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  1. #41
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    Default 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.



  2. #42
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    Default 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...



  3. #43
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    Default 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



  4. #44
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    Default 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.



  5. #45
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    Default 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.



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    Default 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!



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    Default 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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what to do with this 3+ axis dental cnc machine

what to do with this 3+ axis dental cnc machine