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January 5, 2012 / Kasey Mirecki

PID – Tuning A Control Loop

Tuning a control loop – PID – is the adjustment of its control parameters (gain/proportional band, integral gain/reset, derivative gain/rate) to the optimum values for the desired control response. Stability (bounded oscillation) is a basic requirement, but beyond that, different systems have different behavior, different applications have different requirements, and requirements may conflict with one another.

PID tuning is a difficult problem, even though there are only three parameters and in principle is simple to describe, because it must satisfy complex criteria within the limitations of PID control. There are accordingly various methods for loop tuning, and more sophisticated techniques are the subject of patents; this section describes some traditional manual method for loop tuning.

Designing and tuning a PID controller appears to be conceptually intuitive, but can be hard in practice, if multiple (and often conflicting) objectives such as short transient and high stability are to be achieved. Usually, initial parameters need to be adjusted repeatedly while PID is running until the closed-loop system performs or compromises as desired.

Some processes have a degree of non-linearity and so parameters that work well at full-load conditions don’t work when the process is starting up from no-load; this can be corrected by gain scheduling (using different parameters in different operating regions). PID controllers often provide acceptable control using default tunings (or auto-tuning), but performance can generally be improved by careful tuning, and performance may be unacceptable with poor tuning.

Manual Tuning
If the system must remain online, first set Ti and Td values to zero*. Increase the Kp until the output of the loop oscillates, then the Kp should be set to approximately half of that value for a “quarter amplitude decay” type response. Then increase Ti until any offset is corrected in sufficient time for the process. However, too much Ti will cause instability. Finally, increase Td, if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much Td will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly; however, some systems cannot accept overshoot, in which case an over-damped closed-loop system is required, which will require a Kp setting significantly less than half that of the Kp setting causing oscillation.

Effects of increasing a parameter independently

Parameter

Rise time

Overshoot

Settling time

Steady-state error

Stability

Kp

Decrease

Increase

Small change

Decrease

Degrade

Ti

Decrease

Increase

Increase

Decrease significantly

Degrade

Td

Minor decrease

Minor decrease

Minor decrease

No effect in theory

Improve if Td small

* Use minimum value possible in case PID Functions (F355 & F356) doesn’t allows you to use zero as a valid number. (PLC can error out in case zero is not allowed but is being used in the logic and PID is triggered)

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