In the past, freshly welded components for exhaust gas systems for a major automation supplier were measured in a tactile measuring machine and tested for leaks in a separate, manual inspection gauge. A fully automatic system from Ziemann & Urban now performs this task during the cycle time of the welding robot – and thus more than 20 times faster than previously. This is made possible by a Motoman robot from Yaskawa, equipped with a stereo camera head.
Welded components for vehicle exhaust gas systems must satisfy stringent requirements in terms of dimensional accuracy and gas tightness. Quality assurance in this field is correspondingly important – and often time-consuming. The complete measurement of all required parameters calls for a measurement laboratory with a tactile measuring machine and takes around 15 minutes.
The fully automatic inspection solution developed and implemented for a supplier to one of the world’s largest car manufacturers by system supplier Ziemann & Urban GmbH Prüf- und Automatisierungstechnik, based in Moosinning near Munich, works with the same degree of precision, but much faster. Within a matter of seconds, it identifies the component by means of a laser-scanned data matrix code, performs a 3D measurement of the geometrical characteristics in the vehicle coordinate system and carries out a leak test, even for components with complex shapes. The inspection can thus be completed within the cycle time of the welding robot.
The system is a fully integrated and self-contained special inspection machine. It performs the 100-percent measurement of the components to correspond exactly with the production cycle of the robotic welding equipment, i.e. with a cycle time of approximately 40 seconds per component. The inspection booth is designed as a solid welded steel frame with service doors and thus suitable for use in the harsh conditions of an automated welding shop in 3-shift operation. The components to be inspected pass through this inspection cell on a conveyor system with a total of ten component-specific workpiece carriers. For the purpose of traceability these are fitted with RFID tags that can be read at each inspection position. The workpiece carriers are lifted off in a defined manner at two inspection positions.
3D measurement with robot-guided camera head
The 3D measurement is carried out at the first position by a stereo camera head. The system combines two high-resolution GigE cameras with LED ring lights that can be flashed separately for incident illumination and a class 2M cross line laser. Each of the two cameras – calibrated to each other – takes a picture of the same feature.
A 6-axis Motoman MH5 LF robot from Yaskawa ensures that the cameras can reliably reach every point on the component: installed in an inverted position, the extremely flexible jointed-arm robot rotates completely around the component. It approaches a total of over 20 specified features of the component such as sockets, flanges and holder plates, by point-to-point motions to enable the corresponding measurement points to be recorded by the cameras. The measurement resolution is 0.05 mm (50 µm).
A reference measurement is carried out before each measuring operation to ensure absolute positioning accuracy of the robot. In addition, the operator can quickly and easily verify at any time that the system is fully functional, for example after a change of shift or maintenance. A series part firmly screwed to a workpiece carrier serves as a reference point. The latter must be loaded manually and is automatically detected by the robot. For calibration purposes, the reference component has a separate table of values with stricter tolerances.
The ZU-Vision image processing software from Ziemann & Urban uses the customer’s 3D vehicle coordinate system as a basis for each measuring point. The component-specific system points, the so-called reference point system (RPS), are virtually calculated back to the nominal position. The system uses the results of this transformation for automatic correction of the remaining measurement points. This means that the components can be positioned relatively imprecisely on the workpiece carriers without affecting the measurement accuracy. This greatly simplifies loading of the system, which is currently carried out manually.
Leak-testing and visualization
Leak-testing of large-volume components is performed at the second inspection position. In the first step, all flanges and connections are sealed with cylinders. Each component requires ten of them – an unusually high number. A calibrated laminar flow meter then checks the component for possible leaks using compressed air. Faulty parts are automatically marked with a printed “Rework” label specifying the nature of the fault. These NOK parts are subsequently ejected separately at a defined position. OK parts are automatically transported on the workpiece carriers to the unloading position and transferred to the next processing stage.
For statistical evaluation and complete traceability of the individual parts, the measurement results for every component are stored as separate files in the customer’s CAQ system. In addition to the component number and ID of the workpiece carrier on which the component was inspected, the file also contains all nominal dimensions, actual dimensions and tolerances of the measured values.
As measuring equipment, the inspection system is subjected to a measurement system analysis (MSA) for machine capability (measurement system capability index Cg/Cgk) and repeatability (%RR/GRR). All calibration operations are fully automated. Just like other sensitive and function-critical setting steps, they are protected against unauthorized access by means of multi-level user management.
In the course of cyclical measurement and test equipment monitoring, after every component the system performs a reference measurement at the home position of the robot. This ensures self-monitoring of the system. The measurement results supplied are thus verified. Deviations, e.g. resulting from a robot crash, loosening of a camera/lens or other mechanical deviations, are detected immediately. The system issues a warning and stops automatically.
PC-based control and visualization functions are carried out by the ZU-Control software on a 19" industrial computer with an uninterruptible power supply (UPS). The system is operated using a swivel-mounted 23" touch screen control panel. In addition to system visualization, a measurement value history can be retrieved on the touch screen. The individual system parts communicate with each other using EtherCAT and TCP/IP.
Summary and outlook
With a fully automatic inspection system for welded components, Ziemann & Urban GmbH Prüf- und Automatisierungstechnik offers a much faster alternative to conventional, tactile component inspection. A flexible Yaskawa Motoman handling robot forms an integral part of the solution. It ensures that the camera head can reliably reach all of the more than 20 measuring points. The system has already been designed to accommodate future modifications and expansions: with its simple workpiece carriers and the versatile robot, the concept offers great flexibility for future component variants. For new or modified features, new points can be added to, or modified in, the robot program and image processing system with little effort. Furthermore, the system is also already equipped for automatic loading by robots in the event of increasing batch sizes.
