Is controlled rather than the individual variables. In Eq. 1, u and d are physical variables, not deviation variables.
Typical applications of ratio control include:
1. Setting the relative amounts of components in blending operations
2. Maintaining a stoichiometric ratio of reactants to a reactor
3. Keeping a specified reflux ratio for a distillation column
4. Holding the fuel-air ratio to a furnace at the optimum value.