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PICList Thread
'Servo Laser Control'
1997\12\10@140312 by Tim Kerby

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Hi
For my higher (Scottish version of A level) Computing Studies investigation
I am looking into the control of laser light shows by computers.  As well
as contacting various companies, I want to build a PIC based servo
controller.  This will accept RS232 from a PC giving the following data

H0 to H255 - Horizontal position from leftmost (0) to rightmost (255)
V0 to V255 - As horizontal but vertical control.
S0 or S1 - shutter closed (1) and open (0)

These will be in the form of a bite of ascii as an identifier (H, V or S)
followed by the value.
This will be carried out on the 16C84 which is easily adequate for this
processing.

To save me time on coding and working out how to do the serial control and
the servo control I would like some help.  This is because I only have 20
hours from start to finish (cruel computing teachers - who can do much in
this time).

I would like people to send well documented serial and servo control
software that they use and know works.  Some theory on the servo side would
be nice too.  What really annoys me is the lack of details such as pinout
to the devices.  Circuit diagrams would be even nicer.

I understand that servos have power and a PWM input line that translates to
position but can the pic drive the PWM line directly?  The servos draw a
max of 500ma but I presume this is from the power lines.


My plan is this

1 All initialisation carried out
2 Start values are placed in a variable (to center the servos 080H)
3 Loop with the correct pulsing to both servos.  BTW how do you do both at
once?
       Set shutter appropriately (solenoid)

Interrupt: On change of serial line
1 Read serial inputs (all 3) and store in 6 bytes of memory
2 Look at identifier and place each variable into appropriate register
(called in main loop)
3 Return to the start of stage 3 above with new data.



Can anyone help?  If you have an include or header to do serial and servo
stuff then this would be best but I can make do with any well documented code.



Send to my private email (spam_OUTtim.kerbyTakeThisOuTspamukonline.co.uk) if you are attatching
code.  This wont clog the list.


Thankyou very much

Tim Kerby



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1997\12\10@153147 by Andrew Mayo

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What cruel teachers you have. 20 hours is enough to do some preliminary
research. In any event real servo systems have overshoot and damping
issues which you need to consider. Start with the application note on
Microchip's page which shows how to build a servo-controlled DC motor
system that accepts commands much like yours. Note that it was built
around a 17C42 which has hardware PWM.

If you get it done in 20 hours, consider a career in embedded systems.
You are clearly a genius and will dominate the field. Best of luck <g>

{Quote hidden}

1997\12\11@170750 by Tim Kerby

picon face
Hi
I should be able to sneak in lots of extra time at home!  Anyway, it is
only a demo of computer control I am producing.  I'm not intending to do
super high quality light shows with the stuff!  Anyway, I will publish full
details on my web site (due to be completely redone over the xmas break)
along with my report.  For a 20th of the total exam marks it isn't much but
I should be able to produce something decent.  Most of the report will be
taken up with analysis of how this is done professionally with only a small
portion on the demo (which will take most of my time!).

As for a career in embedded systems, I am seriously considering it.  I have
been programming the PIC for over two years now and I know 6502 really well
(it is still in the school course work in a lot of detail).  I can
understand 68HC11 but I have no programming tools for it so that has gone
by the board.  My choice of career would be robotics, specialising in
artificial intelligence, which would combine all the things I love.


Tim Kerby


At 09:20 11/12/97 +1300, you wrote:
{Quote hidden}

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1997\12\11@212443 by Andrew Mayo

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I am quite curious as to how they control the little mirrors they sweep
the beam with etc, as this seems not to be well-documented - I'd be
interested to hear how you go.

{Quote hidden}

1997\12\11@234844 by tjaart

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Andrew Mayo wrote:
>
> I am quite curious as to how they control the little mirrors they sweep
> the beam with etc, as this seems not to be well-documented - I'd be
> interested to hear how you go.

