Calculating Speed with an accelerometer

[Zak]

Mac wrote:

OK, well, that is interesting, but if you were trying to convince me that
the LDRS uses local oscillators on their rockets, then detects Doppler
shift from the ground, you haven't exactly succeeded. I will have to
reserve judgement. ;-)

What could work is transmit some frequency to the ehicle, double or
triple it in frequency there and radiate it back.

This should give a nice phase difference to measure. Multiple receivers
on the ground can obviously use the same reference signal.

If interference is not a problem somehow and range is limited the
frequency tripler can be rather simple and even passive.




Thomas

 

[James Beck]

In article <pan.2005.03.10.02.40.42.981277@bar.net>, foo@bar.net says...

On Wed, 09 Mar 2005 16:29:29 +0000, James Beck wrote:

In article <pan.2005.03.09.04.26.40.360674@bar.net>, foo@bar.net says...
On Tue, 08 Mar 2005 15:06:43 +0000, James Beck wrote:

In article <pan.2005.03.08.02.51.36.678287@bar.net>, foo@bar.net says...
On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:

On 7 Mar 2005 13:39:25 -0800, benn686@hotmail.com wroth:

Im thinking of a senior project for next year, and thought of making a
module that calculates velocity and then can transmit the data
wirelessly. I could demonstrate it on a r/c car, rocket, etc.

For a rocket, the easiest way would be to include a small transmitter in
the rocket. The transmitter's output frequency would be doppler shifted due
directly to velocity. A receiver on the ground would measure the shift. Both
transmitter and receiver would need appropriate frequency stability.

Jim

There was a thread about this right here in this newsgroup a while back.
IIRC, it is not at all obvious that this is the best approach or that it
will even work.

--Mac


I think that the LDRS uses such a system.
It is how they get accurate measurements on the 1200MPH models some of
these guys have.

Jim

I don't know what ldrs is. A google search didn't really enlighten me. It
obviously is some kind of rocket society, but I didn't see anything about
using oscillators and Doppler effects for estimating altitude.

--Mac


LDRS : Large and Dangerous Rocket Society.
I saw the unit being used on one of the Discovery Channel specials.

Jim

OK, well, that is interesting, but if you were trying to convince me that
the LDRS uses local oscillators on their rockets, then detects Doppler
shift from the ground, you haven't exactly succeeded. I will have to
reserve judgement. ;-)

I'm not trying to convince you of anything.
Just reporting what I saw. I tried locating the info and could not.
The info is worth every penny you paid for it

Jim

 

[Dan Major]

Jumping in rather late in the thread...

If I remember correctly (and I'm sure I do), the rocket velocities and peak
altitude were measured by a radio telemetry unit, probably an R-DAS
(http://home.iae.nl/users/aed/rdas/).
The problem with using a flying transmitter and ground-based receivers to
mesure the doppler shift is that the shift is so small for practical
transmitter frequencies and the velocities the rockets travel at. The R-
DAS used barometric pressure and accelleration for its measurements.

On a similar note, it was just about 11 years ago when I presented a poster
at AAHPERD* on the use of accellerometers in measuring human performance.
Until then no one had come up with a way of continuously measuring power
output, especially instatanious power and power/distance ratio. We hooked
up accelerometers to different types of stationary exercise equipment (leg
press, Smith machine, etc) and logged the data into a computer. We only
recorded the accelleration over time, and post run calculated power,
velocity, distance, etc, using Excel.

*AAHPERD=American Association of Health, Physical Education, Recreation,
and Dance.