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Optical Speedometer

 The vehicular equivalent of an optical mouse, for accurate speedometerage
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Regular car speedometers can never know the exact speed of a vehicle without knowing how quickly the wheels rotate combined with the precise circumference of the tyres.

An optical speedometer system would be on the underside of the car, seeing the surface of the road move beneath it, in much the same way as the surface of a desk might pass under an optical mouse; assuming the technology would work at this scale without having to be dangerously close to the road surface.

Using this system to calculate mileage might be easily foiled by a mounted sheet, to fool the system into thinking the road is not moving, thus stopping mileage increment.

—clausage, Jun 20 2002

Solid State Accelerometers http://www.digikey....=99649&Cat=30540265
cheap, low-drift accelerometers sold there (no, I don't work for this firm.) [dsm, Jun 21 2002, last modified Oct 04 2004]

Afro's "Faster Roads" idea http://www.halfbake...22Faster_22_20Roads
Using road stripe reflectivity to calibrate speed (enhanced contrast) in conjunction with Afro's device could lead to some interesting discussions with speed cops... [resurgere, Oct 04 2004]

Ski Speedometer http://www.halfbake...a/Ski_20Speedometer
[waugsqueke, Oct 04 2004]

Direct optical velocity measurement http://www.corrsys-...optical_sensors.htm
I assume these use a laser to measure perpendicular velocity (it is common to do this is the steel industry) [Ling, Jun 13 2005]

[link]






       Keep the manufacturer-recommended diameter tires on your car and you'll be close enough. A doodad would have to have a lot more usefulness than this to get me to pay more for my car.
—phoenix, Jun 20 2002
   

       I've pondered this question since I was a kid and knew little about physics. Now I'm all grown up, I'm still a kid, but I do now know there is an easier way, and the optical approach could be the answer. An optical device might suffer the problems of road grime covering its sensor. Mounting the device above, or angled to aim at the tread of a tyre, and havng it measure its rotation, presents a problem in that you would be travelling faster and further when turning right compared to left, for example. Having 2 devices mounted above the left and right tyres, and computing the differential (no pun) might produce the most accurate speed reading.   

       Australian Design Rules specify a maximum 10% speedometer accuracy error(oxymoron?), yet the police will fine you for 3kph over the speed limit. Unless the road system is changed to operate at a max of 30kph, or everyone has an opto-speedo installed, waive the fines...
—Johnny Mash, Jun 20 2002
   

       I do not understand why a speedometer should be made using anything other than a solid-state accelerometer chip (which are now dirt-cheap and accurate enough (a few percent) for this job.)
—dsm, Jun 20 2002
   

       Question for [clausage]: how big a difference in the speed measurement could the inaccuracies hinted at in the description make?
—polartomato, Jun 20 2002
   

       I saw an article about a GPS based speedometer recently. Looking for the site.
—senatorjam, Jun 21 2002
   

       [dsm]: Accelerometers with small enough drift (DC offset/noise) to be used as speedometers are not dirt cheap.
—wiml, Jun 21 2002
   

       Senatorjam: The sampling rate on GPS isn't quite fast enough to provide useful realtime speed data. Trust me - it's one of the things I'm working on at the moment.   

       dsm: Yes, that's waht we're using to make up for the shortcomings of GPS.   

       Polartomato: We have measured errors as large as 17%   

       wiml: You can factor out most of the errors by using two cheap accelerometers and some fancy software. It works.   

       Phoenix: see my reply to Polartomato.   

