There are three main types of inspection that occur in the factory today: (1) Offline – First article inspection; (2) At-line – Random or reject sampling inspection; and (3) Inline – 100% part inspection. Of the three, 100% part inspection (a.k.a. 100% quality control) is the ultimate goal for manufacturers because it means every single part on the assembly line is inspected, and either verified to be acceptable or rejected.
Inline inspection is usually accomplished using a laser line profiler scanning moving parts on a conveyor (see image below).
Offline: First-Article Inspection
When a machine vision system is slow, manufacturers are forced to use offline inspection methods. The intent with offline inspection is to verify first articles are manufactured correctly and assume that production equipment will behave within tolerances for long production periods. This method takes the first-article as a representation of the whole, and is essentially blind to potential quality issues caused by dynamic process wear and tear on the production line.
At-line: Random or Reject Sampling
Using a faster inspection solution allows engineers to perform random spot checks on the production floor. At-line processes involve isolating random parts from the production line for inspection. In addition, at-line inspection is used to review rejected parts at a dedicated at-line measuring station in order to identify what step in the production process is faulty. While it cannot provide 100% quality control, at-line inspection detects quality issues during production and allows for the quarantine and rework of parts before they leave the factory.
Inline: 100% Part Inspection
100% part inspection becomes possible once inspection methods can achieve scan rates that match production speeds. Full automation can then be achieved with production optimized to minimize rework and error-proofing in place to monitor factory equipment performance.
Laser Triangulation and Structured Light
Today’s most effective inline inspection solutions leverage 3D sensor technologies such as laser triangulation or structured light (fringe projection). Both of these technologies offer non-contact scanning of parts and generate the high-resolution 3D scans required for feature measurement and verification.
The Advantages of Using 3D
2D vision alone cannot achieve 100% quality control because it does not scan a part for geometry. Critical features related to shape are missed as a result. In summary, 3D offers:
• Volumetric measurement (X,Y, and Z-axis) provides shape and position related parameters
• Contrast invariant, ideal for inspecting low contrast objects
• Immune to minor lighting variation or ambient light
• Higher repeatability due to integrated optics, lighting, and pre-calibration
• Simpler to build multi-sensor setups for large object inspection
Stay tuned for part 2 coming soon!
In the meantime we invite you to download our Inline Metrology white paper. This paper explores the differences between lab-based metrology and industrial inline inspection solutions—and how 3D smart sensors effectively combine these approaches to deliver 100% quality control through inline metrology.