Emerson VSG & VSSG Spécifications télécharger pdf (Page 21)

100

Slide Valve Actuator Operation

The slide valve actuator is a gear-motor with a posi-

tion sensor. The motor is powered in the forward

and reverse directions from the main computer in

the control panel. The position sensor tells the main

computer the position of the slide valve. The main

computer uses the position and process information

to decide where to move the slide valve next.

The position sensors works by optically counting mo-

tor turns. On the shaft of the motor is a small alumi-

num “photochopper”. It has a 180 degree fence that

passes through the slots of two slotted optocouplers.

The optocouplers have an infrared light emitting di-

ode (LED) on one side of the slot and a phototransistor

on the other. The phototransistor behaves as a light

controlled switch. When the photochopper fence

is blocking the slot, light from the LED is prevented

from reaching the phototransistor and the switch is

open. When photochopper fence is not blocking the

slot, the switch is closed.

As the motor turns, the photochopper fence al-

ternately blocks and opens the optocoupler slots,

generating a sequence that the position sensor mi-

crocontroller can use to determine motor position by

counting. Because the motor is connected to the slide

valve by gears, knowing the motor position means

knowing the slide valve position.

During calibration, the position sensor records the

high and low count of motor turns. The operator tells

the position sensor when the actuator is at the high

or low position with the push button. Refer to the

calibration instructions for the detailed calibration

procedure.

The position sensor can get “lost” if the motor is

moved while the position sensor is not powered. To

prevent this, the motor can only be moved electrically

while the position sensor is powered. When the posi-

tion sensor loses power, power is cut to the motor. A

capacitor stores enough energy to keep the position

sensor circuitry alive long enough for the motor to

come to a complete stop and then save the motor

position to non-volatile EEPROM memory. When

power is restored, the saved motor position is read

from EEPROM memory and the actuators resumes

normal function

This scheme is not foolproof. If the motor is moved

manually while the power is off or the motor brake

has failed, allowing the motor to free wheel for too

long after the position sensor looses power, the ac-

tuator will become lost.

A brake failure can sometimes be detected by the

position sensor. If the motor never stops turning after

a power loss, the position sensor detects this, knows

it will be lost, and goes immediately into calibrate

mode when power is restored.

Slide Valve Operation

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Emerson VSG & VSSG Spécifications Page 21

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