AVR Freaks

Try this:

1 MPH = 5280 feet per hour
5280 * 12 = 63360 inches per hour
63360 / 6.28 = 10089.17 ticks per hour
10089.17 / 60 = 168.15 ticks per minute
168.15 / 60 = 2.8 ticks per second

So you get 2.8 ticks per second for every mile an hour you are going.

1 mph = 2.8
2 mph = 5.6
3 mph = 8.4
4 mph = 11.2
5 mph = 14
6 mph = 16.8
7 mph = 19.6
8 mph = 22.4
9 mph = 25.2
10 mph = 28
11 mph = 30.8
12 mph = 33.6

So anything beyond 3 ticks per second and you know you are going at least 1 mph. Anything beyond 6 ticks is at least 2 mph. 9 ticks, 3mph

12 ticks, 4 mph
14 ticks, 5 mph
17 ticks, 6 mph
20 ticks, 7 mph
23 ticks, 8 mph
26 ticks, 9 mph
28 ticks, 10 mph

So if you want speed, just count ticks per second and write code that does this:

IF ticks > 3 THEN speed = 1
IF ticks > 6 THEN speed = 2
IF ticks > 9 THEN speed = 3
IF ticks > 12 THEN speed = 4
IF ticks > 14 THEN speed = 5
IF ticks > 17 THEN speed = 6
IF ticks > 20 THEN speed = 7
IF ticks > 23 THEN speed = 8
IF ticks > 26 THEN speed = 9
IF ticks > 28 THEN speed = 10

No math involved, short and guaranteed to work.

Of course you have to have as many IF/THEN statements as miles per hour you want to measure but I bet it will be a more processor efficient process than doing floating point and it will be just as accurate.

Good luck,

Some Guy

distance=rate x time. 30mph=20ft/sec=240in/sec=9.6 tire revs/sec (576 tire rpm), and about 38 sensor tics per sec (4 per rev). You can measure the ms between tics or count how many in an interval of one sec, which ever way is easier. Both use a timer.

Your analysis is correct. But, why perform two multiplies of 60?

Multiplying 60 x 60 = 3600 so, just perform one multiply by 3600 and save some computation time and FLASH...

Also, 8" x 3.141593 = 25.132741", which will only be a small amount more accurate but, none the less, more accurate. You did not specify your end application so, that minute accuracy difference may not matter.

But, as the tire diameter and Pi are both constants, and, as you already know that you are going to use 4 ticks per revolution, 25.132741" / 4 = 6.283185", 6.283185" can simply be a constant, as well.

Not that it matters but... Oddly, 6.283185" = 2 x Pi and is directly related to radians! So, you could also express the rotational data as radians per second, radians per minute and, radians per hour.

Now your equation becomes:

      Ticks * 6.283185"
MPH = ----------------- * 3600S
             12"

Generally, if the sample period was over a 1 second interval and you accumulated 4 ticks:

      4 Ticks * 6.283185"
MPH = ------------------- * 3600S = 1.427996MPH
             12"

But:

6.283185" * 3600S 
----------------- = 1884.955592 <--- This is a constant...
       12"

So:

      Ticks x 1884.955592 
MPH = ------------------- 
              5280'

So for Integers, if the sample period was over a 1 second interval and you accumulated 4 ticks:

      4 Ticks x 1884.955592 
MPH = --------------------- = 1.427997MPH
              5280'

If you are going to use floating point:

1884.955592 
----------- = 0.356999
   5280'

So here:

MPH = 0.356999 x Ticks

If the sample period was over a 1 second interval and you accumulated 4 ticks:

MPH = 0.356999 x 4 Ticks = 1.427997 MPH

Finally:
As all but the sensor Ticks resolve to constants, the sample interval can be recalculated to reflect MPH directly - with out any math calculations at all.

As I chose a 1 second sample interval to determine MPH and, as MPH for a 1 second sample interval = 1.427997 MPH, simply take the ratio of 1 MPH and the 1.427997 MPH value that was calculated in the above examples.

                 1 MPH
Sample Rate = ------------ = 0.700282 Seconds
              1.427997 MPH

So, for the direct reading interval method, the time between samples will be 0.700282 seconds. That is, if you use a sample rate of 0.700282 seconds (700.282 miliseconds), the result will be direct reading (in MPH) without any math at all.

Incedentally, this was the very reasom I initially chose a 1 Second sample interval.

I use the direct reading sample interval method in PLC programming quite often in my "Day Job."

Edit:
Please note the formula below that was produced by jayjay. Now compare that to my floating point example above...

jayjay's example formula confirms my example and, my example confirms his formula, as well.

jayjay1974 wrote:

Which can be calculated using integers as:

mph=(ticks*357)/1000

      4 Ticks * 6.283185"
MPH = ------------------- * 3600S = 1.427996MPH
             12"

What do you output to? Needle? LED display? LCD display? Might be cool to also have a watt meter or batt charge meter. Does the phrase Peukert Effect ring a bell?