Understanding Proportional Controller Offset Error

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Discussion Overview

The discussion centers around the concept of offset error in proportional controllers, particularly in the context of PID (Proportional, Integral, Differential) control systems. Participants explore the reasons behind the occurrence of offset errors and the role of the integral term in addressing these errors.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions why a proportional controller always results in an offset error.
  • Another participant suggests that the integral term is intended to compensate for the offset error, explaining that a pure PD controller makes smaller corrections as it approaches the set point, leading to an offset.
  • This participant also notes that determining the appropriate integral term requires running the process to identify the offset.
  • A third participant seeks clarification on the definition of offset error.
  • One participant defines offset error as the situation where a PID-controlled system stabilizes at a point that is a small distance away from the set point.

Areas of Agreement / Disagreement

The discussion reveals some agreement on the definition of offset error and the role of the integral term, but there is no consensus on the best approach to address the offset or the implications of using different controller types.

Contextual Notes

Participants express varying interpretations of terminology related to PID controllers, indicating potential limitations in communication and understanding within the discussion.

mpm166
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Why is it that a proportional controller will always result in an offset error?
 
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Isn't that what the I (integral) term is supposed to compensate for?

Here is the way I see it. A Pure PD (proportional, Differential) controller makes smaller corrections as you approach the set point. When you get inside of a certain band the corrections are essentially zero, so you will have an offset. The integral term is designed to compensate for this offset by summing (integrating) the error over many control cycles. To set the integral term you need to let the process run to determine the offset then set an appropriate integral term. It seems like everyone who does PID controllers use slightly different terminology, it would be a lot easier to set up PID if they could standardize the terminology.
 
What exactly is meant by an offset error?
 
The offset is when a system under PID (Proportional Integral Differential) control stabilizes a small distance from the control set point. So if you were using a temperature PID controller with a set point of 100 deg, the system would stabilize at 98.
 

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