Controller Performance Deteriorates When Switching Time Domains

After you obtain a well-tuned, continuous-time controller using PID Tuner, you can discretize the controller using the Time Domain selector button in the PID Controller block dialog box. Sometimes, the resulting discrete-time controller performs poorly or even becomes unstable.

How to Fix It

In some cases, you can improve performance by adjusting the sample time by trial and error. However, this procedure can yield a poorly tuned controller, especially where your application imposes a limit on the sample time. Instead, if you change time domains and the response deteriorates, click Tune in the PID Controller block dialog box to design a new controller.

Note

If the plant and controller time domains differ, PID Tuner discretizes the plant (or converts the plant to continuous time) to match the controller time domain. If the plant and controller both use discrete time, but have different sample times, PID Tuner resamples the plant to match the controller. All conversions use the tustin method (see Continuous-Discrete Conversion Methods.