A design has been developed for manufacturing multispeed transmissions that are small enough to be used with minimotors - electromagnetic motors with power ratings of less than 1 W. Like similar, larger systems, such as those in automobiles, the proposed mechanism could be used to satisfy a wider dynamic range than could be achieved with fixed-ratio gearing. However, whereas typical transmission components are machined individually and then assembled, this device would be made using silicon batch-fabrication techniques, similar to those used to manufacture integrated circuits and sensors.

Until now, only fixed-ratio gear trains have been available for minimotors, affording no opportunity to change gears in operation to optimize for varying external conditions, or varying speed, torque, and power requirements. This is because conventional multispeed gear-train geometries and actuation techniques do not lend themselves to cost-effective miniaturization. In recent years, the advent of microelectromechanical systems (MEMS) and of micromachining techniques for making small actuators and gears has created the potential for economical mass production of multispeed transmissions for minimotors. In addition, it should be possible to integrate these mechanisms with sensors, such as tachometers and load cells, as well as circuits, to create integrated silicon systems, which could perform closed-loop speed or torque control under a variety of conditions. In comparison with a conventional motor/transmission assembly, such a package would be smaller and lighter, contain fewer parts, consume less power, and impose less of a computational burden on an external central processing unit (CPU).

This Miniature Transmission could be regarded as a flattened version of a conventional three-speed automatic transmission. The components would be fabricated by micromachining.

Like conventional multispeed transmissions for larger motors, miniature multispeed transmissions would contain gears, clutches, and brakes. However, the designs would be more amenable to micromachining and batch fabrication. Gear stages would be nestled one inside the other (see figure on page 14b), rather than stacked one over the other, creating a more planar device. Actuators and the housing would be fabricated on separate layers. The complex mechanical linkages and bearings used to shift gears in conventional transmissions would not be practical at the small scales of interest here. Promising alternatives might include electrostatic-friction locks or piezoelectric actuators. For example, in the transmission depicted in the figure, piezoelectric clamps would serve as actuators in clutches and brakes.

The structures would be aligned and bonded, followed by a final etch to release the moving parts. The entire fabrication process can be automated, making it both precise and relatively inexpensive. The end product is a "gearbox on a chip," which can be "dropped" onto a compatible motor to make an integrated drive system.

This work was done by Indrani Chakraborty and Linda Miller of Caltech for NASA's Jet Propulsion Laboratory. NPO-20316


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Miniature multispeed transmissions for small motors

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Motion Control Tech Briefs Magazine

This article first appeared in the June, 1998 issue of Motion Control Tech Briefs Magazine.

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