Speed sensor detection

[jgd10]

Hi All, I'm working on a project that requires me to design a speed sensor for a flywheel connected to a drive belt. I'm somewhat of a beginner programmer with PICs and need some advice.

The PIC is receiving its input from a Hall Effect sensor connected to the PIC and a magnet on the flywheel. The Hall Effect emits a 5V signal until disrupted by the magnetic field and then drops to about 0.2 volts. The flywheel spins from 2000 up to 4500 rpms.

My question is this: what is the best approach to detect this signal from the Hall Effect sensor? There are several interrupts options (external interrupts, Port change interrupts) along with other options. I don't want to reinvent the wheel, but just need some recommendations from experts out there.

Please advise.

[Gabriel]

Read the pics datasheet.
Make sure it has a capture and compare hardware
Search the forum for examples
Do math..

You are done.
_________________
CCS PCM 5.078 & CCS PCH 5.093

[jgd10]

This seems like a great choice. Reviewing the datasheet this is what I came up with, let me know if I'm on the right track.

1. Place CCP1 in capture mode
2. When an event occurs on pin CCP1, capture time from TMR1 store in variable prevTime
3. Next event on pin CCP1, capture time and store in currTime
4. Use prevTime and currTime to calculate the speed

Thanks

[PCM programmer]

Yes and there are many threads in the forum archives which show how
to do this and discuss every possible problem.

[jgd10]

Thanks for all the help, I've made lots of progress over the past week. I did find quite a few code examples.

I did have a few questions though. On post http://www.ccsinfo.com/forum/viewtopic.php?t=29963
there was a question in regards to the L in the frequency equation.

[PCM programmer]

Data sheet.
http://ww1.microchip.com/downloads/en/DeviceDoc/30292D.pdf

[jgd10]

I understand, but it also states that:

[temtronic]

PIC peripherals, like the timers and UARTs ,will work without their interupts enabled. However if the interupt is enabled then a bit(flag) is set in a register and the PIC is made aware of it.
Similar to getting mail delivered.The mailman puts letters(usually bills!) in your mailbox, so your mail has come. If he doesn't raise the flag, you don't know there's mail unless you look.
Interupts are a fast,automatic way to KNOW some action has been done instead of having the PIC in a continous 'is it done yet ?' loop.


The 'L' indicates that the variable is a Long( 16 bits).The timer register is 10 bits wide, so without the 'L', 2 bits are missing,giving wrong results and actions.

hth
jay

[jgd10]

I see said the blind man.

Thank you.

[asmboy]

there are actually 3 ways to deal with an INT flag

1 - set up interrupt handler, and accept the side effects of doing so

2- POLL INT flag in main or in dedicated functions, where the main side effect is latency proportional ( roughly) to the inverse polling frequency

3- ignore ( and in the case of a timer for instance, read the timer value directly - to make process decisions )

each of these approaches has its uses

[gpsmikey]

A couple of other things to consider in your program - think about adding an "average" of the last 8 or so readings so that your "speed" is not jumpy if there are slight variations in the sensor signal. Also, if you are going to be displaying the result, keep your display update rate (if digital) to less than 2 or 3 times/second. Too many people try to update a digital display 100 times / second - the end result is if there is any variation, your eyes can't read the silly thing.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

[PCM programmer]