Despatch P1400+ Temperature Controller - Manual (Page 98)

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Valve Motor Drive Control Type: VMD Controller Definition
Valve Motor Drive Controllers are designed to control Modulating Valves using a special “Open
Loop” Valve Motor Drive (VMD) PI control algorithm. Output signals are provided to move the valve
further open, or further closed when the process is higher or lower than the desired setpoint. When
on setpoint, no output is required to maintain control unless load conditions change. This known as
Three-Point Stepping control.
Valve Position or Flow Indication is possible if an Auxiliary Input option module has been fitted and
configured for this purpose.
Also refer to
Auxiliary Input , Modulating Valve , Open Loop VMD , PI Control , PID , Setpoint and
Valve Position Indication .
Valve Position or Flow Indication Type: VMD Controller Definition
The Valve Motor Drive Controllers do not require any kind of position feedback in order for the PID
algorithm to correctly control the process. However, where feedback or flow level signals are
available, they can be displayed as a percentage (
0000
to
100 100 100 100
) of the possible valve opening or flow
level. Valve Position Indication is shown in the Operator Mode lower display in place of the Setpoint
when the Display Strategy is set to
7 .
Valve Position Feedback is usually provided by means of a potentiometer linked to the valve.
Potentiometers can be directly connected to the Full Auxiliary Input (Option Slot B only).
Flow meters typically have linear 0-20/4-20mA or 0-5/0-10V signals, which can be used with either
the Full Auxiliary Input or the Basic Auxiliary Input (Option Slot A only) of the
1
/
16
Din VMD
Controllers.
Even when position feedback is provided in this way, the information is not used by the Open Loop
VMD control algorithm when positioning the valve, avoiding problems associated with faulty
feedback signals.
Also refer to
Auxiliary Input , Display Strategy , Open Loop VMD , PID , Set Valve Closed Position,
Set Valve Open Position, Setpoint, and Valve Motor Control.
Valve Open Limit Type: VMD Controller Parameter
When Valve Position Indication is to be used on Valve Motor Controllers, this parameter
provides a “clamp” on the upper valve position, which the controller will not attempt to drive the valve
past. It can be set between
100 100 100 100
(fully open) and the Valve Closed Limit value +1, expressed as a
percentage of the valve’s “fully open” position.
The Auxiliary Input must correctly scaled using the Set Valve Open and Closed parameters before
using this parameter.
Display code =
PiLL , default setting =
100 100 100 100
.
Also refer to Auxiliary Input , Set Valve Closed Position , Set Valve Open Position , Valve Close
Limit , Valve Motor Control and Valve Position Indication .
VMD Type: VMD Controller Parameter
- Refer to Valve Motor Control .