measurement and instrumentation_lecture6

8/12/2019 Measurement and Instrumentation_lecture6

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MEASUREMENT AND INSTRUMENTATION

BMCC 3743

PC BASED

DATA-ACQUISITION SYSTEMS

Mochamad Safarudin

Faculty of Mechanical Engineering, UTeM

2010


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Measurement process

Analogue signal conditioning - done

DIGITAL SIGNAL CONDITIONING

Sensor/transducerSignal

conditioning

Recorder/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 designand fabricate

ex: IC, low cost, mass product

compare to capacitor etc

2. Ease of data recording, storage and

displayex: digital voltmeter provides a

direct numerical display

of voltage compared with analog

voltage that has to be

visually interpolated if the pointeris between two scales

3. Inherently noise resistant


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COMPUTER AS AMEASUREMENT SYSTEM


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Contents

Components of computersystems

Representing numbers in computersystems

Components of data-acquisitionsystems

Configuration of data-acquisitionsystems


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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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COMPUTERIZED DATA ACQUISITION SYSTEM


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Typical computer components

Central processing unit (CPU)

Program (software)

Random access memory (RAM) - ROM

Mass storage systemmagnetic 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


Representing numbers in

computer systems Components of data-acquisition

systems



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Examples (binary/decimal)

1. Convert the 8-bit binary number01011100 to decimal

2. Find the 8-bit binary number with thesame value as that of the decimal

number 92.


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1. 01011100

N10=0(2

7

)+1(2

6

)+0(2

5

)+1(2

4

)+1(2

3

)+2(2

2

)+0(2

1

)+0(2

0

)=0+64+0+16+8+4+0+0

=92

2. By a series of divisions by 2

remainder2 92

2 46 0

2 23 0

2 11 12 5 1

2 2 1

2 1 0

0 1

LSB

MSB

Answer:

1011100 but weare asked for 8 bit:

01011100


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Most commonly represented using:

2s complement binary

Procedure:

1. Convert the integer to binary as if it were positive

2. Invert all of the bitschange 0s to 1s and 1s to 0

3. Add 1 LSB to the final resulte.g.convert92 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 0as MSB and negativenumbers have 1 as MSB

In a computer often a special code is used : ASCIIAmerican

Standard Code for Information Interchange, e.g. k= 011010112=

10710

What about negative number?


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ASCII Characters


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Contents


Representing numbers in computer

systems

Components 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 switchcomputer

will ask MUX to select a particular channelto be read and processed, sequentially.

Can have crosstalk errors and transfer

accuracy.

Illustration of a multiplexer

In this figure, channel 1 is connected


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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 2Npossible values

If N , no. of possible output states , henceresults more accurate


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Types of ADCs

Unipolar single-slope integrating converter(ramp typequite slow, not very accurate)

Successive-approximations converter(quite fast

typical 12-bit completes a conversion in 1025s)

Parallel or flash or half-flash converter(thefastestcan 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 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


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Example


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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/21) and max. negative output is

(-2N/2)

The output of an offset binary or simple binary converteris given by

where output will range from 0 to (2N-1) max.

2

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 equation

is 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

voltsVV

N

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

voltage

2. 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 zero

4. 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

00244.02

)10(105.0

12

errorresolutioninput

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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1. Rn=2nRf

2. When the switched is closed, inflows to the summing

bus

f

n

R

n

Rn

R

v

R

vi

2

3. The op-amp converts the currents to voltages

k

n

nfo iRv1

4 bit D/A converter


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Example:

A digital code 1011 (equivalent to 11) for the circuit above

with Rf= 5 kW and vs=-10 V. then

i1=-1 mA

i2=0

i3=-1/4 mA

i4=-1/8 mASumming these currents and multiplying by Rfgives

Vo=6.875 V which is 11/16 of the full scale (ref) voltage


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Contents


Representing numbers in computer

systems

Components of data-acquisition

systems

Configuration of data-acquisition systems


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Configuration of data-acquisition

systems

Internal single board plug in system (PCI slot)

External system (USB, IEEE1394, Ethernet,

RS-232, GPIB)

