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'[EE] Low cost encoder. Is it a compass?'
2005\02\16@184430 by James Newton, Host

face picon face
Is this just a compass chip mounted above a magnet on a motors shaft?

http://www.encoderdevices.com/ed_15/ed_15.htm

It can't be that simple, can it?

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2005\02\16@193022 by Russell McMahon

face
flavicon
face
> Is this just a compass chip mounted above a magnet on a motors
> shaft?
> http://www.encoderdevices.com/ed_15/ed_15.htm
> It can't be that simple, can it?

Why not.

A GMR sensor (or two) gives output proportional only to angle of
field. Add some basic electronics and you have a position encoder.

2005\02\17@045759 by Alan B. Pearce

face picon face
>Is this just a compass chip mounted above a magnet on a motors shaft?
>
>http://www.encoderdevices.com/ed_15/ed_15.htm
>
>It can't be that simple, can it?

I guess the sensor chip consists of a pair of hall effect sensors at right
angles to each other, which could probably produce the 4096 step accuracy
they talk about, with a bit of on chip processing.

2005\02\17@050555 by Michael Rigby-Jones

picon face


>-----Original Message-----
>From: .....piclist-bouncesKILLspamspam@spam@mit.edu [piclist-bouncesspamKILLspammit.edu]
>Sent: 17 February 2005 09:58
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Low cost encoder. Is it a compass?
>
>
>>Is this just a compass chip mounted above a magnet on a motors shaft?
>>
>>http://www.encoderdevices.com/ed_15/ed_15.htm
>>
>>It can't be that simple, can it?
>
>I guess the sensor chip consists of a pair of hall effect
>sensors at right angles to each other, which could probably
>produce the 4096 step accuracy they talk about, with a bit of
>on chip processing.

That's how I would imagine it to work, but I'm intrigued to learn how
thaey have managed to remove any non-linearities in the system.  I may
be miles off the mark, but I'd have thought that the output from a hall
sensor would not be proportional to the angle of a magnet, and
furthermore that the output would be heavily dependant on the relative
mounting  points of the magnet and sensor.  Trying to get a totally
repeatable system that dosen't require a lot of calibration must be
tricky, at least with 4096 counts per revolution.

Regards

Mike

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2005\02\17@092312 by Mike Hord

picon face
> That's how I would imagine it to work, but I'm intrigued to learn how
> thaey have managed to remove any non-linearities in the system.  I may
> be miles off the mark, but I'd have thought that the output from a hall
> sensor would not be proportional to the angle of a magnet, and
> furthermore that the output would be heavily dependant on the relative
> mounting  points of the magnet and sensor.  Trying to get a totally
> repeatable system that dosen't require a lot of calibration must be
> tricky, at least with 4096 counts per revolution.

I just went through all of this on a much smaller scale, with GMR sensors
instead of Hall effect.  Repeatability with Hall effect is much more doable;
GMR sensors have nasty levels of hysteresis.

At any rate, if you keep a magnet at the same distance from a Hall effect
sensor and change the angle of the magnet field (say, by rotating the
magnet), you'll find that the field varies in a manner which would easily
allow you to measure the relative position of the magnet- with one caveat.

The measurement is going to vary sinusoidally from full scale to 50% of
full scale.  However, with only one sensor, your readings will be identical
at either side of the circle.  This can be accounted for by checking whether
the reading is increasing or decreasing- first derivative test, IIRC.

I guess I should also mention that this is assuming that the axis of the
magnetic field is perpendicular to the axis of rotation.  From the design
of this product, I'd say they intend for you to bore a hole (with a lathe,
probably) in the center of the shaft of your motor, then put a magnet in
that hole which is cylindrical.  The magnet should be polarized not from
flat face to flat face but along some axis perpendicular to the center.

Those aren't impossible to find; wondermagnet.com sells quite a few
sizes of those, and in quite powerful magnetic alloys (?proper term?)
as well.

I think I'll put some math-heavier stuff in another message...

Mike H.

2005\02\17@103334 by Mike Hord

picon face
Okay, math heavier.  This is going to suck, doing in plain text, but
here goes.

