Lean machines shrink the assembly lineLean machines shrink the assembly line
August 11, 1997
Changing markets, shrinking cycles, and rapidly evolving industrial-control standards drive today's automated assembly system design. While price and performance remain important, user/builder adaptability and deployment issues are more critical than ever to design. As a result, modern assembly systems have leaner structures and tooling that integrate with PC and serial I/O technology.
Among the most recent examples: Chrysler Corp.'s Kokomo, IN, plant opted for D720 Series Open Automation Computers from Eaton Corp.'s Cutler-Hammer automation division instead of PLCs for its new transmission carousel. Ingersoll-Rand Automated Production (Farmington Hills, MI) designed and built the assembly stations installed early this summer at Chrysler's plant.
Ingersoll-Rand chose Cutler-Hammer Automation, the program coordinator, as its standard industrial PC supplier. "In addition to supplying components and computers, Cutler-Hammer takes overall responsibility for the installation's success," says Cutler-Hammer's project manager Ed Gatt.
"We wanted to maximize machine up-time, move away from PLC technology, and greatly reduce floor space requirements for the system," says Kokomo's plant manager Ken Moore. "This new technology has helped us meet all three goals."
Automakers want lower costs, improved visual management, and increased productivity. But they also want controls that ease installation, configuration, and development of maintenance and diagnostics screens for the operator interface (OI). "Standardized controls on every station let us develop screens, then use them on several different platforms," says Ingersoll-Rand's manager of engineering John Newman. "And control modularity really saves labor during installation and final runoffs."
Assembly operations are basically the pay point for automakers. "Increased competition, drives demand for more cost-effective performance. While 'leaner' design strategies reduce costs, boosting performance means dropping the traditional, metal-cutting 'bigger-is-better, move the tool to the work,' philosophy," says Newman.
Bring the work to the tool. Improving visual management on the factory floor means reducing size. To do that, Ingersoll-Rand engineers made a fundamental change in design philosophy. New automated assembly stations reverse the tool/work engagement dynamics. Where earlier machines moved the assembly tools to the work, new systems bring the work to the tools. "Bringing the part to the tool decreases cable and hose fatigue," explains Newman, "reducing maintenance and downtime."
The design also reduces the number of system components. "Traditionally," Newman notes, "a column and pneumatic slide support the tool head. Four pulleys, two chains, and a slug of counterweights provide counterbalance. The new design eliminates these components, shrinking station size by about 25%. Fewer components usually translates into improved mean time between repair (MTBR) and mean time to repair (MTTR)."
Other design changes include eliminating the heavy base, column, slide, and wraparound guarding of earlier fastener-tightening-and-assembly stations. Today, engineers designing stations use stress analysis and FEA modeling to optimize the structure. Components such as extruded rectangular cross members, I-beams, and C-channels provide strength where it's needed and reduce the overall weight. Where previous stations required large cranes for positioning, fork trucks can easily cart around today's stations.
Likewise, localized guarding may prove better for operator safety than the traditional wraparound guarding. Instead of totally isolating the machine from operators and technicians, "Pinch-point guarding" protects only safety-critical areas. This technique cuts costs because it makes the machine easier to repair and maintain. "Easy access for maintenance offers added safety by eliminating the temptation to lock out, defeat, or remove protective guarding in order to access components during repairs," Newman explains.
Serial I/O buses cut control components. These design advances make assembly mechanics smaller, lighter, more modular and more reliable. But what about controls?
Ingersoll-Rand's senior project engineer John Stencel points out that the use of serial I/O buses, not the substitution of PCs for PLCs, drives open architecture. Serial I/O technology, says Stencel, also reduces the number of components. Control enclosures that previously occupied about 36 cu ft now require only about 6 cu ft including the PC.
Every new station going into Chrysler's Kokomo plant has a computer connected to the TCP/IP Ethernet network. Although each computer connects to its I/O through an Interbus-S link, Ingersoll-Rand's system permits the use of any PC board, I/O platform, or PLC, says Stencel.
Ingersoll-Rand mounts IP65-type modules from Harrisburg, PA-based Phoenix Contact directly into the machine without an additional enclosure. According to Stencel, these modules handle all I/O, such as proximity switch cables, that aren't wired directly back to the enclosure. They reduce wiring and require no additional terminals or termination points.
Bus-independent I/O from Brown Deer, WI-based WAGO Corp., handles I/O in the enclosure. Its modular blocks consume less space because they contain only two or four inputs or outputs and can accommodate various voltages. An interchangeable bus coupler on the front end enables quick changes from one bus system to another without rewiring, making it easy to adapt as new technologies evolve, according to WAGO's product manager Charles Cook.
Each station pulls off power and communications as required from a flat-design, three-pod cable developed jointly by Ingersoll-Rand and a supplier. "The cable saves on hardware costs because it eliminates disconnects and power drops at every station," Newman notes. One pod supplies 480V, another 120V, and the third supplies 24V for I/O and communications. "Now, rather than cutting each wire to length before tying it to a terminal box, we use plug-in cables at each station," Newman adds.
