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PICList Thread
'[OT] Laser Show. It was [OT] Stepper Motors'
1999\04\23@190847 by Wagner Lipnharski

picon face
Michael Shiloh wrote:
>
> >From: Wagner Lipnharski <spam_OUTwagnerlTakeThisOuTspamEARTHLINK.NET>
>
> >Sometime ago I was playing with lasers reflected in mirrors attached to
> >fast stepper motors, and I went through all this situations, noise,
> >heat, speed, torque, then after a week I started to use "galvos" and
>
> Never heard of "galvos". What are they?
>
> Michael

Question: What is a galvo?

Remember a current meter, the needle, coil and magnet, it moves the
needle from one side to another when you apply current to the coil. All
the analog multimeter or needle instrument use a galvo system to move
the needle.  

In the laser show area, the word "galvo" means a device like that, but
stronger, with nice torque that has a bigger coil, and a light weight
moving core system (and shaft) that can rotate a mirror several degrees
very, very fast.

A galvo is somehow like a motor, but it don't do a complete rotation, it
just rotate the shaft according to the current applied to the coil(s),
with a maximum range of degrees, relative to the applied current, never
a full rotation. They are usually very expensive (more than $150), and I
really don't see any other use than for laser shows or to reflect a
light beam.  There are some galvo manufacturers, including Swiss made.
It is very easy to burn a galvo, since its impedance is very low, power
hungry, so a very good electronic power bridge with sensors and control
must be used to avoid to smoke $200 in a second.

Galvos can respond and rotate its axis very quickly, even with an
aluminum first face reflective mirror attached to it.  In the laser show
industry, two galvos with mirrors assembled in 90¡ (one to another) can
produce a XY coordinate and reflect a laser beam to a distant surface to
produce images, letters and so on.  Each XY position (coordinate)
defined by both galvos stopped is named "point".  The efficiency of a
laser show equipment is based in "kPoints" (means thousand points per
second), so bigger this index more expensive and good and fast is the
unit.  Starting from 4kPoints you can draw a nice mickey mouse face
dancing, more kpoints are necessary to produce a nice presentation with
several effects.  A complete 4kpoints system with the high power laser
(100mW or more), and interface to a PC with software, can cost more than
$3,000.

Disney Epcot use several light and laser show over the Promenade Lagoon
every night along with fireworks and synchronized to the music show. I
don't even guess how kpoints are those equipments, but must be a high
number, since they project pretty dancing and moving scenes over a water
curtain created by sprays.  The green laser beams are very powerful,
every night I can see them playing to the open sky from my house, 15
miles from Epcot, even during the Florida's very dry season (no much
moisture in the air to reflect the laser). One night at Epcot I was
aprox 20 meters from one laser exit, it was a very strong bright green
beam, 3 to 5 inches diameter, bright like a 1kW lamp filament, I can not
even guess the laser power.

I guess that system cost a little bit more than $3000  :)

Wagner

1999\04\23@215404 by Wagner Lipnharski

picon face
Well, I know this is an interesting subject, and I hope to be not
pestering people's life here.  It is very common to feel attraction for
this kind of experiences, reflect light or laser beam, and play with
music, images, letters and so on. It is very exciting, and very much,
mainly for hobby people, a lot in this list. Using a PIC to control it
should be hours taking adventure.  Some last recommendations to who
wants to go for this challenge, and to reduce several other posts about
it, here it goes:

1) There are some listings in the net just dedicated to this subject,
just search for light show, or laser show.

2) What you need is just few degrees of a mirror movement, since it just
means a big movement when projected at meters of distance.

3) Forget to use speakers with mirrors or something other miraculous
solution, it simply does not work.  Speaker's cone movement does not
creates an angle, it moves straight, even in the round movement close to
the outside of the cone is very difficult to build it, frequency
response is not linear and other problems. Straight movements doesn't
amplify the movement, just transport it. If the cone moves straight
1/8", this would be what the laser beam would move at the wall.

4) Stepper motors are pretty nice to test, play and see something, but
it is not fast enough even to write a stable low flickering "HI" in the
wall.  Also, forget cheap stepper motors with more than 3¡ per step, you
would not have resolution, and the image would "open" too wide in the
wall.  Using a 3.7¡ (if I remember well) per step motor with letters
with 8 steps it means more than 24¡ for a whole letter, it opens half
meter for each meter distance from the wall to the laser. 1:10 relation
is good, so less degree per step and less steps per image or letter.

5) Forget about using a reduction gear-box at the stepper motor to
reduce the angles/step, it would generate slack, image shaking and will
reduce mirrors speed, terrible idea.

6) Several other tests were done;
       
Solenoid attracting pins with mirrors hinged to it.

Solenoid bending a thin reflective metal place.
       
Can of black oil with speakers attached on the bottom and sides,
generating ondulations on the oil surface, laser reflects on it,
creating a somehow nice patterns, dirty job and not useful at all.

None of above solutions can't even get a little close to what a galvo
can do.

Somebody once said that compressing water inside a glass could change
its reflective light angle, since its mass would change, so the angle, I
don't know what the guy was thinking, possibly control pressure inside a
bottle, ... not fast enough.

