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The parallel form allows the user to enter all parameters as Gains. In all cases, larger gain values result in faster
output response.
Parameter
Description
Units
K
p
Proportional Gain
unitless
K
i
Integral Gain
1/seconds
K
d
Derivative Gain
seconds
Output (%) = K
p
e(t) + K
i
f
e(t)dt + K
d
de(t)
dt
Integral Value Management
To determine the integral component of the PID calculation, the controller software must maintain a running total
of the accumulated area under the error curve (Current Integral). The sign of the value added to the accumulated
Current Integral during each cycle may be positive or negative based on the current Direction setting as well as the
relative values of the current process reading and the set point.
Override Control
The Current Integral accumulates when the output is set to Auto mode. If the controller is switched to Off mode,
the value no longer accumulates, but it is not cleared. Therefore, PID control will resume where it left off if the
controller is switched from Off back to Auto. Similarly, accumulation of the Control Integral will be suspended if
the output is interlocked and resume after the lock-out is removed.
Bumpless Transfer
When the output is switched from Hand to Auto mode, the controller calculates a value for the Current Integral
using the current error to generate the same output percent as the Hand Output setting. This calculation does not
use the Derivative tuning setting to minimize errors from momentary fluctuations in the input signal. This feature
ensures a smooth transition from manual to automatic control with minimal overshoot or undershoot as long as the
user sets the Hand Output percentage close to the value that the process is expected to require for optimal control
in Auto mode.
Wind-up Suppression
The Current Integral value that is accumulating while the output is set to Auto can become very large or very small
if the process value remains on the same side of the set point for a prolonged period of time. However, the control-
ler may not be able to continue to respond if its output is already set to the minimum or maximum limits (0-100%
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.
