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Tuning a PID controller
After a control system is implemented the controller settings have to be adjusted until the control system performs according to the user's specifications. This process is called controller tuning.
Trial and error tuning
The most basic method of tuning is the trial and error method. This method involves adjusting the proportional gain, the integral time and the derivative time until the performance is satisfactory. The three settings are often adjusted separately in order to see the effects of the different settings. This process can be time consuming.
It can be difficult to get started using the trial end error method. What kind of gains and times should one start out with? A typical approach for tuning a PID controller can be sumarized as follows:
Step 1. Eliminate integral and derivative action by setting the derivative time to its minimum value (zero) and the integral time to its maximum value.
Step 2. Set the proportional gain to a low value (0.5) and enable the controller.
Step 3. Increase the proportional gain by small increments until continuous cycling occurs after a small set-point or load change. The term "continuous cycling" refers to a sustained oscillation with constant amplitude.
Step 4. Reduce the gain by a factor of two.
Step 5. Decrease the integral time until continuous cyclin occurs again. Set integral time to three times this value.
Step 6. Increase derivative time until continuous cycling occurs. Set derivative time to one-third of this value.
The proportional gain that results in continuous cycling in Step 3 is called the ultimate gain. In performing the experimental test to find the ultimate gain, it is important that the output does not saturate. If saturation occurs it is possible to get continuous cycling even though the gain is higher than the ultimate gain. This would then result in a too high gain in Step 4.
Disadvantages of the trial and error method include:
* It is quite time consuming if a large number of trials are required in order to find the ultimate gain and the integral and derivative times that result in continuous cycling.
* The method is not applicable to processes that are open-loop unstable because such processes are typically unstable at both high and low gain values and are stable for intermediate gain values.
* Some simple processes do not have an ultimate gain.
Ziegler-Nichols method
The Ziegler-Nichols methods of controller tuning are the "closed loop" and the "open loop" method. The closed loop method is quite similar to the trial and error method:
Steps 1-3 are the same as in the trial and error method.
Step 4. Take note of the ultimate gain Kpu, and the ultimate period Tu. The ultimate period is the period of the oscillations.
Step 5. Calculate controller settings according to this table:
Controller | Kp | Ti | Td |
-----------+---------+--------+------+
P | 0.5Kpu | inf. | 0 |
PI | 0.45Kpu | Tu/1.2 | 0 |
PID | 0.6Kpu | Tu/2 | Tu/8 |------------------
For more info on tuning and PID control systems follow this link:
http://www.jashaw.com/pid
-- Roy Vegard Ovesen
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