The PID Tuner considers as the plant all blocks in the loop between the PID Controller block output and input. The blocks in your plant can include nonlinearities. Because automatic tuning requires a linear model, the PID Tuner computes a linearized approximation of the plant in your model. This linearized model is an approximation to a nonlinear system, which is generally valid in a small region around a given operating point of the system.
By default, the PID Tuner linearizes your plant using the initial conditions specified in your Simulink® model as the operating point. The linearized plant can be of any order and can include any time delays. The PID tuner designs a controller for the linearized plant.
In some circumstances, however, you want to design a PID controller for a different operating point from the one defined by the model initial conditions. For example:
The Simulink model has not yet reached steady-state at the operating point specified by the model initial conditions, and you want to design a controller for steady-state operation.
You are designing multiple controllers for a gain-scheduling application and must design each controller for a different operating point.
In such cases, change the operating point used by the PID Tuner. See Opening the Tuner.
For more information about linearization, see Linearizing Nonlinear Models.