1. Universiti Malaysia Perlis
ENT164
SENSORS & MEASUREMENT
Lecture 5: Computerized Data-Acquisition System (DAS/DAQ)
AHI
Sem 2, 2011/2012

2. Crittercam http://animals.nationalgeographic.com/animals/crittercam/
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Sem 2, 2011/2012

3. A data acquisition (DAQ) system is a collection of add-on hardware and software components that allow the computer to receive real-world information from sensors. It consists of
Sensors
I/O terminal panel(s)
DAQ board(s)
Program/Software
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Sem 2, 2011/2012

4. Example of Computer DAQ System
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Sem 2, 2011/2012

5. Computer System – PC for DAQ
Typical PC case
Data-acquisition board with eight analog input channels and two analog output channels also showing an analog signal connection box. Manufactured by National Instruments.
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Sem 2, 2011/2012

6. Computer System – Components of PC
Central Processing Unit (CPU)
Program (Software)
Random Access Memory (RAM)
Mass Storage System
Display
User Input Devices (keyboards, mouse, etc)
Printers and Plotters
AHI
Sem 2, 2011/2012

7. Everyday number format – base 10 (decimal)
Computer System – Representing Numbers in PC
Everyday number format – base 10 (decimal)
Computer number format – base 2 (binary)
Information stored in Computers is stored in flip-flops which have two possible states (“0” or “1”)
Multiple series of flip-flop required to represent a number.
MSB and LSB?
2’s complement?
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Sem 2, 2011/2012

8. Bit
The smallest unit in digital signal is the bit, a contraction of the more descriptive phase of binary digit.
A bit is a single element in digital signal, having only two possible states: on (indicating 1) or off (indicating 0).
Off On 1
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Sem 2, 2011/2012

9. Bytes
Bits are organized into larger units called bytes, the basic unit of information in a computer system.
A basic byte contains 8 bits. The total amount of information it can convey is 28 (=256) possible combinations.
Off On 1
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Sem 2, 2011/2012

10. Example 4.1: Convert the 8-bit binary numbers to 01011100 to decimal
4.2:
Find the 8-bit binary number with the same value as that of the decimal value 92.
4.3:
Convert the decimal -92 to an 8-bit 2’s complement binary number.
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Sem 2, 2011/2012

11. Other binary codes - ASCII
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Sem 2, 2011/2012

12. Sample and Hold Subsystems
DAQ Components
Multiplexers (MUX)
Basics of A/D
Practical A/D
DAC
Sample and Hold Subsystems
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Sem 2, 2011/2012

13. DAQ Components - Multiplexers (MUX)
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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Sem 2, 2011/2012

14. DAQ Components - Multiplexers (MUX)
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Sem 2, 2011/2012

15. DAQ Components – Basics A/D
Analog vs Digital
Represent 5V in analog and digital device. How?
Textbook page 89
Most transducers generate (output) an analog signal.
Example: temperature sensor.
Analog-to-Digital Converter A/D) is required.
Definition (Wiki) : An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device that converts a continuous quantity to a discrete time digital representation.
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Sem 2, 2011/2012

16. DAQ Components – Basics A/D
Sensor output (V)
Bulb 1
Bulb 2
V < 2.5
Off
2.5 ≤ V < 5.0 On
5.0 ≤ V < 7.5
V ≥ 7.5 V
How this above table relate to A/D???
Refer page 90
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Sem 2, 2011/2012

17. DAQ Components – Basics A/D
Characteristic of A/D
Number of bits to represent the output
Input range
Conversion speed
Number of bits:
greater the bits, greater number of possible output states and high accuracy.
Input range:
range of analog input voltages over which the converter will produce a representative digital output.
Unipolar: same sign analog input (i.e. 0 to10V)
Bipolar: both positive and negative (i.e. 0 to ±5V)
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Sem 2, 2011/2012

18. DAQ Components – Basics A/D
Conceptual simple type of A/D: Ramp A/D converter process
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Sem 2, 2011/2012

19. DAQ Components – Basics A/D
Conceptual simple type of A/D: Circuit
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Sem 2, 2011/2012

20. DAQ Components – Basics A/D
Example 4.4
A 4-bit single slope integrating A/D converter has an input range of 0 to 10V. Compute the digital output for an analog input of 6.115V.
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Sem 2, 2011/2012

