Slide Valve Actuator Troubleshooting Guide - Emerson Vission Control Manual

Slide Valve Actuator Theory of 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 con-
trol panel. The position sensor tells the main com-
puter the position of the slide valve. The main com-
puter uses the position and process information to de-
cide 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 alternately
blocks and opens the optocoupler slots, generating a
sequence that the position sensor microcontroller 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 position sen-
sor loses power, power is cut to the motor. A capacitor
stores enough energy to keep the position sensor cir-
cuitry 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

Slide Valve Actuator Troubleshooting Guide

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 actuator will
become lost.
A brake failure can sometimes be detected by the posi-
tion 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.
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