What is pid control and what is pid controller

The so-called PID control is a method to make the controlled physical quantity approach the control target quickly and accurately in a closed-loop control system. The PID control function is one of the important fields of inverter application technology and an important technical means for the inverter to exert its excellent performance.

The designers, operators and maintainers of variable frequency speed regulation products should be fully familiar with and master the basic theory of PID control.

Introduction to PID Control

At present, the level of industrial automation has become an important indicator of the modernization level of all walks of life. At the same time, the development of control theory has also gone through three stages: classical control theory, modern control theory and intelligent control theory. Typical examples of intelligent control are fuzzy fully automatic washing machines, etc. Automatic control systems can be divided into open-loop control systems and closed-loop control systems. A control system includes a controller, a sensor, a transmitter, an actuator, and an input and output interface. The output of the controller is added to the controlled system through the output interface and the actuator; the controlled quantity of the control system is sent to the controller through the sensor, the transmitter, and the input interface. Different control systems have different sensors, transmitters, and actuators. For example, a pressure control system must use a pressure sensor. The sensor of the electric heating control system is a temperature sensor. At present, there are many PID controls and their controllers or intelligent PID controllers (instruments), and the products have been widely used in engineering practice. There are various PID controller products. Major companies have developed intelligent regulators with PID parameter self-tuning functions, in which the automatic adjustment of PID controller parameters is achieved through intelligent adjustment or self-correction and adaptive algorithms. There are pressure, temperature, flow, and liquid level controllers that use PID control, programmable controllers (PLCs) that can implement PID control functions, and PC systems that can implement PID control, etc. Programmable controllers (PLCs) use their closed-loop control modules to implement PID control, and programmable controllers (PLCs) can be directly connected to ControlNet, such as Rockwell's PLC-5. There are also controllers that can implement PID control functions, such as Rockwell's Logix product series, which can be directly connected to ControlNet and use the network to implement its remote control function.

PID debugging steps

No control algorithm is more effective and convenient than the PID regulation law. Now some fashionable regulators are basically derived from PID. It can even be said that PID regulator is the product of other control and regulation algorithms.

Why is PID so widely used and long-lasting?

Because PID solves the most basic problem to be solved by automatic control theory, that is, the stability, rapidity and accuracy of the system. By adjusting the parameters of PID, the system's load capacity and anti-interference ability can be taken into account under the premise of system stability. At the same time, the integral term is introduced in the PID regulator, and the system adds a zero integral point, making it a first-order or higher-order system, so that the steady-state error of the system step response is zero.

Due to the great differences in the controlled objects of the automatic control system, the parameters of PID must also change accordingly to meet the performance requirements of the system. This brings considerable trouble to users, especially for beginners. The following is a brief introduction to the general steps of debugging PID parameters:

1. Negative feedback

Automatic control theory is also called negative feedback control theory. First, check the system wiring to determine that the system feedback is negative feedback. For example, in the motor speed control system, when the input signal is positive and the motor is required to rotate forward, the feedback signal is also positive (in the PID algorithm, error = input-feedback), and the higher the motor speed, the larger the feedback signal. The same method applies to other systems.

2. General principles of PID debugging

a. When the output does not oscillate, increase the proportional gain P.

b. When the output does not oscillate, reduce the integral time constant Ti.

c. When the output does not oscillate, increase the differential time constant Td.

3. General steps

a. Determine the proportional gain P

When determining the proportional gain P, first remove the integral and differential terms of the PID, generally set Ti=0 and Td=0 (see the PID parameter setting instructions for details) to make the PID a pure proportional adjustment. The input is set to 60%~70% of the maximum value allowed by the system, and the proportional gain P is gradually increased from 0 until the system oscillates; conversely, the proportional gain P is gradually reduced from this time until the system oscillation disappears, and the proportional gain P at this time is recorded, and the proportional gain P of the PID is set to 60%~70% of the current value. The proportional gain P is debugged.

b. Determine the integral time constant Ti.

