What is PID control?

  In engineering practice, the most widely used regulator control law is proportional, integral and differential control, or PID control, also known as PID regulation. PID controller has been developed for nearly 70 years. It has become one of the main technologies of industrial control with its simple structure, good stability, reliable work and easy adjustment. When the structure and parameters of the controlled object can not be completely mastered, or when the precise mathematical model is not available, the structure and parameters of the system controller must be determined by experience and field debugging when other technologies of the control theory are difficult to adopt. The application of PID control technology is most convenient. That is, when we do not fully understand a system and controlled object, or we can not get the system parameters through effective measurement means, it is most suitable for PID control technology. PID control, in practice, also has PI and PD control. PID controller is based on the error of the system, using proportional, integral and differential to calculate the control volume to control.

  

 Proportional (P) control

Proportional control is the simplest way of control. The output of the controller is proportional to the input error signal. When proportional control is applied, the system output has steady-state error (Steady-state error).

 

Integral (I) control

In integral control, the output of the controller is directly 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, it is called a steady-state error or a system with Steady-state Error. In order to eliminate the steady-state error, the integral term must be introduced into the controller. The integral term depends on the integral of time, and the integral term increases with time. In this way, even if the error is very small, the integral term will increase with the increase of time, which drives the output of the controller to make the steady-state error further reduced until equal to zero. Therefore, the proportional plus integral (PI) controller can make the system stable without any steady-state error.

 

Integral (D) control

In differential control, the output of the controller is directly proportional to the differential of the input error signal (i.e. the rate of change of the error). The automatic control system may be oscillatory or even unstable in the process of overcoming the error adjustment. The reason is that there is a large inertia component (link) or delay component, which has the effect of restraining error, and its change is always lagging behind the change of error. The solution is to make the change of the effect of restraining error "advance", that is, when the error is close to zero, the effect of restraining the error should be zero. That is to say, it is often not enough to introduce only the "proportion" item in the controller. The function of the proportional term is only the amplitude of the magnification error, and the current need to increase the "differential term", which can predict the trend of the error change, so that the controller with proportional plus differential can make the control effect equal to the control error in advance. Zero or even negative value, thus avoiding serious overshoot of the controlled quantity. Therefore, the proportional plus derivative (PD) controller can improve the dynamic characteristics of the system in the process of adjusting the object with larger inertia or lag.

 

Parameter tuning of PID controller

Parameter tuning of PID controller is the core of control system design. It is based on the characteristics of the controlled process to determine the proportionality coefficient, the integration time and the differential time of the PID controller. The engineering tuning methods of PID controller parameters include the critical ratio method, the response curve method and the attenuation method. The three methods have their own characteristics. The common point is that the parameters of the controller can be adjusted according to the empirical formula of the project. However, the controller parameters obtained by any method need to be adjusted and perfected in practice. The critical proportion method is generally used now. The steps of tuning PID controller parameters by using this method are as follows: (1) first select a short sampling period to make the system work; (2) only join the proportional control link until the system has a critical oscillation on the step response of the input, and notes the proportional amplification factor and the critical oscillation period; (3) in a certain control. The parameters of PID controller are obtained by formula calculation.In actual debugging, only one experience value can be roughly set, and then modified according to the adjustment effect.

In actual debugging, only one experience value can be roughly set, and then modified according to the adjustment effect.