Hanna Instruments HI520 - Page 98

Control Modes & Algorithms 98
The example graph given here was obtained by dosing an alkaline solution to a weak acid solution in a tank.
For this, the initial settings have been:
Maximum slope = 3 pH/5 minutes =
0.6 pH/minute
Control period = Tx = approx. 7 minutes
Deviation = Tx * 0.6 = 4.2 pH
Reset time = Tx / 0.4 = 17.5 minutes
Rate time = Tx * 0.4 = 2.8 minutes
pH
Tx
3 pH
5 min.
To Time
Figure 42: Tunning PID Parameters, Dosing an Alkaline Solution to a Weak Acid
PID Control
Following graphs exemplify how input parameters work.
Control out is proportional with the Set point variance, the sum of previous control errors and an estimation of
the future ones.
SP
Control period
mode Low
Deviation
Process value
Relay
out
Analog
out
PID control
Figure 43: PID Control Mode Low, Relay & Analog Out
Relay On time has a guaranteed minimum to prevent stressing the actuators electrically or mechanically.