90
either higher or lower. This procedure is repeated for
up to nine breakpoints, but if any scale value is set to
100% if automatically becomes the last in the series.
Also refer to Indicator, Multipoint Scaling Enable and
Process Variable.
Offset
Type: Controller Parameter
Offset is used to modify the measured process vari-
able value and is adjustable in the range ±input span.
Use this parameter to compensate for errors in the dis-
played process variable. Positive values are added to
the process variable reading, negative values are sub-
tracted. This parameter is in effect, a calibration adjust-
ment; it MUST be used with care. Injudicious use could
lead to the displayed value bearing no meaningful rela-
tionship to the actual process variable. There is no front
panel indication of when this parameter is in use.
Display value = OFFS, default value = 0.
Also refer to Input Span, Process Variable and Tare.
On-Off Control
Type: Controller Denition
When operating in On-Off control, the output(s) will turn
on or off as the process variable crosses the setpoint in
a manner similar to a central heating thermostat. Some
oscillation of the process variable is inevitable when
using On-Off control.
On-Off control can be implemented only with Time Pro-
portioning Control (Relay, Triac or SSR driver output),
by setting the corresponding proportional band(s) to
zero. On-Off operation can be assigned to the Primary
output alone (secondary output not present), Primary
and Secondary outputs or Secondary output only (with
the primary Output set for time proportional or current
proportional control).
On-Off control cannot be used on Valve Motor Drive
controllers.
Also refer to Differential, PID, Process Variable, Prima-
ry Proportional Band, Secondary Proportional Band,
Setpoint, Time Proportioning Control and Valve Motor
Drive Control.
On-Off Differential (Hysteresis)
Type: Controller Parameter
Refer to Differential.
Open Loop VMD
Type: VMD Controller Denition
An “Open Loop” PID control algorithm does not re-
quire a position feedback signal from the valve in order
to correctly control the process. Instead, the Process
Variable’s deviation from the Setpoint is used to decide
how long the valve open or close outputs should be
energized (in relation to the Motor Travel Time) in order
to bring the process under control.
Even when position feedback is provided for Valve Po-
sition Indication, the controller does not use this signal
when positioning the valve, so problems associated
with faulty feedback signals are avoided.
Also refer to Modulating Valve, Motor Travel Time, PID,
Process Variable, Setpoint, Valve Position Indication
and Valve Motor Drive Control.
Overlap/Deadband
Type: Controller Parameter
Defines the portion of the primary and secondary pro-
portional bands (Pb
-
P + Pb
-
S) over which both out-
puts are active (Overlap), or neither is active (Dead-
band). It is adjustable in the range -20% to +20% of
the two proportional bands added together. Positive
values = Overlap, negative values = Deadband.
This parameter is not applicable if the primary output is
set for On-Off control or there is no Secondary Output.
If the Secondary Output is set for On-Off, this parame-
ter has the effect of moving the Differential band of the
Secondary Output to create the overlap or deadband.
When Overlap/Deadband = 0, the “OFF” edge of the
Secondary Output Differential band coincides with the
point at which the Primary Output = 0%. ).
Display code = OL, default value = 0%.
Also refer to Differential, On-Off Control, Primary Pro-
portional Band and Secondary Proportional Band.
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