Sep 14, 2005 ... EN341 Electronic Instrumentation and Control. September 14, ... characteristics
of an instrument is essential for selecting the most ..... H.S. Kalsi.
University of Moratuwa Department of Electronic and Telecommunication Engineering
Electronic Instrumentation and Control
EN341 Electronic Instrumentation and Control 2005
September 14,
1
University of Moratuwa Department of Electronic and Telecommunication Engineering
Performance Characteristics A knowledge of the performance characteristics of an instrument is essential for selecting the most suitable instrument specific measuring jobs § Static characteristics § Dynamic characteristics EN341 Electronic Instrumentation and Control 2005
September 14,
2
1
University of Moratuwa Department of Electronic and Telecommunication Engineering
Comparison of Characteristics Static Static characteristics are obtained via a calibration process. Considered for instruments which are used to measure fixed process conditions. § Fixed via calibration.
Dynamic The response of an instrument as the measured variable changes at the input. Eg. Slowness and sluggish response of instrument. Fixed via compensation.
EN341 Electronic Instrumentation and Control 2005
September 14,
3
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
4
2
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
5
University of Moratuwa Department of Electronic and Telecommunication Engineering
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.
EN341 Electronic Instrumentation and Control 2005
September 14,
6
3
University of Moratuwa Department of Electronic and Telecommunication Engineering
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) EN341 Electronic Instrumentation and Control 2005
September 14,
7
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
8
4
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
9
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Introduction to Instrumentation and control – A.K. Ghosh § Main reading § pp. 4 - pp. 19
§ Optional reading § pp. 23 – pp. 37 EN341 Electronic Instrumentation and Control 2005
September 14,
10
5
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
11
University of Moratuwa Department of Electronic and Telecommunication Engineering
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) EN341 Electronic Instrumentation and Control 2005
September 14,
12
6
University of Moratuwa Department of Electronic and Telecommunication Engineering
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) EN341 Electronic Instrumentation and Control 2005
September 14,
13
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
14
7
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
15
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
16
8
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
17
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Introduction to Instrumentation and control – A.K. Ghosh § Main reading § pp. 42 - pp. 68 § pp. 276 – pp. 304
EN341 Electronic Instrumentation and Control 2005
September 14,
18
9
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
19
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
20
10
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
21
University of Moratuwa Department of Electronic and Telecommunication Engineering
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.
EN341 Electronic Instrumentation and Control 2005
September 14,
22
11
University of Moratuwa Department of Electronic and Telecommunication Engineering
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
EN341 Electronic Instrumentation and Control 2005
September 14,
23
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
24
12
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Introduction to Instrumentation and control – A.K. Ghosh § Main reading § pp. 276-282
§ Optional Reading § Control system document on the web. EN341 Electronic Instrumentation and Control 2005
September 14,
25
University of Moratuwa Department of Electronic and Telecommunication Engineering
Control System Analysis § Objectives § Transient response adjustment § Steady state characteristics adjustment. § Stability compensation.
§ We are trying to obtain a desired response.
EN341 Electronic Instrumentation and Control 2005
September 14,
26
13
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
27
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
28
14
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
29
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
30
15
University of Moratuwa Department of Electronic and Telecommunication Engineering
Block Diagram Reduction § Essential part of the mathematical modeling. § Rules can be used. § Rules are based on moving the feedback point on the diagram.
EN341 Electronic Instrumentation and Control 2005
September 14,
31
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
32
16
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Electronic Instrumentation by H.S. Kalsi § Main reading § 1.1 to 1.7
§ Optional reading § 1.8 to 1.12 EN341 Electronic Instrumentation and Control 2005
September 14,
33
University of Moratuwa Department of Electronic and Telecommunication Engineering
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) EN341 Electronic Instrumentation and Control 2005
September 14,
34
17
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
35
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
36
18
University of Moratuwa Department of Electronic and Telecommunication Engineering
Measurement Quantities (Cont..) § Modulated Signals: AM and FM modulated signals. Instrument should handle spurious signals. § Decibel measurements: Manageable measurement. § dBm, dBV etc .
EN341 Electronic Instrumentation and Control 2005
September 14,
37
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
38
19
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
39
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
40
20
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
41
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Electronic Instrumentation by H.S. Kalsi § Main reading § 3.1 to 3.5
§ Optional reading § 3.6 to 3.9 EN341 Electronic Instrumentation and Control 2005
September 14,
42
21
University of Moratuwa Department of Electronic and Telecommunication Engineering
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.
EN341 Electronic Instrumentation and Control 2005
September 14,
43
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
44
22
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Electronic Instrumentation by H.S. Kalsi § Main reading § 4.1 to 4.11
§ Optional reading § None. EN341 Electronic Instrumentation and Control 2005
September 14,
45
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
46
23
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. § It’s a reliable technique as long as the frequency and the shape are not varied. EN341 Electronic Instrumentation and Control 2005
September 14,
47
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
48
24
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
49
University of Moratuwa Department of Electronic and Telecommunication Engineering
Supplementary Reading § Electronic Instrumentation by H.S. Kalsi § Main reading § 4.12 to4.19
§ Optional reading § 4.20 to 4.26 EN341 Electronic Instrumentation and Control 2005
September 14,
50
25
University of Moratuwa Department of Electronic and Telecommunication Engineering
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. EN341 Electronic Instrumentation and Control 2005
September 14,
51
University of Moratuwa Department of Electronic and Telecommunication Engineering
Signal Sources § § § § §
Models, and ground plane Sine wave sources Imperfections in sine wave sources Function generators Arbitrary waveform generators.
EN341 Electronic Instrumentation and Control 2005
September 14,
52
26