A solution could be a (thin) polished mirror fastened to the voice coil
>from hard drive (per axis) I made a PID solenoid controller a few years
ago (16C71) and used a Maxtor voice coil to test it. I was amazed at the
hammering these little beasts can take without even heating up (1A 12V).
They are extremely fast as well...

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1997\12\12@000954 by patrick_roberts

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Andrew Mayo wrote:
>
> I am quite curious as to how they control the little mirrors they sweep
> the beam with etc, as this seems not to be well-documented - I'd be
> interested to hear how you go.
My be I am way out here but the first thing I thought of was a electric
mirrior on a car.

PR

1997\12\12@060120 by Keith Howell

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I believe that for the kind of thing you describe, the lasers are deflected by
cunning gadgets.

You know how a simple ammeter works: they're like that except the needle is
replaced by a tiny mirror.
The laser shines at a mirror, and angular deflection is proportional to current.
The light beam becomes a very long zero-mass needle!
You still have the inertia of the coil and mirror, but this is still a lot
faster and simpler than steppers

You use a pair to scan in x and y directions.

What kind of show did you have in mind?
The arrangement above can generate nice Lissajous figures.

If you want to do some kind of raster scanning, then typically you'd use a
rotating octagonal mirror to do the
repetitive sweeps. Tomorrow's World showed this principle with a mix of red
green and blue lasers to produce a
cinema type of display. Always focused too.

1997\12\14@075105 by Tim Kerby

picon face
Hi
I think by this tou are meaning galvanometers.  They cost more than servos
to get decent ones and I am not planning any serious shows with the
equipment.  As I said before, it is mainly a demo of computer control of
the beam.  By using servos, I can turn the project into something else when
I am finished.  As for raster and other scanning methods, it is way too
complex for a short project.  I saw the tomorrows world program a while
ago.  They projected a flower didn't they?


Tim Kerby

At 10:51 12/12/97 +0000, you wrote:
>I believe that for the kind of thing you describe, the lasers are
deflected by
> cunning gadgets.
>
>You know how a simple ammeter works: they're like that except the needle is
> replaced by a tiny mirror.
>The laser shines at a mirror, and angular deflection is proportional to
current.
{Quote hidden}

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1997\12\15@152306 by Alan King

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Homemade (IE cheap!) galvos:

 Glue a front surface mirror (a broken fax machine has lots of this)
onto the top flat of a plastic bottle cap from coke etc.  Wrap 30-50
turns of #30 magnet wire (RS has this) around the cap near the upper
flat surface, so it's about inline with the mirror looking edge on.
Push a needle through the cap just below the mirror so the needle is
running just below the flat surface/mirror so the needle becomes your
axis.  (Actually, do the needle before the wire.)  I forget what to use
as the bearing for the needle points but you should be able to get it
from here.  Use a strong magnet on either side for the field and apply
voltage to the coil.  Voila, a galvo for a few bucks or less, vs. $100+
for anything commercial.
 Of course this is on the net somewhere, just don't have the URL handy.

Alan


Tim Kerby wrote:
{Quote hidden}

1997\12\15@170936 by XYGAX

picon face
The best scource for this is an old (ancient) Phillips laser vision disk
player they have two galvanicly mounted front silvered mirrors and a visable
light laser and the HT generator to make laser work they are also obselete may
be worth trying to find one in sombodys junk box

Cheers Steve.....

1997\12\15@182441 by William Chops Westfield

face picon face
The read/write head of CDROM drive has a pretty impressive set of degrees of
fine motion that I image are used for focus and "fine" movement.  Has anyone
experiemented with these?  It might make "reasonable" patterns (given the
addition of a visible laser) at a 'reasonable' distance, and it certainly
ought to be pretty fast...

BillW

1997\12\15@183957 by Andrew Mayo

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A very interesting idea indeed. Now, where's an old CDROM drive.....

