Walchem W600 Series Instruction Manual page 78

dt
de(t)
Parallel
The parallel form allows the user to enter all parameters as Gains. In all cases, larger gain values result in
faster output response. This form is used in the WebMaster controller and is used internally by the Control
Module.
Parameter
K
p
K
i
K
d
Output (%) = K
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 accu-
mulated 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 controller 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 mini-
mum or maximum limit during normal control operations. But with this wind-up suppression feature, over
shoot will be minimized should that situation occur.
Delta Time Between Readings
Difference Between Current Error & Previous Error
Description
Proportional Gain
Integral Gain
Derivative Gain
e(t) + K
p i
Units
unitless
1/ seconds
seconds
f
e(t)dt + K
d
74
seconds
% of full scale
de(t)
dt