Which controller switches abruptly between two states with hysteresis to avoid high-frequency switching?

• A. On-Off Controller
• B. Proportional Controller
• C. Integral Controller
• D. Derivative Controller

Answer: (A)

This question tests bang-bang control with hysteresis, where the actuator flips between two extreme states to keep the process value within a band and avoid rapid relay switching. In this approach, two thresholds are set: when the measured variable falls below the lower limit, the actuator turns fully on; when it rises above the upper limit, it turns fully off. The gap between these thresholds—the hysteresis band—means small fluctuations near the setpoint don’t cause constant on-off cycling, reducing high-frequency switching.

This is why it’s the best fit here: the output is not a continuous scale but two discrete states, chosen specifically to prevent rapid switching that could wear relays or cause instability. Proportional, integral, and derivative controllers modulate the output in a continuous range based on error (and its history or rate of change), which can still produce chattering or smoother control rather than abrupt two-state behavior.