West Control Solutions MRC 8000 - Manual (Page 41)

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4.1.2.4 TIME PROPORTIONING CONTROL
Time Proportioning Control can be implemented on controllers provided with SPST relay or
SSR driver output(s). Time proportioning can be programed for output 1 and/or 2 for each
pen. Time proportioning control is accomplished by cycling the output On and Off when the
process value is within the proportional bandwidth selected at a prescribed time period. The
time period is selected in the Tune mode by adjusting Ct1 and/or Ct2. The On time is a
percentage of the Cycle Time.
Example: Calculated output % = 40%; Cycle Time adjustment = 20 seconds
Output On Time = .4 x 20 = 8 seconds
Output Off Time = .6 x 20 = 12 seconds
4.1.2.5 CURRENT PROPORTIONING CONTROL
Current Proportioning control provides a proportional current output in response to process
value and setpoint. The current output can be selected for direct or reverse operation. Direct
current output control is typically used for cooling applications. The current output will
increase as the process value increases within the proportional bandwidth selected. The
reverse current output control is typically used in heating applications. The current output
will decrease as the process value increases within the proportional bandwidth selected.
The instrument can be programmed to provide 0 to 20mADC or 4-20mADC current output(s).
The output selected is dependent upon the final control element being used in the process.
The output 1 and/or output 2 LED will be lighted whenever the Current Proportional outputs
are selected.
4.1.2.6 POSITION PROPORTIONING CONTROL
Position Proportioning control can be implemented on those controllers provided with two
SPST relay outputs or two SSR Driver outputs and Slidewire Feedback option for the
respective pen. This control implementation can be performed on each pen.
Position proportioning control permits the use of PID control where the final control element is
a modulating device such as a motorized valve. In this form, each of the two required relays
or SSR Drivers will be used to control the valve. One output will be used to open the valve,
the other is used to close the valve. The slidewire feedback is used to provide a signal
relative to the valve armature position to the instrument.
As with the other proportioning control forms, the process input, tuning parameters and the
setpoint are used by the control algorithm to calculate the output % required to correct for the
deviation between setpoint and process.
Proportional Output 1 Action P1EC on error condition does not apply to position proportioning
control. In an error condition, the unit goes only to 0% output (fully closed).
With Position Proportioning control, it may be necessary to adjust the Sensitivity Tune SEnS
mode parameter to reduce or eliminate oscillations of the motor around setpoint. If oscillation
occurs, increase the SEnS value until the motor stops oscillating. If the differential between
the Open and Closed rotation is too large, then decrease the SEnS value. Also, for proper
Position Proportioning operation, it is necessary to specify the actuation time of the valve or
damper from full open to full closed. If the motor has a stroke duration of 60 seconds, change
the value in the Cycle Time parameter Ct1 to 60. This ensures that controller will move the
motor for the proper amount of time when making adjustments.