how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.


--

Dan H.

"dan" wrote in message
...
how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.


--

Dan H.

I ran a flame-cutter several decades ago that used an optical trace - the
thing had to have a fairly high contrast between line and background, and it
had an "ey" about the size of a coffee cup. The whiz-kids who kept it in
operating order (as it was an *old* model) said that it had a set of pickups
surrounding the central sensor, and they read "more" or "less" and the
movement was controlled by that.

I personally favored the "witchcraft" explanation however, because if a big
gob of slag blew out of the burn-table and hit the paper in just the right
place, even uttering harsh imprecations and vile epithets wold not save the
job. This usually happened after the job was 65% cut, and generally when
cutting 8-up in 8" steel. (No, it was just about impossible to save it and
restart).

I spent two nights shoveling out the table while the factory-folk installed
a tape head and controlller on it and two weeks while they debugged it, and
then went on to significant wealth at piece-rate.

Flash

"dan" wrote in message
...
how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.


it is probably a photocell - a long time ago (inthe 60s) there was an
article in popular electronics about building a robot that would follow a
line - it used a single photo cell and a light bulb, no lenses. it would
follow one side of a piece of tape - you set a threshold (it had one
transistor, a CK722, and one sensitive and expensive relay) - when the
voltage out of the photocell was greater than some amount, the relay
energized and it drove one motor, making the thing turn left, when it was
less than the threshold, the relay dropped out and that would energize the
other motor making the thing turn right.

with the addition of a lens and maybe one more transistor, this could be of
use, no?

Flash wrote:


(snip)

I personally favored the "witchcraft" explanation however, because if a big
gob of slag blew out of the burn-table and hit the paper in just the right
place, even uttering harsh imprecations and vile epithets wold not save the
job. This usually happened after the job was 65% cut, and generally when
cutting 8-up in 8" steel. (No, it was just about impossible to save it and
restart).

I spent two nights shoveling out the table while the factory-folk installed
a tape head and controlller on it and two weeks while they debugged it, and
then went on to significant wealth at piece-rate.

Flash



I was talking to a torch operator (late 70s) when his machine took off
after a fly walking across the template!!!! Moved about a half an inch
before the fly flew away then the unit just moved in a small circle
looking for the line. Oscar saw this happening and shut the torches down
before too much bad happened.

Darn funny though, he said it happened often. The drafting machine used
for the templates was 30' long and 8' tall hung along the wall in IE.

Matt

dan wrote:
how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.


One method is to use two photocells, squinted a bit, one to one side,
the other towards the opposite side. They have overlapping fields of
view. Ratio of the two sides can find a line position. In the center the
two signals are equal, and if line is one one direction one cell
decreases signal while other increases.

In article ,
(dan) wrote:

how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Although it was easily distracted, as other posters have noted.

Joe Gwinn

What's that Lassie? You say that Flash fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sat, 6 Dec 2008 23:48:13 -0500:

I ran a flame-cutter several decades ago that used an optical trace - the
thing had to have a fairly high contrast between line and background, and it
had an "ey" about the size of a coffee cup. The whiz-kids who kept it in
operating order (as it was an *old* model) said that it had a set of pickups
surrounding the central sensor, and they read "more" or "less" and the
movement was controlled by that.

Thanks.

That is kind of what I figured for the steering of the pickup head.
But how does it convert the position of the pickup head to the X and Y
drive motors. I could make something follow a line easy enough. But
when it comes to going around a corner that would require one(or both)
of the drive motors to change direction, that's when I get lost.

If I had an X Y table (or gantry), that was controlled by a pointer
that you could rotate to change the direction of travel, I could make
a line following eye to move said pointer and trace out a shape on
paper.
--

Dan H.

What's that Lassie? You say that Bill Noble fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sat, 6 Dec 2008 22:18:09 -0800:

it is probably a photocell - a long time ago (inthe 60s) there was an
article in popular electronics about building a robot that would follow a
line - it used a single photo cell and a light bulb, no lenses. it would
follow one side of a piece of tape - you set a threshold (it had one
transistor, a CK722, and one sensitive and expensive relay) - when the
voltage out of the photocell was greater than some amount, the relay
energized and it drove one motor, making the thing turn left, when it was
less than the threshold, the relay dropped out and that would energize the
other motor making the thing turn right.

with the addition of a lens and maybe one more transistor, this could be of
use, no?

Actually I get how you could use a photocell to sense a line. And I
get how to use that to steer something. I could build a toy car that
would follow a line. No problem.

But I don't know how to get an X Y table to move when it needs to
change directions of one or both of the axes.

On the tracer I saw, the 'eye' rotated to follow the line. The X and
Y drives were controlled by the direction that the 'eye' was pointing.
That's the part I'm not sure of. How to control two axis, forward and
backward as needed, to move in the direction that the eye is pointed.

