104
CONTROL MODES
Figure 38
illustrates how the response overshoot can be improved with a proper Rate-time setting.
RATE TIME COMPENSATES FOR RAPID CHANGES
Time
t
0
Low rate
Proper rate setting
SP
pH
Figure 38:
Derivative Function with pH Probe Connected
Tuning PID Parameters using relay on/off controlled device
PID parameters have to be adjusted to a user’s process variables. Values for PID parameters depend
on the installing process characteristics e.g. overall liquid volume, recirculated flow, dosed reagent
concentration, flow mixing, process buffering, electrode’s response time.
Optimum values for PID parameters can be tuned (adjusted) after an experimental tuning procedure.
To get the best possible control, a “trial and error” tuning procedure must be first performed. Below
listed five parameters can be adjusted to achieve a fast response time and a small overshoot:
Set point
Deviation
Reset time
Rate time
Control period
Note
Users have to disable the derivative and integrative actions by setting the Rate time to
0 and Reset time to maximum. Control period and Set point need to be at maximum value.
Deviaton needs to be set at minimum value.
Please note that this procedure allows for a rough setting of the PID parameters only; and therefore,
would not fit all processes. Reset time and Rate time parameters should be set by technical personnel
only.
1. Turn On controller; set the log interval to 10s.
2. Start with a solution that has a pH or mV value different enough from the dosed liquid (e.g. a
minimum of 3 pH or 150mV difference).
3. Turn On the dosing device at its maximum capacity and note down the starting time to correlate
with controller real time clock taken from the daily log files.
4. The pH or mV will start to vary and subsequently will reach a maximum rate of change (slope).
