GM Transmission Assembly: Integrated, Decentralized Control
New control and information system methods on the GF6 six-speed, front-wheel-drive transmission line at General Motors Powertrain in Toledo, Ohio are causing a weeks-to-hours reduction in time needed for manufacturing line changes (see related article in February Control Engineering: Decentralized Control Gives GM Manufacturing Flexibility). And, GM engineers are also enjoying a range of other benefits of highly integrated yet decentralized automation control. Transmission lines there now integrate diagnostics, RFID systems, safety systems and CNC control solutions.
Through the Siemens Automotive Center of Competence in Troy, MI, GM got the PLCs, CNCs, HMIs, RFID system and high-level Ethernet protocol, Profinet, to run on the GM network. Overlaying this hardware and communications topology, Elite Engineering of Rochester Hills, MI, delivered its Flexible Assembly Configuration System (FACS). Siemens then created SIFACS, which largely focuses on integrating the core PLC software blocks and station functionalities with the RFID tags on each of the workpiece pallets to become the hub of the information management system for the line.
RFID gets things started
As a workpiece proceeds through the transmission assembly line—delivered by an automated guided vehicle (AGV), in most cases—each pallet is equipped with a radio-frequency identification (RFID) tag. Reinhold Niesing, engineering manager on the project for Siemens, said, “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.”
Niesing said 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,” he said.
The dynamic mode of operation for this RFID system includes the fact that information at subsequent line stations must be read “on the fly” without any line stoppage. This is often seen in conventional packaging, shipping or other line applications for RFID; all data are read as the tag passes by the antenna. In less sophisticated applications, however, the signal can degrade over time and after a number of reads.
To avoid this, two interface protocols are supported: ISO 15693 (an open standard) and a proprietary Siemens-developed standard, Simatic RF300. According to Siemens technical specialists, Simatic RF300 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, non-stop 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.
Critical for a continuously moving line such as these, the Siemens Simatic control systems are executing motion commands read from the RFID devices at 8,000 bytes/sec—far in excess of the ISO 15693 standards for read and write performance.
Throughout the metalcutting process in the plant, mostly in gear and spline forming, hobbing, grinding and finishing, CNC technology is on dozens of machine tools. Most of the machines are controlled by Sinumerik 840D, the highest-level CNC offered by Siemens. The CNC controller processes the particular part dimensions in the cutting area of the machine and coordinates all motion control and movements into and out of the machine.
The controllers work 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, without 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.
Failsafe systems, safety network
Safety features are numerous in the GM Toledo plant, resulting in a complete failsafe system across all Siemens Simatic PLC, I/O devices and safety-integrated 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 safe I/O and failsafe drives are part of the Siemens Totally Integrated Automation (TIA) protocol, which provides comprehensive system diagnostics. These can guide maintenance staff to the fault location and mitigate downtime.
Since the drives, starters and machine safety are integrated into the multi-functional machine mount I/O system, Simatic ET 200pro, the overall engineering complexity is reduced. 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. There is nonetheless a very high degree of integral protection, to IP65/67 standards. The fact that an enclosure is not required also helped save on the total cost of the project for GM.
One automated assembly station, Hanwha, produces the various sub-assemblies of the transmission, as other lines produce the components that go into the sub-assemblies. Adding a station requires simply adding a PLC with the standard SIFACS logic, adding desired process devices, and downloading an eFACS configuration, according to GM controls engineer Greg Nazareth. In contrast to the traditional zone control, this reconfiguration is not a building block concept; rather, the instructions being given impact the entire line, Nazareth said.
Nazareth worked with the GM controls team, headed by Ron Goeckerman, to implement FACS with the host server.
By contrast, 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 used the same software to execute quick tooling changes, machine sequence variations, line balancing and report tracking. Operators received training from Siemens and Elite Engineering for these tasks.
All part build histories, troubleshooting, and machine debugging are recorded for further analysis, and diagnostics in the system are highly integrated. Matthew Thornton and Jeremy Bryant of Siemens devised “pre-made templates and blocks important to the powertrain build process as our starting point.”
Thornton noted the importance of placing the critical performance data on all the HMI panels in the system for easy operator access: “With all motion and safety functions integrated into the drives, there was no need to build a separate troubleshooting architecture for what would be a more traditional safety network of relay cabinetry.” In the safety communications area, GM is reviewing another Siemens option for open safety communications technology on distributed automation systems.
Vision system integration
SIFACS HMI screen extensions with Cognex Vision View further permit constant monitoring by both a team leader and a conveyor controller, eliminating a PC dedicated to a vision system.
Process improvement tools
Process improvement tools and process efficiency tools, provided with the FACS, enable both process and production engineers to collect data and fine-tune the system in real time, keeping build status and cycle time information always current. Line and station balancing can likewise be achieved on-the-fly, with complete process efficiency, operator loading, anticipated cycle time, and even individual process operation time calculations being made, charted, displayed, and rapidly analyzed by the team leader or station control personnel, in a hierarchy of need-to-know, need-to-act protocol.
Jim Remski is Industry Manager, Automotive Powertrain, for Siemens Industry Inc., Siemens Industrial Automation.
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