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  1. #37
    Community Moderator ger21's Avatar
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    Default Re: Best components?

    Closed loop steppers won't be any more accurate than open loop steppers.

    Gerry

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    Default Re: Best components?

    Quote Originally Posted by ger21 View Post
    Closed loop steppers won't be any more accurate than open loop steppers.
    Using closed loop would be the closest thing to a commercial setup using actual servos. With the encoders, any missed steps would be compensated for. Additionally, having an encoder allows the control to see any uncommanded input and is able to move the motor back to the held position.

    So I guess I need clarification to why you think it wouldnt be more accurate. Or am I missing something?



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    Community Moderator ger21's Avatar
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    Default Re: Best components?

    Additionally, having an encoder allows the control to see any uncommanded input and is able to move the motor back to the held position.
    LinuxCNC and Dynomotions KMotionCNC are the only hobby controls that can do closed loop operation.

    But even then, closed loop steppers close the loop at the drive, not the control.


    With the encoders, any missed steps would be compensated for.
    Not necessarily.
    Steppers miss steps because they don't have enough power to move to the commanded position.

    If an open loop stepper stalls, then an identically sized closed loop stepper will also stall, usually causing the drive to fault. There's no extra power to move it back to the correct position like a servo has.
    Now, if you are just talking about a few missed steps here and there, then yes, the closed loop steppers will be able to make those up.
    But a properly sized open loop stepper system should never lose steps in the first place. Imo, any machine that loses steps at any time is basically worthless.

    So, yes, it's potentially more accurate, but a properly designed open loop system should not be any less accurate. If that makes sense.

    It's really not that much more money to go with much more powerful AC servos vs closed loop steppers.

    Gerry

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    Default Re: Best components?

    Quote Originally Posted by ger21 View Post
    LinuxCNC and Dynomotions KMotionCNC are the only hobby controls that can do closed loop operation.

    But even then, closed loop steppers close the loop at the drive, not the control.
    Makes sense. So there is no feedback loop to the control from the drive then?

    Quote Originally Posted by ger21 View Post
    It's really not that much more money to go with much more powerful AC servos vs closed loop steppers.
    While I agree, I think true servo drives on a smallish type router would quickly move into the overkill range. Thats why I was exploring closed loop steppers.



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    Default Re: Best components?

    No, not until you get into the much higher end controls, typically with analog servos. Most if not all step/dir closed loop drives close the loop in the drive.
    While I agree, I think true servo drives on a smallish type router would quickly move into the overkill range.
    That depends on what you consider overkill.
    With steppers, you always trade speed for resolution, so any application that requires fairly high speeds would benefit from servos, imo.

    In the DIY world, it's all about your expectations. A lot of people will be happy with a <$1000 machine, while many others would be unhappy with anything less than a $5K-$8K machine. Strictly from a performance standpoint.

    Gerry

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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Quote Originally Posted by Outlawtaz View Post
    Makes sense. So there is no feedback loop to the control from the drive then?
    On most systems no. That doesnt mean you can't build such a system though. As pointed out if you are building a stepper based system the need to close the loop isnt that great.


    While I agree, I think true servo drives on a smallish type router would quickly move into the overkill range.
    Not really. There are whole ranges of servos solutions out there that would never be useful on a router. Finding suitable servos for a router sized machine isnt that difficult. The problem for most of us is the cost.
    Thats why I was exploring closed loop steppers.
    Closed loop steppers are a new beast on the market. At least they are new to me. Honestly i dont see the point as these days you can buy servo drives that take step and direction control effectively emulating stepper behavior.

    In any event id like to back up a bit here, there is a lot more to precision and repeatability than the electroniics. Once a stepper or servo achieves an angular position it is up to the mechanical system to repeat and give you the precision you want.

    I spent years working on lathes machining optical parts and have to say it can be extremely challenging to achieve high precision results. You can have the finest encoders and servos that exist but you are still limited by what is happening in the mechanical world. Now that is an extreme world motion control wise compared to a router but the point here is that you need a good mechanical design to achive high precision and repeatability. The controls are part of the solution.



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    Default Re: Best components?

    Quote Originally Posted by wizard View Post
    In any event id like to back up a bit here, there is a lot more to precision and repeatability than the electroniics. Once a stepper or servo achieves an angular position it is up to the mechanical system to repeat and give you the precision you want.

