MEASUREMENT AND INSTRUMENTATION BMCC 4743

1
MEASUREMENT AND INSTRUMENTATIONBMCC 4743

• LECTURE 7 COMPUTERISED DATA-ACQUISITION SYSTEMS

Mochamad Safarudin Faculty of Mechanical
Engineering, UTeM 2008
2
Recap from previous lecture

• Measurement process
• Analogue signal conditioning - done
• DIGITAL SIGNAL CONDITIONING

Signal conditioning
Recorder/display/processor
Sensor/transducer
measurand
3
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)

4
Why digital?

• Digital electronics easier to design and
  fabricate
• ex IC, low cost, mass product compare to
  capacitor etc
• 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

5
COMPUTER AS A MEASUREMENT SYSTEM
6
Contents

• Components of computer systems
• Representing numbers in computer systems
• Components of data-acquisition systems
• Configuration of data-acquisition systems

7
Components of computer systems
Display
CPU and RAM
Printer
Digital input-output (ports or expansion bus)
Mass storage (disk drives)
Keyboard
8
Typical computer components

• 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

9
Contents

• Components of computer systems
• Representing numbers in computer systems
• Components of data-acquisition systems
• Configuration of data-acquisition systems

10
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
• 1 byte 8 bits

4 bit binary number
MSBMost Significant Bit LSB Least Significant
Bit
MSB
LSB
11
Examples (binary/decimal)

1. Convert the 8-bit binary number 01011100 to
   decimal
2. Find the 8-bit binary number with the same value
   as that of the decimal number 92.

12

• 01011100
• N100(27)1(26)0(25)1(24)1(23)2(22)0(21)0(2
  0)
• 0640168400
• 92
• By a series of divisions by 2
• remainder
• 2 92
• 2 46 0
• 2 23 0
• 2 11 1
• 2 5 1
• 2 2 1
• 2 1 0
• 0 1

LSB
Answer 1011100 but we are asked for 8
bit 01011100
MSB
13
Most commonly represented using2s complement
binary
What about negative number?

• Procedure
• Convert the integer to binary as if it were
  positive
• Invert all of the bits change 0s to 1s and
  1s to 0
• Add 1 LSB to the final result
• e.g. convert 92 to an 8-bit 2s complement
  binary number
• answer from previous, 01011100
• invert 10100011
• 1 LSB 101000112 12 become 10100100
• Note that, positive numbers always have 0 as MSB
  and negative
• numbers have 1 as MSB
• In a computer a special code is used ASCII
  American Code for Information Interchange, e.g. k
  011010112 10710

14
ASCII Characters
15
Contents

• Components of computer systems
• Representing numbers in computer systems
• Components of data-acquisition systems
• Configuration of data-acquisition systems

16
Components of data-acquisition systems

• Multiplexer
• Simultaneous sample-and-hold subsystem
• ADCs
• DACs

17
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.

18
Simultaneous sample-and-hold subsystem

• 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

19
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

20
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)

21
Unipolar single-slope integrating converter

1. A fixed reference voltage is used to charge an
   integrator at a constant rate
2. The integrator output voltage then increase
   linearly with time
3. A digital clock (counter) is started at the same
   time that the charging is begun
4. The integrator output voltage is compared
   continuously with the analog input voltage using
   a comparator
5. When the integrator voltage exceeds the analog
   input voltage, digital clock is stopped
6. The count of the digital clock is the digital
   output of the A/D converter

22
Example
23
Formula to estimate A/D converter digital output

• The output of a 2s-complement, given the
  analogue input voltage, is
• 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
• where output will range from 0 to (2N-1) max.

24
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
25
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
• The quantisation error is thus 0.5 LSB

26
Successive-approximations converter (most common
type)

• 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
27
Example
28
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
The resolution uncertainty of 0.00244 is the
best that can be achieved
Comment if input voltage0.1 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
29
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.

30
4 bit D/A converter

• Rn2nRf
• When the switched is closed, in flows to the
  summing bus

• The op-amp converts the currents to voltages

31
Example A digital code 1011 (equivalent to 11)
for the circuit above with Rf 5 kq and vs-10 V.
then i1-1 mA i20 i3-1/4 mA i4-1/8 mA Summing
these currents and multiplying by Rf gives
Vo6.875 V which is 11/16 of the full scale
(ref) voltage
32
Contents

• Components of computer systems
• Representing numbers in computer systems
• Components of data-acquisition systems
• Configuration of data-acquisition systems

33
Configuration of data-acquisition systems

• 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.