46
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
HI504
. 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
.
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
This procedure allows only a rough setting of the PID pa-
rameters and could not fit all the processes .
It is suggested that I and D parameters be set by technical
personnel, because their inadequate values may cause
undesired behaviors of the system .
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) .
1 . Starting from a solution with a pH or mV value different
from the dosed liquid (at least a 3 pH or 150mV differ-
ence) turn on the dosing device at its maximum capacity
without the controller in the loop (open loop process) .
Note the starting time .
2 . After some delay the pH or mV starts to vary . After more
delay, the pH or mV will reach a maximum rate of change
(slope) . Note the time that this maximum slope occurs and
the pH or mV value at which it occurs . Note the maximum
slope in pH or mV per minute . Turn the system power off .
3 . On the chart draw a tangent to the maximum slope point
until intersection with the horizontal line corresponding
to the initial pH or mV value . Read the system time delay
Tx on the time axis .
