Differential movement streamlines motion stage

Goleta, CA--Laptop and palm computers, CD and mini-disk players, cellular phones, or advanced medical testing and biotechnology--few associate such glitzy modern conveniences with something as mind numbing as motion control. Yet without precise positioning, such advances wouldn't be nearly so accessible, or abundant. Achieving two-axis positioning for such high-speed applications usually means stacking a couple of linear-motor stages on top of one another. But the redundancy of such schemes creates larger than necessary moving mass, which presents inefficiencies in terms of stage acceleration, cost, and assembly--something that just doesn't suit the lean and agile manufacturing paradigm of today.

The typical linear-motor-driven, two-axis stage achieves the primary axis of motion with a magnet track, a coil-driven carriage with bearing assembly, and a feedback device. The secondary motion axis, stacked orthogonal to the first, consists of essentially identical components. So each axis of motion requires a separate magnet track, carriage, bearing, and feedback device, which increase the system's moving mass. As a result, stacked stages require a larger primary-axis motor to move the mass of the secondary stage in addition to the mass of the load. Another drawback is that the extra moving mass, combined with high acceleration and deceleration rates, typically requires a very heavy frame, bolted to the floor, for stability--another faux pas in the world of agile manufacturing.

In contrast, Bell-Everman Inc.'s patent-pending flying-V structure represents a whole new class of multi axis stages. The design incorporates multiple independent carriages on the same primary axis to achieve movements in multiple directions. This completely eliminates the need for a separate magnet track, and linear scale for the second axis of motion. With speeds to 5 m/sec and 4g acceleration in both axes, the KAOS(TM) multi-axis linear servo stage, as it is called, represents not just a step, but a flying leap, forward for high-speed pick-and-place, pipetting, and testing applications.

It all boils down to a simple design with fewer parts and less moving mass, says Michael Everman, co-founder of the six-year-old firm. In fact, KAOS stages can have 1/10 the moving mass of stacked designs with equivalent acceleration. This means you can not only downsize motors to save money but also drastically reduce the support structure requirements, making high-speed positioning on the desktop possible. KAOS can make very aggressive moves, without a heavy support frame, he adds, and doesn't shake everything on the floor.

Major components include a linear bearing assembly that guides two independent carriages, each with its own moving coil running along a single rare-earth magnet track. Each primary carriage uses a submicron read head to sense a lone strip of encoder tape providing position feedback. Two secondary ball-retainer type linear bearing/carriage assemblies tie each primary carriage to the flying-V, or moving part of the stage.

The V-shaped plate is key to the design since it mounts on top of the secondary bearings, protracting a specific angle (typically in the 30-90 degrees range) between them. Thus concurrent carriage motion moves the V-plate in the primary axis direction, parallel to the magnet track, while differential carriage motion forces the V-plate to move in a direction perpendicular to primary axis.

Everman's biggest challenge in developing KAOS was finding standard off-the-shelf components to create a scalable build-up technique for applications with a wide variety of envelope requirements. That's why every KAOS chassis starts with a stable cast-tooling plate for its base. Side extrusions and end plates, available in a bare, nickel, powder coat, or ano-dized finish, box in the chassis structure. Flexibility in design allows the company to quickly customize the product to meet different needs. For example, motors from Normag, Trilogy, Kollmorgen, or Anorad are available, as are high-performance controls from Delta Tau Data Systems Inc. Rennishaw endcoders and THK bearings are standard, along with igus or Kableschlepp cable carriers, depending on the application.

"The design allows us to be very fleet of foot," says Everman. "Just in the last year, we've developed a half dozen flavors of KAOS." Standard configurations include XY and XZ in small and medium sizes, XYZ, and ZRTheta.

Additional details...Contact Michael Everman, Bell-Everman Inc., 82 Aero Camino, Goleta, CA 93117; Tel: (805) 685-1029; Fax: (805) 685-6787; E-mail: [email protected].

Michael Everman

CEO and Director of Engineering Michael Everman has been a mechanism designer for 22 years, nine of which he spent in the aerospace industry. He has been awarded seven U.S. Patents, with the KAOS patent pending; five of these involving high-precision rotary and linear motion devices, most notably the 12-ft diameter rotary joints for Space Station Freedom for which he received a second place in the "Excellence in Design" competition in 1988. He has also received NASA commendations for the hardware design of several space dynamics experiments, two of which were part of highly successful Space Shuttle flights. His current efforts are toward completing a wide array of multi-axis KAOS incarnations, and expanding on a variety of other differential motion mechanisms.