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15858Re: [SeattleRobotics] Re: SRF08 beam angle

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  • warrenhb
    Dec 2, 2003
      From http://www.robot-electronics.co.uk/htm/sonar_faq.htm:
      | Q. How can I narrow the beam width?
      | A. You can't! This is a question which crops up regularly, however there
      | is no easy way to reduce or change the beam width that I'm aware of. The
      | beam pattern of the SRF04/8 is conical with the width of the beam being a
      | function of the surface area of the transducers and is fixed. The beam
      | pattern of the transducers used on the SRF04/8, taken from the
      | manufacturers data sheet, is shown below.

      You could imagine trying to cut the beam width by putting something
      in the way. Since there is a separate emitter and detector, you might be
      able to absorb or deflect part of the outgoing beam away from the path
      to the detector, or shield the detector so it would detect a narrower angle.
      This probably wouldn't work because of diffraction - you end up making
      a smaller emitter, but it acts as a point source, like dropping a rock in a
      pool of water, which could produce an even wider beam. Shielding the
      detector could produce the same result, for the same reason a radio
      antenna has a similar pattern for transmit and receive.

      Thinking about how you improve the directivity of a radio antenna, you
      might be able to make a phased array of emitters and/or detectors, but
      that sounds expensive and would require redesigning the circuitry to
      work with multiple transducers. How about a parabolic dish? That
      sounds doable! There's a calculation that should tell you how much
      improvement in directivity you would get due to a dish a given number
      of wavelengths in diameter. I don't know it off hand, but you might be
      able to search for it on-line. If you find the version for microwaves, you
      could substitute the speed of sound for the speed of light to convert it
      to ultrasonics. If you don't find the calculation on-line, try the ARRL
      handbook.

      You could put a dish on either the emitter, the detector, or both. Putting
      a dish on the emitter would improve the signal strength; putting one on
      the detector would improve the immunity to other emitters. You might
      be able to mount the pair at the focus of a single large dish - that way
      you wouldn't have to take the unit apart. This would be pretty easy to
      test! Get a Big Ear or similar dish-based microphone, or look for a
      suitable kitchen dish or barbeque lid, etc. Anything roughly
      hemispherical should be close enough to parabolic to produce some
      improvement.

      However, a dish could make your robot a lot larger. There might be a
      way to make an ultrasonic lens, but the dish sounds easier to do with
      readily available materials. To make a lens, you need to find a material
      for which the speed of sound is different from the speed in air, that
      is transparent to the sound. If the speed of sound is faster, you would
      make a concave lens to focus the beam; if the speed of sound is slower,
      you would make a convex lens. To get an idea of how much smaller this
      could be than an equivalent dish, consider the new lens-based headlights
      compared to the traditional sealed-beam headlights.

      By the way, an old headlight reflector might be a good dish to try. Either
      trim the front off a dead sealed-beam headlight, or go to a junk yard and
      get the reflector from one of the newer "European style" headlights that
      are not sealed-beam.

      Doug Bell

      ----- Original Message -----
      From: "Karim Virani" <karim@...>
      To: <SeattleRobotics@yahoogroups.com>
      Sent: Monday, December 01, 2003 2:55 PM
      Subject: RE: [SeattleRobotics] Re: SRF08 beam angle


      | Is there any physical way to confine the angle further - without throwing
      off the results? I'd like to reject everything outside of about 7 to 10
      degrees from center, 15 to 20 degree total cone.
      |
      | Karim
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