What type of controller has only proportional and integral functions?

A controller that has only proportional and integral functions is known as a PI controller. This type of controller combines the proportional control action, which occupies the present error, and the integral control action, which sums the past errors over time. By using both the proportional and integral functions, the PI controller helps to eliminate steady-state error and enhances the system's ability to respond to changes or disturbances.

The proportional component contributes to the immediate reaction to the current error, while the integral component helps to correct any accumulated offset by integrating the error over time. This combination allows for improved control performance, particularly in processes that require stability and minimal oscillation without introducing derivative action.

The other controller options feature different combinations of control actions. A PD controller includes proportional and derivative functions for improving response times and reducing overshoot. A P controller consists solely of a proportional function, focusing on immediate response without correcting for steady-state error. A PID controller, which stands for proportional, integral, and derivative, includes all three functions to offer comprehensive control over a process by addressing present, past, and future errors.