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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,
where the first term represents the proportional action, the
second is the integrative action and the third is the deriva-
tive action .
Proportional action can be set by means of the Proportional
Band (PB) . Proportional Band is expressed in percentage of the
input range and is related to Kp according to the following:
Kp = 100/PB .
The proportional action is set directly as “Deviation” in pH
and mV units respectively . Relation between Deviation (D)
and PB is:
D = Range * PB/100
Each setpoint has a selectable proportional band: PB1 for
setpoint1 and PB2 for setpoint2 . Two further parameters
must be provided for both setpoint:
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 .
