Instructions
9.4 PID basic functions
PID control
Function Manual, 03/2017, A5E35300227-AC
499
To suppress a minor sustained oscillation due to the manipulated variable quantization (for
example, in pulse width modulation with PULSEGEN) a dead band is applied to the
(DEADBAND) control deviation. With DEADB_W = 0.0, the dead band is switched off.
PI step controller algorithm
The instruction TCONT_S operates without position feedback. 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 shaper (PULSEOUT) that generates the pulses
for the control valve. Adapting the response threshold of the three-step element reduces the
switching frequency of the controller.
Weakening of the P-action when setpoint changes occur
To prevent overshoot, you can weaken 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
As in the case of the continuous controller, a value of PFAC_SP < 1.0 can reduce the
overshoot if the motor run time MTR_TM is small compared with the recovery time TA and
the ratio is TU/TA < 0.2. If MTR_TM reaches 20% of TA, only a slight improvement can still
be achieved.
A disturbance variable can be added at the DISV input.
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