Are there any GPS-modules from where it is possible to get out speed more frequently than once / second?
- 10 Hz would be the ideal rate.
- I'd like to measure speeds on vehicles, typically 50 km/h. Speeds less than 10 km/h or over 150 km/h are not important.
- The location information is not important.
- Low power consumption is important (battery powered unit).
This sounds like a good job for an accelerometer.
You get speed from a GPS by looking at where you are at one time, and where you are at another, and dividing distance by time. If you wanted
10 Hz readout at 50 km/h, then you'd be looking at a change of position of 14 meters per second, or 1.4 meters per update. If your GPS could measure to 14 centimeters in a tenth of a second, then you would be able to tell whetehr you were going 50 km/h or 55 km/h. But you can't get that good with civilian GPS.So you should use a GPS to tell you how fast you are going over one second, and an accelerometer to tell you that during the third tenth of that second, you were going 2 km/h faster than average...
-- David M. Palmer dmpalmer@email.com (formerly @clark.net, @ematic.com)
If you can connect to the OBD connector, I believe the VSS information is available at that point (and you can power off the vehicle's electric system, too).
That's what I am afraid too, but the device should work without _any_ installation.
GPS does not measure speed as a change of position / time, but using doppler effect. I do not see why a device could not provide more than one measurement / second. A civilian GPS should be able to measure speed to 0.1 nautical miles accuracy.
Not every vehicle has an OBD-connector, so that cannot be used in this case.
How? This would only give you acceleration, not speed. Of course you could integrate acceleration, but this will give you drift errors.
That's not how a GPS receiver determines speed. A GPS measures the doppler shift of the satellite signals to measure speed, so the indicated speed is instantly accurate.
Meindert
In my area, at least, it is a federal mandate that all vehicles have an OBD connector.
Having exhausted that possibility, how much are you allowed to modify the vehicle? A magnet on the wheel and a Hall effect sensor in the vehicle body would do the same thing. (But this is quite challenging to keep working longterm).
You can also use a MEMS accelerometer attached to the wheel hub with double-sided tape. Count the peaks on one axis...
I don't think that's quite correct. GPS calculates speed by using doppler shift (sort of). I'm not a GPS expert - just a heads-up that you may want to research further.
Commercial velocity measurement systems are available that claim 7 updates/second with 0.1 mph accuracy.
-- Al Balmer Sun City, AZ
To what kind of precision?
Keep in mind that GPS devices typically use Doppler to measure speed
--- that basically means to compare the speed of your vehicle to the speed of light. That's a dynamic range of 3*10^7 you're talking about, and now you want to get 10 readings of that per second, on a severely restricted power budget? That's not particularly likely to happen. Not for civilian-grade hand-held GPS units running on batteries, anyway.
-- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
One clarification, I mean an accelerometer plus a GPS unit. Also, a compass would help, if you don't want to work things out from context or if the road is extremely twisty.
You don't need any installation beyond what you need for the GPS unit. They can be in the same box, however it is not installed.
It works out to the same thing, in theory. "By looking at the radio signals, I determine that I have moved 20 cm (= 1 wavelength) in the past second" is equivalent to "I have detected a doppler of 1 cycle/second, so I am moving at 20 cm/s".
In practice, working in the frequency domain uses different techniques that working in the time domain. (The difference between theory and practice is that, in theory, there is no difference.)
A device can report many measurements per second. It usually doesn't because it gains accuracy by using a low-pass Kalman filter, and so the additional reports are little better than inter/extrapo-lations.
A speed of 0.1 mph measured over 0.1 seconds corresponds to 5 mm, or less than 10 degrees of carrier phase. GPS signals are very weak (-130 dBm, like a microwatt EIRP a mile away) and it is very hard to reliably measure such a weak signal to such high phase accuracy in such a short time, even without real-world complications such as multi-path.
