Jim Carr sez:
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| Exactly. Tilting the accelerometer causes two errors. A component
| of the acceleration of gravity will now be along that axis of the
| accelerometer and be interpreted as a change in velocity, and the
| actual acceleration of the vehicle will no longer be parallel to
| the axis of the accelerometer and part will get missed. The
| properties of the trig functions make the first effect big and the
| second small.
Jay Mitchell wrote:
>
> This is also a problem in a car with no pitch or roll going up or
> down a grade or cornering on a curve with camber. The best you
> can do using only acceleromters is to resolve accelerations in
> the plane of the pavement.
Which is what you want to know for racing or navigation. As I
neglected to mention and you imply here, you must have three
accelerometers (and/or other data) to convert raw readings into
useful ones. Although I did not give the equations for doing
that, they are similar to what I talked about before. You only
have to assume that local gravity does not change. ;-)
> This is why the g-analyst has 3
> accelerometers on orthogonal axes. The others all make do with
> 2-axis accelerometer systems.
If, as you write below, the g-analyst does not do the correction
for you, I would say that this may be why they have three
accelerometers but that they may not make full use of them.
Or perhaps I don't quite understand the calibration setup
and what it does.
> The g-analyst has a calibration setup that includes estimates of
> roll and pitch in degrees/g. There's a table in the manual giving
> typical values of these parameters for different types of cars
> ranging from large family sedans to formula cars.
So should we assume that GEEZ! has a built-in table of expected roll
and pitch for measured G values so it can do the correction without
knowing what the vertical load is (the G-Cube is stated to have only
two accelerometers)? It would seem that this is the only thing it
can do. If so, my comments below apply to adjustments other than to
the "loose nut behind the wheel".
> It's not
> perfect, but you can account for pitch angle and resolve the
> actual deceleration to a new axis that isn't necessarily
> coincident with any of the accelerometer axes through relatively
> simple rotation of axes mathematics.
Exactly, which is what I did in my example. However, if you have
an approximate calibration of that type built in for a given car,
then an adjustment that changes the amount of roll or pitch for
some maneuver will change the G reading even if the true Gs in the
"pavement reference frame" stay the same. In particular, the
deduced reading could drop while the true Gs increase if the other
changes mean the car does not roll as much as before.
It would be important to know what GEEZ! does internally about
this so one would do the right thing with comparison data.
Jim Carr
BS 1993 Miata & Old Fartz physicist
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