When we are working with two-level control say the primary and secondary level of the grid, I have four PID controllers, two at the primary section and two at the secondary section. Now, for tuning purposes what should be the order, and how we can tune each controller co-officiants?
Rinku Kumar
Take the following steps:
1. Switch to MANUAL mode in the loop and run few bump tests in the controller OP.
2. Determine the process model (gain, time constant, and dead time) and use the appropriate tuning methods to generate the new set of parameters based on the process dynamics.
If you know the desired behaviour of each of your nested controllers then start with the lowest level one and then work up to your highest level. This means that each controller is being tuned to include the previously tuned controller so you should only have to do each one once. Otherwise you may need to retune to take into account any changes.
In terms of the tuning itself there are many possible techniques. If you have access to the equipment and can 'fiddle' then I'd recommend one of the ZEIGLER NICHOLS TUNING techniques (as Qamar Ul Islam suggests above). Otherwise you can construct a Routh array to determine co-efficient values.
Dear Rinku Kumar,
A cascade regulation is made up of two nested loops, the internal loop must be much faster than the external loop. The system can be broken down into two subsystems linked by a measurable intermediate quantity. A first loop, the slave loop, has as regulated quantity; this magnitude is intermediate. The second loop, the master loop, has the set magnitude. The controlled variable of the cascade control controls the setpoint of the slave control.
For more information about this subject, please take a look at links on topic.
https://www.mathworks.com/help/slcontrol/ug/tuning-multi-loop-control-systems.html
Article PID Controller Tuning to Obtain Desired Closed-Loop Response...
Article A Simultaneous Tuning Method for Cascade Control Systems Bas...
Best regards
First you need to understand that the anticipated behaviour of the two levels involved. As clearly iterated by most contributors, the inner loop should be faster than the external loop , hence it is expected that the set point for the external loop should be used to determine that of the inner. To anticipate good control performance, you can manually use the controlled variable of the nested control to effectively control the inner loop control. Or use the Zeigler Nichols auto-tuning techniques as suggested.
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