SCUV – Step controller unit with velocity input

Block SymbolLicensing group: STANDARD
PIC

Function Description
The block SCUV substitutes the secondary position controller SCU in the step controller loop when the position signal is not available. The primary controller PIDU (or some of the derived function blocks) is connected with the block SCUV using the block inputs mv, dmv and SAT.

If the primary controller uses PI or PID control law (CWOI = off), then all three inputs mv, dmv and SAT of the block SCUV are sequentially processed by the special integration algorithm and by the three state element with parameters thron and throff (see the TSE block, use parameters ep = thron, epoff = throff, en = -thron and enoff = -throff). Pulse outputs of the three state element are further shaped in such a way that the minimum pulse duration time dtime and minimum pulse break time btime are guaranteed at the block outputs UP and DN. The parameter RACT determines the direction of motor moving. Note, the velocity output of the primary controller is reconstructed from input signals mv and dmv. Moreover, if the deviation error of the primary controller with icotype = 4 (working in automatic mode) is less than its dead zone (SAT = on), then the output of the corresponding internal integrator is set to zero.

The position pos of the valve is estimated by an integrator with the time constant trun. If signals from high and low limit switches of the valve are available, they should be connected to the inputs HS and LS.

If the primary controller uses P or PD control law (CWOI = on), then the deviation error of the primary controller can be eliminated by the bias ub manually. In this case, the control algorithm is slightly modified, the position of the motor pos is used and the proper settings of thron, throff and the tracking time constant tt are necessary for the suppressing of up/down pulses in the steady state.

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 overall control function of the SCUV block is obvious from the following diagram:

PIC

The complete structures of the three-state controllers are depicted in the following figures:

PIC

PIC

Inputs

mv

Manipulated variable (controller output)

double

dmv

Controller velocity output (difference)

double

ub

Bias (only for P or PD primary controller)

double

SAT

Internal integrator reset (connected to the SAT output of the primary controller)

bool

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

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

pos

Position output of motor simulator

double

MR

Request to move the motor

bool

off ..

Motor idle (UP = off and DN = off)

on ...

Request to move (UP = on or DN = on)

Parameters

thron

Switch-on value   0.0 0.02

double

throff

Switch-off value   0.0 0.01

double

dtime

Minimum width of the output pulse [s]   0.0 0.1

double

btime

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

double

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

CWOI

Controller without integration flag

bool

off ..

The primary controller has an integrator (I, PI, PID)

on ...

The primary controller does not have an integrator (P, PD)

tt

Tracking time constant   0.0 1.0

double

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