As assembly technique, it not only demanded constant physical effort but also caused a lot of noise. Besides there was no way of ensuring and documenting the correct execution of the screw tightening process. Totally new, mobile EC nutrunning stations have now replaced the former nutrunning systems. They not only relieve the worker physically. They also produce a lot less noise and introduce a previously unknown intelligence, which ensures a consistent quality level with a high repetitive accuracy.
Every nutrunning station is designed as a suspended unit, which runs in an aluminium section rail and is driven within the assembly station motor. The feed is synchronised exactly with the movement of the push-pallet so that the workstation automatically follows the position of the workpiece carrier on which the drive unit is positioned. At the end of the line of an assembly station the nutrunning station automatically returns to its home position and is positioned exactly parallel to the next workpiece. The exact position of the nutrunning station is defined by perforated rails running parallel to the carrier section, which supply an absolute path-actual value. The S7-200 of the individual nutrunning stations communicates with the plant PLC over a radio line consisting of a leak wave conductor and radio modems. The path-actual value of the skid conveyor is synchronised with the nutrunning station among other things and the drive is controlled accordingly. The communication between the Siemens frequency converter Sinamics and the scanning head on the perforated rail of the nutrunning station takes place via RS485 interfaces. A manual forward and reverse of the nutrunner unit, which the worker can control with buttons, is also possible within the respective workstation in addition to the fully automatic operating mode. The nutrunning station runs on four rollers. It is driven by a Vulkolan wheel, which is pressed against the carrier rail by a spring. If a resistance of more than 150 N occurs - for example due to overloading or collision - the preliminary drive of the assembly is stopped automatically. The frictional connection between the motor and the drive wheel is made by an electromagnetic clutch, which automatically breaks the connection in the event of a power failure or at the press of a button to stop the screwer station from moving.
Visual user guidance
Every nutrunning station incorporates a PC on which the program for guiding the worker and controlling the screw tightening process. A color LCD with touchscreen forms the user interface. At the beginning of the work process the drive unit to be processed appears on the display (different motor-gear combinations are mounted in mixed mode). As soon as a joint has been made correctly, the color of the screw concerned changes to green. If, on the other hand, the system has registered a NOK status, the joint is marked red and an appropriate warning is displayed. During the tightening process itself several parameters are recorded by the electromechanical nutrunners (for example the torque with which the screw is tightened) and compared with the respective setptions from the screws database. If all the joint of an assembly station have been made correctly, the nutrunning station automatically returns to the home position and is ready for the next drive unit. If one of the joints is faulty, the worker can correct it afterwards providing his work cycle gives him enough time to do so, or he has to acknowledge the end of the work cycle manually.
The data of every single joint are transferred to the screws data server and saved additionally in the PC of the nutrunning station. These data are also transmitted wirelessly by the radio link of radio modem and leak wave conductor. The PC of the nutrunning station receives the ID number of the respective powertrain from the plant PLC for every new work cycle. This tells it among other things which engine-gear combination currently has to be processed and the appropriate user guidance menu is displayed.
Eliminating faults and errors
The Siemens specialists have come up with a special solution for the workstations at which different sized screws are used. One example of this are the joints of the on the gear shaft. The necessary sockets of different sizes are located in a selector box with a LED indicator. The worker is promoted to change the socket at just the right time in the work procedure and a flashing LED ensures that he chooses exactly the right socket. If he takes out the wrong socket, the assembly process is blocked and an error message appears. The further process is also blocked and an error message displayed, if the socket is not put back in the right place in the selector box after screwing. Time-wasting operation errors are therefore largely ruled out and clocking of the assembly line is guaranteed. The final stage in the drive unit assembly is a workstation, which serves to check all the previous work steps and repair any defective joints. The user guidance on the LCD shows all the work steps and highlights those for which a not yet corrected defective screw connection has been registered. Since the majority of screw connections are okay as a rule, the worker at this station has enough time to deal with the unsolved problems. When he taps a workstep marked defective on the touchscreen, the appropriate user guidance menu opens and the worker can see at a glance which joint needs rework.
Fully automatic marriage
The fully assembled powertrain units finally run on a roller conveyor into the adjacent assembly hall, where they are added to the married chassis and body. First they are placed on a new workpiece carrier, on which the whole chassis including rear axle and exhaust are then assembled. In the next step, the chassis components positioned exactly on the workpiece carrier are fit precisely into the body lowered from above and then connected to the chassis in several nutrunning stations. Where numerous manual processes were necessary for connecting the chassis and body in the previous Passat production, which sometimes entailed undergonomic overhead work, this process now runs fully automatically and without manual intervention.
For Siegfried Rossdeutscher, head of production planning assembly, the VW factory in Emden, the production technology for the new Passat is a decisive step towards more ergonomic workplaces and a further increase in precision. In addition, the new nutrunning stations for the powertrain assembly also integrate a continuous quality assurance directly in the manual work process for the first time.
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| Posted : 10/26/2005 |
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