How would you detect a stuck actuator in a DDC loop?

• A. Actuator position is ideally aligned with the commanded signal at all times
• B. The only way to detect is by measuring the process variable directly
• C. Observe actuator position not matching commanded signal; check for mechanical binding, check feedback sensor issues
• D. Replace the actuator immediately without diagnostics

Answer: (C)

The main idea is that a stuck actuator shows up when the signal the controller sends to position the device doesn’t result in a corresponding change in the actuator’s actual position. In a DDC loop, the controller commands a position or opening, and a healthy actuator moves toward that value while sending back a feedback signal. If the actuator can’t move (stuck), the commanded position and the actuator’s real position won’t match, revealing a fault.

To diagnose, you compare the commanded position with the feedback-position reading. A mismatch indicates a problem. Then you investigate mechanical causes—binding, seized bearings, jammed linkage, or any obstruction in the actuator path. You also verify the feedback sensor itself; a faulty sensor can report an incorrect position and simulate a stuck actuator even if the mechanism is free. This approach directly targets the symptom (mismatch plus possible mechanical or sensor issues) and guides you to the root cause, rather than relying on process-variable measurements alone or jumping straight to replacing components.