A Single CCD Station Covers 64 Battery Case Features
The lower case of an EV battery pack is structural: if its assembly is wrong, the pack is at risk. Hence full inspection rather than sampling, and a feature count worth planning around. This part carries more than 64 features to verify: 12 rivet nuts, 12 blind rivets, 18 mounting points, 22 datum points, plus assembly bolts, riveted brackets and machined holes. Density like that defeats visual inspection: dozens of small features, close together, for a whole shift.
Why 8 percent escapes is a feature-density problem
The features are numerous, small and repetitive, so attention decays long before the shift ends, and a missed feature does not announce itself. Such defects rarely fail at once; vibration and impact load them until they do. Once one leaves the plant the cost is a pack rework or a safety event, which is why full inspection has replaced sampling here.
Three kinds of verdict, three imaging designs
The features are not one problem. Assembly bolts, riveted brackets and machined holes are presence checks. Rivet nuts are state checks: pulled or not, tilted or not, fully seated or not. Mounting points and datum points are tolerance checks in millimetres. One camera and one light source cannot serve all three.
- Presence: a clean silhouette in a zone view decides there or not there.
- State: existence, angle and height are read separately, separating a missing nut from a tilted one and from one standing proud of the reference face.
- Tolerance: the feature centre is extracted and compared with a calibrated reference to see whether it falls inside the band.
Zoning turns a large part into small fields of view
Several high-resolution CCD cameras cover the part zone by zone with custom high-brightness illumination. Resolution is chosen per zone between 5 and 20 megapixels, because a rivet nut and a machined hole need very different pixel density. Each field of view then holds only a few features and one verdict type, which lowers the complexity of every frame the algorithm interprets.
An inspection list binds every feature to a camera, a shooting position and a decision rule: any point left uninspected means the part is not released, which turns a feature list into a guarantee.
Telecentric lens and backlight for the tolerance points
Perspective error is the enemy of quantitative imaging. A telecentric lens removes most of it and a high-brightness backlight turns the contour into a sharp boundary, so the measured centre is stable. The two light types divide the work: custom illumination for zone lighting, backlight for contour points, avoiding the glare and shadow that corrupt a decision.
Weight as a second opinion
A transfer mechanism moves the part between inspection and conveying positions so attitude is identical each cycle; a weighing station and MES communication close the loop around it. Weighing checks the whole part: missing bolts or brackets show up as a weight deviation even where a zone image was ambiguous. Vision sees the detail, weighing sees the whole part, MES carries both into production data.
318 seconds, and the 8 to 0.2 percent figure
The full-check cycle is about 318 seconds per part, subject to site acceptance results. The station runs 100 percent inline inspection; escapes moved from roughly 8 percent under manual inspection to below 0.2 percent, and 2 to 3 inspectors per shift were released, saving RMB 300,000 to 500,000 in labour and about RMB 800,000 in quality cost a year, subject to site acceptance results. A new checkpoint adds a shooting position in one zone or a rule in the list, never a new architecture.
Before a 64-feature list is signed off
How are a missing, a tilted and a loose rivet nut told apart?
By three image features: no nut at the hole, contour rotated against the hole axis, and nut face standing above the reference plane.
How is a 64-feature list guaranteed complete?
Every feature is bound to a zone camera and a rule; the part is released only when all points have a verdict, and one missing point marks it abnormal.
Does weighing replace any vision check?
No. It is a whole-part cross-check for gross omissions and triggers a recheck, never a pass.