It will be the day that we will see a light beam angle controlled by non
mechanical ways.

If you need to buy some laser pointers to play and can't find in your
neighborhood stores, take a look at my webpage (I sell them) <g>.

wagner.
http://www.ustr.net

1999\04\24@033814 by Alan King

picon face
 Take a plastic bottle cap.  Stick a needle through it just behind the
flat face for the axis.  Wind 25 or 30 turns of #30 magnet wire just
behind the needle and leave some leads and glue.  (My step here to get
rid of the excess bottle cap below the coil..)  Then glue a front
surface mirror to the flat of the cap.  Old / broken fax machines or
scanners are a great source of fs mirror.  Suspend this on the points
between two strong magnets and you have a simple cheap galvo to play
with until you decide you need a real one..  And use the money you saved
to buy more PICs to control it with!
Alan


Wagner Lipnharski wrote:
>
> Michael Shiloh wrote:
> light beam.  There are some galvo manufacturers, including Swiss made.
> It is very easy to burn a galvo, since its impedance is very low, power
> hungry, so a very good electronic power bridge with sensors and control
> must be used to avoid to smoke $200 in a second.

1999\04\24@172632 by Sebastian Garcia

flavicon
face
-----Mensaje original-----
De: Wagner Lipnharski <.....wagnerlKILLspamspam@spam@EARTHLINK.NET>


|A galvo is ...
| They are usually very expensive (more than $150), and I
|really don't see any other use than for laser shows or to |reflect a light
beam.

Do you have any document with more information about the galvos (or an URL)? I'm
thinking on try to home-make one.
I don't know how hard it can be; Have You experience on trying it?

|... Each XY position (coordinate)defined by both galvos |stopped is named
"point".


I can't understand it.
Reading your description about the galvos I've supposed that the movement of the
device is continuous between two fixed position limits. But "point" sounds like
a discretization of the possible positions. In which way it is performed this
discretization ?

Thanx,

Sebastian.-

1999\04\24@175759 by Wagner Lipnharski

picon face
You apply current, the galvo moves in one direction or another,
according to the polarity of the current, you stop the current the galvo
stops the movement.  So, you can "position" the galvo shaft at a
specific angle, thus, called "point".  Suppose you move the galvo from
the extreme "left" to the extreme "right" in 20 stops points, and you go
forward and backward doing it 200 times per second, you have 4000
"points".  Well, things are not so easy, you need to break the inertia
for better performance when want the galvo to stop fast.

It is not so easy to produce a good one home made. First you need to
produce the moving parts; shaft, coil or magnets as light as possible
for high speed response. Second, different from a motor that can rotate
fast, a galvo goes back and forward, there is a inertia involved.

Here I go doing free advertisement for this one:
http://www.svskits.com/Laser/92-000.htm they have a kit for less than
$140. (I hope they send a free galvo to me... :)

Wagner.


Sebastian Garcia wrote:
>
> -----Mensaje original-----
> De: Wagner Lipnharski <wagnerlspamKILLspamEARTHLINK.NET>
>
> |A galvo is ...
> | They are usually very expensive (more than $150), and I
> |really don't see any other use than for laser shows or to |reflect a light
> beam.
>
> Do you have any document with more information about the galvos (or an URL)? I
'm
> thinking on try to home-make one.
> I don't know how hard it can be; Have You experience on trying it?
>
> |... Each XY position (coordinate)defined by both galvos |stopped is named
> "point".
>
> I can't understand it.
> Reading your description about the galvos I've supposed that the movement of t
he
> device is continuous between two fixed position limits. But "point" sounds lik
e
> a discretization of the possible positions. In which way it is performed this
> discretization ?
>
> Thanx,
>
> Sebastian.-

1999\04\24@181508 by William Chops Westfield

face picon face
I would think the head positioner from a broken hard disk drive would
make a fine galvo-like movement, although perhaps with a more limitted
range of motion than a "real" galvo.

BillW

1999\04\24@191838 by paulb

flavicon
face
William Chops Westfield wrote:

> I would think the head positioner from a broken hard disk drive would
> make a fine galvo-like movement, although perhaps with a more limitted
> range of motion than a "real" galvo.

 Ha!  You beat me to it.  I was going to point out that "galvos"
(Galvanometers) are exactly what they stepped up to(!) from stepper
motors in hard drives, only they happen to call them "voice coil"
drivers.  (Difference from a speaker?  The voice coil traverses a curved
path!)

 And they have *really* powerful magnets.

 So, all those trashed hard drives (well, many of them) contain the
mechanism ready-built, *plus* the driving electronics if you can only
dissect it out, even including the (re-) programmable controller
(usually a 68HC11 or Intel 80196).  And they have much the *same* range
of motion as a laser galvo - from 60 to 90 degrees.

 If *only* you could get circuit diagrams for the things!
--
 Cheers,
       Paul B.

1999\04\24@200034 by Wagner Lipnharski

picon face
> William Chops Westfield wrote:
> I would think the head positioner from a broken hard disk drive would
> make a fine galvo-like movement, although perhaps with a more limitted
> range of motion than a "real" galvo.

