Combination drive-wheel mechanism and travel-sensor mechanism

Abstract

Traction apparatus for driving a vehicle and accurately sensing the distance traveled, in which a drive mechanism is encircled by a separate ring-shaped encoder wheel. The encoder wheel, which is part of a travel-sensing mechanism, nests annularly with the drive mechanism so that the width of the combination is minimized. The travel-sensing mechanism can be assembled to the vehicle without disturbing the drive mechanism. There is enough radial clearance in the toroidal encoder wheel to permit the drive mechanism to move up and down within the encoder wheel. The encoder wheel has approximately the same outside diameter as the drive wheel, so certain common-mode measurement errors cancel. The encoder wheel and the drive wheel are approximately concentric and closely spaced laterally to minimize measurement errors that would otherwise occur upon turning of the vehicle. The tire of the encoder wheel is much narrower than that of the driving wheel, so the encoder wheel has a more constant effective lateral location during turning of the vehicle, which increases accuracy of travel measurement. An electronic encoder is offset from the encoder wheel and is belt-driven to provide a mechanical amplification that improves the measurement's resolution. A belt sprocket on the encoder wheel has spaced-apart groups of teeth that engage the toothed belt.