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General FAQ on Instrumentation

Fequently Asked Questions

Question: What is PID control?

 

Answer: PID can be described as a set of rules with which precise regulation of a closed-loop control system is obtained.  Closed loop control means a method in which a real-time measurement of the process being controlled is constantly fed back to the controlling device to ensure that the value which is desired is, in fact, being realized.  The mission of the controlling device is to make the measured value, usually known as the PROCESS VARIABLE, equal to the desired value, usually known as the SETPOINT.  The very best way of accomplishing this task is with the use of the control algorithm we know as PID.

In its basic form, PID involves three mathematical control functions working together: Proportional-Integral-Derivative.  The most important of these, Proportional Control, determines the magnitude of the difference between the SETPOINT and the PROCESS VARIABLE (known as ERROR), and then applies appropriate proportional changes to the CONTROL VARIABLE to eliminate ERROR.  Many control systems will, in fact, work quite well with only Proportional Control.  Integral Control examines the offset of SETPOINT and the PROCESS VARIABLE over time and corrects it when and if necessary.  Derivative Control monitors the rate of change of the PROCESS VARIABLE and consequently makes changes to the OUTPUT VARIABLE to accomodate unusual changes.

Each of the three control functions is governed by a user-defined parameter.  These parameters vary immensely from one control system to another, and, as such, need to be adjusted to optimize the precision of control.  The process of determining the values of these parameters is known as PID Tuning.

PID Tuning, although considered "black magic" by many, really is, of course, always a well-defined technical process.  There are several different methods of PID Tuning available, any of which will tune any system.  Certain PID Tuning methods require more equipment than others, but usually result in more accurate results with less effort.

Download the following PDF Document for a detailed explanation of PID Tuning and the various available methods used. Unfortunately, the author of this excellent and useful document is unknown to us.

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