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Checking the speed of a shaft at high rpm?


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bytraper



Joined: 31 Oct 2005
Posts: 195


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Checking the speed of a shaft at high rpm?

Hi guys, anybody know of an easy way to do this?

I was thinking of having a pulse generator on the end of the shaft (like those used on gearboxes) but I dont know how reliable they would be at high rpm...

Another idea was to use a optical system with a mouse type wheel counter to check the speed..

Anybody know if these would work? or could suggest a better way to calculate the rotational speed of a round shaft at say 5000-10000 rpm?

Cheers!

Post Tue Nov 08, 2005 1:15 pm 
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Grotto



Joined: 30 Aug 2005
Posts: 38
Location: Morisset NSW


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Speed checking

If you're driving the motor with a clean DC signal, and youve got
access to an Oscilloscope or a multimeter with a Frequence range,
You may be able to get a reading off the small voltage spikes
being generated and thereby get a frquency.
The Osc would work best, but you may get lucky with the multimeter too,
I just stick mine straight across the power wires and it sees the spikes
and tells me their frequency.

Once youve gotten a frequency the formula is RPM = ( 1 / F ) / 60
or the more basic RPSeconds = 1 / F

Hopfully this helps.
Cya
Grotto
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........CEO Cyberdyne Systems

Post Tue Nov 08, 2005 1:27 pm 
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bytraper



Joined: 31 Oct 2005
Posts: 195


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Hmmm I never knew that, thanks man

But I need to read the pulses from inside the bot while its operating, it doesn't have to read the speed in rpm, it just need it to return pulses or whatever to a pic chip I'll have running, I just need to constantly know the speed of the shaft so I can change some varibles in the bot !

Cheers!!

Post Tue Nov 08, 2005 1:35 pm 
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Knightrous
Site Admin


Joined: 15 Jun 2004
Posts: 8511
Location: NSW


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A hall sensor and a magnet mounted on the shaft will give you a fairly accurate RPM count.
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Post Tue Nov 08, 2005 2:15 pm 
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Grotto



Joined: 30 Aug 2005
Posts: 38
Location: Morisset NSW


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Speed sensing

Hmmm. I see your need now.

As TDT says, a magnet and hall sensor, or even a reed switch
may be able to read that fast but I'm not sure.
Assuming the weight of the magnet isnt too high to throw your
balance out and cause like bad vibes man. Cool

Another idea, use an IR led & photo-transistor looking at
shiny/dark patches painted on the shaft.
Giime a sec and I'll find a circuit of how to hook it to a pic,
Ive got one somewhere here.... Shocked


I'll be back
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"The future is not set. There is no fate but what WE make."
........CEO Cyberdyne Systems

Post Tue Nov 08, 2005 4:57 pm 
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Grotto



Joined: 30 Aug 2005
Posts: 38
Location: Morisset NSW


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Sensor Circuit diagram

Sorry, I still have figured out how to post an image yet.... Embarassed
(Any advice on posting pics in these messages anyone?)

If you look in my photo gallery in this forum you'll find it,
Its called "Infrared Sensor Circuit", and its the only photo there so far.

http://www.robowars.org/forum/album_pic.php?pic_id=1078

If youve got any questions about it, let me know.

Grotto
_________________
"The future is not set. There is no fate but what WE make."
........CEO Cyberdyne Systems


Last edited by Grotto on Mon Nov 14, 2005 11:46 am; edited 1 time in total

Post Tue Nov 08, 2005 5:56 pm 
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DumHed
Experienced Roboteer


Joined: 29 Jun 2004
Posts: 1219
Location: Sydney


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I made a basic optical tacho once by connecting a photo transistor to a cable with a 3.5mm microphone plug on the end, plugged it into a computer, and used a freeware frequency counter program that uses the sound card.

It worked really well, as long as the photo transistor is the right way around (due to the power being supplied across the mic connections to power a condenser mic)
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Post Tue Nov 08, 2005 10:51 pm 
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