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  1. #21
    Hi Devmonkey

    I have been discussing alignment with a good mate who is planning a very nice servo driven router, he may pop in here. We built an arduino driven device to sense a stretched wire buy mechanical contact. He did the electronics I did the mechanical s. Being driven by the arduino we were able to cycle it thousands of times. the attached dial indicator never varied by more than a couple of tenths 90% of the time. occasionally there was an error probably caused by a dirty contact. or maybe some mechanical stiction.

    The software looked for a break of contact. First finding the wire on contact then slowly moving away.
    A couple of times per cycle.

    The mechanical system used a 10:1 lever, a flexure and a stepper driven with 4:1 reduction M8 (1.25mm pitch) screw drive. It was accurate to better than .0001"

    I would like to try using a webcam as you have described instead of the electrical contact and compare the results. Being non contact it should not have the dirty contact issues.

    I would love to try the software you have worked on if you are willing to share it? . Is it running on a PC? or a micro?

    Regards
    John

    This photo is a little unclear. I still have the unit I will take some better photos. It was made from scrap material I had to hand as a test of concept only prototype.

    Click image for larger version. 

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    Last edited by John McNamara; 15-08-2019 at 02:38 PM.

  2. #22
    I was wondering about how a taught wire might be used for measurements and very much like the arrangement John describes. I have a box full of Arduinos waiting for something to do.
    Joe, One idea to throw into the melting pot... If two wires are stretched one above the other, separated by a distance slightly less than the diameter of the laser beam (wrapped around two suitably chosen drill shanks for example), can your sensor be used to accurately show the relative illumination of the two wires as the beam moves horizontally along them? Quite small variations of height might show up as significant variations of illumination brightness or width on the wires.

    Kit
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  3. #23
    Hi Kitwin

    I would like to avoid a laser altogether. Instead ideally I would like to use the shadow of the wire over the sensor backlit with a dispersed light source. This would avoid any focussing issues. There would be a shadow formed on the sensor with a normal distribution characteristic darkest in the center. to lightest near the two edges.

    Borisovs paper mentioned earlier uses omron sensors in this manner, no optics.

    As the camera sensor is only about 2mm square. with .008" wire I can imagine that the workable measurement range would be about 1mm at best. This is where the device I made would help. It has a range of about 5mm, It could be used to center the sensor. Once this is done the current count of the stepper would represent the null point on the sensor.

    These counts could be plotted along the wire giving a chart of the flatness of the object being measured.

    For longer distances over a metre the catenary sag of the wire will become noticable. This can be computed as the measurements are taken along the wire assuming the drive is stepper or a servo with an encoder.

    Alternatively glass optics, I guess however there are many hurdles to a perfect arrangement.

    Note
    One of the very practical reasons for using wire is that it can be positioned very accurately prior to measurements being taken. Both endpoints can be fixed and very accurately aligned to the surface being measured. and once fixed they stay put.

    I also have a Taylor Hobson alinement telescope. It works well but not to .0001" of an inch. (.002" is achievable) Setting it up buy aligning the targets is no easy matter. An hour can easily pass. If a target is knocked you have to start again. pointing a laser will be the same. Using both systems there is no obvious error. It can only be found out by checking.

    When I did work on my lathe I used stretched wire mounted at each end on the bed. Here I used a Microscope. A digital camera sensor should be a lot more accurate. Scroll down to near the bottom of the following link.

    https://www.model-engineer.co.uk/for....asp?th=136771

    Regards
    John
    Last edited by John McNamara; 16-08-2019 at 05:10 AM. Reason: A few typos!

  4. #24
    John,
    I had a look at the other thread on this forum where you described the taught wire system on your lathe and also scanned through Borisov's paper, though I must admit to not studying all 170 pages in detail. Cunning use of cheaply available sensors and I can well understand not wanting to involve adjustments to both the wire and the laser. I was thinking more of Joe's desire to check the horizontal flatness of the beam from his commercial laser device when the 2 wire idea came to me.
    As someone who plays with a DIY CNC router as just as one of several hobbies I'm always on the lookout for ways of accurately aligning my machine without having to buy several expensive measurement devices that will only ever get used a handful of times. There's also a great deal of satisfaction from devising a method which is inexpensive but effective... which I haven't quite managed yet. I only cut wood, for the present at least, so my current aim is achieve 0.1mm accuracy. So I shan't be worrying about the sag in the middle of a 1200mm taught wire anytime soon

    Kit
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  5. #25
    Just a quick one but surely if you are going to use a image sensor it is easier to reference off the perfectly straight laser? I'm also not sure why the stepper motor part is needed, wouldn't you just read the error straight off the image sensor and convert pixels to um?

