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  1. #1
    Join Date
    Jan 2016
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    120

    Closing the loop and controllers

    Hi guys,

    I have been researching a controller upgrade for my mill (Novakon Pulsar) for a pretty long while, and I have a question regarding servos and controllers. It seems that there are several different controller options out there that handle servos differently - some still send step/dir information to the servo drivers which close the loop internally, some can send analog outputs to drives and receive the encoder input in the controller, and some are in between. I apologize that this question is a bit vague, but when selecting a controller for servos, is there a performance increase for closing the loop in the controller vs the drive? And does a controller that sends step/dir vs analog basically negate the advantages of using servos? My goal is to choose a controller that will get the best performance out of the servos.

    Thanks for your advice!

    Micah

  2. #2
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    Dec 2013
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    5717

    Re: Closing the loop and controllers

    Closing the loop in the controller and using an analog signal has been the industry standard for the last 30 years or so. Today the highest performance systems use an all digital system over a network, using one of a few communication protocols.

    I still use the analog method and close the loop in the controller, and it's my preference to put the feedback device directly on the load for maximum positioning accuracy.

    With the hobby class servos that use step & direction signals and close the loop in the drive, there is still some advantage in that the torque curve is much better than a stepper.
    Jim Dawson
    Sandy, Oregon, USA

  3. #3
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    Re: Closing the loop and controllers

    Thanks for your input Jim. Do you have recommendations on analog hardware/software controllers?

  4. #4
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    5717

    Re: Closing the loop and controllers

    Quote Originally Posted by burbingus View Post
    Thanks for your input Jim. Do you have recommendations on analog hardware/software controllers?
    My pleasure to help out.

    Edit: I need to work on my reading comprehension.

    Centroid products would deserve a hard look.
    Jim Dawson
    Sandy, Oregon, USA

  5. #5
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    Jan 2016
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    120

    Re: Closing the loop and controllers

    I had looked at the Oak - I was a little put off that the software add ons weren't included and seemed pretty expensive, but I need to learn more about it. Thanks again!

  6. #6
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    Dec 2013
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    5717

    Re: Closing the loop and controllers

    I finally looked at what a Novakon Pulsar is.

    For a budget upgrade DMM Tech servos/drives, Mesa or Dynomotion Kflop/Kanalog products, and Mach3 or LinuxCNC might be good options.

    Another good but more expensive option for a controller is Galil Motion Control products, the 18x6 or 40x0 product lines. Compatible with Mach3 and DMM products. I use Galil and DMM products, along with my own CNC software, on my machines.
    Jim Dawson
    Sandy, Oregon, USA

  7. #7
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    Jul 2003
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    1753

    Re: Closing the loop and controllers

    I would second linuxcnc and mesa.. Probably the least expensive solution. (software is free and interface hardware is inexpensive)

  8. #8
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    Jan 2016
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    120

    Re: Closing the loop and controllers

    Thanks guys. I think at the top of my list are LinuxCNC/Mesa, Mach3or4/Galil, KMotion/KAnalog. I'm trying to look at the hardware specs and the other pros and cons of each. Thanks for your recommendations - your previous posts have also been extremely helpful in trying to figure this all out.

  9. #9
    Join Date
    Jan 2005
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    15362

    Re: Closing the loop and controllers

    Quote Originally Posted by burbingus View Post
    I had looked at the Oak - I was a little put off that the software add ons weren't included and seemed pretty expensive, but I need to learn more about it. Thanks again!
    For what you are doing closing the loop in the servo drive is all you would need to do, you would not notice any difference if you closed it in the control, just the headache of setting it up and the extra cost for no gain

    Dmm and Acorn you won't get any better, other than free software like Linux CNC
    Mactec54

  10. #10
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    Dec 2003
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    24216

    Re: Closing the loop and controllers

    Quote Originally Posted by burbingus View Post
    Thanks guys. I think at the top of my list are LinuxCNC/Mesa, Mach3or4/Galil, KMotion/KAnalog. I'm trying to look at the hardware specs and the other pros and cons of each. .
    Like Jim I use mainly ±10vdc analogue control and use a controller that closes the loop rather than just handing positioning over to the drives alone, there are a few advantages, the likes of electronic gearing of axis allowing the for accurate synchronized threading or any other method where axis synchronization is reqd.
    This helps in placing the controller on par with the higher end Industrial versions
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  11. #11
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    Nov 2013
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    4282

    Re: Closing the loop and controllers

    Hi,
    I use an open loop controller and Step/Direction to closed loop servo drives (Allen Bradley) and it works very well indeed.

