R
There is a buffer tank, which is continuously fed into. There is no knowledge about the inlet flow, but it is supposed to be steady. One of the requirements is to keep the level in the tank at a defined setpoint, using a variable speed pump on the outlet of the tank.
Another requirement is to keep the outlet flow steady, +/-10 % is the maximum fluctuation accepted.
The solution I found is to use a cascade control, having the level as the master, and the flow as the slave loop. Like this I will have the level controlled. By limiting the max output of the master PID to flowSP + 10% and the min output of the master PID to flow SP - 10%. By having a set of parameters for the flow PID which is slow enough(without overshoot), I will have also the second requirement fulfilled.
However, I am wondering if there is another/better solution to this problem. because this isn't really a cascade control, as the primary variable in the end is the flow, and not the level.
I am aware of the fact, that if the flowSP doesn't match the inlet flow rate, the buffer tank will fill up. but for the moment this situation is ignored, only short period disturbances of the inlet flow have to be covered by the control strategy.
Another requirement is to keep the outlet flow steady, +/-10 % is the maximum fluctuation accepted.
The solution I found is to use a cascade control, having the level as the master, and the flow as the slave loop. Like this I will have the level controlled. By limiting the max output of the master PID to flowSP + 10% and the min output of the master PID to flow SP - 10%. By having a set of parameters for the flow PID which is slow enough(without overshoot), I will have also the second requirement fulfilled.
However, I am wondering if there is another/better solution to this problem. because this isn't really a cascade control, as the primary variable in the end is the flow, and not the level.
I am aware of the fact, that if the flowSP doesn't match the inlet flow rate, the buffer tank will fill up. but for the moment this situation is ignored, only short period disturbances of the inlet flow have to be covered by the control strategy.