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PICList Thread
'[EE] measuring rotation rate'
2007\05\18@013813 by David Cary

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Dear piclisters,

What are good methods for measuring rotation rate?

I have a rotating device.
Currently I measure how fast it rotates using a
one-pulse-per-revolution optical sensor,
and then assume that it rotates at a fairly constant rate between pulses.

Lately I wonder -- is it really rotating at a constant rate?
Or are there significant angular accelerations and decelerations between pulses?

I think an angular rate gyro should be able to directly measure such
angular acceleration.
The fastest one I've found so far advertises "300 degrees per second!".
I suppose nearly 1 revolution per second is nice.
But my device spins around 50 revolutions per second.

I suppose some sort of rotary encoder (along with accurately timing
the width of each pulse) could possibly work.
But all the off-the-shelf rotary encoders seem to assume they are at
the center of rotation.
It would be more convenient if I could bolt something to the outside,
with a pattern ring about a foot in diameter glued to the stationary
part and a sensor bolted to the rotating part. (Or vice versa).
(Perhaps replacing, the one-pulse-per-revolution sensor I have now).

Suggestions?

--
David Cary
http://massmind.org/techref/member/DAV-MP-E62a

2007\05\18@015528 by Lee Jones

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face
> What are good methods for measuring rotation rate?
>
> I have a rotating device.  Currently I measure how fast it rotates
> using a one-pulse-per-revolution optical sensor, and then assume
> that it rotates at a fairly constant rate between pulses.
>
> Lately I wonder -- is it really rotating at a constant rate?
> Or are there significant angular accelerations and decelerations
> between pulses?
>
> I suppose some sort of rotary encoder (along with accurately timing
> the width of each pulse) could possibly work.

> It would be more convenient if I could bolt something to the outside,
> with a pattern ring about a foot in diameter glued to the stationary
> part and a sensor bolted to the rotating part. (Or vice versa).
> (Perhaps replacing, the one-pulse-per-revolution sensor I have now).

Construct a flat disk with identical evenly spaces slots around
the outside edge.  Install disk on shaft, hopefully with little
wobble or runout.  Put an LED on one side of disk, shining through
slots.  Put a photo diode on other side of disk.  As shaft rotates,
photo diode output is train of pulses.

Non contact, so limiting factors are response speed of photo diode
and computational ability of microprocessor (at some speed, it will
start to miss pulses -- upper bound determined by number of slots,
shaft speed, any hardware buffering, CPU speed, quality of software,
species of dead fish(*), etc).

(*) I hadn't seen it mentioned on PIClist in a while, so ... :-)

                                               Lee Jones

old PIClist
output is train of pulses
of

2007\05\18@022809 by Jon Chandler

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In order to tell if there are accelerations/decelerations in the rotation
rate, use a rotary encoder as described below, and run the pulse stream into a
frequency/voltage converter.  At constant speed, the outout will be a DC
voltage.  Variations in speed show up as shifts in DC level.  

Jon




On Thu, 17 May 2007 22:56:24 -0700, Lee Jones wrote
{Quote hidden}

> --

2007\05\18@025857 by PICLIST

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face
Some current automotive applications use a missing tooth wheel for this. A
toothed wheel is either bolted to the front (or rear) or the crankshaft, or the
teeth are part of the crankshaft itself. A magnetic reluctance coil is used to
create pulses as the toothed wheel goes by.

The missing tooth is used to determine absolute position.

The automotive computers very carefully monitor the changes to engine RPM as it
turns. Changes in rotational speed can indicate a misfire (this is reported via
the OBDII diagnostics system). They are also sometimes compared with ABS brake
sensors at the wheels as a sudden decrease in rotational speed combined with a
sudden drop in tire speed can indicate the vehicle just hit a bump and did not
misfire.

Now that I think of it, the ABS wheel speed sensors are very much like what you
are looking for.

{Original Message removed}

2007\05\18@033406 by Russell McMahon

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> Lately I wonder -- is it really rotating at a constant rate?
> Or are there significant angular accelerations and decelerations
> between pulses?

What they said.
ie as others have suggested, a brute force and easy method is a multi
pulse per turn sensor.
Many possibilities - slotted disk, optically striped track, multipole
magnet, ... .
You could even have a semicontinuous method by having a sinusoidally
(or other) modulated sensor and measure the rate of change of signal.

BUT, can it in fact alter significantly in a single turn?
If you know moment of inertia and energy sources / sinks then you can
determine the maximum possible range of speed changes within a single
cycle. That will then tell you whether you need to know more
accurately.

Knowing what it is that you are measuring would help us comment.




       Russell


2007\05\18@035614 by Richard Prosser

picon face
Aten't there anenometers that work like this? With a  oversize cup and
a reference marker they can determine wind direction from the velocity
variations.

RP

On 18/05/07, Russell McMahon <spam_OUTapptechTakeThisOuTspamparadise.net.nz> wrote:
{Quote hidden}

> -

2007\05\18@092135 by Paul Hutchinson

picon face
> -----Original Message-----
> From: .....piclist-bouncesKILLspamspam@spam@mit.edu On Behalf Of Richard Prosser
> Sent: Friday, May 18, 2007 3:56 AM
>
> Aten't there anenometers that work like this? With a  oversize cup and
> a reference marker they can determine wind direction from the velocity
> variations.

There is one, the Rotavecta Wind Transducer from Raymarine (formerly
Raytheon Marine Division), I believe it is patented.

Paul  

>
> RP
>

2007\05\18@112832 by Tony Smith

picon face
> > Aten't there anenometers that work like this? With a  
> oversize cup and
> > a reference marker they can determine wind direction from
> the velocity
> > variations.
>
> There is one, the Rotavecta Wind Transducer from Raymarine
> (formerly Raytheon Marine Division), I believe it is patented.


It's not a bug, it's a design feature!  Imagine being in on that meeting.  

"So, your new anenometer is crap, the bearing are dodgy & the cups have poor
tolerances."
"Err... since the wobbles are consistant, we figure it'll measure wind
direction."
"Hmmm, brilliant - quick, to the patent office!"

Tony

2007\05\18@113356 by Bob Blick

face picon face

--- David Cary <d.cary+2004spamKILLspamieee.org> wrote:

> I have a rotating device.

Is it a secret device?

> Currently I measure how fast it rotates using a
> one-pulse-per-revolution optical sensor,
> and then assume that it rotates at a fairly constant
> rate between pulses.

Probably an incorrect assumption.

>
> Lately I wonder -- is it really rotating at a
> constant rate?

Of course not. Whether you see it or not depends on
your method of measurement and its resolution.

> Or are there significant angular accelerations and
> decelerations between pulses?

Ahh, back to the secret part. What's significant to
you  and your "device"?

Sorry, I'm not being grumpy or anything, you've just
given us no metrics, but want an opinion.

Cheerful regards,

Bob

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