User Guide

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 derivative

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 setpoints:

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.

Error

100%

Proportional Band

Controller

output