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 trun parameter specifies the time it takes for the motor position to change by one unit. 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.

This block propagates the signal quality. More information can be found in the 1.4 section.

Input

sp	Setpoint (output of the primary controller)	Double (F64)
pv	Controlled variable (valve position)	Double (F64)
HS	Upper end switch	Bool
LS	Lower end switch	Bool
MUP	Manual UP signal	Bool
MDN	Manual DN signal	Bool
mdv	Manual differential value	Double (F64)
DVC	Differential value change command	Bool
MAN	Manual or automatic mode	Bool
	off .. Automatic mode on ... Manual mode

Parameter

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 output pulses [s] $↓$ 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 $↓$ 0.0 $⊙$ 10.0	Double (F64)

Output

UP	The UP signal (up, more)	Bool
DN	The DOWN signal (down, less)	Bool
de	Deviation error	Double (F64)

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