21
In HI21 and HI22 the proportional action is set directly
as “Deviation” in pH and mV units respectively. Relation
between Deviation (D) and PB is:
D = Range * PB/100
Each setpoint has a selectable proportional band: PB1 for
setpoint1 and PB2 for setpoint2. Two further parameters
must be provided for both setpoints:
Ti
= Kp/Ki,
reset time
, measured in minutes
Td
= Kd/Kp,
rate time
, measured in minutes.
Ti1 and Td1 will be the reset time and rate time for setpoint1,
while Ti2 and Td2 will be the reset time and the rate time
for setpoint2.
TUNING A PID CONTROLLER
The proportional, integrative, derivative terms must be
tuned, i.e. adjusted to a particular process. Since usually
the process variables are not completely known, a “trial
and error” tuning procedure must be applied to get the
best possible control for the particular process. The target
is to achieve a fast response time and a small overshoot.
Many tuning procedures are available and can be applied
to
HI21.
A simple and profitable procedure is reported in
this manual and can be used in almost all applications.
The user can vary five different parameters, i.e. the setpoint
(S1 or S2), the deviation (D1 or D2), the reset time, the rate
time and the proportional control mode period T
c
(from 1
to 30 minutes).
Note
User can disable the derivative and/or integrative action
(for P or PI controllers) by setting Td = 0 and/or Ti = MAX
(Ti) respectively through the setup procedure.
SIMPLE TUNING PROCEDURE
The following procedure uses a graphical technique of
analyzing a process response curve to a step input.
Note
Connecting an external device (chart recorder or PC) to the
controller, the procedure is easier and doesn’t need the use
of hand plotting the process variable (pH or mV).