After the proportional gain P is determined, set a larger initial value of the integral time constant Ti, and then gradually reduce Ti until the system oscillates. Then, in reverse, gradually increase Ti until the system oscillation disappears. Record Ti at this time, and set the integral time constant Ti of PID to 150%~180% of the current value. The debugging of the integral time constant Ti is completed.

c. Determine the integral time constant Td.

The integral time constant Td generally does not need to be set, and can be set to 0. If it needs to be set, the method is the same as that for determining P and Ti, and take 30% when there is no oscillation.

d. The system is debugged with no load and load, and then the PID parameters are fine-tuned until the requirements are met.

Implementation of PID Control

1. PID feedback logic

The names of feedback logics of various inverters are different, and there are even similar names with opposite meanings. When designing the system, the manual of the selected inverter should be used as the standard. The so-called feedback logic refers to the control polarity of the feedback signal detected by the sensor on the output frequency of the inverter. For example, in the central air-conditioning system, the return water temperature is used to control the output frequency of the inverter and the speed of the water pump motor. When heating in winter, if the return water temperature is low, the feedback signal decreases, indicating that the room temperature is low, and it is required to increase the output frequency of the inverter and the motor speed, and increase the flow of hot water; while in summer cooling, if the return water temperature is low, the feedback signal decreases, indicating that the room temperature is too low, and the output frequency of the inverter and the motor speed can be reduced. Reduce the flow of cold water. As can be seen from the above, the feedback signal decreases when the temperature is low, but the frequency change direction of the inverter is required to be opposite. This is the reason for introducing feedback logic. The function selection of several inverter feedback logics is shown in Table 1.

2. Enable the PID function

To realize the closed-loop PID control function, the PID function should be preset to be valid first. There are two specific methods: one is to preset through the function parameter code of the inverter. For example, for the Convo CVF-G2 series inverter, when parameter H-48 is set to 0, there is no PID function; when it is set to 1, it is ordinary PID control; when it is set to 2, it is constant pressure water supply PID. The second is determined by the state of the inverter's external multi-function terminal. For example, for the Yaskawa CIMR-G 7A series inverter, as shown in Figure 1, select any one of the multi-function input terminals S1-S10, and preset the function code H1-01 ~ H1-10 (corresponding to terminals S1-S10) to 19, then the terminal has the function of determining whether the PI control is valid. This terminal and the common terminal SC are invalid when "ON" and valid when "OFF". It should be noted that most inverters have both of the above two preset methods, but a few brands of inverters have only one of them.

In some systems where the control requirements are not very strict, sometimes only using the PI control function and not starting the D function can meet the needs. The debugging process of such a system is relatively simple.

3. Target signal and feedback signal

In order to stabilize a physical quantity in the frequency conversion system at the expected target value, the PID function circuit of the frequency converter will continuously compare the feedback signal with the target signal, and adjust the output frequency and the speed of the motor in real time according to the comparison result. Therefore, the PID control of the frequency converter requires at least two control signals: the target signal and the feedback signal. The target signal mentioned here is the electrical signal corresponding to the expected stable value of a physical quantity, also known as the target value or given value; and the electrical signal corresponding to the actual value of the physical quantity measured by the sensor is called the feedback signal, also known as the feedback quantity or current value. The functional diagram of PID control is shown in Figure 2. There is a PID switch in the figure. The PID function can be enabled or disabled by setting the function parameters of the frequency converter. When the PID function is enabled, the operating frequency is determined by the PID circuit; when the PID function is disabled, the operating frequency is determined by the frequency setting signal. The working state of the PID switch, action selection switch and feedback signal switching switch is determined by the setting of the function parameters.

