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IndustryArena Forum > Community Club House > General Off Topic Discussions > what to do with this 3+ axis dental cnc machine
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  1. #41
    Join Date
    Aug 2009
    Posts
    230

    Re: what to do with this 3+ axis dental cnc machine

    I gotta say, it calms be a bit that even someone who know what they are doing messes up a board every now and then.
    You don't want to know how many PCB "coasters" I've got around here...

    So when you thought you fried it, what were the symptoms? Would it just not register on USB or take a sketch upload?
    Intermittently wouldn't be recognized properly by the OS; wouldn't be recognized properly by the Arduino IDE (didn't show up in the "Ports" menu); when it did show up the upload process would fail; killing the Arduino IDE and relaunching it along with a reset of the board would end up with two entries in the system USB tree for the board... it was a mess. I had also thought it might be a Catalina issue; owing to the fact that I had just upgraded the OS on that machine (and there was an open issue for the Leonardo bootloader being nonfunctional on Catalina); but given that it seems to be working now I don't think that was the issue.

    I ask because I have several boards that I think would work if I got a new bootloader on them, but I had tried that a while back with no success.
    Depends on the exact board and symptoms. ARM based Arduinos and the Leonardo use a direct USB connection; all the others use some sort of USB-serial chip (FTDI, CH340, ATMega32u2, etc).

    If it's a USB-serial chip based Arduino, the board won't show up at all when plugged in to a known-working USB port & cable, and you know that the PC side has working drivers on it; then the serial chip is dead. If it's a ATMega32u2, then that might have gotten corrupted (and is re-flashable, there's an ICSP header on the board specificaly for this); but if it's one of the others then it's more-or-less non-repairable.

    If it's a direct-USB Arduino, or the symptom is anything other than not showing up at all; then software corruption on the board is a possibility; re-flashing the bootloader & a sketch with a programming tool might help.

    Or... they might be toast.

    Some of those boards still run the sketch that is on it, but wont connect to take a new upload.
    Highly probable that it's either the USB-serial chip (if it has one) or the bootloader.

    What board do you select when uploading a sketch on your clone?
    I've been selecting the "Arduino/Genuino Zero (Native USB Port)". As far as I know, the practical difference between most of them is just the Arduino-pin to CPU-register (and thus physical pin) mapping. However, the listings for "Programming Port" are actually setup to connect to a programmer/debugger module (the Atmel-ICE is one, and the Atmel Embedded Debugger that's on the official Arduino Zero and M0-Pro boards is another).

    Re: soldering station.
    Well, I was in a bit of a chicken-and-egg situation, so I ended up just buying a T-12 compatible one from Circuit Specialists yea many years ago... it still works, so I haven't replaced it. Only thing I don't like about it is that it always comes up at 350c when it's turned on...

  2. #42
    Join Date
    May 2009
    Posts
    26

    Re: what to do with this 3+ axis dental cnc machine



    well my soldering station is finally done...man that was a hassle. photos here.

    so that took way too long, but I did learn quite a bit that will be directly relevant to these motor controllers.

    Firstly, opamps are frustrating, they seem so simple yet cause me sooo many headaches.

    Just for fun, I tried to make my own reference voltage for reading temp. I used a tl421 to create a 2.5v reference. At the time, I was unclear about the precision relative to what the aruduio could do. I now know that TL421b would have been significantly more precise. After the little work on the breadboard, I had nice looking 2.5v reference on my scope and proceeded to start tuning. But my read values were not very consistent and that made tuning the PID very challenging. So I went back to the scope to look at the amplified signal from the RTD in the iron and it looked real smooth. Went back to reference voltage and zoomed waaaayyy in on time to reveal what I think is called a sawtooth waveform. Very consistent, but definitely not constant. Is this normal? Is that where the 10% tol on the TL421 comes from? I ask all this because I feel a very good reference voltage will be critical to the motor drivers functioning at their best.

    The other major learning curve was PID tuning. I do hope its a touch easier to do with the motor.

  3. #43
    Join Date
    Aug 2009
    Posts
    230

    Re: what to do with this 3+ axis dental cnc machine

    I can't seem to find the datasheet for the TL421... Digikey, Mouser, and Octopart don't turn up anything; and google seems to be under the impression that it's a front-loader for a Kubota tractor.

    As for the saw-tooth, I'd need to see the datasheet, as well as the schematic of the circuit before I could hazard much of a guess as to what is going on. Op-amp circuits can oscillate; but my understanding of it is that usually it takes some external components to make a saw-tooth - by itself, you usually get a sine wave if it breaks into oscillation.