> ----------
> From:         William Chops Westfield[SMTP:EraseMEbillwspamCISCO.COM]
> Reply To:     pic microcontroller discussion list
> Sent:         Tuesday, December 16, 1997 12:20 PM
> To:   RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU
> Subject:      Re: Servo Laser Control
>
> The read/write head of CDROM drive has a pretty impressive set of
> degrees of
> fine motion that I image are used for focus and "fine" movement.  Has
> anyone
> experiemented with these?  It might make "reasonable" patterns (given
> the
> addition of a visible laser) at a 'reasonable' distance, and it
> certainly
> ought to be pretty fast...
>
> BillW
>

1997\12\15@215225 by paulh

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On Mon, 15 Dec 1997, William Chops Westfield wrote:

> The read/write head of CDROM drive has a pretty impressive set of degrees of
> fine motion that I image are used for focus and "fine" movement.  Has anyone
> experiemented with these?

On the two that I've taken apart there are 2 degrees of freedom in the
fine motion system.  There is a little lens with a focal length of 1 or 2
millimeters.  One degree of freedom is up and down, the other is side to
side.  One drive did side to side with a pivot point, the other drive used
a 4 bar linkage.  Both systems used coils of fine copper wire.

I mounted a mirror on the mechanism that pivoted.  Since I didn't have an
active feedback system, I could move the mirror rapidly, but it wasn't
very controlled.  I could move it slowly enough that it didn't oscillate.
I just used a potentiometer, a PIC and PWM would have been much spiffier.

Unfortunately only one of the drives had a pivoting movement.  With 2
pivoting movements, I could make a rather small X-Y system.

The motions might be small enough that open loop control would be usefull
if I were projecting a small image on a wall several feet away.  I think
the currents involved are small enough for the PIC to drive the coils
directly through a current limiting resistor.  The coils need bipolar
drive, so it would take 2 pins for each degree of freedom.

Would capacitors help the PWM, and if so, how do I figure out the best
size?  This is a job for my trusty old 20 Mhz oscilliscope (just trying to
bring in another thread).  Further experiments will wait for another day
or another experimenter.  I don't see much point with only one degree of
freedom.  I'll have to wait until I get matching broken CD-ROM drives.

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1997\12\16@083535 by Kolesnikoff, Paul

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A recent Circuit Cellar (I think September or October) had an article on
building a laser billboard. Check out their web site at
http://www.circuitcellar.com. The author used a speaker coil to move one edge
of a hinged mirror. Depending on the range of motion that you need, this
could be a cost effective means of moving a mirror.

The direction of movement is controlled by the polarity of an applied
voltage and the distance by the magnitude of the applied voltage. The
magnitude could be determined using your PWM output and a capacitor. If
you wanted to avoid messing with polarity, just set the neutral position
of the speaker to point the laser to one edge of the desired range of
motion and use a single polarity voltage to deflect the laser where you
want it.

Good Luck
Paul K

1997\12\17@220447 by Alan King

picon face
Fine is the key word.  Very little angular deflection.  Not too useful
unless you need a 1" pattern on the wall on the other side of the room..


Andrew Mayo wrote:
{Quote hidden}

1997\12\18@002335 by Andrew Mayo

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Of course, you could bounce the laser beam between two parallel mirrors
- this provides a virtual path much longer than the folded path of the
beam, so when it finally exits the mirrors, it will have travelled a
much larger distance and hence the deflection at the far end would be
greater. Of course, there's some beam loss at each bounce, but good
surface-silvered mirrors would minimise that, I suppose.

{Quote hidden}

1997\12\18@002348 by Peter van Hoof

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I would use one of the speaker types that has a dome shaped mirror on it's
voice coil with a shallow entry angle you should be able to get a nice
deflection off these


just my $0.02

Peter

----------
From:   Alan King
Sent:   Wednesday, December 17, 1997 10:06 PM
To:     EraseMEPICLISTspamEraseMEMITVMA.MIT.EDU
Subject:        Re: Servo Laser Control

Fine is the key word.  Very little angular deflection.  Not too useful
unless you need a 1" pattern on the wall on the other side of the room..


Andrew Mayo wrote:
{Quote hidden}

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