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Principles and Applications of Cascade Control: Yunnan Changhui Instrument Manufacturing Co., Ltd. illustrates, through practical examples, the principles of cascade control, the selection of cascade control instruments, and the methods for setting parameters in cascade control systems. The purpose of using cascade control in industrial applications is to improve response to external disturbances and to limit the control range of the load. 1. Improving disturbance response through feedback control: The purpose of using a temperature controller for measurement and control is to enable the state of the controlled object (the measured temperature) to quickly approach the target value (such as the set temperature). For any disturbance, the difference between the target value and the state of the controlled object is often brought to zero. So, how does feedback control suppress the disruption in the control state caused by disturbances? As shown in Figure 1, the temperature control of the medicinal solution using a thermostat is illustrated as follows. Figure 1: Simple feedback control system. Components of a simple feedback control system’s instrumentation include: 1. Field sensors; 2. Actuators (control valves, frequency converters, etc.); 3. Single-loop temperature controllers. For applications such as temperature control where there is a large time lag, it is advisable to use regulators equipped with artificial intelligence or fuzzy control algorithms as well as self-tuning capabilities, in order to prevent overshooting and under-shooting in situations with large time lags. This simple temperature feedback control system uses the opening degree of a valve to adjust the steam flow, thereby controlling the temperature of the liquid medication. The main disturbances include changes in steam temperature and steam pressure. If feedback control is used when such disturbances occur in this control system, it is difficult to quickly resolve the confusion in response caused by the disturbances, as the controller cannot take corrective action before these effects manifest themselves as changes in the temperature of the liquid medication. Figure 2: Response when steam temperature decreases. 2. Improving disturbance response by combining with feedforward control. In the control system shown in Figure 1, if it is possible to measure changes in steam temperature and if the appropriate correction amount (valve opening output MV) required for such changes can be determined, then using this in combination with feedforward control can mitigate the effects caused by variations in steam temperature, compared to using only feedback control. Figure 3: Feedforward control system. The components of a feedforward control system include: 1. Field sensors; 2. Actuators (control valves, frequency converters, etc.); 3. Single-loop PID regulators. In applications involving large time lags such as temperature control, it is advisable to use regulators equipped with artificial intelligence or fuzzy control algorithms as well as self-tuning capabilities, in order to prevent overshooting and under-shooting under such conditions. 4. Addition and subtraction operation modules. Figure 3 shows a feedforward control system that detects the temperature of steam. When there are changes in this temperature, these changes can be corrected promptly, thereby limiting the extent of variations in the temperature of the liquid being processed. However, since the temperature of the liquid medicine is not affected by the steam flow rate, it is generally difficult to accurately determine the steam flow rate (the relationship between disturbances and changes in the temperature profile), which limits the possibility of improving control performance through the use of feedforward control. Furthermore, since the feedforward control system in Figure 3 only compensates for the steam temperature, it has no effect on changes in steam pressure. 3. Improving the response to disturbances through cascade control. In the control system shown in Figure 1, the assumed disturbances are “changes in steam temperature” and “changes in steam pressure.” In either case, such changes are reflected as variations in the energy supplied to the liquid medicine, resulting in changes in the temperature of the liquid medicine. Therefore, it is assumed that even if the temperature or pressure of the steam changes, as long as the energy supplied to the liquid medicine by the steam remains constant, the temperature of the liquid medicine should be able to remain at a constant value. From this perspective, it is a system that detects the temperature at the lower part of the liquid tank supplying energy to the liquid, and uses this value in the control circuit for regulation. Figure 4: Configuration of a cascade control system. The instruments that make up a cascade control system include: 1. Field sensors; 2. Actuators (control valves, frequency converters, etc.); 3. Single-loop PID regulators. In applications involving large time lags, such as temperature control, it is advisable to use regulators equipped with artificial intelligence or fuzzy control algorithms as well as self-tuning capabilities, in order to prevent overshoot or under-control in such conditions. 4. External setpoint regulators. As shown in Figure 4, the two regulators operate in series, with the output of one regulator serving as the setpoint for the other regulator. This control method, which involves multiple loops, is called “cascade control.” Its operation is as follows: The MASTER controller is used to ensure that the “temperature of the liquid medication” reaches the desired level by determining the temperature at the bottom of the liquid medication tank, and it outputs the target temperature for the SLAVE controller. The thermostat SLAVE is designed to determine the \"opening degree output\" so that the \"temperature at the bottom of the liquid medicine tank\" reaches the target temperature set by the thermostat MASTER. Figure 5 Block diagram of the cascade control loop: SV: Target set temperature of the liquid; FL: Flow rate of steam; SVs: Target set temperature at the bottom of the liquid tank; PVS: Measured temperature at the bottom of the liquid tank; MV: Opening degree output of the flow control valve; PV: Measured temperature of the liquid; G1: Response characteristic of the temperature at the bottom of the liquid tank to the steam flow rate; G2: Response characteristic from the temperature at the bottom of the liquid tank to the temperature measurement point of the liquid. In a cascade control system, once there is a change in the temperature or pressure of the steam, the SLAVE controller detects this change as a temperature change at the bottom of the liquid tank. In order to make the temperature difference between this value and the set temperature of the liquid tank determined by the MASTER controller equal to “0”, the opening degree of the control valve is adjusted. It can be seen that this system can suppress temperature disturbances caused by external perturbances more quickly than a simple feedback control system as shown in Figure 1. Figure 6: 4. Limiting the control range of the load through cascade control. Figure 7: Single-loop temperature control system. Figure 8: Cascade temperature control system. If the control output value MV is set at 100%, the heater may burn out. To keep the heater temperature below 600°C, the cascade control shown in Figure 8 is required. If you want to set the input range of the subordinate temperature controller to 0–600°C, then an MV output of 0%–100% from the main temperature controller corresponds to a target value SV of 0–600°C for the subordinate temperature controller, which allows the heater’s temperature to be kept below 600°C. 5. Summary of cascade control: I’m not sure if I understand how cascade control improves the disturbance response characteristics Figure 9 Block diagram of the cascade control loop. As shown in Figure 9, there are two control loops in the cascade control system. The loop that uses the control quantity (PV) as the feedback target is called the “MASTER loop”, while the loop located inside the “MASTER loop” is called the “SLAVE loop”. 6. To achieve good control results in cascade control, the following conditions must be met: ① A secondary loop can be established. ②The perturbation occurs within the secondary loop. ③Regarding the delay in the response speed of the primary control object, the delay in the response speed of the secondary control object is smaller. Please note that if the conditions are not as stated above, cascade control cannot effectively improve the disturbance response to disturbances occurring in the main circuit, and it will not achieve good control results. The ability to skillfully tune PID parameters and put automatic control systems into automatic mode reflects the automation skills of engineering technicians; however, many people do not truly master PID control and PID parameter tuning.