21. DAQ Components – Basics A/D
In reality, there are simple ways to estimate the digital form of an analog output.
Page 93 and table 4.2 for explanation.
Some formula could also be used, as in Figure 4.7
Example 4.5
Example 4.6
Example 4.7 – input resolution error
Errors in A/D:
Systematic error: linearity, zero and sensitivity errors
Thermal-stability error: can effect zero, sensitivity and linearity.
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Sem 2, 2011/2012

22. Formulas to estimate A/D converter digital output.
Figure 4.7:
Formulas to estimate A/D converter digital output.
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Sem 2, 2011/2012

23. DAQ Components – Practical A/D
Commonly used A/D in DAQ – successive-approximations converter employs an interval-halving technique (iterative bisections calculations).
A series of known analog voltages are created and compared to the analog input voltage, before halved to approximate its digital representation.
Repeated iteratively.
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Sem 2, 2011/2012

24. DAQ Components – Practical A/D
Figure Block diagram of a successive-approximations A/D converter. Copyright 1990 Instrument Society of America. From Computer-based Data Acquisition Systems by J.Taylor.
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Sem 2, 2011/2012

25. DAQ Components – Practical A/D
Example 4.8
An 8-bit A/D unipolar successive-approximations converter has an input range of 0 to 5V. An analog voltage of 3.15V is applied to the input. Determine the digital output.
Answer:
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Sem 2, 2011/2012

26. Solution for Ex. 4.8 Trial digital output (Do) Do X ∆V Pass/Fail
Actual Digital Output
(128)
2.5
Pass
(192)
3.75
Fail
(160)
3.125
(176)
3.438
(168)
3.281
(164)
3.203
(162)
3.164
(161)
3.144
End
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Sem 2, 2011/2012

27. DAQ Components – Practical A/D
Type and Advantage of Practical A/D
Parallel or Flash A/D
Fastest conversion
Completed algorithms in one step
As fast as 10ns
Block diagram in Figure 4.10
Half-Flash Converter
two flash converters
Complete 12-bit conversion in about 100ns
Dual-slope integrating converter
Used in digital voltmeters
Most accurate type of A/D
Noise is averaging out (eliminate)
Conversion time slower compared to previous two.
Figure 4.11(a0 and (b)
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Sem 2, 2011/2012

28. DAQ Components – Practical A/D
Figure Block diagram of a parallel A/D converter. Based on S. Franco, Design with Operational Amplifiers and Analog Integrated Circuits, McGraw-Hill, Used with permission of McGraw-Hill Inc.
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Sem 2, 2011/2012

29. DAQ Components – Practical A/D
Figure Block diagram of a dual-slope integrating A/D converter: (a) block diagram;
(b) integration process. Copyright 1990 Instrument Society of America. From Computer-based Data Acquisition Systems by J.Taylor.
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Sem 2, 2011/2012

30. DAQ Components – D/A
Even though purpose of measurement is to collect data or display them, but sometime the result of measurement could serve as control function.
Most devices used to control are analog device, eg solenoids, heaters, valves, etc etc.
Remember again computer are digital, and input TO computer from DAQ is converted to digital using A/D.
Thus, for control function, the computer is required to convert them back to analog.
Such device are known as Digital-to-Analog Converter, D/A or DAC.
Schematic is shown in Figure 4.12, 4 bit and 4 switches.
Generally, electrical power from D/A are small, and insufficient to control any control devices (solenoids, valves, etc). Normally it is amplified.
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Sem 2, 2011/2012

31. DAQ Components – D/A
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Sem 2, 2011/2012

32. DAQ Components – Simultaneous Sample & Hold Subsystems
When using a multiplexer.
Recall principle of multiplexer.
Recording all channels or its subset in precisely same time could be done by using Simultaneous Sample & Hold Subsystems
Figure 4.13 shows the subsystem.
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Sem 2, 2011/2012

33. DAQ Components – Sample & Hold Subsystems
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Sem 2, 2011/2012

34. DAQ Configuration Software for DAQ
Internal single board plug-in system
External system
Virtual instrument
Digital storage oscilloscope
Data loggers
Software for DAQ
Commercial Software Package
Example??
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Sem 2, 2011/2012

35. Some of DAQ boards available in market
Some of DAQ boards available in market. > Keithley products > National Instrument (NI-DAQ) > Agilent > etc etc
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Sem 2, 2011/2012

36. AHI
Sem 2, 2011/2012

37. AHI
Sem 2, 2011/2012

38. AHI
Sem 2, 2011/2012

39. What kind of DAQ did u used for your lab?
What kind of software?
THANK YOU
AHI
Sem 2, 2011/2012