What is Ratio control system?

Ratio control systems are usually used chemical factories, where more than one chemical should mix in a specific ratio.

Ratio control is a special type of feedforward controller. The objective of the ratio control is two maintain a ratio between two process variable without considering the external disturbance.

In the above-shown figure, there is a process with two process variables. The variables are usually flowrates, one is fixed as manipulated stream and the other flow is fixed as the disturbance stream.

Whatever the external load disturbance cause the manipulated stream, a ratio control keep a fixed ratio between the two or more variables irrespective if possible setpoint change.

In case of flow ratio control, the flow of both streams are measured using flow transmitter and the values are applied to a divider. The divider calculates the ratio between the two streams and applied to the ratio controller. Ratio controller compares the actual value with a set point to take a control action on a valve.

Even if there is an external disturbance in the manipulated or disturbance stream, a proportional change in ratio stabilizes the process. The ratio of one rate to the other can be changed by adjusting the gain of the secondary controller.


Process design and operations often calls for keeping a certain ratio two or more flow rates. One of the flows in a ratio-control scenario, sometimes called the master flow or wild flow, is set according to an external objective like production rate. The ratio controller manipulates the other flow to maintain the desired ratio between the two flows. The flow controlled by the ratio controller is called the controlled flow. For example, when treating drinking water with chlorine, the water is the wild flow, and the chlorine is the controlled flow.

There are two fundamentally different designs for ratio control. One of them is the correct design, the other one does not work in practice. You could read more here