Controlling waterflow vs buffering water

Hi all,
We have a large watertank and from that large watertank we try to supply the water to our watertreatment as constant as possible. But we also like to hold the level in the tank rather low so we can hold the rainwater if it rains a lot.

For the moment we are using a PID controller and we try to maintain a constant level (40%), so we still have the buffer for the rainwater. The proportional gain is very low and the integral large, so the controller goes very slow.

Do any of you gurus have a sugegstion what would be the best method for this?

Thanks and have a good weekend

We have a similar situation. We want to process our waste water as slowly as possible to remove the most contaminants, but every night at around 11 pm, we want our holding tank to be as low as possible when sanitation begins, because they can quickly overfill the tank!

We have a PID that is based on flow, and will acheive the setpoint in gallons per minute. Then we change the flow setpoints depending on both time of day and tank level. We only have one flowmeter, so the only way to tell how much water is coming into the system, is to compare the change in level against the outflow rate. Basically, we increase the GPM target until the rate of change in the level is what we want for that period of the workday.

We also know the minimum speed that our pump can run efficiently, so we clamp the low speed command above that value, and below the value which can overflow the next process downstream. If our tank level gets below 10%, we stop the pump, and don't start it again until it gets above 12% to help avoid short cycling the pump.

Rather than trying to acheive a tank level, you may want to regulate based on flow rate, and modify the target rate depending on tank levels.

oh boys please try fuzzy logic, it can use time and other inputs.