Your robot stopped at the lift again: seven failure modes and how to isolate them

First, separate "the robot cannot" from "the lift will not"

Before touching the robot, watch one failed attempt and note exactly where it stops: outside the lift, in the doorway, inside the car, or at the destination floor. That single observation eliminates half the list below. A robot that stops outside the lift is not talking to the lift. A robot that stops inside the car has a navigation or floor-detection problem. A robot that arrives but will not leave has a map problem.

1. The dry contact is not dry

The most common fault is a lift interface that was wired to a signal with voltage on it, or wired to a common that is shared with the alarm circuit. It works during commissioning, then fails when the building does something else on the same circuit. Test: measure the contact with a multimeter while the car moves. It should be a clean open or closed, with no floating voltage.

2. The lift maintenance company changed a parameter

Lift controllers get firmware updates and parameter changes during routine maintenance. A floor-lockout setting, a door-dwell time reduced from 8 to 4 seconds, or a new fire-service test schedule will all break an integration that was working. Test: ask the lift contractor for the last service date and whether any parameter was changed. This costs one phone call and saves a site visit.

3. Door dwell time is shorter than the robot's entry sequence

A robot approaching a lift slows, confirms the car is present, aligns and enters. That sequence takes 4 to 6 seconds. If the door closes at 4, the robot will either stop short or be nudged by the door and abort. Test: hold the door open manually with the call button and see whether the robot enters cleanly. If it does, the problem is timing, not navigation.

4. Wi-Fi drops in the lift shaft

A robot that loses network inside the car may stop and wait for a heartbeat that never arrives, because the access point is in the corridor. Test: put a phone on the same SSID and watch the signal inside the car. If it drops below about -75 dBm, add an access point in the lift lobby or a repeater in the car.

5. Reflective surfaces and glass doors

Lidar returns from mirrors, polished stone and full-height glass create phantom obstacles. The robot does not fail at the lift; it fails near the lift, in the lobby, and the operator reasonably blames the lift. Test: cover the glass with paper for one run. If the robot passes, you have found it. The fix is map editing or a lidar-compatible film, not a new robot.

6. The map has drifted

Mapping is a snapshot. Renovate a lobby, move a plant, add a signage totem, and the stored map no longer matches the building. The robot then localises incorrectly and refuses to enter the car because it believes the car is elsewhere. Test: watch the robot's position indicator on the map during a manual drive. If the marker does not track the machine, re-survey the floor.

7. Someone is holding the lift

In a busy hotel, a guest presses the call button while the robot is waiting, the car leaves, and the robot times out and returns to its dock. The logs show a lift timeout and the operator concludes the integration is broken. Test: check the timestamps against the lift's own event log. If the car was called away, the fix is a queueing rule or a second attempt, not hardware.

What to send us if it is still failing

• The robot's log file for one failed run, with timestamps.
• Where exactly it stops: outside, doorway, inside, or destination floor.
• The lift vendor, model and last service date.
• Whether it ever worked, and what changed on the day it stopped.
• A phone photo of the lift interface wiring.

With those five items we can usually name the cause before anyone travels. Roughly half of the lift calls we handle are resolved remotely, which is why we ask for the log before the flight.