Transcript MEASUREMENT AND INSTRUMENTATION BMCC 4743

MEASUREMENT AND INSTRUMENTATION
BMCC 4743
LECTURE 7: COMPUTERISED
DATA-ACQUISITION SYSTEMS
Mochamad Safarudin
Faculty of Mechanical Engineering, UTeM
2008
Recap – from previous lecture

Measurement process
Sensor/transducer
measurand
Signal
conditioning
Recorder/display/
processor

Analogue signal conditioning - done
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DIGITAL SIGNAL CONDITIONING
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ANALOG AND DIGITAL
•Most measurands originate in analog form
•Analog signal varies smoothly in time, without discontinuty
•Example: 220 V ac, 60 Hz power line voltage
Example of analog signal
•Digital information is transmitted and processed in form of bits
•Each bit defined by one or other of two predefined “logic level”
•The time interval assigned to it called bit interval
•Most common two logic states is predetermined voltage levels
(say 0 and 5 V dc)
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Why digital?
1. Digital electronics easier to design and fabricate
ex: IC, low cost, mass product compare to capacitor etc
2. Ease of data recording, storage and display
ex: digital voltmeter provides a direct numerical display
of voltage compared with analog voltage that has to be
visually interpolated if the pointer is between two scales
3. Inherently noise resistant
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COMPUTER AS A
MEASUREMENT SYSTEM
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Contents

Components of computer
systems

Representing numbers in computer
systems
Components of data-acquisition
systems
Configuration of data-acquisition
systems
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Components of computer systems
Display
Digital
input-output
(ports or expansion bus)
Printer
CPU and RAM
Mass storage
(disk drives)
Keyboard
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Typical computer components

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Central processing unit (CPU)
Program (software)
Random access memory (RAM) - ROM
Mass storage system – magnetic tape
recorder, magnetic disk drive, optical disk
drive
 Display/monitor/screen
 User input device (keyboard, mouse,
joystick,etc)
 Printers and plotters
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Contents

Components of computer systems

Representing numbers in
computer systems

Components of data-acquisition
systems
Configuration of data-acquisition
systems

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Representing numbers in computer
systems

Computers use bistable flip-flops to
store information, which have only 2
possible states: on (1) or off (0)
E.g. 1001 2
MSB

LSB
4 bit binary number
MSB:Most Significant Bit
LSB: Least Significant Bit
1 byte = 8 bits
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Examples (binary/decimal)
1.
2.
Convert the 8-bit binary number
01011100 to decimal
Find the 8-bit binary number with the
same value as that of the decimal
number 92.
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1. 01011100
N10=0(27)+1(26)+0(25)+1(24)+1(23)+2(22)+0(21)+0(20)
=0+64+0+16+8+4+0+0
=92
2. By a series of divisions by 2
remainder
2
92
LSB
2
46
0
2
23
0
2
11
1
2
5
1
2
2
1
2
1
0
MSB
0
1
Answer:
1011100 but we
are asked for 8 bit:
01011100
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What about negative number?
Most commonly represented using:
2’s complement binary
Procedure:
1.
Convert the integer to binary as if it were positive
2.
Invert all of the bits – change 0’s to 1’s and 1’s to 0’
3.
Add 1 LSB to the final result
e.g. convert –92 to an 8-bit 2’s
complement binary number
answer: from previous, 01011100
invert
10100011
+1 LSB
101000112 + 12 become 10100100
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Note that, positive numbers always have 0 as MSB and negative
numbers have 1 as MSB
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In a computer a special code is used : ASCII – American Code for
Information Interchange, e.g. k = 011010112 = 10710
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ASCII Characters
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Contents


Components of computer systems
Representing numbers in computer
systems

Components of dataacquisition systems

Configuration of data-acquisition
systems
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Components of data-acquisition
systems
Multiplexer
 Simultaneous sample-and-hold
subsystem
 ADCs
 DACs
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Multiplexer (MUX)
Works as an electronic switch –
computer will ask MUX to select a
particular channel to be read and
processed, sequentially.
 Can have crosstalk errors and transfer
accuracy.
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Simultaneous sample-and-hold
subsystem
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Need to be used to record data from
different channel of MUX, precisely at
the same time.
e.g. Measuring tire forces using 6
component force transducers
simultaneously
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Analogue-to-Digital Converters
Converts continuous analogue
waveform into discrete digital signals
 Examples: audio amplifiers, TV, output
voltage from transducers, etc
 Output of ADCs has 2N possible values
 If N , no. of possible output states ,
hence results more accurate
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Types of ADCs