It might work, with restricted speed, precision is not a problem since a
hard disk has hundreds of cylinders so possible stop points. The mirror
should be attached vertically right above the "motor shaft" as an
extension of it (like your hand in your arm), so it would "rotate" over
itself. To control the voice coil it's another story... :)

Angle range is not a problem, 10 to 20 degrees is more than enough.
Remember that the laser would be reflected from the first mirror and
then by the second. If the angle range of the first is to big, the beam
reflection would slip out of the second mirror.  The adjustment is
simple, moving the first mirror all the way in the range, the beam
should touch the second mirror face in any position this one is, doesn't
matter what. So, the first motor should not deflect much.

Example, 11 degrees in both mirrors, would represent a ( sin(11¡)=0.19 )
relation of 5:1, for every 5 meters this system is distant from the
projection screen (wall) it represents one meter of laser scan
projected, plenty and nice.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\04\24@220837 by Brian Whittaker

flavicon
face
Hi All
 The secret to a good light show is the driver circuitry! Even the best Galvo w
ill not draw the square corners necessary for good
text and graphics without a matched PID servo amp. Mechanically a Galvo can be r
epresented as a weight connected by a spring to a
motor. If the motor moves smoothly the weight will pretty much follow. But when
the motor moves with a jerk the weight at first will
not move (the spring stretches) then it will slowly accelerate. When the motor s
tops suddenly the weight keeps moving ( the spring
compresses). This is called under and over shoot. Also the weight does not stop
moving when the motor does. It will oscillate about
the stop point with each cycle being slightly smaller until it finally comes to
a stop. This is called ringing
A good servo amp generates drive signals that compensate for ringing, under, an
d, over shoot. It gets the Galvo to move exactly the
way you want it to. To do this you need a computer. Most servo amps are actually
analog computers. They need to find the difference
between the actual mirror position and the desired mirror position. This require
s a sensor on the Galvo to measure the actual mirror
position.
You can make a digital servo amp. Parallax has posted sample PIC machine code f
or a one. No stamp code has been posted, to my
knowledge, however.
PID stands for Proportional, Integral, and Differential. It is a kind of servo
amp and this is a quick look at how it works.  The
difference between the actual and desired positions is called the proportional c
omponent.
The proportional component is applied to the Galvo and it starts to move.
The proportional signal changes as the Galvo moves.
The servo amp calculates the first Integral and first Differential of the chang
ing proportional signal.
These two components are scaled and added to the proportional signal.

The integral component insures that the system comes to a smooth and complete s
top without oscillations, when the Galvo has reached
the desired position. The integral component is not very important in light show
applications because the text must be refreshed
25-30 times per second to avoid flicker, just like a television. The Galvo never
actually stops moving for more than a few
microseconds.
The first differential component allows the servo amp to see into the future. N
o not magic! You see the future every day. For
example, you are driving an automobile down the street. You see a stop sign and
apply the brakes. What really happened is you looked
into the future and saw that you would need to stop when you got to the sign, so
you applied the brakes before you actually got to
the stop sign.

BTW I work for the company wagner so generously plugged. That $140 includes the
PID amp Two galvos with feedback and a computer
interface. Just the galvos and PID amp are $124. Both are in kit form, but the g
alvos are pre-assembled . . nobody could do that at
home!. It is for advanced builders and takes about 15 hours to assemble.

--------------------------------------------
Brian Whittaker SVSKITS.COM
Voice: 510-582-6602 FAX: 510-291-2218
.....SVS_KITSKILLspamspam.....MSN.COM URL: http://www.svskits.com
--------------------------------------------

{Quote hidden}

1999\04\24@230947 by Gerhard Fiedler

picon face
At 19:11 04/24/99 -0700, Brian Whittaker wrote:
> A good servo amp generates drive signals that compensate for ringing,
>under, and, over shoot.

i'm not sure whether pid control is actually the most adequate way to avoid
ringing and overshoot (the mechanical, spring-like part of it you
described). i would guess that limiting the change of the output, in some
smart way (ie. depending on how far away from the final point it is), might
do the same thing, without feedback. but that might be too fast to do it
with a micro, i don't know.


>The integral component insures that the system comes to a smooth and
>complete stop without oscillations, when the Galvo has reached
>the desired position.

actually it is the differential part that is responsible for smooth stops,
and the integral part often is responsible for oscillations :)  it's
function is to compensate the position error inherent in a proportional (or
a pd) regulator.

>The first differential component allows the servo amp to see into the
>future.

that's not quite true... control would be easier if we actually had a
device that could do this :)  what it does is it projects the immediate
past into the immediate future, and if both are the same, it works in the
way that it slows down early enough to make for a smooth stop and avoid
overshoots. since both (the immediate past and the immediate future) are
often approximately the same, it often works approximately. but they are
not always the same, and it's not always good enough to work
"approximately." that's where we needed something that actually could look
into the future :)

ge

1999\04\25@001302 by Alan King
picon face
 Closer to a voice coil, galvos spin on axis and VC go in and out
generally.  The coil itself in a VC drive moves, even though the whole
head assembly spins in modern ones.  I will try and put up some pics.
Everyone needs to see what was inside an old bernoulli drive.  They were
quite insane!
Alan


>   Ha!  You beat me to it.  I was going to point out that "galvos"
> (Galvanometers) are exactly what they stepped up to(!) from stepper
> motors in hard drives, only they happen to call them "voice coil"
> drivers.  (Difference from a speaker?  The voice coil traverses a curved
> path!)
>
>   And they have *really* powerful magnets.

