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Basic Knowledge of Automatic Control (I) – Basic Concepts. Automatic control refers to the use of specialized instruments and devices to form a control system, which replaces manual operation by humans in order to adjust the parameters of air conditioning systems and keep them at specified values or vary them according to predetermined patterns, thereby meeting the requirements of the air-conditioned space. The methods used in domestic automatic control today involve first measuring the deviation of the control parameters from the set value, and then, based on this deviation, using the control system to eliminate the effects of disturbances so that the control parameters return to the set value (or within the allowable range). (II) Composition of automatic control systems: Currently, electric control is widely used in air-conditioning automatic control systems. Such a control system can be represented by the block diagram shown below: Figure: Block diagram of the automatic control system. Due to external disturbances, the control parameters of the controlled object change; these changes are detected by sensitive elements and transmitted to the control mechanism (regulator). The regulator then commands the actuator to make the control mechanism operate, thereby adjusting the load on the controlled object and bringing the control parameters back to their original set values. On the main circuit that supplies power to the actuator, a switching mechanism is often installed to ensure stable regulation, thereby allowing intermittent power supply to the actuator. (III) Common terms for automatic regulation 1. Regulation parameters (also known as controlled parameters) Parameters that need to be kept at a constant value or allowed to vary within a specified range are called regulation parameters. The control parameters in air conditioners mainly include temperature, humidity, pressure, as well as water level, and so on. 2. A set value (also known as a fixed value) is an unchanging value or range of variation that is predetermined for a control parameter as needed, and is referred to as a set value. For example, it is specified that the room temperature should be maintained at 23±0.5°C; this value (i.e., the range of 22.5–23.5°C) constitutes the setpoint (range) for the room temperature control system. 3. Deviation: The difference between the actual value of a tuning parameter and the specified value is called deviation. For example, if the controlled temperature (set value) is specified at 20°C, but the actual temperature is 21°C, then the difference of 1°C between them is considered the deviation. 4. Perturbations: Factors that can cause deviations in the control parameters are called perturbations or disturbances. Factors in air conditioners that cause changes in the temperature of the conditioned room, such as changes in outdoor temperature, changes in the temperature of the supply air, and changes in residual heat inside the room, are all disturbances to the room temperature. The function of automatic adjustment is precisely to eliminate the effects of disturbances and keep the adjustment parameters constant or within the required range. 5. Control object: A place where it is necessary to keep the value of the control parameters within a specified range of variation is called a control object. In air conditioning systems, all the components where air parameters need to be adjusted are considered control targets, such as the constant-temperature chamber, the room outlet, and the area after the secondary heater, among others. 6. Sensing elements: Components that measure and reflect the magnitude of control parameters are called sensing elements. In air conditioners, the main components are temperature-sensing elements, that is, devices for measuring temperature, such as thermistors. In addition, there are also humidity sensing elements, pressure measuring elements, and water level indication devices, etc. 7. Regulator: A secondary instrument or device that receives signals from sensitive elements and commands the actuator to operate; these are collectively referred to as regulators. 8. Actuator: A device that receives instructions from the control mechanism (regulator) and drives the regulator to operate is called an actuator. Such as contactors, motors, voltage regulators, and so on. 9. Control mechanism: A mechanism that directly affects and regulates the parameter being controlled is called a control mechanism. Such as electric heaters, two-way and three-way water valves, air valves, and so on. (IV) Characteristics of the control object The control object is the entity that the automatic control system serves. Its characteristics directly affect the performance of the automatic control system. These features are ； 1. Load of the object: When the regulation process is in a stable state, the energy that flows into or out of the regulated object per unit time is called the load of that object. For example, when the air temperature in an air-conditioned room remains constant, the amount of heat that flows into or out of the room per unit of time represents the load on the air conditioner. At this point, the heat flowing out is in balance with the heat flowing in. Due to external disturbances, the load on the system in question changes (for example, in the case of an air-conditioned room, changes in outdoor temperature lead to changes in the amount of heat dissipated to the outside), thereby disrupting the original energy balance. This results in changes in the control parameters, and thus a regulation process begins, with the aim of altering the input or output energy of the system so that a new balance is achieved and the control parameters return to their set values. It can be seen that the changes in the load of the regulated object are directly related to the requirements for the automatic control system. If the load on the object changes at a fairly rapid pace, then the automatic control system needs to have high sensitivity, so that it can start making adjustments even when the deviation in the control parameters is small, in order to restore balance quickly. Conversely, the requirement for the sensitivity of the automatic control system is not necessarily that high. Generally, the load on air-conditioning systems changes relatively slowly. 