Unipolar single-slope integrating converter
(ramp type – quite slow, not very accurate)
 Successive-approximations converter (quite
fast – typical 12-bit completes a conversion in
10 – 25 μs)
 Parallel or flash or half-flash converter (the
fastest – can be 10 ns, using lots of
comparators)
 Dual-slope integrating converter (used in
digital voltmeter)
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Unipolar single-slope integrating converter
1.
2.
3.
4.
5.
6.
A fixed reference voltage is used to charge an integrator at a constant rate
The integrator output voltage then increase linearly with time
A digital clock (counter) is started at the same time that the charging is begun
The integrator output voltage is compared continuously with the analog input
voltage using a comparator
When the integrator voltage exceeds the analog input voltage, digital clock is
stopped
The count of the digital clock is the digital output of the A/D converter
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Example
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Formula to estimate A/D converter digital output
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The output of a 2’s-complement, given the analogue input
voltage, is
 Vi  Vrl N  2 N
Do  int
2 
Vru  Vrl
 2
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where max. positive output is (2N/2 –1) and max. negative
output is (-2N/2)
The output of an offset binary or simple binary converter is given
by
 Vi  Vrl N 
Do  int
2 
Vru  Vrl

where output will range from 0 to (2N-1) max.
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Example:
From example before, estimate the digital output for
6.115 V analog input to A/D converter
Answer:
Since this is a simple binary devices the second equation
Is applicable:
 6.115 0 4 
Do  int
x2   int(9.78)  10
 10  0

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Quantisation error

Resolution uncertainty (or treated as
random error, analogous to the reading
error of a digital display) due to output of
ADC with discrete steps, given by
Input resolution error =

Vru  Vrl
 0.5
volts
N
2
The quantisation error is thus ±0.5 LSB
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Successive-approximations converter (most common type)
1.
2.
3.
4.
A series of known analog voltages are created and compared to the analog input
voltage
In the first trial, a voltage interval of one-half the input span is compared with
the input voltage
If the input voltage is in the upper half of the range, the MSB is set to1; otherwise
it is set to zero
This process is repeated with an interval half the width of the interval used in the
first trail to determine the second MSB and so forth until LSB is determined
Successive aproximation
method for 4 bit A/D
converter
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Example:
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Example:
A 12-bit A/D converter has an input range of -10 to +10 V.
Find the resolution error of the converter for the analog input.
Answer:
Using above equation
10  (10) 
input resolutionerror  0.5
  0.00244
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
The resolution uncertainty of ±0.00244 is the best that
can be achieved
Comment: if input voltage=0.1 V (low end of input range),
The quantization error would represent 2.5% of the reading,
which is probably not acceptable. The input signal should be
amplified probably before the signal enters the converter
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Digital-to-Analogue Converters
Converts discrete digital signals into
continuous analogue waveform
 Examples: To operate heaters or valves
under computer control
 Similar specs as ADCs, i.e. depends on
no. of input bits, analogue output range
and conversion speed.
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4 bit D/A converter
1. Rn=2nRf
2. When the switched is closed, in flows to the summing
bus
in 
vR
v
 nR
Rn 2 Rf
3. The op-amp converts the currents to voltages
k
vo   R f  in
n 1
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Example:
A digital code 1011 (equivalent to 11) for the circuit above
with Rf= 5 kq and vs=-10 V. then
i1=-1 mA
i2=0
i3=-1/4 mA
i4=-1/8 mA
Summing these currents and multiplying by Rf gives
Vo=6.875 V which is 11/16 of the full scale (ref) voltage
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Contents




Components of computer systems
Representing numbers in computer
systems
Components of data-acquisition
systems
Configuration of dataacquisition systems
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Configuration of data-acquisition
systems
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General overview of DAS configuration is
given by
– Plugging one or more DAQ circuit boards
(includes a MUX and an ADC with an amplifier)
into the bus of a PC
– PC turns into digital oscilloscope
– GPIB (General Purpose Interface Bus) or IEEE488
system
– Process control high performance computers
– Distributed DAS – latest development for process
control where it use modular components close to
the sensors.
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