Project: Laser Cutter (CNC)

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Re: Project: Laser Cutter (CNC)

Postby Jerry » Mon Feb 14, 2011 1:34 am

I beefed up the Y motor mount. Silver soldered a piece of stainless to unitize. Much more rigid now.

I also got the Y axis linear encoder strip mounted down and machined an adapter to mount the reader head to the back of the carriage.

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IMGP6549 by macona, on Flickr

Once that was done I attempted to tun the Y axis motor. It just would not tune. Complaining about the encoder and all sorts of stuff. One time I tried to tune it I got a weird noise from the motor and that was all she wrote. Looks like I fried a winding. Swapped drives to make sure and got the same result. So it was time to redesign. I was thinking of various other drive methods like belts and stuff. I went to a friends surplus store and picked up a couple small DC servos. One was a MicroMo/Faulhaber 3863 series motor. Just so happens it has the same hole pattern as the old maxon flat motor. I had to mill out the shaft hole a bit but it seems to work. I also had to turn the shaft down to 3mm from 6mm to install the old pulley. I ended up using a 5C collet in the lathe to drive the motor while the motor housing was held in the steady rest. The shaft was hardened so I used a nice polished carbide insert to shave it down. Put a dab of loctite on the shaft and put the pulley in place.

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IMGP6540 by macona, on Flickr

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IMGP6542 by macona, on Flickr

I had left extra pads for a connection to a DC servo in case I couldnt get the brushless motor to work. I installed a connector on the board and wired up the motor. Ran the configure routine and auto tune and it tuned perfectly. Stinking powerful motor. Rated over 200 watts. I do need to machine a touch from the end support to clear the motor still. You can see the motor (black) in this shot:

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IMGP6547 by macona, on Flickr
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Mon Feb 14, 2011 1:48 am

So now that the motors are tuned it was time to get this thing talking to a PC. That took some doing. First I made up a cable to connect to a DB-25 for the parallel port. Then it was software time. These drives are unlike any drive I have used before. They use programming environment similar to C to program the advanced features where they store the program on flash. It took hours of going through the documentation to gather all the commands I needed to make up a program that would set the drive to pulse and direction mode. Also with the encoder of the Y axis the interpolator multiplied the resolution time 2 so it gives me a 10 micron resolution, 2.5 micron in quadrature. Thats about .00009" per step which is crazy for an application like this. Luckily there the a pulse multiplier built in. Setting that to 4x it allowed me to match the X axis resolution of 10 microns in quadrature, around .0008". It also allowed me to kick the speed up on the Y axis. It will now do about 1000ipm.

Once I got the commands figured I got things going with an old 3ghz P4 I had lying around. Here is the machine running the roadrunner program that is in mach at about 400ipm. The X axis is quite a bit slower than the Y, much more massive and a weaker motor but it still does pretty good.



A few more pics:

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IMGP6534 by macona, on Flickr

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IMGP6539 by macona, on Flickr

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IMGP6548 by macona, on Flickr
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Tue Feb 22, 2011 3:27 am

Been spending the last week or so debugging things. X axis would just take off in the positive direction randomly. But only when commanded from the parallel port. If I send move commands in the control software via serial it moves how it should. Y axis would not hold position. If I move it falls short in the return to the original position.

It turned out both problems had a common theme... Noise.

X axis was getting noise in the step line when going in the positive direction. (Why positive only, I dont know) The noise was being picked up as step signals and the axis would take off until it tripped an internal overspeed limit. Luckily there is built in filtering. I adjusted it up to match the encoder filtering and everything started working as it should.

Y axis was a bit of a pain. First I had an issue with the multiplier. When I looked at the position status in the control software it was showing some odd position data, it was not multiplying right. Seems it does funny things when it is set in nvram and program code. Eliminated the multiplier in code and it moved pretty much as it should. Still it was not returning to the same spot after a move. If I rapid moved from 0 to 5 inches it would move further and then when I return back it would come up about 1/8" short. Weird thing was the slower I ran this test the more it would be off. Normally with an encoder you loose pulses and the motion is more than commanded. This was the opposite, it was receiving extra pulses while moving so it would come up short. I took my scope and put it on the lines from the pickup head to the interpolater board and found all sorts of noise. I disconnected the motor and a lot of it disappeared. I was getting all sorts of harmonics from the PWM from the motor drives. I tried using the built in filtering but it just wouldnt cut it. While the probe was connected I took a little ceramic .01 uf cap and put it from the signal line to the frame ground. That killed about 90% of the noise. I pulled the arm board off and soldered caps from each signal line to ground. I also solder in a couple 1uf caps near the connector to the encoder on the gantry board. That seemed to fix the problem, almost. It was moving the right distance in one direction but falling short in the other. I took the scope and put it on the TTL output of interpolator board. There was a strange glitch to ground in one channel and in only one direction. I then put the scope on the 1v p-p lines from the encoder. As I moved the axis one of the channels had a glitch where it would suddenly drop out for a few microseconds before resuming. I swapped out the reader head with a new one and that fixed the last remaining problem with the Y axis. It now moves exactly where it should, at least better than I can measure, or care to. 100 steps per mm, 2000steps per sec per sec acceleration, and will do over 1100ipm.