--

Dan H.

What's that Lassie? You say that Don Stauffer fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 11:06:06 -0600:

One method is to use two photocells, squinted a bit, one to one side,
the other towards the opposite side. They have overlapping fields of
view. Ratio of the two sides can find a line position. In the center the
two signals are equal, and if line is one one direction one cell
decreases signal while other increases.

I get the eye part. And how to steer the pickup head. But how to
control the X and Y axes to move in the direction that the pickup head
is pointing? That is where I get lost. If the axes didn't have to be
able to change direction, then I see how they could do it with a
non-rotating head that just controls the speed of one axis to keep up
with the other.

But in a tracer, the X and Y axes need to be able to reverse as needed
to follow the line. And the speed of each needs to be adjusted to
keep the torch constant as the lines angle changes.

--

Dan H.

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 13:03:03 -0500:

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Do you mean that's what rotated the pickup head? And the position of
the pickup head controlled the X and Y for the gantry?

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?
--

Dan H.

In article ,
(dan) wrote:

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 13:03:03 -0500:

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Do you mean that's what rotated the pickup head? And the position of
the pickup head controlled the X and Y for the gantry?

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 19:22:55 -0500:

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn


Joe,... you're my hero. I get it.

I didn't think I would be able to understand a stationary head
tracker. Especially from one posting alone.

Just one thing. What makes the axes move at all?
Once centered on the line, why does it follow the line. And witch
way does it go? On a vertical line would it go up or down?

And do you know anything about the rotating head type.
I get how the tracker head sees the line and turns to follow it, but
how is the rotation of the tracker head turned into X and Y outputs?
Is it like the stationary head sensors, but fed by something attached
to the rotating head?

Thanks a million,



--

Dan H.

One more thing Joe,

Many of the tracers that I've seen on the WWW, can trace a solid
silhouette.

How does that work?
--

Dan H.

On Sun, 07 Dec 2008 19:22:55 -0500, Joseph Gwinn
wrote:

In article ,
(dan) wrote:

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 13:03:03 -0500:

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Do you mean that's what rotated the pickup head? And the position of
the pickup head controlled the X and Y for the gantry?

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn

That gets displacement error signals for X and Y, but provides no
motion in the direction of the line, only correction for offsets
perpendicular to it.

I think there must be some means for maintaining set speed in a
direction parallel to the line. This could be done with a form of
difference engine, or it might use more straighforward computation of
Vx^2 + Vy^2 and Vy/Vx. The difference engine might require a start
at known speed and known direction. Could have been analog or
digital. Microprocessors have been around since the mid-'70's, CNC
since the late '50s, and electronic, mechanical and even pneumatic
ways of doing such computations have been known since World War II.

"dan" wrote in message
...
What's that Lassie? You say that Joseph Gwinn fell down the old


Joe,... you're my hero. I get it.

I didn't think I would be able to understand a stationary head
tracker. Especially from one posting alone.

Just one thing. What makes the axes move at all?
Once centered on the line, why does it follow the line. And witch
way does it go? On a vertical line would it go up or down?

And do you know anything about the rotating head type.
I get how the tracker head sees the line and turns to follow it, but
how is the rotation of the tracker head turned into X and Y outputs?
Is it like the stationary head sensors, but fed by something attached
to the rotating head?

Thanks a million,



--

Dan H.


Dan,

ISTR that when I would start the burn, I had to manually set it to a preheat
position, manually set X and Y vectors toward an engagement of the pattern
trace AT A DIAGONAL, so it would pick up the line and run from the oblique
angle, rather than the acute. Then, I would mash a button to begin cutting,
and when it had completed the pierce, engage the travel.

Flash

Flash wrote:


Just one thing. What makes the axes move at all?
Once centered on the line, why does it follow the line. And witch
way does it go? On a vertical line would it go up or down?

Dan H.


The one I was around had CW/CCW switch, CW would move the torch away
from the operator (X+/Y+), and CCW would move the torch towards the
operator (X-Y-). The torch could be adjusted to "offset" the line a bit
for machining later. So CW cut the part larger than the template and CCW
cut an inside feature smaller than the template. Unlike the one Flash
used the pierce was done from a 90 to the cut line and a pointer on top
of the eye was adjusted to "go that way" until it ran across a line,
when it hit the line it would go either CW or CCW to the direction of
travel.

Once in a while Oscar had to steer the torch by the pointer on the top
of the "eye unit" because if the torch got distracted after the "start
travel cycle" and lost the line the eye would quickly "spin in a small
circle" trying to find it.

It was kind of funny watching the pointer going round and round, (I
suppose "not" funny for Oscar though)

Matt



Dan,

ISTR that when I would start the burn, I had to manually set it to a preheat
position, manually set X and Y vectors toward an engagement of the pattern
trace AT A DIAGONAL, so it would pick up the line and run from the oblique
angle, rather than the acute. Then, I would mash a button to begin cutting,
and when it had completed the pierce, engage the travel.