    I spent years working on lathes machining optical parts and have to say it can be extremely challenging to achieve high precision results. You can have the finest encoders and servos that exist but you are still limited by what is happening in the mechanical world. Now that is an extreme world motion control wise compared to a router but the point here is that you need a good mechanical design to achive high precision and repeatability. The controls are part of the solution.
    Oh, I totally agree. One of my customers when I was still working on machines used a Haas mill with a 4th axis rotary and an indexable spindle attachment to cut 0.005" wide by 0.003" deep threads on a stylus for friction stir welding research. He had to use an bore scope to verify accuracy and a razor blade to do deburr. My role in this insanity? Dialing in his machine to accomplish this. So, I understand how repeatability and precision works.

    Knowing that the electronics are more precise than what I am trying to accomplish effectively takes that part of it out of the equation. Then, I can concentrate on my mechanical tolerances. I'm just trying to narrow the weak spots so I can concentrate my design on the remaining limits



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    Default Re: Best components?

    I kinda yhink we are of the same mold that is a lot if time spent dialing in machines to do things the original manufacture never considered.

    Sadly im not a mechanical engineer so all i can offer up is experience in automation and machine tools. Generally what you see with DIY machines is a lack of robustness. So when designing your machine it might be a good idea to ask your self will this part do its job. If you have the training you can of course run structural analysis codes against the proposed design. If not the general arraingemnt of these machines is well known. Pay particular attention to the gantry as it is critical to machine operation.



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    Default Re: Best components?

    Quote Originally Posted by wizard View Post
    I kinda yhink we are of the same mold that is a lot if time spent dialing in machines to do things the original manufacture never considered.

    Sadly im not a mechanical engineer so all i can offer up is experience in automation and machine tools. Generally what you see with DIY machines is a lack of robustness. So when designing your machine it might be a good idea to ask your self will this part do its job. If you have the training you can of course run structural analysis codes against the proposed design. If not the general arraingemnt of these machines is well known. Pay particular attention to the gantry as it is critical to machine operation.
    Well, I'm gonna concentrate on the robustness. That's why I dont want to worry about electronics. I bought a kit at the end of last week for a test bed. Its aluminum, but the machine I will build for myself will be entirely steel. I'm planning on just using the kit to test electronics until I'm happy with a setup.

    As for my bigger DYI router, I'm at the point where I'm trying to figure out how I can machine the frame to minimalize welding... That's the toughest part right now. If I can figure that out, then I can machine it out, bolt it together and not have to worry about normalizing a 500 pound structure to get rid of the welding stresses.



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    Default Re: Best components?

    Quote Originally Posted by wizard View Post
    The issue of control software / hardware apparently is a very personal thing so when you ask questions focused on which is best you will likely get widely varying answers.

    For example the absolute cheapest route, for a respectable controller, is likely GRBL running on Arduino. It is very limited due to the micro processor it is built upon. Do realize though that ports to ARM controllers exist. My suspicion is that you wouldn't be happy going this route due to exposure to high end controllers but this is the viable low end.

    LinuxCNC might be comparable to what you are familiar with capability wise. I do however believe that they have painted themselves into a corner trying to run realtime off commodity PCs. This mainly due to issues if I/O on modern PCs. In any event well worth a look.

    I have to agree with the above comments related to Mach 3/4. The only good thing here is support for external motion controllers.

    Centroids DIY solutions, including Acorn, are very interesting if you like the Centroid way of doing things. They offer what i would have to say is the closest thing to a professional solution at the asking price point. Several people have gone this route recently so try looking for those threads. All indications are surprisingly good stability right out of the box.

    One thing not often considered here is buying a CNC controller directly from China. The costs are surprisingly low for FANUC emulating or "ripping off" controllers. The biggest issue in my mind is communications which has me reluctant.

    This is just a quick overview there are probably a dozen more solutions for CNC on DIY machines. This doesn't even consider all of the reprap controllers out there.

    Lastly you could always buy a used Fanuc controller considering that you know your way around them.


    Please can you explain what you meant about Linux CNC issues from running realtime off modern PCs? What is the problem with this approach? I am asking because I am about to install Linux CNC and go through a learning curve. I don't want to find a bunch of issues that mean I have to go through the learning process again with other software...

    Everything I read here (before yesterday) suggested that Linux CNC works without issues and at least as well as other hobby level software solutions. Are we talking about a theoretical problem or one that people are actually experiencing on a regular basis?

