Calculating Speed with an accelerometer

[Roger Hamlett]

The phase of a received RF signal changes as you get near the surface of

the earth. Since the goal is to measure the distance this would
normally
be purely a confounding effect. You may want to check to see if the
phase
relationship between two signals could give you the needed information.

Or you could just calculate an adjustment for this. It is a fairly well
measured effect.

Best Wishes

 

[James Beck]

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

 

[Thomas Magma]

OK now we're thinking outside the box!

I like the microphone idea. I'll just add onto it if I may.
The RC car will have a certain pitch at different velocities...so why not
just FFT (and filter) the sound of the car to determine velocities. Even
when the batteries start to die, the sound of the gears will remain constant
based on any given velocity.

You could use just a pick-up mic to listen to the car, or for extended
range, use a cheap wireless mic.

Thomas

 

[Ken Smith]

Or you could just calculate an adjustment for this. It is a fairly well
measured effect.

Yes it is well measured, but that is mainly because it varies from place
to place. If it didn't, people wouldn't still be measuring it.

 

[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. ;-)

--Mac

 

[Zak]

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]

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.