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    Re: Digital Sextant
    From: JC Sutherland
    Date: 2002 Apr 2, 20:04 +0100

    Quoting Vic Fraenckel :
    
    > Dan Allen wrote:
    > | I wonder what kind of silicon sensors exist that act as artificial
    > horizons,
    > | and what kind measure angles, and how accurately?  The digital
    > compasses
    > | recently referred to are accurate to 2 degrees.  For a sextant the
    > angle
    > | needs to be measured to a few seconds of arc to be competitive.
    >
    > In discussing the Digital Sextant, one must NOT confuse a digital
    > compass
    > with the method of measuring altitude. A sextant is NOT used to
    > measure
    > azimuth so to quote the accuracy of a digital compass module as an
    > argument
    > against the possibility of building a Digital Sextant clouds the issue.
    > I
    > have personally used digital optical encoders that can measure an angle
    > to
    > 360/4096 degrees = 0.088 degrees = 5.27 minutes. The encoder resolves
    > 360
    > degrees into 4096 counts. While this does NOT come close to what would
    > be
    > needed to make the Digital Sextant, it is a device I am familiar with.
    > There
    > exists encoders that have even higher resolution.
    >
    > There also exists a integrated circuit device (Analog Devices ADXL202)
    > which
    > can measure it's own relationship to the the earth's gravitational field
    > and
    > who's output is a measure of the angles it makes with the
    > gravitational
    > field in two axes.  I have used this device as well..
    >
    > I have interfaced both devices to a microcontroller with ease and made
    > useful measurements with them. Perhaps a Digital Sextant is not so far
    > fetched after all!
    >
    > Vic
    > ________________________________________________________
    >
    > Victor Fraenckel - The Windman                 vfraenc1{at}nycap.rr.com
    > KC2GUI
    > www.windsway.com
    >
    >       Home of the WindReader Electronic Theodolite
    >                                Read the WIND
    >
    Perhaps I can make a suggestion. Instead of using an encoder to measure the
    index mirror angle ...  Attach a stepper motor to the micrometer drum and drive
    the drum in steps by a computor interface which counts the steps moved.
    The gearing of the motor will determine what fraction of a turn of the drum each
    step represents up to I think 200 steps for direct drive. As this would be 1/200
    deg per step, resolution would not be a problem.
    With the right electronics the observer would only need a up/down button to
    drive the index arm to the position of alignment. An end of track switch for
    each end of the arc would be needed to protect the sextant and some means of
    disabling the clamp that allows quick movement of the index arm is needed to
    avoid the pulse counter getting out of step with the scale.
    
    Clive Sutherland
    
    Oxford UK
    
    
    

       
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