Despatch P1400+ Temperature Controller - Manual (Page 86)

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Also refer to Cascade Control , Retransmit Output , Remote Setpoint , Serial Communications ,
Setpoint
Modulating Valve Type: VMD Controller Definition
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. Some modulating valve motors require linear (mA or VDC) signals to position the
valve. These require standard Process Controllers (using PI control).
Offset Type: Controller Parameter
Offset is used to modify the measured process variable value and is adjustable in the range ±input
span. Use this parameter to compensate for errors in the displayed process variable. Positive values
are added to the process variable reading, negative values are subtracted. This parameter is in
effect, a calibration adjustment; it MUST be used with care. Injudicious use could lead to the
displayed value bearing no meaningful relationship 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 Definition
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 Proportioning 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 , Primary 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 Definition
An “Open Loop” PID control algorithm does not require 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 energised (in relation to the
Motor Travel Time) in order to bring the process under control.
Even when position feedback is provided for Valve Position 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 .