What happens when the proportional band of a controller is set to the minimum position?

When the proportional band of a controller is set to the minimum position, the control system operates in an on/off manner, which is characteristic of a feedback control loop. In a control system, the proportional band determines the range of error over which the controller will respond proportionally to changes. Setting the proportional band to its minimum effectively means that any deviation from the setpoint results in an immediate correction.

This on/off action can lead to oscillations around the setpoint, as the controller will quickly engage and disengage to try and maintain the desired level. While this method can be effective in some situations, it lacks the smooth control typically needed in many process applications, where a more refined adjustment is beneficial.

In contrast, a wider proportional band allows for more gradual adjustments and is better suited for maintaining stability. Therefore, when the proportional band is at its minimum, the system defaults to a more binary approach, hence the label of on/off action.