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| Balda AG develops and manufactures components and assemblies for high tech complex plastic parts. The main target markets are future growth industries such as Infocom, Automotive and Medical. One of the core products are casings for mobile telephones that are produced for nearly all major mobile phone OEM customers such as Siemens, Nokia, Sony Ericsson etc. The increasing demand from OEM’s for high quality products combined with shorter development cycles requires the usage of the most modern inspection technology. Therefore the Balda plant at Bad Oeynhausen (Germany) invested in a 3D laser scanner based inspection system to monitor the quality of the products. |
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Balda has installed a Metris LC15 scanner system integrated on a Werth CMM to verify the mobile phone casings. The main application of the system is first article inspection and troubleshooting on assembly issues. Examples of problems that may occur during assembly are misalignment of holes in the casings, or rear and front parts of the casings that won’t fit together to required standards. These problems are mainly due to molding process faults, such as shrinkage or flow problems. Typically a complete casing exists of about 16 subassemblies that all need to be inspected. Beside the inspection for mobile phone first article check off, the system is also used to inspect samples of the production series and medical products.
A CMM based laser scanning system
Before the current inspection system was installed, the dimensional verification was performed using a Werth coordinate measuring machine (CMM) equipped with a tactile probe, point laser and vision probe. Due to the speed limitations of tactile probing, a fully detailed dimensional verification of the free-form casings was very time consuming. Furthermore, tactile probing can cause deformations or deflections of the injection molded PC / ABS casings or other flexible items such as small rubber gaskets.
To overcome these limitations, Balda installed a Metris LC15 laser scanning system, to be used in conjunction with the classic tactile probing method. The non-contact laser scanning is a much faster process than touch probing and has the advantage that possible deformations due to touching are avoided.
A dedicated laser scanner for high detail scanning is developed in co-operation with Balda. The LC15 has a field of view of 15mm by 15mm, with a maximum resolution of 20μm, and point accuracy of ± 8μm. The data is collected by triangulation using a laser light source and a CCD camera for detecting the reflected beam.
The data is transferred to the PC through the Renishaw Multiwire that is integrated in the CMM. The actual verification of the scanned point cloud data against the CAD model requires dedicated dimensional inspection software that can process large point clouds and import the CAD system’s format. Since verification projects may occur multiple times per week, an important requirement for the system is the time performance of the complete system at Balda.
The scanning process
The first step in the verification process is setting up the laser scanning system. The system is able to scan most materials and surface colors by varying the laser intensity. A typical problem rises when transparent casings need to be scanned. These parts require the user to spray the part with a thin white layer. The part can be positioned on the CMM table without a fixture, but generally a dedicated clamping tool is used.
Scanning the inside of a casing is performed by the use of reference spheres. The point clouds of both sides are then merged based on the reference spheres.
The scanning software allows the inspection operator to easily qualify the PH10M orientations that correspond to the different angles and views to scan the part. For every orientation, the quality operator specifies the path for the scan: the start and end of the scan, the width of the area, and the minimum overlap between different passes determine the scanning area. To capture every detail of the casing, the operators often use more than 50 different orientations. The inspection engineer also specifies the interscanline distance by setting the desired CMM speed. A casing is scanned in about 2 hours. The main advantage of the 3D scanning is that all part information is available for future analysis. When inspecting using a touch trigger probe, it is often the “value between two measured points” that is needed, but where no data is available.
Recording the different paths into a macro and running it on similar parts automate the complete scanning workflow.
Verification of the digital model against the reference CAD Model
The scan result is a dense digital copy of the physical part, represented by a point cloud. The point cloud often exists of millions of points and is therefore pre-processed before the actual inspection. First, points that are not part of the model are cut from the pointcloud. The scanning area may contain parts of the scanning environment, such as the CMM table or fixture. Next, scatter reducing and 3D curvature dependent filtering is applied to obtain a reduced pointcloud that still accurately describes the casings. Mostly a reduction factor of 2 to 5 is obtained.
The processed digital model is then aligned against the reference CAD model. Two methods are basically used. In N-points alignment the point cloud and CAD model are aligned by selecting points in corresponding areas in both models. This free form alignment can be refined in a second step by using a best-fit alignment, where the overall distances between part and CAD are optimized. The inspection software also offers a Reference Positioning System for feature-based registration, especially useful when reference spheres are available. This way the operator can immediately measure in the CAD coordinate system.
Next, the pointcloud is compared to the CAD model. The inspection operator can input the tolerances specified by the designer. At Balda these tolerances are typically specified in the range of –0.3 to 0.3 mm for global verification and smaller for detail verification. The result is a full deviation analysis color plot of the part. The verification engineer can indicate the hotspots using customizable deviation fly-outs.
Beside the complete pointcloud, also sections in all directions can be analyzed and reported. Alternatively, the inspection software can be used as a virtual CMM to check features on the digital copy of the physical model, e.g. the distance between two holes.
The reports are typically sent electronically to the verification engineers who accept or reject the incoming product batch and liaise with the mold makers, providing them with a detailed analysis of the inspection. The data is also sent to the mobile phone OEMs who use it for further inspection using the same Metris software.
The future of 3D digitizing at Balda
As Balda is proud to use this high technology inspection system, they also promote laser scanning as a value-adding step in the quality process. Currently Balda is already expanding the use of the Metris laser scanner to other application domains. Another application where the Metris scanner is used is the inspection of medical parts, such as the injectors for diabetics. Typically these pen injectors consist of more than 15 parts with critical assembly constraints. Good designs combined with carefully inspected first article products reduce the risk of possible assembly and operating errors.
Conclusion
The Metris laser scanner based inspection system enables Balda to get a detailed overall view of dimensional errors in first article mobile telephone casings in a very short time. The parts are digitized using a laser scanner that captures about 20.000 points per second. The dense digital copy of the physical part is aligned and compared to the reference CAD model to detect and analyze the problem areas in a very short time. The pointcloud model and reports are delivered to the mobile phone OEM as part of an improved quality process, to prevent in early stage, any possible error.
Download: Balda AG performs 3D laser inspection of mobile phones (PDF file)
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