A
I'm having some serious trouble with flow/level control in a large tank. The tank is fed constantly, the tricky bit is that it's aboard ship and as the boat rolls, the level indicator changes value, and the PI-controlled pump ramps up and down to compensate. These variations in flow speed are hurting our process.
These variations in the apparent tank level have a period of about five minutes and an amplitude of between +- 2 to 5 cm depending on what kind of seas the ship is in.
Flow in is governed by a magmeter, at a constant rate of around 5m3/hr. The tank has a cross sectional area of 7m2 and the pump can run at between 2 and 7 m3/hr. So an apparent ten centimeter level change from ship's roll represents a volume equal to about ten minutes of system flow or an hour and a half of the inlet and discharge pumps running within 10% of each other (an acceptable value).
Right now we have a guess-and-tested P-I loop implemented to control the discharge pump. It maintains level well but at the cost of ramping the pump up to full speed then down to stopped, and repeating. Decreasing the integral component only makes it worse. Really, we don't care about the level in the tank varying +- 10 cm, so long as the flow is more or less constant.
Big tank, small pump, lots of signal noise. What should I do?
These variations in the apparent tank level have a period of about five minutes and an amplitude of between +- 2 to 5 cm depending on what kind of seas the ship is in.
Flow in is governed by a magmeter, at a constant rate of around 5m3/hr. The tank has a cross sectional area of 7m2 and the pump can run at between 2 and 7 m3/hr. So an apparent ten centimeter level change from ship's roll represents a volume equal to about ten minutes of system flow or an hour and a half of the inlet and discharge pumps running within 10% of each other (an acceptable value).
Right now we have a guess-and-tested P-I loop implemented to control the discharge pump. It maintains level well but at the cost of ramping the pump up to full speed then down to stopped, and repeating. Decreasing the integral component only makes it worse. Really, we don't care about the level in the tank varying +- 10 cm, so long as the flow is more or less constant.
Big tank, small pump, lots of signal noise. What should I do?