Hanna Instruments HI 700 Series - Manual (Page 12)

23 22
SIMPLE TUNING PROCEDURE
The following procedure uses a graphical technique of ana-
lyzing a process response curve to a step input.
1. Starting from a solution with an EC or TDS value quite
different from the dosed liquid, 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 (T
0
) the EC or TDS starts to vary. After
more delay, the EC or TDS will reach a maximum rate of
change (slope). Note the time that this maximum slope
occurs and the EC or TDS value at which it occurs. Note
the maximum slope in EC or TDS 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
EC or TDS value. Read the system time delay Tx on the time axis.
4. The deviation, Ti and Td can be calculated from the following:
• Deviation = Tx * max. slope (EC/TDS)
• Ti = Tx / 0.4 (minutes)
• Td = Tx * 0.4 (minutes).
5. Set the above parameters and restart the system with the
controller in the loop. If the response has too much over-
shoot or is oscillating, then the system can be fine-tuned
slightly increasing or decreasing the PID parameters one
at a time.
Note Connecting an external device (e.g. chart recorder) to the
controller, the procedure is easier and doesn’t need the use
of hand plotting the process variable (EC or TDS).
The proportional action is set through the setup procedure as
“Deviation” in percent of full scale of the selected range.
Each setpoint has a selectable deviation: D1 for setpoint1
and D2 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 the process vari-
ables are not typically 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 the EC/TDS controllers. 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.