-- David M. Palmer dmpalmer@email.com (formerly @clark.net, @ematic.com)
I read all 10 replies before I responded. I think a lot of what they say is correct in that the higher update rate will not buy you a lot due to the filtering done on the values returned, but certainly there may be applications that still need the frequent updates. I have recently worked with two brands of GPS modules. By modules I mean very small surface mount devices that are used much like a chip. You provide an antenna connection, power and an LVTTL level serial interface to a processor. The module will spit out a variety of reports including speed and will do so at 4 updates per second. These modules are designed for embedding in a cell phone (aren't they all at this point?) and so the size and power are very low. I belive they are about 100 mW.
The one we picked for our project was from Fastrax called the iTrax03S. The second runner up (and a very *close* second it was) is the LEA-4S from uBlox. uBlox is the company that is putting up the money for the Antaris devices from Atmel. I believe Atmel is the fab house and they have an agreement to sell the chips to markets that uBlox is not pursuing. uBlox seems to be selling Antaris 4 based modules while promoting the next gen uBlox 5 "all on one" chip at 8x8 mm. Add the crystals, the SAW filter, antenna and 50 mW of power and you have a working GPS unit!
On rally-cars, motorcycles, snowscooters, bicycles, go-karts, trucks and busses too? I have understood that OBD-connector is mandatory on registered passanger cars only.
It seems that the 10 Hz is not easily possible, so anything better than the typical 1 Hz would help. The typical speed precision of 0.1 nautical miles / h would be ideal, but on worst case I could accept precision of
1 km/h.
Thanks, I'll check these ones.
Without _any_ installation I mean the the device could be "thorwn in". The device can be in any angle, which means that simple accelerometers cannot be used.
I could accept lower update rate and/or lower accuracy. The typical update rate (1Hz) is however too low.
Jyrki Holopainen wrote in news:v3wWg.257$ snipped-for-privacy@reader1.news.jippii.net:
Some low cost modules output raw data (pseudoranges, integrated carrier ranges and instantaneous Doppler) at 10Hz (e.g. the u-blox Antaris based receivers). I do not know of modules that are able to output position and velocity with 10Hz, Probably their processor lacks power to do the calculations at this rate. But the math is not too difficult, so if you add your own processor you could calculate position and velocity at 10Hz using the raw data. I have done some tests with the u-blox TIM-LP (see
Cheers,
Sam
less
Un bel giorno Jyrki Holopainen digitò:
If you just need the speed, you could try to use the so called "raw data". Most GPS modules are able to output some intermediate values (basically pseudoranges, ephemeris and doppler shift) at a higher speed than the normal update rate; the really heavy calculations (triangulation and coordinate transformation) are made after.
You need to check on your GPS manual/datasheet if it has the ability to output raw data (and perhaps to disable the triangulation/trasnformation algorithms to save power). Then you need some very strong basics on GPS, since you have to convert the doppler shift of each satellite signal into a speed value.
-- emboliaschizoide.splinder.com
A lot of the UBLOX modules will update at 4Hz. I've found that adequate for navigation at speeds up to about 20m/second (~45mph).
Mark Borgerson
That's gonna be *very* hard to find. You're asking for 1e-9 precison in an uncontrolled environment.
A precision of 1 km/h during 0.1 seconds corresponds to less than 0.03 meters of position tolerance. You're asking for a GPS that's principally accurate to an inch, but just refuses to tell you about it. Or, to put it differently: you're requiring the frequency of a very faint GHz wave to be counted to less than one cycle time.
And as if that wasn't enough, the unit will be getting different Doppler-shifted frequencies from different satellites, all mixed into a single antenna signal, so it also has to read the data contained in those streams to know where the satellite with the dominant signal is, so you can know its Doppler signal.
-- Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de) Even if all the snow were burnt, ashes would remain.
If you have 3 accelerometers at right angles, you can have the device at any angle, as long as it doesn't slide around (which would also be a problem in the GPS-only case). It will have to teach itself which direction corresponds to forward acceleration, and depending on the vehicle type (e.g. how much sideslipping it does, how fast it turns...) a compass would be useful for converting among north, direction of motion, and the derived nominal forward.
-- David M. Palmer dmpalmer@email.com (formerly @clark.net, @ematic.com)
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