Chemtrac SCM2500XRD User Manual

Chemtrac Systems, Inc. Page 36
need for a controller with PID "tuning constants". The Proportional Band, Integral Time,
and PID Rate functions provide a way to eliminate cycling due to the lag time between
coagulant additions and streaming current sensing.
The following are description of parameters used in the PID control loop algorithm:
q Process Variable: The controlled process, meaning the value at the Controller
used to look at the process input (SCV).
q Setpoint: The target for the process variable, meaning the value at the setpoint
input.
q PID Output: The correction signal produced by the PID loop algorithm. The
output is used to control the process that effect the process variable.
q Proportional Band: The value of the property proportional constant (KP).
KP = Output range / effected process range.
q Integral Time: The value of the Integral constant (KI) expressed in repeats per
minute.
KI = Component of the loop output reacts to the amount setpoint error.
q PID Rate: The value is a function design to improve the PID loop response.
q Maximum Output: The maximum value produced by the PID loop output.
q Minimum Output: The minimum value produced by the PID loop output.
The Controller’s PID loop is designed for reverse acting; process variable goes down will
cause the output to go up. To set the loop to be direct acting, the minimum and
maximum values need to be swapped; set minimum at maximum value and set
maximum at minimum value.
The following describe each PID parameter in detail.
Proportional Band
The proportional band constant setting is dependent upon the response of the controller
or and also, to some degree, the lag time. The more responsive the Controller, and/or
the longer the lag time, the higher the proportional band constant should be set. Only
through trial and error can any tuning constant be correctly established. To test
proportional band constant setting, 10 to 30% changes in raw water flow rates can be
made. Take note of the Controller’s response as the SCV changes.
q If the response of the pump output seems to be large for the amount of SCV
change you are seeing, increase the proportional band constant setting and re-
test.
q If the Controller is making too small of an adjustment for the SCV change you are
seeing, the proportional band constant is set too high and should be reduced and
then re-tested.
The important thing to remember about the proportional portion of PID is that
proportional only changes the Pump Output as the SCV is changing. Or in other words,
it responds to the change in deviation as it occurs between the actual SCV and the
setpoint and does not respond to a deviation if it is not changing. So, if you notice that
as the SCV goes from a reading of 0 (setpoint) to –10 that the pump output went from
50% to 60%, and you know from experience that a 10% change in Pump output
produces a 20 unit change in the SCV, then you can assume that the proportional band
constant is set too low and needs to be increased. In the above example, you would
know that if the SCV went from 0 to –10 that the pump output would need to increase
approximately 5% (since 10% produced a 20 unit change) and you would adjust the

Chemtrac SCM2500XRD Specifications

General

Brand	Chemtrac
Model	SCM2500XRD
Category	Controller
Language	English