At the GF6 six-speed, front-wheel transmission line at General Motors Powertrain in Toledo, OH, a new front-wheel-drive transmission line for smaller, more fuel-efficient vehicles such as the Chevy Malibu and Chevy Cruze is currently ramping up to its initial goal of 2,200 units per day. A closer look reveals the method used to program this line, implement changeover, stage the workpiece flow, perform all machining and secondary operations, and assemble the finished transmissions.
During the development of this line, GM engineering contacted its controls suppliers to investigate ways to significantly reduce the workflow through the line, as well as enable faster changeover, reduce reprogramming, and prevent situations where one out-of-spec machine could cause complete shutdown. Another key driver in the development of the GF6 line was the need to minimize maintenance time by installing programmable logic controllers (PLCs), drives, and component pallet recognition devices outside the conventional cabinetry found on traditional assembly lines. In addition, controllers were distributed throughout the system, which allowed for removal of typical zone controllers and, in turn, substantially increased system flexibility.
Following investigation into the process for the new line, the major obstacle remained changeover and the need for a more flexible yet highly automated system of transmission builds. In consultation with the controls provider on the existing six-speed, rear-wheel-drive line; Siemens Automotive Center of Competence (Troy, MI); and third-party software provider Elite Engineering (Rochester Hills, MI), a deterministic study was undertaken and the result was the line now in production. Siemens provided the PLC, CNC, human-machine interface (HMI), RFID, and its high-level Ethernet protocol, Profinet, to run on the GM network. Overlaying this hardware and communications topology, Elite Engineering delivered its Flexible Assembly Configuration System (FACS), complemented by Siemens to create its SIFACS solution, wherein all the control elements for every assembly operation and test station would be fully integrated. SIFACS largely focuses on the integration of the core PLC software blocks and functionalities of the individual stations with the RFID tags on each of the workpiece pallets, according Jim Remski, manager of powertrain activities for Siemens.
The result of this collaboration was the Transline HMI Lite CE package that provides uniform user interface for operational and diagnostic functions on the vast majority of the various machine tools, transfer lines, robotics, assembly machines, sensing devices, and vision systems throughout the facility. As Michael Grass, project manager for Siemens, explained, “The best part is that the package can be customized to meet specific user needs and preferences. It provides our SIFACS system of configurable assembly automation very useful information, as the two systems complement each other quite well.”
RFID Gets Things Started
As a workpiece proceeds through the line, having been delivered by an automated guided vehicle (AGV) in most cases, each pallet is equipped with an RFID tag. “The key here is the data throughput in the system, as it directly impacts the cycle time or takt time (maximum allowable time to produce one finished part or product) of the line,” explained Reinhold Niesing, engineering manager for the project at Siemens. “The tags must be able to function in static mode, whereby the data on the part must be read before the process begins. Model number, serial number, and build status information are all contained in the tag. The faster we read the information, the faster the process begins.” In the dynamic mode of operation for this RFID system, the information at subsequent line stations must be read “on the fly” without any line stoppage, as is often seen in conventional packaging, shipping, or other line applications for RFID. In this case, all data are read as the tag passes by the antenna.
Often, in less sophisticated applications, the signal can degrade over time and number of reads. Here, two interface protocols are supported: ISO 15693 (open standard) and a proprietary Siemens-developed standard, Simatic RF300. The latter uses a state-of-the-art chip paired with highly optimized communications to achieve the faster data read/write rates. Large amounts of data (64kB) are handled in faster cycle times, while the overall RFID solution is applied in a high-speed, nonstop environment. One of the key drivers in the system is the fact that each RFID tag has both EEPROM and FRAM. The 20-byte EEPROM is actually designed to be a one-time programmable memory chip (OTP), a security feature that was deemed most desirable by GM for this application. Meanwhile, the FRAM can be written and rewritten many times for optimum utilization of the hardware over time. Despite this level of sophistication in the RFID hardware, the system easily communicates over the existing Profinet, Profibus, and other common protocols.
