Which of the following is a typical drawback of open-loop control?

• A. Requires sensors for operation
• B. Susceptible to disturbances and model errors
• C. Provides exact positioning under all loads
• D. Always slower than closed-loop

Answer: (B)

Open-loop control executes a fixed set of commands without checking the actual output. Because there’s no feedback to correct errors, any disturbances or inaccuracies in the model will cause the system to deviate from the intended behavior, and nothing will automatically bring it back on track. This makes it inherently sensitive to disturbances and model errors—the typical drawback. In environments with friction changes, payload variations, or external forces, the commanded action won’t align with what actually happens, since there’s no sensor-based adjustment. The other statements don’t describe this weakness: open-loop doesn’t inherently require sensors, so needing sensors isn’t a drawback; it doesn’t guarantee exact positioning under all loads (that requires feedback); and being slower than closed-loop isn’t a universal rule—speed depends on the specific task and control strategy.