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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