Around 700 Hz. So if the oscillator pulls 70 Hz at max g-force, the velocity error is 10%. 70 Hz is 0.03 PPM. You won't find a small oscillator that will hold 0.03 PPM with even 1G of acceleration change.
So un-bullshit.
John
Do the math.
John
That's exactly the way oscillators are tested for g sensitivity. Gotta do all directions, of course.
John
--
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.
So if you flip it over on the bench it will change twice that?
Easy enough to try.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
On Tue, 08 Mar 2005 02:51:37 GMT, Mac wroth:
However, it's head and shoulders above an accelerometer approach.
What was the consensus concerning why doppler shift woldn't work?
Jim
On Tue, 8 Mar 2005 03:04:44 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken Smith) wroth:
Bullshit. 2.4 GHz rf sources are readily available. What's the doppler shift at that frequency for 200 MPH velocities?
Jim
I read in sci.electronics.design that James Meyer wrote (in ) about 'Calculating Speed with an accelerometer', on Tue, 8 Mar 2005:
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. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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.
-- -- kensmith@rahul.net forging knowledge
Both
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
On 2005-03-07, snipped-for-privacy@hotmail.com wrote: | 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.
This sounds like fun!
| 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.
A number of posters have mentioned that there is an issue with using a
2D-accelerometer and not being able to detect rotation in the measurement plane. A really neat web site that has what looks like a great hobbyist part to solve this isScroll down to the IMU Combo Board. Note that they have a link under that product to:
Which is a GPL-licensed project creating autopilot software and hardware for an R/C helicopter. The software has a Kalman filter and utilizes GPS as well as INS and magnetic heading detection.
sparkfun.com has some really amazingly neat stuff, if you ask me. I haven't ordered anything yet, but I will.
| 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.
Some systems have tables of the earth's magnetic field declination for the locale and use a three-axis magnetometer.
| Anybody have any thoughts on this? Are there other approaches better | suited?
I have theories but no practice on this. It would be great if you shared your practice back to this ng or to the autopilot-devel list.
My armchair advice would be: filter vibrations mechanically first, by using hard rubber mounts for the sensor board. Then filter vibrations with an R/C network and DC output with a decoupling cap.
Have fun!
Heath
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
Both
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
Yes, almost exactly. It usually is the direction that puts the disk of the crystal horizontal that gives the biggest change.
Also, crystals tend to shift the first time you run them through shake and environmental testing. I'd expect that the crystal will shift if the RC car is allowed to crash into a wall.
-- -- kensmith@rahul.net forging knowledge
In article , Roger Hamlett wrote: [...]
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.
-- -- kensmith@rahul.net forging knowledge
This might work. You will still need to deal with bias (offset).
I don't see what good the foam does. I would rather have the thing rigidly mounted to a rigid frame rolling on very hard wheels. Any freedom of motion between the accelerometer and the vehicle will just lead to errors.
Touching a hot soldering iron is a great learning experience, too.
The gyros might be OK for providing yaw information over the short term, but I still don't think the accelerometers will work very well.
Everybody comes into a project with a different set of skills and experiences. It is my opinion that unless the OP has a lot of talent or some really good people on his team, he needs to keep it simple or he will not submit a working project at the end of the allowed period.
If the floor is even half a degree out of level, the error will be totally intolerable after 60 seconds. If my calculation is correct, the velocity error will be something like 5 meters per second,(using v = at, with a being 9.8m/s * sin(0.5 deg)) and the distance error, using d=0.5at^2 is over 100 feet.
If he really wants to do telemetry, maybe he could just put a microphone on an RC car, digitize the audio, and send it over the wireless link. It would sort of be a spy-mobile. A camera might be even better.
Or why not just take speed off of the axle or motor?
I'm just trying to save Ben's butt.
If an accelerometer MUST be used, some ground truth needs to be applied to rein in the drift. Maybe the car could be stopped periodically (every couple of seconds) at which point a message would be sent to the u-controller, somehow, telling it that the current bias readings on the accelerometer should be zeroed out.
--Mac
There are lots of GPS receivers with much more frequent updates. They might be too expensive, though. A slow GPS combined with full inertial sensors might actually work, too, since the GPS data can pull the IMU back toward reality. But I think the scope of the project would then be too large to be realistic, unless Ben buys a packaged combination of IMU and GPS.
--Mac
If you're interested in speed, GPS receivers measure that directly via the satellite doppler shifts. It's a lot more accurate than anything you could hope to achieve by position difference.
A position is required for this internally in the receiver, but it doesn't have to be very accurate. 100m position error gives you better than 0.1m/s velocity error for most constellations.
As someone else mentioned, most GPS receivers will output measurements only once per second. However, inside a classic tracking receiver, the doppler shifts are available at sample rates of 100-1000 per second. You can buy more expensive GPS units which can afford to run the maths more frequently, giving you 2-10 updates per second.
Once per second might be enough, as the speed output is the momentary speed at the time of the timestamp. Make sure you disable any smoothing filters in the receiver.
The professional approach to your problem uses GPS for long term calibration and accelerometers+gyroscopes for short term accuracy. Look up "sensor fusion" and Kalman filters.
Kind regards,
Iwo
Or you could just calculate an adjustment for this. It is a fairly well measured effect.
Best Wishes
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.