AIA: Flexible Robot Cells Help Differentiate Car Differentials
ZF Achsgetriebe GmbH (Thyrnau, Germany) is a subsidiary of ZF Passau GmbH. The company develops and manufactures drivetrains.
ZF Achsgetriebe contacted Prowin Produktionstechnologie GmbH to build an automated line for assembling and testing automotive differentials. The assembly line needed to perform continuous process assurance, be flexible with regard to changes in part counts, and be efficient and economical.
The entire system consists of two independent workpiece carrier systems that link the automated and manual stations.
At the washing station, individual parts of the differentials are manually picked and fitted to the workpiece carrier. To provide reliable traceability of all individual parts built into the differentials, parts data are acquired by scanning material tags and stored in a database. The raw parts data, process data and measurement results are referenced to the individual components by means of the workpiece carrier number. At each processing station, the workpiece carrier is identified, and the associated data is read from the database.
Following successful processing at each of the stations, the data set is updated with the processing status and measurement results and written back to the database. The entire history of each acceptable differential is labeled with a serial number and can be retrieved at any time. This includes batch numbers and manufacturer data for the individual parts, process and production data, the plant code, the programs used for the robots, and measurements.
Following successful fitting of the components to the wash workpiece carrier, a camera system is used to automatically check that all parts are present and correctly positioned.
Following this, the workpiece carriers are transported to the in-line washing machine, where various cleaning operations are performed.
The individual parts are transferred from washing to assembly by Stäubli's (Faverges, France) RX six-axis industrial robots.
Using special grippers, the individual parts are removed from the wash workpiece carrier and placed on the assembly workpiece carrier. Besides the handling tasks, the parts are accurately positioned on the carrier by a Stäubli RX130 robot equipped with a laser. A second Stäubli RX90 robot uses a microfog oiling system to oil the parts before they are placed on the workpiece carrier.
When the individual parts are transferred from the wash workpiece carrier to the assembly carrier, the data set for each individual part is copied to the assembly circuit and again assigned to the number of the assembly workpiece carrier. Even if mixed components are placed on the carrier, the data still references the correct final product.
The individual parts are preassembled at the two semiautomated manual workstations in an off-line arrangement. Each manual workstation is equipped with a "trend display," which prompts the worker in making the correct parts selection. In addition, the worker can retrieve all relevant measurement data for the differential.
An additional automated station verifies the results from the manual workstations. This station checks that all of the individual parts are correctly assembled.
Following this, the two preassembled halves of the differential are assembled automatically. During this process, the teeth of the axle bevel gear must correctly mesh with the remainder of the two halves of the differential. To secure the differential, a spring pin is automatically fed to the assembly and pressed in place using force and displacement monitoring.
At this point, all acceptable differentials are transported in-line to a computer-controlled test system and automatically docked to the tester. The characteristics that critically affect the quality of a differential are recorded and evaluated during a test run. Parts that are unacceptable are automatically fed back to a reassembly station. At this station, criteria are displayed for the worker to allow appropriate reassembly. Following rework, the differentials can again be routed to the test stand.
At an automated labeling station, acceptable differentials are labeled using a scratch embosser. A vacuum aspiration system prevents contamination of the differentials during labeling. The differentials are then manually packed.
Because the system requires quality assurance and data storage, a PC-based control concept is used. The individual station controllers consist of industrial PCs, each containing an integrated statistical process control expansion card. Each station has a touch display that provides a convenient user interface for operating the system. All station computers are connected to a central database server. This database system records all current operational data for the facility to allow operational and error analyses to be made rapidly and conveniently from a master office. It is also possible to perform searches in the stored operational data for the facility. The database system also records all of the workpiece data, referenced to serial numbers and production periods, to ensure the reliable traceability of each individual differential produced in the facility. In both systems, the robots are integrated via the station controllers and a ProfiBus interface for each robot. This makes it possible to precisely analyze the operational state of the robots.
For more information on robots, call 864-433-1980 or visit www.staubli-robotics.com.