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IndustryArena Forum > CNC Electronics > Stepper Motors / Drives > Closed loop hybrid stepper motors.
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  1. #1
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    Sep 2016
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    Closed loop hybrid stepper motors.

    Will I still have to gear down my motor when I use a closed loop stepper instead of a normal one?
    My machine will have a GT2 belt with a 20teeth pulley so 40mm per rotation.
    I will microstep the motor 10-20 maybe.
    Some motor/deiver set like that.

    https://www.ebay.de/itm/DE-STOCK-NEMA23-23SSM2440-Closed-Loop-Stepper-motor-2-9N-M-4A-Driver-HBS57-LONGS/263916291272?_trkparms=aid%3D222007%26algo%3DSIM.M BE%26ao%3D1%26asc%3D20180213162448%26meid%3D021b32 cd93cf4e6eb23ae6bc38d664d8%26pid%3D100930%26rk%3D1 %26rkt%3D8%26sd%3D263326838489%26itm%3D26391629127 2&_trksid=p2056116.c100930.m5375

    My other option is a nema 23 with a planetary gear 5:1 or 15:1 reduction.
    Freies schiff 3 stücke 4-Blei NEMA 23 reduktion getriebe Stepper Motor, Getriebe verhältnis 15:1, 20n. m
    https://s.click.aliexpress.com/e/b834MfFE

  2. #2
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    Re: Closed loop hybrid stepper motors.

    Hi,
    if you have to gear a normal stepper down to do the job you'll have to gear down a closed loop stepper as well.

    Being closed loop does not make it faster or more powerful or more anything really.....its just advertising hype.

    Craig

  3. #3
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    Quote Originally Posted by joeavaerage View Post
    Hi,
    if you have to gear a normal stepper down to do the job you'll have to gear down a closed loop stepper as well.

    Being closed loop does not make it faster or more powerful or more anything really.....its just advertising hype.

    Craig
    But being closed loop it will just try to reach it's position by applying more steps if necessary?
    In that video it looks like the closed loop does indeed have more power?

    https://youtu.be/vNjPo0wq8xI

  4. #4
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    35538

    Re: Closed loop hybrid stepper motors.

    But being closed loop it will just try to reach it's position by applying more steps if necessary
    If it doesn't have enough power to maintain its position, then how will it have enough to regain it's position??
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

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

  5. #5
    Join Date
    Feb 2008
    Posts
    644

    Re: Closed loop hybrid stepper motors.

    The difference is that a closed loop step motor/drive will recover position after a momentary
    stall while open loop stepper will stall and have to be rehomed to continue

    Other advantages of closed loop step motor systems are:

    1. Lower power consumption/higher efficiency
    ( a open loop drive/motor spends most if its time pulling outwards or pushing inwards on the rotor to no great effect
    other than generating heat, a closed loop drive/motor always pulls sideways to generate the required torque )
    2. Higher accuracy via position feedback
    3. Reliable stall detection

    A closed loop step drive system will not change the basic stepmotor torque/speed characteristics,
    as this is determined by the number of poles, inductance and drive voltage.
    The high number of poles in a stepmotor (typically 50 poles) is what limits the maximum speed
    compared to normal 4/8 pole AC servos

  6. #6
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    Re: Closed loop hybrid stepper motors.

    Hi,

    2. Higher accuracy via position feedback
    I agree, a normal open loop stepper will hold position accurately and with high torque in full step and half step positions
    only whereas a closed loop stepper can hold an intermediate position given the resolution of the feedback.

    3. Reliable stall detection
    I agree, a closed loop stepper can detect loss of position and may even attempt to regain it. A good, well specified
    and tuned open loop stepper does not lose steps so detecting a stall is of limited value.My open loop steppers have not lost
    a step in four years except if I was doing something stupid so I certainly wont be paying extra for something I don't need.

    1. Lower power consumption/higher efficiency
    I disagree, a stepper be it closed loop or otherwise is a variable reluctance motor and thus consumes power maintaining a magnetic
    flux and subsequent holding torque. Feedback changes that not one jot.

    Craig

  7. #7
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    Re: Closed loop hybrid stepper motors.

    Hi,
    compare the two torque/speed curves attached.

