What is direct acting and reverse acting conroller?

The output of a proportional controller is "directly" proportional to the error. The error is the difference between the measured variable and the set point. Therefore, if the error increases, the output of the controller is expected to increase.

The relationship between the controlled variable and the measured variable is considered here. The actuator manipulates the controlled variable.

Direct Acting Controller

If the control output is increased by a positive error, the controller is said to be direct acting.

Reverse Acting Controller

In the opposite case, the controller is said to be reverse acting when a positive error decreases the control output.

The process configuration determines whether a controller must be direct or reverse.

Explanation of Direct and Reverse action with the help of an Example

For example, it depends on the placement of the control valve in the case of a tank level control.

If the valve controls the flow out of the tank, we would like to see a positive error (tank level high) in order to increase the control output, open the valve and leave more fluid out of the tank.

However, if the valve controls the flow into the tank, a reverse - acting controller would be used to respond to a high level by closing the valve and reducing the flow into the vessel.