2. Transfer coefficient of the object: The change in the control parameter that occurs when the load of the object changes by one unit of energy is called the transfer coefficient, denoted by K. For example, the transfer coefficient of a room refers to the change in dew point temperature that occurs when the temperature changes by 1℃, under certain conditions of air volume. The transfer coefficient of a water heater is the change in air temperature that takes place when the temperature of the hot water changes by 1℃. The transfer coefficient of a controlled room is the change in room temperature (usually referring to the control point) that occurs when the supply air temperature changes by 1℃, under certain conditions of air supply volume. In summary, assuming that a temperature change of Δθf in the load of the object causes a temperature change of Δθ in the object itself, then K = Δθ/Δθf. When the transfer coefficient K is small, the deviation of the control parameters from their set values is small when disturbances disrupt the equilibrium state, allowing the automatic control system to maintain balance easily. Conversely, if the transfer coefficient is large, the deviation of the control parameters from their set values is large, making it difficult for the system under control to maintain balance. 3. The time constant of the controlled object (also known as the response time) represents the time required for the control parameter to maintain its initial rate of change and adjust its value to a specified level, when there is a maximum change in the load on the controlled object; this time is denoted by T. The reciprocal of the response time is called the sensitivity of the object; it represents the rate of change of the control parameter when there is a maximum change in the load on the controlled object. They indicate the speed at which the control parameters change when the load of the object being controlled changes. A long response time (low sensitivity) means that even if there is a large change in heat (perturbation), the temperature will only change slowly; conversely, a short response time (high sensitivity) indicates that the room temperature changes rapidly, with low thermal inertia. In air conditioning, the time constant T of a system is determined as the time it takes for the cooling or heating load of that system to change suddenly (in a stepwise manner) from one steady value to another; it is the time required for the control parameter – temperature – to reach 63.2% of the final steady value after the load undergoes this step change, starting from the original steady value. 4. Object lag (also known as delay): When the load on an object changes, the control parameters do not change immediately; instead, they start to change after a certain delay. This delay period is referred to as the lag time, denoted by τ. For example, in an air conditioning system, as soon as the power to the electric heater is turned on, the temperature at the control point of the room being cooled does not rise immediately; it takes some time before the temperature starts to increase, during which time heat is transferred and the air is mixed. The lag of the regulated object has an adverse effect on the regulation process; it reduces the stability of the control system, increases the deviation of the control parameters, and prolongs the regulation time. Figure: Ascending variation curve of the control object. In summary, an ideal control object is one with minor load variations, a low transfer coefficient, and a short lag time. (5) Characteristics of the regulator: In an automatic control system, the regulator acts like a human brain; it is responsible for receiving signals and issuing action commands. It has the following main features: 1. Adjustment range refers to the operating range of the regulator, that is, the range within which the regulator can function when the adjustment parameters vary from one value to another; this is generally indicated by the scale values on the dial. 2. Accuracy class: It represents the basic error inherent in the instrument itself, and refers to the maximum absolute error Δx (the difference between the instrument’s reading and the actual value being measured) that may occur under normal operating conditions, expressed as a percentage of the instrument’s rated value XH (the maximum scale value on the dial, i.e., the full scale value). There are two ways of expressing this: one is in terms of percentages, for example, if a gauge has an accuracy of 1%, and the other is by using the numerical value represented by the percentage – that is, 1% is referred to as a grade 1 gauge, 0.5% as a grade 0.5 gauge, and so on. Obviously, the smaller the number, the smaller the instrument error. Since the position of the gauge error Δx is unpredictable, the smaller the measurement value, the greater the error. For example, for a level 1 sensor whose temperature measurement range is 0–50°C, the maximum possible error Δx could be 50×1% = ±0.5°C. This means that an error of 0.50°C can occur whenever measuring temperatures within the 0–50°C range; obviously, the error of 0.5°C at 0°C is much greater than the same 0.50°C error at 50°C. Therefore, to make full use of the instrument’s accuracy and improve measurement precision, it is advisable to choose instruments with a small range, or to ensure that the instrument is frequently operating within more than 1/2 of its full range. 3. The insensitive zone (dead zone) is the deviation range of the control parameter from the set value that does not trigger any adjustment action, as shown in the figure. It is generally expressed as a percentage of the full scale. For example, in a regulator with a scale ranging from 0 to 50°C and a sensitivity range of 0.5%, the temperature value corresponding to this sensitivity range is 50×0.5% = 0.25°C. This means that when the actual temperature falls within a deviation range of 0.25°C around the set value, the regulator does not generate any output signal. Obviously, the smaller the non-functional area, the more sensitive the instrument. Figure: Zone of poor sensitivity of the instrument. 