1999\04\25@034820 by William Chops Westfield

face picon face
While some older disk drives used 'voice coil' positioners that looked an
awful lot like the middle of a speaker (coil of wire on form, around a round
magnet), I think the current technology is a lot closer to "galvos" as being
discussed here.  Typically, there is a flat coil of wire positioned between
two (very strong) magnetic poles, with a pivot and the actual heads on the
other side.  It's definately angular rather than linear movement...

BillW

1999\04\25@103930 by Mike Keitz

picon face
On Sat, 24 Apr 1999 20:00:00 -0400 Wagner Lipnharski
<EraseMEwagnerlspam_OUTspamTakeThisOuTEARTHLINK.NET> writes:
>> William Chops Westfield wrote:
>> I would think the head positioner from a broken hard disk drive
>would
>> make a fine galvo-like movement,

>It might work, with restricted speed, precision is not a problem since
>a
>hard disk has hundreds of cylinders so possible stop points.

In the hard disk, they are counting on being able to constantly determine
the position of the galvo by reading the signals from the disk.  Using
lots of feedback makes any mechanism work a lot better.

I haven't seen a laser-show galvo, but I assume it has a spring or
permanent magnet exerting a force to return the mirror to the center
position.  This feature allows it to work without feedback.  Applying a
constant current to the coil causes the mirror to (eventually) assume a
position proportional to the current.  Of course, applying some lead and
lag compensation will let it move a lot faster, but at the heart of the
driver circuit is a current-output amplifier.

A disk drive galvo doesn't have any centering mechanism.  If you applied
a constant current to it, it would just move all the way to the end.
It's a "pure" integrator and there's no way to keep it from drifting to
the end without obtaining feedback of the actual mirror position somehow.


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1999\04\25@182828 by Wagner Lipnharski

picon face
Mike Keitz wrote:
> In the hard disk, they are counting on being able to constantly determine
> the position of the galvo by reading the signals from the disk.  Using
> lots of feedback makes any mechanism work a lot better.
>
> I haven't seen a laser-show galvo, but I assume it has a spring or
> permanent magnet exerting a force to return the mirror to the center
> position.  This feature allows it to work without feedback.  Applying a
> constant current to the coil causes the mirror to (eventually) assume a
> position proportional to the current.  Of course, applying some lead and
> lag compensation will let it move a lot faster, but at the heart of the
> driver circuit is a current-output amplifier.
>
> A disk drive galvo doesn't have any centering mechanism.  If you applied
> a constant current to it, it would just move all the way to the end.
> It's a "pure" integrator and there's no way to keep it from drifting to
> the end without obtaining feedback of the actual mirror position somehow.

Well, yes and no, I really don't know much about hard disk low level
formatting, I am not so sure that all the cylinders are pre-written.  A
voice coil system used at IBM 3330 disk storage use a sophisticated
digital electronics that accelerate, course and breaks the heads right
over the desired track with small fine adjustments. It is just a loose
voice coil mechanism running straight in and out over bearing rods, with
a huge fixed magnet inside (magnet weights more than 50 lb.).

Spring loaded galvos create a soft reaction turning sharp edges in
rounded ones, what is not a desired effect.  If you think that it is
just a laser show, not a surgical operation, doesn't matter so much if
the final image has some offset errors.  Good software control is a
requirement to a good operation and at least one side of the galvos can
have a "home position" optical sensor for a "positional reset" when
necessary.  Optical feedback could be used, as a zebra ribbon or
something like that to feedback the microcontroller about the head
movement. The problem with this solution is that the galvos are working
at maximum possible speed and there is almost no time for correction or
position trimming. This solution is necessary at XY precision tables or
laser cutting.

This is the main reason why there is *no* TV projection using lasers and
galvos. Even using rotating heads it requires mechanical sync with the
image, what is... very difficult at 15750 lines/second. The use of delay
lines, digital delays, or a complete frame capture for a delayed
projection could help to reduce this effect. Well, they may exist, but I
never heard of them.

I always wanted to have my computer screen projected at the wall using
more than 800k dots... :) then the 14" monitor goes to... trash.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\04\25@213639 by Rob

flavicon
face
>This is the main reason why there is *no* TV projection using lasers and
>galvos. Even using rotating heads it requires mechanical sync with the
>image, what is... very difficult at 15750 lines/second. The use of delay
>lines, digital delays, or a complete frame capture for a delayed
>projection could help to reduce this effect. Well, they may exist, but I
>never heard of them.
>
>I always wanted to have my computer screen projected at the wall using
>more than 800k dots... :) then the 14" monitor goes to... trash.

