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 [DPRG] Speedometer Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] Previous message: [DPRG] Speedometer Next message: [DPRG] Speedometer Subject: [DPRG] Speedometer From: Sluggy! sluggy9912 at swbell.net Date: Mon Jul 10 01:40:07 CDT 2006 ``` --- Balter Wiggin wrote: > What i am planning is a > combination RPM counter by means of a magnetic > encoder, and a speedometer, > probably fashioned through some modification of a > tachometer. The purpose of > this is to successfully measure the distance and > average speed of a bicycle > piloted around White Rock Lake. With the one sensor, you can measure distance travelled, as you established later in your message. You get one "encoder tick" for every wheel rotation. If your wheels is 26", then each encoder tick will represent 81.6816 (2pi x radius (2x3.1416) x 13) However, by precisely timing the interval between ticks, you can also determine the speed. Let's work the math out in reverse. If you have a 26in wheel and you're going 1 mph, that would work out to be 63,360 inches per hour (5280 feet times 12 inches per foot times 1). One wheel rotation means you have travelled a distance equal to the circumference of the wheel. Divide that by the wheel circumference 81.6816 and that would be 775.695 ticks per hour. Counting ticks per hour is not a very practical speedometer, so divide that number by 3600 (the number of seconds in an hour) and you get a much more manageable .215 ticks per second. Take the reciprocal of that (1 second divided by .215 ticks per second) and we're getting a really usable number, 4.651 seconds, or 4651 milliseconds per tick for 1 mph. Armed with this number, we can now measure the number of milliseconds per wheel rotation tick, get the reciprocal and multiply by 4651 and you get miles per hour directly. For example, if it's 1500 milliseconds between ticks (1/1500) x 4651 = 3.1 mph. If it's 561 milliseconds between ticks (1/561) x 4651 = 8.29 mph. For the entertainment value, if it's 4 minutes between ticks 1/(4x60,000) x 4651 = 0.0193 mph :) In practice, a 26in wheel is almost never exactly 81.6816 inches in circumference. Even as the tire wears, the circumference will shrink. In the speedometer application, small errors in the circumference measurement won't make much difference, however, if the diameter is off by 1/10 inch, the circumference calculation is off by nearly 1/2 inch. Two rotations is off by about an inch. 30,000 rotations is off by about 1/4 mile. To correct for that, the most practical approach is probably to directly measure the circumference of the wheel rather than calculating it. Use a sewing tape to measure the circumference of the wheel. This will be much closer to the real number needed for an accurate calculation of speed. Another approach is to passively roll the wheel a practical number of rotations, 10 for example, on a smooth surface and measure the distance travelled with a 100' tape, then divide by the number of rotations. My little Taiwanese bicycle speedometer is calibrated by entering the centimeters circumference of the wheel. It included a chart showing the figure to use for various brands of tire :) Robert --- tibi habere mamma. Ego malle mamma ab femina. ``` Previous message: [DPRG] Speedometer Next message: [DPRG] Speedometer Message index sorted by: [ date ] [ thread ] [ subject ] [ author ] More information about the DPRG mailing list