The PID control system's PID function is implemented by the PID controller or the internal function program module of the DCS system. Understanding some basic concepts related to PID control can help PID beginners quickly become familiar with controller applications and successfully tune PID parameters in automatic control systems.
1. Quantity being adjusted
The controlled quantity is the value that reflects the actual fluctuation of the controlled object. The controlled quantity changes frequently.
2. Setting value
The setpoint of a PID controller is the value that the controlled variable is expected to reach. The setpoint can be fixed or variable.
3. Control output
The control output refers to the command issued by the PID controller after calculating the changes in the controlled variable, instructing the external actuator to act according to its requirements. There are other components between the PID controller and the actuator, such as limiters and servo amplifiers. Limiting is usually performed within the PID controller; if the PID, limiter, and servo amplifier functions are integrated into a single instrument, it constitutes a valve position control PID controller; if the servo amplifier and limiter are integrated into the actuator, it constitutes an intelligent actuator.
4. Input deviation
Input deviation is the difference between the controlled variable and the set value.
5. P (proportion)
P stands for proportional effect, which simply means the input deviation is multiplied by a coefficient.
6. I (Integral)
I stands for integral, which simply means integrating the input deviation.
7. D (differential)
D stands for differential, which simply means performing a differential operation on the input deviation.
8. Basic Formula of PID
In simple terms, the PID controller parameter tuning process involves first setting the system to a pure proportional action, gradually increasing the proportional action to allow the system to oscillate, recording the proportional action and oscillation period, then multiplying the proportional action by 0.6, and appropriately extending the integral action.
KP=0.6Km
KD=KP×π/4ω
KI=KP×ω/π
In the formula, KP is the proportional control parameter; KD is the integral control parameter; KI is the derivative control parameter; Km is the proportional value at which the system begins to oscillate; and ω is the frequency of oscillation in polar coordinates.
9. Single circuit
A single-loop system is a control system with only one PID controller.
10. Cascade
One PID controller is not enough; cascading involves connecting two PID controllers in series to form a cascade control system, also known as a dual-loop control system. In a cascade control system, the PID controller has a primary controller and a secondary controller.
In a cascade control system, the PID controller that adjusts the controlled variable is called the master controller, and the PID controller whose output directly commands the actuator is called the slave controller. The control output of the master controller is fed into the slave controller as its setpoint. The master controller uses a single-loop PID controller, and the slave controller uses an external setpoint controller.
11. Positive effect
For a PID controller, the effect of the control output increasing as the controlled variable increases and decreasing as the controlled variable decreases is called PID positive action.
12. Positive effect
For a PID controller, the effect of the control output decreasing as the controlled variable increases and increasing as the controlled variable decreases is called PID positive action.
13. Dynamic Deviation
During the adjustment process, the deviation between the adjusted value and the set value changes at any time, and the deviation between the two at any given moment is called the dynamic deviation.
14. Static Deviation
After the adjustment stabilizes, the remaining deviation between the controlled variable and the setpoint is called the static deviation. Eliminating the static deviation is achieved through the integral action of the PID controller.
15. Callback
The regulator's adjustment function indicates that the adjusted quantity begins to change from rising to falling, or from falling to rising, which is called a pullback.
