Proportional P-only control

The controller switches off when the process variable exits the dead band on the high side and does not turn on again until PV is outside the dead band on the low side. The frequency of cycling is reduced, but the deviation from the set point is increased. If the dead band is reduced then the frequency of cycling is increased, but deviation from set point is decreased. 

With an on-off controller, cycling cannot be eliminated. When a large lag is present in the process, the deviation from the set point may not be perceptible, since the amount of time per cycle is longer. If this is acceptable, then an on-off controller can be used. However, in order to eliminate cycling completely, another control mode would need to be implemented. 

Proportional control is the simplest continuous control mode that can damp out oscillations in the feedback control loop. This control mode normally stops the process variable PV from cycling, but it does not necessarily return it to the set point. 

In equation form, the output of a proportional controller is proportional to the error (where error is the deviation of the measurement CV from the set point SP)  

For a controller output of zero, what will the level do The level will begin to drop. To stop this movement F and Fo must equal 50 per cent again. If a linear relationship is assumed between inflow and controller output, then for F = 50% we will have MV = 50%. 

---