The arbitrary field strength from a magnet dipole is

H~ = (m/4*pi*(R' )^3)(Rhat*2cosX + Xhat*sinX)

where H~ is the magnetic field strength vector at the sensor,
m is the magnetic moment of the dipole, R' is the magnitude
of the distance from the dipole, Rhat is the unit radius vector
and Xhat is the unit vector of deflection from the axis of the
magnetic field.  This is in spherical coordinates; the Yhat
vector plays no role since it represents rotation about the
magnetic field axis.  Classically, X should be theta and Y
should be phi, but including those in a plain text e-mail is
just asking for trouble.

Anyway, in this case, we have a magnet and sensor, between
which the distance isn't changing, we can write off the first
portion of the RHS, (m/4*pi*(R' )^3), as a constant k.  That
leaves

H~ = k(Rhat*2cosX + Xhat*sinX)

The single sensor in this case gives us only magnitude, not
direction.  So we're really interested in |H~|.  Nothing in the
magnitude operation is going to alter or remove those two
sinusoidal functions, so the resulting measurement will
clearly be sinusoidal.

The current position along a sinusoid can be fixed if we
know magnitude and slope, so the sensor must be both
reading the current value and comparing it to prior values
to establish the current position.

It has to have some help from the user to figure out which
direction it's turning, though, because without that info,
the sensor won't know if it's gone, say, 10 degrees
clockwise versus 350 degrees anti-clockwise.

I hope this is clear enough.  It's been a while since I did
too much of this sort of thing.

Mike H.

2005\02\17@132708 by James Newtons Massmind

face picon face
Yep, that fried my brain... But I'm very interested in this...

Does "unit radius vector" (Rhat) mean the distance from the magnet to the
hall effect sensor?

Xhat is then the value we want to solve for? No, it must be X we are
interested in. So I don't understand what "unit vector of deflection from
the axis of the magnetic field" means. Help?

Once I understand the math a bit better I can start the hell of trying to
solve for X and convert it into code.

The need for the slope could be removed by having another sensor at 90' to
the first?

Aside from being slightly less than human with regard to math (RAH says
"people who  don't understand at lease calculus are at best sub-human") I'm
also pretty clueless on analog electronics.

Does anyone have a schematic for connecting a hall effect sensor to a PIC in
such a way that the reading is available as an analog value? All I can find
are go/no-go sort of interfaces where the sensor is being used to count
pulses.


---
James.



> {Original Message removed}

2005\02\17@135622 by Mark Scoville

flavicon
face


{Quote hidden}

2005\02\17@143807 by Mark Scoville

flavicon
face
Sorry. I was reading this thread and accidently replied with an "empty"
response. I think it was when I hiccuped after a big swig of soda pop. Don't
beat me up too bad.

Please accept my most humble apology.

-Mark



2005\02\17@144019 by Mike Hord

picon face
> Does "unit radius vector" (Rhat) mean the distance from the magnet to the
> hall effect sensor?

> Xhat is then the value we want to solve for? No, it must be X we are
> interested in. So I don't understand what "unit vector of deflection from
> the axis of the magnetic field" means. Help?

First, a note on the names.  If you're confused, they aren't Rhat as
in "rat", but as in "are hat".  My physics prof pronounced them
as "are hat" or "eks hat", and so I naturally do too.

Xhat and Rhat are usually written as X or R with a little ^ over the top.
These are unit vectors; for example, in cartesian coordinates, Xhat,
Yhat, and Zhat would be vectors of length 1 in the positive direction of
the X, Y, and Z axes, respectively.  

It becomes far less intuitive in radial or spherical coordinates.  To
visualize spherical coordinates, imagine a point in space (the origin).  
Any other point can be described by its distance from the origin (R),
rotation away from some meridian (theta)(analogous to longitude on
Earth), and by ascension or declination above or below some
equatorial plane (phi)(analogous to latitude on Earth).  Consider the
magnetic field of Earth: ideally (assuming Earth is a perfect sphere
with the magnetic poles at the geographic poles), at all points on the
equator (different values of theta), the magnetic field strength is the
same.  Move in phi or R (move towards one pole or the other, or
move closer or farther from the center of the dipole) and you'll see a
difference.

