www.examquestionpaper.in Department of Electronics and Instrumentation Engineering CL7202/ INDUSTRIAL AUTOMATION QUESTION BANK-Part-A UNIT I DESIGN OF SIGNAL CONDITIONING AND TRANSMISSION 1. List few common signals conditioning operation. a. Amplification b. Filtering c. Isolation d. Electrical Isolation e. Linearization f. Cold Junction Compensation g. Excitation 2. What does a filter as signal conditioner do in a measurement system? Filtering is the most common signal conditioning function, as usually not all the signal frequency spectrum contains valid data. The common example is 60 Hz AC power lines, present in most environments, which will produce noise if amplified. 3. State the two types of excitations used in bridge circuits and give its advantages. Voltage Excitation of Bridges The alternative method for measuring bridge circuits with the dataTaker is the voltage excitation with ratiometric measurement. The principal of the method is that the bridge is excited by a constant voltage source, and the bridge output voltage is measured as a ratio of the measured excitation voltage. Constant Current Excitation of Bridges The constant current excitation method of bridge measurement has a constant current of 2.500 mA or 250.0 µA flowing in the bridge circuit for excitation. The bridge sensitivity and zero is independent of the length of leads used to connect the bridge to the dataTaker. In some cases the bridge output can have greater linearity and reduced temperature sensitivity for constant current excitation, than for voltage excitation. 4. What are the two drawbacks of differential amplifier that are solved in instrumentation amplifier? For transmission purpose for long range, noise also gets superimposed along the original wave. Therefore specific cables are used to reduce noise. Disadvantages of this circuit include the inability to operate at unity gain, a decreased common-mode voltage range as circuit gain is lowered, and poor ac common-mode rejection. 5. High ‘Q’ in bandpass filter is dangerous. Why? Now, instead of talking about the bandwidth of the filter, we can use the Q which gives us an idea of the width of the filter in musical terms. This is because, as we increase the centre frequency, we have to increase the bandwidth proportionately to maintain the same Q. Notice however, that if we maintain a centre frequency, the smaller the bandwidth gets, the bigger the Q becomes, so if you’re used to talking in terms of musical intervals, you have to think backwards. The bigger the Q, the smaller the interval. Remember that you can have a very high Q, and therefore a very narrow bandwidth for a bandpass filter. All of the definitions still hold, however. The cutoff frequencies are still www.examquestionpaper.in www.examquestionpaper.in the points where we’re 3 dB lower than the maximum value and the bandwidth is still the distance in Hertz between these two points and so on. 6. What is the basic principle of shielding electric field interface? When an electromagnetic wave propagating in one material encounters another material with different electrical properties, some of the energy in the wave is reflected and the rest is transmitted into the new material. For example, consider the electromagnetic plane wave, Einc, incident upon an infinite slab of material as illustrated in Figure 1. The wave propagates in free space in the x direction until it strikes the material, which has intrinsic impedance, ηs. 7. Why do you require signal conditioners in PC based measurement systems? It is primarily utilized for data acquisition, in which sensor signals must be normalized and filtered to levels suitable for analog-to-digital conversion so they can be read by computerized devices. Other uses include preprocessing signals in order to reduce computing time, converting ranged data to boolean values, for example when knowing when a sensor has reached certain value. 8. Draw a quarter bridge circuit and give its output voltage. 9. List the disadvantages of using inverting amplifier for amplifying bridge outputs. But the real drawback to the inverting amplifier is the amplifier's input impedance, which is equal to R1. As we saw with voltage dividers, we need to take a circuit's impedance into account when using it as part of a larger system of circuits. We need each successive circuit stage to have an input impedance at least 10 times greater than the output of the one preceeding it, to prevent loading. Since the inverting amplifier's input impedance is equal to R1, there may be times we'd be forced to pick unusually large resistors for our feedback loop, which can cause other problems. www.examquestionpaper.in www.examquestionpaper.in The solution to our impedance worries lie in the Non-Inverting Amplifier, also made with an op-amp and negative feedback 10. What is meant by active filter? An active filter is a type of analog electronic filteer that uses active components such as an amplifier. Amplifiers included in a filter design can be used to improve the performance and predictability of a filter,[1] while avoiding the need for inductors (which are typically expensive compared to other components). An amplifier prevents the load impedance of the following stage from affecting the characteristics of the filter 11. List the characteristics of thermocouple. 