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Re: Calculating Speed with an accelerometer


On Tue, 08 Mar 2005 10:12:42 -0600, John Fields

>wrote:
>
>>On Tue, 8 Mar 2005 03:04:44 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith)
>>wroth:
>>
>>>[...]
>>>>Right. It might be difficult to get an oscillator that's sufficiently
>>>>g-insensitive to make this work.
>>>
>>>An atomic clock or Laser may be required.
>>>
>>>--
>>
>>    Bullshit.  2.4 GHz rf sources are readily available.  What's the doppler
>>shift at that frequency for 200 MPH velocities?
>
>---
>If the observer is aligned axially with the direction of travel of the
>transmitter,
>
>
>                 fC
>    f' = ---------
>              C +/- v
>
>
>where f is the frequency of the CW carrier radiated by the transmitter
>      f' is the oserved frequency
>      C is 3.0E9 m/s, and
>      v is the velocity of the transmitter.
>
>
>So, at with v = 200MPH (89.3 m/s)
>
>
>               2.4E9Hz * 3.0E9m/s
>    f' = --------------------- = f +/- 71Hz.
>               3.0E9m/s +/- 89.3m/s
>

---
Oops...

C = 3.0E8, so f' ~ f +/- 700Hz.

--
John Fields


Re: Calculating Speed with an accelerometer


'Calculating Speed with an accelerometer', on Tue, 8 Mar 2005:
>On Tue, 8 Mar 2005 03:04:44 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith)
>wroth:
>
>>[...]
>>>Right. It might be difficult to get an oscillator that's sufficiently
>>>g-insensitive to make this work.
>>
>>An atomic clock or Laser may be required.
>>
>>--
>
>       Bullshit.  2.4 GHz rf sources are readily available.  What's the doppler
>shift at that frequency for 200 MPH velocities?
>
It's velocity-dependent. You need to specify the relative direction as
well as the speed. (;-)
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
We've slightly trimmed the long signature. Click to see the full one.
Re: Calculating Speed with an accelerometer


>On Tue, 8 Mar 2005 03:04:44 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith)
>wroth:
>
>>[...]
>>>Right. It might be difficult to get an oscillator that's sufficiently
>>>g-insensitive to make this work.
>>
>>An atomic clock or Laser may be required.
>>
>>--
>
>    Bullshit.  2.4 GHz rf sources are readily available.  What's the doppler
>shift at that frequency for 200 MPH velocities?

Why does 200MPH qualify as the speed we care about?  200MPH is kind of
fast for a over land vehicle.

The g-shift in an oscillator is dependant on the frequency of said
oscillator.  ie: it is a constant % change.  As a result, increasing the
frequency doesn't help.

You've also neglected the fact that the doppler only gives you the
component along direct line of sight.

Also 2.4GHz is not such a great idea in a cluttered environment.  You will
get drop outs and multipathing.

--
--
snipped-for-privacy@rahul.net   forging knowledge



Re: Calculating Speed with an accelerometer



> On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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



Re: Calculating Speed with an accelerometer



>On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
>
>> On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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

    However, it's head and shoulders above an accelerometer approach.

    What was the consensus concerning why doppler shift woldn't work?

Jim




Re: Calculating Speed with an accelerometer



>    What was the consensus concerning why doppler shift woldn't work?

Do the math.

John



Re: Calculating Speed with an accelerometer



> On Tue, 08 Mar 2005 12:54:17 +0000, James Meyer wrote:
>>>On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
>>>> On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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.
>>>
>>>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.
>>
>> However, it's head and shoulders above an accelerometer approach.
>>
>> What was the consensus concerning why doppler shift woldn't work?
>>
> The amount of Doppler shift at any sane velocity is less than the
> stability/accuracy of any practical oscillator.
>
> Cheers!
> Rich
I think you could do this, by phase locking the onboard oscillator, to a
distant radio station. Given you are looking for vertical motion, which
would be perpendicular to the radio signals, this should give the required
stability.

Best Wishes




Re: Calculating Speed with an accelerometer


[...]
>I think you could do this, by phase locking the onboard oscillator, to a
>distant radio station. Given you are looking for vertical motion, which
>would be perpendicular to the radio signals, this should give the required
>stability.