So when I say unit vector of deflection from the axis of the magnetic
field, I'm saying that it is a vector, of unit length (1), which takes an
otherwise scalar value and assigns it some meaning.  That is, the sinX
portion contributes based on the angle between the position of the
sensor and the central axis of the magnetic field.  It's tough to
visualize a unit vector of dimension whose magnitude is stated in
terms of an angle.

> Once I understand the math a bit better I can start the hell of trying to
> solve for X and convert it into code.

May not be necessary.  After all, the magnet will change character over
time, and two different magnets in different devices will not be identical.

What you're really after is maxima and minima.  When the value is at its
maximum, it is 180 degrees rotated from its minimum.  At 50% of the
difference, it is halfway between.  A few easy to calculate points in the
middle (30 degrees, 45 degrees, and 60 degrees spring to mind) may
allow for your device to self-calibrate.

The math I did above was just a demonstration to show that rotating the
magnet will produce a time-varying field, and that that field is in fact a
sinusoid.

> The need for the slope could be removed by having another sensor at 90' to
> the first?

Yes.  At the possible expense of making things harder, since now you're
dealing with two sensors, and doing math between the two.  I'm not
convinced it's necessary.

> Aside from being slightly less than human with regard to math (RAH says
> "people who  don't understand at least calculus are at best sub-human") I'm
> also pretty clueless on analog electronics.

This probably won't take a lot of heinous analog work.  Probably some filters,
some amplification, but nothing that can't come straight out of AoE or a little
Radio Shack Forrest Mims book.

> Does anyone have a schematic for connecting a hall effect sensor to a PIC in
> such a way that the reading is available as an analog value? All I can find
> are go/no-go sort of interfaces where the sensor is being used to count
> pulses.

There are commercially available analog output Hall effect sensors.  A company
called Melexis makes one that is actually programmable; part number
MLX90215 (IIRC).  I looked at using one for the project which has had me
thinking about magnetic fields so much, but even it turned out to be too
insensitive for my application.  I ended up with the GMR sensor, which was
VERY sensitive (rotation in Earth's field produces significant signal
deflection)
but has nasty hysteresis and can easily (very easily) be damaged by exposure
to excessive magnetic fields (even large ferrous objects coming to close- for
example, a screwdriver).

The MLX90215 is available from Digikey, with several selectable gains, for
about $10.  Or, if you prefer, you can buy a $900 programmer.  Or, contact
Melexis and they'll (if you plead enough) send you the programming specs
for the chip and you can knock together your own programmer (I did that;
it wasn't too tough).

Mike H.

2005\02\17@144310 by Mike Hord

picon face
> The MLX90215 is available from Digikey, with several selectable gains, for
> about $10.  Or, if you prefer, you can buy a $900 programmer.  Or, contact
> Melexis and they'll (if you plead enough) send you the programming specs
> for the chip and you can knock together your own programmer (I did that;
> it wasn't too tough).

I'll add to that (I guess) that if you find yourself wanting (for some
unknowable
reason) my primitive kluged together programmer, I'll send you the plans and
executable for it.  I don't think I ever did draw it out; might be a
good thing to
do.

It is EXTREMELY primitive.  Really.  Don't say I didn't warn you.

Mike H.

2005\02\17@152511 by Stephen R Phillips

picon face

--- James Newtons Massmind <KILLspamjamesnewtonKILLspamspammassmind.org> wrote:

> Yep, that fried my brain... But I'm very interested in this...
>
> Does "unit radius vector" (Rhat) mean the distance from the magnet to
> the
> hall effect sensor?

I think you need to understand it in terms of circular coordinates.

>
> Xhat is then the value we want to solve for? No, it must be X we are
> interested in. So I don't understand what "unit vector of deflection
> from the axis of the magnetic field" means. Help?
>
X is a cartesian coordinate value no? I think you are looking for a
radial measurement. So in your case you are solving for alpha or the
angle.  The magnetic field strength changes as the magnetic field
rotates about an axis.

