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Function of process controller

The process controller function enables a measured process variable to be controlled to a reference (setpoint) value.

Features

• Process controller calculation in speed controller cycle
• Process controller as PI controller with Kp adaptation
• Process controller actual value selectable via selector
• Filtering and offset correct of reference and actual values
• Process controller output can be connected to different points in the general control structure
• Process controller is usable in all control modes
  
  

Controller structure

Bild: Schematic of process controller

Vorgehensweise: Process controller setup

• Set process controller reference value:
  P 2666 CON_PRC_REFVAL: Reference input in user units (this parameter can be written cyclically over a field bus)
• Scaling of process controller reference value:
  P 2667 CON_PRC_REFSCALE; The reference P 2666 CON_PRC_REFVAL can be scaled (taking into account the user units), see Application Manual, "Scaling".
• Select actual value sources:
  P 2668 CON_PRC_ACTSEL: The actual value source must be set to the desired reference source (e.g. field bus). The field bus writes the actual value to parameter
  P 2677 CON_PRC_ACTVAL_FIELDBUS
• Select offset
  Optional, P 2669 CON_PRC_ACTOFFSET: Setting of an offset for actual value calibration
• Scaling of process controller actual value:
  P 2670 CON_PRC_ACTTF; filter time for actual value filter [ms]. The actual value is smoothed via the integral-action time of the PT-1 filter. (taking into account the user units)
• Inversion of the control difference
  P 2665 CON_PRC_CDIFF_SIGN: Adaptation of control difference sign
• Activate process controller:
  P 2681 CON_PRC_CtrlWord: Control word bit 0 = 1 (process controller active). This bit must be reset after every restart. The bit is not stored in the data set.
• Optimization of controller setup:
  P 2659 CON_PRC_Kp: Controller gain
  P 2660 CON_PRC_KP_SCALE: Scaling of gain
  P 2661 CON_PRC_Tn: TN integral-action time: If the integral-action time is set to the permissible maximum value, the I-component of the controller is inactive (10000 ms = off).
  
  
  
• Offset for the process controller output
  P 2662 CON_PRC_REFOFFSET: Then the totalled variable is connected via a limitation to the output of the process control loop. The user can parameterize the limitation via parameter
  P 2663 CON_PRC_LIMPOS for the positive limit and
  P 2664 CON_PRC_LIMNEG for the negative limit.
  
  
  
  

Rate limiter

Downstream of the control variable limiter there is another limitation which limits the changes to the control variable per sampling segment. By way of field parameter

P 2680 CON_PRC_Rate Limiter the limitation of the control variable steepness per millisecond can be parameterized. By way of index (0) the limitation is active in standard process controller operation. By way of index (1) reduction of the I-component is activated (see table).

With P 2672 CON_PRC_OUTSEL = 3 the process controller delivers an additive position reference value. The rate limiter limits the possible control variable change. The control variable change each time interval by the process controller results in a speed change on the motor shaft.

Example

The amount of the process controller to change the speed on the motor shaft should not be higher than 100 revolutions per minute. To achieve this, the value of parameter P 2680 (0) CON_PRC_Rate Limiter must be parameterized with a value corresponding to the user unit. The unit of this parameter is [x/ms]. The x stands for the respective unit of the process controller output variable. In this example the control variable (additive position reference) has the unit "Increments"

(see also parameter P 270 MPRO_FG_PosNorm). This parameter indicates how many increments correspond to one motor revolution.

Conversion from [rpm] to [Inc/ms]

n_change = 100 rpm

P 0270 MPRO_FG_PosNorm in inc/rev

Internal position resolution = 1048576 inc/rev (default)

To reduce the I-component, the same method is applicable

P 2680(1) CON_PRC_Rate Limiter(1) [Inc/ms]).

P 2680 CON_PRC_Rate Limiter = n _change*1048576 *1/60000

P 2680 [Inc/ms] = 100 [rpm] * P 0270 [Inc/rev] * 1/60 [min/s*] * 1/1000 [s/ms]

Scope signals for visualization of the process control loop

Tabelle: Scope signals

Process controller parameters

Tabelle: Process controller parameters