    The sort of specs people talk about here for PCs to run CNC software are relatively basic which means that CNC / motion control software is not particularly demanding on a computer's resources. My CNC box is a relatively high end workstation running a fast liquid cooled Xeon processor and a Maxiums graphics package. I was thinking this would be overkill.... or at least enough (for a dedicated CNC control)....



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    Default Re: Best components?

    Quote Originally Posted by Goemon View Post
    Please can you explain what you meant about Linux CNC issues from running realtime off modern PCs? What is the problem with this approach? I am asking because I am about to install Linux CNC and go through a learning curve. I don't want to find a bunch of issues that mean I have to go through the learning process again with other software...

    Everything I read here (before yesterday) suggested that Linux CNC works without issues and at least as well as other hobby level software solutions. Are we talking about a theoretical problem or one that people are actually experiencing on a regular basis?

    The sort of specs people talk about here for PCs to run CNC software are relatively basic which means that CNC / motion control software is not particularly demanding on a computer's resources. My CNC box is a relatively high end workstation running a fast liquid cooled Xeon processor and a Maxiums graphics package. I was thinking this would be overkill.... or at least enough (for a dedicated CNC control)....
    Linuxcnc IS_THE_MOTION_CONTROLLER whereas in a Mach3 with a smooth stepper it is the external card that is the motion controller. So Linuxcnc requires PC hardware that has low latency. This is much more important with a parallel port system as the step pulses are generated in software and have to arrive on time with precise timing. These pulses are run on a base thread. The rest of the motion control is done on the servo thread at 1000 times a second. If you choose an ethernet card interface such as the Mesa 7i76e or Mesa 7i92, or a Mesa 5i25/7i76 or 6i25/7i76 combo the step generation is offloaded to the hardware. This means the base thread is not even created. Latency only becomes a problem if something is delayed enough to make the servo thread to fire late and this is reported as an error by LInuxcnc. The latency of a machine is a function of the hardware and BIOS. Many power saving functions cause high latency. A high spec machine may be overkill. a simple Celeron is enough.

    Comments that LInuxcnc is held back by requiring old hardware ar ill informed. I'm running it on a USFF Gigabyte BRIX N3160 PC about 5" square with an ethernet 7i76e.

    Rod Webster
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    Quote Originally Posted by Goemon View Post
    Please can you explain what you meant about Linux CNC issues from running realtime off modern PCs? What is the problem with this approach?
    First understand that there are numerous ways to configure LinucCNC so not every issue applies to every computer.

    In sny event i would suggest spending some time on the Linuxcnc forums to get an idea of the issues seen with various desktop PCs. You will also find there well reccomended boards that are known to work well eith Linuxcnc.
    I am asking because I am about to install Linux CNC and go through a learning curve. I don't want to find a bunch of issues that mean I have to go through the learning process again with other software...
    All software has a learning curve and the curve with Linuxcnc is pretty steep. What you get gor that learning curve is a hgh performance machine controller known to work well. At the hobby level there is little competeing with Linuxcnc except possibly Centroids solution.
    Everything I read here (before yesterday) suggested that Linux CNC works without issues and at least as well as other hobby level software solutions.
    I would suggest it works far better than most hobby oriented solutions. It in fact is commercial grade in my opinion. You just need to make sure you have a compatible motherboard.
    Are we talking about a theoretical problem or one that people are actually experiencing on a regular basis?
    Go to the Linuxcnc forums to get the details but people have had issues with certain PCs. You can run with confidence by choosing a motherboard that is known to be trouble free.
    The sort of specs people talk about here for PCs to run CNC software are relatively basic which means that CNC / motion control software is not particularly demanding on a computer's resources.
    For the most part this is true. With Linuxcnc you do benefit from a bit more horse power than a 6502 though. Remember you have a realtime Linux installation to support.
    My CNC box is a relatively high end workstation running a fast liquid cooled Xeon processor and a Maxiums graphics package.
    That right there concerns me as it dounds like you are teesking things to run beyond stable. Generally i like to see a dedicated machine for each CNC implementation.
    I was thinking this would be overkill.... or at least enough (for a dedicated CNC control)....
    You can certainly test the machine out but id make sure it is dialed back for reliable operation. Possible gotchas with Xeon boards though include interference from any network management hardware on the board.



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