Gripper withdrawal can disturb step-lap tips placed earlier in the stack. Observe those neighbouring features as well as the newly released sheet to qualify the withdrawal motion.
Map the neighbouring features
Identify exposed tips and edges within the tool’s approach and withdrawal envelope. Record their positions immediately before placement, after release and after the tool clears. Register those observations to stationary stack references so that camera movement is not counted as part displacement. [1]
Include the actual stack stage. A tip that is well supported later in the sequence may be unsupported when the robot first passes it. The test must reproduce that vulnerable condition, including the layer geometry and any normal restraint.
Compare paths through their physical effects
| Withdrawal feature | Observation | Mechanism to distinguish |
|---|---|---|
| Near-tip tool clearance | Local tool and tip positions through retraction | Contact or snagging |
| Release transition | Residual attachment and tip response | Suction or incomplete detachment |
| Airflow near the stack | Tip motion with a suitable airflow control | Aerodynamic disturbance |
| Settled stack | Final position compared with the initial state | Persistent displacement versus reversible motion |
Change one path feature at a time where practical. Compare alternative clearance direction or sequence while retaining the same part, release condition and stack state. If tool geometry, speed and airflow all change together, the comparison may select a useful combined route but cannot identify which intervention resolved the disturbance.
Keep transient and permanent movement separate
A tip can vibrate and return, slide into another position or develop residual bending. Retain the motion trace and the settled shape. A final image alone misses a temporary excursion that could contact another component; a transient peak alone does not establish permanent misalignment.
Check measurement visibility throughout the relevant interval. If the tool hides the tip while contact could occur, use another view or a suitable measurement method. Assuming the hidden interval was uneventful would leave the most important part of the test unobserved.
Residual suction, mechanical contact and airflow are candidate disturbance mechanisms. Isolate them experimentally; the robot-metrology source does not establish their amplitude or identify the cause in this cell.
Use catalogue gauge 0.23 mm [2] and an assumed permitted temporary normal motion 0.10 mm during withdrawal. In an assumed commissioning trace, a neighbouring tip lifts by one gauge: 0.23 mm, or 2.3 times the allocation. Reject that path even if final height recovers. Observe the full withdrawal interval and compare release timing, approach/peel direction and support state; lowering only the final tool target does not resolve the transient.
Qualified withdrawal depends on local sheet support, residual vacuum, contact cleanliness and overlap friction. Record these states and inspect joint registration after the gripper has completely cleared the stack.
Select withdrawal speed after confirming neighbouring-tip stability across the intended part families and stack stages. Keep that retained geometry alongside cycle-time results.
References
- G. Qiao and B. A. Weiss, Industrial Robot Accuracy Degradation Monitoring and Quick Health Assessment, Journal of Manufacturing Science and Engineering 141, 2019. DOI: 10.1115/1.4043649.
- Nippon Steel Corporation, Grain-Oriented Electrical Steel Sheets, catalogue D004je, April 2020, specification and typical-property tables.