Virtual Instruments (modular) ex: Labview

from NI

PC turns into digital storage oscilloscope

Data loggers (simpler and specialized)

ex: flight data recorder in airplane

Several configuration of DAS


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INTERNAL SINGLE BOARD PLUG IN SYSTEM

PCI DAQ


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EXTERNAL SYSTEM

USB DAQ from NI


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VIRTUAL INSTRUMENTS

LABVIEW from NI
http://www.ni.com/images/labview/us/userinterfaces_large.jpg


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DATA LOGGER


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EXAMPLE:

How many bits are required for a digital device to represent the decimal

number 27541 in simple binary? How many bits for 2s complement binary?

Solution:

The following table presents the maximum decimal number versus the number of bits for simple

binary:

No. Bits Max. Dec. No. Simple Binary

12 212-1 = 4095

13 213-1 = 819114 214-1 = 16383

15 215 -1 = 32767

16 216-1 = 65535

Consequently, 15 bits are needed to represent 27541 in simple binary. For a two's complement

binary number, the MSB will be zero so 16 bits will be required.


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An A/D converter is to operate with a full-scale voltage of 10V. How many

bits should be employed to obtain a resolution of 0.01 %?


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A 12-bit A/D converter has an input range of 8 V, and the output code is

offset binary. Find the output (in decimal) if the input is:

a. 4.2 Vb. -5.7 V

c. 10.9 V

d. -8.5


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Solution:

(d) Since -8.5V falls outside the input range, D0will take the minimum value:

DV V

V V

in rl

ru rl

N

0 2

int

int.

int .

4 2 8

8 82

3123 2

3123

12

D012

5 7 8

8 82

int

.

int .5888

589

D0122 1 4095

N = 12

Vru = 8V

Vrl= -8V

Vin= input voltage(a)By second equation

(b)By second equation :

(c) Since 10.9V falls outside the input range, Dowill have the maximum output:

D0 0


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A 12 bit A/D converter has an input range of 10V and an amplifier at the

Input with a gain of 10. The output of the A/D converter is in 2s complement

Format. Find the output of the A/D converter if the input to the amplifier is:

a. 1.5 Vb. 0.8 V

c. -1.5 V

d. -0.8 V


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16382

22

)10(10

)10(8int

2

22int

1212

NN

rlru

rlin

oVV

VVD

16382

22

)10(10

)10(8int

2

22int

12

12

N

N

rlru

rlin

o VV

VVD

We need the first equation to solve this problem

(a)When the 1.5V signal is amplified with a gain of 10, it becomes 15V which exceeds the input range

of the A/D converter (it is saturated). According to the formula, the maximum output is 2N/2-1 = 212/2-1

= 2047

(b)With the gain of 10, the input becomes 8V. The output, in decimal, is then:

(c) When amplified, -1.5V results in an input to the A/D converter which is

below the input range (it is saturated). The largest negative output is2N/2 = -

2048

(d)With the amplifier, this voltage results in an input to the A/D of8V. The

output is then:


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A 3.29 V signal is input to a 12 bit successive approximations converter withan input range of 0 to 10 V and simple binary output. Simulate the successive

approximation process to determine the simple binary output


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The reference voltage increment is:

V input spanN 2102

0 002441412

.

Trial digital output (D0) D0V Pass/Fail Actual digital output

100000000000 (2048) 5.0 F

010000000000 (1024) 2.5 P 010000000000 (1024)

011000000000 (1536) 3.75 F 010000000000 (1024)

010100000000 (1280) 3.12 P 010100000000 (1280)

010110000000 (1408) 3.44 F 010100000000 (1280)

010101000000 (1344) 3.28 P 010101000000 (1344)

010101100000 (1376) 3.36 F 010101000000 (1344)

010101010000 (1360) 3.32 F 010101000000 (1344)

010101001000 (1352) 3.30 F 010101000000 (1344)

010101000100 (1348) 3.2901 F 010101000000 (1344)

010101000010 (1346) 3.286 P 010101000010 (1346)

010101000011 (1347) 3.289 P 010101000011 (1347)

The output is 010101000011 or 1347 in decimal.


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Thank You
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