Ziegler-Nichols
Method:
- First, note whether the required proportional control gain is positive or
negative. To do so, step the input u up (increased) a little, under manual
control, to see if the resulting steady state value of the process output has
also moved up (increased). If so, then the steady-state process gain is
positive and the required Proportional control gain, Kc, has
to be positive as well.
- Turn the controller to P-only mode, i.e. turn both the Integral and
Derivative modes off.
- Turn the controller gain, Kc, up slowly (more positive
if Kc was decided to be so in step 1, otherwise more
negative if Kc was found to be negative in step 1) and
observe the output response. Note that this requires changing
Kc in step increments and waiting for a steady state in the
output, before another change in Kc is implemented.
- When a value of Kc results in a sustained periodic
oscillation in the output (or close to it), mark this critical value of
Kc as Ku, the ultimate gain. Also, measure
the period of oscillation, Pu, referred to as the ultimate
period. ( Hint: for the system A in the PID simulator, Ku
should be around 0.7 and 0.8 )
- Using the values of the ultimate gain, Ku, and the
ultimate period, Pu, Ziegler and Nichols prescribes the
following values for Kc, tI and
tD, depending on which type of controller is desired:
|
Kc |
tI |
tD |
P control |
Ku/2 |
|
|
PI control |
Ku/2.2 |
Pu/1.2 |
|
PID control |
Ku/1.7 |
Pu/2 |
Pu/8 |
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This page is maintained by Tomas B. Co (tbco@mtu.edu). Last
revised 1/12/1999.
Tomas B. Co
Associate Professor
Department of Chemical Engineering
Michigan Technological
University
1400 Townsend
Avenue
Houghton, MI
49931-1295
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