4. Target value setting

How to transmit the command information of the target value (target signal) to the inverter? Various inverters have chosen different methods, and generally there are two solutions: one is the automatic conversion method, that is, when the inverter preset PID function is effective, its frequency setting function in open-loop operation is automatically converted to the target value setting. Such as Yaskawa CIMR-G 7A and Fuji P11S inverters in Table 2. The other is the channel selection method, such as Convo CVF-G2, Senlan SB12 and Puchon P17000 series inverters in Table 2.

The above introduces the input channel of the target signal. Next, we need to determine the size of the target value. Since the target signal and the feedback signal are usually not the same physical quantity, it is difficult to make a direct comparison. Therefore, the target signal of most frequency converters is expressed as a percentage of the sensor range. For example, the air pressure of a gas tank is required to be stable at 1.2MPa, and the range of the pressure sensor is 2MPa. The percentage corresponding to 1.2MPa is 60%, and the target value is 60%. In the parameter list of some frequency converters, there are parameters corresponding to the upper and lower limits of the sensor range. For example, Fuji P11S frequency converter sets parameter E40 (display coefficient A) to 2, that is, the upper limit of the pressure sensor range is 2MPa: parameter E41 (display coefficient B) is set to 0, that is, the lower limit of the range is 0, and the target value is 1.2. That is, the pressure stability value is 1.2 MPa. The target value is the absolute value of the expected stability value.

5. Feedback signal connection

Various frequency converters have several frequency setting input terminals. Among these input terminals, if one has been determined as the input channel of the target signal, the other input terminals can be used as the input terminals of the feedback signal. You can select one of them through the corresponding function parameter code. See Table 3 for the selection of several typical frequency converter feedback signal channels.

6. Presetting and adjustment of P, I and D parameters

(1) Proportional gain P

The PID function of the frequency converter uses the difference between the target signal and the feedback signal to adjust the output frequency. On the one hand, we hope that the target signal and the feedback signal are infinitely close, that is, the difference is very small, so as to meet the accuracy of the adjustment; on the other hand, we hope that the adjustment signal has a certain amplitude to ensure the sensitivity of the adjustment. The way to solve this contradiction is to amplify the difference signal in advance. The proportional gain P is used to set the amplification factor of the difference signal. The parameter P of any frequency converter gives a settable value range. Generally, during the initial debugging, P can be preset to a middle-large value. Or temporarily default to the factory value, and then fine-tune according to the actual situation when the equipment is running.

(2) Integration time

As mentioned above. The larger the proportional gain P is, the higher the adjustment sensitivity is. However, due to the inertia of the transmission system and the control circuit, the adjustment result cannot stop immediately when it reaches the optimal value, resulting in "overshoot", and then adjustment in reverse, overshoot again, and oscillation. For this reason, the integral link I is introduced, and its effect is to make the difference signal amplified by the proportional gain P gradually increase (or decrease) within the integral time, thereby slowing down its change speed and preventing oscillation. However, if the integral time I is too long, when the feedback signal changes sharply, the controlled physical quantity will be difficult to recover quickly. Therefore, the value of I is related to the time constant of the drag system: when the time constant of the drag system is small, the integral time should be shorter; when the time constant of the drag system is large, the integral time should be longer.

(3) Derivative time D

The differential time D is based on the rate of change of the differential signal, giving a corresponding adjustment action in advance, thereby shortening the adjustment time and overcoming the defect of delayed recovery due to too long integration time. The value of D is also related to the time constant of the drag system: when the time constant of the drag system is small, the differential time should be shorter; conversely, when the time constant of the drag system is large, the differential time should be longer.

(4) Adjustment principles of P, I, and D parameters

The presets of P, I, and D parameters complement each other. The following fine adjustments should be made at the operating site according to the actual situation: If the controlled physical quantity oscillates near the target value, first increase the integral time I. If there is still oscillation, appropriately reduce the proportional gain P. If the controlled physical quantity is difficult to recover after the change, first increase the proportional gain P. If the recovery is still slow, appropriately reduce the integral time I and increase the differential time D.