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Digital Tachometer

IP.com Disclosure Number: IPCOM000092162D
Original Publication Date: 1968-Oct-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Campbell, RR: AUTHOR [+3]

Abstract

Light from a source is chopped by grating lines on a rotating surface and detected by a masked photocell to provide digital tachometer output signals. Transparent disk 1 is mounted on a device, not shown, whose rotational speed is to be measured. The front face of disk 1 contains a series of opaque radial grating lines 2. Reflective coating 3 is applied to the rear surface of 1. Light from source 4 follows path 5 outside of grating lines 2 and is reflected by coating 3 back through lines 2 to photocell 6. Mask 7, with a grating pattern identical to and aligned with a portion of 2, is positioned between 1 and 6. The transmitted light is chopped by interaction of 2 and 7, producing a variable signal, the frequency of which is proportional to the rotational speed of 1.

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Digital Tachometer

Light from a source is chopped by grating lines on a rotating surface and detected by a masked photocell to provide digital tachometer output signals. Transparent disk 1 is mounted on a device, not shown, whose rotational speed is to be measured. The front face of disk 1 contains a series of opaque radial grating lines 2. Reflective coating 3 is applied to the rear surface of 1. Light from source 4 follows path 5 outside of grating lines 2 and is reflected by coating 3 back through lines 2 to photocell 6. Mask 7, with a grating pattern identical to and aligned with a portion of 2, is positioned between 1 and 6. The transmitted light is chopped by interaction of 2 and 7, producing a variable signal, the frequency of which is proportional to the rotational speed of 1. The device generates two signals, separated electrically by 90 degrees and, thus, direction of rotation is indicated by the order in which the two signals appear. In another arrangement, drum 8 is the rotating member. The outside surface of 8 is composed of a series of alternate reflecting lines 9 and nonreflecting lines 10. Mask 11 has grating lines spaced identically to and running parallel to lines 9. A see-through arrangement with 11 and 12 mounted inside 8 is provided by having alternate opaque and transparent lines for 9 and 10.

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