An office building's lift carries a small vibration sensor, and every trip it makes is recorded as one reading. An autoencoder squeezes each reading through a narrow bottleneck and rebuilds it, and any trip whose reconstruction error exceeds raises an alarm for the maintenance crew. That threshold was chosen because only about of ordinary trips exceed it.
The autoencoder was trained on the six months of recorded trips that ran right up to deployment, all logged while the lift was judged to be working normally. No faults were labelled.
Three months after deployment, the crew reviews a typical trip of each kind:
| trip | reconstruction error |
|---|---|
| an ordinary trip | |
| a motor bearing starting to wear | |
| a door dragging on its track as it closes | |
| a worn guide roller — a hard knock every time the car passes a rail joint |
The knocks are the biggest spikes on the trace, and they have never once set off the alarm. The maintenance log shows this lift's guide rollers wear quickly: over the past year a worn roller has been found and replaced several times, each time after it had been knocking for weeks.
Which explanation accounts for the last row?
Select all that apply.