Week 9 - Data Acquisition Systems (DAQ)
Week 9 - Data Acquisition Systems (DAQ)
Week 9 - Data Acquisition Systems (DAQ)
Date: 01/02/2012 Prepared by: Maizatul Nurul Bariah Ahmad Email: maizatulnurul@unimap.edu.my/0194052335
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Data
acquisition systems are products or processes used to collect information to document or analyze some phenomenon to capture basic electro-mechanical phenomena by measuring the electrical output from a variety of transducers
Designed
Transducer 1
Signal Conditioner 1
Analog Data
Transducer 2
Signal Conditioner 2
Transducer 3
Signal Conditioner 3
M U L T I P L E X E R
Transducer 4
Signal Conditioner 4
Computer Processing
1. 2. 3. 4. 5.
(a) (b)
Sensors and Transducers Signals Signal Conditioning DAQ Hardware Driver and Application Software
Driver Software Application Software
Sensors and Transducers A device that converts a physical phenomenon into a measurable electrical signal the ability of a data acquisition system to measure different phenomena depends on the transducers to convert the physical phenomena into signals measurable by the data acquisition hardware. Transducers are synonymous with sensors in data acquisition system. There are specific transducers for many different applications.
1.
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2.
Signals
The appropriate transducers convert physical phenomena into measurable signals. However, different signals need to be measured in different ways. For this reason, it is important to understand the different types of signals and their corresponding attributes. Signals can be categorized into two groups: Analog Digital
Analog
Signal
Exist at any value with respect to time Eg: voltage, temperature, pressure, sound and load Primary characteristic: level, shape, and frequency
Digital
Signal
Cannot take any value with respect to time Has two possible levels: high and low Useful information measured from a digital signal state and rate.
3)
Signal Conditioning Sometimes transducers generate signals too difficult or too dangerous to measure directly with a DAQ device. Signal conditioning is essential for an effective data acquisition system for instance, when dealing with high voltages, noisy environments, extreme high and low signals, or simultaneous signal measurement too difficult to measure directly with a data acquisition device. Maximize accuracy of a system, allow sensors to operate properly and guarantees safety Types of signal conditioning amplification, attenuation, isolation, bridge completion, simultaneous sampling, sensor excitation, multiplexing.
4)
DAQ Hardware
DAQ hardware acts as the interface between the computer and the outside world It primarily functions as a device that digitizes incoming analog signals so that the computer can interpret them. Other data acquisition functionality includes:
Analog input/output Digital input/output Counter/timers Multifunction a combination of analog, digital, and counter in a single device
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Driver and Application Software Driver Sofware Software transforms the PC and the DAQ hardware into a complete data acquisition, analysis, and presentation tool.
5)
Without software to control or drive the hardware, the DAQ device will not work properly. Driver software is the layer of software for easily communicating with the hardware. It forms the middle layer between the application software and the hardware. Driver software also prevents a programmer from having to do register-level programming or complicated commands in order to access the hardware functions.
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Application Software The application layer can be either a development environment in which you build a custom application that meets specific criteria, or it can be a configuration-based program with preset functionality.
Application software adds analysis and presentation capabilities to driver software. To choose the right application software, evaluate the complexity of the application, the availability of configuration-based software that fits the application, and the amount of time available to develop the application. If the application is complex or there is no existing program, use a development environment. Eg: LabView, Matlab, Visual Basic, etc..
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Introduction to ADC An electronic device that converts analog signals to an equivalent digital form Digital Computer: Binary (discrete) values Physical World: Analog (continuous) values Example: Temperature, Humidity, Pressure Output: Voltage or Current Microcontroller? -----> Digital Therefore, ADC is needed to translate (convert) the analog signals to digital numbers
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ADC
CPU
DISPLAY
ADC Resolution
n-bit 8 10 12
Assuming VREF = 5V * Step Size (Resolution): is the smallest change that can be discerned by an ADC
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ADC Reference Voltage (Vref) Vref: Input voltage used for the reference voltage The voltage connected to this pin , with the resolution of the ADC chip, dictate the step size Example: If we need the analog input to be 0 to 4 volts, Vref is connected to 4 volts Digital data output: 8-bit (D0-D7), 10-bit (D0-D9)
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Digital data output (in decimal): 8-bit (D0-D7)= 256 10-bit (D0-D9) = 1024
An
electronic component found in many data acquisition devices that produce an analog output signal To convert digital values to analog voltages Performs inverse operation of the Analog-toDigital Converter (ADC)
Digital Value
DAC
Analog Voltage
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Types
The reference voltage is constant and is set by the manufacturer. The reference voltage can be changed during operation.
Non-Multiplier DAC
Characteristics
Comprised of switches, op-amps, and resistors Provides resistance inversely proportion to significance of bit
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Rf = R
I
MSB
2R
4R
8R
LSB
Vo
-VREF
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Rf = R
I
R 2R 4R 8R
Vo
VREF
CLEARED
-VREF
( 1
1 )2 = ( 15 )10
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Rf = R Weighted Resistors based on bit R Reduces current by a MSB factor of 2 for each bit
I
2R 4R 8R
Vo
LSB
-VREF
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Result:
B3 B2 B1 B0 + + + I = VREF R 2 R 4 R 8R
VOUT
B2 B1 B0 = I R f = VREF B3 + + + 2 4 8
Bi = Value of Bit i
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More
Generally:
VOUT = VREF
Bi = Value of Bit i n = Number of Bits
Bi
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VREF
MSB
LSB
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Same
input switch setup as Binary Weighted Resistor DAC All bits pass through resistance of 2R
MSB
VREF
LSB
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The less significant the bit, the more resistors the signal muss pass through before reaching the op-amp
LSB
MSB
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The current is divided by a factor of 2 at each node Analysis for current from (001)2 shown below
I0 2
R R 2R R
I0 4
2R R
I0 8
2R 2R
I0
VREF
B1
B2
B0
VREF VREF = I0 = 2 R + 2 R 2 R 3R
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VREF B2 B1 B0 = + + 3R 2 4 8
Bi = Value of Bit i
Rf
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I0
VREF
I0
VREF Op-Amp input Ground
B0
B2
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Binary Weighted
R-2R
Only 2 resistor values Easier implementation Easier to manufacture Faster response time
Limited to ~ 8 bits Large # of resistors Cons Susceptible to noise Expensive Greater Error
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Most computers perform instructions sequentially. Thus, computer take data from sensors one at one time using device called multiplexer (MUX) A concept of electronic switch, selecting particular channels, read and the process them. Figure 4.4 shows a mechanical analog to the DAS, switches are semiconductor devices (eg: transistors) Subject to errors: Crosstalk Transfer accuracy
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