It *IS* possible to do this with an AO modulator... afaik, it *seems* that
the bending of the laser light is caused by very strong magnetic x-y yokes,
as in a television..
I believe a system combining the yoke type deflection with various mirrors
could achieve some amazing results..
There is even a site on the net that tells how to hijack a monitor yoke to
do this..

Now, the real question is, how in the H*** did Ford Aerospace manage to
develop full color laser volumetric holographic projection back in 93?

As a sidenote, rumor has it that the govt stopped funding the project
promptly thereafter.
I saw ONE article on it, then never another ever again..

Beware what you wish for.. ;)

Rob

1999\04\25@223351 by Mike Keitz

picon face
On Sun, 25 Apr 1999 20:55:21 -0400 Rob <cyborg0spamspam_OUTGLOBALEYES.NET> writes:

>>I always wanted to have my computer screen projected at the wall
>using
>>more than 800k dots... :) then the 14" monitor goes to... trash.
>
>It *IS* possible to do this with an AO modulator... afaik, it *seems*
>that
>the bending of the laser light is caused by very strong magnetic x-y
>yokes,
>as in a television..

I think this is possible.  You'd only need one axis of fast scanning, the
vertical scanning could be done easily with a rotating mirror.  But a
laser TV is an expensive proposition because it would need 3 bright
lasers, one of each primary color.  It may not be possible to send
multiple colors through the same scanner and have them stay aligned
together, in that case 3 scanners may be needed.

The new "LCD shutter" type video projectors are quite good.  There are
models that do VGA 1024 x 768.  It doesn't seem very practical for office
use unless you have a large office.

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1999\04\25@224745 by Sean Breheny

face picon face
I really don't understand this. How can a static magnetic field bend a beam
of photons?
I would appreciate it if someone could explain this to me.

Thanks,

Sean


At 10:29 PM 4/25/99 -0400, you wrote:
>>It *IS* possible to do this with an AO modulator... afaik, it *seems*
>>that
>>the bending of the laser light is caused by very strong magnetic x-y
>>yokes,
>>as in a television..
>

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
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1999\04\25@225409 by g.daniel.invent.design

flavicon
face
Mike Keitz wrote:
<cut>
> >It *IS* possible to do this with an AO modulator... afaik, it *seems*
> >that
> >the bending of the laser light is caused by very strong magnetic x-y
> >yokes,
> >as in a television..
>
> I think this is possible.  You'd only need one axis of fast scanning, the
> vertical scanning could be done easily with a rotating mirror.  But a
> laser TV is an expensive proposition because it would need 3 bright
> lasers, one of each primary color.  It may not be possible to send
> multiple colors through the same scanner and have them stay aligned
> together, in that case 3 scanners may be needed.
<cut>
"as in a television"--> attraction/repulsion of electrons, *not*
photons.

Alignment ?     Lenses will give different refraction for different
frequencies (spectra) of light, mirrors however will give same angle of
reflection for same incident angle regardless of colour/spectra.

regards,
Graham.

1999\04\25@230445 by g.daniel.invent.design

flavicon
face
Sean Breheny wrote:
>
> I really don't understand this. How can a static magnetic field bend a beam
> of photons?
> I would appreciate it if someone could explain this to me.
>
> Thanks,
>
> Sean

Behold ! the power of imagination unbound by practical knoledge.

(it sounds like magic to me too.)
regards,
Graham Daniel.

1999\04\25@231312 by Wagner Lipnharski

picon face
Mike Keitz wrote:
> I think this is possible.  You'd only need one axis of fast scanning, the
> vertical scanning could be done easily with a rotating mirror.  But a
> laser TV is an expensive proposition because it would need 3 bright
> lasers, one of each primary color.  It may not be possible to send
> multiple colors through the same scanner and have them stay aligned
> together, in that case 3 scanners may be needed.

To generate a TV image tracing frame with laser scan is not a problem.
Two faceted drums, one rotating in such speed that 60 faces sweep the
image top down, while another doing the same for 15750 faces left to
right.  Problem is;  in a 54" diagonal screen, it has 525 lines of aprox
16 inches, what goes to 12075 inches (306 meters) of "plotted" laser
before it loops back to trace the next frame.  For example a 5mW laser
would be very dim, almost imperceptible, so it would require powerful
lasers. Try to imagine concentrating a bright TV imagine in a single
point via special lenses, it would be very bright, right? that should
the be laser brightness to do the opposite.  The second problem is that
a full image frame needs to be digitized, stored and then synchronized
to the laser drums, it requires 4MHz ADC's, and lost of sync from the
source image would be a... disaster... :)

Yes, it is possible to combine different lasers through the same scanner
and lenses, in fact, this is done by a lens named "collimated lens".

There is another problem, the lasers should be modulated at frequencies
around 4MHz to generate the image nuances.  Lasers used to fiber optics
communication can reach much higher frequencies, but they cost... I am
not sure about regular low cost lasers.

Rob wrote:
> It *IS* possible to do this with an AO modulator... afaik,
> it *seems* that the bending of the laser light is caused by
> very strong magnetic x-y yokes, as in a television..

