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 project example on hand shows the application of a continuous controller with pulse width modulation to a simulated plant. The target of the application is showing the aspects of engineering and commissioning of controllers with pulse width modulation that are particularly important in industrial applications, in order to achieve fast dynamics and proper operation in all operation modes.
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.
Valid for PCS 7 V7.1, in principal transferable to V7.0 from SP1.
|Content of the downloads||
|Documentation - Configurating of Continuous Control with Pulse Width Modulation|
|08/2010 V1.0||First release|