80
by default). This condition is referred to as Control Wind-Up and can result severe overshoot or undershoot after a
prolonged upset has ended.
For example, if the process value remains far below the set point despite a control output being pinned at 100%,
the Current Integral will continue to accumulate errors (wind-up). When the process value finally rises to above the
set point, negative errors will begin to decrease the Current Integral value. However, the value may remain large
enough to keep the output at 100% long after the set point is satisfied. The controller will overshoot the set point
and the process value will continue to rise.
To optimize system recovery after wind-up situations, the controller suppresses updates to the Current Integral
that would drive the output beyond its minimum or maximum limit. Ideally, the PID parameters will be tuned
and the control elements (pump, valves, etc.) will be sized properly so that the output never reaches its minimum
or maximum limit during normal control operations. But with this wind-up suppression feature, overshoot will be
minimized should that situation occur.
Output Details
The details for this type of output include the pulse rate in %, HOA mode or Interlock status, input value, current
integral, current and accumulated on-times, alarms related to this output, relay type, and the current control mode
setting.
Set Point
Numeric entry of a process value used as a target for PID control. The default value,
units and display format (number of decimal places) used during data entry are defined
based on the Input channel setting selected.
Gain
When the Gain Form setting is Standard, this unitless value is multiplied by the total
of the proportional, integral, and derivative terms to determine the calculated output
percent.
Proportional Gain
When the Gain Form setting is Parallel, this unitless value is multiplied by the normalized
error (current process value versus set point) to determine the proportional component of
the calculated output percent.
Integral Time
When the Gain Form setting is Standard, this value is divided into the integral of the
normalized error (area under the error curve), then multiplied by the Gain to determine
the integral component of the calculated output percent.
Integral Gain
When the Gain Form setting is Parallel, this value is multiplied by the integral of the
normalized error (area under the error curve) to determine the integral component of
the calculated output percent.
Derivative Time
When the Gain Form setting is Standard, this value is multiplied by the change in error
between the current reading and the previous reading, then multiplied by the Gain to
determine the derivative component of the calculated output percent.
Derivative Gain
When the Gain Form setting is Parallel, this value is multiplied by the change in error
between the current reading and the previous reading to determine the derivative
component of the calculated output percent.
Reset PID Integral
The PID Integral Value is a running total of the accumulated area under the error curve
(Current Integral). When this menu option is selected, this total is set to zero and the
PID algorithm is reset to its initial state.
Minimum Output
Enter the lowest possible pulse rate as a percentage of the Maximum Stroke Rate set
below (normally 0%).
Maximum Output
Enter the highest possible pulse rate as a percentage of the Maximum Stroke Rate set below.
Maximum Rate
Enter the maximum pulse rate that the metering pump is designed to accept
(10 – 2400 pulse/minute range).
Input
Select the sensor to be used by this relay
Direction
Set the control direction. This setting is used to determine the sign of the calculated error
(current process value versus set point) and allows flexible control with only positive
values for all PID tuning parameters.