Download - En 341 Presentation 2004
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1University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
1
Electronic Instrumentation and Control
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
2
Performance CharacteristicsA knowledge of the performance
characteristics of an instrument is essential for selecting the most suitable instrument
specific measuring jobs
Static characteristics Dynamic characteristics
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2University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
3
Comparison of CharacteristicsStatic
Static characteristics are obtained via a calibration process.
Considered for instruments which are used to measure fixed process conditions. Fixed via calibration.
DynamicThe response of an
instrument as the measured variable changes at the input. Eg. Slowness and sluggish response of instrument.
Fixed via compensation.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
4
Static Characteristics Accuracy: Degree of exactness between the
measured and expected values.(A1) Precision is related to accuracy: Accuracy
sometimes means precision. However precision measurements may not be accurate.(A2) Significant figures is also a quantity
representing accuracy. This is the error of representation.
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3University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
5
Static Characteristics Sensitivity: The smallest change in the
measured variable it responds too. This is dO/dI. This is a relationship between the input and the output. (A3) Reproducibility: Consistency and
repeatability of measurements. Successive values should not change. This determines the precision of an instrument.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
6
Useful Static Quantities Expected value: The design value or most
probable value. Measured Value: The actual value that the
instrument indicates. Error: Deviation of the true value from the
desired value.
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4University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
7
Undesirable Static Characteristics Drift: Change of the instrument reading
over a period of time. Dead Zones: Instrument is not responsive Hysteresis: Difference in loading and
unloading. Threshold: Input required from zero position to
indicate value. Resolution: Over and above the threshold input,
the minimum increment in input to produce a perceptible output.(A4)
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
8
Static Errors Human Gross (human mistakes of reading and
recording), Misuse, Observational (due lack of knowledge to use the instrument)
Random Systematic Instrumental errors such as inherent short
comings and loading effects. Environmental errors.
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5University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
9
Loading Errors Voltmeters should be applied in shunt and should
have high input impedance (A5) Ammeters should be applied in series and should
have a low input impedance. Loading avoids unnecessary voltage drops due to
current drawn by the instrument. However for maximum power to be transmitted
the condition is resistance being matched.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
10
Supplementary Reading Introduction to
Instrumentation and control A.K. Ghosh
Main reading pp. 4 - pp. 19
Optional reading pp. 23 pp. 37
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6University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
11
Dynamic Characteristics Describes the behavior of the system with
time with some input given to the system. The behavior of the system is represented
via a differential equation/transfer function. Dynamic response characterizes the system. Idealized inputs (step, impulse etc.) is used
to obtain the dynamic response.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
12
Dynamic Characteristics Speed of response: it is the rapidity with
which the instrument response to a change in the measured quantity. Fidelity: The degree to which an instrument
indicates the changes in the measured variable without dynamic error (ability to faithfully reproduce)
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7University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
13
Dynamic Characteristics Lag: It is the retardation in the response of
an instrument to changes in the measured variable. Dynamic Error: It is the difference between
the true value of a quantity changing with time and the value indicated by instrument, if no static error is assumed (Note the difference with static error slew rate)
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
14
Entities Required to Study the Dynamic Characteristics
Transfer function: Determines the type of instrument. A cascade of transfer function is possible. (A6) Response: Time domain analysis is required
to obtain the dynamic response. Bode Plots: System characterization in the
frequency domain to obtain response.
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8University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
15
Instrument Categorization Zero Order: Obeys an algebraic equation First Order: Dynamic relation between the
input and the output of the instrument is characterized by a first order DE. Second Order: Dynamic relation between
the input and the output of the instrument is characterized by a second order DE. Others are a combination of the above.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
16
Zero Order Instruments (A7) Linear relationship between input and
output. No distortions at the output. No time lag of any sort between the input
and output. This is considered to be the ideal dynamic
response. Example is a potentiometer.