"Before, only the mechanics were truly modular," says Stencel. "Now, the control system is modular as well, making each station complete and independently functional. This speeds up integration because each station arrives at the customer plant fully validated for production."
Another advantage: minimal ducting. "Installing ductwork is labor intensive for both the customer and the builder," Stencel points out. After runoff at the builder prior to customer shipment, the ducting and headers are torn down then reinstalled at the end-user facility. Plug-in cable minimizes connections and saves time and money during final installation, since contractors don't have to reattach as many wire harnesses or validate as many connections, Stencel says.
PCs provide the information. Each station's computer includes a color 10.4-inch flat-panel touchscreen display. A passive backplane provides easy access to six half-size ISA slots. Drawer-mounted floppy and hard drives, plus an external parallel port for a CD ROM drive, lets users load software quickly with a portable CD device.
A NEMA 4/4X/12 panel-mount steel bezel disconnects without tools from the unit's electronic module for easy mounting and fast access or swap out. "Swapping the electronics module, reloading the station software, and resetting the Ethernet address takes just a few minutes," says Stencel.
Every D720 employs a 100 MHz Pentium Pro microprocessor card, Ethernet card, Interbus-S card, 1.2 GB IDE hard drive, and up to 32 MB of battery-backed DRAM. "Some station's need signature analysis hardware/software for air pressure decay, press force, and gauging functions. To add these functions to older stations required separate computers connected to the PLC. Now we combine many features into one box," Stencel says.
The station and zone controllers run Visual Logic Controller (VLC) 3.0 from Steeplechase Software (Ann Arbor, MI) and Microsoft Windows NT 4.0 for Workstations. Zone controllers employ faster CPUs with more contact capacity and larger hard drives. They serve primarily as data concentrators for historic data storage.
Accessing a vendor's file server with its startup procedures, the latest preventive maintenance and other techniques will be possible from any station, says Kokomo's electrical engineering supervisor Mike Taylor. Similarly, Chrysler can install--and tightly integrate--third-party Windows packages without adding another station computer, he adds.
The ability to load station prints, maintenance manuals, bills of material, and in-house reorder codes allows troubleshooting without need for a laptop. Where practical, Chrysler plans to train operators and tradesmen on the line using visual training materials loaded into the station PCs.
Since the advent of Windows NT for factory-floor automation, issues of achieving hard real-time control have been at the forefront. To address these issues, Steeplechase's VLC is built on an iRMX-based, real-time operating system (RTOS) called INTime developed by RadiSys (Hillsboro, OR).
In fact, Steeplechase president Mike Klein deliberately caused an NT crash, known as "the blue screen of death," and a hard-disk failure at the product's March introduction in Chicago, demonstrating that VLC would continue to run despite both problems. A rigid wall between the RTOS and Windows environments prevents Windows from pre-empting control functions or causing the computer to crash. A Chrysler operator using the computer's touchscreen will work only within Windows and never see or touch the RTOS.
To provide deterministic and repeatable control, Ingersoll-Rand and Chrysler select fixed RTOS scan intervals between 1 and 500 mS. Low-priority Windows tasks are executed during the free time between each read/execute/write scan. While existing Chrysler PLC-based assembly carousels typically scan every 35 mS, scans well under 5 mS exist on some of the new stations.
Chrysler chose Windows NT for its robustness, rigid separation between applications, full 32-bit functionality, superior graphics, security features, and built-in TCP/IP Ethernet software. Compared to conventional PLC control, the major advantage of distributed, open-architecture PC control at Chrysler should be lower life-cycle costs and the opportunity for timely technological advancement through the use of largely off-the-shelf, industry-standard hardware and software.
Windows-based displays have the same look and feel throughout the plant, and the wide familiarity with PCs among young engineers is making PC technology universally accepted and understood, says Klein. This should translate into better quality products, higher machine uptime, and greater output not only at Chrysler but in numerous production settings, he concludes.
5 rules of PC-based control
When implementing a PC-based control solution, special attention must be given to the function and capabilities of the operating system. There are a variety of approaches, but to be a viable alternative to PLCs in a control application, all PC-based control systems must:
Provide deterministic operation. Control must be treated as the highest priority and insure a predictable, repeatable response.
Survive a Windows crash. A machine control program must survive a General Protection Fault in Windows, or the 'Blue Screen of Death' in NT, and continue to operate in a safe manner.
Be isolated from poorly behaved Windows applications and drivers. The control system cannot be adversely affected by unstable applications or drivers.
Survive a hard disk crash. It is essential that deterministic control is not interrupted in the event that a hard disk crashes or is removed.
Be based on a proven real-time engine. The control engine must have a proven track record in mission-critical applications.
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