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    Micro accelerometer module: was [4225] automatic celestial navigation.
    From: George Huxtable
    Date: 2008 Jan 26, 17:04 -0000

    Frank Jones wrote, in Navlist 4225, 5 December 07-
    
    | I recently noted in the November 22, 2007 issue of EDN (Electronic
    | Design News - <>), page 19 a brief announcement
    | regarding a MEMS-based (microelectromechanical-system)
    | inclinometer apparently capable of accurately measuring the
    | deviation from vertical by sensing the downward G force.  The
    | device/s is described as an Analog Devices (<>)
    | ADIS16209.
    |
    | All the key elements are obviously available to designers for
    | construction of an advanced sextant now, even one that
    | automatically tracks objects day or night.  I suspect a gimballed
    | arrangement would make the software engineering easier although
    | it might be avoided at the expense of much more complicated
    | tracking schemes.  I recently saw optical sensors for similar
    | systems go for several hundred dollars each on eBay.  The cost of a
    | 'new' sensor like these in small quantities would be much pricer.
    ====================
    
    And I made a somewhat dismissive comment in [4240]-
    
    "If they have somehow succeeded in disentangling the "downward G force" from
    all the other accelerations that apply to a sextant in a vessel in waves on
    the sea, then there may be a future in it. But I doubt if they have. In
    which case, it would suffer from all the defects of a pendulum or spirit
    level or other form of artificial horizon, that render them all unusable at
    sea."
    
    ===================
    
    However, such a device may be of interest for checking tilt and measuring
    accelerations, so I have investigated a similar 3-axis accelerometer, not
    the Analog Devices chip that Frank Jones referred to, but one from another
    maker, the MEMS inertial sensor type LIS3L06AL from ST Microelectronics.
    
    Here is what I have gleaned from its data sheet.
    
    It's a 3-axis accelerometer in a tiny module, only 5 x 5 x 1.6mm, with an
    analogue output.
    
    It expects to be powered from supply rails at 0v and +3.3volts, from which
    it draws about 1 milliamp.
    
    It can be set to a low-sensitivity mode, but I will deal only with its
    high-sensitivity mode, in which it's more likely to be used.
    
    Three pins, one for each axis, provide an output voltage corresponding to an
    acceleration (or gravity) along that axis. With zero acceleration, the
    output is at nominally the mid-point voltage between the supply rails,
    therefore +1.65 volts. An acceleration of 1g along any axis adds or
    subtracts 0.66 volts to the output corresponding to that axis. The maximum
    acceleration it can work with is 2g in any direction, though it can survive
    accelerations far higher without damage.
    
    So, when it's laid flat, and stationary, the X and Y outputs will both be at
    nominally +1.65 volts, the Z output at +0.99v  or +2.31v, depending on which
    way up it is.
    
    However, those values are somewhat imprecise. At zero-g, each output could
    be anywhere between 1.6 and 1.7 volts, and its sensitivity might be 1% or
    more from the stated value, and these can change significantly with
    temperature. So these devices are unlikely to be useful in demanding
    applications, such as in an integrating accelerometer, though they might be
    handy for detecting gross tilting errors, of a degree or more.
    
    There also exist similar devices which incorporate an ADC converter, so
    providing digital outputs, but I doubt if their internal sensor is any more
    precise. I have not investigated such sensors from Analog Devices.
    
    One application in which I understand they have been used is in portable
    hard-disk memory units. It senses, by the absence of a g-force in any
    direction, that the unit has been dropped and is in free-fall, and does its
    best to apply emergency shutdown procedures before it hits the ground!
    
    I have acquired one of these modules with the idea of cobbling up some sort
    of testbed to try it out, but so far I have been daunted by the challenge to
    my soldering skills presented by its tiny surface-mount connection pads. It
    cost me about $20.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    George.
    
    
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