2.x for kbob's maker space

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Re: 2.x for kbob's maker space

Postby bdring » Tue Oct 30, 2012 12:50 pm

Those look pretty good. I like the way you cheated the max size. I have printed sideways holes before with pretty good results. I usually run a drill through to clean them up, but in this case the size of the hole is not critical. You should post them on Thingiverse with a title like "2.x Laser Tube Brackers for Small Printers".
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Re: 2.x for kbob's maker space

Postby garyacrowellsr » Tue Oct 30, 2012 6:35 pm

Yes, I'd appreciate it if you would post your .scad files somewhere; Thingiverse, whatever.

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Re: 2.x for kbob's maker space

Postby kbob » Thu Nov 01, 2012 8:12 pm

garyacrowellsr wrote:Yes, I'd appreciate it if you would post your .scad files somewhere; Thingiverse, whatever.

Gary


It took me a couple of days, but I put it up on Thingiverse.

http://www.thingiverse.com/thing:33579


Have fun!
Bob
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Driving a Stepper Motor

Postby kbob » Sun Nov 11, 2012 10:23 am

Tonight I reached a milestone on the software side of this project. I'm able to drive a stepper motor, accelerating, decelerating, and reversing. This is my own software stack running on an Azteeg X3 with Allegro stepper driver chips.

I'd done this earlier using the Arduino library. The part that is new is that I'm generating the stepper motor pulses directly from one of the timer/counter circuits on the AVR. That means that the step pulses are controlled very precisely by the counter/timer. The counter/timer is clocked at 16 MHz, the AVR's CPU frequency. When I have three axes going (X, Y, and laser pulses), the precise timing means that the X and Y will track each other closely and the laser pulses will be evenly spaced. Right now, it's just a single axis.

I mentioned acceleration. Each time the counter overflows, it pulses the Allegro chip, and it also triggers an interrupt. The interrupt service routine loads the counter with its next overflow limit. By varying the overflow limit, I increase or decrease the motor's speed. At present, I am using a canned acceleration ramp.

I will shoot some video when it's daylight.
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Stepper Video

Postby kbob » Mon Nov 12, 2012 12:11 am

Here's a short video on the Ewww-Toob.



Here are some more photos of the test bed I made. It was a completely unnecessary diversion but a good chance to learn a little about OpenSCAD and 3D printing.

The blue plastic carriage was an attempt to create the gantry end in plastic. I think it would have worked after a couple of iterations, but the other end with the motor was going to be tricky since it has overhangs in every orientation. Also, the printer I have access to is too small, so I was going to have to impose on a friend who has a bigger printer for every iteration.

IMG_9288.JPG


IMG_9264.JPG


IMG_9269.JPG


IMG_9272.JPG


IMG_9274.JPG
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Azteeg Wiring Issue

Postby kbob » Sat Nov 17, 2012 8:24 am

I ran into a setback tonight. I think I have a workaround for it.

If you've been following my story, you know that I'm using an Azteeg X3 (Arduino Mega 2560 + RAMPS compatible) for motor and laser control, and that I'm writing my own software from scratch. Furthermore, I've designed the control program so that the stepper pulses (and later, the laser pulses) are all driven off the timer/counter circuits in the ATmega.

I've been doing development so far using a single stepper motor driven by the Z stepper driver. I chose the Z because the Z STEP pin happens to be one of the timer-controlled pins. That is not true for any of the other four stepper drivers on the Azteeg. A week or so ago, I soldered a 2 inch lead onto a SureSteppr's STEP pin, and also put some female headers on pins D4, D5, D6, and D11. The plan is (well, was) to use D5, D6 and D11 for X STEP, Y STEP, and laser pulse, respectively. Those pins are all able to be driven by a 16 bit timer/counter.

So tonight I hooked it up, just to verify that it works and that my code to parameterize pin assignments works. It did not work. It appears that pins D5 and D6 are not connected to anything. It's possible that I burned through a trace while soldering on the header. But I identified the through hole where the trace goes through the board, and I can't see any voltage there either. I don't think I've shorted the pins to ground, but I'm not certain -- the battery in the multimeter died while I was working tonight.

If I can figure out how to resuscitate D5 and D6, I will. I would like to use those pins, as they're in a tidy group. If I can't, though, I can fall back to pins D2 and D7. D7 is on the EXP3 header and D2 is the X MAX limit switch, which I don't need.

And that's how I spent my Friday night.
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Re: 2.x for kbob's maker space

Postby royco » Sat Nov 17, 2012 7:18 pm

"I'm writing my own software from scratch" I want to know more :-)

The D4, D5 D6 and D11 are mosfet driven switches and not directly connected to the corresponding pins. If you need access to those pins you may have to tap from the resistor pad connected to the gate of the Mosfet. I would suggest maybe using the J11 expansion header instead if possible for your application. It was placed there for an additional stepper driver control. Which reminds me to finish cleaning up the schematic so I can place it on the wiki.

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Wondering what power supply to get

Postby kbob » Tue Nov 20, 2012 9:04 pm

I'm thinking about a power supply for the controller and motors. (Not the laser. I will probably get the laser tube and power supply together.) The 2.x BOM calls for a 24V power supply from Bart; Bart doesn't appear to be selling power supplies any more.

