The Jack and Jill of Ergonomics (PLM Strategies Column)1 May, 2008 By: Kenneth Wong
How Ford uses human-motion modeling for manufacturing-process simulation.
Remember Jack and Jill from the nursery rhyme, the unwieldy pair that went up the hill to fetch a pail of water? As you might recall, Jack fell down and broke his crown, and Jill came tumbling after. Well, guess what? They're working for Ford now.
In this article
Jack and Jill take turns crawling under the newest Ford models. As they lie on their backs, they try to reach for oil filters. On the plant floor, they negotiate rooms in the nooks and crannies of the assembly machineries. Meanwhile, Ford ergonomists carefully note the level of fatigue and injury risk produced by their repetitive motions. Then the automaker incorporates what it has learned from its observations into the design of the next generation of vehicles and the layout of the next factory wing.
Jack and Jill can't feel pain the way humans do. They're digital models. They can twist their anatomically detailed spines, shoulders, and hands without being subjected to the physical discomfort we would. Instead, they provide numerical feedback about the amount of force they feel in various joints.
The workers on the plant floor installing the satellite radio antenna on the Ford Edge or the cargo-management system on the Ford F-Series may never get to meet Jack and Jill face to face. But, for the ease with which they're able to do their job, they owe a lot to the falls and tumbles that those two have taken on their behalf.
Jack and Jill are part of Tecnomatix, a digital manufacturing suite from Siemens PLM Software. The pair was conceived and brought to life at the Center for Human Modeling and Simulation at the University of Pennsylvania in the 1980s and 1990s. They have gained many new skills since being commercialized in 1996. In their Tecnomatix incarnation, they're designed to help manufacturers better understand the human-performance factors in their production workflow. (The product is officially marketed under the name JACK, but rest assured, Jill is part of the package.)
Here's how JACK works, according to its creators. First, you build a virtual environment that corresponds to your factory setup. You can do this by importing the geometry of CAD models in a variety of formats, including VRML or IGES. Ford imports its production machineries as DirectModel JT format, a widely adopted lightweight 3D standard that works with most popular CAD systems. Because detailed CAD geometry can be overkill for real-time simulation (specifically to study movements and collisions), the software allows you to parse the JT data to a manageable level.
Figure 1. Digital prototyping enters a new phase as manufacturers begin to use virtual avatars to verify and eliminate safety and ergonomic issues on the plant floor.
Next, you define your human avatar's size and shape to represent your worker population, position him or her in the digital world, assign a task, and let the experiment run (figure 1). By watching the way the avatar interacts with the mechanisms around it, you can better understand the following issues:
- 1. What the worker can reach
- 2. The strength and time required to perform a specific task
- 3. Whether the worker will be exposed to elevated risk of musculoskeletal injury
- 4. Whether the task produces fatigue
See Them Run
Allison Stephens, an ergonomics technical specialist for Ford's vehicle operations manufacturing engineering, gave Jack and Jill a coming-out party of sorts. In November 2007, she introduced the digital duo at Siemens PLM's Digital Manufacturing Symposium in Dearborn, Michigan.
"Human simulation is part of our virtual manufacturing process," explained Stephens. In this process, using a mix of virtual vehicle prototypes, virtual machineries, and digital human models, Stephens and her team try to answer basic questions about the assembly process:
- 1. Can an operator put his or her hand around a component?
- 2. Can an operator reach the component?
- 3. Is an operator strong enough to install the component?
In an initial test, Ford ergonomists simply insert a static human model into the digital environment. If the result shows that the operator is in collision with nearby structures, the team moves ahead with a more comprehensive analysis.
Using the same type of motion-capture technology used by video game developers, Ford ergonomists place a real human operator in a virtual space representing the production environment.
See Them in Context
To do the same type of design and process verifications in the past, Ford ergonomists had to wait until the physical prototype was available and then use a real human operator to study his or her posture and the risk of injury.
"Computer programs to calculate the impact of human motion have existed for nearly 20 years now," said Stephens. "Not long ago, we could take a photograph of an assembly worker, digitize his or her joints, then use something like a free-body diagram to figure out the forces applied on the joints. But now, the technology has progressed so much that we can actually place the digital human model in the virtual environment — and do all this in real time."
This allows ergonomists to observe the digital model in context, right alongside the virtual vehicle and work-station (figure 2). "So I can now see, for instance, whether Jack's hand is colliding with the fender in the back when he's reaching for a door," explained Stephens.
Figure 2. The ability to see the virtual model in context — right alongside the vehicle and the workstation — allows ergonomists to redesign the product or the task, if necessary, to minimize on-the-job injuries.
Easier simulation produces a cas-cading effect on the product development cycle. "Because we can do our verification early, without waiting for a physical prototype, we're now two years ahead of where we used to be," Stephens said.
In the past, by the time the prototype was ready for safety and ergonomic evaluation, the design was frozen. That left Stephens and her team with limited options if they discovered a nagging issue. "Mostly, we had to decide what we could live with and what we couldn't," Stephens said.
Now, with the ability to run tests roughly 24 months in advance, she can make suggestions to the design engineers on how to reshape the vehicle (for example, move the protruding fender approximately 5 centimeters) for worker safety and assembly-line efficiency.
Under the U.S. Government's Occupational Safety and Health Administration (OSHA), employers such as Ford are required to provide "safe and healthful working conditions for working men and women" (OSH Act of 1970, www.osha.gov).
An employer may be cited for ergonomic violation if, for example, the task an employee is required to perform exposes him or her to spinal compression forces beyond the acceptable tolerance (based on data compiled by the National Institute for Occupational Safety and Health, NIOSH, www.cdc.gov/niosh).
"3,400 N [newton] is NIOSH's recommended limit," said Stephens. "So JACK software can actually calculate the compression force based on the digital model's posture and compare the results with NIOSH standards."
In other words, if Ford is ever asked to prove that its manufacturing practices are OSHA compliant, Jack and Jill can testify on behalf of the automaker.
JACK on Film
Perhaps the best value that JACK offers is its visual appeal. The simulation created with the software can be output as screen captures and movie clips in AVI format. Stephens might have trouble swaying a budget-conscious executive by citing the spinal compression force to which the assembly worker would be subjected. "But when I play back the AVI file to the management team, the engineers, and the project managers," said Stephens, "they suddenly see the awkward posture of the digital model (figure 3), and they say, 'Oh yeah, we so need to change that.'"
Figure 3. This view of a worker s posture speaks volumes about the practicality of a task.
Jack and Jill hide somewhere in the hard drive, awaiting their next assignment.
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