Red-lion TCU User Manual download pdf (Page 67)

Velocity Mode Valve Control

The Velocity mode of the Valve Positioner option is a special valve 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 output power directly, as in Position mode. Subsequently, as long as there

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., all contribute to the need for closed loop control. The steampressure is

controlled by a TCU with Valve Positioner option. The TCU maintains

constant hot water temperature by controlling the position of 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 also to provide for a control deadband. The effective control

deadband expressed in percent of controller output is:

Effective deadband (in percent )

Vont

.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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Red-lion TCU User Manual Page 67