Ratio Control
Ratio control holds two flows or quantities in a fixed proportion by driving one to track a scaled version of the other.
Maintaining a proportion
Many processes require two streams to be combined in a constant ratio: fuel and air for combustion, reactant proportions in mixing, or coolant blended in a set fraction. Ratio control designates one stream as the wild or uncontrolled flow, often set by an upstream demand, and manipulates the other, the controlled flow, so that their ratio stays at the desired value regardless of how the wild flow varies.
How it is implemented
The wild flow is measured, multiplied by the desired ratio to compute the setpoint for the controlled flow, and a flow controller drives the controlled stream to that moving setpoint. As the wild flow rises or falls, the controlled-flow setpoint scales with it automatically, preserving the proportion. This is a feedforward structure on the wild flow combined with feedback on the controlled flow.
Design considerations
Accurate flow measurement is essential, since ratio errors propagate directly from measurement errors. Nonlinear flow sensors, such as differential-pressure meters whose reading scales with the square of flow, must be linearized before the ratio is applied, or the ratio will drift with operating point. Ratio control is frequently the inner element of a larger scheme: a higher-level controller adjusts the ratio setpoint itself, for instance trimming an air-fuel ratio based on measured exhaust composition.
- One wild flow is measured; the other is manipulated
- Controlled-flow setpoint equals ratio times wild flow
- Preserves proportion as the wild flow varies
- Requires accurate, linearized flow measurement
Ratio control combines naturally with cascade control: the ratio block sets the inner flow loop's target while an outer loop, such as composition or temperature, adjusts the ratio. This layering is standard in combustion and blending processes.
For a design-stage plant requiring two streams in fixed proportion under varying demand, ratio control would hold the mixture correct automatically, evaluated in simulation. It is a simple, robust, and pervasive process-control technique.