12. Compare the characteristics of RTD and Thermistor. RTD Thermistoor 1. RTD is made up of metals. Thermistor is made up of semiconductor Materials 2. Metals have Positive Temperature Semiconductor materials have Negative Temperature Coefficient (PTC) of resistance. Hence, the Coefficient (NTC) of resistance. Hence, the resistance resistance of RTD increases with an increase in of a thermistor decreases with an increase in temperature and decrc eases with a decrease in temperature and increases with a decrease in temperature. temperature. 3. The resistance temperature characteristics of The resistance temperature characteristics of thermistor RTD's are linear. are highly nonlinear. 4. It is less sensitive to temperature compared to It has large temperature coefficient of resistance i.e. thermistor. thermistor highly sensitive to temperature compared to RTD. 5. But, it has-a wide operating temperature It has low operating temperature range compared to range i.e., minus 200 to plus 650°C. RTD i.e., minus 100 to plus 300°C. 6. RTD's are relatively larger in size. Size of thermistors are small . 7. They are costlier as compared to thermistor. They are not costlier as compared to RTD. 8. They have low self resistance. They have high self resistance. Thus, they require shielding cables to minimize 9. RTD's provide high degree of accuracy and interference problems. long term stability. Thermistors also provide an accuracy of ± 10. They are used in laboratory and industrial 0.01°C. applications. They are widely used for dynamic temperature measurement. 13. Give the characteristics of IC temperature sensor. www.examquestionpaper.in 14. What is the basic function of a sensor in measurement system? A sensor is a device that detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal; for example, a thermocouple converts temperature to an output voltage. But a mercury-in-glass thermometer is also a sensor; it converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. 15. What do you mean by sensor self heating? How do you avoid this? Temperature sensors can exhibit a phenomenon known as self-heating. When a temperature sensor is self heated, the sensor itself contributes heat to raise its' own temperature. When using thermistor circuits, you want to minimize self-heating. You do that by minimizing the current through the thermistor. Given a choice, choose the situation where the thermistor has the smallest amount of current flowing through it. 16. Why is latch needed in DAC? A latch is an example of a bistable multivibrator, that is, a device with exactly two stable states. These states are high-output and low-output. A latch has a feedback path, so information can be retained by the device. Therefore latches can be memory devices, and can store one bit of data for as long as the device is powered. As the name suggests, latches are used to "latch onto" information and hold in place. Latches are very similar to flip-flops, but are not synchronous devices, and do not operate on clock edges as flip- flops do. 17. List the characteristics of pH sensor. 18. List the features of pneumatic transmitters a) As this pressure transmitter is of pneumatic type, it can be used in an explosive and corrosive environment. b) As this pressure transmitter is of force balanced type, the displacement of the pressure element and linkage mechanism hardly exist, making it rigid construction possible as a whole. Accordingly, precision regarding vibration www.examquestionpaper.in prevention, linear resistance, hysteresis, etc. can be maintained for a long period of time. c) The element of the pressure sensing portion is built in the transmitter body, making the transmitter extremely small and performing zero adjustment easily from the outside without removing the cover and without any effect on the span.. 19. What is oversampling and where it is employed? In signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than the Nyquist rate. Theoretically a bandwidth-limited signal can be perfectly reconstructed if sampled above the Nyquist rate, which is twice the highest frequency in the signal. 20. List the types of ADC with its advantages. a) direct-conversion ADC or flash ADC b) successive-approximation ADC c) ramp-compare ADC d) Wilkinson ADC e) integrating ADC (also dual-slope or multi-slope ADC) f) delta-encoded ADC or counter-ramp g) pipeline ADC (also called subranging quantizer) h) sigma-delta ADC (also known as a delta-sigma ADC) i) time-interleaved ADC j) ADC with intermediate FM stage PART-B 1.