Open Loop Tuning

The Zeigler Nichols Open-Loop Tuning Method is a way of relating the process parameters - delay time, process gain and time constant - to the controller parameters - controller gain and reset time. It has been developed for use on delay-followed-by-first-order-lag processes but can also be adapted to real processes.

The method is outlined below.

Look at the open loop response of the process to a step change in the manipulated variable.
Evaluate
- The steady-state gain, (y2 - y1) / (u2 - u1)
- The time delay, Td
- The time constant, Ts
Look at the diagrams below to see how this differs between ideal and real processes.
Ideal Process

Real Process
Finally substitute these values into the table below to obtain the relevent controller parameters.

Controller Type Gain Reset Derivative

P (Ts ) / (Td ) - -

PI (0.9 Ts ) / (Td ) 3.3 Td -

PID (1.2 Ts ) / (Td ) 2.0 Td 0.5 Td

Controller Type	Gain	Reset	Derivative
P	(Ts ) / (Td )	-	-
PI	(0.9 Ts ) / (Td )	3.3 Td	-
PID	(1.2 Ts ) / (Td )	2.0 Td	0.5 Td

Please move Back to previous page