Principles and characteristics of PID control

PID is proportional differential control. You can refer to the automatic control course for detailed introduction! In temperature control, positive action and negative action mean heating when positive action is heating and negative action is cooling control when negative action is cooling 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.

1

Open loop control system

An open-loop control system is one in which the output of the controlled object (controlled quantity) has no effect on the output of the controller. In this control system, there is no reliance on feeding back the controlled quantity to form any closed loop.

2

Closed-loop control system

The characteristic of a closed-loop control system is that the output of the controlled object (controlled quantity) of the system will be fed back to affect the output of the controller, forming one or more closed loops. A closed-loop control system has positive feedback and negative feedback. If the feedback signal is opposite to the system set value signal, it is called negative feedback. If the polarity is the same, it is called positive feedback. Generally, closed-loop control systems use negative feedback, also known as negative feedback control systems. There are many examples of closed-loop control systems. For example, a person is a closed-loop control system with negative feedback. The eyes are sensors that act as feedback. The human body system can make various correct actions through continuous corrections. If there are no eyes, there will be no feedback loop, and it will become an open-loop control system. For another example, when a truly automatic washing machine has the ability to continuously check whether the clothes are clean and automatically cut off the power after washing, it is a closed-loop control system.

3

Step response

Step response refers to the output of the system when a step input (step function) is added to the system. Steady-state error refers to the difference between the expected output and the actual output of the system after the system response enters the steady state. The performance of the control system can be described by the three words stable, accurate, and fast. Stable refers to the stability of the system. For a system to work properly, it must first be stable. From the perspective of step response, it should be convergent; accurate refers to the accuracy and control precision of the control system, which is usually described by steady-state error (Steady-stateerror), which represents the difference between the steady-state value of the system output and the expected value; fast refers to the rapidity of the control system response, which is usually quantitatively described by the rise time.

4

Principles and characteristics of PID control

In engineering practice, the most widely used regulator control law is proportional, integral, and differential control, referred to as PID control, also known as PID regulation. The PID controller has been around for nearly 70 years. It has become one of the main technologies for industrial control due to its simple structure, good stability, reliable operation, and convenient adjustment. When the structure and parameters of the controlled object cannot be fully mastered, or an accurate mathematical model cannot be obtained, and other technologies of control theory are difficult to adopt, the structure and parameters of the system controller must be determined by experience and on-site debugging. At this time, it is most convenient to apply PID control technology. That is, when we do not fully understand a system and the controlled object, or cannot obtain system parameters through effective measurement methods, PID control technology is most suitable. PID control, in practice, also has PI and PD control. The PID controller uses proportional, integral, and differential to calculate the control quantity for control based on the system error.

Proportional (P) control

Proportional control is the simplest control method. The output of the controller is proportional to the input error signal. When only proportional control is used, there is a steady-state error in the system output.

Integral (I) control

In integral control, the output of the controller is proportional to the integral of the input error signal. For an automatic control system, if there is a steady-state error after entering the steady state, the control system is called a system with a steady-state error or simply a system with a steady-state error. In order to eliminate the steady-state error, an "integral term" must be introduced in the controller. The integral term depends on the integral of the error over time, and the integral term increases as time increases. In this way, even if the error is very small, the integral term will increase with time, which drives the controller's output to increase and further reduce the steady-state error until it is equal to zero. Therefore, the proportional + integral (PI) controller can make the system have no steady-state error after entering the steady state.

Derivative (D) control

In differential control, the output of the controller is proportional to the differential of the input error signal (i.e., the rate of change of the error). The automatic control system may oscillate or even become unstable during the adjustment process to overcome the error. The reason is that there are large inertia components (links) or lag components, which have the effect of suppressing the error, and their changes always lag behind the changes in the error. The solution is to make the change of the effect of suppressing the error "ahead", that is, when the error is close to zero, the effect of suppressing the error should be zero. That is to say, it is often not enough to introduce only the "proportional" term in the controller. The role of the proportional term is only to amplify the amplitude of the error, and what needs to be added at present is the "differential term", which can predict the trend of the error change. In this way, the controller with proportional + differential can make the control effect of suppressing the error equal to zero or even negative in advance, thereby avoiding serious overshoot of the controlled quantity. Therefore, for controlled objects with large inertia or lag, the proportional + differential (PD) controller can improve the dynamic characteristics of the system during the adjustment process.

5

PID controller parameter tuning

PID controller parameter tuning is the core content of control system design. It determines the proportional coefficient, integral time and differential time of the PID controller according to the characteristics of the controlled process. There are many methods for PID controller parameter tuning, which can be summarized into two categories:

The first is the theoretical calculation setting method:

It is mainly based on the mathematical model of the system and determines the controller parameters through theoretical calculation. The calculated data obtained by this method may not be directly used, and must be adjusted and modified through actual engineering.

The second is the engineering setting method:

It mainly relies on engineering experience and is carried out directly in the control system test. The method is simple and easy to master, and is widely used in engineering practice. The engineering tuning methods of PID controller parameters mainly include critical proportion method, reaction curve method and attenuation method. The three methods have their own characteristics, and their common point is that they are tested and then the controller parameters are tuned according to the engineering experience formula. However, no matter which method is used, the controller parameters obtained need to be finally adjusted and improved in actual operation.

The critical proportion method is generally used now. The steps for adjusting the PID controller parameters using this method are as follows:

(1) First, preselect a sampling period that is short enough for the system to work;

(2) Only add proportional control link until the system shows critical oscillation in response to the input step, and record the proportional gain factor and critical oscillation period at this time;