Please note: This is a technical article, intended for those of you that are eager to learn. You don't need to understand all of this to be able to utilize the PID controller that will be developed by espresso.liljenberg.net.

PID is the abbreviaton for Proportional Integral Derivative. This type of controller uses feedback from the process it is controlling and takes action based on the error. The error is the difference between the user-defined setpoint (SP, the desired value) and the measured process variable (PV, i.e. the actual value).

The P,I and D elements of the PID controller refers to particular actions taken on the error:

• Proportional: The error multiplied by a gain, Kp. Kp affects the process stability: too low and we will never reach the setpoint; too high and the process will oscillate.
• Integral: The integral of the error multiplied by a gain, Ki. In many systems Ki is responsible for driving error to zero, but to set Ki too high is to invite oscillation or instability or integrator windup or actuator saturation.
• Derivative: The rate of change of the error multiplied by a gain, Kd. Kd determines the system response: too high and the process will oscillate; too low and the process will respond sluggishly. It should be noted that the derivative action amplifies noise that may occur in the error signal.

Tuning a PID controller involves adjusting Kp, Ki, and Kd to achieve the "optimal" system response.

For an exhaustive description and the mathemetical theory of a PID controller, please visit the Wikipedia on the subject.

The PID controller developed by espresso.liljenberg.net is a "Direct Acting, Type C PID Controller", visit Bestune's site to see what that means.