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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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optical pattern tracers
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. |
#2
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optical pattern tracers
"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 |
#3
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optical pattern tracers
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 |
#4
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optical pattern tracers
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. |
#5
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optical pattern tracers
"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? |
#6
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optical pattern tracers
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. |
#7
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optical pattern tracers
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. |
#8
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optical pattern tracers
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. |
#9
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optical pattern tracers
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#10
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optical pattern tracers
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. |
#11
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optical pattern tracers
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#12
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optical pattern tracers
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. |
#13
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optical pattern tracers
"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 |
#14
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optical pattern tracers
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#15
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optical pattern tracers
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. |
#16
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optical pattern tracers
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#18
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optical pattern tracers
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 |
#19
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optical pattern tracers
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 |
#20
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optical pattern tracers
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. |
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