A robot cell reports that the robot is waiting for a part. Production calls the robot technician. The robot controller shows no fault, and the robot program is sitting at a normal wait instruction. The maintenance technician works backward through the handshake. The robot-ready and cell-ready signals are good, but the part-present handshake never arrives.
The upstream PLC logic is waiting on a photoeye that is physically blocked by a bent guide, even though no part is present. Once the guide is corrected and the photoeye aligned, the handshake completes and the robot cycles. The cell stopped at the robot because that is where the sequence became visible, but the root cause was upstream. Interfaces between machines often produce misleading ownership boundaries.
What Mattered
- A waiting device may be healthy.
- Follow handshakes backward to the first missing condition.
- Physical product guides can create sensor and logic symptoms.
- Do not let departmental ownership replace system thinking.
Questions to Ask Yourself
- Which early clue had the highest information value?
- What tempting but unsupported action could have wasted time?
- Which measurement or observation moved the diagnosis across a system boundary?
- What should be changed so the same failure is detected earlier next time?