       Johnny mash: The answer to the optical obscuration problem is to use ultrasonics (which require a lot of filitering and noise extraction in that enviroment) or microwaves (don't work well on some road surfaces, we've found). Both will work OK with substantial contamination/occlusion of the emitter/reciever. Ultrasonics can be upset by water films and spray in some circumstances, but are less bulky than the microwave kit.
—8th of 7, Jun 21 2002
   

       what is the speed limit of the optical detector then? Say for a mouse. Anyone happen to know?
—neuronet, Feb 05 2003
   

       How about using radar? That's what "they" use.
—snarfyguy, Feb 05 2003
   

       You need a reliable reflective target as your reference point. See my comment above re microwaves (= radar).
—8th of 7, Feb 05 2003
   

       I see. Back to the drawing board then.
—snarfyguy, Feb 05 2003
   

       I have a need for a device that can, very accurately (about ±1cm error per 10m), measure the distance a vehicle has traveled regardless of the vehicle speed. Cumulative error over a long distance isn't an issue, though. Rather, every x number of meters, say every 5 meters, the system can send a signal and then reset itself to count to the next 5 meters and send a signal and so on.   

       What's best? Accelerometer? 5th wheel?
—bristolz, Feb 05 2003
   

       I would have thought the 5th wheel would be more accurate as variants of it are the thing I've seen used most often by highway engineers in the UK
—oneoffdave, Feb 05 2003
   

       [bristolz] I used to run auto road rallies quite a bit. (The type that are run with street-legal vehicles on ordinary roads). To be competitive at the top tiers you need an odometer that can read to 1/1000 of a mile or better.   

       What many rallyists use is a home-bakeable setup. It requires a reed switch attached to the frame of the car near where the rim of one of the rear wheels passes. Then you glue on the rim of the wheel a set of evenly spaced electromagnets. The better accuracy you want, the more magnets you add, but ensure they are equally spaced around the rim.   

       As the wheel rotates, the magnets pass the reed switch and generate a pulse. This is then fed into a computer that can count the pulses and convert them to distance traveled. (You can even use a conventional parallel port as the input with the right connector, no special hardware required).   

       This can measure distance traveled quite accurately, but it does require a calibration step at the beginning of each measurement run to calculate the number of pulses per foot.
—krelnik, Feb 05 2003
   

       I went off and read about people who use bicycle computers adapted for the car. They recommended magnets every 15°. I need something that stays calibrated for a long distance, like 1000 miles or more.
—bristolz, Feb 06 2003
   

       [krelnik] - would it be possible to acheive the same thing by tapping into the ABS sensors?   

       // sampling rate on GPS isn't quite fast enough to provide useful realtime speed data // [8th of 7] - do you need to look at your speedometer more often than every two seconds?   

       My speedometer crapped out (fluctuates wildly). GPS now lives on the dashboard and we get along fine.
—lurch, Feb 06 2003
   

       [bristolz] Road rallies often last 1000 miles, so that would work. Your main problem with calibration is extreme changes in temperature which causes the tires to expand/contract. (Even that isn't a big deal with modern tires, though if you are running an antique car with authentic tires it can be a BIG problem). 15 degrees per magnet sounds about right.   

       [lurch] Don't know anything about how ABS senses wheel movement but its certainly an interesting idea.
—krelnik, Feb 06 2003
   

       >I do not understand why a speedometer should be >made using anything other than a solid-state >accelerometer chip (which are now dirt-cheap and >accurate enough (a few percent) for this job.)   

       OK, but don't drive the odometer from it unless you want mileage accumulated on vehicle transporters, tow-trucks, motorail and ferries to count towards your service intervals!
—SteveAdams, Feb 07 2003
   

       C'mon, [Steve]. Just load 'er on the transport backwards.
—lurch, Feb 07 2003
   

       bristolz, a nice link for you ^
—Ling, Jun 13 2005
   

       Thanks!
—bristolz, Jun 13 2005
   

       [krelnik] I don't get how counting revolutions of the wheel is going to remain accurate enough over the sorts of distances [bristolz] is talking about, particularly if the wheel is driven, which is why highway engineers seem to use a fifth wheel. I imagine wheel slip is going to cause more of an error than expansion or contraction of the tyre due to temperature - look at the recent experience of the Mars rover. I think you'd also need encoders on the steering to account for turns - that is going to be a pretty complicated set of calculations.
—AbsintheWithoutLeave, Jun 13 2005
   
      
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