SCU – Step controller with position feedback

Block SymbolLicensing group: STANDARD
PIC

Function Description
The SCU block implements the secondary (inner) position controller of the step controller loop. PIDU function block or some of the derived function blocks (PIDMA, etc.) is assumed as the primary controller.

The SCU block processes the control deviation sppv by a three state element with parameters (thresholds) thron and throff (see the TSE block, use parameters ep = thron, epoff = throff, en = -thron and enoff = -throff). The parameter RACT determines whether the UP or DN pulse is generated for positive or negative value of the controller deviation. Two pulse outputs of the three state element are further shaped so that minimum pulse duration dtime and minimum pulse break time btime are guaranteed at the block UP and DN outputs. If signals from high and low limit switches of the valve are available, they should be connected to the HS and LS inputs.

There is also a group of input signals for manual control available. The manual mode is activated by the MAN = on input signal. Then it is possible to move the motor back and forth by the MUP and MDN input signals. It is also possible to specify a position increment/decrement request by the mdv input. In this case the request must be confirmed by a rising edge (off on) in the DVC input signal.

The control function of the SCU block is quite clear from the following diagram.

PIC

The complete structure of the three-state step controller is depicted in the following figure.

PIC

Inputs

sp

Setpoint (output of the primary controller)

Double (F64)

pv

Controlled variable (position of the motorized valve drive)

Double (F64)

HS

Upper end switch (detects the upper limit position of the valve)

Bool

LS

Lower end switch (detects the lower limit position of the valve)

Bool

MUP

Manual UP signal

Bool

MDN

Manual DN signal

Bool

mdv

Manual differential value (requested position increment/decrement with higher priority than direct signals MUP/MDN)

Double (F64)

DVC

Differential value change command (off on)

Bool

MAN

Manual or automatic mode

Bool

off ..

Automatic mode

on ...

Manual mode

Outputs

UP

The "up" signal

Bool

DN

The "down" signal

Bool

de

Deviation error

Double (F64)

Parameters

thron

Switch-on value   0.0 0.02

Double (F64)

throff

Switch-off value   0.0 0.01

Double (F64)

dtime

Minimum width of the output pulse [s]   0.0 0.1

Double (F64)

btime

Minimum delay between two subsequent output pulses [s] to do   0.0 0.1

Double (F64)

RACT

Reverse action flag

Bool

off ..

Higher mv higher pv

on ...

Higher mv lower pv

trun

Motor time constant (determines the time during which the motor position changes by one unit)   0.0 10.0

Double (F64)

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