GM runs in top gear with AS/RS sequencing
By sending a "perfectly mixed" flow of General Motors trucks to assembly, this handling system balances work loads and helps maximize productivity within a 2-hour JIT window.
By Tom Feare -- Modern Materials Handling, 8/1/1998
Down in Texas, "big'' comes with the territory. Texans like to point with pride to the biggest this or that. But far more than just bragging rights is involved for the world's largest automaker at its assembly plant in Arlington, Texas.At this General Motors facility a huge automated storage/retrieval system (AS/RS) holds truck bodies. It's making a major difference in GM's ability to maintain maximum productivity on an assembly line. That line, moreover, runs on a tight, 2-hour just-in-time schedule with parts suppliers to GM.
For this AS/RS, however, the term "storage" doesn't fit. No vehicle body remains "stored" in the system for more than a few hours. "Staging" and "sequencing"-with a heavy emphasis on the latter-are the more appropriate terms to describe what the system does-and what it produces for GM in the way of benefits. Computer "smarts" based on complex work scheduling rules pick what's sequenced, what's not.
"This system is paramount to our success in achieving a lean manufacturing approach to production," says Bill Burns, senior electrical engineer, at the GM plant. "We simply couldn't achieve the same results with the way we used to handle vehicles coming from our highly automated painting operations."
Balancing the work load
Adds Bob Murday, director, manufacturing engineering, at the GM plant: "When you're building a variety of products with multiple option packages like we do, it's extremely important to balance the work load going to general assembly." Minutes saved moving painted vehicles along the line-and making more efficient use of workers' time and skills-all add up to reduced costs.
Because this GM plant averages 2,600 vehicles assembled each week by its labor force of more than 1,900 workers, balancing work-in-process (WIP) to the assembly line rises to a highly skilled and complex art. The goal: achieving a "perfect mix" of WIP. Computer hardware and software programmed with complex rules for WIP scheduling and the swift response time and reliability of the AS/RS (Acco Systems) make it all happen.
Just the right mix of vehicles with a higher job content (in terms of assembly line tasks) has to be sequenced by the system together with other, lower work load vehicles-all of which will then be sent in a designated sequence to the GA (general assembly) area.
A high labor content vehicle, for example, might be a 4-wheel-drive GMC Yukon with several option packages ordered by the customer. Less labor is required for, let's say, assembly of a basic, 2-wheel-drive Chevrolet C/K pickup truck. Thus, here's the extreme case that GM seeks to avoid: Sequencing, in a row, four 4-wheel drive vehicles, all loaded with options and moving them to assembly in this order. This far-less-than-perfect sequence would put extra work pressure on assembly line employees.
The color(s) on the individual vehicles coming down the line from the AS/RS to undergo assembly also must match up with work orders as prioritized by GM manufacturing management from among incoming customer orders, adding to the complexity of work load scheduling.
Finally, GM has to perform this balancing act within the constraints of sending out, via electronic data interchange (EDI) links, a two-hour "heads up" alert to the automaker's just-in-time suppliers of seats and other parts. That listing (or "broadcast list," to use GM's jargon) tells JIT suppliers that a particular model of a specific type of GM vehicle in a certain color scheme with selected options will arrive in assembly in just 120 minutes, followed by other models similarly specified in the listing. About 75 vehicles will be listed.
Staging, sequencing details
GM, after studying its materials handling options, decided to install the AS/RS in a rack-supported structure adjacent to existing manufacturing operations. The company made the move over a five-month period, starting in late 1996, as the plant switched over from auto to truck production.
Here's the basic concept behind how the new system works: The AS/RS stages entire bodies-hood, fenders, cab or passenger compartment, and, for trucks, the pickup beds-for the "perfect mix" of GM truck and sport utility vehicles needed to move ahead to assembly at any point in time.
Computer hardware and software provide this system with the "smarts" necessary to sequence a balanced stream of work-in-process to the line. "Before general assembly bolts any option onto a GM truck," says Burns, "that move has been planned out hours earlier by the computer."
After receiving its specific color finish(es) on the robotic paint line upstream, each vehicle goes into the AS/RS where it 'rides' on its own skid.
Although the system contains 192 rack "bins" or sequencing positions, in practice it handles only up to 176 active skids in the bins. These skids either are loaded, holding WIP vehicles or they are "empties," serving temporarily as buffer skids. The remaining 16 AS/RS positions are set aside for maintenance purposes and inbound and outbound stations.
Skids loaded with vehicles are not in the AS/RS for long-certainly not long enough for the GM trucks to gather much dust or to get any dings and nicks on their shiny new paint finishes. The AS/RS will handle a total of some 60 inbound load (staging) and outbound (sequencing) jobs each hour.
Buffering capability
Besides serving as a means to control the "perfect mix," the AS/RS fulfills another important purpose, says Murday. It provides a "banking" or buffering capability between the highly automated paint shop and assembly.
"In the event that we have a problem in the paint shop delaying or stopping this part of our manufacturing process, we can still keep assembly line employees busy working on vehicles already painted and coming from the AS/RS."
It's also important to realize that, as he explains, the AS/RS acts as a "re-sequencer." Vehicles entering this system do not leave in the same order that they arrived.
Taking some risks
Installing this AS/RS involved taking some risks, recalls Burns. Similar systems exist elsewhere in the auto industry and some also within GM. But the approach hadn't been tried before in truck manufacture within GM.
Helping justify the risk were the limits of the prior staging system for painted vehicle bodies. The bodies rode on skids on an in-line roller conveyor system, with 12 lanes from which to pull an individual body. But only the first job in the lane among five jobs could be pulled.
Were the risks worth the investment in the new AS/RS? "Yes," say Burns and Murday.
The new system, Burns declares, "was a leap in technology compared to conveyors. We now have the choice and freedom to structure our assembly operations for the most competitive position."
"We also have more flexibility in our manufacturing operations now, and greater flexibility for future GM assembly at this plant," adds Murday.
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