Control Philosophy for Input and Output

Hi All,

I am looking for the best way to control a system that has 2 inputs and 1 output (sludge feed In + Polymer dosing and one output which is sludge discharge). We have two process variable and one control variable "polymer dosing", so which the best to maintain the sludge discharge "output" to meet my set-point. Is one loop PID can do that?

I managed to control the speed pump of the polymer (the control variable) based on the feedback of the suspended solid sensor (one loop) as the output goes less than my set-point; the speed pump increase to compensate and vice versa so now how can i add another process variable which is sludge feed in an account.

Still PIK Ok or i need to look at fuzzy logic or any more advanced control?

Many Thanks