Ahhh, geekspeak! Some time, I love this list. :)
"Jay Mitchell" <jemitchell@compuserve.com> wrote:
> Jim Carr sez:
>> 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.
I had been aware of this, but wan't aware of the magnitude of the error until I
worked out the math myself. Kinda suprising to see a .3G induced error in
magnitude on a 1G "real" value.
> 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.
True enough, and in fact I had already decided prior to this thread that one of
the things to work into my event routine would be to make a sketch of the course
while I was walking it, and make note of camber directions.
However, it's probably worth pointing out that for most types of analysis, the
absolute values coming from the accelerometers aren't important. What is
important is the position, shape, and relative values between events on the
trace.
As long as the errors due to pitch/roll etc are consistent to each other, the
absolute values don't really matter.
It's also probably worth noting that when it comes to performance, the speed
line is king. The goal is not to optimise for acceleration, it's to optimise the
area under the speed curve. The acceleration traces are there to help evaluate
why the speed curve is shaped the way it is.
> you need to get the longitudinal accelerometer
> perfectly level when the car is on level ground - will contribute
> to the same tpye of error. Again, the g-analyst accounts for
> that.
So does the Edelbrock system, incidently.
> The g-analyst has a calibration setup that includes estimates of
> roll and pitch in degrees/g.
The Edelbrock system does not - yet. I'll be talking to their programmers about
adding this feature.
Incidently, although mine doesn't have one, there IS a provision in the
Edelbrock system for a third accelerometer - or one could be mounted to the car,
and use one of the analogue channels to log it.
> The ideal system would also include a gyroscope-stabilized horizon
> indicator as well
There's a web page out there done by a Formula SAE team that details their track
mapping system, and the troubles they had with it. They wound up incorporating a
GPS and a magnetic compass (for slip angle determination) into their logger, and
got very impressive results.
I actually looked into the compass sensor they used, but it's $700. :(
In any case, I doubt any accelerometer-based system that will be anywheres near
affordable will be capable of generating track maps with enough resolution to
determine line from. The track map feature is useful to act as a reference to
help relate the strip chart (where the real information is) to a rough
geographic reference, and that "red line = slowing, green line = accelerating"
feature of GEEZ! is pretty nifty, but that's pretty well it. Line determination
needs accuracy to the order of centemetres.
One cool idea I had though would be to set up one of those road race lap trigger
beacon thingies at the start and finish lights at major races. Then, anyone with
a data logger could use those beacons as start/stop log triggers. That'd be
_real_ handy.
How much do those systems cost?
DG
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