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Optical Tachometer with Electronic Switch Circuitry

IP.com Disclosure Number: IPCOM000079854D
Original Publication Date: 1973-Sep-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 80K

Publishing Venue

IBM

Related People

Palmer, RS: AUTHOR [+2]

Abstract

A system is described which produces a continuous electrical signal, which is proportional to linear velocity over essentially infinite range. In addition, the proportionality constant (gain) has an exceptionally wide range needed for high-performance disk drives, which seek up to 100 ips and follow at less than 0.01 ips.

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Optical Tachometer with Electronic Switch Circuitry

A system is described which produces a continuous electrical signal, which is proportional to linear velocity over essentially infinite range. In addition, the proportionality constant (gain) has an exceptionally wide range needed for high- performance disk drives, which seek up to 100 ips and follow at less than 0.01 ips.

The system consists of two optical position transducers, differentiators and electronic switch circuitry to process the signals, to obtain the desired velocity information.

The optical tachometer physically consists of a light source 1, (typically a light-emitting diode (LED)) a stationary mask assembly 2, a reflective pattern 3 fastened to the moveable object and three photosensors 4a, 4b, 4c. There are two signal sensors in space quadrature with each other by means of an offset in the reflective pattern between Channels A and B. Channel C (light path C) is unobstructed by the reflective pattern, and controls the intensity of the light source to correct for aging of the light, sensitivity changes of the photosensors or any unwanted vertical runout in the reflector. The mask has multiple slots and the sensors measure over several reflective patterns, so slight imperfections in a single pattern will be averaged by the other reflective patterns illuminated by light source 1 and viewed by sensors 4. The resultant output from the A and B photosensors is shown in Fig. 2, as the grid 3 patterns move Vast the stationary mask. The signal is periodic over each light/dark pattern on the reflective grid, and is adjusted to be as linear is possible.

The system relies on the fact that at any given position at least one of the outputs will be in its linear range, and output can be electronically switched on and off as it passes in and out of this linear range.

Fig. 3 illustrates one method for electronically linearizing and combining the two photosensor channels, although there are other configurations that accomplish the same objective. Photosensor signals from 4a and 4b are first amplified by amplifiers 21 and 22, the output's derivative with respect to time is taken by differentiators 23 and 24. The outputs from 23 and 24 represent velocity of the moving grid 3, but with inverted polarities on odd tracks and only over the linear portion of their displacement curves.

The velocity signal which is linear, must be selected and corrected for proper polarity as the grid moves from track-to-track. The switching points for this selection are detected by level detectors 25 and 26, one for each channel, each with two outputs, light and dark corresponding to the signal level that becomes nonlinear due to maximum light or minimum light (dark). When AL is detected by 25, the velocity signal from B is selected and since its slope is the wrong polarity, it is inverted by the four channel selection amplifier 27.

Note that AL can be detected going in either direction and it will always se...