Position sensor and circuit for optical encoder

Details Position sensor and circuit for optical encoder Position sensor and circuit for optical encoder

2001-09-07

2003-03-04

Le, Que T. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231140, C250S231160, C250S208100, C356S616000, C356S617000, C356S619000, C341S009000, C341S011000, C341S013000

Reexamination Certificate

active

06528783

ABSTRACT:

TECHNICAL FIELD
This invention relates to electronic circuits, and in particular to opto-electronic circuits forming part of the sensor component in linear or rotary encoders for the measurement of linear or angular displacement respectively.
BACKGROUND
Typically in such encoders, one or more fixed sources of electromagnetic radiation (EMR) are arranged to illuminate a graduated planar or cylindrical surface. The markings on the graduated surface comprise regions of high and low reflectivity (or, alternatively, high and low transmissibility) to the EMR and the reflected (or transmitted) component of the EMR is arranged to impinge on a fixed opto-electronic sensor which detects the time-dependent or spatially distributed intensity pattern of the incident EMR, and hence provide measurement of the relative position of the graduated surface. Both such “reflective” and “transmissive” encoder versions are used commonly in industry and consumer products today for measurement of linear or angular displacement, although the latter arrangement is more common.
The opto-electronic sensor incorporated in such encoders typically employ four photodiodes and a “quadrature interpolation” method, well known in the art, is used to increase the positional measurement resolution well above the pitch of markings on the respective graduated planar or cylindrical surface of the encoder. In U.S. Pat. No. 4,410,798 (Breslow) and U.S. Pat. No. 5,235,181 (Durana et al.) various implementations are described where the measurement resolution is increased many times higher than the pitch of the markings, being limited by the accuracy of the marking pitch, width and edge quality, the positional accuracy of photodiodes providing the quadrature signals and the signal to noise ratio of these photodiodes. The disadvantage of these systems are, that in order to provide measurement accuracy in the order of microns, very high quality components, “micron accuracy” mechanical and assembly tolerances, and high quality markings are needed. As an example, in both the above prior art patents, the phase error of the quadrature signals is equal to the positional error of the discrete photodiode detectors, plus the positional error of the markings, plus the errors due to the electrical and optical noise, mismatch of the detectors, and other components in the signal processing system.
The electronic circuit according to the present invention seeks to overcome some of these disadvantages by extensive over-sampling of the incident EMR via an opto-electronic sensor which has one or more arrays of multiple photodiode detectors, each array of photo-diode detectors simultaneously spanning many pitches of the pattern, of incident EMR impinging on the array. In this specification “pitch” of the pattern is defined as the distance between adjacent regions of maximum EMR intensity of the pattern of incident EMR impinging on the array of detectors, and directly relates to the pitch of the markings on the graduated surface or surfaces from which the EMR was reflected (for a reflective encoder) or through which the EMR was transmitted (for a transmissive encoder). As a result the measurement accuracy is higher than the positional accuracy of any single detector in the array, and indeed the positional accuracy of a single pattern pitch. The positional accuracy of the resulting quadrature pair signals is not determined by mechanical and assembly tolerances, but by the positional accuracy of the “very large scale integration” (VLSI) process used for the manufacture of the photodiode arrays and can be as low as 0.1 um economically with today's silicon fabrication processes. According to the present invention, the opto-electronic circuit for the encoder sensor component does not rely on the use of expensive and highly accurate marking processes for the manufacture of the encoder, and is able to tolerate local imperfections in the graduated surface(s) of the encoder, even damaged or entirely missing areas of markings.
It is an aim of this invention to provide a very accurate opto-electronic relative position measurement circuit comprising less accurate (and hence lower cost) elements. A sensor component of an encoder, employing an opto-electronic circuit according to the present invention, will typically have >100 times higher resolution than the pitch of the markings on the graduated surface whose relative position is to be measured, >10 times higher resolution than the pitch of the array of photodiode detectors, and >10 times higher measurement accuracy than the positional accuracy of any of the individual detectors in the array.
Extensive over-sampling of the incident EMR pattern impinging on the array, and massively-parallel collective computation within the opto-electronic circuit, results in the relative position measurement resolution being determined by the pitching accuracy of the detector array, rather than being limited by the pitching accuracy of the encoder marking graduations. In addition, by measuring many pitches of the incident EMR pattern, the accuracy of the relative position measurement is theoretically n
p
times higher than the positional accuracy of any individual pitch of the incident EMR pattern, where n
p
is the number of pattern pitches being sampled by the detector array. The advantage of this approach is that it does not require an expensive graduation marking process to achieve “submicron” resolution in relative position measurement of the sensor component. Furthermore, assuming analog (for example photodiode) detectors are used in the array, the measurement technique provides subpixel resolution, and can economically supply 10-100 times higher resolution than that of the detector array pitch. The signal to noise ratio of the relative position measurement is typically more than n
d
times larger than the signal to noise ratio of the individual detectors, where n
d
is the number of detectors in the array. This is a clear advantage because, using today's VLSI silicon fabrication processes, one or more arrays consisting of thousands of detectors can be implemented economically on a single chip.
The computation that is needed for the relative position measurement is mainly an inner-product operation, which is executed very efficiently by a capacitive circuit. The capacitive correlator circuits, according to the present invention, carry out the necessary computation in a massively-parallel single operation, achieving the highest possible processing speed and hence minimum processing time. The computing accuracy is related to the accuracy of the capacitors within the capacitive correlator circuits which is the best controlled parameter of VLSI circuits, providing the most area-efficient solution for the type of circuit architecture. Also the massively-parallel, analog inner-product computation block minimizes the energy needed to perform the computation, requiring as little as 0.1% of the power dissipation of corresponding digital microprocessor solutions.
The electronic circuit, according to the present invention, is suitable for very high resolution relative position measurement over a range corresponding to one pitch of the incident EMR pattern. To obtain high resolution absolute position measurement extending beyond this one-pitch range, the circuit can be used in combination with simple, lower resolution absolute bar code measurement techniques. For example, the graduation markings may be encrypted using a variable width marking (eg. a binary thin/thick marking) to the graduations or, alternatively, a separate bar-code marking graduation on the planar or cylindrical surface in the encoder can be employed. Many different bar-code marking encryption techniques are commonly used in industry and consumer products.
SUMMARY OF INVENTION
The present invention consists in an electronic circuit comprising a longitudinally disposed array of electromagnetic radiation (EMR) detectors, two or more correlator units, and a phase angle computing unit, the circuit enabling measurement of the relative position o

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