The almost universal system of dressing doors off the line and reuniting them with the right body requires sophisticated handling and software systems, as seen at BMW’s Dingolfing plant
In recent years, all processes across automotive production have dramatically changed. Arguably, final assembly has changed the most, with an increased number of different models being built on a single line, while customer-driven production requires rapid turnaround of incoming orders. Such changes are only possible by ensuring that facilities are manned by well trained production personnel and that plants are regularly updated.
In one such update, a flexible conveyor and storage system at the BMW plant in Dingolfing, Germany, picks up all doors from the assembly line and reunites them just-in-sequence with the correct body. The driving force behind this new storage and handling system is a software architecture called zenon.
In a production environment such as this, zenon is an integral part of the assembly process. In operation, the software both automates conveyor operation, while delivering a visual representation of the complete system. This is not a standard conveyor system, but a series of elements that comprises a complete transport system, with electric vehicles (EFZ), suspended electric trains (EHB) and lifts.
After painting, the doors are removed from the body shells and taken by lifts to the storage level; the doors for different vehicle types are all stored together. The electric trains then transport the doors for further processing, initially to the flexible store and then, when requested, onto the door fitting station.
At the same time, an electronic ‘overview’ is provided: all doors are labelled (model, chassis number, door position, etc.), and all load-carrying media are provided with RFID tags. The location of any individual door is known at all times. Located at various points in the plant are distributed read/write stations that control and monitor the suspension gear routes; these can be uniquely identified at each point. Users can read out the data, but also change and update it as necessary. Even with this equipment, presenting the doors at the right time and in the right place presents a particularly difficult logistical challenge, and an extremely complex task for the control system.
Random temporary storage – reliable control
The previous solution door kitting solution, with sequential storage before and after door assembly, required an excessive amount of floorspace. After analyzing the existing system, BMW decided to work with control specialist company Staudinger and mechatronics experts from Rofa Rosenheimer Förderanlagen to develop a totally new concept.
The crucial factor behind the success of the new storage system is the sophisticated control technology, which features a number of components from Siemens. Each load carrier on the electric overhead conveyor has an RFID chip. Moby-type read/write stations assign each of these storage locations a comprehensive individual data record to uniquely identify the doors. The advantage of Moby-type RFID technology is that the data can be read and updated at any point when the doors pass a read/write station. Additionally, once the storage locations have been fitted on the suspension racks, they can be reset and rewritten any number of times. This provides the required flexibility to load the racks on the conveyor with doors in any order. Mechanically, the racks can be adjusted quickly and easily for different door models. In conjunction with a total of 122 read/write stations, the racks are directed through the flexible store in different directions and on changing routes using points in the electric conveyor rails. The read/ write units and the points are controlled and monitored by decentral distributors. All 76 of the stations have terminals, some of them Simatic PC 677, for reading and editing system parameters and status. Simatic ET 200pro interface modules integrate the read/write units for the RFID store and the motor controllers for the point drives into the data network.
Complex but clearly visible control processes
A primary objective of the door conveyor project was to implement and visually represent the complex control processes so that the plant manager or maintenance engineer can instantly see the locations of vehicle bodies and doors. The flexible storage and conveyor system has a full graphic display, showing the operator all the strands of conveying and storage. Each box in a storage strand represents one door or a set of two or four doors. The colour provides information about the vehicle type and the status of the door’s manufacture (unfinished door, fully-assembled door, empty vehicle).
In order for the doors to be fitted, a robot transfers them to the suspended electric train for pre-assembly. This process, too, is visualised within zenon. Once the doors have been completed on the dedicated production lines, they are moved back to storage to await the bodies. A sequence list is maintained to keep track of when cars are ready to have their doors fitted. Light-grey boxes in the visualisation are unoccupied spaces in the storage and conveyor system. Before doors pass into the lines, an automated check is carried out to determine whether a line of doors of the same type already exists. If it does, then the door will be stored there in order that separate lines are created for each model. There is also a colour to indicate when doors are re-fitted in the vehicle – like door removal, this is a process that is performed manually. When doors are removed from the storage and conveyor system, the empty hangers return to the door removal station to complete the cycle. The zenon overview display enables operators and maintenance engineers alike to zoom in on any detail of the plant while also keeping the entire plant on the screen. “The overview display is for me one of zenon’s best functions as it hugely increases user-friendliness and enables the plant manager to work efficiently and retain an overview, no matter how complex the plant”, says Markus März, Control Software Designer at Staudinger, responsible for visualisation systems.
Networking and consistency
The basic idea behind networked visualisation is that operators can perform all necessary tasks at any time and from any position, defining parameters, switching units on and retrieving alarms and trend data. Zenon’s consistency, in combination with its unique network technology, opens up various options for integrating and optimising production and information flows.
Staudinger took advantage of this consistency when configuring the visualisation system. The system planners have integrated the standard project specified by BMW and used only the small part of it (the fault reporting system) that they really needed, greatly reducing the cost. Object oriented parameter definition makes system configuration very simple; as soon as an object has been centrally defined, it is available for use throughout the system. Whenever any item is modified, it is handed down quickly, securely and accurately without any further input.
Allowing for all eventualities
The zenon application further enables data sets to be modified, moved or deleted, while it allows the data system to be directly accessed. With the help of the relevant operation the controller can now determine which subsequent processes are necessary. “This entire flexible storage system is very user-friendly, while demonstrating the high level of automation and efficiency made possible by this software”, explains Markus März.
To implement their solution, März and his colleagues had to calculate all possible eventualities in order to illustrate the necessary processes. If a door is included in the wrong conveyor line due to a defect in the routing information, then this must be immediately corrected. The visualisation includes a waste-paper basket that the user can use to throw away any records that are not needed. Using drag-and-drop techniques, the responsible person can then move the door data to the correct position in the system. März: “This entire project is visualised using zenon. With all its technically refined hardware and software solutions, it is a truly state-ofthe- art automation system which not only works extremely reliably, but is also unparalleled in terms of flexibility.”