    In my opinion and how I visualise this is you are only ever measuring errors to a known reference you don't need to be parallel to it whether it is a wire or a laser or a precision straight edge. What you are trying to do is ensure these errors lie on a straight line, or in the case of two rails in the same plane.

    If you want to use the cmos sensor as a shadow camera without optics my software would work if I inverted the image intensity as you'd get a guassian shadow from a round wire strongly back lit. Mechanically with no optics you need the wire very close to the sensor which may complicate things both wrt protecting the sensor but also aligning your wire in the first place. I would just rebuild a USB microscope at fixed focus in an aluminium block.Ideally you need to measure in 2D not 1D so you can set it straight in both dimensions at the same time, this can be done with two microscopes setup orthogonally on the same block. With a normal dot laser (if you are worried about twist then use a cross laser) pointed directly at an image sensor you are measuring 2D with a single sensor, with a wire you would need two sensors as you cannot 'look' down the wire. The cheap £15 microscopes on Amazon at mid resolution have a depth of field that could maintain focus on your wire if your setup error is within 0.5mm or so. Just be aware that you have to remount the guts of it in something rigid as the assembly that lets the optics slide in and out and the part that holds the sensor pcb can wobble. The latter reason is why i've dropped the optics all together as it simplifies mounting it.
    Last edited by devmonkey; 16-08-2019 at 11:14 PM.

  6. #26
    No more blutack!

    A few happy hours with the milling machine and I've finished a rigid sensor mount. I surfaced the base plate so it the two faces are planar, then drilled it for the hiwin block. Then milled a true cube of Delrin with 6 sides surfaced in planar pairs, machining a cavity in one face to house the camera. Finally a window was milled. The glass in the window is a microscope slide cover slip, extremely thin, not sure how long it will last.The cube was drilled in two directions so it can be rotated 90 degrees for using the laser as a straight edge rather than a plane.

    The cube is loaded onto the base plate with a bolt and a spring so it can still be rotated about the Z axis when in use, this means I can transfer it from one rail to another and pivot it back to look at the laser. Hopefully since I surface milled the contact surfaces it will remain in plane when rotated.

    For reference that is a 20mm hiwin block. I managed to get a smudge on the sensor, doesn't really impact performance.

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    Last edited by devmonkey; 17-08-2019 at 05:29 PM.

  7. #27
    Hi Devmonky
    Hi all

    I agree that you could use the output from the sensor to directly plot position.
    Using a simple webcam with a 2mm sensor to centre a .008" piano wire or laser, the effective measurement range would be maybe 1mm. The active area of the sensor needs to be found in some way, unless the wire or beam is positioned within the active area all the time.

    As you can see below, I have been thinking of making a device that flattens surfaces by "Pecking" using a small cutter stepped over a work piece. The overlapping pecks will create a flat surface that in the end will look like an engine turned surface. Another way of thinking about it is as if the surface was scraped. As with scraping practice the last step is to lightly stone the surface to remove burrs and correct any high spots.

    When the positioning device shown earlier was being designed the range of motion was considered - what sort of materials would be measured and for my purposes flattened? I was thinking rolled 50-100mm square steel sections, weldments etc. I reasoned that maybe a 5mm range of motion would be desirable to flatten these objects. Large bends in thin walled tube would obviously not work if severely bent, and would be cut right through without the addition of some sort of built up face made from Steel, Aluminium or Epoxy, standard practice in industry.

    My design intent was to design a simple machine to flatten objects that could be built from very low-cost materials. There are numerous posts on the Web regarding the creation of flat surfaces on DIY built CNC Routers Mills and other machines, various methods are used, some quite costly. Many are not entirely successful.

    As I described earlier the test jig uses a lever flexure and screw driven by a stepper motor. I used the same mechanism to position a grinding jig to within a repeatable mechanical accuracy of .0001" 2.5 microns. I know it works. It has a range of motion of 5mm.

