessary
adaptations can be performed autonomously based on conditions within the system, in the system's
environment, or in system QoS policies defined by end-users.

• The increased viability of open systems architectures and open-source availability – By their very nature,
systems developed by composing separate components are more open than systems conceived and developed
as monolithic entities. The focus on interfaces for integrating and controlling the component parts leads
naturally to standard interfaces. In turn, this yields the potential for multiple choices for component
implementations, and open engineering concepts. Standards organizations such as the OMG and The Open
Group have fostered the cooperative efforts needed to bring together groups of users and vendors to define
domain-specific functionality that overlays open integrating architectures, forming a basis for industry-wide
use of some software components. Once a common, open structure exists, it becomes feasible for a wide
variety of participants to contribute to the off-the-shelf availability of additional parts needed to construct
complete systems. Since few companies today can afford significant investments in internally funded R&D, it
is increasingly important for the information technology industry to leverage externally funded R&D sources,
such as government investment. In this context, standards-based DOC middleware serves as a common
platform to help concentrate the results of R&D efforts and ensure smooth transition conduits from research
groups into production systems.

For example, research conducted under the DARPA Quorum program [Dar99] focused heavily on CORBA
open systems middleware. Quorum yielded many results that transitioned into standardized service definitions
and implementations for Real-time [OMG00B, Sch98A] and Fault-tolerant [Omg98a, Cuk98] CORBA
specification and productization efforts. In this case, focused government R&D efforts leveraged their results
by exporting them into, and combining them with, other on going public and private activities that also used a
standards-based open middleware substrate. Prior to the viability of common middleware platforms, these
same results would have been buried within a custom or proprietary system, serving only as the existence
proof, not as the basis for incorporating into a larger whole.

• Increased leverage for disruptive technologies leading to increased global competition– Middleware
supporting component integration and reuse is a key technology to help amortize software life-cycle costs by:

1.Leveraging previous development expertise, e.g., DOC middleware helps to abstract commonly reused
low-level OS concurrency and networking details away into higher-level, more easily used artifacts and
2.Focusing on efforts to improve software quality and performance, e.g., DOC middleware combines
various aspects of a larger solution together, e.g., fault tolerance for domain-specific objects with real-
time QoS properties.

When developers needn’t worry as much about low-level details, they are freed to focus on more strategic,
larger scope, application-centric specializations concerns, such as distributed resource management and end-
to-end dependability. Ultimately, this higher level focus will result in software -intensive distributed system
components that apply reusable middleware to get smaller, faster, cheaper, and better at a predictable pace,
just as computing and networking hardware do today. And that, in turn, will enable the next-generation of
better and cheaper approaches to what are now carefully crafted custom solutions, which are often inflexible
and proprietary. The result will be a new technological economy where developers can leverage:

1.Frequently used common components, which come with steady innovation cycles resulting from a multi-
user basis, in conjunction with
 2.Custom domain-specific capabilities, which allow appropriate mixing of multi-user low cost and custom
development for competitive advantage.

• Potential complexity cap for next-generation complex systems – As today’s technology transitions run their
course, the systemic reduction in long-term R&D activities runs the risk of limiting the complexity of next-
generation systems that can be developed and integrated using COTS hardware and software components.
The advent of open DOC middleware standards, such as CORBA and Java-based technologies, is hastening
industry consolidation towards portable and interoperable sets of COTS products that are readily available for
purchase or open-source acquisition. These products are still deficient and/or immature, however, in their
ability to handle some of the most important attributes needed to support future systems. Key attributes
include end-to-end QoS, dynamic property tradeoffs, extreme scaling (large and small), highly mobile
environments, and a variety of other inherent complexities. Complicating this situation over the past decade
has been the steady flow of faculty, staff, and graduate students out of universities and research labs and into
startup companies and other industrial positions. While this migration helped fuel the global economic boom
in the late ‘90s, it does not bode well for long-term technology innovation.

As distributed systems grow in complexity, it may not be possible to sustain the composition and integration
perspective we have achieved with current middleware platforms without continued R&D. Even worse, we
may plunge ahead with an inadequate knowledge base, reverting to a myriad of high-risk independent
solutions to common problems. Ultimately, premium value and competitive advantage will accrue to those
who master the patterns and pattern languages
[Sch00b] necessary to integrate COTS hardware and DOC middleware to develop low cost,
complex distributed systems with superior domain-specific attributes that cannot be bought off-
the-shelf at any given point in time.