# optical rpm measuring w. frequency counter

Discussion in 'Electronic Basics' started by J.T., Sep 20, 2003.

1. ### J.T.Guest

Hi
does anyone have easy and accurate way of measuring rpms of diesel motor (or
any spinning device) optically.
Idea is to glue little reflecting tape to shaft and optically read pulses,
then convert pulses for frequency counter to display rpms
any help, schematic ??
thanks
J.T.

2. ### BaphometGuest

Not optical but....
http://www.e-insite.net/ednmag/contents/images/30499di.pdf

3. ### Dana RaymondGuest

Suck devices are off-the-shelf

4. ### GarethGuest

One way to do this is to use a strobe with an adjustable frequency. You
adjust the frequency of the strobe until the spinning device appears to
be still. The rate of rotation should then be the strobe frequency,
i.e. if the strobe frequency is 10 Hz, the rate of rotation is 10*60 =
600 RPM.

One problem with this method though is that the spinning thing will have
rotated a whole number of times in one strobe period, not necessarily
once, so in the above example the speed could be 1200, 1800 or... RPM so
you need to check at a few other strobe frequencies to be sure, or start
the strobe at the highest possible frequency and work down, the first
frequency that appears to be still should then be the correct one.

5. ### CFoley1064Guest

Hi
Hi, J.T. The problem with measuring this type of thing optically is that dust,
dirt, grime, and the thousand slings and arrows of outrageous fortune are all
conspiring to dirty your sight lines up.

Many high speed inductive prox switches can detect a cam follower signal up to
1KHz..Unless your motor or spiinning device goes faster than 60,000 RPM, all
you have to do is power up the prox switch (usually requires 12 to 24VDC),
point it at a piece of ferrous metal that's sticking out a little, and use a
pullup resistor from the output to your + (for NPN or current-sinking type
devices -- be sure to specify). You then just hook up your frequency counter
to the output, and you're good to go. Inductive prox switches are available
surplus, if you look around. View in fixed-font...

VCC
+
o-----------o
| |
| .-.
.-----. | |
| + | | | R
| | '-'
| |OUT |
| - o--------o----------------o
'-----' To Freq Counter
|
=== .------o
GND |
|
===
GND

created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

Remember to choose R such that you don't exceed max current.

Good luck.
Chris

6. ### SailorGuest

Yes this is a way of doing it..
But make also a mark on motor where spinning part passes. So when your
making your measurements you have a reference point. It wil make it much
easier to see if the picture is still or not.
I've done this may times to measure fan speeds

Greetings to all

7. ### SpajkyGuest

if the shaft has a some outpointing part like this letter: (Q)

spinning Q -magnetised coil-amplifier-frequency counter

the principle is the same like for guitar pick up or original Hammond
organ tone generator; just that outpointig shaft part has to be iron-
can be mounted additionally to the shaft (& will function just like a
guitar string close to its magnetic pick up (electrical guitar
normally!) producing pulses while shaft rotates!
cheap & effective way to do it!

-- Regards, SPAJKY
& visit - http://www.spajky.iscyber.com
Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
E-mail AntiSpam: remove ##

8. ### Don KellyGuest

----------------
Stripes on the shaft and an optical pickup will work. Accuracy depends on
the number of stripes.The more stripes, the better the accuracy and counting
should be for a fixed interval of 1 to 10 seconds (experience with a counter
set to its normal frequency measuring mode gives disappointing accuracy -).
60 stripes on a shaft at 1000rpm will give 100 pulses per second. Counting
for 1 second will get you to 1% accuracy on the basis of +/- 1 count. A 10
second count gives 0.1% accuracy for the 10 second average. This longer term
average also eliminates the effects of torque pulsations.
There are other options- A disk attached to an end of the shaft(if
available) with 60 black and 60 clear segments can be used with a light and
photocell works well. One or two pulses/revolution will not give decent
accuracy so the magnetic option is not good unless you can mount some form
of toothed wheel.

9. ### Baronvonrex420Guest

I bought a digital photo tachometer from TNC electronics in Woodstock NY for
less than \$50 USD. Just point and measure. These are used for model airplane
tuneups measuring the speed of the prop. He made minor mods to get me the
resolution I needed at no charge.
ph 914-679-8549

An alternative anyway.