Mitsubishi stamps out inefficiencies

A new fleet of automatic guided vehicles is the carmaker's path to improved uptime and materials flow in its body panel stamping shop.

By Roberto Michel, Contributing Editor -- Modern Materials Handling, 11/1/2004

The tipping point for a new automatic guided vehicle (AGV) system at Mitsubishi Motors North America (MMNA) came down to a lack of reliability and efficiency. The legacy AGV system was inflexible and getting harder to maintain, making it increasingly unreliable. Worse yet, these shortcomings sometimes left transfer presses starved for materials, causing costly downtime and limiting productivity.

Since it opened in 1988, MMNA's Normal, Ill.-plant has used automated materials handling systems. Some of those systems, including two automated storage and retrieval systems (AS/RS), continue to serve the plant. However, the original wire-guided AGV system in the plant's stamping area had long been problematic, says Barry Hansis, MMNA staff engineer.

The legacy AGVs were controlled by a mainframe that was difficult to reprogram if a route change was needed. The units also needed recharging two to three times per shift, says Hansis.

The three vehicle system was designed with one vehicle as a backup, explains Hansis. When parts on one AGV failed and could not be replaced, it became increasingly difficult to ensure reliable materials flow to the transfer presses.

'When we were running with just two AGVs, if we took one of them offline—say to move material back out of a press—we were down to one AGV to service two cranes and six transfer presses,' says Hansis. 'It didn't work out very well. We would have presses waiting for materials. With transfer presses of the type we have, every minute they are not running costs a tremendous amount of money.'

Today, those presses have a reliable flow of steel blanks to manufacture body panels with a new PC-based AGV system (AGV Products). The plant is also upgrading and integrating its software system for production management and scheduling of the presses. That computer project already has yielded an 'electronic bingo board' that monitors the status of work and assets in the stamping area. When completed next year, the integrated system will link together all key materials handling and production equipment.

The end result already, says Hansis, is an improved flow of materials for stamping operations. He continues, 'we are still moving blanks to presses with AGVs, but we are doing it more reliably and efficiently than we did with our original system.'

Materials flow in the stamping operation begins with delivery of steel coils to a staging area adjacent to two blanking presses. The presses cut the coils into blanks, and a crane stores them in an AS/RS. The new PC-based AGVs then move the blanks to one of six transfer presses, which form body panels from the blanks. Panels are stored in a second AS/RS. From here, panels are delivered to the body shop as needed.

While the flow is fairly simple, there are constraints to deal with, says Hansis. First of all, the stamping operation now accommodates production for six different vehicles, including the Mitsubishi Eclipse, Spyder, Galant and Endeavor as well as the Chrysler Sebring and the Dodge Stratus. When the plant first opened, only two vehicle models were built there.

'We don't have excess storage space for inventory buffers, or extra press capacity like some plants do. So we have to maximize the running of the presses while keeping the inventory low,' says Hansis. 'It's vital to have a smooth flow of materials coming into the plant, getting blanked, going to press lines, out onto the main high-rise [AS/RS for panels] and out to the body shop.'

There were several problems with the old wire-guided AGVs that have been overcome with the new system, says Hansis.

For one, the new AGVs use special batteries that recharge every time a vehicle returns to its home position. 'Opportunity charging is working well,' says Hansis. 'We absolutely do not change batteries anymore. The AGVs run themselves.'

The new system's laser signaling and PC-based controls software also make it more flexible than the previous system. 'The new system has a digital map of the shop, and the AGV paths are represented by what looks like dots along a line,' says Hansis. 'You can grab one of those dots and drag and drop it to change the route. If we have a new piece of equipment to put down, or want to deliver to a different spot, it's very easy to set up. I can change a route now in about 20 seconds.'

The laser-guided system also gets more positioning data than the previous system. Each AGV has a laser unit mounted on its mast, which emits a signal that is bounced back by reflective targets throughout the shop. Then an onboard computer calculates the vehicle's exact coordinates. The combination of reliable batteries, more data, and a Windows-based user interface to control the AGVs allows MMNA to be more flexible and precise in the way that materials are staged for the transfer presses, says Hansis.

'Each transfer press has three overflow buffers used to temporarily hold stacks of blanks,' he says. 'These are used, for instance, if we need to back a stack out of a press. The old AGV system was limited in that it could only move a stack from one press to its own buffer. To do that, we needed the third AGV, so we sometimes ended up having to do manual stack returns, which waste time and labor. But now we can move any stack from any press to any overflow buffer. We have a lot of flexibility in these scenarios.'

The new AGVs, which went on line in January of this year, have improved the efficiency of the stamping areas, including a reduction in sheet rack downtime of 43%. As important as the new AGVs have been, says Hansis, they are just one part of an effort to improve production management in stamping operations.

The other major improvement thrust is a custom-developed software system that schedules and manages press operations. Dubbed the 'production computer project,' phase two of the deployment will replace an older, stand-alone scheduling system for running the presses with one that is integrated to the automated storage systems and AGVS. It also takes into account current scrap and usage data at the presses, rather than relying on historical data. The legacy press scheduling tool, says Hansis, runs on older VAX computer, while the new system is PC-based.

'With phase two, we are going to start taking all this information that is out there—our inventory from the high-rises (AS/RS), our scrap consumption, and production for service parts—and calculate that against what needs to be produced for the vehicles coming down the line to create our production schedule. It's expected to give a more coordinated way of keeping our inventories right and keeping the body shop supplied. There hasn't been a central clearinghouse for all this information, and that's what the new software system will do,' says Hansis.

An early outgrowth of the software project is an equipment monitoring function. The new shop status screen can be viewed on the system's PC clients, and also via a large (8 × 12 feet) electronic 'bingo board' on the shop floor that displays the same information.

'It shows the status of each major piece of equipment, and if it's down-say for a die change—it will indicate why,' says Hansis. 'It's a way of monitoring the stamping shop from remote locations.'

Once phase two is completed, which is expected by September 2005, Hansis says the scheduling of stamping operations should see further efficiencies, especially fewer die changes. But that's just an extension of what has already been accomplished with the new AGVS.

'Before, we had presses waiting, and that's not a good condition. The material gets there when they need it now,' says Hansis.

Click on the icon to read how PPG uses AGVs in manufacturing.
(PPG moves to hands-off handling - January 2004)