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9University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
17
First Order Instruments (A8) System is characterized by a first order
ODE. Time constant determines the response. Step, Ramp and Impulse responses are used
to characterize the operation of the instrument. A temperature measuring system can be
given as an example.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
18
Supplementary Reading Introduction to
Instrumentation and control A.K. Ghosh
Main reading pp. 42 - pp. 68 pp. 276 pp. 304
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
19
Compensation Dynamic characteristics can be altered by
compensation. Need to know control strategies. Stability is an issue when controlling the
equipment. Hence learning control theoretic approach is
useful.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
20
Recap Instruments can be using stand alone or in groups. Instruments have static and dynamic
characteristics. Static properties (errors) can be avoided by
calibration. Dynamic properties (errors) can be avoided by
compensation. Dynamic characteristics are based on order of the
instrument. The control is based on adjusting these characteristics.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
21
Control Control as a measure of compensation. Control as a method to maintain the physical
quantity we are measuring. Control as a way of understanding the internal
operation of the instrument so that on can wisely choose an instrument. (fast instruments vs. slow instruments) Control as a way of keeping a group of
instruments stable.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
22
Control Systems Open Loop Fig 15.1 Cannot compensate for external
disturbances. Cannot be automated. In the figure the temperature has to be
adjusted manually. No tracking of input signals.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
23
Control Systems Closed Loop Automatic maintenance of signal
conditions. Physical parameters of the system is used
for automated tracking. Automatic feedback control system as in
Fig 15.3
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
24
Properties of Closed Loop Control Lowers the gain of the system. The system characteristics will depend on the
feedback factor for large open loop gains. Lowers the system dependency on the process
transfer function. Drift cushioning. O/P impedance is lowered. BW is increased. The effect of disturbance is accounted for.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
25
Supplementary Reading Introduction to
Instrumentation and control A.K. Ghosh
Main reading pp. 276-282
Optional Reading Control system
document on the web.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
26
Control System Analysis Objectives Transient response adjustment Steady state characteristics adjustment. Stability compensation.
We are trying to obtain a desired response.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
27
Transient Response Design We shall use a quantitative measure of
transient response. We will analyze a systems existing transient
response. We shall seek to adjust the design
parameters to yield a desired transient response.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
28
Steady-State Design Steady state accuracy is the most important
parameter. This is also based on the transient response. We shall define quantitative measure for
steady state accuracy. We shall design corrective measures to
reduce the steady state error.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
29
Stability Natural response - does not depend on the
input. If this grows out of proportion system instability occurs. Need to be controlled. Forced response depends on the input.
Controlled via controlling the input. Measures of stability will be defined and
then methods learnt how to derive these.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
30
Analysis Sequence Determine physical system from
requirements. Transform physical system into a
schematic. Construct a mathematical model. Perform block diagram reduction. Analysis and design for the parameters
previously mentioned.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
31
Block Diagram Reduction Essential part of the mathematical
modeling. Rules can be used. Rules are based on moving the feedback
point on the diagram.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
32
Pole-Zero Plot and Root Locus Poles and zero locations on the complex
plane determine the response of the system. Real axis poles generate exponential responses. Complex poles generate oscillatory responses.
The movement of the system poles with the variation of the gain of the system is plotted using the Root Locus.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
33
Supplementary Reading Electronic
Instrumentation by H.S. Kalsi
Main reading 1.1 to 1.7
Optional reading 1.8 to 1.12
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
34
Measurement Quantities AC: average, rms, peak etc. Crest factor: Vo-p/Vrms. Waveforms with
high CFs require the measuring instrument to tolerate very large peak voltages while simultaneously measuring the much smaller rms value. Phase: Phase drifts are important in some
measurements (eg. Lissajous figures)
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
35
Measurement Quantities (Cont..) AC Power: Instantaneous power and
average power. Any AC waveforms that have the same rms value will cause the same power to be delivered to a resistor. Non sinusoidal waveforms: Fourier
components. Harmonics: Multiple of fundamental
frequency.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
36
Measurement Quantities (Cont..) Square wave: Testing of amplifiers. Pulse train: A pulse train generates
harmonics with amplitudes that are dependent on the duty cycle. Can be used to measure BW with the energy concentrated below 1/tau. Combined AC and DC: Instrument should
handle this.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
37
Measurement Quantities (Cont..) Modulated Signals: AM and FM modulated
signals. Instrument should handle spurious signals. Decibel measurements: Manageable
measurement. dBm, dBV etc .
