Proportional control action is the most basic form of modulating control. In this control mode, the correction signal (or output from the controller) is directly proportional to the deviation or error between the measured variable (controlled condition) and the desired set point. The larger the deviation, the stronger the corrective response by the controller.

Mathematically:

$$Output\:\alpha\:Error$$

A proportional controller is suitable for processes where small, continuous adjustments are required and the process dynamics are relatively stable. A common application is in temperature control systems, where proportional action can effectively maintain the temperature close to a desired set point with minimal oscillation. It can also be used in pressure regulation, level control, and flow control systems.

The main disadvantage of proportional-only control is the presence of an offset or steady-state error. Since the controller output is proportional to the error, a finite error is required to maintain a specific output. This means the system may not reach the exact set point but will stabilise at a point close to it, depending on the proportional gain. Therefore, proportional control alone cannot eliminate steady-state error.