Subject: PID loops - defined

From: (Ken Robinson)

PID (Proportional, Integral, and Derivative) Loops use algorithms to calculate an appropriate response to varying conditions on the output. The Proportional term is based on the error, or the difference between the analog input and the output in percent. The Integral term takes into account the amount of time the input has been away from the setpoint. The Derivative, or differential term, is based on the rate at which the input is approaching the setpoint, and is used to prevent overshoot. Using the classic Proportional only control will allow the output to oscillate around the setpoint. What shows up as a change in the output of the loop is the sum of the P, I, and D Term, as well as the interval, interval being the time period between successive solutions of the loop. One other parameter is important, and similar to pneumatic systems. Bias is input to kind of a preset to tell the loop where to start. Keep in mind that you must consider many factors when tuning a loop. A Pneumatic loop controlling building static is probably the hardest to tune correctly. If the system is setup correctly, not only will the vortex, VFD, or whatever is controlling the static be stable, the entire system will be stable and not "HUNT". Also keep in mind that "Pneumatic" / DDC systems are "sloppy", control is based on slow changes, incremently made, and allowed to settle for a fairly long period of time. Hope this helps.
Been in controls tooooooo long. BYE
Ken Robinson

From: (Richard Eiden)

In answer to "What are the PID terms and how are they tuned" I submit the following:

PROPORTIONAL RESPONSE: The output of a proportional controller is changed in proporation to the (offset) proportional error. This error is the difference between the setpoint and the current value of the controlled variable. This is the traditional pneumatic controller.

INTEGRAL ACTION: The output of the integral control response is as the name implies, it adds up the total proporational error over time. The output is expressed in a unit of time. This time is the time required for the integral action to duplicate the total proportional error. This duplication of error acts to eliminate the proportional error. This action is used to eliminate large offset errors caused by wide, unacceptable throttling ranges caused by stablizing a proportional control response.

DERIVATIVE ACTION: The output of a derivative control response is based on the rate in which the output signal is changing over time. Essentially the need for this action in HVAC controls is very rare and limited. This response can be thought of as suspending controller sensitivity for a limited time so that the controller can quickly respond to the changing controlled variable.

GENERAL COMMENTS: The output responses are cumulative. If you critically tune a controller with proportional action and then wish to add integral action you must first reduce the proportional action. If this is not done the controller will be unstable. There are many books on manually tuning PID controllers, and one of the widely known methods is the Zigler- Nicoles (sp?) method.

I hope this answers the original question. I hope others will add their own comments to this topic.


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