West Control Solutions Pro-EC44 - Manual (Page 92)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 85
Position Feedback
In VMD mode this instrument uses a boundless (open-loop) 3-point stepping algorithm. It
does not require any kind of position feedback in order to correctly control the process and
can therefore avoids problems associated with faulty feedback signals.
However, where valve feedback is available it can still be displayed in a bar-graph as a
percentage open (0 to 100%). Position feedback is usually provided by means of a
potentiometer mechanically linked to the valve. The output of a related flow meter can also
be used to indicate the relative valve position. Flow meters typically have linear 0-20/4-20mA
or 0-5/0-10V signals. To display the position/flow signal the 2nd input is must be configured
for this purpose.
The input is adjusted and scaled to show 0 to 100% representing valve fully closed to fully
open, or a flow rate equating to fully closed to fully open. The valve position scaling
parameters are set in the Input Configuration sub menus
– see page 46.
Valve Limiting
When valve position/flow indication is in use, the signal can be used by the controller to limit
the valve movement. Upper and/or lower limits can be set beyond which it will not attempt to
drive the valve. The valve open and close limits are set in the Control Configuration sub
menu
– see page 50.
CAUTION: These limits must be used with care. They are effectively control
power limits. Do not set values that prevent proper control of the process!