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I. Basic Knowledge Section 1. What is measurement? Answer: Measurement is the process of comparing a known standard quantity with an unknown physical quantity being measured. 2. How are measurements classified? Answer: In terms of form, it can be divided into direct measurement, indirect measurement, and combined measurement. 3. What is measurement error? Answer: During any measurement process, due to the imperfections in the measurement methods, as well as issues related to the measurement equipment, the measurement environment, and a person’s visual acuity, certain errors are inevitable. These errors distort the measurement results, resulting in a difference between the measured values and the true values. This difference is what is known as measurement error. 4. What are the calibration methods for instruments? Answer: 1. Indication comparison method; 2. Standard material method; 5. What are the three modes of heat transfer? Answer: Conduction, convection, radiation. 6. Into which three regions are the output characteristics of a transistor divided? Answer: Cut-off region, amplification region, saturation region. 7. What are the components of an actuator? Answer: The actuator consists of an actuating mechanism and a control valve. 8. What is PID control? Answer: PID control refers to the function of control instruments that include proportional (P), integral (I), and derivative (D) actions. 9. What are the components of a dynamic coil temperature gauge? Answer: The indicator-regulating type consists of three parts: a moving-coil measuring mechanism, a measuring circuit, and an electronic regulating circuit. An indicator type consists of only two parts: a moving-coil measuring mechanism and a measuring circuit. 10. What is sensitivity? Answer: Sensitivity is the ability of a measuring instrument to respond to the object being measured; it indicates how sensitive the instrument is to changes in the parameter being measured. 11. What factors affect the electromotive force of a galvanic cell? Answer: The material of the thermocouple and the temperature difference between its two junctions. 12. What are the conditions for a thermocouple to generate an electromotive force? Answer: The materials of the two electrodes are different, and the temperatures at the two contacts are also different. 13. What is the measurement range of a pressure-type thermometer? Answer: 0~300℃ 14. Why cannot the *1 setting of a multimeter be used to test whether a diode is good or bad? Answer: Excessive current can easily damage the diode. 15. What are the consequences of a leak in the pressure guiding tube of a differential pressure transmitter? Answer: Leaks on the positive pressure side will cause the instrument reading to be low, while leaks on the negative pressure side will cause the reading to be high. II. Control Theory Section 1. What is closed-loop control? Answer: A closed-loop control system is a system that controls based on the deviation between the controlled quantity and the set value. 2. In closed-loop control, what parameters are commonly used to measure control quality? Answer: When analyzing the performance of a closed-loop control system, the rise time and overshoot time are commonly used to measure the system’s speed, while the overshoot and damping ratio are used to indicate the system’s stability. The magnitude of the steady-state error represents the system’s accuracy. 3. What is sequential control? Answer: According to the requirements of the production process, achieving regular control over digital signals is referred to as sequential control. 4. What is the function of a transmitter? Answer: The function of a transmitter is to convert the quantity being measured by the sensing element into the signal type required by the controller. 5. What are the advantages of the function of a proportional regulator? Answer: The advantage of a proportional regulator is its fast response time; the regulating action takes effect immediately. That is, when a deviation signal is input, the output of the regulator changes proportionally to the deviation right away, allowing for rapid suppression of fluctuations in the parameter being regulated caused by disturbances. The greater the output deviation signal, the stronger the corrective action applied; this is a notable feature of proportional controllers. 6. What is the role of the error detection circuit in electric PID control? Answer: The function of the deviation detection circuit is to compare the input signal with a given signal, and to send the resulting deviation signal to the input terminal of the modulated amplifier. 7. Briefly describe the overshoot process in a PID control system. Answer: When interference occurs, differential control acts immediately; at the same time, proportional control also plays a role in reducing the magnitude of the deviation. Then integral control comes into effect, and this system has a stronger ability to eliminate deviations compared to a proportional-integral system. After a brief period of damped oscillation, the PID control eliminates the deviation and enters a new stable state. 8. Briefly describe the signal sources of the three-impulse feedwater control system. Answer: In the three-impulse feedwater control system, the controller receives three input signals: the main signal, which is the drum water level H; the feedforward signal, which is the steam flow rate D; and the feedback signal, which is the feedwater flow rate W. Among them, steam flow and feedwater flow are the main factors causing changes in the drum water level. Once a disturbance that leads to changes in the drum water level occurs, the control system acts immediately, enabling effective control of these level changes. 9. What are the general principles for selecting regulators in a hierarchical control system? Answer: (1) The secondary loop should enclose the location where strong disturbances are introduced. (2) The secondary loop should enclose as much of the component with a large time constant as possible. 3) The primary and secondary regulators should be selected as regulators with different control laws. 10. Why cannot differential adjustment be used alone? Answer: The output of a simple differential regulator can only reflect the rate of change of the error signal, not the magnitude of the error itself. The regulation achieved by such a regulator cannot eliminate the steady-state error; therefore, a differential regulator cannot be used alone, and it must always be combined with proportional or integral control mechanisms. 11. What control method is used for water supply control? Answer: Cascade three-shot control. 