Adding LiDAR to Hold an AR Overlay on Long Rail Bodies
Rail vehicle body weldments are large and stand on the shop floor. They carry many assembly features, and externally visible errors such as a missing part, a wrong part, a reversed part or an obviously displaced part affect the next operations directly. The old routine had an inspector walking the part with drawings, checking item by item: large part, many features, long duration and easy to miss something. Moving the part to a dedicated inspection area costs money and time.
On a long part, the camera loses itself
The body weldment is long and slender, and a tablet camera easily loses its position during a long walk, so the overlaid view drifts. That is the most prominent difficulty here and the most underestimated: once the overlay drifts, the shop floor cannot tell whether a mismatch on screen is a problem with the part or a problem with the alignment.
LiDAR covers the stretch where image features run out
The answer is a tablet terminal with a LiDAR depth sensor, using depth to strengthen spatial positioning alongside real-time registration of CAD against the live image. That improves tracking stability over a long structure and keeps the overlay reliable while walking.
For selection this is a prerequisite rather than an option. Ask about the depth sensor configuration and long-structure tracking stability: an ordinary tablet tends to lose position partway along a long workpiece.
Four classes of problem appear in the overlay
- A part that should be fitted is missing.
- A part is fitted in the wrong position.
- A part is fitted the wrong way round.
- A part is visibly out of place.
All four are externally visible and qualitatively decidable. The system overlays design CAD onto the live image by spatial position, alignment depending on feature matching plus depth, and the overlay updates as the inspector's viewpoint changes. A problem is marked on the spot and recorded, with the position and the image evidence saved together.
Working region by region keeps the far end in view
Checking advances region by region, covering every exposed assembly feature of the weldment. As the inspector moves the tablet the overlay keeps updating, so each region finishes with a clear conclusion rather than a memory to be written up at the end. On a long part the classic failure is looking at the front and forgetting the back, and zoning suppresses it.
What remains after 620 seconds
A check takes about 620 seconds and produces an archived report, subject to site acceptance results. Compared with reading drawings item by item, this solves two things at once: the design state is presented directly on the live view, so the criterion is more consistent, and the record is electronic, so the problem position, image and conclusion are traceable.
The scheme suits appearance checking of oversized, hard-to-move parts in rail, shipbuilding and similar industries. It is a screening tool, not a measuring machine.
What belongs to a CMM
AR comparison is a fast qualitative appearance check for missing, wrong, reversed or obviously displaced parts; a CMM is high-precision quantitative measurement. The division is clear: screen with AR, confirm doubtful spots with metrology. Putting qualitative judgment on AR and leaving quantitative judgment to metrology is the basic split on parts this size, and it is also the boundary most often confused on site.
Walk-through questions
Why insist on a LiDAR tablet?
Image features alone drift on a long structure. Depth and image features share the positioning task so the overlay stays aligned across a long walk.
How are the regions divided?
Region by region over all exposed assembly features, each finished before the next begins.
Can AR replace a CMM?
No. AR screens qualitatively, a CMM measures quantitatively, and the screening comes first.