Having regulated dead time and static gain, the model time constant must be successively regulated by observing the command signal U(t) record.
If we call
the model delay, two illustrations appear:
-
model is lesser than
practice
-
model is greater than
practice
This figure illustrates the two cases.
Signal Speed When OL_TIME <
Process
The following figure describes the signal speed.
Signal Speed During OL_TIME >
Process
The following figure describes the signal speed.
Closed Loop Time Constant Selection
Having determined the model to be used, the closed loop time constant selection remains to be made. Its value is dependent on the response speed of the required closed loop.
For processes that correspond to a first order model with delay, by choosing a time constant CL_PERF relation between 1.05 and 1.15, the system response is improved without running the risk of destabilising the process.
CL_PREF = OL_TIME / required closed loop time constant.
All CL_PERF increases correspond to an increase in response speed (it is a more important action of the adjustment unit), but also to an increased sensitivity to modeling errors.
