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182 UDC 3300 Controller Product Manual 4/00
5.16 Three Position Step Control Algorithm
Introduction
The Three Position Step Control algorithm (Loop 1 only) allows the
control of a valve (or other actuator) with an electric motor driven by two
controller output relays; one to move the motor upscale, the other to move
it downscale, without a feedback slidewire linked to the motor shaft.
Accutune SP or SP+PV tuning does not function with this algorithm.
Accutune TUNE will operate with this algorithm.
Estimated motor
position
Models DC330X-EE-XXX-X0, DC330X-AA-XXX-X0
The Three Position Step control algorithm provides an output display
(OUT) which is an estimated motor position since the motor is not using
any feedback. Although this output indication is only accurate to a few
percent, it is corrected each time the controller drives the motor to one of
its stops (0 % or 100 %).
It avoids all the control problems associated with the feedback slidewire
(wear, dirt, noise). When operating in this algorithm, the estimated “OUT”
display is shown to the nearest percent (i.e. no decimal).
Accurate motor
position
Models DC330X-EE-XXX-X2, DC330X-AA-XXX-X2
In the event that an accurate and repeatable indication in motor position is
required, Position Proportional model’s slidewire input can be used to
read the motor position and display it on the lower display as “POS” while
still operating in the Three Position Step control mode.
The slidewire must be calibrated for this to operate correctly.
Displaying the motor
position
Table 5-29 lists the procedure for displaying the motor position.
Table 5-29 Procedure for Displaying the 3PSTEP Motor Position
Step Operation Press Action
1
Access the
displays
LOWER
DISPLAY
until you see:
Lower Display
Upper Display
The PV value
POS = 3PStep motor position with
slidewire connected
or
OUT = Estimated 3PStep motor
position when no slidewire exists
Power-up output
When the controller powers up after a power outage, the position of the
motor will correspond to whatever was configured at the Control Set Up
function prompt PWR OUT, selection LAST or F’SAFE. Refer to Section
4.10, Table 4-9 for definition of each selection.