> Once I understand the math a bit better I can start the hell of
> trying to solve for X and convert it into code.
>
I suggest grabing your old algebra book and a physics book.  Then draw
up a model of what you are looking at (IE grab a pencil and paper).  By
model write down your 'Given' IE you have a magnet, you have a sensor
that is location in a specific place etc. Then write what you are
sensing, in this case magnetic field strength AND it's direction (I
hope  you are otherwise it would be a really frivolous exercise in
futility :) ).  Then comb through your old physics book and look at
information reguard magnetic fields and the field strength relative to
a bar magnet given a point on a cartesian plane.  Convert that
information to circular coordinates relative to the center of the
magnet.  Your sensor is essentially a POINT on a circle that the magnet
rotates within.

> The need for the slope could be removed by having another sensor at
> 90' to the first?
>
I suggest looking up a 'resolver' and the associated information
reguarding it. That's really what they have, they just changed the
information and obfuscated it.  Analog devices has had a resolver to
encoder IC available for a number of years. I suggest you look into
this because that's really all they have, it's nothing new at all.  In
fact I HIGHLY recomend you look at analog devices motor control
information, it should be very helpful.
www.analog.com/Analog_Root/static/marketSolutions/motorControl/index.html
This application note might help as well (smile).
http://www.analog.com/UploadedFiles/Application_Notes/394309286AN263.pdf

> Aside from being slightly less than human with regard to math (RAH
> says "people who  don't understand at lease calculus are at best
> sub-human") I'm also pretty clueless on analog electronics.
>
That would include all of electronic systems? Digital has analog
behavior :) especially at high clock rates or heck even low clock rates
just depends on the application :)

> Does anyone have a schematic for connecting a hall effect sensor to a
> PIC in such a way that the reading is available as an analog value?
> All I can find are go/no-go sort of interfaces where the sensor is
> being used to count pulses.
>
These are digital hall affect sensors, they have a hysteries (think
Schmidt trigger input from a linear hall affect sensor) input from the
hall device that changes it's output state when the magnetic flux
increase beyond or below preconfigured threshhold values.  They use
diffrent gain values in the hall sensor to create different thresholds
for toggling the output of the Schmidt trigger.

Have fun! I'm sure you'll have no troubles thinking this through. :)


Stephen

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2005\02\17@161445 by Russell McMahon

face
flavicon
face
> Aside from being slightly less than human with regard to math (RAH
> says
> "people who  don't understand at lease calculus are at best
> sub-human") I'm
> also pretty clueless on analog electronics.

Just goes to show that RAH spouted garbage with the best of them when
he wanted to (and maybe, like most of us, also sometimes when he
didn't want to).

Anyone with an average IQ and many below that COULD understand
calculus IF it was explained intuitively to them. Many don't get this
opportunity for whatever reason.

FWIW, the turn of phrase "... at best ..." is especially elitist and
unfortunate.


       RM

2005\02\17@192242 by James Newtons Massmind

face picon face
Ah, I think I have it. The spherical vectors may have components of X, Y and
Z if converted to Cartesian coordinates, and the unit spherical vectors
always have a length of one. As X changes (the motor shaft rotates) the
magnetic field strength changes based on the sin and cos of X but only as it
relates to the alignment of the magnet with the sensor (Rhat and Xhat).

And after all that thought, yes, it makes sense that only the max and min
matter anyway.

The MLX90215 is interesting although the cost is more than what I expected.
If you publish your programmer I'll put it up on the site so other people
can make use of it.

http://www.analog.com/UploadedFiles/Application_Notes/394309286AN263.pdf was
an interesting read

I'm thinking of trying something much simpler to start.. Just an LC
oscillator with a little coil for the L placed next to an arm on the shaft
of the motor. Then see if I can count the number of oscillations per unit
time and detect changes as the motor is turned. The nice thing about an osc
like that is if I drive it hard enough, the signal at the input pin is
basically digital.

---
James.



> {Original Message removed}

2005\02\17@200109 by Russell McMahon

face
flavicon
face
> I'm thinking of trying something much simpler to start.. Just an LC
> oscillator with a little coil for the L placed next to an arm on the
> shaft
> of the motor. Then see if I can count the number of oscillations per
> unit
> time and detect changes as the motor is turned. The nice thing about
> an osc
> like that is if I drive it hard enough, the signal at the input pin
> is
> basically digital.

