20
During proportional control the process controller calculates the relay activation time
at certain moments t
0
, t
0
+T
c
, t
0
+2T
c
etc. The ON interval (the shaded areas) is then
dependent on the error amplitude.
With the integral function (reset), the controller will reach a more stable output around
the setpoint providing a more accurate control than with the ON/OFF or proportional
action only.
The derivative function (rate action) compensates for rapid changes in the system reducing
undershoot and overshoot of the EC or TDS value.
During PID control, the ON interval is dependent not only on the error amplitude but even
on the previous measurements.
Definitely PID control provides more accurate and stable control than ON/OFF controllers
and it is best suitable in system with a fast response, quickly reacting to changes due
to addition of low or high conductivity solution.
An example of how the response overshoot can be improved with a proper rate action
setting is depicted in the following graphic.
where the first term represents the proportional action, the second is the integrative
action and the third is the derivative action.
EC
RATE ACTION COMPENSATES FOR RAPID CHANGES
t
PID TRANSFER FUNCTION
The transfer function of a PID control is as follows:
Kp + Ki/s + s Kd = Kp(1 + 1/(s Ti) +s Td)
with Ti = Kp/Ki, Td = Kd/Kp,
t
0
t
0
+T
c
t
0
+2T
c
t
0
+3T
c