I know that photons can be deviated by a very strong gravitational
field, as the Sun changes the trajectory of star's light a little
minutes of a degree.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\04\26@004511 by Rob

flavicon
face
bite me

Magnets may be incorporated in HeNe lasers for several reasons including the
suppression of IR spectral lines to improve efficiency (such as it is!) and
to boost power at visible wavelengths, for the stabilization of the beam,
and to control its polarization. There are no doubt other uses as well.

Rob


{Original Message removed}

1999\04\26@005752 by Sean Breheny

face picon face
Hi Rob,

I am not trying to say this is wrong, I am just saying that I have never
heard of static magnetic fields having any big effects on photons. AFAIK,
to take a very mathematical view of it,a static magnetic field just drops
out when the derivatives are taken in Maxwell's equations.

How does this work? There are things that I don't understand about
lasers,I'm sure. I could much more easily buy the idea that a static
magnetic field affects the gas atoms in a HeNe which in turn affects the
emitted photons,than I could buy the idea that a static magnetic field
affects photons in free space (or air).

Please explain further.

Sean


At 12:04 AM 4/26/99 -0400, you wrote:
{Quote hidden}

| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
KILLspamshb7KILLspamspamcornell.edu ICQ #: 3329174

1999\04\26@014334 by Rob

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face
Sorry about the unprofessionalism of the "bite me" statement.
I get like this when an engineer who makes twice as much as I do (im an
engineering tech) tries to tell me all electrical networks are linear, with
a straight face I might add... Not to mention he says this when we are
working in radio freqs
Im usually not prone to that sort of thing, but the sort of close-mindedness
of so called "engineering" folk at where I work has really rubbed me wrong
lately.
BTW, I said AFAIK, not IM SURE.
Doesnt anyone remember physics? If you produce a strong enough field of any
type of energy, you can get any type of effect you want.
Anything is possible.. it just depends on practicality and how badly you
want it.

Read this
http://www.eio.com/repairfaq/sam/laserioi.htm#ioimdf0

It states that "These are devices to modulate or deflect a beam of (usually
laser) light without any moving parts (at least at the macroscopic level).
If a longitudinal standing wave is induced in a cell with transparent sides
containing a suitable medium, the resulting density variations will act like
a diffraction grating to light passing through it. This can be used to
modulate or deflect a laser beam at a very high rate. "

So, apparently, its using the standing waves generated by the field.


Definitely NOT the most concise faq, but pretty good.
Rob

1999\04\26@015958 by Sean Breheny

face picon face
Hi again Rob,

At 01:02 AM 4/26/99 -0400, you wrote:
>Sorry about the unprofessionalism of the "bite me" statement.
>I get like this when an engineer who makes twice as much as I do (im an
>engineering tech) tries to tell me all electrical networks are linear, with
>a straight face I might add... Not to mention he says this when we are
>working in radio freqs

It's ok, I understand. After all, I am an EE student who has also been a
hobbyist for several years before coming to school. 'nough said. :)

>Im usually not prone to that sort of thing, but the sort of close-mindedness
>of so called "engineering" folk at where I work has really rubbed me wrong
>lately.
>BTW, I said AFAIK, not IM SURE.
>Doesnt anyone remember physics? If you produce a strong enough field of any
>type of energy, you can get any type of effect you want.
>Anything is possible.. it just depends on practicality and how badly you
>want it.
>

Hmmm. Well, yes, I can see that you would get strange higher-order effects
at VERY high field densities, but I don't think this comes into play with
any field you could hope to generate with non-superconducting magnets.

{Quote hidden}

I haven't read the whole thing yet, but at least in that section, if I am
reading it correctly,it isn't talking about magnetics at all. Instead,it is
talking about MECHANICAL standing waves,caused by piezo elements.
Aparently,it is causing variations in the path that the light takes through
a medium (such as water or Lithium Niobate) by applying mechanical stress
waves at RF frequencies.

>So, apparently, its using the standing waves generated by the field.
>
>
>Definitely NOT the most concise faq, but pretty good.

Yes, it is fascinating,thanks for the URL!

>Rob
>

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
RemoveMEshb7TakeThisOuTspamcornell.edu ICQ #: 3329174

1999\04\26@021250 by Rob

flavicon
face
>Hi Rob,
>
>I am not trying to say this is wrong, I am just saying that I have never
>heard of static magnetic fields having any big effects on photons. AFAIK,
>to take a very mathematical view of it,a static magnetic field just drops
>out when the derivatives are taken in Maxwell's equations.

Yep, you are correct. Magnetic effects become insignificant at these
frequencies.

Well, gas WAS the key... I totally discounted the fact that there is AIR
within the yokes through which the laser passes.. Since air is the medium,
we use magnetism to modulate the AIR, thereby modulating the medium through
which the laser passes, thereby modding the laser beam itself. Duh. Some of
them also vibrate a crystal, although the great thing about the ao modulator
is that your losses are less with air.
I knew it seemed a bit off, thats why I said it "seems".
Well, there now, we all know now.
Mind you, you dont get HUGE deflections out of this process, but you can get
random access deflections with infinite resolutions and extremely high
speed.
BTW, using the same type of process, it is possible to use very strong mag
fields to "squash" a laser beam increase the intensity per cm, but, once
again, you are using the air as a sort of lens, and using the fields to
align the air molecules for the desired fx.