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.

--
--
snipped-for-privacy@rahul.net   forging knowledge



Re: Calculating Speed with an accelerometer


> 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




Re: Calculating Speed with an accelerometer


>> 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.

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.

--
--
snipped-for-privacy@rahul.net   forging knowledge



Re: Calculating Speed with an accelerometer


>
> > On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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


Re: Calculating Speed with an accelerometer



>> On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
>>
>> > On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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



Re: Calculating Speed with an accelerometer


>
> >> On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
> >>
> >> > On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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



Re: Calculating Speed with an accelerometer



>> On Tue, 08 Mar 2005 15:06:43 +0000, James Beck wrote:
>>
>> >> On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
>> >>
>> >> > On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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.  ;-)

--Mac



Re: Calculating Speed with an accelerometer


>
> >> On Tue, 08 Mar 2005 15:06:43 +0000, James Beck wrote:
> >>
> >> >> On Tue, 08 Mar 2005 00:59:55 +0000, James Meyer wrote:
> >> >>
> >> >> > On 7 Mar 2005 13:39:25 -0800, snipped-for-privacy@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


Re: Calculating Speed with an accelerometer



> 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


Re: Calculating Speed with an accelerometer


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.


Re: Calculating Speed with an accelerometer



> 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.
>
> The wireless data transmission isnt the problem, the speed calculation
> is what Im thinking about. Analog Devices (ADXL202) and ST have
> accelerometers that output a PWM signal that a micro could then count
> over time and determine velocity.
>
> But since theres no absolute reference coming from the accelerometer,
> it seems like the calculated velocity could easily get out of sync over
> time and with stop/go bumpy acceleration.
>

This is the understatement of the year.

> Anybody have any thoughts on this?  Are there other approaches better
> suited?
>
> Ben

The accelerometer idea won't work at all, except maybe in a rocket, as
suggested by someone elsewhere in the thread.

I suggest you forget about the accelerometer and use GPS or ultrasonics,
or just use the rotation speed of the axle or motor or something.

--Mac



Re: Calculating Speed with an accelerometer


If I went with a GPS approach, would a GPS have enough precision to
resolve between an rc car moving pretty short distances, on the order
of a tens of feet maybe?

If the resolution is there, it seems plausable that changes in position
could be timed.



Re: Calculating Speed with an accelerometer



> If I went with a GPS approach, would a GPS have enough precision to
> resolve between an rc car moving pretty short distances, on the order
> of a tens of feet maybe?
>
> If the resolution is there, it seems plausable that changes in position
> could be timed.

The position reported by the GPS has plenty of precision. But there is a
lot of error. How the error varies with time is something I'm not too sure
about, but that is what you need to know. For example, if you imagine
logging positions from a stationary GPS over half an hour, you would
expect that the reported position would wander during that half hour. But
if it never wanders more than a foot or two over the course of a minute,
then GPS might work. If it jumps around 20 feet per second, then it
probably won't work for you.

There are differential GPS's that will definitely work, but for that,
you have to buy two, and they are expensive, and require either a radio
link or data post-processing. These things will give you sub-inch
precision and relative accuracy.

The way they work in the post processing mode is that instead of just
solving for position, they record all the GPS information from all the
satellites they can track. Later the post processing software figures out
the position of the mobile GPS with respect to the stationary one to very
high resolution.

If you have a real-time radio link between the two units, then they
transmit enough information back and forth to do the same thing, but the
mobile unit then does it itself, internally, in real time.

I wonder whether you could just use two regular GPS's, with one on the
vehicle, and one stationary, but nearby. Then whatever hardware is
receiving the messages from the RC car could also receive messages
from the stationary GPS, and subtract the two GPS positions (vector wise)
to get a vector from the known location of the stationary GPS.

There are a lot of reasons why this might not work very well, but it is a
thought. I doubt it would work any worse than a single GPS, which still
might work OK, especially if you have a good view of the sky.

Good luck. Don't let the scope of this project get out of control.
Otherwise you'll never finish!  ;-)

--Mac



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