97 Control Modes & Algorithms
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. Deviation needs to be set at
minimum value.
Please note that this procedure allows for a rough setting of the PID parameters only and 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 10 s.
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 150 mV 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).
5. At this stage, stop dosing reagent.
6. Transfer the log file on a USB flash drive.
7. Connect to a PC and download the data from the USB flash drive and prepare the process graphic.
8.
On the chart draw a tangent to the maximum slope point until it intersects with the horizontal line corresponding
to the initial pH or mV value. Read the system time delay (Tx) on the time axis.
9. The deviation, Reset Time and Rate Time can be calculated from the following:
Deviation = Tx * max. slope (pH or mV)
Reset time= Tx / 0.4 (minutes)
Rate time= Tx * 0.4 (minutes)
10.
Set the above parameters and restart the system. If the response has too much overshoot or is oscillating,
the system can be fine‑tuned by slightly increasing or decreasing the PID parameters one at a time.
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