The exsting system has a cascaded control with volume flow rate as quantity of major interest, being controlled in primary loop and controller in secondary loop controlls the valve position. Both are PI controllers. I want to design optimal controller for this system, so can I just replace outer(primary) PI controller with optimal controller with inner PI controller still controlling valve position or I should redesign whole as one optimal controller?
we can use any type of controller in cascade or in loops, sure you can modify the PI with Optimal controller or whole system with optimal controllers. but one thing you must maintain, the control resolution of inner loop must be higher than outer loop, otherwise it shows wrong results, because only after the inner loop correction the outer loop works with that values. so generally the advanced strategies are used for inner loops.
In cascade control each control loop has its own controller no matter of which type. Cascade control is mainly used in process control but also in other industrial control applications like industrial robots. The respective outer loops deliver the setpoints for the inner control loops. A common rule is that the inner loops handle the faster subprocesses of the total process and the outer loops the slower subprocesses. That's why the sample rates of the inner loops should be higher than those of the outer loops. An example can be found in robot control where the most inner loop is responsible for the current control of the actuator. The next loop is responsible for the speed control and the next for the position control. In a well structured cascade control system the respective inner control loops can be handled as a closed stable system with a specific time constant. This assumption makes the design of the control parameters of every control loop easier to handle.
I subscribe to Rainer's explanation any kind of controller but the INNER LOOP IS CONTROLLER AS THE FASTEST ONE.
There are two ways you can approach this. One, You need to ensure that the control scheme of the inner loop is is of a higher resolution than the outer in order not to miss any change in the process variable. Also, the response of the control valve needs to be fast such as not to miss out in any process change. PI controller can do that for you. Second, you can avoid using two loops and go at once for an optimal control scheme that is robust and has an observer. If you have a few more of this in your plant, I recommend you go for a small DCS (Distributive Control System).
- Badges
- Science topic
- Similar topics
- Engineering
- Control Theory