What is ratio control and give an example application?

Ratio control maintains a fixed ratio between two process variables by having one variable (wild) set the setpoint for another (controlled) through a ratio station. Example: maintaining air-to-fuel ratio in a combustion system - fuel flow is wild flow, air flow is controlled to maintain stoichiometric ratio. Another example is blending two streams in fixed proportion. Ratio station multiplies wild flow by ratio factor to generate controlled flow setpoint.