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
38
Loading Effects Loading is caused by the external
instrument load and the internal source resistance. If the instrument loads the circuit correct
measurements cannot be taken. Voltage measurements should have infinite
instrument resistance and for current measurements it should be zero.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
39
Bandwidth Limitations Any instrument has an operating BW. The instrument BW is defined as the
frequency at which the instruments response has decreased by 3dB. For some instruments even if you have
exceeded BW, it may still be usable. Eg. Frequency counter.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
40
Bandwidth Limitations (Cont..) The use of BW limitations for DC
measuring and AC measuring equipment is important. For non sinusoidal waveform measuring the
BW of the instrument should be high. Otherwise some harmonics will fall outside the measuring BW. Eg. Square wave. Such equipment are expensive.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
41
Rise Time Limitation Ideal waveforms have instantaneous jumps in the
waveform. Practical waveforms do not have these. The instrument may also not be capable of
jumping to voltage levels instantaneously. Rise time=0.35/BW The instrument should have a rise time
significantly smaller than the rise time being measured.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
42
Supplementary Reading Electronic
Instrumentation by H.S. Kalsi
Main reading 3.1 to 3.5
Optional reading 3.6 to 3.9
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
43
DC Voltmeter Ideal voltmeter will have no internal
resistance. A practical voltmeter has a shunt resistor. Fig 1.0: Measuring set up. Basic range can be increased by using an
external multiplier resistance.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
44
Chopper type DC Voltmeter Used to measure small voltages. The DC voltage is chopped into an AC
voltage of frequency 100-300Hz. The residual DC is blocked using a capacitor. Immune to drift problems. Input impedance is high. Figure 2.0. Construction.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
45
Supplementary Reading Electronic
Instrumentation by H.S. Kalsi
Main reading 4.1 to 4.11
Optional reading None.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
46
AC Voltmeter Unless otherwise stated the AC voltmeter is
usually calibrated to read RMS values. For AC meters, the BW is critical. The accuracy
of the meter is usually defined using the BW. Usually AC meters are calibrated to read sine
waveforms and factors should be used for non sinusoidal waveforms. Meters may have coupling capacitors to block DC.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
47
Average Responding Meter This is a low cost version of the AC
voltmeter. Figure 3.0 Construction. The average value is found and calibrated to
read the RMS. Only valid for sine waves in rectified form. Its a reliable technique as long as the
frequency and the shape are not varied.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
48
Peak Responding Meter The difference with the previous type is the
manner in which the diode and the capacitor is used. Figure 4.0 Construction. The Peak voltage is calibrated to read RMS. The advantage is that the diode circuitry can be
moved into the probe so that the equipment will take no AC waveforms and hence no BW limitations.
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
49
True RMS Voltmeter Complex waveforms are best measured
using a true rms voltmeter. Expensive. The meter is based on a meter indication by
sensing the waveform heating power using thermo couples. Heating is based on the rms value.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
50
Supplementary Reading Electronic
Instrumentation by H.S. Kalsi
Main reading 4.12 to4.19
Optional reading 4.20 to 4.26
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University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
51
Digital Voltmeters Desirable features Types Ramp technique Low cost, easy to design Single ramp requires excellent characteristics. Large errors possible due to noise.
Dual slope integrating type DVM Accuracy of the measured voltage is independent of the
integrating time constant. Independent of the oscillator frequency. Noise performance is good.
University of Moratuwa
Department of Electronic and Telecommunication Engineering
EN341 Electronic Instrumentation and Control September 14, 2005
52
Signal Sources Models, and ground plane Sine wave sources Imperfections in sine wave sources Function generators Arbitrary waveform generators.