Rob

1999\04\26@110841 by John Payson

flavicon
face
|To generate a TV image tracing frame with laser scan is not a problem.
|Two faceted drums, one rotating in such speed that 60 faces sweep the
|image top down, while another doing the same for 15750 faces left to
|right.  Problem is;  in a 54" diagonal screen, it has 525 lines of aprox
|16 inches, what goes to 12075 inches (306 meters) of "plotted" laser
|before it loops back to trace the next frame.  For example a 5mW laser
|would be very dim, almost imperceptible, so it would require powerful
|lasers.

Hmm... what about using 125 (or 250 or 500) lasers, each of which
handled one scan line?  The required modulation rate could then be
reduced considerably (using a frame buffer to capture sample the
incoming video at 640 dots/line, 14.1818MHz, and retransmit it at
60 lines/sec, or about 4Khz.  Modulating a laser at that slower
speed shouldn't be nearly as hard as modulating at 4+Mhz, plus it's
probably a lot easier to modulate a small laser than a big one.
True you'd need dozens of lasers, but 240 lasers at $10ea (for diode
lasers, which should be quite adequate for many circumstances) would
only be $2400.

Depending upon the design of the optics, if 240 lasers are used it
may be possible to do interlacing mechanically by using a mirror that
shifted up and down (slightly) 60x/second.  As for color, if one had
a white laser (contradiction in terms, I know) a rotating color wheel
could probably yield somewhat decent results.  Otherwise, I think the
best approach is to wait for blue diode lasers.

1999\04\26@111252 by John Payson

flavicon
face
|4) Stepper motors are pretty nice to test, play and see something, but
|it is not fast enough even to write a stable low flickering "HI" in the
|wall.  Also, forget cheap stepper motors with more than 3? per step, you
|would not have resolution, and the image would "open" too wide in the
|wall.  Using a 3.7? (if I remember well) per step motor with letters
|with 8 steps it means more than 24? for a whole letter, it opens half
|meter for each meter distance from the wall to the laser. 1:10 relation
|is good, so less degree per step and less steps per image or letter.

What about using a stepper motor with a fairly big step size and
keeping one of the coils energized full-time?  It would seem that
the current in the other coil should then correspond pretty well
to position (in the static case; obviously the motor has mass and
will take time to move when the current changes).  Of course, if
you're using the motors like this and want them to work quickly,
you may want to put some dry ice in the box. >:*3

1999\04\26@114414 by Michael Rigby-Jones

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face
I don't think modulating the laser at 4 MHz would be any problem at all.
The slowest device we manufacture is capable of 100MHz and 2.4GHz is
considered fairly average.  Even higher frequency's are possible although
the laser is not usualy directly modulated.

Regards

Mike Rigby-Jones
Nortel Networks (Optoelectronics)

> {Original Message removed}

1999\04\26@114621 by Dave VanHorn

flavicon
face
> What about using a stepper motor with a fairly big step size and
> keeping one of the coils energized full-time?  It would seem that
> the current in the other coil should then correspond pretty well
> to position (in the static case; obviously the motor has mass and
> will take time to move when the current changes).  Of course, if
> you're using the motors like this and want them to work quickly,
> you may want to put some dry ice in the box. >:*3

Barcode scanners scan this way, but they are only trying to get a scan rate
of maybe 10 scans/sec. It's a reasonable approach, but the mass of the core
magnet is still a problem.

1999\04\27@175131 by Brian Whittaker

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face
> On Saturday, April 24, 1999 8:01 PM, Gerhard Fiedler wrote:
> I'm not sure whether PID control is actually the most adequate way to avoid
> ringing and overshoot (the mechanical, spring-like part of it you
> described). I would guess that limiting the change of the output, in some
> smart way, might do the same thing, without feedback.

 Well I agree in theory! To change the velocity of a fixed mass by a fixed amou
nt will take the same amount of energy every time
you do it. So if you know the galvo's velocity and mass you can calculate how mu
ch energy to apply to get it to move and stop where
you want.
 If you drive the galvo with a fixed current source then energy is just a funct
ion of time. If you know the acceleration profile of
the galvo then:

Velocity = time * Acceleration
Position = time * Velocity
OR
Position = time squared * Acceleration
If you know position then you do not need feedback

 The problem is knowing exactly the performance of your galvo. How fast does it
accelerate.  There are many factors. Here are some:

 The acceleration of a galvo is non linear. It depends on the relationship of t
he rotating coil's magnetic field, to the fixed
magnet's field. This relation ship varies as the galvo turns and the fields chan
ge position.

