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Unit 1: Basic Concepts of Instruments I. Measurement, Measurement Error, Direct Measurement, and Indirect Measurement 1. What is measurement error? Answer: The difference between the measured value and the true value is called measurement error. 2. What are direct measurement and indirect measurement? Answer: Direct measurement refers to the situation where the parameter being measured is directly compared with a certain standard quantity. Indirect measurement involves substituting the directly measured data into a specific formula to calculate the value of the parameter being measured. 3. What is measurement? Answer: It is the process of converting and transmitting the signal of the parameter to be measured, and comparing it with the corresponding unit of measurement; this is what is called measurement. II. Instrument error, drift, sensitivity, and sensitivity limit. 1. What are the allowable error of a meter, its accuracy, and its accuracy class? Answer: The allowable error refers to the maximum percentage error that is permitted for a meter under specified normal conditions. The accuracy of a gauge refers to the value obtained by removing the percent sign (%) from the allowable error. The accuracy grade is a series of standard percentage values that are **uniformly specified**. 2. What is instrument drift? Answer: It refers to the difference between the readings obtained when using the same instrument to measure a certain parameter in both forward and reverse directions, under constant external conditions. 3. What are the sensitivity and detection limit of a measuring instrument? Answer: Sensitivity is used to indicate the degree to which a measuring instrument is responsive to changes in the parameter being measured. Sensitivity is expressed as the ratio of the change in the reading indicated by the instrument to the change in the parameter being measured that causes this change. The sensitivity limit refers to the smallest change in the parameter being measured that causes a visible change in the instrument’s reading. 4. What is the dynamic error of a measurement system? Answer: It refers to the difference between the instrument reading and the actual value of the parameter being measured, when the information related to that parameter is in a state of change within the measurement system. III. Pressure, engineering atmosphere, physical atmosphere, gauge pressure, absolute pressure. 1. What is pressure? Answer: Pressure is a force that acts vertically and evenly on a unit area. 2. What is engineering atmosphere? Answer: Engineering atmosphere is a unit commonly used in industry; it represents the pressure generated by 1 KG of force acting vertically and evenly on an area of 1 square centimeter, and it is expressed in kgf/cm2. 3. What is physical atmospheric pressure? Answer: Physical atmospheric pressure is the force generated by a mercury column 760 mm high, with a density of 13.5951 grams per cubic meter and a gravitational acceleration of 980.665 centimeters per second squared, acting on a surface. IV. Piezoelectric effect, pyroelectric effect, and piezoresistive effect. 1. What is the piezoelectric effect? When certain crystals are subjected to pressure and undergo mechanical deformation, opposite charges are generated on their two opposing faces. This electrical phenomenon that arises due to deformation in the absence of an external electric field is known as the piezoelectric effect. 2. What is the magnetostrictive effect? When a ferromagnetic material is under pressure, not only do the mechanical stresses within the material change with the applied pressure, but the material’s magnetic permeability also changes; this phenomenon is known as the magnetostrictive effect. 3. What is the piezoresistive effect? When a semiconductor crystal is under pressure, the symmetry of its crystal structure is temporarily altered, which in turn changes the way the semiconductor conducts electricity, resulting in a change in its resistivity. This effect is known as the piezoresistive effect. V. Analog display instruments, digital display instruments, and image display instruments. 1. Analog display instruments: These use the deflection angle or displacement of a pointer (recording pen) to simulate and display the continuous changes in the parameter being measured. Their disadvantages include a complex mechanism, low precision, and slow speed. Analog display instruments include: moving-coil display instruments, self-balancing display instruments, and automatically balanced display instruments. 2. Digital display instruments: Display the value of the parameter being measured in digital form, offering high precision and fast speed. 3. Image display instrument: It displays information directly via a screen using graphics, characters, curves, and numbers. Unit 2: Measurement Principles and Operations of Common Instruments 1. Measurement principles and operations of thermal resistors. Principle: It utilizes the temperature measurement principle based on the linear relationship between the resistance value of a thermistor and temperature within a certain temperature range. Common thermal resistors include: platinum resistors, copper resistors, manganese resistors, and carbon resistors. Operation: When measuring the resistance signal, thermoresistors use a three-wire system; this is done to reduce measurement errors caused by the resistance of the wires during transmission. When checking its resistance value, pay attention to which two wires are common wires. 