Technical data

General notes
Using BUMP If BUMP (control bit 8) is set to zero, the changeover from manual to
automatic operation is bumpless, i.e. the system error, however large
it may be, is corrected only by the I action. If, however, you have
selected TI = TA/TN = 0 (P or PD controller) the system error does
not cause a change of the manipulated variable when the changeover
takes place.
You can prevent this by setting BUMP = 1. This means that a system
error is corrected quickly when there is a manual-to- automatic
changeover, irrespective of TI = 0. The manipulated variable jump
that results corresponds to the value of the system error, which means
that it is not arbitrary in the sense of a disturbance of the controller
operation.
Displaying MERK, bits 0
and 1
Bits 0 and 1 of MERK can be displayed if required to show that the
manipulated variable (for velocity algorithm, the control increment)
lies between the upper and lower limits. Since these bits are evaluated
by the algorithm for disabling the I action, you cannot overwrite them.
Note
You must not reload the controller data blocks DB x + 1 during
cyclic operation.
Cascade control If two or more controllers are cascaded, remember the following
points:
•• If the cascade is split, either all the controllers have to change to
manual operation simultaneously to prevent any controller drift
due to the I action or at least the controller of the outer loop must
be operated manually to ensure that the last manipulated variable
corresponding to the setpoint of the inner loop is retained or
changed to a safe value.
•• If you want to close the cascade, both loops should operate at the
same time in the automatic mode or at least the inner loop to
ensure that the manipulated variable of the outer loop is taken as
the setpoint.
Switching to manual mode If the control system is disconnected from the controller and directly
adjusted at the actuator following the changeover to manual operation,
the manipulated variable obtained must be supplied to the controller
via the manual input. This ensures that when you change from manual
to automatic operation, the controller output will correspond to the
manipulated variable set during manual operation. In the case of the
velocity algorithm, this will be the change in the manipulated variable.
6
OB 250/251: Closed-Loop Control/ PID Algorithm
CPU 928B Programming Guide
C79000-B8576-C898-01
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