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.
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.
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
(see Continuous-Discrete Conversion Methods (Control System Toolbox).