What is a PID Controller?

A Proportional-Integral-Derivative (PID) controller is a control loop feedback mechanism widely used in industrial control systems and a variety of other applications requiring continuously modulated control. A PID controller continuously calculates an error value as the difference between a desired setpoint (SP) and a measured process variable (PV) and applies a correction based on proportional, integral, and derivative terms.

How the PID Calculation Works

The calculation involves three separate parameters: the Proportional, the Integral, and the Derivative values. The Proportional value determines the reaction to the current error, the Integral value determines the reaction based on the sum of recent errors, and the Derivative value determines the reaction based on the rate at which the error has been changing.

By tuning these three constants in the PID controller algorithm, the controller can provide control action designed for specific process requirements. The response of the controller can be described in terms of its responsiveness to an error, the degree to which the controller overshoots the setpoint, and the degree of system oscillation.

Understanding the Components

Proportional (Kp): This produces an output value that is proportional to the current error value. A high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable.

Integral (Ki): This accounts for past values of the error and integrates them over time. For example, if there is a residual error after the application of proportional control, the integral term seeks to eliminate the residual error by adding a control effect due to the historic cumulative value of the error.

Derivative (Kd): This is an estimate of the future trend of the error, based on its current rate of change. It is sometimes called "anticipatory control" because it effectively seeks to reduce the effect of the error by exerting a control influence generated by the rate of error change.

FAQs about PID Tuning

What is the Ziegler-Nichols method?

The Ziegler-Nichols method is a heuristic tuning method used to determine the gains for a PID controller. It involves setting the integral and derivative gains to zero and increasing the proportional gain until the system reaches ultimate oscillation.

Why is my PID system oscillating?

Oscillation usually occurs when the proportional gain (Kp) is too high or if there is too much lag in the system. Reducing Kp or increasing the derivative term (Kd) can often help stabilize the system.