1.PID Controllers
Quick rules for selection:
a.Offset is acceptable ; use P-control
b.Fast process, offset undesirable: use PI-control
c.Slow process, offset undesirable: use PID-control
2.Controller Tuning Methods
a. Ziegler-Nichols Method
i.Set controller to Proportional only control mode
ii.Obtain the ultimate gain, Ku, and the corresponding ultimate period,
Pu, by slowly increasing
process gain until process sustains periodic oscillation
iii.Using Ku and Pu, evaluate the controller parameters using the Ziegler-Nichols
tuning table.
b. Autotuning Relay Method
i.Connect the feedback to an on-off controller:
If process gain is positive: U = uo+h (if y<yset), uo-h (otherwise)
If process gain is negative: U = uo-h (if yyset) uo+h (otherwise)
ii.Wait until the process attains a limit cycle
iii.Using the amplitude of the limit cycle, a, evaluate the approximate
ultimate gain, Ku =
(4/p)(h/a). Also record the period of oscillation of the limit cycle as
the ultimate period, Pu.
iv.Using the calculated Ku and Pu, use the Ziegler-Nichols tuning table.
c. Cohen and Coon Method:
Procedure:
Function Description |
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Step Function |
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Impulse Function |
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Ramp Function |
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Sine Function |
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Cosine Function |
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Exponential Function |
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Power Function |
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