【What isPIDControl?】
In practical engineering applications, the most widely used regulator control law is proportional, integral, and derivative control, referred to asPIDcontrol, also known asPIDregulation. The PID controller has a history of nearly70years since its inception. With its simple structure, good stability, reliable operation, and easy adjustment, it has become one of the main technologies in industrial control. When the structure and parameters of the controlled object cannot be fully grasped, or an accurate mathematical model cannot be obtained, and other control theory technologies are difficult to adopt, the structure and parameters of the system controller must be determined based on experience and on-site debugging. At this time, the application ofPIDcontrol technology is the most convenient. That is, when we do not fully understand a system and the controlled object, or cannot obtain system parameters through effective measurement means, PID control technology is most suitable. In practice, there are alsoPIandPDcontrol methods besides PID control. ThePIDcontroller calculates the control quantity using proportional, integral, and derivative based on the system error to perform control. 
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 (Steady-state error). 
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 said to have a steady-state error or be simply referred to as an error system (System with Steady-state Error). To eliminate the steady-state error, an"integral term"must be introduced into the controller. The integral term for the error depends on the integral over time; as time increases, the integral term increases. Thus, even if the error is small, the integral term will increase with time, pushing the controller output to increase, further reducing the steady-state error until it equals zero. Therefore, a proportional+integral (PI) controller can make the system have no steady-state error after entering the steady state. 
Derivative (D) Control 
In derivative control, the output of the controller is proportional to the derivative (i.e., the rate of change of the error) of the input error signal. Automatic control systems may experience oscillations or even instability in the process of overcoming errors. The reason is the presence of large inertia components (links) or lag(delay)components, which have the effect of suppressing errors, and their changes always lag behind the changes of errors. The solution is to make the change of the error suppression effect"ahead of time", that is, when the error approaches zero, the error suppression effect should be zero. This means that introducing only the"proportional"term in the controller is often insufficient; the role of the proportional term is only to amplify the amplitude of the error, while what is currently needed is to add the"derivative term", which can predict the trend of error change. Thus, a controller with proportional+derivative (PD) can advance the error suppression control effect to zero or even negative values, thereby avoiding severe overshoot of the controlled variable. Therefore, for controlled objects with large inertia or lag, proportional+derivative (PD) controllers can improve the dynamic characteristics of the system during the adjustment process. 
PID Controller Parameter Tuning 
PID controller parameter tuning is the core content of control system design. It involves determining the values of the proportional coefficient, integral time, and derivative time of the PID controller based on the characteristics of the controlled process. The engineering tuning methods for PID controller parameters mainly include the critical proportionality method, reaction curve method, and attenuation method. Each of these three methods has its own characteristics, and their common point is to tune the controller parameters according to empirical engineering formulas through experiments. However, the controller parameters obtained by any of these methods need to be finally adjusted and perfected in actual operation. Currently, the critical proportionality method is generally adopted. The steps for tuning PIDcontroller parameters using this method are as follows:(1)First, pre-select a sufficiently short sampling period for the system to operate;(2)Add only the proportional control link until the system exhibits critical oscillation in response to a step input, and record the proportional gain and critical oscillation period at this time;(3)Calculate the parameters of thePIDcontroller through formulas under a certain control degree. 
In actual debugging, only an empirical value can be roughly set first, and then modified according to the adjustment effect.