distance speed time

[Melanie]

Really you have to work out sensible limits and then choose the kind of methodology you are going to use...

If your mechanical shaft/disc is spinning every 5uS it has a rotational speed of 12,000,000 (twelve MILLION) rpm! This I would like to see... (I'd stand there with a stopwatch and measure the length of time until your mechanical contacts disintegrate and bearings turn to dust!). If you are measuring Wind-Speed, there is a direct correlation between the wind pushing the Cup, the Diameter of the Vane and the shaft rotation. Assuming a total vane diameter of 200mm, a single pulse per rotation every 5uS (and not taking any friction into account) a bit of math calculates a wind speed of 281,100 mph (two-hundred and eighty-one THOUSAND)... methinks a pitot tube would struggle with that one!!!!

Now, you really only have two choices - and each has it's limitations...

(a) You COUNT the number of PULSES in a given set time... this method is good for FAST Data, but falls apart for slow Data (eg you're counting one second but it takes 17 days for the shaft to rotate!).

(b) You COUNT TIME between the start of one Pulse and the start of the next... this method is good for SLOW Data, but falls apart for fast Data because of the speed limitation of the PICs.

[brianD]

thanks melanie for your comments,but it was not what i actually meant.
obviously the shaft won't turn that fast.

i get where you are going with this,but all i was looking for was a sensible formula within the chip.everything else was hypothetical.



thank you for the link mackrackit

[Melanie]

Well, start by deciding your limits of measurement...

For the sake of argument, let's over-simplify this...

If I was doing wind-speed down here on Earth, then I'd choose limits between 0 and say 150mph.

Next I'd decide how I'd measure the Wind-Speed... So I'm going to have a Vane diameter of 200mm... That means one rotation equates to 628mm of travel...

150mph equates to 67 metres per second...

67 metres per second divides down to about 107 pulses per second.

Is this good enough for what I want? If I was doing it, then I'd probably say No - mainly because I'd want more pulses per second so I could improve the resolution (unless you are going to have a really long sampling time which then means you won't be able to measure gust). And so the design process continues.

Only once I know what the worst-case scenario is, would I then decide on how my PIC is going to process this information - that's the easy part!

You want to run with a 20MHz PIC... probably total overkill for this application.

I certainly wouldn't go with a shaft with a contact on it (because contact bounce will give you nightmares)... I'd consider a magnet bonded to the shaft with a Hall-effect switch, or a slotted vane and an optical sensor (best option for improving resolution at minimal cost whilst still keeping the shaft balanced for reduced frictional losses).