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  1. #161
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    Re: Jerk Control in machines

    Quote Originally Posted by peteeng View Post
    Hi MT - I agree that particular video was not good. But if you look at the Tinyg closer its algorithms are very impressive and can be implemented in other systems as they are open source. The TinyG approach is what I expect best practice open loop motion to be. To go further, closed loop approaches need to be used and these are appearing in 3D printing systems at the cost level that I operate at. The big machine players have proprietary open and closed systems that do amazing things in terms of motion control but they are outside the scope of my machines. Milli my first mill will use servos which is a new world for me.... Peter

    In a slight tangent there are a couple of old threads in which flywheels and eccentrics were added to steppers to improve motion (by way of mechanical damping I expect). I can't find if anyone concluded that this was purely a flywheel effect and the eccentrics (small free weights in cavities in the flywheel) where not needed or the eccentrics were functional? Anyone comment on that? Peter
    Yes, I remember the weighted wheels that were attached to the stepper motors, it's like everything people try to make things work better, but there are tradeoffs so what appears to work well, may not be as good as it seems, or it would have stuck and been implemented to the stepper motor, or someone would have been manufacturing them as an add on.

    There are servos that do not work well, so when you go that route careful what you chose.

    Any quality Ac Servo Drive is closing the loop, this does not have anything to do with the control, the control only has to send a signal for it to move, Servo Drives are very sophisticated.
    Mactec54

  2. #162
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    Re: Jerk Control in machines

    Hi All - Starting the think about the next build and this is what I shall be using as the controller. This is the first time I have seen centripetal accel discussed. Peter

    https://www.youtube.com/watch?v=d68U80HeKc4

  3. #163
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    Re: Jerk Control in machines

    Quote Originally Posted by peteeng View Post
    Hi All - Starting the think about the next build and this is what I shall be using as the controller. This is the first time I have seen centripetal accel discussed. Peter
    Thats a big improvement for a setup like this but does not come close to adaptive jerk control.
    Mactec54

  4. #164
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    Re: Jerk Control in machines

    Hi Martec - Please explain what you mean by adaptive jerk control? By nature all motion control in machines is "adaptive" there has to be some look ahead done and some change (adaption) of motion (path, velocity or accel) to make it smoother yet maintain the set path tolerance. Peter

    image summary
    ideas - describes in words what is happening in many controllers
    adaptive - shows a typical calculation flow chart
    jerk limit - shows the screen for Buildbotics path jerk limit setting
    centripetal accel - shows Buildbotics screen for setting the centripetal accel which they call junction accel. Not sure why they call it that, I have asked Doug about that, waiting for a comment...

    Buildbotics do comment that centripetal accel can be dominate and override the path accel (tangential accel). As centripetal (radial) accel is V(squared)/r the velocity can dominate or as R is the divisor if it is small it can create large accels. The vector sum of the tangent and radial accels has to be calculated and limited to provide smooth motion ie limit inertial forces. This is done by smoothing the path within the tolerance to "control" the accels and jerk.... its not simple math!!

    Some advanced systems don't just look ahead and adjust once they run forward and back through the path smoothing several times. This maybe what you are calling adaptive? Peter

    ARggghhhh!! Buildbotics have got back to me and they are out of stock due to the microprocessor shortage and don't know when the next run will be available! bummer!! I'll be patient and look at the 3D printer controllers a bit more...

  5. #165
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    Re: Jerk Control in machines

    Hi All- I think the 3D printer controllers have gone past the router/mill controllers with input shaping ie jerk control via accel feedback loop.

    https://www.youtube.com/watch?v=er7q-CJL1lc

    I'll have to try to get the one tree octopus to run as a router controller... Peter

  6. #166
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    Re: Jerk Control in machines

    Quote Originally Posted by peteeng View Post
    Hi Martec - Please explain what you mean by adaptive jerk control? By nature all motion control in machines is "adaptive" there has to be some look ahead done and some change (adaption) of motion (path, velocity or accel) to make it smoother yet maintain the set path tolerance. Peter

    image summary
    ideas - describes in words what is happening in many controllers
    adaptive - shows a typical calculation flow chart
    jerk limit - shows the screen for Buildbotics path jerk limit setting
    centripetal accel - shows Buildbotics screen for setting the centripetal accel which they call junction accel. Not sure why they call it that, I have asked Doug about that, waiting for a comment...

    Buildbotics do comment that centripetal accel can be dominate and override the path accel (tangential accel). As centripetal (radial) accel is V(squared)/r the velocity can dominate or as R is the divisor if it is small it can create large accels. The vector sum of the tangent and radial accels has to be calculated and limited to provide smooth motion ie limit inertial forces. This is done by smoothing the path within the tolerance to "control" the accels and jerk.... its not simple math!!