Now that I got that fixed I made up a plate to support the gantry board and guide the wires in the cable tray. Took some aluminum and milled it to the shape I wanted and mounted. I also yanked the hall effect cables from the cable bundle as they will not be needed.

Another thing I did was to stiffen up the Y axis. It was running on single truck on the small 10mm linear rail. The truck is preloaded but it is still lacking in stiffness. There is an auxiliary rail just below it with a couple more trucks. So I took some measurements and made a small plate to tie the two trucks together, I also milled away some of the part the truck mounted to to make up for the increased thickness. Tying these two together increased the stiffness substantially.

Now that it moves how it should I have found a problem that is causing me some problems. When the X axis moves the whole arm flexes. I have traced it down to the upright support that rises from the linear bearings. Much of it has been milled away for cable pass though and lightening. This has also made it pretty weak. Normally it would be fine because the original machine probably had S-curve acceleration and deceleration which helps a lot. Mach, EMC, and all the other cheap laser controls use a trapezoidal acceleration/deceleration curve that induces a lot of "jerk" into the machine which is very apparent in this set up. The far end wobbles back and forth like a leaf in the wind!

So, somehow I need to make things more rigid. So far I have two Ideas. One is to machine some plates to fill in the hollowed out areas on the X axis arm. Bolt through and probably epoxy them into place. Then Also make a heavy lid for the back side to box it in as well as I can. If that does not work Its on to plan 2. That is using a cable/belt and pulley system to drive the other side of the gantry. Another thing that might help is replacing the round rail guide system on the undriven side with some linear rails. That ought to give it a little more stiffness.

Now to a point where I need to get the laser and power supply. Also need to start scrounging up some mirrors. Its going to take more than I thought due to how I want to install the tube. Looks like I will need about 5 of them. Smaller than normal as well due to space constraints.

Here's a pic of the gantry board and support. The extra connectors are for things like home and limit switches.

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IMGP6555 by macona, on Flickr
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Re: Project: Laser Cutter (CNC)

Postby Doug Coulter » Tue Feb 22, 2011 9:56 am

I'd almost have to bet you've got some ground loop noise that's the true cause of those issues. A floating scope measuring between ground here and ground over there would show that if it's the case. It's amazing how much noise can get coupled from some wires running motor-level currents into low level logic type stuff just by transformer effect when you're not really careful about that. Some re-routing or re arrangement of how grounding is done might do wonders, but if the caps really fix it, it ain't broke neither.

Sounds like rigidity is going to be an issue -- wasn't that designed for a case where it just moved to a place, carrying something light, then could pause before dispensing whatever? So in the original design it didn't have to be right except some time after motion-stop? You don't want a hot laser wafting around like a leaf in the wind! Hope some simple fix helps with that one...but might just move the problem. Can you adjust your length of time the trapezoid takes? You might hit less mechanical harmonics that way, at the price of some speed.

OF course, you can bulk up any part of this that doesn't have to move for it to work. But adding too much mass to anything moving will just move the mech resonance lower in frequency -- which might mean that if it "rings" for X cycles, they take longer to settle down (and adding mass may also increase the cycle count by having more mass to loss ratio). I've had good luck (with simpler things) using something resistive damping mechanically -- inelastic bending -- to cut way down on things like that. Lead (but too heavy) or this stuff they sell car audio fanatics (tar between two soft Al thin sheets) work pretty well at changing flexure to heat quickly. I've used the latter on gun barrels to tune out the oscillations. I get funny looks at the range, then do the demo with the stuff on and off -- it's a pretty big deal in that venue too (and then people want to buy some of it from me).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Tue Feb 22, 2011 12:42 pm

No, it was definitely noise from theservo drives. I killed power to the Y and Z drives and it mostly went away. It was pretty high frequency noise and you could see harmonics of the main signal. I was worried I might get something with the flex cable running right next to each other and no shielding.

I have tried lowering the acceleration and deceleration ramp times but it gets pretty sluggish. Mach3 has something called the Tempest planner that Art was playing around with. It added some modified accel/decel curves. Tried that and even that did not help alot.

I can see the twist in the riser arm. I think cladding it to build it up ought to help. If not I will just add a belt drive to the other side. Really shouldn't be too difficult.
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Re: Project: Laser Cutter (CNC)

Postby Doug Coulter » Tue Feb 22, 2011 2:32 pm

So, no motor current-carrying wires are ground? That's the only case where they wouldn't contribute to ground loop noise else it would be from magnetic coupling if the wires weren't twisted together (so they make a transformer "turn"). Yeah, stiffening up the non-moving stuff should be pretty easy. Gets a lot trickier with the moving stuff since you tend to add more inertia unless you just replace existing materials with lighter but stronger stuff.

