MEASUREMENT AND INSTRUMENTATIONBMCC 4743

LECTURE 7: COMPUTERISED DATA-ACQUISITION SYSTEMS

Mochamad Safarudin

Faculty of Mechanical Engineering, UTeM

2008

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Recap – from previous lecture

Measurement process

Analogue signal conditioning - done

DIGITAL SIGNAL CONDITIONING

Sensor/transducerSignal

conditioningRecorder/display/processormeasurand

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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 fabricateex: IC, low cost, mass product compare to capacitor etc

2. Ease of data recording, storage and displayex: digital voltmeter provides a direct numerical displayof voltage compared with analog voltage that has to bevisually 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

CPU and RAM

Printer

Display

Mass storage(disk drives)

Keyboard

Digital input-output

(ports or expansion bus)

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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

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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

1 byte = 8 bits

MSB LSB

4 bit binary number

MSB:Most Significant BitLSB: Least Significant Bit

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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.

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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 2remainder

2 922 46 02 23 02 11 12 5 12 2 12 1 0

0 1

LSB

MSB

Answer:1011100 but weare asked for 8 bit:01011100

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Most commonly represented using:2’s complement binaryProcedure:

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

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

What about negative number?

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ASCII Characters

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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 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 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. A fixed reference voltage is used to charge an integrator at a constant rate2. The integrator output voltage then increase linearly with time3. A digital clock (counter) is started at the same time that the charging is begun4. The integrator output voltage is compared continuously with the analog input

voltage using a comparator5. When the integrator voltage exceeds the analog input voltage, digital clock is

stopped6. 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

The output of a 2’s-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.

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22int

NN

rlru

rlio VV

VVD

N

rlru

rlio VV

VVD 2int

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From example before, estimate the digital output for 6.115 V analog input to A/D converter

Example:

Answer:Since this is a simple binary devices the second equationIs applicable:

10)78.9int(2010

0115.6int 4

xDo

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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 =

The quantisation error is thus ±0.5 LSB

voltsVVN

rlru

25.0

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Successive-approximations converter (most common type)1. A series of known analog voltages are created and compared to the analog input

voltage2. In the first trial, a voltage interval of one-half the input span is compared with the

input voltage3. If the input voltage is in the upper half of the range, the MSB is set to1; otherwise

it is set to zero4. 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 aproximationmethod for 4 bit A/Dconverter

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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

00244.02

)10(105.0

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errorresolutioninput

The resolution uncertainty of ±0.00244 is the best thatcan 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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1. Rn=2nRf

2. When the switched is closed, in flows to the summing bus

Rf

v

R

vi

nR

n

Rn 2

3. The op-amp converts the currents to voltages

k

nnfo iRv

1

4 bit D/A converter

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Example:A digital code 1011 (equivalent to 11) for the circuit abovewith Rf= 5 kq and vs=-10 V. then

i1=-1 mAi2=0i3=-1/4 mAi4=-1/8 mASumming 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 data-acquisition systems

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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.