12. In feedwater control, which one is the main signal, which one is the secondary signal, and which one needs to be inverted? Answer: The drum level is the primary signal, while the feedwater flow rate and steam flow rate are secondary signals; the steam flow rate signal must be inverted. 13. What is negative feedback? Answer: It refers to the situation where the feedback signal sent back from the output terminal has a phase opposite to that of the input signal; the two signals cancel each other out, thereby reducing the amplitude of the input signal. 14. What conditions must be met to achieve a disturbance-free switchover in an automatic system? Answer: The output of the regulator equals the set value. III. Measurement Element Section 1. What are the main temperature measurement elements used in our factory’s operations? Answer: There are thermocouples and thermal resistors. 2. On what principle do thermocouples work? Answer: Thermocouples operate by utilizing the thermoelectric effect. 3. What is the measurement range of the thermal resistor? Answer: -200°C to 650°C. 4. What are the components of an industrial ordinary thermocouple? Answer: An industrial ordinary thermocouple consists of four parts: thermal electrodes, an insulating tube, a protective tube, and a terminal box. 5. What flow measurement instruments does our factory use? Answer: 6. What are the components of a differential pressure flow meter? Answer: Throttling element, pressure lead, differential pressure gauge. 7. What are the standard throttling elements? Answer: Nozzles, orifice plates, and venturi tubes. 8. What is the function of a compensation wire? Answer: The function of a compensation wire is to place the cold end of the thermocouple in a location that is far away from the heat source and where the ambient temperature is stable. 9. What are the two components of a digital voltmeter? Answer: By what control are the analog-to-digital conversion section and the counting section 10, as well as the electronic potentiometer amplification stage, used to determine the rotation direction of the reversible motor? Answer: What is the main function of phase 11 of the input signal and an automatic balance bridge? Answer: Used in conjunction with a thermistor to display and record temperature. 12. What model of differential pressure transmitter is mainly used in our factory? Answer: DDZ—II differential pressure transmitter 13. What is the signal type used in DDZ—II instruments? Answer: 0–10 mADC. 14. What quantities should be introduced for compensation in the measurement of gas flow rate? Answer: Temperature and pressure. 15. What parameters should be taken into account for compensation in the measurement of feedwater flow rate? Answer: What are the advantages and disadvantages of a temperature 16 standard orifice plate? Answer: The advantages are easy processing, material savings, and low cost ； The downside is the high pressure loss. 17. What is the working principle of a throttling device? Answer: Throttling devices such as orifice plates or nozzles with varying cross-sections are installed inside the pipeline. When fluid flows through these throttling elements, the flow narrows, resulting in a static pressure difference before and after the throttling element. By utilizing the relationship between pressure difference and flow velocity, it is possible to determine the flow rate further. 18. What is the function of a temperature transmitter? Answer: Used in conjunction with thermocouples or thermal resistors to convert temperature or temperature difference into an electrical current signal. 19. Why is a three-wire connection used for thermal resistors? Answer: To overcome the effect of the resistance of the connecting wires. 20. Why does the thermocouple input circuit need to have cold-junction temperature compensation? Answer: This is because when using thermocouples, it is always desired for the temperature at the cold end to remain constant. However, this is difficult to achieve in industrial production. Therefore, a cold junction compensator is used. When the output of the thermocouple changes due to variations in the cold end temperature, the output of the bridge circuit should also change, with the magnitudes of these changes being equal but in opposite directions. In this way, the measurement errors caused by changes in the cold end temperature of the thermocouple can be compensated for. 21. What is static pressure error? Answer: When the same pressure is applied to both the positive and negative pressure chambers of the transmitter, the output voltage of the transmitter changes as the static pressure varies. This error caused by static pressure is known as static pressure error. 22. What is non-linearity? What is the cause of it? Answer: The phenomenon in which the input pressure and output pressure of a transmitter do not follow a linear relationship is known as non-linearity. The reason is the conversion error of the measuring element. 23. What are the accuracy grades of pressure gauges commonly used in industry? Answer: 0.5, 1, 1.5, 2, 2.5, 4, 24. What is the function of the measuring element in a differential pressure transmitter? Answer: Convert the differential pressure signal into a force signal. 25. What components does a U-tube manometer consist of? Answer: What is the working principle of glass tubes, scales, liquid columns, and pressure-type level gauges? Answer: Different liquids have different dielectric constants; when the liquid level changes, the capacitance of the capacitor located within the liquid or that of the container itself changes. By measuring these changes, it is possible to determine the amount by which the liquid level has changed. 27. What are the advantages and disadvantages of ultrasonic level gauges? Answer: Ultrasonic level gauges can perform both point-based and continuous measurements. They facilitate the transmission of telemetry and remote control signals, exhibit high adaptability, have no strict requirements regarding the condition of the medium, and do not require any moving parts. Their propagation speed is not directly related to the dielectric constant, conductivity, or thermal conductivity of the medium. Additionally, they are cost-effective and durable, making them easy to use and install. The disadvantage is that it cannot measure the level of liquids containing bubbles and suspended particles; it is also difficult to take accurate readings when there are large fluctuations in the liquid surface, which leads to measurement errors. 28. On what principle is an electronic automatic balance bridge built? Answer: Principle of bridge balance. 29. What are the key components of a voltage regulator? Answer: Zener diode 30. Is the electronic automatic balance bridge a balance bridge? Answer: No. Source: www.chinanics.com] China Mechanics Network – an e-commerce platform in the mechanical and electrical sector!