The controllers in ratio control are usually P-I type. This is in order to achieve zero steady state error for maintaining the desired ratio. Derivative action is avoided because the flow is always noisy.
Example -2
We have mentioned earlier that ratio control is a special type of feedforward control. We shall elaborate this point in the following example [4]. Effects of control actions in feedback only mode, feedforward only mode and feedforward-feedback mode will also be elaborated in the context of control of a blending process.
Consider a blending system where two streams are blended; one is uncontrolled wild stream that acts as disturbance and another is controlled and acts as the manipulated variable. The ratio of these two streams affect the quality of the output of the process.