2. Principle and operation of thermocouple measurement. Principle: When two different metals are connected at one end, a potential difference is established between their other ends, and within a certain range, this potential difference has a linear relationship with temperature. Operation: A two-wire system is used; during inspection, its resistance value should be very low. 3. Measurement principle and operation of pressure transmitters. Principle: Thin-film metal strain measuring element. Operation: When wiring, pay attention to the positive and negative poles of the connections. When in use, the manual valve should be opened slowly to avoid sudden shocks to the diaphragm box. Check for any leaks at the instrument connections to prevent measurement errors. When removing it, the manual valve should be closed first; once it is confirmed that the valve is closed, the gauge should be removed slowly, while shaking the transmitter to allow any remaining gas to escape. 4. Working principle of differential pressure level gauge. Principle: The diaphragm of the silicone-oil-filled measuring cell is connected via two coupling pins; the differential pressure generates a force that causes the torsion rod to deform. This torsion rod is placed in front of film strain gauges arranged in a Wheatstone bridge, which converts the pressure difference into a balanced voltage signal. The electronic amplifier converts the signal from the measurement chamber into a two-wire 4–20 mA DC output signal and can provide a Wheatstone bridge. Operation: When wiring, pay attention to the positive and negative poles of the connections. When in use, first open the balance valve of the transmitter, close the manual valves in the high-pressure and low-pressure chambers, open the manual valves on the pressure lead lines of these chambers, slowly open the manual valves in the high-pressure and low-pressure chambers, and then close the balance manual valve. Unit 3: Basic Concepts of Automatic Control Systems I. Controlled object, controlled parameter, actuating parameter, and control channel. 1. What is a control object? The equipment or machines that are controlled during the production process. 2. What is a parameter to be adjusted? It refers to the parameter within the object being regulated that needs to be kept within a predetermined range and adjusted accordingly. 3. What are regulating parameters? They are parameters that act on the regulated object and help to stabilize the regulated parameter. 4. What is interference? It refers to external factors that disrupt the equilibrium state of a system, thereby causing changes in the parameter being regulated. 5. What is an interference path? It refers to all the components between the point where interference is generated and the parameter being affected. II. Regulation principles, transmitters, regulators, actuators. 1. What is a regulation law? It refers to the pattern of change over time between the output signal of a regulator and its input signal. When studying the regulation law of a regulator, it is disconnected from the system to separately examine the relationship between its output signal and input signal. When analyzing the regulation law of a regulator, a step signal is usually applied to its input; that is, when a certain deviation occurs suddenly, the behavior of the output signal in response to this step input signal is examined. The regulation law of a regulator actually represents its dynamic characteristics, which can be described in the form of a transfer function. The basic control laws for regulators are proportional (P), integral (I), derivative (D), and their combinations. 2. What is a transmitter? The role of a transmitter in automatic monitoring and control systems is to convert various process parameters, such as physical quantities like pressure, differential pressure, temperature, flow rate, liquid level, and composition, into corresponding standard signals. These signals are then sent to indicator recorders, calculators, and controllers for display, recording, and control. Classified by the parameters being measured, transmitters mainly include differential pressure transmitters, pressure transmitters, temperature transmitters, flow rate transmitters, etc. Composition: It usually consists of an input conversion section, an amplifier, and a feedback section. The input conversion section contains sensitive components whose function is to sense the parameter being measured and convert it into an intermediate analog value. The intermediate quantity can be physical quantities such as voltage, current, displacement, and force. The feedback section converts the transmitter’s output signal into a feedback signal. The amplifier amplifies the difference between the intermediate analog signal and the feedback signal, and converts it into a standard output signal. 3. What is a regulator? A regulator typically performs a PID calculation on the deviation between the input signal and a setpoint signal, and sends the result as a unified signal to the actuator to achieve automatic control. A regulator must have two key components: one for detecting deviations and another for performing PID calculations. The deviation detection circuit is generally referred to as the input circuit. The deviation signal is usually in voltage form, so the input signal and the reference signal are compared in voltage form within the input circuit. If the input signal is a current, it must be converted into the corresponding voltage through a precision resistor. The input circuit must also be equipped with a switch for switching between internal and external supply circuits, a forward-reverse action switch, and a deviation indicator, among other components. The PID operation circuit is the key component that enables the regulator to exert its regulating function; it consists of an amplifier and a PID feedback circuit. 