The automotive industry is putting great effort into surface quality assurance of body components and bodies-in-white. Experienced inspectors find defects on the surfaces, like dents, bumps, sink marks, waviness, constrictions and cracks using their eyes, hands or a whetstone, and classify these according to their relevance for the paint. Problems are posed by those defects in particular that are hardly recognizable on the sheet metal surface and, after further work steps like cathode immersion coating and painting, become visible and thus customer relevant. Also, any judgment of a surface is subjective, the auditing grade varying with each auditor. There is a potential cost reduction, if relevant surface defects are identified as early as possible and objectively. Daimler AG at their Sindelfingen plant are using the ABIS II surface inspection system as a central auditing station.

Having become operational in November 2008 in the press plant of Daimler Mercedes-Benz Cars (Sindelfingen), the optical surface inspection system ABIS II has been used (fig. 1), which shows the highest degree of automation and the greatest component flow, compared to other plants at other automobile manufacturers. During the inspection sequence (see fig. 2), a component is placed on the conveyor belt (1), enters the inspection cell (2), and, after its orientation being captured by the single camera above the belt and after the adjustment of the robot path, is inspected by the ABIS II surface inspection system, to be transported out the inspection cell on the exit belt. Near the exit belt, the surface inspection result is shown on a large visualizing screen (fig. 3) Whether the sheet metal part is okay, or needs re-working, or is waste, is shown to the person taking the sheet metal parts off the exit belt by the report on the visualizing screen. With the classic manual auditing, it takes at least 15 minutes to inspect a complete side frame; using the ABIS II system, this task will be finished in less than two minutes.
The core of the system is the ABIS II optical surface inspection system. ABIS stands for Automatic Body Inspection System, being a system specifically developed to meet the requirements of an industrial environment. The sensor concept is based on the so-called projected-fringes technology, projecting a periodic grid onto the object, and the stripe pattern being registered by a camera placed perpendicularly to the surface. Applying the one-frame technology, with an extremely short exposure time of 0.1 ms, guarantees negligibility of vibration and the surrounding light typically encountered during manufacturing in factory buildings (e.g. press and body plant). Also, there are no moving parts in the sensor, ensuring high acceleration without loss of calibration stability. Integrated classification of surface defects is a decisive part of the data processing, to allow the application of quality criteria during the subsequent decision processes. Analysis system software of ABIS allows defining defect features with their resulting auditing values, which can be set by the auditors according to the customer-specific product quality. This, as not every surface defect found is customer-relevant and needs to be re-worked.

The user of the ABIS II ­automated surface inspection system at the press plant in the Sindelfingen plant of Daimler use it for several types of tasks:
- Imaging the agreed-upon surface limit patterns, thereby installing a central control station with objective criteria.
- In case of discussions about problematic spots on a component, there can be found an objective assessment of the quality by individual checks.
- When operating the system automatically, grading of individual press lots can be carried out in an economic way. Lots can be between 100 and 2,000 parts. There is sufficient space around the plant for load carriers and fork lift traffic.
- The surface inspection system, used in random sampling, is integrated into the press plant quality control circuits and thus will facilitate feed-back on the surface quality after a tool change.

The concept of the system, as now operating with Daimler at Sindelfingen, was created in close co-operation ­between the planners and operators at Daimler and specialists of Steinbichler Optotechnik. The first layout plans (fig. 4) included an overhead door, while at a later stage a sectional gate was decided on, in order to simplify servicing of the orientation capturing camera above the conveyor belt. Using robot simulation the optimal robot position, i. e. distance to conveyor belt and pillar height, was established, so that the sensor could be positioned for capturing even the largest possible parts (side frames) in all positions on the belt.
A special feature of the system at Daimler is an optional extension to inspect parts of complete bodies. For this purpose, the gate will
be opened, the body-in-white will be positioned in front of the robot, and the ABIS II sensor, in a secure mode, will be positioned by the robot near the parts to be inspected. This additional mode facilitates inspecting those surfaces that cannot be placed onto the belt as a component after assembly. Side frames, fenders and roofs can, in an assembled state only be inspected on the complete body. The reason for this is the requirement to examine and follow-up surface quality even after the individual process steps, i.e. component, assembled state, cathode immersion coating. Experience has shown that defects appearing to necessitate re-working on a component, sometimes lose their relevance during subsequent processes. But the opposite (particularly in later visible areas) has also been known: surface defects, initially not rework-relevant, have become clearly visible after cathode immersion coating.
The ABIS II surface inspection system installed in the press plant of Daimler at Sindelfingen presently offers the highest degree of automation, the greatest flexibility and work-flow capacity. The requests made by and requirements of Messrs. Dr. Frank Weber, Center Press Plant Manager, and Christoph Schlott, Manager Press Plant Quality Assurance, were implemented in both design and construction of the machine. For Steinbichler Optotechnik GmbH at Neubeuern, supplier (and general contractor) of the machine installation, the customer's satisfaction is a welcome confirmation. As expressed in Schlott's words, "The concept of the machine installation offers exactly what we wished for."