Converting period to Miles Per Hour

[SFOdesigns]

Hi,
I'm utilizing the 18F46K22 on a project. I'm using all of the CCP modules for other purposes.

My problem is that I cannot figure out a formula to calculate miles per hour using the period between two positive pulses. The pulses are generated by rotor bolts on a wheel. There are 5 bolts, and the circumference of the wheel is 78.4 inches. It seems to me that there should be a formula that will allow me to measure the period using an interrupt driven routine, and the calculate miles per hour. But I don't know how.

Can someone help? I'm using PicBasic Pro.

Roy

[Acetronics2]

Hi, Roy

1 wheel turn = 78.4 inches ... 5 bolts per turn

15.68 inches per pulse

1 mile is 63360 inches ... = 4040.816 pulses ( or 1 034 449 / 256 ... )

Done.

BTW ... does PBP Silver edition support the 46K22 ???

Alain

[Amoque]

Adding to Alain's expert analysis of distance.

RATE * TIME = DISTANCE, so... If 1 rotation takes 1 second then:

78.4 inches per second * 60 seconds = 4704 inches per minute. Multiply by 60 minutes to find distance per hour = 282,240 inches per hour or (dividing by 63,360 inches in one mile) = 4.45 MPH.

-OR-

[INCHES PER SECOND] * 3600 / 63360 = MPH

The above works only if the inches in one second are known. So, we must also know how to calculate the [INCHES PER SECOND]... Take the example of .4 seconds per rotation:

To calculate the [INCHES PER SECOND] multiply the reciprocal (1/X) of rotation period by the wheel diameter (78.4) to find:
1 / .4 = 2.5 rotations per second. 2.5 rotations (2.5 * 78.4) = 196 inches = [INCHES PER SECOND]. Now, complete the formula above (11.136 MPH) . Not remarkably this is 2.5 times the 4.45 MPH figure calculated for 1 rotation per second - allowing for rounding.


DISCLAIMER: The accuracy of these figures is only as correct as the time base from which they are figured. IE: timer overflow vs atomic clock.

[Acetronics2]

Hi, Amoque

Here we meet the precision problem with reciprocal calculations ....

sooo ... I would suggest, depending on the pulse count or the pulse duration, to choose the value that gives the minimum error to the result.

I.E. ... @ low speed, let's consider pulse duration ( big number gives better resolution ) and @ High speed let's consider the pulse count.

But ... as our "friend" didn't tell what he wants to measure ( snail or car racing ??? ) ...

Alain

[Amoque]

Hope is not lost, Alain. He may be back as he left his answer here - only leaving his wallet would motivate him to return more promptly. We need only wait for him to notice then... nab him and we shall have answer!

It may be we may cajole him into revealing where the [leftover] pumpkin pie is hidden as well!

[Acetronics2]

http://sfodesigns.com/aboutsfodesigns.html

that could match the question ... and create some ...

Alain