Flash

In article ,
Don Foreman wrote:

On Sun, 07 Dec 2008 19:22:55 -0500, Joseph Gwinn
wrote:

In article ,
(dan) wrote:

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 13:03:03 -0500:

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Do you mean that's what rotated the pickup head? And the position of
the pickup head controlled the X and Y for the gantry?

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn

That gets displacement error signals for X and Y, but provides no
motion in the direction of the line, only correction for offsets
perpendicular to it.

I think there must be some means for maintaining set speed in a
direction parallel to the line. This could be done with a form of
difference engine, or it might use more straighforward computation of
Vx^2 + Vy^2 and Vy/Vx. The difference engine might require a start
at known speed and known direction. Could have been analog or
digital. Microprocessors have been around since the mid-'70's, CNC
since the late '50s, and electronic, mechanical and even pneumatic
ways of doing such computations have been known since World War II.

You are right that the above mechanism would not move unless propelled
somehow.

As others have mentioned, the operator sets up and starts the cut. The
controller is programmed to move at a constant linear speed, and the
servo keeps the head on the line as the curve is followed.

Joe Gwinn

In article ,
(dan) wrote:

One more thing Joe,

Many of the tracers that I've seen on the WWW, can trace a solid
silhouette.

How does that work?

I can think of a number of ways. One way has a 9-sensor grid and
detects edges, but I suspect that given the expense per sensor channel
back then, it wasn't done that way.

They may have servoed to a constant output (not zero) from the head, but
this requires knowing something about the orientation of the edge, which
is circular.

Ill think about it.

Joe Gwinn

In article ,
(dan) wrote:

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 19:22:55 -0500:

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn


Joe,... you're my hero. I get it.

I didn't think I would be able to understand a stationary head
tracker. Especially from one posting alone.

Just one thing. What makes the axes move at all?
Once centered on the line, why does it follow the line. And witch
way does it go? On a vertical line would it go up or down?

It's propelled at a constant speed as set by the operator, and the servo
only follows the line.


And do you know anything about the rotating head type.
I get how the tracker head sees the line and turns to follow it, but
how is the rotation of the tracker head turned into X and Y outputs?
Is it like the stationary head sensors, but fed by something attached
to the rotating head?

A rotating head can use one or more eccentric sensors to do the work of
all four sensors in my example (or nine sensors in the larger array for
edge following), so long as the sensor head rotates fast enough that the
curves to be followed are sampled very densely.

Joe Gwinn

On Dec 8, 9:08*am, Joseph Gwinn wrote:
In article ,

(dan) wrote:
One more thing Joe,

Many of the tracers that I've seen on the WWW, can trace a solid
silhouette.

How does that work?

I can think of a number of ways. *One way has a 9-sensor grid and
detects edges, but I suspect that given the expense per sensor channel
back then, it wasn't done that way.

They may have servoed to a constant output (not zero) from the head, but
this requires knowing something about the orientation of the edge, which
is circular.

Ill think about it.

Joe Gwinn

Video cameras are cheap these days, ditto computers to do the image
processing. Software is cheaper than hardware these days.

Stan

dan wrote:

how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.




It has been quite a while since I worked on an optical tracer head. It
had some miniture lamps the illumininated the line, I think there were
four of them around the bottom of the tracing head. There was a mirror
that was rotated by a motor that directed the focused image of the line
onto two sensors. There was a magnet on the rotating shaft that held the
mirror that gave the angular position of the mirror. Each axis was a
separate circuit and as the mirror would scan across the line, first
from left to right and then right to left the sum of the two scans was
fed into the amp for the servo drive motor. If the head was drifting to
the right the combinded reference voltage would go higher and move the
head to the left and vice versa. The same action operated the other
axis. The speed of the cut was regulated by a control the set the gains
of both amps.


John

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 11:08:34 -0500:

Many of the tracers that I've seen on the WWW, can trace a solid
silhouette.

How does that work?

I can think of a number of ways. One way has a 9-sensor grid and
detects edges, but I suspect that given the expense per sensor channel
back then, it wasn't done that way.

They may have servoed to a constant output (not zero) from the head, but
this requires knowing something about the orientation of the edge, which
is circular.

Ill think about it.

I think it would work similar to the line following rotating head, but
look for max contrast, not max balance, between a pair of photo cells.


--

Dan H.

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 11:03:32 -0500:

That gets displacement error signals for X and Y, but provides no
motion in the direction of the line, only correction for offsets
perpendicular to it.