If you are after revolution counting then a magnet and inductor will
give you an essentially sinusoidal output. If you feed the inductor to
a differential amplifier (as simple as a single opamp and not much
else) then you will get a high amplitude square wave. Fails at low
enough speeds when signal level gets too low wrt amplifier offset
error, but is liable to be usable in practical situations. More turns,
stronger magnet, smaller gap all allow lower speeds.


       RM

2005\02\17@211101 by James Newtons Massmind

face picon face
No, I was thinking of an oscillator. I know from my dads old tube radios
that LC or "tank" circuits will tune an astable multiviberator (oscillator)
to a frequency that can be changed by passing a metal core though the coil
of the inductor. Most old radios used variable capacitors, but at least one
that I remember seeing was tuned by rotating another coil inside the main
coil.

Simple metal detectors are two oscillators setup so that one is affected by
the presence of metal and the other is not. The difference between the two
is an audio "beat" frequency which is amplified and fed to the speaker.
http://home.clara.net/saxons/bfo.htm

Obviously, the change in frequency isn't much, but perhaps it is enough that
a fast processor can detect it. If not, a beat frequency oscillator isn't
much to add.. All the components are low cost.


---
James.



> {Original Message removed}

2005\02\17@215603 by Russell McMahon

face
flavicon
face
>> If you are after revolution counting then ...

> No, I was thinking of an oscillator.

I could see that you were THINKING of an oscillator.
The question (mine anyway) is, what are you trying to achieve?
Looking back at the thread I see it wasn't posed as an application
question but as a "is this yyy just an xxx" so we seem to have
wandered into something without every having had a requirement
specified in any way. (Unlike the more usual threads where the
trequirement is not specified well).

What is the (technology and possible-solution free) aim of the
exercise?


       RM


2005\02\18@091400 by Mike Hord

picon face
> What is the (technology and possible-solution free) aim of the
> exercise?

A shaft-position encoder, with only one sensor, which is completely
non-contact and where the shaft's current position can easily be
determined even if the encoder's electronics completely fail and are
restarted with no knowledge of what's currently happening.

At least, that's what I'm thinking of. ;-)

Mike H.

2005\02\18@142351 by Peter L. Peres

picon face


On Thu, 17 Feb 2005, James Newtons Massmind wrote:

{Quote hidden}

If you are trying to make something accurate then surplus selsines
(transmitters) could get you a fair distance in the right direction for
relatively little money (I saw some for $5). A selsine is a rotary LVDT
with 3 phases. You can use only two phases if needed. These have the
potential to resolve better than 0.1 degrees if in good condition.

Peter

2005\02\18@145131 by steve

flavicon
face
> > What is the (technology and possible-solution free) aim of the
> > exercise?
>
> A shaft-position encoder, with only one sensor, which is completely
> non-contact and where the shaft's current position can easily be
> determined even if the encoder's electronics completely fail and are
> restarted with no knowledge of what's currently happening.
>
> At least, that's what I'm thinking of. ;-)

A.K.A. a Contactless Throttle Position Sensor.

Steve.

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2005\02\18@162557 by James Newtons Massmind

face picon face
part 1 3237 bytes content-type:text/plain; (decoded 7bit)

The point of an encoder is encoding the position of a shaft with minimal
hardware expense.

Optical encoders are fine, but the ones that give an absolute position tend
to be fairly expensive. Homemade versions can use quadrature and vernier
methods to gain accuracy at very low cost, but without absolute position:
www.massmind.org/io/sensor/pos/enc/quadrature.htm
http://www.massmind.org/new/letter/news0312

Originally the device I linked to looked good, but appears to be fairly
expensive as well. The basic heart of the device (an analog hall effect
sensor) is better than $10 and would still need a uC to correct the signal,
etc...

After reading the appnote for that base device, I was inspired by their
description of a magnetic coil pickup and a recent interest my son had in
metal detectors.

I wondered if a very basic metal detector couldn't be used to measure the
distance of an arm attached to the motor shaft. A uC could manage automatic
calibration based on minimum and maximum signal and signal correction to
provide an angular measurement.

The base component cost for a BFO metal detector is pennies.
http://home.clara.net/saxons/bfo.htm is probably more than needed, I've
attached a less complex version.

