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  1. #1
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    Closed Loop CNC

    I have a few questions about closed loop CNC.
    1. Do existitng softawre / hardware handle closed loop CNC. Or are they running in open loop mode like just send steps and hope that the motors reach their position?
    2. In case of DC servos with encoders attached, how do the drives use the encoder information. Because AFAIK the even the DC servo drives have step / dir inputs. So if a servo drive receives 3 steps will it map it to degrees and rotate the motor to go to that absolute position based on the encoder feedback?
    3. Do motion controllers handle encoder feeback or is it the drives responsibility to do that?

    I am asking this because I need to used high power motors for at least 2 of my axis. The motors would be rated at least 1.5 to 2 KW so I'm not sure whether you get such high power steppers. I'm currently considering to go in for either a regular AC motor with VFD. But for this I would need a VFD with position control and I don't know how well VFD's handle position control or possibly I'm thinking of going in for the DMM Ac servo's / drive combo.

    Thanks in Advance

  2. #2
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    Re: Closed Loop CNC

    First of all, I would question whether you really need that much power on the axis drives. Seriously. You should experiment a bit with one axis and a load.

    Secondly, do not try to compare modern digital CNC systems with those from (say) 30+ years ago. The older designs were a rats nest of technologies, every one different.

    1: Different SW packages handle closed loop in different ways. Modern digital CNCs tend towards either open loop steppers or closed loop (ie encoder) servo motors.
    With steppers, you just have to 'calibrate' your machine parameters: peak velocity and peak acceleration. Stay within those limits and you should be OK.
    With encoder-controlled servo systems, the motor driver handles the 'closed loop' bit. If all is well, then all is well. If the motor lags too far the driver senses this and flags the controller with a fault. The system stops. You do not need the controller itself to handle the closing of the loop.

    2: Modern CNCs are digital. They do not use analog control; they use digital controls. And that falls neatly into the Step/Dir paradigm. The controller does the mapping, and tells the motor driver to go 100 steps that-away. If you are moving 2 (or more) axes together, the controller handles the synchronisation.

    3: see above.

    You have a good range of servo motors to call on. Brushed DC motors, BrushLess DC motors, AC servo motors, and probably more.

    Cheers
    Roger

  3. #3
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    Re: Closed Loop CNC

    Roger, Thanks for the reply.
    Apart from Dynomotion, Kflop / Kanalog system are there motion controllers available that close the loop. I mean close the loop at the motion controller level and not at the drive level ?

  4. #4
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    Re: Closed Loop CNC

    There may be, but I do not know of any.
    The motion controller does not need to close the loop itself; it only needs to know that its path commands have been obeyed.

    Cheers
    Roger

  5. #5
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    Re: Closed Loop CNC

    Quote Originally Posted by RCaffin View Post
    There may be, but I do not know of any.
    The motion controller does not need to close the loop itself; it only needs to know that its path commands have been obeyed.

    Cheers
    Roger
    Does the controller know that its paths / commands have been obeyed in current motion controllers like Smoothstepper or UC400ETH?

  6. #6
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    Re: Closed Loop CNC

    Yes, emphatically so.
    If they are not obeyed the motor driver will fault - fast.

    This is how thousands of CNCs, including most hobby ones, work every day.

    Cheers
    Roger

  7. #7
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    Re: Closed Loop CNC

    I have used CSLabs CSMIO/a in the past. AFAIK they only work with Mach3/4. They are easy to set up and accept the encoder signals thus closing the loop, The "a" implies an analogue demand signal. I believe they do digital versions too but they are not cheap but they do work right out of the box without the need for programming etc apart from setting the plug in parameters for servo tuning and other factors. You could also look at Galil's offerings most of which work well with Mach3 and 4. They are usually even more expensive unless you can buy secondhand. Often in the UK industrial controllers such as Fanuc Heidenhain and Fagor amongst others come up for sale but you need to do your homework on these.

  8. #8
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    Re: Closed Loop CNC

    Quote Originally Posted by RCaffin View Post
    Yes, emphatically so.
    If they are not obeyed the motor driver will fault - fast.

    This is how thousands of CNCs, including most hobby ones, work every day.

    Cheers
    Roger
    How would the controller know that the Drives / motor combo has obeyed the command unless three is an encoder feedback connected back to the controller. I have not often seen encoders being connected to Steppers at least at the hobby level.

  9. #9
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    Re: Closed Loop CNC

    Quote Originally Posted by Bdrangdo View Post
    I have used CSLabs CSMIO/a in the past. AFAIK they only work with Mach3/4. They are easy to set up and accept the encoder signals thus closing the loop, The "a" implies an analogue demand signal. I believe they do digital versions too but they are not cheap but they do work right out of the box without the need for programming etc apart from setting the plug in parameters for servo tuning and other factors. You could also look at Galil's offerings most of which work well with Mach3 and 4. They are usually even more expensive unless you can buy secondhand. Often in the UK industrial controllers such as Fanuc Heidenhain and Fagor amongst others come up for sale but you need to do your homework on these.
    Thanks for the info Bdrangdo. Would you consider Kflop also closing the loop at controller level, currently that's on my list of to buy stuff.
    On another forum post I found that only EMC can close loop dircetly in software with special hardware. Can Kflop close the loop similar to EMC, what I like about Kflop is that they provide API's to link to their hardware and we can write programs in C, C# to interface with them.
    The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so.. but from what I see on these forums is that many hobbyists are asking questions on Kflop and Dynomotion products.

  10. #10
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    Re: Closed Loop CNC

    Linuxcnc is a closed loop pc based cnc controller. Free and open source.

    LinuxCNC

    Because the motion controller is inside the computer the loop is closed there.

    sam

  11. #11
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    Re: Closed Loop CNC

    Quote Originally Posted by ZeroBacklash View Post
    The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so..
    No - the documentation is always getting updated. There is just a lot of it... Sometimes hard to find what you want.

    I am guessing you are referencing this thread. https://forum.linuxcnc.org/32-docume...d?limitstart=0

    Linuxcnc is constantly checking actual following error and will stop if it is violated (you set the following error limits). Also - it has path following tolerance.

  12. #12
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    Re: Closed Loop CNC

    Quote Originally Posted by ZeroBacklash View Post
    Thanks for the info Bdrangdo. Would you consider Kflop also closing the loop at controller level, currently that's on my list of to buy stuff.
    On another forum post I found that only EMC can close loop dircetly in software with special hardware. Can Kflop close the loop similar to EMC, what I like about Kflop is that they provide API's to link to their hardware and we can write programs in C, C# to interface with them.
    The only issue that worries me is that someone on another forum said that they had not updated their documentation for 6 years or so.. but from what I see on these forums is that many hobbyists are asking questions on Kflop and Dynomotion products.
    I have only dabbled with Kflop, they do can close the encoder loop dependent on the model. If you are using an analogue demand signal I believe you need a separate board which bumps up the price. You will need to have a basic understanding of C to get the most out of them. I have no experience of EMC or if Kflop supports it. Many EMC users speak of using Mesa cards. You have many options open to you and that can make it even more confusing. One thing that the CS labs unit has going for it is the fact that is a much more robust unit with a more industrial feel with well made interface units on it rather than the basic circuit boards made by Dynamotion. Horse for courses I guess.

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