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Thickness measurement

  • US 4,053,234 A
  • Filed: 02/18/1975
  • Issued: 10/11/1977
  • Est. Priority Date: 02/18/1975
  • Status: Expired due to Term
First Claim
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1. In an apparatus for use in measuring the position of a surface in a direction normal to said surface, as said position is related to a datum, wherein a beam of light is projected onto said surface along a first axis with the light reflecting from said surface along a second axis, said axes being in a common plane, the improvement comprising:

  • a photodiode strip comprising a plurality of photodiodes positioned sequentially along a line, said strip being positioned transverse to said second axis with said line in said plane, whereby a change in the position of said surface in said direction will cause said second axis to move along said line with the result of a change in which of the photodiodes are illuminated by the reflected light; and

    means connected to said photodiode strip for determining which photodiodes are illuminated to a predetermined degree, said means being arranged to repetitively scan the strip in a given direction to determine the voltage output of each diode in sequence along said line to thereby create a composite voltage signal which is a composite, as related to time, of the voltage output of each photodiode, as related to the direction of scan said beam having a leading edge and a trailing edge, said means connected to said photodiode strip producing a square wave initiated by that part (TA) of the composite voltage signal corresponding to the voltage output of the first diode to be illuminated to said degree and terminated (at TS) by the termination of said scan, the width of said beam between said leading edge and said trailing edge being sufficient to simultaneously illuminate a plurality of said diodes, said means determining which of the diodes, with respect to the direction of scan, is the first to be illuminated to said predetermined degree thereby determining the position of the reflected beam along said line, said means producing a sensible indication of the position of the reflected beam along said line calibrated in terms of the dimension between the part of said one surface at which the axes converge and said datum, said sensible indication being related to the time difference between said signals TA and TS.

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