4. What is an actuator? In automatic control systems, the role of an actuator is to receive control signals from the control unit, thereby causing a corresponding change in the opening degree of the control valve. Thereby achieving the purpose of regulating flow rate. III. Feedback, positive feedback, negative feedback. 1. Feedback: refers to returning the system’s output signal to the input in a certain manner. 2. Positive feedback: refers to feedback in which the added feedback signal increases the system’s input signal. 3. Negative feedback: Refers to feedback in which the feedback signal reduces the system’s input signal. IV. Composition of the automatic control system. An automatic control system consists of four components: the controlled process, the control valve, the measuring and transmitting device, and the controller. V. Ratios, integrals, derivatives. 1. Proportional control law (P): It refers to a proportional relationship between its output signal and the deviation amount. The advantage of proportional control characteristics is its fast response speed; the control action takes effect immediately, as once a deviation signal is input, the output of the controller changes proportionally to that deviation right away. The greater the input deviation signal, the stronger the output regulation effect, which is a notable feature of a proportional regulator. 2. Integral control law (I): Its output signal is proportional to the integral of the error signal. When a deviation exists, the output signal of the integral regulator will keep increasing or decreasing over time; only when the input deviation is zero does the output signal cease to change and stabilize at a certain value. The speed at which the output signal of the regulator changes is proportional to the magnitude of the input error and the integration rate; the direction of the output change is determined by the sign of the error. 3. Differential regulation law (D): It refers to the situation where its output signal is proportional to the rate of change of the error signal. This type of regulator is used in systems; even if the deviation is small, as long as there is a trend of change, it can make adjustments immediately. Hence, it is referred to as “proactive” regulation. However, its output can only reflect the rate of change of the deviation signal, not the magnitude of the deviation; moreover, the adjustment result does not eliminate the deviation, so such an adjuster cannot be used alone. It must often be combined with proportional or integral control laws to form a PD or PID controller. Unit 4: Principles, Classification, Characteristics, and Functions of Actuators I. Principles of actuators. 1. Working principle of electric actuators: The actuator consists of a servo motor, mechanical reduction gear, and a position transmitter. The actuator receives the output signal from a servo amplifier or electric operator, controls the forward and reverse rotation of the servo motor, and after passing through a mechanical reducer, generates an output torque to drive the adjustment mechanism to move. Meanwhile, the position transmitter converts the angular displacement of the actuator into a corresponding 4-20 mADC signal, which serves as an indication of the valve position and is fed back to the input of the pre-amplifier as a position feedback signal to balance the input signal. 2. Working principle of pneumatic actuators: Pneumatic actuators receive the air pressure signal generated by a pneumatic regulator or valve positioner, and convert it into the corresponding linear displacement of the actuator rod, thereby driving the control mechanism to function. II. Classification and characteristics of actuators. Actuators can be classified into three main categories based on the type of energy they use: pneumatic, electric, and hydraulic. ⑴Actuators operated by pneumatic actuators are called pneumatic actuators or pneumatic control valves; (2) Actuators operated by electric actuators are called electric actuators or electric control valves; (3) Actuators operated by hydraulic actuators are called hydraulic actuators or hydraulic control valves. Features: 1. Pneumatic actuators have advantages such as simple structure, reliable and stable operation, high output force, ease of installation and maintenance, low cost, as well as fire and explosion resistance, and they are widely used in industries such as petroleum, chemicals, metallurgy, and power generation. The drawback is large latency, making it unsuitable for long-distance transmission (the transmission distance is limited to within 150 meters). To overcome this drawback, an electric/pneumatic converter or an electric/pneumatic valve positioner can be used, so that the transmission signal is electrical while the on-site operation is pneumatic. 