I think there must be some means for maintaining set speed in a
direction parallel to the line. This could be done with a form of
difference engine, or it might use more straighforward computation of
Vx^2 + Vy^2 and Vy/Vx. The difference engine might require a start
at known speed and known direction. Could have been analog or
digital. Microprocessors have been around since the mid-'70's, CNC
since the late '50s, and electronic, mechanical and even pneumatic
ways of doing such computations have been known since World War II.

You are right that the above mechanism would not move unless propelled
somehow.

As others have mentioned, the operator sets up and starts the cut. The
controller is programmed to move at a constant linear speed, and the
servo keeps the head on the line as the curve is followed.

Joe Gwinn

I can live with the idea of the rotating head to track the line, with
the four cell arraignment to move the axes in the right direction.

I'm almost tempted to try to cobble one up. But I don't think I could
make one that could do more than prove the concept.

I think I might keep an eye out on e-bay.

Thanks all for your replies
--

Dan H.

dan wrote:
What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 11:03:32 -0500:


That gets displacement error signals for X and Y, but provides no
motion in the direction of the line, only correction for offsets
perpendicular to it.

I think there must be some means for maintaining set speed in a
direction parallel to the line. This could be done with a form of
difference engine, or it might use more straighforward computation of
Vx^2 + Vy^2 and Vy/Vx. The difference engine might require a start
at known speed and known direction. Could have been analog or
digital. Microprocessors have been around since the mid-'70's, CNC
since the late '50s, and electronic, mechanical and even pneumatic
ways of doing such computations have been known since World War II.

You are right that the above mechanism would not move unless propelled
somehow.

As others have mentioned, the operator sets up and starts the cut. The
controller is programmed to move at a constant linear speed, and the
servo keeps the head on the line as the curve is followed.

Joe Gwinn


I can live with the idea of the rotating head to track the line, with
the four cell arraignment to move the axes in the right direction.

I'm almost tempted to try to cobble one up. But I don't think I could
make one that could do more than prove the concept.

I think I might keep an eye out on e-bay.

Thanks all for your replies

Maybe this type of thing would be simpler to cobble up
http://www.lowbucktools.com/flamecut.html

What's that Lassie? You say that David Billington fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Tue, 09 Dec 2008 00:18:26 +0000:

dan wrote:
What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 11:03:32 -0500:


That gets displacement error signals for X and Y, but provides no
motion in the direction of the line, only correction for offsets
perpendicular to it.

I think there must be some means for maintaining set speed in a
direction parallel to the line. This could be done with a form of
difference engine, or it might use more straighforward computation of
Vx^2 + Vy^2 and Vy/Vx. The difference engine might require a start
at known speed and known direction. Could have been analog or
digital. Microprocessors have been around since the mid-'70's, CNC
since the late '50s, and electronic, mechanical and even pneumatic
ways of doing such computations have been known since World War II.

You are right that the above mechanism would not move unless propelled
somehow.

As others have mentioned, the operator sets up and starts the cut. The
controller is programmed to move at a constant linear speed, and the
servo keeps the head on the line as the curve is followed.

Joe Gwinn


I can live with the idea of the rotating head to track the line, with
the four cell arraignment to move the axes in the right direction.

I'm almost tempted to try to cobble one up. But I don't think I could
make one that could do more than prove the concept.

I think I might keep an eye out on e-bay.

Thanks all for your replies

Maybe this type of thing would be simpler to cobble up
http://www.lowbucktools.com/flamecut.html

I saw that, and did consider it. But the kerf of my plasma cutter is
so narrow that it would need to have a magnetic spindle that is .060
dia. Or make the patterns undersize. Not worth the effort for 1-2
pieces.

I may make a space frame style arm that can be attached to the table
of one of the milling centers(cnc) at work. It would hold the torch
out side of the enclosure, over a scrap barrel. I wouldn't be able to
tie in to the control to start/stop the cutter, but that's OK.

Or could I. There is an interface for a Hass rotary table. I could
use that to press a switch and start cutting. Hmmm.




--

Dan H.

What's that Lassie? You say that john fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 18:04:38 -0500:

It has been quite a while since I worked on an optical tracer head. It
had some miniture lamps the illumininated the line, I think there were
four of them around the bottom of the tracing head. There was a mirror
that was rotated by a motor that directed the focused image of the line
onto two sensors. There was a magnet on the rotating shaft that held the
mirror that gave the angular position of the mirror. Each axis was a
separate circuit and as the mirror would scan across the line, first
from left to right and then right to left the sum of the two scans was
fed into the amp for the servo drive motor. If the head was drifting to
the right the combinded reference voltage would go higher and move the
head to the left and vice versa. The same action operated the other
axis. The speed of the cut was regulated by a control the set the gains
of both amps.

Thanks John. This is the first description involving a mirror I've
heard of. Makes sense. No slip rings to go bad or introduce noise.

--

Dan H.