accelerometer based dyno ideas

[Chris Conlon]

At 11:07 AM 1/28/99 -0800, Orin Eman wrote:

>> At 08:24 AM 1/28/99 -0500, Dan Llewellyn2 wrote:
>
>> >you can get up to speed are rare.  If they incorporated the data
>> >from a G-field measuring device, like an Analog Digital ADXL05,
>
>Yes, but you still need the speed input since the accelerometer
>cannot tell the difference between acceleration and gravity...

Yeah, sort of. There are a lot of intermediate levels depending on
how many simplifying assumptions you make. If you require the road
to be flat (not necessarily level) and suspension travel to be
effectively nil, you can subtract out the known constant 1g gravity
vector. (This assumes a 2-axis accelerometer, which the adxl202 is.)

If you try to solve the more realistic problem, and allow nonflat
roads and suspension travel, it gets trickier. For my own amusement
I've tried to see how much of that you could reliably factor out,
but haven't gotten too far.

>As far as the horsepower calculation is concerned, you would
>use the acceleration from the accelerometer and the speed from
>another sensor...

If you have the speed, from RPM, VSS or whatever, I guess you could
use that info in combination with the accelerometer info to
compensate for nonlevel (and perhaps nonflat?) roads. I haven't
really thought about this yet.

In any case you still need mass plugged in as a given, right?
To get horsepower rather than acceleration I mean.


>> One thing I was gonna mention is the Analog Devices ADXL202, a
>> second generation, 2 axis +/- 5g accelerometer. It's even easier
>
>I looked at it when it first came out and the specs didn't look
>very good to me.  It's bad enough getting .01g resolution out
>of an ADXL05 with any reasonable bandwith and the 202 looked worse
>to me.

I agree that getting a high resolution*bandwidth product out of the
xl05 is not easy, but I don't see how it's any worse for the xl202.
The noise spec is exactly the same for both parts. Maybe if you needed
to use the PWM output on the xl202 that would make it tougher, but
the raw analog signal is easily available too.  I'm not trying to
irritate anyone here, maybe there's just something I'm missing?

   Chris C.