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Arrangement for measuring or detecting a change in a retro-reflective element

  • US 5,666,037 A
  • Filed: 04/15/1996
  • Issued: 09/09/1997
  • Est. Priority Date: 07/02/1993
  • Status: Expired due to Term
First Claim
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1. An arrangement for indicating a change in a reflective element of a radiation-permeable medium having a detection side disposed opposite the reflective element, the arrangement comprising:

  • at least two measured sections for generating, reflecting and receiving radiation for measurement, each measured section including;

    a radiation source disposed adjacent the detection side of the medium for generating radiation for transmission through the radiation-permeable medium thereby resulting in generated radiation;

    a sensor-active surface on the reflective element for reflecting at least a portion of the generated radiation thereby resulting in reflected radiation emerging from the detection side of the medium; and

    a radiation receiver disposed adjacent the detection side of the medium at a distance from the radiation source for receiving the reflected radiation, the radiation receiver having an output for producing a detection signal representing received radiation;

    a switch arrangement operatively coupled to the at least two measured sections for periodically actuating respective ones of the measured sections in a successive, repeating switching sequence according to a predetermined switching sequence frequency, wherein the detection signal includes a modulation frequency corresponding to the predetermined switching sequence frequency;

    an adjusting device operatively coupled to the at least two measured sections for adjusting the detection signal so that, in a rest state of the sensor-active surface of each measured section, a part of the detection signal corresponding to each measured section has an average amplitude value equal to an average amplitude value of parts of the detection signal corresponding to other ones of the measured sections;

    a filter circuit connected to the radiation receiver of each measured section for producing a filtered detection signal; and

    an evaluation device having an input for receiving the filtered detection signal and producing an output signal in dependence of a difference between parts of the filtered detection signal corresponding to respective ones of the at least two measured sections.

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