en 341 presentation 2004

1University of Moratuwa

Department of Electronic and Telecommunication Engineering

EN341 Electronic Instrumentation and Control September 14, 2005

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Electronic Instrumentation and Control

University of Moratuwa



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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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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.




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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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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.




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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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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)




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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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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.




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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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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.




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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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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)




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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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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.




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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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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.




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Main reading pp. 42 - pp. 68 pp. 276 pp. 304

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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.




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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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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.




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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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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




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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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Main reading pp. 276-282

Optional Reading Control system

document on the web.




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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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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.




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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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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.




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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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Block Diagram Reduction Essential part of the mathematical

modeling. Rules can be used. Rules are based on moving the feedback

point on the diagram.




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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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Supplementary Reading Electronic

Instrumentation by H.S. Kalsi

Main reading 1.1 to 1.7

Optional reading 1.8 to 1.12




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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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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.




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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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Measurement Quantities (Cont..) Modulated Signals: AM and FM modulated

signals. Instrument should handle spurious signals. Decibel measurements: Manageable

measurement. dBm, dBV etc .




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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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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.




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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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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.




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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.




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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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Optional reading None.




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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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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.




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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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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.




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Main reading 4.12 to4.19


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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.




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Signal Sources Models, and ground plane Sine wave sources Imperfections in sine wave sources Function generators Arbitrary waveform generators.

en 341 presentation 2004

Documents

measured value

instrument reading

design value

true value

desired value

actual value

probable value

threshold input