2. Electric actuators have advantages such as fast operation, suitability for long-distance signal transmission, and ease of use in conjunction with computers. Generally speaking, electric actuators are not suitable for fire and explosion-proof applications. However, by using an explosion-proof structure, the goals of fire and explosion prevention can also be achieved. III. Selection principles for air-open control valves and air-close control valves: The selection principle is to ensure the safety of the equipment and operators in the event of a disruption in the pressure signal. IV. The function of electric/gas valve positioners. Functions: 1. Improve the sensitivity and accuracy of pneumatic actuators, as well as their static characteristics. The following factors that affect the sensitivity and accuracy of pneumatic actuators can all be reduced. a. Instability of the diaphragm and spring in the actuator section, as well as friction in the various moving parts. b. The unbalanced force that results when the pressure difference before and after the control valve is too large. c. The resistance to the movement of the valve stem caused by the high viscosity of the regulating medium, or by the presence of suspended matter and solid particles. 2. Increase the movement speed of the valve stem to reduce the transmission lag of the system. Unit 5: Concepts of explosion-proof instruments and explosion-proof measures 1. Flameproof instruments. It refers to instruments whose casings can withstand the pressure resulting from an explosion inside, such that an internal explosion does not cause an external explosion; these instruments are marked with d. 2. Intrinsically safe instruments. It refers to instruments whose electrical circuit systems, whether in normal operation or in a faulty state, do not generate sparks or heat effects sufficient to ignite the specified explosive mixture; such instruments are labeled as: ia. 3. Safety barriers, types of safety barriers. Safety barrier: It is installed in the control room and serves as a device that connects the instruments in the control room with those on site. On one hand, it transmits signals; on the other hand, it controls the flow of energy in hazardous areas so that it remains below the ignition energy of explosive gases or mixtures, thereby ensuring the system’s safety with regard to sparks. Types of safety barriers: a. Resistive safety barriers: Resistive safety barriers utilize the current-limiting effect of resistors to keep the energy flowing into a particular area below a critical level, thereby achieving explosion protection. b. Zener safety barrier: It operates on the principle of the reverse breakdown behavior of Zener diodes. c. Relay-amplified safety barrier: It evolved from resistive safety barriers; it utilizes the high input impedance of an amplifier to increase the impedance of the current-limiting resistor connected in the input circuit, thereby ensuring the generation of safe sparks. d. Isolated safety barrier: It limits the amount of energy that can enter hazardous areas through measures such as isolation, voltage limiting, and current limiting, in order to ensure the generation of safe sparks. The main measures include: insulation and energy limitation. IV. Precautions for using explosion-proof instruments. 1. Check whether the instrument enclosure has an EX mark, and verify that the explosion-proof marking complies with the regulations regarding hazardous substances at the site. 2. Intrinsic safety transmitters must be equipped with a safety barrier in order to be used in hazardous environments. 3. The instrument enclosure must have proper grounding. 4. In hazardous situations, the power must be turned off first before the cover can be opened. 5. The specifications for the incoming cable of the intrinsically safe transmitter are specified by the safety barrier with joint certification. Unit 6: Basic Electrical Knowledge I. Basic Electrical Concepts 1. Ohm’s Law: The current I flowing through a resistor R is proportional to the voltage U applied across it. Its mathematical expression is: U = R * I². The Ohm’s law for a complete circuit: A complete circuit refers to a single loop; if the electromotive force of the power source in this loop is E, its internal resistance is r, and the load resistance is R, then the current is equal to the electromotive force divided by the total resistance of the loop. Its mathematical expression is: I = E/(R + r). 3. Kirchhoff’s laws: (1) Current law: At any given instant, the sum of all currents entering a node is equal to the sum of all currents leaving it. ⑵Voltage law: Around any closed loop, the algebraic sum of the voltages across its segments is zero. 4. Series and parallel connection of resistors: Series connection: When resistors are connected one after another in a row, it is called a series connection. Its characteristic is that the same current flows through each resistor. The equivalent resistance R of a series of resistors is the sum of all the individual resistances. R=R1+R2+R3+. . . +Rn in parallel: One end of each resistor is connected together, and the other end is also connected together; this is called parallel connection. Its characteristic is that each resistor is subjected to the same voltage. The reciprocal of the equivalent resistance in parallel equals the sum of the reciprocals of each individual resistance. 1/R=1/R1+1/R2+1/R3+. . +1/Rn II. Working principle of frequency conversion speed control for three-phase asynchronous motors. The speed of the synchronously rotating magnetic field generated by the stator windings in a three-phase asynchronous motor is: n0=60f/p revolutions per minute. Here, f is the frequency of the alternating current, and p is the number of pole pairs in the winding. In fact, the motor rotates at a speed of n, which is slightly different from n0