Glossary · Control theory and process control
Ratio control
Also known as: Ratio loop
German: Verhältnisregelung
In process control, ratio control keeps one process variable, usually a flow, in a fixed or adjustable ratio to another, by using the measured value of the leading variable multiplied by the ratio as the setpoint of the following controller.
- Control theory
- Process industry
In one sentence
Ratio control keeps one flow in a set ratio to another by deriving the follower's setpoint from the leading flow.
Example
In a burner, the air flow controller receives the measured fuel flow multiplied by the air-to-fuel ratio as its setpoint.
How it applies
- Engineering: The leading flow is often uncontrolled (wild flow) or set by production rate. The ratio may come from a Recipe or from an outer quality controller that trims it. Combustion control adds cross-limiting so air leads on load increases and lags on decreases.
- Operation: Operators set the ratio rather than the follower flow. The HMI should show ratio setpoint, actual ratio and both flows.
- Documentation: Define the ratio unambiguously, including units and basis (mass or volume, which flow is numerator). An Engineering unit mix-up between kg/h and m³/h in a ratio can shift a product out of specification.
Ratio control vs. cascade control
In Cascade control, an outer controller sets the inner loop's setpoint. In ratio control, a measured variable does, multiplied by a ratio. Ratio control is often combined with a cascade on top.