Plant Cannot Be Linearized or Linearizes to Zero

When you open PID Tuner, it attempts to linearize the model at the operating point specified by the model initial conditions. Sometimes, PID Tuner cannot obtain a nonzero linear system for the plant as seen by the PID controller.

How to Fix It

If the plant model in the PID loop cannot be linearized or linearizes to zero, you have several options for obtaining a linear plant model for PID tuning. The following table summarizes some of the options and when they are useful.

ApproachUseful WhenMore Information
Linearize at a different operating point

There is a known operating point suitable for tuning, such as:

  • A simulation snapshot time at which the plant is in a linearizable steady state.

  • Known state values or a previously trimmed operating point at which the plant is linearizable.

Tune at a Different Operating Point
Import a linear model of the plant to PID TunerYou have an LTI model of the plant at the desired operating condition for tuning in the MATLAB® workspace. In PID Tuner, in the Plant menu, select Import.
Tune the controller using simulated plant frequency-response dataThe plant is not linearizable in any operating condition suitable for tuning.Design PID Controller from Plant Frequency-Response Data
Use system identification to estimate a linear plant model from measured or simulated response dataYou have System Identification Toolbox™ software. An advantage of this approach is that it yields an analytic plant model that you can use for further analysis.Interactively Estimate Plant from Measured or Simulated Response Data

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