Gavztheouch's Mk2 Laser

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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Wed Jul 04, 2012 1:31 pm

Using a laser to monitor a laser. Cool. That reminds me of a joke. Paraphrased: My laser has it's own laser, and even its laser is cooler than your laser. :)

I'd be tempted to replace that Arduino with an op amp comparator to set the trip level voltage from the photocell. Of course, I don't have an Arduino in my parts bin, but I have lots of inexpensive op amps. Use what you have. And as Steve Ciarcia would say on the issue of what to implement in hardware and what to implement in software... "I program in SOLDER."

My first inclination would generally be to avoid designs where two motors could be out of sync and rack the gantry at a harmful angle, but given the advantages of the dual motor design, this looks like a very appropriate solution to me. The Hadron printer I'm building uses two Z stepper motors driving a threaded rod on each end. Fortunately, I don't think those motors are powerful enough to damage the stouter Hadron mechanics because the Hadron is small and the motors don't have the leverage that you have on a long laser gantry.
Apparently, I didn't build that! :-)
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Wed Jul 04, 2012 9:17 pm

Glad you like it Liberty. I know very little about electronics so I thought an arduino would be the simplest way. There's plenty of documentation for various projects on the web, but I agree the simplest elegant solution is always the best but for some reason I want to include an Arduino with a lcd screen just for the heck of it. :D

Here are some aluminium parts fresh off the router. Each part takes about 1 hour 15 mins, and I need 10 separate pieces (2 of each). Because it's aluminium I need to baby sit it, which takes all the fun out of it, watching it slowly bite its way round the table quickly loses any appeal. On the plus side the parts look great and have plenty of strength in them to resist bending. :P

P1070445.jpg


P1070440.jpg
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Wed Jul 04, 2012 10:19 pm

gavztheouch wrote:Because it's aluminium I need to baby sit it, which takes all the fun out of it

Yeah, but those parts look great!
:)

When I have a big laser job, I web surf (often BuildLog.net) while babysitting the laser. Yet another advantage to having internet access in the shop and Firefox on every PC based CNC machine tool.

I'm going to go work on my Hadron Ord Bot. I've been away from it for a week or so, and I'm eager to wrap it up.
Apparently, I didn't build that! :-)
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Wed Jul 04, 2012 10:46 pm

If you'd like to repurpose that Arduino (as the controller for a 3D printer!), here's an op amp based laser trip wire detector circuit that definitely SHOULD work. I didn't actually build it, but it's a simple circuit. The values aren't critical. They're just voltage dividers. You could use a 10K potentiometer, or a 47K, or 1K. Once you determine the correct ratio, you could substitute fixed resistors. If you need more current than the op amp can supply, you can add a transistor to the output to amplify the current. Swap the + and - inputs to the op amp and it's a laser detector instead of a laser absent detector.

LaserTripWireDetectorSchematic.jpg
Inexpensive op amp circuit to detect a low power laser beam. Could be used to determine if a malfunction has occurred and one of two motors has stopped, which would rack the gantry on a laser engraver and possibly damage the hardware.
Apparently, I didn't build that! :-)
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Fri Jul 06, 2012 7:36 pm

Cool thanks for the sketch, I will try and mock one up when my breadboard arrives. I made an Arduino based version last night and it works great, now I have the electrical part roughed out I will need to look into the mechanicals, I already have a rough idea of how its going to work but the devil may well be in the detail with this one.

I have managed some more progress with the ball screws brackets here is a picture of the motor and fixed end ball screw mount.

mount2.jPG
mount.jPG


I am happy with how rigid these are and once they are bolted to the frame things can only get stiffer.
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Ball screw Machining

Postby gavztheouch » Sat Jul 07, 2012 10:39 pm

Ball Screw Machining!

Image
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Ball Screw Test Fit

Postby gavztheouch » Fri Jul 13, 2012 5:22 pm

A few days ago I test installed one of the finished ball screws.

P1070465.jpg

P1070464.jpg

P1070467.jpg
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Postby gavztheouch » Tue Jul 17, 2012 10:22 pm

Good news and bad news today. The good, the ball screws and servo motors are working, the bad, I still have "backlash" style symptoms. :(

I am a little disappointed in the end result but It has been a great learning curve machining the ball screws and mounts and I feel it has been worth it converting from belts. The one glimmer of hope I have right now is the backlash style errors get worse at speed, between speeds of 1 moving up to 2 in the dsp the backlash more than doubles. Unlike the belts the screws cannot deform/stretch under speed so I would guess the errors are coming from the motor losing it position and not backlash in the screw? or possibly a bug in the DSP controller. Maybe I should switch back to mach 3 and test my vectors again.
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Re: Gavztheouch's Mk2 Laser

Postby Liberty4Ever » Wed Jul 18, 2012 3:08 am

You have a light load and a relatively heavy ball screw, but the screws seem fairly long. Are you seeing screw windup with fast acceleration? Ball screws have a large pitch, so it wouldn't take much torsional deflection to result in some axial motion that would appear similar to dynamic backlash. The other possibility I can conceive would be flexing in relatively long and unsupported ball screws. Either of these problems should be dynamic problems, causing following error when in motion, but not a loss of accuracy when the axis has reached a destination and come to a stop, and any twisting or bending of the ball screw has been eliminated. Are you seeing following error based on observations of cut quality, or are you measuring positioning errors where the commanded motion doesn't match the actual motion after the axis has stopped moving?

One trick for eliminating screw windup is to put an encoder on each end of the ball screw to measure the windup (difference between the two encoders) and compute the axis position based on the screw windup evenly distributed across the length of the screw. The farther the ball nut is from the motor, the larger the windup error. But I doubt your controls are up to that sort of ball screw windup compensation.

Of course, in an open loop stepper motion system, losing steps is always a possibility. More or less microstepping could help, or slowing the motion (probably not what you want to do), or more motor current, or bigger stepper motors. Occasionally, faster motion will prevent lost steps if the steady state speed results in harmonics the stepper motor doesn't like, and the ramp up and ramp down isn't in the harmonic zone long enough for the amplitude to build to a level that causes missing steps.
Apparently, I didn't build that! :-)
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Re: Gavztheouch's Mk2 Laser

Postby gavztheouch » Wed Jul 18, 2012 5:19 pm

Thanks for your thoughs Liberty your help is appreciated. Today I switched from the DSP to Mach3 and surprise :o my vectors were much nicer. Visually you really need to look for the signs of any backlash. I now have a measured backlash value of 0.05mm in the Y which is great.

P1070468.JPG


Here is a similer backlash snake to the one igull was using to diagnose his backlash problems in another thread. You can see all horizontal lines line up quite well. As I was using mach3 I can tweak my backlash comp, you can also use this to give your system artifical backlash. I gave it a value of 0.2mm in the Y. This made all the lines visually off by quite some bit. I gradually lowered the value of backlash comp, watching the lines start to realign themselves. Around 0.02-0.03 I could not see any backlash at all and removing the backlash comp gave what you see in the pic, perfectly decent vectors.

So now I need to figure out what is wrong with the DSP for it to make such a mess of the vectors, there is also a good chance there was also nothing wrong with my last belt setup. :oops:
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