The only real difference between a trapezoid ramp and a sine is in the second differential at the corner being discontinuous.
Might not be such a big deal, depending. At any rate, glad to see this thing coming along. Looks totally cool!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Tue Feb 22, 2011 3:43 pm

No. All the motor wires are isolated. But the PWM freq is default at 22khz which can creep into anything.

S-Curve makes a heck of a difference. But the math to do them on the fly in motion control is tough and only the higher end systems have it. Those transitions really hit depending on the machine.
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Tue Mar 08, 2011 3:21 am

I ordered the laser tube and power supply combo from Cole Tech on ebay on Feb 24th, it shipped the 25th and DHL tried to deliver it the 28th. I got it on the 1st. Tube was intact. The only damage was to the power supply HV lead strain relief. A little glue will fix that. Did a quick rig up and did a couple short bursts and it looks like it is good to go.

I have decided to mount the tube and aiming laser to an aluminum base to build an optical rail of sorts. This will allow me to prealign the co2 and diode laser and drop the whole assembly in place when I am done. I will also mount two of the beam steering mirrors to the plate as well. Due to the design I am going to have to use 5 mirrors to the get beanm to the lens. Whee...

The rail is made from some 1/2" thick x 4" wide aluminum strip I had lying around. I still need to machine the tube mounts. There will be room for the red diode and beam combiner optics as well as room for a beam expander should the need arise.

I have also sorted out the cooling system for the most part. I have some small 18v stainless gear pumps from a Dialysis machine I took apart. Good flow and pressure. I also have a copper/stainless heat exchanger that was from the cooling loop on a LASIK machine. Combine that with a small reservoir and a flow switch that should complete things. Need to order a couple fittings as well.

The one thing that has kind of stumped me is getting rid of the rigidity issue I mentioned in the earlier posts. After looking at beefing up the riser support I have decided to abandon that idea. I put a indicator on the base near the linear guide trucks and measured movement down there when the beam was flexed. So I think even adding extra metal to strengthen the support will still leave me with a floppy axis. So I have decided to use two timing belts, one on each side of the X axis, attached to a common shaft. Hopefully this will tie in both sides together akin to a rack and pinion drive set up. I ordered the longest kevlar 5/16" MXL timing belts I could from SDP-SI and also two of the belt clamps. I will cut them to length with the clam attached to the gantry. One end of the machine will have bearing and pulleys on the common shaft and the other will have idler pulleys with tensioners. I have not decided what to use for the shaft. I want to keep the mass down but the stiffness up so it does not torque. I may use a piece of titanium rod I have had lying around.

Couple more pics from my iphone. First is of the cooling system parts, second is of the tube and power supply as well as the rail it will mount on.

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Laser cutter cooling system by macona, on Flickr

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Laser and power supply by macona, on Flickr
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Fri Jul 20, 2012 2:22 am

This was a post from last may:

I have not been doing much on the laser lately. I got the optics. Mirrors, beam combiner, and lens. I also made up the mount for the laser tube. I turned one end of the mount into a optical breadboard so I can easily mount my steering optics and combiner. We had a batch of stuff going to anodizing at work so I sent it out with it to be anodized. I mounted the tube to one room to give me more room in case I ever want to try building/buying a beam expander.

Next step is to get the combiner optics mounted as well as the laser diode and get them running coaxial with the laser tube.

Image
IMG_0153 by macona, on Flickr


Image
IMG_0152 by macona, on Flickr
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Re: Project: Laser Cutter (CNC)

Postby Jerry » Fri Jul 20, 2012 2:22 am

OK, after a year of doing nothing with the laser I have started back up on it. First thing I did was solve the rigidity issue. The one unsupported end of the short axis flopped around in the breeze with anything resembling a decent acceleration rate. It would be nice if mach supported S-curve accel and decel, but thats life. So what I did was tie both sides of the gantry together using timing belts on a common free running shaft. This makes it pretty rigid. I had to mill out a recess on the upright for the shaft bearing and then tig a cross member between the two rear supports and mill another hole for that bearing. Getting the two holes concentric with each other with no solid point of reference was, well, interesting. Lost of measuring with 1-2-3 blocks and I managed to get it figured out. For the belts I used the longest belts SDP-SI had and cut them down. It is cheaper this way than buying open ended belting by the foot. I used a splice kit to hold the belt down to the gantry. Idler wheels were made from aluminum and timing pulleys for the other end to tension the belts. Once the belts were installed and tensioned I glued them in place to help hold them. It seems like it will work, it made the gantry very rigid. Time will tell.

Shaft side, back of machine:
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IMG_2279 by macona, on Flickr

idler side, back of machine:
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IMG_2280 by macona, on Flickr

belt splice, front of gantry:
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IMG_2281 by macona, on Flickr
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