Overview

Instruments can be considered as systems, and there are only a few structural schemes
employed in the construction of such systems; one such scheme is the use of feedback. The general
mode of operation of feedback-measuring systems is ascertained, and the reasons for applying feedback
to a measurement situation are established. Using feedback, it is possible to improve accuracy and
speed of measurement, reduce the effect of interfering and modifying inputs, and allow remote
indication and non-contact measurement. The property of inversion assists instrument design, and can
provide for digital indication. The common balance variables are listed, and a wide range of instruments
is discussed, making explicit the general properties of feedback.

Introduction/concept

Application of the feedback principle had its beginnings in simple machines and instruments,
some of them going back 2000 years or more (Mayr 1970). In fact, the ancient water clock is the earliest
known device for feedback control. It was invented in the third century Be by a Greek mechanic called
Ktesibios working in Alexandria, and he employed a float regulator. Records also exist of water clocks
using float valves in the ninth century.

The invention of the feedback amplifier (Black 1934) was considered by Greig (1950) to be
probably the most important single influence on measurement technique from a conceptual and
practical point of view in the period that followed. Many instruments incorporating feedback techniques
for the measurement of a wide range of variables are available today, and the future extensive
development and use of feedback-measuring systems seems certain.

One can think of a feedback system as a system which tends to maintain automatically a
prescribed relationship of one system variable to another by comparing functions of these variables and
using the difference as a means of control. The main characteristic of a feedback system is its closed-
loop structure. The author has chosen to call a measuring system in which feedback is the basic
structural arrangement, a feedback-measuring system Oones 1974), and has defined a
feedbackmeasuring system as one that measures a variable by using error sensing through a closed loop
Oones 1977).

Example of Feedback System

Home furnace control system must control the temperature in the room and kept it constant.

As in open loop system a timer is used to switch on the furnace for some time and then switch it
off, accuracy is not obtained.

This is because the system does not act according to the room temperature but according to a
preset value of time.

A closed loop control system takes care of this problem. The feedback unit (the main component of the
closed loop control system) senses the room temperature and accordingly turns on or off the furnace.

The feedback unit feeds the output back to a comparator which is provided with a reference value with
which the output is compared to generate an error signal. This error signal generates the required
control action.

In the home furnace control system, a temperature sensor is used to sense the temperature in the room
and this is feedback to an error detecting device. The error detecting device compares the room
temperature with the reference value.
If it detects that the room temperature is higher than the reference value, if generates a signal to switch
off the furnace. On the other hand, if it detects that the room temperature is lower than the reference
value, it generates a signal to switch on the furnace. This has been shown in the figure.

Hence it is clear that a system with feedback (closed loop control system) is efficient than that of a
system without feedback (open loop control system).

Simple discrete feedback systems, such as sensors to detect end-of-stroke on pneumatic cylinders, or
the use of discrete handshaking signals between equipment instead of timers to adjust controller
output, improves control and monitoring. The same is true for properly designed closed-loop feedback
on a process.

losing the loop with feedback improves control, measuring, and monitoring of packaging, process, and
custom machinery. Checking the actual output condition and adjusting the commanded output helps
machinery automatically adapt to changing conditions. Open-loop systems save money initially, but will
almost always be less efficient and not as repeatable, resulting in a higher total cost of ownership.

Advantages of feedback control

Using feedback in closed-loop systems improves control by automatically adjusting the
controller output to reduce the error. This helps reduce the effects of dynamic
disturbances. Feedback also adds stability to an unstable process, ensuring a
repeatable and reliable control loop. Table 1 lists some of the main reasons to add
closed-loop feedback to a machine or process.

Many processes have been manually “tweaked” for years, with operators adjusting the
controller output to reduce the error. With today’s sensor and controller technology,
many of these open loops can make use of feedback and a controller to improve
operation.

Reducing human involvement in the feedback loop greatly reduces process variations,
and allows for continuous improvement as control loop parameters can be continually
adjusted to optimize control. These adjustments can be made automatically by
various loop tuning software algorithms and programs, or manually by experienced
operators. In many cases, a combination of the two methods is used, with operators
evaluating recommended changes from loop tuning software, and implementing
recommendations judiciously.

Use of feedback in 24/7 operations can reduce process variations and changes that
may occur at shift changes as different operators put their own spin on manual loop
control. It can also reduce the number of operators needed, or allow operators and
other plant personnel to concentrate on other areas such as optimizing operations.

With automatic control enabled by feedback and change control functionality enforced in
the controller, process repeatability is improved along with output quality.
Applications for feedback control systems
Although sensor feedback and closed-loop control is taught in most engineering
programs, some skimp on sensors to save upfront costs, often neglecting the long-term
benefits of closed-loop control. Improving efficiency and reducing process waste are
real benefits of feedback in control systems; although they are harder to evaluate than
upfront costs, they still must be carefully calculated and considered.

Improving precision, increasing throughput, and enhancing quality are possible with
proper feedback. For example, maintaining a steady, reliable head pressure of product
in a filling machine’s hopper is achieved with feedback sensors, and is a simple solution
to improve accuracy. Providing a consistent and regulated supply of ingredients
improves product quality. Careful, automatic control of material feed reduces spillage
and increases productivity, and required changes are automatically enabled by proper
feedback and design of closed-loop control.

A programmable logic controller (PLC) or programmable automation controller (PAC)

provides logic in many closed-loop systems, and provides significant configuration
options. Smart relays and low-cost PLCs can be configured to respond to feedback
programmatically. Control of the percent on time of a contactor controlling a heating
element is an example of simple closed-loop control. Although this type of feedback and
control works in many applications, it does not respond to input disturbances as well as
more sophisticated means, such as proportional-integral-derivative (PID) control or
advanced control methods.

High-end PLCs
Higher end PLCs will have built-in PID control, which will provide a much higher degree
of control, particularly if the PID parameters are set correctly. The PID control output is
generally sent to an analog output channel, or may be used as a time proportion on a
discrete output. With analog control, the output can also be used as a setpoint to
another loop for cascade control.

When designing a control system, feedback and closed-loop systems can greatly
enhance performance. Identify areas where feedback can improve and optimize
machine operation and process control. Carefully select from the wide variety of
available process control and measurement sensors, and make sure each loop is
programmed with the right parameters. Following these steps will improve operation of
your machines and processes, while reducing labor requirements and enhancing
quality.

– Bill Dehner is an engineer with AutomationDirect; edited by Mark T. Hoske, content

manager, Control Engineering, mhoske@cfemedia.com.