    Using the controller to close the loop is a venerable technique but is slowly fading away as closed loop servo drives become ever smarter and smarter
    and at cheaper prices.

    Increasingly in industrial practice distributed motion control is becoming the norm. In that circumstance each servo drive not only closes the loop with its own
    servo but handles limits, homing and generates its own motion commands under synchronous communication with other drives/servos connected
    to the machine via the likes Ethercat or Profibus.

    Bang for your buck I would go for Delta or DMM servos/drives. In which case a wider range of cheaper open loop controllers than the more traditional closed loop controllers.

    Craig

  12. #12
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    120

    Re: Closing the loop and controllers

    Quote Originally Posted by mactec54 View Post
    For what you are doing closing the loop in the servo drive is all you would need to do, you would not notice any difference if you closed it in the control, just the headache of setting it up and the extra cost for no gain

    Dmm and Acorn you won't get any better, other than free software like Linux CNC
    Hey Mac, why wouldn't there be a noticeable difference in closing in the driver versus the controller?

  13. #13
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    Jan 2016
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    120

    Re: Closing the loop and controllers

    Thanks for the additional advice and replies. There's a lot to think about. It seems that maybe some of the advantages in choosing between closing the loop in the controller versus the driver may depend on hardware specs of the controller - control refresh rates, D/A resolution and update frequency, and whatnot.

    I really appreciate your help as I learn more about this!

    Micah

  14. #14
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    Re: Closing the loop and controllers

    Quote Originally Posted by burbingus View Post
    Hey Mac, why wouldn't there be a noticeable difference in closing in the driver versus the controller?
    You tell me, modern Servo drives can do a better job, both ways are doing the same thing, one less complicated than the other, I have done them both ways, and I prefer to close the loop in the servo drive, you can do it any way you like, but I would be cutting parts while you will be trying to get your system tuned, and if there are complication's with your system you may never get it perfectly tuned
    Mactec54

  15. #15
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    Nov 2013
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    4282

    Re: Closing the loop and controllers

    Hi,
    I agree with Mactec54. Modern AC servo drives have really flexible control options.

    For instance my Allen Bradley servo drive, and it is by no means the latest model, has a thermal model of my servo built in but also has 10 point magnetic
    model to simulate magnetic saturation. These sorts of details are not available for most servos or most controllers, thus the closed loop servo drive is
    materially better at predicting thermal and magnetic performance.

    The only time I would regard a loop closed by the controller to be a distinct advantage is if you were using a linear scale or
    feedback device OTHER THAN an encoder fitted onto the servo.

    Craig

  16. #16
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    Feb 2008
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    644

    Re: Closing the loop and controllers

    Quote Originally Posted by joeavaerage View Post
    Hi,
    I agree with Mactec54. Modern AC servo drives have really flexible control options.

    For instance my Allen Bradley servo drive, and it is by no means the latest model, has a thermal model of my servo built in but also has 10 point magnetic
    model to simulate magnetic saturation. These sorts of details are not available for most servos or most controllers, thus the closed loop servo drive is
    materially better at predicting thermal and magnetic performance.

    The only time I would regard a loop closed by the controller to be a distinct advantage is if you were using a linear scale or
    feedback device OTHER THAN an encoder fitted onto the servo.

    Craig
    Actually there are many advantages of having the controller close the loop or at least have bi-directional communication with the drives:

    1. Absolute encoder support (very awkward with step/dir interfaces)
    2. Real time load, and smart following error monitoring
    3. Complex Feed forward, the controller is aware of whats coming next the drive is not ( feedback is great but its always too late! )

    Most higher end CNC systems have position feedback to the controller

  17. #17
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    15362

    Re: Closing the loop and controllers

    Quote Originally Posted by PCW_MESA View Post
    Actually there are many advantages of having the controller close the loop or at least have bi-directional communication with the drives:

    1. Absolute encoder support (very awkward with step/dir interfaces)
    2. Real time load, and smart following error monitoring
    3. Complex Feed forward, the controller is aware of whats coming next the drive is not ( feedback is great but its always too late! )

    Most higher end CNC systems have position feedback to the controller
    ( 1 ) is incorrect I do it all the time, and with many different high end servo drives Absolute Encoder I have never had a problem with

    ( 2 )Following error is also covered when the servo drive has an encoder feedback to the control for monitoring any error which most quality servo drives have

    ( 3 ) There is not difference if the control is looking at the encoder which most good closed loop servo drives can send the encoder position to the control, if it is to late as you say you would have a fault which does not happen, Do you mean Look-ahead or something else,

    The drive does not know what's coming next even if the loop is closed in the control both ways the drive does not do anything until it gets a signal to do something from the control
    Mactec54

  18. #18
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    Feb 2008
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    644

    Re: Closing the loop and controllers

    ( 1 ) is incorrect I do it all the time, and with many different high end servo drives Absolute Encoder I have never had a problem with

    How does the controller know the initial position at startup to avoid homing? This is not possible with just step/dir

    ( 2 )Following error is also covered when the servo drive has an encoder feedback to the control for monitoring any error which most quality servo drives have

    Right, this requires bidirectional communication

    ( 3 ) There is not difference if the control is looking at the encoder which most good closed loop servo drives can send the encoder position to the control,
    if it is to late as you say you would have a fault which does not happen, Do you mean Look-ahead or something else,

    The drive does not know what's coming next even if the loop is closed in the control both ways the drive does not do anything until it gets a signal to do something from the control[/QUOTE]

    Yes, but the controller can send a smarter signal because it does effectively have lookahead

    This is why most high end CNC and robotic systems have position (and maybe velocity and torque) feedback to the controller

  19. #19
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    Jan 2005
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    15362

    Re: Closing the loop and controllers

    Quote Originally Posted by PCW_MESA View Post
    ( 1 ) is incorrect I do it all the time, and with many different high end servo drives Absolute Encoder I have never had a problem with

    How does the controller know the initial position at startup to avoid homing? This is not possible with just step/dir

    ( 2 )Following error is also covered when the servo drive has an encoder feedback to the control for monitoring any error which most quality servo drives have

    Right, this requires bidirectional communication

    ( 3 ) There is not difference if the control is looking at the encoder which most good closed loop servo drives can send the encoder position to the control,
    if it is to late as you say you would have a fault which does not happen, Do you mean Look-ahead or something else,

    The drive does not know what's coming next even if the loop is closed in the control both ways the drive does not do anything until it gets a signal to do something from the control
    Yes, but the controller can send a smarter signal because it does effectively have lookahead

    This is why most high end CNC and robotic systems have position (and maybe velocity and torque) feedback to the controller[/QUOTE]

    You still have the same feed back to the control either way you want to do it, the control is only looking at the Encoder in both cases, whether you close the loop in the Drive or the Control
    Mactec54

  20. #20
    Join Date
    Dec 2003
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    24216

    Re: Closing the loop and controllers

    Quote Originally Posted by burbingus View Post
    Thanks for the additional advice and replies. There's a lot to think about. It seems that maybe some of the advantages in choosing between closing the loop in the controller versus the driver may depend on hardware specs of the controller - control refresh rates, D/A resolution and update frequency, and whatnot.
    Micah
    I guess I was spoiled as I came into the CNC field from the 'Top' end installing and commissioning Fanuc and Mitsubishi, etc, For my self build units I used Galil Motion, which to me was the closest thing to the 'big boys'.
    It is quite enlightening to watch the Galil educational video's by Dr Jacob Tall.
    There is one on the usage of a dual feedback system that is required when you use a Scale and an encoder on the motor to eliminate problems that occur if servo backlash exists (oscillation etc), splitting the PID loop between the two and how to tune it....
    The Galil is also capable of look-ahead motion. The 32 bit microprocessor, has feed-forward and notch filter, encoder feedback up to 12Mhz!
    I found the gearing ability very handy, for example using two servo motors for the Table X axis to synchronize them
    Incidentally the very simple analogue drives that Galil sell are relabeled Advanced Motion Controls, simple transconductance amplifiers.
    Just my 2¢
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

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