Operating modes

Selecting the operating modes

There are three operating mode selectable through the man and halt elements:

Operating mode	man	halt
Automatic	0	0
Manual mode	1	0 or 1
Halt	0	1

Automatic operating mode

In automatic mode, the manipulated variable Y is determined through the discrete PID closed-loop control algorithm subject to controlled variable PV and reference variable SP. The manipulated variable is limited by ymax and ymin. The control limits are also limits for the Anti-windup reset.

Manual mode

In manual mode the manually manipulated value YMAN is passed on directly to the manipulated variable Y. However, the manipulated variable is limited by ymax and ymin. Internal variables will be manipulated in such a manner that the controller changeover from manual to automatic (with I component enabled) can be bumpless. The control limits are also limits for the Anti-windup reset.

In this operating mode the D component is automatically set to 0.

Halt mode

In halt mode the control output remains unchanged; the function block does not influence the manipulated variable Y, i.e. Y = Y(old). The internal sizes are tracked in such a way that the controller (on connecting to the I component) bumplessly proceeds from its current position. The control limits are also limits for the Anti-windup reset. The halt operating mode is also useful for setting the control output Y via an external operator device, whereby the internal components are tracked correctly in the controller.

In this operating mode the D component is automatically set to 0.

Switching from automatic to manual

The changeover from automatic to manual is normally not bumpless, since output Y can take on any value between ymax and ymin, and Y goes directly to YMAN at the changeover.

There are two possibilities if, nevertheless, a bumpless changeover from automatic to manual is required:

Switching via MOVE

Using function MOVE set the value of YMAN to the value of Y:

The MOVE function is only performed when the PID controller is in automatic mode (Mode.man = 0). If only one changeover from automatic to manual takes place it is bumpless, as the value of YMAN is equal to the value of Y in this cycle. In the manual mode the value of YMAN can slowly be changed.

Switching via VLIM

If you do not want to change YMAN, e.g. because it is a constant, then the previous solution can be implemented using a velocity limiter (function block VLIM):

In automatic mode (MPID.man = 0) the velocity limiter is in manual mode (MOVE function). That way the PID controller manual value (YMAN from PID) can be set to the Y value of the PID controller via the velocity limiter manual value (YMAN from VLIM). If only one changeover from automatic to manual takes place, it is bumpless, as the value of YMAN (of the PID) is equal to the value of Y (of the PID) in this cycle. The PID controller YMAN value, starting at your adjustment value (Para.rate), are compared with the actual manual value (on VLIM) beginning with the next cycle.