System integration: Flexible assembly system delivers in-sequence automotive cooling modules
Pre-assembly of components leads to flexible, accurate assembly of multiple combinations sequentially. RFID helps information flow. Machine vision guides robots that check quality. PLCs provide control, from order to shipping.
Manufacturing air conditioning and engine cooling systems for passenger and commercial vehicles accurately, in the proper sequence is a priority for Behr GmbH & Co. KG, of Stuttgart, Germany. The company produces automobile and truck engine cooling systems in its Charleston, SC, facility, using programmable logic controllers (PLCs), radio frequency identification (RFID) tags, and machine-vision-guided robotics to check quality.
One of Behr’s customers in the truck industry asked that assembled cooling modules be provided to its final assembly plant locations. The customer also requested that Behr supply these completed cooling modules just-in-sequence (JIS), meaning that each module would be custom built in sequence for a particular truck, with Behr’s line operators pre-assembling the correct components on each module.
To satisfy customer requirements, Behr designed a module assembly line capable of building any cooling module with any set of components at any time. Behr turned to system integrator Control Infotech for help designing and installing the PLC control system required to run the assembly line and ensure that the line operators could completely and accurately build, inspect, and load the cooling modules into shipping containers.
Moving materials, information
With their new flexible assembly system, Behr can electronically accept information for each build order directly from the customer and transmit it to the PLC system for execution. Once a module is finished, the PLC system sends a data telegram with all applicable manufacturing and test data back to the Behr mainframe.
The PLC control system is responsible for coordinating the operators’ efforts at the various assembly, test, and packing stations as well as for moving materials between stations by means of carriers attached to an overhead conveyor. After leaving the last assembly area, the carriers move into a robotic vision inspection area where the completed modules are inspected prior to leak testing and packing.
A Rockwell Automation ControlLogix 5500 coordinates the cooling module assembly line conveyor controls, data manipulation, work instructions, parts verification, fastening control, and fastening verification.
The PLC system controls the overhead conveyor system via a series of proximity switches, limit switches, and solenoids. The PLC receives a data telegram consisting of all the required parts, assembly, and build information provided by the Behr mainframe via an Ethernet connection.
The PLC then forwards the data telegram to the individual carriers via an RFID network that uses antennas communicating with the PLC via EtherNet/IP, an ODVA Common Industrial Protocol (CIP) using the Ethernet physical layer. The PLC initializes the RFID tag for each carrier before it enters the assembly area then reads the tag as the carrier passes through each assembly station’s entry and exit points.
The PLC also records the build data for each module at each station, including the carrier identity, the ID of the operator assigned to that station, and a station function result. The station function result indicates whether or not build data was successfully captured from the RFID tag, the work-complete button was pressed, a module was rejected by an operator, no work was required at this particular assembly station, or required work was not completed because of an error at a previous station. This data is subsequently uploaded to the Behr mainframe as the module is packed at the packing station.
Everything in place: equipment, information
Each assembly station is equipped with all the equipment and information that the line operators need to build the cooling modules:
• An operator-build console with a touch screen monitor for displaying work instructions and system messages and for inputting operator information;
• A cordless barcode scanner for verifying the parts being assembled;
• Work-complete, emergency-stop, and module-reject buttons; and
• An electric torque screwdriver (gun) located at stations where fasteners are required.
A cooling module enters an assembly station where line operators attach parts as directed by the work instructions automatically displayed on the operator console.
All are interfaced to the PLC system so that when a carrier enters an assembly station and is identified via its RFID tag, the PLC can display the appropriate part assembly instructions and direct the operator to the correct bin for parts. The PLC also reads the screw program information out of the RFID tag and properly arms the torque gun based on the required torque and angle characteristics and the required number of screws.
The PLC also performs a series of cross-checks to verify that the correct parts are being correctly assembled at each station. The first cross-check verifies that the proper part was selected and scanned by the operator using the station barcode scanner. If not, a red screen LED lights beside the work instructions.
Another cross-check verifies the number of screw operations that have been performed with the torque gun. The final cross-check verifies that the operator has acknowledged that the work is complete by pressing the work-complete button on the operator console. If the required barcode verifications, screw count, and operator acknowledgement are not all received by the PLC by the time the module reaches the end of the assembly station, the carrier is not allowed to proceed to the next station.
Inline quality control via robot-guided machine vision
A series of quality tests are performed once a module has passed through all eight assembly stations. A rework loop and a rework station accommodate any rework required to solve any assembly quality issues.
A robot-guided vision system visually inspects up to 25 points on each side of each module and returns good or bad results to the PLC. To accommodate varying module configurations, the specific points to be inspected on a particular module are read from the build data previously downloaded to the carrier’s RFID tag. Passed and failed images are displayed on a 17-in. touch monitor. Failed images are also saved for display to the rework operator who is required to view each image and either accept it as a true reject or mark it as a false reject resulting from a vision system error.
Front and rear vision inspection stations using Fanuc robots and Cognex cameras verify that all of the components have been properly positioned and fastened onto the completed cooling module assembly.
Leak test stations check for leaks in the radiators and associated hoses or the condensers and associated hoses. The PLC reads the leak test requirements for the current module from the carrier’s RFID tag. Due to the flexibility of the line, the PLC has multiple programs available for leak-testing the various cooling module assembly radiators and condensers.
The station operator is required to physically connect the testing hoses as the module comes into the station. As the carrier exits the leak test station, test data is downloaded to the carrier’s RFID tag.
RFID tag provides information for packing
If a module passes the visual inspection and leak tests, the carrier moves to the packing station. As the module enters the packing station, the PLC reads the carrier’s RFID tag and displays the proper shipping container and the module’s proper placement within the shipping container. This allows the correct ordering to be maintained as the finished modules are shipped to the customer. Shipping containers stay on the packing operator’s screen until they are full. Multiple shipping containers can be shown on the screen simultaneously.
Area: 200,000 ft
Once a module is ready for packing, the PLC sends the module’s complete assembly and test history up to the Behr mainframe which returns a shipping label for the module. The label is printed at the packing station and is applied to the module as it is loaded into the appropriate shipping container. Empty carriers are then sent back to the beginning of the line to be re-initialized with a new module order.
Benefits: any product, any order, one PLC-based system
The flexibility of its new PLC-based assembly and work instruction system allows Behr to build any model of cooling module in any order as specified by the customer. The material and information-handling chores are all coordinated by one PLC-based automation system responsible for all operations from order entry through shipping. A separate manufacturing execution system (MES) is not required.
Data tracking has also been eliminated by distributing all work instructions and test data to the carriers themselves rather than a virtual information network. Synchronizing product information with a specific cooling module is greatly simplified.
A central repository for work instructions and illustrations also allows Behr to update its module assembly information for the entire line from a central location. Behr can now provide its customers with just-in-sequence manufacturing services that would not otherwise be possible.
Vance VanDoren, Ph.D., P.E., is consulting editor for Control Engineering ; Oliver Weise is with Behr Heat Transfer, Charleston, SC.
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