Logic Blocks All Around
The overall thrust of the line development, according to George Jewell, the GM engineer responsible for the implementation of the FACS online at the Toledo plant, was to have consistent, even identical logic blocks at every station. This would allow, as is seminal to the FACS architecture, immediate successive modifications to be made in the machine or assembly operations performed, throughout all stages of the line. When rebalancing was needed, when an upturn/downturn in current production was required, or when an entirely new model came onto the line, the changeover needed to happen in hours, rather than weeks, as was the industry norm. From the utility perspective, the run-time component in the system would function without the full configuration system being online, further complementing a decentralized architecture.
Currently, GM uses the FACS at various plants in Mexico, China, India, Thailand, Korea, and the U.S. — and soon in Canada and Eastern Europe — for the production of transmissions, engines, and even the generator on the Chevy Volt. These products can be manufactured, assembled, and tested all within the same flexible control architecture, while supporting standardized GM processes.
Typical Station Dynamics
All manual workstations on this line have the same download received to a PLC, provided by Siemens in its Simatic lines. While not reliant on the server network in a deterministic mode, the manual stations nonetheless utilize the same software to execute quick tooling changes, machine sequence variations, line balancing, and report tracking. Operators received training from both Siemens and Elite Engineering personnel for these tasks. All part build histories, troubleshooting, and machine debugging are recorded for further analysis.
Throughout the metal-cutting process — mostly in the gear and spline forming, hobbing, grinding, and finishing — CNC technology is onboard dozens of machine tools. Most of the machines are controlled by Sinumerik® 840D, the highest-level CNC offered by Siemens. The control not only processes the particular part dimensions in the cutting theater of the machine, but also coordinates all motion control and movements into and out of the machine. Working in tandem with the other hardware and communication network software in the line, for example, ring gears cut on a Wera Profilator machine are indexed from one station to the next, in timed sequences, to coordinate with predetermined production requirements. This operation occurs in a fully automated mode, requiring no operator intervention, except for maintenance and planned inspections.
Likewise, in the machining of valve bodies and transmission cases, each step of the process is controlled by the Siemens CNC to produce the required components in the proper sequence for subsequent assembly and testing operations. During those subsequent operations, other motion control devices and software solutions provided by Siemens execute, monitor, and control the assembly process through the SIFACS solution set.
Through a decentralized and cabinet-less design, GM achieves highly integrated RFID control with easy access and true out-of-the-box solutions for the control architecture installed on this line. A Profinet solution provides GM with a high-performance, reliable network with minimum bandwidth impact or additional network load achieved at this plant, all with no special hardware required.
Safety First and Last
Safety features are numerous, resulting in a complete failsafe system across all Siemens Simatic PLC, I/O devices, and safetyintegrated drives. All safety devices are networked over Profisafe protocol, a certified safety network, eliminating time-consuming and difficult-to-maintain traditional hardwired safety connections. All I/O failsafe drives are part of the Siemens Totally Integrated Automation (TIA) protocol. Since it is fully integrated, this protocol provides comprehensive system diagnostics, which can help guide maintenance staff to exact fault location and mitigate downtime. Since the drives, starters, and machine safety are integrated into the multifunctional machine mount I/O system, Simatic ET 200pro, the overall engineering complexity is reduced because of simplicity in panel design, wiring architecture, and seamless integration to the project-level hardware configuration, which is reduced due to the totally integrated automation design. For service requirements in the event of a fault, hot-swapping of an I/O module is possible during operation, without switching off the entire station.
Between the two lines, GM Toledo has invested $872 million on its six-speed, rear- and front-wheel-drive transmission production at the 2-million-square-foot facility. The rear-wheel-drive Hydramatic 6L80 transmission is now joined by the GF6 units being produced on this new line under the FACS control solution that supports flexible manufacturing while driving standard processes.
This article was written by Jim Remski, Business Manager for Siemens Automotive. For more information, Click Here