West Control Solutions Pro-EC44 - Manual (Page 227)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 220
Modulating Valve
A valve that can be positioned anywhere between fully closed and fully open by means of an
incorporated motor. A typical application would be controlling temperature in a furnace
heated by gas burners. The controller moves the valve to the desired position in order to
control the gas flow.
If the valve motor is directly driven with Open and Close outputs from the controller feeding
power to the motor, valve motor drive (VMD) control mode must be used. Some modulating
valves have positioning circuitry incorporated that requires linear (mA or VDC) signals to set
the position. These use the standard control mode (using PI control) instead of VMD mode.
Also refer to Linear Outputs; PI Control and Valve Motor Drive Control.
Motor Travel Time
The Motor Travel Time parameter is used in Valve Motor Drive control mode. It must be set
to the time the valve takes to travel from one physical end stop to the other. This time is used
by the VMD algorithm wh
en calculating how long to energise the “Valve Open” or “Valve
Close” outputs in order to bring the process on to control.
It is important that the time set accurately reflects the time taken to travel between the
physical limits, otherwise the control can be severely impaired. The motor travel time may be
stated in
your valve supplier’s specification or the valve can be timed from the fully closed to
fully opened position. The controller can be placed in Manual Mode to assist with the timing
of valve movement.
Also refer to Manual Mode Enable
Multi-Point Scaling
If the process input is connected to a linear input signal, multi-point scaling can be enabled in
the input configuration sub-menu. This allows the linearization of non-linear signals.
The scale input limits define the values shown when the input is at minimum and maximum
values, and up to 15 breakpoints can scale input vs. displayed value between these limits. It
is advisable to concentrate the break points in the area of the range that has the greatest
amount of non-linearity, or the area of particular interest in the application.
Also refer to: Input Configuration; Linear Input; Process Input; Scaled Input Lower Limit and
Scaled Input Upper Limit.
mVDC
This stands for millivolt DC. It is used in reference to the linear DC millivolt input ranges of
the main process inputs. These can be 0 to 50mV or 10 to 50mV
Also refer to: Input Range; Linear Input; mADC; Process Variable and VDC
On-Off Control
When operating in On-Off mode, the control output(s) turn on or off as the process variable
crosses the setpoint in a manner similar to a simple thermostat. Some oscillation of the
process variable is inevitable when using on-off control. The amount of oscillation is mainly
defined by the process characteristics, but is also affected by the on-off differential setting.
On-off control can be implemented only with Relay, Triac or SSR driver outputs. It can be
assigned to the primary output alone (secondary output not present), primary and secondary
outputs or to a secondary output only (with the primary output set for time proportional or
continuous control). On-off Control is selected by setting the corresponding proportional
band(s) to on-off.
Also refer to: Continuous Control; On-Off Differential; PID; Process Variable; Primary
Proportional Band; Secondary Proportional Band; Relay; Setpoint; SSR Driver; Time
Proportioning Control and Triac.