Which type of control does a proportional-only controller NOT employ?

A proportional-only controller does not employ exact maintenance of the set-point because it operates on the principle of generating an output that is proportional to the error, which is the difference between the set-point and the measured variable. This means that the controller will attempt to adjust the output based on how far the current measurement is from the desired set-point, but it will not actively maintain the set-point at all times.

In practical terms, this means that while a proportional controller will adjust the output in response to changes in the process variable, it cannot eliminate the steady-state error completely. As a result, there will typically be fluctuation around the set-point rather than exact maintenance. The controller can only reduce the error but not necessarily eliminate it, leading to a situation where the process variable can oscillate around the intended set-point rather than being precisely maintained at that value.

On the other hand, the other types of control mentioned do play a role in proportional-only systems: on-off control triggers a full output or none, feedback comparison is essential for determining the error, and proportional output adjustment is the core function of this controller type. Thus, while it has merits in certain applications, exact maintenance of set-point is not a characteristic of a proportional-only controller's operation