Siemens SIMATIC S7-1200 Function Manual page 225

In the following image, the transient oscillation is illustrated in the stationary state:
Linearity and operating range
The process response must be linear across the operating range. Non-linear response
occurs, for example, when an aggregation state changes. Tuning must take place in a linear
part of the operating range.
This means, during tuning and normal control operation non-linear effects within the
operating range must be insignificant. It is, however possible to retune the process when the
operating point changes, providing tuning is repeated in the close vicinity of the new
operating point and non-linearity does not occur during tuning.
If a specific static non-linearity (e.g., valve characteristics) is known, it is always advisable to
compensate this with a polyline to linearize the process response.
Disturbance in temperature processes
Disturbances such as the transfer of heat to neighboring zones must not affect the overall
temperature process too much. For example, when optimizing the zones of an extruder, all
zones must be heated simultaneously.
PID control
Function Manual, 03/2017, A5E35300227-AC
Using PID basic functions
7.3 TCONT_CP
225