Physicist's Notes · Moving Laser Positioning

Six-step verification, four drift patterns, annual recalibration cycle.

Motorised position-reproducibility verification protocol — six steps that the physicist runs annually to confirm the moving laser system still arrives at the same coordinates within ≤ 1 mm. This post walks the protocol, the four drift patterns to watch for, and the Saxsons annual recalibration service cycle.

Six-step verification protocol

Annual ≤ 1 mm reproducibility check

1

Pick test positions

Choose at least 5 test positions across the rail travel — extremes + centre + 2 mid-positions.

Cover the full operational range; drift typically shows at the rail extremes first.
2

Drive to position

Operator enters the coordinates; motorised drive moves the laser carriage to the position; record arrival.

Time the arrival; slow arrival can indicate worn drive belts or bearing drag.
3

Verify cross-hair

Place a precision target at the expected cross-hair position; measure laser-vs-target offset with a micrometre or precision rule.

Use a thermally stable target (Invar or aluminium); plastic targets warp under ambient temperature changes.
4

Drive away and return

Drive the laser away to a different position, then return to the original position. Measure the offset again.

Round-trip reproducibility — a single drive-to test misses the failure mode where drift accumulates on return trips.
5

Compare against tolerance

Offset must be ≤ 1 mm at each test position, for both drive-to and round-trip. Document the worst-case offset.

The worst-case position is the lifecycle-replacement trigger when it exceeds the tolerance twice in successive annual cycles.
6

Document and file

Record positions, measured offsets, pass/fail and annotate any failed position. Sign and file in the annual QA dossier.

AERB inspection reads the dossier; document the protocol so the inspector can verify the result is repeatable.

Four drift patterns

Diagnosing the root cause

Single-axis drift on one rail: typically a worn drive belt or shifted encoder coupling on that rail. Saxsons service-visit replaces the belt; recalibrates the encoder.
Bilateral lateral drift (both left + right rails drift equally): typically the room reference frame has shifted — recheck the CT-simulator isocentre before assuming the lasers drifted.
Sagittal-only drift (ceiling laser drifts, laterals stay): typically the ceiling-rail mount has shifted, often from HVAC vibration. Re-anchor the ceiling rail; recalibrate.
Drift increases over the rail length (small at centre, large at extremes): typically encoder-count drift — the motorised drive thinks it has moved further than it actually has. Saxsons service recalibrates the encoder.