Explain the difference between a Proportional (P) control and a Proportional-Integral (PI) control in DDC.

• A. P provides immediate correction proportional to error; PI adds integral action to eliminate steady-state error over time.
• B. P control uses integral action to reduce long-term error.
• C. PI control uses only proportional action.
• D. P control drives the actuator to the final value without overshoot by itself.

Answer: (A)

A P controller provides a correction that is proportional to the current error, so the system responds immediately to how far off the target you are. However, it often leaves a constant offset when there’s a steady disturbance or bias, because there’s no mechanism to account for past errors or learn from them.

A PI controller adds an integral term that sums past errors over time. This accumulated action gradually adjusts the control signal to drive the error toward zero, eliminating steady-state offset. In other words, the proportional part handles present deviation quickly, while the integral part corrects long-term bias by considering the history of error.

So the main difference is that proportional action gives immediate correction based on the current error, and integral action in the PI controller removes persistent error over time by integrating past errors. (Be aware that improper tuning can cause slower response or overshoot, and integral windup can be a concern in discrete implementations.)