Ratio Control System

Ratio control system is a technique where in variable is manipulated to keep it as a ratio proportional to another ratio control system is a special type feed forward control system widely used in the process industries. The objective of ratio control system is to maintain the ratio of two variables at a specified value.

In this figure the ratio control system consists of the flow transmitter which senses the flow rate of the first pipe and second flow control. The flow controller controls the flow of the second pipe with respect to the flow in the first pipe.

Example of ratio control system

A common example is when the ratio of two reactants must be controlled is shown in the figure. One of the flow rates is measured but allowed to float, that is , not regulated. The outer flow rate is both measured and adjusted. The outer flow rate is both measured and adjusted to provide the specified control ratio. The flow rate of reactant A is measured and added with appropriate scaling, to the measurement of flow rate B. the controller reacts to the resulting input signal by adjustment of the control valve in the reactant B input line.

Application of ratio control system:

Blending operations
For holding the fuel-air ratio of reactants of the optimum.
Maintaining a stoichiometric ratio of reactance of a reactor.
Keeping a specified reflux ratio for a distillation column etc.

Cascade Control System

It there is more than one control systems are added together in a process the control system is called cascade control system.

Block diagram of cascade control system

The inherent interaction that occurs between two control systems in many applications is used in one control loop determined by measurement of a different variable for which interaction exists.

Two measurements are taken from the system. Each is used in its own control loop. Set point of inner loop is decided by controller output of outer loop. Cascade control provides better control of the outer loop variable than is accomplished through a single variable system.

Example of Cascade control system

The temperature controller generates the correction signal based on the actual temperature. This correction signal is used as the set point input to a flow controller. The flow controller signal positions the control valve, which regulates the flow of hot liquid from the vessel.

If temperature of heating tank fluid rises above the set point the temperature controller generates pneumatic signal which acts as the set point to the flow controller. Flow controller closes the control valve that decreases the flow of heating fluid so as to get the desired heating effect. Similarly the heating fluid temperature falls below the desired heating effect.

Advantages of cascade control system

The flow controller is much faster than the temperature controller due to much shorter dead time in the secondary loop than in the original single loop system.

The temperature controller with an integral mode remains in the design to ensure zero for all disturbance source.