SCU – Step controller with position feedback

Block SymbolLicensing group: STANDARD

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 $sp - pv$ 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 $\to$ on) in the DVC input signal.

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

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

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 $\to$ 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 $\to$ higher pv on ... Higher mv $\to$ 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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