eXtended Transport System (XTS) during the optical quality inspection of cylindrical metal parts
For complex component shapes, optical inspection systems typically require aseries of cameras in serial or parallel stations in order to be able to check allpertinent criteria. The Sova-VIS 16 system from Gefasoft Automatisierung undSoftware GmbH in Regensburg, which inspects diffusors for car airbags, is noexception. Since they are safety-critical components of the car airbags, the cupshaped,drawn sheet metal parts, which are produced in various versions withdiameters of approx. 60 millimeters, must be subjected to detailed inspection oftheir interior and exterior surfaces.
XTS compensates for time differences and simplifiessystem expansion
While some inspection tasks require only a single picture, others require sophisticatedlighting systems and multiple images. The interior wall of the diffusor isa particularly critical surface that must be inspected for even the smallest imperfectionswith dimensions of only a few tens of micrometers. To achieve the necessaryimage resolution, line scan cameras are used before the part is trimmed.The trimming process alone takes approximately 2 seconds, which means thatthe image acquisition times vary greatly between the individual stations.
If all camera stations were linked via a rigid conveyor system like a rotaryindexing table, the station with the longest image acquisition time would determinethe shortest cycle time of the entire system and limit the throughput rateaccordingly. With the flexibility of the XTS solution, on the other hand, fasterinspection stations can be served at a higher rate, while slower stations – ofwhich more have been installed – can be supplied with components simultaneously.
Gefasoft marketing manager Georg Schlaffer explains: “With the Sova-VIS16 system, the very heterogeneous imaging times of the individual steps causedus to look for a machine layout with a flexible, asynchronous transport systemthat would enable us to perform the job efficiently in a limited amount of space.
The XTS from Beckhoff with its independently controllable movers meets theserequirements perfectly. Despite the presence of slower inspection stations, wewere able to achieve an average cycle time of 1.9 seconds. The XTS also enabledus to implement this in a footprint that is much smaller than comparable solutionsin order to meet the end customer’s space requirements.”
Engineering director Christian Schärtl adds: “Because of the high demand forthese parts, the end customer specified a cycle of less than 2 seconds. Sincetwo of the inspection steps take almost 4 seconds, these stations had to beduplicated and parallelized in order to meet this goal, and the flexibility of theXTS made that easy. Other solutions with carriers and switches based on beltsor chains would have required much more complex mechanics and would havebeen slower while taking up more space.”
Because of the many camera inspection stations, the total length of the XTSline amounts to approx. 11 meters (36 feet) with 30 movers. Because of therelatively heavy movers, the line was equipped with a special guidance systemmade by Hepco.
The mass of approx. 4 kilograms (8.8 lbs) per mover was neededto accommodate the handling requirements with units for lifting and rotating,because the diffusors must rotate precisely in front of the cameras and be ableto be moved in two layers (“hat” and “cup”) so that their top and bottom canbe inspected.
From the beginning, the goal was to use a single inspection machine to fullyautomate the previously manual process, which ranges from picking up theparts from a pallet to stacking or sorting them via a robot. Distributing theinspection tasks over multiple independent machines was not an economicaloption for the end customer. Besides the complex logistics between the machinesand their increased space requirements, it would also have been tooexpensive to split up the feeding process of the diffusors from bulk containers.Georg Schlaffer points out another significant advantage of the XTS: “With theXTS we can integrate additional processing stations along the line with greatflexibility. This ensures that the system can be adapted to future requirements.
One inspection system for the entire process
After the surface of the components has been degreased, they are suppliedto the inspection machine as bulkware in mesh pallets. A gripper takes theparts from a conveyor and passes them to the XTS. To ensure proper orientationfor the workpiece holder of the XTS mover, the transfer station has acamera that detects the diffusor’s positional angle as well as its type. Eachdiffusor has a batch number embossed in its top, which is checked in thefirst camera station.
After detecting the batch ID, the system checks the interior of the diffusor, whichrequires it to be rotated in the mover from a “hat” to a “cup” position, i.e. withthe opening pointing upward. To do this, the parts are picked up from the moverwith a gripper, rotated 180 degrees and placed back onto the mover. In themeantime, the internal mandrel was withdrawn so that the part is centered andresting on its flange. To inspect the surface of the part’s interior, it is then rotatedin front of a line scan camera. Christian Schärtl explains: “What turned out tobe a major challenge was the limited space, because the camera and four lightshad to fit inside the part, which only has an interior diameter of approx. 60 mm.
To resolve this, we had to design an integrated lighting and camera unit, whichdips into the part while a servomotor docks from the outside to the rotatablemandrel in the mover. In addition, a fast controller had to be developed for the lighting system. Moving the lighting unit into the component and rotating ittakes a little over 2 seconds. To achieve the line cycle time of 1.9 seconds, weinstalled two of these inspection stations.
”The rotating stations are followed by a control station for material deviationsand a station for inspecting the base of the diffusor. The last camera station ofthe first linear XTS section checks the flange for flaws. This means that the firstlinear section holds six camera stations. With duplicate line scan and rotatingstations, this brings the total to nine positions.
After the movers have passed through the 180-degree curve of the XTS, morecamera stations as well as a printing station for marking the diffusors follow onthe other side. The parts are flipped from “cup” to “hat” orientation before twoparallel line scan camera stations check their exterior surface. This is followedby the inspection of the upper side of the flange, a camera-controlled inkjetprinter for marking faulty components that is synchronized with the motion ofthe XTS movers, and a laser for marking the good parts. At the end of the secondlinear XTS section, a fast Stäubli TP80 robot picks up the parts from the moverfor further processing.
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