Configurating of Continuous Control with Pulse Width Modulation
Objective of the Application
A continuous control loop with pulse width modulation consists of a conventional PID controller and a pulse modulation function. The pulse generator block modifies the analogous manipulated variable signal of the controller into a sequence of digital pulses with pulse length (or pulse/break ratio) equivalent to the analogous manipulated variable. This configuration is applied for switching actuators (e.g. heating on/off or valve open/close).
This use case has to be distinguished from step controllers. A step controller is applied for integrating actuators (e.g. motor driven valves) commanded by binary output signals open/stop/close. The function blocks PIDStepL or CTRL_S for step controllers are available in the Advanced Process Library or in the PCS 7 Standard Library, whereas a pulse generator function block does not exist in the PCS libraries. The function block PULSEGEN is provided only in the CFC-Library ELEM_400.
The following issues are discussed in this application note:
- Interaction of a continuous controller and pulse width modulation with regard to controller tuning, parameter specification and choice of the sampling time.
- Interaction of a continuous controller and pulse width modulation with respect to mode switch over.
- Simulation example.
This entry was created with SIMATIC PCS 7 V7.1, the procedure is transferable to PCS 7 V7.0 SP1 and higher versions.
08/2010 First release