Red Lion PCU - Manual (Page 33)

Output Power Limits (OPLO & OPHI)
Enter the safe output power limits for the process. These parameters may
also be used to limit the minimum and maximum controller power due to
process disturbances or setpoint changes, to reduce overshoots by limiting
the process approach level.
OPLO & OPHI - 0 to 100%
If the secondary output option is installed, the limits range from:
OPLO & OPHI - -100 to 100%
With the secondary output option installed, the Lower Limit can be set to
less than 0% to limit maximum secondary output power or set to greater than
0% to limit minimum main control output power. Set the High Limit to less
than 0% to limit minimum secondary output power or greater than 0% to limit
maximum main control output power. When controlling power in the manual
mode, the output power limits do not take effect.
Input Overdrive Preset Power (OPFL)
If an input overdrive signal is detected, the control output(s) default to a
preset power output.
OPFL - 0% (OP1 output full “off”) to 100% (OP1 output full “on”)
If the secondary output option is installed, the range is extended from:
OPFL - -100% to +100%
At 0% both outputs are off, at 100% OP1 is on and OP2 is off, and at -100%
OP2 is on and OP1 is off. The alarm outputs always have an up-scale drive
(+9999), independent of this setting, for an input overdrive signal.
For position mode valve controllers, the valve is positioned according to
the setting of this parameter.
For velocity mode valve controllers, the following actions occur:
Velocity mode: OPFL = 0%, valve CLOSE output activates
OPFL = 100%, valve OPEN output activates
OPFL = any other setting, both valve outputs disable
Output Power Dampening (OPdP)
The output power calculated by the PID controller can be dampened
(filtered) to reduce the controller output activity. Those processes with high
gain and/or derivative times or those processes with a relatively high noise
content can benefit from the dampening action.
OPdP - 0 to 250 seconds
The dampening parameter is expressed as a time constant in seconds.
Increasing the value increases the dampening or filtering effect. A value of
zero disables output power dampening. The amount of dampening to be used
depends primarily on the response time of the process and the amount of final
actuator activity desired. Generally, dampening times in the range of 1/20 to
1/50 of the controller’s integral time (or process time constant) prove to be
effective. Dampening times longer than these may cause controller instability
due to the added lag effect of too much filtering.
In the case where a relatively high dampening time is desired, the
controller’s proportional band may be increased to restore an adequate
stability margin. The Auto-tune procedure of the controller sets the
dampening value appropriate to the characteristics of the process.
ON/OFF Control Hysteresis Band (CHYS)
The controller can be placed in the ON/OFF control mode by setting the
proportional band to 0.0%. The control hysteresis value affects only the main
control output (OP1).
CHYS - 1 to 250 units
The hysteresis band should be set to a minimum value to eliminate output
chatter at the setpoint. Set the hysteresis band to a sufficient level prior to
invoking Auto-Tune. Internal cascade controllers, secondary loop, have a
fixed hysteresis of 1.5% of scaled range.
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