G-Tech meter acceleration calculations

[Dan]

Tom Cloud wrote:
> 
> >I bought a G-Tech meter and it pretty much works.  You have to be on a
> >flat, level road for the thing to work correctly, though.  Since it uses
> >an accelerometer for all of its calculations, it can get thrown off
> >pretty quickly.  For example, if you are going up a hill, the
> >accelerometer will think you are going faster than you really are.  You
> >could even park your car on a hill, and G-Tech will think you are
> >accelerating continuously.  The device is also only accurate for HP,
> 
> I'm confused .... an accelerometer is an "AC-coupled" device.
> I.e. it only detects a "change" in velocity.  If it reads while
> sitting still or when the speed is constant, it must be a weight
> hung on a pot, not a 'real' accelerometer.
> 
> Tom Cloud <cloud at peaches.ph.utexas.edu>


If you turn the GTech meter 90 degress, you measure the constant
acceleration of gravity (1.0G).  May be strange to think of, but, on
earth, you are always under acceleration even sitting still.  So,
parking on a hill, gives you an acceleration if you had previously set
your accelerometer to a level surface.  By turning the accelerometer 90
degrees, it makes a pretty easy check to see if it is set right also
since it has to read 1.0G's to be correct.

You can get much better results by including a vehicle speed sensor
(like tapping into the cruise control) and using the vehicle velocity as
an input to the HP calculations along with the accelerometer
measurements.  That way you can eliminate the cumulative errors that
occur using only the accelerometer.  You can also do some coast down
tests to see drag; if you do a low speed and a high speed coast down,
you can separate the rolling frictional from the air drag also because
the air drag increases non-linearly while the frictional is pretty much
linear.  Lets you figure out how your spoilers, tires and what not are
eating your power.

I bought a two axis temperature accelerometer +/- 2.5G for front to
back, and +/- 1.5G for side to side.  It also has a temperature sensor
so that you can do temperature compensation since they drift some with
temperature.  Also had 5VDC regulation so I could run it off a 9V
battery and not have to worry about the power supply.  Whole package
potted ran $150 from NGT, 914-223-3359 in LaGrangeville, NY.  Owner is
George VanderGheynst (gvanderghe at aol.com) ang he was very helpful.
Outputs are zero to 5VDC so measurement is easy.  Cheap A/D converters
that plug into the printer port of a laptop are available from lots of
sources for $100 or so.  Can't remember at the moment where I bought
mine but can look for the papers if you want.  

Dan