Center seeking control
Center seeking control is a control strategy which can be used when two actuators with different ranges (usually fine valve and coarse valve) of the control action are available. It is often called also valve position control. PID control function is always performed by the fine (small) valve. In this simulator, the coarse (big) valve is controlled by the integrating controller so that the fine valve is in the center of its range. It can be ensured by an appropriate choice of the relay blocks RLY 1 and RLY 2. The integration controller PIDU_I should be sufficiently slow (i.e. its integral time constant must be sufficiently large) so that the main control loop with the PIDU_PV controller remains stable. The position of the coarse valve will be corrected by switching the PIDU_I controller to the automatic mode (MAN=0) whenever the fine valve leaves the specified (recommended) interval (0:25; 0:75) and correction is finished when the (recommended) interval (0:45; 0:55) is reached.
How to start the simulator?
Before starting the simulator (applet) by START button, the sampling period can be set in the field Sampling time. The continous applet running can be stopped by STOP button or paused by PAUSE button. The simulation speed can be adjusted by the slider. The re-init button reset all parameters to initial state.
All parameters of active blocks (non-shadow) can be changed in the dialog which is invoked by cliking at the block. Detailed description of block parameters is available in Function blocks reference manual.