One Size Fits All

Whether you're a sumo wrestler or horse-racing jockey, chances are you'll be comfortable in the new Mirra^™ office chair from Herman Miller, Inc. That's because engineers there used simulation software to analyze the base, arms, pedestal, and other subassemblies for performance with a wide range of body types. The software: Pro/ENGINEER from PTC for CAD, and DesignSpace and ANSYS from ANSYS, Inc.

Engineers established the required performance criteria by instrumenting seating mechanisms with torque and displacement transducers. Once they had done the required balancing of all criteria, they tweaked the design and preferred materials to get the targeted performance.

Among key design elements was a cantilever leaf spring and moving fulcrum tilt mechanism providing resistive force to allow people to lean back comfortably. Larry Larder, director of development and systems implementation, says engineers generated torque curves to represent the force required to support different body types in upright, fully tilted, and midway positions. Then, they wrote a script file to simulate a range of spring and fulcrum combinations to operate within the torque-curve envelope. "We found that we could change the shape of the fulcrum to get different performance," Larder says.

Output from the ANSYS structural analysis runs included spring deflection and stress distributions. That gave engineers insight into each design so they could select and refine the configuration that worked best.

Engineers also used ANSYS to determine the coupled response of the back and its supporting spine based on material characteristics of each part together with the size and geometric pattern of the perforated back. Engineers used analysis to design a single composite plastic structure that delivers the required coupled-deflection response, cuts the parts count for the assembly, and conforms to environmental criteria.

Seating Comfort: Engineers at Herman Miller used ANSYS in development of the cantilever leaf spring and moving fulcrum tilt mechanism in the Mirra chair.