The coil could be small since the metal is very close. The beat frequency is
audio and so well within the sampling range of a PIC or SX. Given sufficient
drive, the signal should be strong enough to trigger a digital input; no A2D
required since we are only interested in the frequency.

The BFO has two coils, in a metal detector, one is the reference and the
other is the "search" coil which is actually held to the ground to detect
metal. In this application, one coil can be placed at 90' to the other so
that as the arm passes closer to one than the other, the variation is twice
what it would be.

There are millions of possible problems and solutions:
- Noise: cover in metal can? Multiple samples with averaging?
- non-linearity: Table correction with interpolation?
- hysterisys: Another layer of correction based on prior position?

But it seems to me to be an interesting idea that has not been explored in
the hobbyist circles.

---
James.



> {Original Message removed}

2005\02\18@164838 by James Newtons Massmind

face picon face
yep

---
James.



{Quote hidden}

> -

2005\02\18@164939 by James Newtons Massmind

face picon face
Ahh... That would be the coils for this idea.

---
James.



{Quote hidden}

> -

2005\02\18@192141 by Tim_S

flavicon
face
these have been around for a couple of years that i know of.
i tried to make something useful but i guess my smarts aren't smart
enough<G>.
i have some devices that work good.but getting the end result is beyond my
expertice.
i wanted to design a multi turn absolute encoder scheme using a pic.
typical motor 1750 rpm.
i put it on the back burner for quite some months.
if anyone is interested e mail me off list and save the bandwidth.



http://www.austriamicrosystems.com/03products/sensor_int_magnetic_sensors.htm

http://www.austriamicrosystems.com/03products/stand_prod_start.htm



2005\02\18@194824 by James Newtons Massmind

face picon face
The austraiamicrosystems part is about $10 again:
http://www.findchips.com/avail?part=AS5040

Are you saying that you tried to use the AS5040? Or that you tried to make
something that did the same thing?

What problems did you run into?

---
James.



> {Original Message removed}

2005\02\18@195657 by Tim_S

flavicon
face
no i used the device. incremental and absolute versions.
my pic experience is well almost null.but still
learning.duh....picklist..yeah i read it alot!
price is about 5 bucks usd.
there is a market at least for me to use the damn thing.
non contact almost lasts forever.
unless u program a trip into the pic.
almost like the check engine light on the vechile programmed to come on
every
thousand miles for service.
----- Original Message -----
From: "James Newtons Massmind" <EraseMEjamesnewtonspammassmind.org>
To: "'Microcontroller discussion list - Public.'" <RemoveMEpiclistEraseMEspamEraseMEmit.edu>
Sent: Friday, February 18, 2005 7:48 PM
Subject: RE: [EE] Low cost encoder. Is it a compass?


{Quote hidden}

>> {Original Message removed}

2005\02\19@112111 by michael brown

picon face

----- Original Message -----
From: "James Newtons Massmind" <RemoveMEjamesnewtonspam_OUTspamKILLspammassmind.org>

> Does anyone have a schematic for connecting a hall effect sensor to a
PIC in
> such a way that the reading is available as an analog value? All I can
find
> are go/no-go sort of interfaces where the sensor is being used to
count
> pulses.

What you are looking for is a *linear* hall sensor.  They give a
proportionate output voltage (usually 1/2 Vcc when no field is present)
The voltage will rise (a predictable amount based on field strength)
when one pole of a magnet is brought near it, and it will drop when the
other pole is presented.

2005\02\25@124759 by kenash1

picon face
James

I have purchased 3 of your controllers and they are built. What sorts of
power is connected where?

I got a big heat sink out of scrap, fitted fans and are ready!

Also, on your schematics it says the two caps at C5 and C6 are 22uF. I got
two 4.7 ones. Just went ahead and soldered them in. Is this right?

Look forward to hearing from you.

Ken
----- Original Message -----
From: "James Newtons Massmind" <RemoveMEjamesnewtonTakeThisOuTspamspammassmind.org>
To: "'Microcontroller discussion list - Public.'" <EraseMEpiclistspamspamspamBeGonemit.edu>
Sent: Thursday, February 17, 2005 6:22 PM
Subject: RE: [EE] Low cost encoder. Is it a compass?


{Quote hidden}

>> {Original Message removed}

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