My Azteeg has a 12-24V boost converter, so I actually need 12V for the motors. The motors are rated for 1.68 amps. 1.68 amps × 3 motors × 24V = 121 watts. The computers, the Azteeg and a Raspberry Pi, run on +5V, probably well under an amp for both. I also am planning to put some LEDs somewhere in the case for illumination, but they will be another fraction of an amp. Call it 150 watts total.

I'm thinking a PC power supply is the way to go. The computers can run on the +5V standby line, and I can switch the main power on and off under software control.

PC power supplies normally exhaust hot air at the end with the power plug. The power plug won't be exposed on the laser cutter; instead it will be wired to some kind of internal power bus. I'm thinking I should position the power supply against the side of the case and cut a hole in the skin so it can draw in fresh air and exhaust the hot air into the enclosure. There won't be a lot of heat -- if it outputs 150W at 80% efficiency, that's 37 watts of waste heat. And when the laser is running, the exhaust fan will be moving a lot of air through the enclosure and the heat shouldn't build up.

If I go that way, I'll have to ensure that the exhaust fan flows much more air than the PC power supply. If the PC PSU is the lowest resistance path for fresh air to enter the enclosure, the PC PSU may not even need to run its own fan. I should get a PSU with a fan thermostat.

An alternative is to let the PSU suck inside air and exhaust it back into the box. The bad thing about that is that the PSU will inhale smoke and particulates and get grody inside. Another alternative is to put the PSU outside the box and run the cables in.

This power supply has caught my eye. SeaSonic SS-350TGM Bulk 350W TFX12V It is high efficiency, thermostatted, and small. 350W is comfortably above my requirement. And this review says the fan doesn't come on below 200W. Because the cables unplug from the PSU, it'd make the wiring a little easier. The downside is that it's not the cheapest at $75 US shipped from NewEgg.

Is the electronics side of the box crowded? There will be a motor/electronics power supply, a laser power supply, a Raspberry Pi, an Azteeg, a power distribution panel, and some wiring. It seems like there's plenty of room -- is there?

Also, did I see something about keeping the laser power supply isolated or shielded from the other electonics?
Bob
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Re: 2.x for kbob's maker space

Postby cvoinescu » Wed Nov 21, 2012 3:19 pm

Your calculation for the motors is a bit misleading. First, it's 1.68 A per winding, and exactly how much your motor can take when two windings are energized should be in its datasheet. Second, the driver works as a chopper; together with the inductance of the winding of the motor, it forms a buck supply, so it can deliver a higher current (1.68 A) at a low voltage (likely 2 to 5 V, check your motor specs) drawing a lower average current from a higher voltage supply (24 V). So you don't need 1.68 A times 3 motors from the power supply; you need only what the drivers draw to supply that current to the motors, which you can't calculate exactly because you don't know the efficiency of the chopper/winding combination. The efficiency is not very good at high speed because of back EMF and what not, but still you don't need the full 1.68 A at 24 V. It doesn't hurt to have a beefier power supply than needed, but my best guess is that 3 A for the motors is plenty.

Standard disclaimer applies. I may or may not know what I'm talking about. Warranty void if opened or seals broken. Past performance does not guarantee future results. May cause nausea, vomiting, headache and warts. Consult your doctor or pharmacist. Laser radiation when open and interlocks defeated.
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Re: 2.x for kbob's maker space

Postby kbob » Wed Nov 21, 2012 5:34 pm

cvoinescu wrote:Your calculation for the motors is a bit misleading. First, it's 1.68 A per winding, and exactly how much your motor can take when two windings are energized should be in its datasheet. Second, the driver works as a chopper; together with the inductance of the winding of the motor, it forms a buck supply, so it can deliver a higher current (1.68 A) at a low voltage (likely 2 to 5 V, check your motor specs) drawing a lower average current from a higher voltage supply (24 V).


Interesting. "Buck Supply" was a new term to me. Wikipedia defines it, though.

My motors are Keling KL17H247-168-4B. The datasheet is here. http://www.automationtechnologiesinc.co ... 168-4B.pdf

The datasheet lists these.
  • Phase 2
  • Rated Voltage 2.8 V
  • Current/Phase 1.68 A
  • Resistance/Phase 1.65 Ω
  • Inductance/Phase 2.8 mH
  • (and some other stuff)
I guess that calculates out to 1 phase (at a time) × 1.68 A/phase × 2.8 V ≅ 4.7 watts when the motor is at rest. Right? That number, as you said, isn't very useful because of back EMF when the motor is turning and losses in the driver chip and who knows what else.

FWIW, if my calculations are correct, the fastest I've seen the motor turn under Arduino control is about 2100 RPM. Above that, it would stall. But my stepper drivers don't have heat sinks yet, so I'm using ~1 A, not 1.68A. But I digress... (-:

The standard answer here would be to grab an old PC power supply off the discard heap. But I'm unique, I think, among PC hobbyists because I never have any old ATX power supplies sitting around. My servers eat them and I have to replace them every couple of years. I think it's because the servers are in a room that gets very warm in the summer. There is nothing to be done — the house was built with an undersized air conditioning unit.

So is the TFX supply I listed complete overkill? It's hard to find quiet, reliable supplies in low wattages.
Bob
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