    I ask all this because I feel a very good reference voltage will be critical to the motor drivers functioning at their best.
    It's less critical than one would think; because of the feedback from the encoders (if it drifts out of position by as little as one encoder count, the feedback loop will command a rotation in the opposite direction until it's back where it's supposed to be). Also, you can include a 10-turn trimmer pot to adjust the "zero point" of the reference voltage to match the zero-RPM output voltage of the SAMD21. However, better is, well, better... so...

  4. #44
    Join Date
    May 2009
    Posts
    26

    Re: what to do with this 3+ axis dental cnc machine

    TL431 ...my bad. I was using it in the most basic configuration, cathode & ref shorted together and to 5v via a ~1000ohm* resistor and then a decoupling cap to ground (*i played with the size of that R and it stretched that pattern out or in, but still had the pattern., over sizing the decoupling cap stabilized the trace, but raised the output voltage to 2.6v)

    here is what I saw initially and thought was decent(initially):


    but then I zoomed in to see this:


    note the bottom trace is the signal line...I initially confused them, you can probably see why. So the problem was, every time I read the sensor, my reference was falling somewhere different on that slopped line, thus leading to less stable reading. The 3.3v ref on the arduino proved very stable on the scope and switching to it resulted in much nicer readings and way less of a headache processing the data.

    It's less critical than one would think; because of the feedback from the encoders (if it drifts out of position by as little as one encoder count, the feedback loop will command a rotation in the opposite direction until it's back where it's supposed to be). Also, you can include a 10-turn trimmer pot to adjust the "zero point" of the reference voltage to match the zero-RPM output voltage of the SAMD21. However, better is, well, better... so...
    after the first read I was going to say something like: "but surely it is better to not introduce additional error".....is that what you were saying at the end of that statement? If so, we are on the same page.

    i should probably get the regulator you suggested in look at it on the scope, may not even be a problem

  5. #45
    Join Date
    Aug 2009
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    230

    Re: what to do with this 3+ axis dental cnc machine

    Is this on a breadboard? I'm just guessing, but I suspect that the total capacitance (components + stray capacitance from the breadboard itself) might be pushing it into oscillation? According to the datasheet, there is a region of instability...

    If you are using it where I think you are - generating the 1.65v offset voltage - then having it be 2.6v won't be a problem, just adjust the 10-turn trimmer to bring the voltage on the wiper down to 1.65.

    "but surely it is better to not introduce additional error".....is that what you were saying at the end of that statement? If so, we are on the same page.
    Well, it's a cleaner design when there's no extraneous errors... even if they don't end up affecting the final end product in a detectable way (i.e, the machined parts come out on-size with good surface finish). What I'm saying, I guess, is that if you can't squeeze out the last little bit of error... don't freak out about it.

  6. #46
    Join Date
    May 2009
    Posts
    26

    Re: what to do with this 3+ axis dental cnc machine

    Is this on a breadboard?
    yes & no --actually I not sure for those particular images. I did testing with my soldered board & a breadboard & saw no change.

    If you are using it where I think you are - generating the 1.65v offset voltage
    my understanding is the internal reference is 2.5v (2.49...) and the way I stumbled into making 2.6v just kind of seems wrong to me. I am not gonna worry about it to much, unless the neg regulator I choose to use shows the same irregularity.

    I have been sorted my electronic comps and found a few adjustable negative voltage regulators---that's just going to create problems right? better to stick with something fixed?

    Well, it's a cleaner design when there's no extraneous errors... even if they don't end up affecting the final end product in a detectable way (i.e, the machined parts come out on-size with good surface finish). What I'm saying, I guess, is that if you can't squeeze out the last little bit of error... don't freak out about it.
    noted...but I might need a reminder on that one, my OCD keeps me trying for perfection, even when its not at all worth it.

  7. #47
    Join Date
    Oct 2020
    Posts
    1

    Re: what to do with this 3+ axis dental cnc machine

    Hello. I'm new to CNC as well. Here is a really good resource I found: https://handtoolsforfun.com/what-can...-a-cnc-router/ Here is another for converting one machine into another: https://handtoolsforfun.com/how-to-t...a-belt-sander/ Good luck!

  8. #48
    Join Date
    Oct 2020
    Posts
    1

    Re: what to do with this 3+ axis dental cnc machine

    coincidentally I'm working on the same project. I have made some progress. For ease of programming, I have an arduino controlling each axis to do PID control as an output to the motor drivers (in current mode) and they receive an input of step / dir like a normal stepper driver would. I'm then using estlcam to control each axis, but you could easily sub for GRBL. I'm not done yet, but I do have the control prototyped and working on one axis, and I'm able to successfully control from estlcam. I'm in the process of building mounting everything to a new frame now. If you're interested in details / code I'd be happy to share.

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