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Thread: Setting up KFlop + KStep closed loop stepper system

  1. #13
    Gold Member TomKerekes's Avatar
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    Hi Eero,

    The System shouldn't be that sensitive to the speed of the PC or what is loaded on it. That is the main idea behind KFLOP to relieve the PC from the hard real time functions.

    Until the most recent Test Versions an Axis being disabled from a Following Error or Limit Switch would end up being reported as a buffer under flow. That is more likely what is happening. Check the KMotion Console Screen to see if there are any messages

    Regards

    TK
    http://dynomotion.com


  2. #14
    Gold Member TomKerekes's Avatar
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    Hi Eero,

    The System shouldn't be that sensitive to the speed of the PC or what is loaded on it. That is the main idea behind KFLOP to relieve the PC from the hard real time functions.

    Until the most recent Test Versions an Axis being disabled from a Following Error or Limit Switch would end up being reported as a buffer under flow. That is more likely what is happening. Check the KMotion Console Screen to see if there are any messages

    Regards

    TK
    http://dynomotion.com


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    Default Re: Setting up KFlop + KStep closed loop stepper system

    Thank you for posting this! It helped me a lot to get my Kflop running.



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    Default Re: Setting up KFlop + KStep closed loop stepper system

    Tom,
    I have a couple of basic questions that might fit this forum topic, which I can't seem to find with searching.
    (big fan of your support and positive demeanor)

    questions related to a cnc router layout.
    1) if I put linear encoders along the axis, like Renishaws or USDigital EM1, can I install dumb steppers(open loop)? Why have closed loop on the stepper if I close the loop to controller directly? Will the Kflop modify the step/dir based on actual feedback, to achieve this?
    2) in high speed machining of wood, plastics and aluminum, (30k RPM, small endmills, like Datron) it seems the cutting forces on the bit are low, meaning the machines speed capabilities are largely tied to just it's masses and motion driving power.
    Could this mean a strong (Poly belt, T5, 50mm wide) could be used using simple driven pulley arrangements (not servobelt concept)? I wonder if the elastic nature of belts would be compensated for. the cost of driving each axis would be halved with belts vs screws.
    3) on a moving gantry machine, with two motors slaved, would each side need encoders and could Kflop manage synchronizing those two, while still handling the XandZ motions? (still thinking of open loop driven motors)

    I'm currently building a simple mach3/BOB/gecko mill, using Parker Daedals 404XR linears and a Hitachi router spindle. my plan is to use this to make the mechanical bits for a larger granite based Datron clone. It's a long road.

    Dave



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    Default Re: Setting up KFlop + KStep closed loop stepper system

    Hi Dave

    Since I just added some glass scales to my Kflop I might be able to answer your question:

    1) The only reason you want to add some glass scales (linear encoders) to your Kflop is if you want to improve accuracy. If your (mechanical) accuracy is good enough without any linear encoders you will do fine without the hassle and cost of additional encoders.
    But if you add linear encoders Kflop will measure the actual position at the glass scales and then pulse your steppers until the destination on your glass scale is reached. This is called a closed loop Step Dir.
    Closed Loop Stepper
    There is also closed loop for controlling servo motors. But that is something different.

    2) from my experience it is advisable to always make the driving of an axis as rigid as possible. The lost rigidity can not be compensated with a better regulation. Manly because of oscillation. Your belts will act like rubber bands and you might picture how difficult it is to position your machine with two rubberbands.

    Let me tell you from personal experience: If you start out with milling you will most likely underestimate the importance of rigidity be the factor of at least two. So if you make it twice as rigid as you think that would be enough for your application it might barely be useful. You will find backlash, freeplay, dynamic deformation where you never thought it was possible. I did most things twice because I had to learn....

