Measuring RPM''s at relatively slow speeds.
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Posts: 46,084
How about a cog on the end of the shaft and an inductive sensor close to the
teeth? Another way is a transparent plastic disc with dark graduation marks
around the edge. One edge of the disc goes around inside a opto fork sensor.
Alex
Original Message
From: smartdim@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=IvdfPpoQiL_JO1D9mLzBZQaBRW6XbX6A7TfWDr4iT4dMtEzPmwfsmHZ227oFfoeb_8jykqgv]smartdim@a...[/url
Sent: Wednesday, 28 May 2003 15:07
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Measuring RPM's at relatively slow speeds.
In a message dated 5/27/2003 10:14:46 PM Pacific Daylight Time,
john@l... writes:
> My application would be less than 100 RPM, but accuracy would be important.
>
> I did manage to find some info on magnets on a shaft, but being I need
> accuracy at low RPM's that doesn't seem to fit my need.
>
> I'm sure its some sort of sensor that reads though holes or something
> (sorry for sounding stupid, but you know what I mean). I can make the
> 'disk' part of it at work, with any configuration I need, but what is the
> 'eye' part? Give me a shove, and I'll go do the searching.
>
> Thanks in advance,
>
> -John
>
>
John,
If you make a disk with many holes in it near the circumference of the disk,
you can use an opto coupler to sense the rotation. The opto coupler is
basically a photo diode and a photo transistor. The photo diode shoots a beam of
Infra Red light at the photo transistor. When the beam is blocked, the photo
transistor is "off" and when the beam is not blocked the phototransistor is
"on".....hence the holes in your disc will alternately block and not block the
beam.
You set the current in the photo diode part, much like you do for an LED,
know the total voltage supply, know the forward voltage of the photodiode, and
set the current using a resistor. Resistor value via ohms law.....270 to 330
ohms will do assuming a 5v supply.
The phototransistor section will be an NPN device. Emitter to ground,
collector gets a resistor to you supply voltage (if using stamp, then 5v).
Resistor
value of 4.7k should work well. The base lead (if so equipped) should get 1k to
ground to help the "turn off" time of the transistor.
You will sense the voltage at the collector. When the beam is blocked, the
collector will read 5 volts, and when the beam is not blocked, ~ 0.2 volts.
These resistor values are simply off the top of my bald head.....but I am
reasonably sure they will work.....
Write back if clarification or help is needed.
<A
HREF="http://dkc3.digikey.com/pdf/T032/1083.pdf">http://dkc3.digikey.com/pdf/T03\
2/1083.pdf</A>
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teeth? Another way is a transparent plastic disc with dark graduation marks
around the edge. One edge of the disc goes around inside a opto fork sensor.
Alex
Original Message
From: smartdim@a... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=IvdfPpoQiL_JO1D9mLzBZQaBRW6XbX6A7TfWDr4iT4dMtEzPmwfsmHZ227oFfoeb_8jykqgv]smartdim@a...[/url
Sent: Wednesday, 28 May 2003 15:07
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] Measuring RPM's at relatively slow speeds.
In a message dated 5/27/2003 10:14:46 PM Pacific Daylight Time,
john@l... writes:
> My application would be less than 100 RPM, but accuracy would be important.
>
> I did manage to find some info on magnets on a shaft, but being I need
> accuracy at low RPM's that doesn't seem to fit my need.
>
> I'm sure its some sort of sensor that reads though holes or something
> (sorry for sounding stupid, but you know what I mean). I can make the
> 'disk' part of it at work, with any configuration I need, but what is the
> 'eye' part? Give me a shove, and I'll go do the searching.
>
> Thanks in advance,
>
> -John
>
>
John,
If you make a disk with many holes in it near the circumference of the disk,
you can use an opto coupler to sense the rotation. The opto coupler is
basically a photo diode and a photo transistor. The photo diode shoots a beam of
Infra Red light at the photo transistor. When the beam is blocked, the photo
transistor is "off" and when the beam is not blocked the phototransistor is
"on".....hence the holes in your disc will alternately block and not block the
beam.
You set the current in the photo diode part, much like you do for an LED,
know the total voltage supply, know the forward voltage of the photodiode, and
set the current using a resistor. Resistor value via ohms law.....270 to 330
ohms will do assuming a 5v supply.
The phototransistor section will be an NPN device. Emitter to ground,
collector gets a resistor to you supply voltage (if using stamp, then 5v).
Resistor
value of 4.7k should work well. The base lead (if so equipped) should get 1k to
ground to help the "turn off" time of the transistor.
You will sense the voltage at the collector. When the beam is blocked, the
collector will read 5 volts, and when the beam is not blocked, ~ 0.2 volts.
These resistor values are simply off the top of my bald head.....but I am
reasonably sure they will work.....
Write back if clarification or help is needed.
<A
HREF="http://dkc3.digikey.com/pdf/T032/1083.pdf">http://dkc3.digikey.com/pdf/T03\
2/1083.pdf</A>
[noparse][[/noparse]Non-text portions of this message have been removed]
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