Pi Step Controller Algorithm - Siemens SIMATIC Manual

Temperature Step Controller FB59 "TCONT_S"
4.1.2

PI Step Controller Algorithm

FB 59 "TCONT_S" operates without a position feedback signal (see block diagram
in Section 4.2, Page 4-5). The I-action of the PI algorithm and the assumed
position feedback signal are calculated in an integrator (INT) and compared as a
feedback value with the remaining P-action. The difference is applied to a three-
step element (THREE_ST) and a pulse generator (PULSEOUT) that forms the
pulses for the valve. Adapting the response threshold of the three-step element
reduces the switching frequency of the controller.
Weakening the P-action when setpoint changes occur (PFAC_SP)
To prevent overshoot, you can attenuate the P-action using the "proportional factor
for setpoint changes" parameter (PFAC_SP). Using PFAC_SP, you can now select
continuously between 0.0 and 1.0 to decide the effect of the P-action when the
setpoint changes:
•
PFAC_SP = 1.0: P-action has full effect if the setpoint changes
•
PFAC_SP = 0.0: P-action has no effect if the setpoint changes
A value for PFAC_SP < 1.0 can reduce the overshoot as with the continuous
controller if the motor run time MTR_TM is small compared with the recovery time
TA and the ratio TU/TA is < 0.2. If MTR_TM reaches 20 % of TA, only a slight
improvement can be achieved.
Feedforward Control
A feedforward variable can be added at the DISV input.
Manual Value Processing (LMNS_ON, LMNUP, LMNDN)
With LMNS_ON, you can change between manual and automatic mode. In manual
mode, the actuator and the integrator (INT) are set to 0 internally. Using LMNUP
and LMNDN, the actuator can be adjusted to OPEN and CLOSED. Switching over
to automatic mode therefore involves a bump. As a result of the GAIN, the existing
error leads to a step change in the internal manipulated variable. The integral
component of the actuator, however, results in a ramp-shaped excitation of the
process.
4-4
PID Temperature Control
A5E00125039-02