Decentralized control becomes the center of efficiency
New control and information system methods on the GF6 six-speed, front-wheel-drive transmission line at General Motors Powertrain in Toledo, OH are causing a weeks-to-hours reduction in time needed for manufacturing line changes.
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 an 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.
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 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. 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 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 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 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.
Since the drives, starters and machine safety are integrated into the multi-functional machine mount I/O system, 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. 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 of Siemens 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.
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.