    Leo



  6. #18
    Gold Member TomKerekes's Avatar
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    Default Re: Setting up KFlop + KStep closed loop stepper system

    Hi Dave,

    if I put linear encoders along the axis, like Renishaws or USDigital EM1, can I install dumb steppers(open loop)? Why have closed loop on the stepper if I close the loop to controller directly? Will the Kflop modify the step/dir based on actual feedback, to achieve this?
    Yes KFLOP will generate additional motor steps in order to get the linear encoders to read the desired value. But the issue is more the dynamic stability of the control loop. Closing the control loop around linear glass scales includes more "bad" mechanical things inside the control loop making it more difficult to control. The main things are backlash/compliance/stiction. So for example say the linear encoder has a small error that the servo wants to correct. This will usually cause the servo to ramp up the motor torque. Eventually the motor will have enough torque to break stiction and begin accelerating. But the linear scales may not yet indicate any thing has moved because of the system's backlash/compliance. So the Servo will continue to ramp up torque further. Eventually when the backlash/compliance is taken out the Axes will move and the Linear scales will report the motion. But at that point the motor may be at such a velocity that it is difficult to avoid a significant overshoot. Most systems like this can be made to work and be stable but only with very low feedback gains (torque is ramped up very very gradually) resulting in poor dynamic performance (errors are not connected quickly and therefore are allowed to grow to larger values).

    With Servo Motors it is almost always necessary to have some feedback from the motor shaft. Either from a tachometer or rotary encoder. This allows the motor to move at a controlled speed through any backlash. Ironically Steppers have some inherent feedback from the motor. Once the motor breaks loose and begins to move the torque will naturally be reduced. You might consider the model of a stepper motor as a spring. Microstepping is like advancing the spring connected to a "stuck" load. The spring will stretch and apply more and more force until the load breaks free and moves. At that point the spring force will naturally be reduced.

    So in summary you may well not need encoder feedback on the steppers. However what performance you will be able to achieve is a complex dynamic problem dependent on the mechanical parameters of your system.

    in high speed machining of wood, plastics and aluminum, (30k RPM, small endmills, like Datron) it seems the cutting forces on the bit are low, meaning the machines speed capabilities are largely tied to just it's masses and motion driving power.
    Could this mean a strong (Poly belt, T5, 50mm wide) could be used using simple driven pulley arrangements (not servobelt concept)? I wonder if the elastic nature of belts would be compensated for. the cost of driving each axis would be halved with belts vs screws.
    I don't have the experience to answer this other than more elastic nature (compliance) is likely to reduce performance. But how much is hard for me to say.

    on a moving gantry machine, with two motors slaved, would each side need encoders and could Kflop manage synchronizing those two, while still handling the XandZ motions? (still thinking of open loop driven motors)
    You should be able to configure KFLOP for the Master Axis to be closed loop and the Slave Axis open loop, but I think it would be better to have both sides closed loop. Otherwise your worst case errors will essentially be the same as if everything was open loop.

    HTH
    Regards

    TK
    http://dynomotion.com


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    Default Re: Setting up KFlop + KStep closed loop stepper system

    thanks! I have a lot to learn, and that's great advice Leo, to think stiffer than I estimate. 2x stiffer,.......that's what she said.....
    The whole granite idea is to help with the tramming and stability.
    My 80/20 framework and the tramming of it has been a real eye opener. Precision granite will solve a lot.

    I've been CAD'ing up ball screw, belt, servo belt etc ideas. I've also begun sorting out a spinning nut ball screw idea.
    I read the specs and you can only spin a ball screw shaft about 50 RPS or so, and there is some inertia to account for.
    The spinning nut concept eliminates some end bearing blocks and can spin faster.

    I'm obsessed in having the Datron results at home and am a designer and fabricator, like most here.
    I just spent 3 years immersed in the 3D printer realm, and am excited to get into this now. (videos on youtube Mitus Prints, and failed Kickstarter X-truder)

    thanks again.

    Dave



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Setting up KFlop + KStep closed loop stepper system
Setting up KFlop + KStep closed loop stepper system