    The mechanism is able to apply a significant force the 10:1 primary lever and flexure provides enough force to easily position the Z carriage. (Flexures are free of backlash although geometric errors have to be considered).

    My plan is to also drive the mechanism with a crude Y motion that guides the sensor head and cutter along the work piece. I imagined using a piece of steel, say a piece of I beam or other available steel section with simple ball bearing rollers guiding a carriage along it, the piece being flattened would be placed upon it. The accuracy of the flattening is determined by the wire and sensor not the beam or mechanism. The only preparation of the support beam needed would be to hand grind and stone the surfaces that the ball bearings run on for smooth running.

    Two possible arrangements are:

    For a horizontally disposed flat surface generator.
    The x axis requires very little travel, maybe 75mm will cover just about any bearing surface I can imagine. I am still thinking on this. Moving the work piece laterally on two small slides would be simple enough for shorter lengths. Longer lengths would be a different matter - they would require mid span support or they will deflect slightly due to cutting forces and gravity. The X axis motion will require high precision, however over a very short distance. The two or more slides will all have to be coplanar.

    For a vertically disposed flat surface generator. (This has always been my preferred option.)

    This solves a number of the design issues of the horizontal arrangement. The y axis can be the same as previously described (except it now has to be described as the Z axis!)

    In this case two alignment wires can be used, one each side of the Z axis support beam, simply suspended with a counterweight (the weights can be placed in a container of thick oil to dampen them).

    Using this arrangement there is no catenary sag at all, the wires will be perfectly coplanar at all times.

    The Cutting head can be mounted on a small precision X axis slide.
    To align the x axis slide to the work piece will require two cameras and two stepper driven positioners as described earlier. The software will need to go through a few iterations to get the two cameras in perfect alignment.

    The work piece can be simply clamped to the Y axis base beam.

    Cost?
    2 web cams
    2 wire alignment steppers.
    2 timing pulleys and a belt

    1 X Axis drive stepper.
    1 X axis 100mm slide, bought or built.
    1 X Axis screw drive - only low resolution is required.

    1 Y axis drive stepper
    1 Y axis timing belt drive, this will be a long belt
    8 or 10 Y axis low cost ball bearings.

    1 Z axis Cutter head drive stepper.
    2 Z Axis 20mm slides bought or built.
    1 Z Axis screw drive (Must have no backlash)

    A computer and interface.

    A piece of scrap steel beam or tube as long as needed, assorted scrap steel.
    Time

    It will depend on what you have in stock?

    Regards
    John

  8. #28
    Thanks John
    Over the years i have been wondering if it would be possible to build an assembly that could grind a reference plane on a machine base but couldn't think how to make it rigid enough to be accurate, this would be much better than the epoxy faff and you could add flatbar to the frame giving you a decent amount of material to tap into. keep up the good work and plenty of pictures when you get it sorted.
    Regards
    Mike

  9. #29
    Quote Originally Posted by John McNamara View Post

    I agree that you could use the output from the sensor to directly plot position.
    Using a simple webcam with a 2mm sensor to centre a .008" piano wire or laser, the effective measurement range would be maybe 1mm. The active area of the sensor needs to be found in some way, unless the wire or beam is positioned within the active area all the time.
    Hi John,

    With a self levelling laser hitting a 2mm square target over the entire machine shouldn't be too hard assuming you have levelling feet on the machine, we all do this or better when we put up a shelf. This assumes we can weld the frame to within 2mm.

    Should also be noted that it is only the vga (640x480) web cams that have this small sensor area I think these are 1/6" diagonal sensors, other similarly priced but higher resolution cams have larger sensor areas. For example the raspberry pi camera (1/4") has a 4.6mm diagonal and another sensor I have the OV2640 has 4.5mm. It seems that the sensors are classified by the diagonal size of the sensor of which the active area with the pixels is smaller, this is why 1/4" > 4.6mm.

    I like the idea of levelling a jig then grinding the surface as an alternative to shimming. What sort of grinder would you use for this?

