Velocity Mode Valve Control
TheVelocity mode ofthe ValvePositioneroption isa specialvalve control
algorithm that does not use a slidewire feedback signal. In this control mode,
the controller responds to changes in output power instead of responding to
the outputpower directly, asin Position mode. Subsequently,as long asthere
is process error, the controller activates the motor control outputs
periodically to eliminate the error.
The valve motor open and close transit time and minimum motor on-time
are required parameters for Velocity mode. The valve motor transit times
should be measured in actual use as they frequently differ f rom the nominal
valve motor times. The minimum on-time is another Velocity mode
parameter.It establishes the control deadband of the controller.Minimum on
times that are too short could cause excessive valve activity. Minimum
on-times that are too long may cause too much error. Velocity mode of the
controller is engaged by setting both Valve Position parameters to 0.0%.
Example:
Steam is used to heat water by passing it through a heat exchanger.
Variations in inlet water temperature, steam pressure, hot water demand,
etc.,allcontributeto theneedfor closedloopcontrol.The steampressure is
controlled by a TCU with Valve Positioner option. The TCU maintains
constant hotwater temperatureby controlling the positionof the valve. No
slidewire feedback is used. Given the time constant of the process is 60
seconds, the Valve Update Time (VUdt) of the controller is set at five
seconds to reduce valve activity. The valve motor open and close transit
times were measured at 20 and 2 5 seconds respectively. The minimum on
time pulse is set at 0.5 seconds to allow for valve motor overrun and
backlash, and alsoto provide for acontrol deadband. The effectivecontrol
deadband expressed in percent of controller output is:
Effective deadband (in percent )
Vont
0
=
.5* (Vopt VCLt )+
=
+
0.5
0.5* (20 25)
= 2.2%
The following data is used to configure the Velocity mode valve control
parameters of the TCU:
VPS1 0.0 Set valve position #1 to 0.0 to engage Velocity mode
VPS2 0.0 Set valve position #2 to 0.0 to engage Velocity mode
Vudt 5 Update valve position, at most, once every 5 seconds
VOPt 20 Set valve motor open time to 20 seconds
VCLt 25 Set valve motor close time to 25 seconds
Vont 0.5 Set valve motor minimum on time (deadband) to 0.5 seconds
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