    Some advanced systems don't just look ahead and adjust once they run forward and back through the path smoothing several times. This maybe what you are calling adaptive? Peter

    ARggghhhh!! Buildbotics have got back to me and they are out of stock due to the microprocessor shortage and don't know when the next run will be available! bummer!! I'll be patient and look at the 3D printer controllers a bit more...
    Note their demo was cutting air, no changing load, they are only setting controlling motor parameters, (lack of microprocessors)

    Controlling the changing inertia acceleration / deceleration, with every move, is what adaptive control is, it is superior to anything that you have posted.


    To simplify tuning while still allowing highest versatility and control over servo behavior, we have developed Adaptive Tuning technology, meaning relative to the inertia load, the servo adapts a wide margin of permissible parameters that will ensure stable control. This allows to easily select the settings for smoothest motion, and fastest response.

    The margin of stability being adaptive is critical for many applications. For example, in machine tool, the dynamic load inertia can drastically change. The load inertia ratio is different at acceleration/deceleration, constant traverse and workpiece processing. The same is true for robotic applications. By maintaining a wide region of stability, the DMM adaptive tuning provides seamless performance transition between these events. Even with the same parameter settings, as the load inertia ratio change, the servo is still well within the stable region.

    Second generation tuning algorithm further improves smoothing with wider frequency range leading to wider domain of inertia load capability.
    Mactec54

  7. #167

    Re: Jerk Control in machines

    To modify acceleration curve from trapezioidal to s-curve on single Axis is not a good thing.
    Better to apply S-Curve to interpolated TCP Point (result of all-axis acceleration quantum quantized to what an axis have to move).

  8. #168
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    Re: Jerk Control in machines

    hy pete, after insistong soooooo much on splines, if you wish, please take a look at how different interpolation methods can satisfy same machining error

    a perfomant cnc can output different interpolation methods, balancing accuracy vs speed / kindly

    www.youtube.com/watch?v=FlieT66N9OM
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  9. #169
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    Re: Jerk Control in machines

    Hi Mactec - Then the Klipper/Bigtreetech is adaptive as it changes the velocity & accel on the go via feedback from the accelerometer. The DMM maybe superior but at a high cost point. As I have said before I build machines at a Maker/DIY level. High performance servo systems with position/vel/accel feedback feed forward are outside the scope of my environment. I expect that the accelerometer system can do as well as the DMM system as the algorithms and basic physics are the same. It may even be better as it includes the whole mechanical system response vs the complex feedback/calculation requirements of the servo type systems. Adaptive, input shaping are words to describe using the second differential of position to create a smooth path to minimise jerk (or jounce, depends were you were brought up) I'll keep looking. Peter

    Hi DK - The video explains what the motion controller is doing. If the path strategy is about minimising curvature (ie max second differential of position) within the "track" ie the machining tolerance then this would be the smoothest least jerk path possible. The trick however is that many paths have transitions and if you max the curvature around one curve this may not max the curvature around the next curve. That's why different race car drivers have slightly different race lines. Their car and personality are slightly different, same as machines. So optimising transitions is a bit of a fuzzy area. The maths is straightforward as the racetrack example shows, the implementation can be a bit tricky. Peter

  10. #170
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    Re: Jerk Control in machines

    Quote Originally Posted by peteeng View Post
    Hi Mactec - Then the Klipper/Bigtreetech is adaptive as it changes the velocity & accel on the go via feedback from the accelerometer. The DMM maybe superior but at a high cost point. As I have said before I build machines at a Maker/DIY level. High performance servo systems with position/vel/accel feedback feed forward are outside the scope of my environment. I expect that the accelerometer system can do as well as the DMM system as the algorithms and basic physics are the same. It may even be better as it includes the whole mechanical system response vs the complex feedback/calculation requirements of the servo type systems. Adaptive, input shaping are words to describe using the second differential of position to create a smooth path to minimise jerk (or jounce, depends were you were brought up) I'll keep looking. Peter
    No not at all, unless it is using a closed loop system between the motor and the Drive, it can't really do much of anything, I'm sure they can make the control execute smoother code, which would help some, the only way is to control the inertia acceleration / deceleration, while it is running the program, the DMM Adaptive control was developed for Heart / lung machines, used for severely injured lungs, it is the only true Adaptive control available in a servo drive unless licensed to another manufacture, there are hundreds of hobby users using DMM servo systems so not out of reach for hobby users today, a lot don't get the full benefit of a servo system because the machines are not built that well.
    Mactec54

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