    I can imagine a jig short jig say 500mm that clamps around a piece of box section that can be micro adjusted for twist and level. I we bolted aluminium to the top of the box section you could probably skim it with a hand router with a small fly cutter in it. Skim the section under the jig then move the jig along, re-align it with the laser/wire then repeat. When you finish one rail then move the assembly to the other without moving the reference laser to bring the other rail into plane. Maybe a wood power planer could be used for the skimming, would be scary though.

    The jig itself needs to be planar as you say but this should be straight forward on a milling machine.

    What are you thinking for differentiating twist from height variation? I think with the laser two image sensors stuck out each side could detect twist same when you tram a mill with two dial indicators on a T bar in the spindle.

    Cheers, Joe
    Last edited by devmonkey; 18-08-2019 at 10:56 AM.

  10. #30
    Quote Originally Posted by devmonkey View Post
    Hi John,

    "With a self levelling laser hitting a 2mm square target over the entire machine shouldn't be too hard assuming you have levelling feet on the machine, we all do this or better when we put up a shelf. This assumes we can weld the frame to within 2mm."

    Assuming a vertical arrangement the only alignment required is to align the rough frame he two counter weighted wires. When a work piece is clamped in position it is also aligned to the wires.

    The concept of using a "metrology frame" in this case for one axis is not new.
    https://www.google.com/search?q=metr...5sCCs1M:&vet=1

    It is important that the alignment wires are in line with the tip of the cutter at the point of cutting. This will greatly reduce any error caused by any angular misalignment of the z axis to the work piece. Abbes principal must always be honored.
    https://www.google.com/search?rlz=1C...71.zasns7Fk5YU

    ------
    "Should also be noted that it is only the vga (640x480) web cams that have this small sensor area I think these are 1/6" diagonal sensors, other similarly priced but higher resolution cams have larger sensor areas. For example the raspberry pi camera (1/4") has a 4.6mm diagonal and another sensor I have the OV2640 has 4.5mm. It seems that the sensors are classified by the diagonal size of the sensor of which the active area with the pixels is smaller, this is why 1/4" > 4.6mm."

    I think sarting with a low cost solution first then if necessary upgrading to a more expensive camera.

    ------
    "I like the idea of levelling a jig then grinding the surface as an alternative to shimming. What sort of grinder would you use for this?"

    I had been reflecting on grinding but had substituted milling, strangely I should have written milling!
    Grinders have a lot of inertia and take a long time to stop, they also make a lot of noise. A Small motor driven milling cutter ideally around 3mm can be fed relatively a lot slower and the motor can stop quickly. I suspect having a running spindle near the wire alignment system may cause the wires to vibrate. For high accuracy results it may be necessary to stop the spindle before each alignment cycle.

    ------
    I can imagine a jig short jig say 500mm that clamps around a piece of box section that can be micro adjusted for twist and level. I we bolted aluminium to the top of the box section you could probably skim it with a hand router with a small fly cutter in it. Skim the section under the jig then move the jig along, re-align it with the laser/wire then repeat. When you finish one rail then move the assembly to the other without moving the reference laser to bring the other rail into plane. Maybe a wood power planer could be used for the skimming, would be scary though.

    The jig itself needs to be planar as you say but this should be straight forward on a milling machine.

    ------
    "What are you thinking for differentiating twist from height variation? I think with the laser two image sensors stuck out each side could detect twist same when you tram a mill with two dial indicators on a T bar in the spindle."

    Two weighted wires suspended vertically will have no twist between the at all. These two wires are our meteorology frame. if we position our cutting stage relative to the wires using two cameras, make a peck cut then move X or Y,, check the cameras are still perfectly centered, make a new cut and so on our accuracy will be determined by the accuracy achieved by the camera system.

    No system is without error however if you consider the above scenario the surface generated will have a surface that comprises a series of physical levels some high and some low. representing the accuracy of the measurement cameras and the positioning system.

    Using a vertical alignment system, the exciting part is that the wires represent a plane that is far flatter than an inspection grade surface plate. The final result will be a surface that is extremely straight overall. With a surface similar to a scraped bearing surface.

    Upon reflection I will make my test setup vertical.

    As you may have guessed I have been thinking on this problem for a long time.

    Cheers, Joe
    Answers above.
    Last edited by John McNamara; 19-08-2019 at 02:08 AM. Reason: More Typos!

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