4. Hysteresis of the regulator: When the control parameters at the location where the measuring element is installed in the controlled system begin to change, it usually takes some time before the regulator causes the control mechanism to respond; this time period is known as the regulator’s hysteresis. The lag of the regulator will cause lag in the automatic control system. 5. Feedback: To ensure the stability of an automatic control system, certain values from the control mechanism (or the output of the controller) are fed back to influence the controller; this is known as feedback. If the adjustment parameter causes a deviation from the given value, and the feedback effect reduces this deviation signal, it is called negative feedback; conversely, if the feedback effect increases the deviation signal, it is called positive feedback. Positive feedback can increase the gain of a regulator, while negative feedback is used to improve the stability of an automatic control system (or regulator). Regulators generally use negative feedback to improve their performance and enhance the stability of regulation. (VI) Types of regulators: Due to the wide variety of industrial parameters, there are also many different types of regulators. On air conditioners, on-off controllers, proportional controllers, and proportional-integral-derivative controllers are commonly used. Briefly described as follows ； 1. Two-position regulator: The operating characteristic of a two-position regulator is that when there is a deviation in the control parameter, its output signal causes the actuator to either turn on or off, thereby bringing the control mechanism to its fully open or fully closed position; as a result, the control parameter fluctuates between two extreme values. Therefore, it is generally used for control objects that allow certain fluctuations in the control parameters, have a long response time and small lag time, and experience infrequent load changes, such as for room temperature control. To improve the quality of two-position control, three-position regulators are also used in air conditioners; essentially, these consist of two two-position control elements that set upper and lower limits, thereby enabling three-position control. It is smaller than the two-digit deviation. The regulators used in automatic air-conditioning control systems are, for the most part, on-off regulators. For example, XCT—102, 112, 122 are available in all series of automatic balance bridges. 2. Proportional regulator: Proportional control is a fundamental control method among various types of continuous control. Its characteristic is that when there is a deviation between the control parameter and the set value, the regulator emits a signal proportional to the magnitude and direction of the deviation; different deviations result in corresponding positions of the control mechanism. In other words, when the adjustment parameter deviates from the set value, the adjustment mechanism moves to a new position; once the deviation is eliminated, it returns to its original position. The operation of the adjustment mechanism depends only on the magnitude of the deviation, and is unrelated to the rate of change of the adjustment parameters or the duration for which the deviation exists. Proportional regulators have a fast tuning speed and good stability; generally, there is no \"oscillation process\", and the tuning parameters can remain stable. However, once the regulation is complete, the regulation parameters cannot return to their original set values; instead, there remains a residual deviation, known as static error. This is because the deviation value of such a regulator is proportional to the position of the regulating mechanism. When the load on the system changes, the regulating mechanism must adjust its position accordingly in order to regulate the amount of flow in or out, thereby achieving a new state of equilibrium. The setpoint of the regulator can be considered as the value corresponding to a certain load on the system. In the new state of equilibrium, the regulating parameter is not necessarily equal to this setpoint (unless the equilibrium state at that time happens to correspond to the setpoint value). Hence, there is a difference between the two. “The “proportional band” is a key characteristic of a proportional regulator. It means: the percentage change in the adjustment parameter required to move the control mechanism from full closed (full open) to full open (full closed), denoted by P. In other words, it is the percentage change in the adjustment parameter required for the control mechanism to move through its full range of operation (from full open to full closed, or from full closed to full open), with the full range of the regulator (from the starting value to the full scale value) considered as 100%. For example, if a temperature regulator has a full range of 0–50°C and the set point is set at 20°C, then the valve that controls the heating medium will close completely when the temperature rises to 21.5°C, and it will open completely when the temperature drops to 18.5°C. In other words, a change in temperature of 21.5–18.5 = 3°C is sufficient to cause the valve to move through its entire range of motion. The proportional band for this regulator is: P = (21.5 – 18.5) / (50 – 0) = 6%. The aforementioned “proportional band” refers to a relative value; sometimes it is referred to as the “true proportional band,” which denotes an absolute value. It represents the value of the control parameter for which the control mechanism moves through its entire range of motion. For example, if a change of 1°C in the temperature to be regulated causes the control valve to move from fully open (fully closed) to fully closed (fully open), then its quasi-proportional band is said to be 1°C. As in the previous example, the proportional band is 6%, while the quasi-proportional band is 3°C. The proportional band of a regulator is generally adjustable. The width (size) of the proportional band indicates the speed at which the control mechanism acts. The narrower the proportional band, the more sensitive the system is to changes in the control parameters; the controller responds more quickly, and the static error is smaller. However, the stability of the system deteriorates, as when P