Robot cell troubleshooting
A robot cell with a hidden fault — some stop the line, the dangerous ones don't. Work the panel, read the robot, and name the fault.
The complaint
Supervisor: “Monthly audit. The cell has run three shifts without a hiccup — just sign off that the stop functions work and we're done.” Arm the cell, run a cycle… and actually test the stops.
Result
Nothing ruled out yet — the PLC view below is live.
What the PLC sees — output bits
A lit coil is an order, not proof the machine moved. Under each coil is what that device is physically doing — when the two disagree, the break is past the output card.
Call it — name the component and how it failed
Open contact, broken wire or loose terminal — the PLC never sees it actuate.
How it works
Using the tool
A robot cell with a hidden fault — and the worst ones don't stop production. You get the complaint, the panel, the cell in front of you, and the ladder online; the logic is healthy and something in the field is lying. Some calls are a cell that won't run. The critical ones are a cell that runs perfectly: a bypassed e-stop looks exactly like a working one until somebody tests it. Work the panel, read the robot's status and its PROG echo, and name the fault before you sign anything.
The method
The method is the same comparison as any I/O troubleshooting — do something physical, watch whether its contact in the online ladder reacts — plus two instruments only a robot gives you. The status bits (RDY / BUSY / DN / FLT) read the handshake in one line: a faulted robot flies FLT, a not-ready one tells you the enable is missing. The PROG echo is command confirmed, not command sent: if the selector is on and the echo reads 0, the selection died between the panel and the robot — you just localized the fault without opening a manual.
And the safety chain adds the rule that makes these calls matter: a running cell proves nothing about its protections. A bypassed e-stop, a welded gate channel, and a dead light curtain all leave production untouched — the only thing that finds them is deliberately testing each stop function, one at a time, the way these scenarios force you to. Sign the audit after the test, never after a good shift.
New to the math? Learn the concept: Robot cell integration