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'[OT] Using a Stepper as a Galvo'
1999\04\24@095132 by

PICsters,
There has been a little bit of discussion on the PIC list about
Laser Light Shows and the use of Galvos.

Did you know that you can use a regular stepping motor as a Galvo?
Most of you probably know that you can half-step a stepper motor
by activating two windings at once. Most of you probably know
that you can microstep a stepper motor by pulse width modulating
one winding while you apply DC to the next winding. What many
people don't realize is that you can ALSO microPOSITION a stepper
motor by applying a fixed DC current to one winding, and a variable
DC current to an adjacent winding. The fixed DC value can be obtained
quite simply by applying the regular rated voltage to one winding.
For the Variable Current Source/Sink, I typically use a DAC
(Digital to Analog Converter) to convert a binary value to a
ground referenced current using an opamp, a power transistor or FET,
and a few other discrete components. Since the "mechanical load"
is usually only a small front surface mirror, you can get by with
a very small stepper motor.

The smaller the physical size of the stepper motor, the faster you
can microposition it in "galvo" mode. This is due to the smaller
MASS of the rotor.

The larger the stepper motor, the more torque you will have.
This is generally only important if you are doing something
*other* than just positioning a small mirror.

The Galvo "range" is about equal to the usual step angle, so
choose the step angle accordingly.

When the stepper galvo is initially powered up you will want the
mirror to assume a "home" position. You can do this by attaching
a physical stop that will restrict the shaft position to a range
of positions that is just slightly larger than the desired
free-movement range. Just remember to keep the MASS as low as
possible so you can achieve maximum speed. My own favorite method
for building a physical stop is to drill a small hole part way
through the shaft of the stepper motor and epoxy a needle
into the shaft. The needle is very strong, which is important.
You want high strength and low mass. A piece of metal with
a hole punched in it is then inserted over the needle and arranged
so that the needle movement is restricted as desired.

**** xy versus arc ****

The stepper galvo moves in an arc. If the arc angle is kept
small, then distortion between angular position and linear
position of the beam on a wall is not too bad. If you choose
to use wider angles, then the distortion can be significant.
You can compensate for this by either using a lookup table,
or by mathematically processing the data to compensate.

**** OTHER USES ****

I have built several other devices that make use of the stepper
galvo. One was a sort of miniature chart recording device for a
seismograph. A small plotter pen was moved in an arc of about
7 degrees, and recorded seismic activity on a slow moving strip
of paper.

In another device I used two stepper galvos to quickly position
a glass slide under a microscope. The xy coordinates of
interesting sections were "memorized" when a keypad was hit
while holding down the "memorize" function button. The user
could then quickly zip back and forth to the areas of interest
at the push of a keypad button.

I'm sure that most of you can come up with some additional
uses for a stepper galvo.

Fr. Tom McGahee

GRAEME SMITH                         email: grysmithfreenet.edmonton.ab.ca
YMCA Edmonton

Address has changed with little warning!
(I moved across the hall! :) )

Email will remain constant... at least for now.

On Sat, 24 Apr 1999, Thomas McGahee wrote:

{Quote hidden}

Kinda reminds me very little of the feeling you used to get with
micro-fishes when you spent the whole day looking at them as you
zipped back and forth finding that one specific part number......

> I'm sure that most of you can come up with some additional
> uses for a stepper galvo.
>
> Fr. Tom McGahee
>

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