 Galvos and mirrors being mechanical will vary slightly from unit to unit. On h
ome made and inexpensive galvos there can be a
relatively large difference between galvos

 Galvos and mirrors, especially home built ones, are not perfectly balanced. Gr
avity will aid acceleration in one direction and
oppose it in another, of course once you cross center the oppose and assist dire
ctions swap

 Because the Galvo are at right angles to each other in order to produce the co
rrect x-y movements one galvo will be effected more
than the other.

 What this all means is that to do it right you must store a lot of information
about the galvo's performance to improve on the
basic PID algorithm. In experiments I performed several years ago I was able to
increase the performance of a test galvo by about
15% over the basic PID filter. I used a table lookup approach and had about 4k e
ntries in the table. The tables output went through
an algorithm that looked at the next two positions as well as the current positi
on. The problem was the system had to be tuned for
each galvo. Change the galvo with another unit of the same make and model and pe
rformance went to 90% of the PID value. That problem
could have been fixed but I stopped the research when I realized that the spring
was the source of most of the problems. Removing
the spring gave a 200% increase in performance. However without the spring, feed
back became necessary to keep the galvo from
slamming into the stops whenever the smallest amount of current was applied.
>
> actually it is the differential part that is responsible for smooth stops,
> and the integral part often is responsible for oscillations :)  it's
> function is to compensate the position error inherent in a proportional (or
> a pd) regulator.

oops! You are correct. My apologies, I should have spent more time proofing befo
re I hit the send button. btw laser galvo's usually
do not use the Integral term because they hardly ever stop moving.

> >The first differential component allows the servo amp to see into the future.
> that's not quite true... control would be easier if we actually had a device t
hat could do this :)

I apologize to any list readers who thought I was trying to say that it was poss
ible to see into the future . . No body can do that
with the possible exception of Bill Gates. . I Only meant that the effect was li
ke being able to see into the future.

1999\04\27@185619 by Scott Newell

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>I apologize to any list readers who thought I was trying to say that it
was possible to see into the future . . No body can do that with the
possible exception of Bill Gates. . I  Only meant that the effect was like
being able to see into the future.
>

Why can't you see the into the future?  I'm assuming most setups are using
a controller to send preprogrammed movements to the galvos.  Why not just
implement a 'look-ahead' to see what moves are coming up?


newell

1999\04\27@195908 by wagnerl

picon face
Scott Newell wrote:
>I apologize to any list readers who thought I was trying to say that it
> was possible to see into the future . . No body can do that with the...

... you can see into the future if your present (not you) is at the
past... :)

probably the same for

... you can never see the present if the past is into the future...

(quite sucking 100% processing mind thoughts, huh?)

20 years ago I did a car electronic ignition timing control, (remember
the vacuum system to advance the spark when you step into gas?) well, it
is difficult to advance an electronic pulse, generate a pulse before it
even exist... :)  so I just delayed the electronic pulses in 90
degrees... "what?  you can delay time, not degrees..." someone could
say, because degrees change in time according to the motor rpm.  Well,
it is not that difficult.  You just need a timing disk reference,
advance the mechanical in 90¡ and delay the electronics in 90¡, so when
you reduce the angular delay in real you are advancing the spark
timing.  After produce some street tests with different motor block
temperatures and some other things as vacuum, and burn tables into an
eprom to be used by the Z80 that was controlling the sparks, it did runs
a little bit better, mainly in cold weather... I think I did somehow an
old parent of the today's ignition control.

So, in 1975 my car was "drawing the lottery numbers at Friday's night,
but only announcing 24 hours later"... :)

Yes, you can predict the galvos movement and change the galvos faith if
you stack delay 3 or 4 commands, so you gently can play god with the
galvo's lifes... well still you can't predict mechanic reactions, but...
:)

... just found today, sadly, that power lasers (30 Watts or more) can
not have a concentrated parallel narrow beam... they *always* use a lens
system to generate the laser focus outside the lens.  To generate a
narrow parallel beam the focus need to be done inside the lens, and I
think a 30W laser focus heat is enough to blow up the lens.  So, you
don't have an "infinite focus" burning beam... :(

1999\04\27@201351 by Brian Whittaker

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On Tuesday, April 27, 1999 3:55 PM Scott Newell wrote

> Why can't you see the into the future?  I'm assuming most setups are using
> a controller to send preprogrammed movements to the galvos.  Why not just
> implement a 'look-ahead' to see what moves are coming up?

 I feel This String has gotten too fat off topic for a PIC Microcontroller disc
ussion list so to kill it A reply was sent to Scott
and not posted to the list. If anybody is interested in continuing the discussio
n Email me off list an I will send a copy.

 I enjoy laser light show topics and wish I could find a list for them. Does an
ybody out there know of one.

. . Brian

1999\04\28@145053 by Gerhard Fiedler

picon face
At 14:54 04/27/99 -0700, Brian Whittaker wrote:
>  What this all means is that to do it right you must store a lot of
>information about the galvo's performance to improve on the
>basic PID algorithm.

when i proposed a non-pid scheme, i didn't mean to =improve= on the pid
algorithm, but to =simplify= the thing by avoiding feedback. but i must
admit that i don't know enough about the construction of the devices
involved to be able to tell whether this would be possible. whether and how
depends, as you say, a lot on the particular construction.

ge

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