    They come from the Leadshine site. The first is a 34 size 95mm long 3.5Nm (nom) open loop stepper. The second is their closed loop stepper.
    34 size, 95mm long and 3.5Nm (nom), ie its the same motor with an encoder fitted.

    Can you tell any substantial difference between the two sets of curves....I can't. Given that they are nearly identical how would you describe this quote
    from your previous post?

    In that video it looks like the closed loop does indeed have more power?
    Craig
    Attached Thumbnails Attached Thumbnails leadshine86cm35.png   leadshinecsm23435.png  

  8. #8
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    644

    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by joeavaerage View Post
    Hi,

    I disagree, a stepper be it closed loop or otherwise is a variable reluctance motor and thus consumes power maintaining a magnetic
    flux and subsequent holding torque. Feedback changes that not one jot.

    Craig

    Feedback changes everything here because a closed loop servo (and closed loop step motors are true servos) only applies as
    much torque=current as required to hold position, not full current all the time its in motion like a open loop stepper.
    In fact closed loop step motors are often dead cool when idle or with low torque loads

  9. #9
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    1762

    Re: Closed loop hybrid stepper motors.

    When you say: " (and closed loop step motors are true servos)", my first inclination is to wholeheartedly disagree. That said, can you point to non-marketing documents or tech bulletins that back up your claim?
    Gary Campbell CNC Technology & Training
    GCnC411 (at) gmail.com www.youtube.com/user/Islaww1/videos

  10. #10
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    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by islaww View Post
    When you say: " (and closed loop step motors are true servos)", my first inclination is to wholeheartedly disagree. That said, can you point to non-marketing documents or tech bulletins that back up your claim?
    Why would you think that a closed loop step motor controller that does Field Oriented Control (FOC) and uses deviation in commanded to actual position
    to modulate the torque plus uses the encoder to determine rotor position for commutation just like any 3 phase AC servo is not a true servo?
    If you look up FOC and step drives you will find a number of references.

  11. #11
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    Re: Closed loop hybrid stepper motors.

    Hi PCW_MESA,

    Why would you think that a closed loop step motor controller that does Field Oriented Control (FOC) and uses deviation in commanded to actual position
    to modulate the torque plus uses the encoder to determine rotor position for commutation just like any 3 phase AC servo is not a true servo?
    If you look up FOC and step drives you will find a number of references.
    I follow you argument regarding FOC....there is however a difference between an AC servo and a closed loop stepper, namely that the AC servo current
    is modulated whereas the vector sum of the currents in a closed loop stepper are constant. The flux angle that results from the current applied is variable
    but the flux strength remains the same.

    As you know AC servos have a flat torque characteristic up to their rated speed unlike a stepper....look at the torque/speed curves I've posted
    previously.....they are very different to an AC servo.

    Craig

  12. #12
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    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by joeavaerage View Post
    Hi PCW_MESA,



    I follow you argument regarding FOC....there is however a difference between an AC servo and a closed loop stepper, namely that the AC servo current
    is modulated whereas the vector sum of the currents in a closed loop stepper are constant. The flux angle that results from the current applied is variable
    but the flux strength remains the same.

    As you know AC servos have a flat torque characteristic up to their rated speed unlike a stepper....look at the torque/speed curves I've posted
    previously.....they are very different to an AC servo.

    Craig
    Not true, most modern closed loop step drives use FOC and modulate the current (this is why they have nearly 0 idle power)

    If you take a standard constant current step drive and close the loop by only changing the flux angle what you say would be true
    but most closed loop step drives do not do this.

    The torque characteristic is not a function of servo/open loop but mostly function of the high pole count and the concomitant
    requirement of high frequency drive, this limits the current at high speeds and reasonable voltages.
    That is, a 50 pole 3 phase AC servo would have similar characteristics...

  13. #13
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    Re: Closed loop hybrid stepper motors.

    Hi,
    my only experience with closed loop steppers comes as a result helping a friend with setting his ones up. They were constant
    current types.

    I am intrigued about your description of a stepper being a high pole count AC machine. I concede that the analogy is good.

    The Achilles heel of any stepper is the loss of torque at speed. This was the problem for which my friend purchased close loop steppers,
    it didn't work.

    We concluded that the marked increase in resolution and loss of position detection were the only real advantages that a closed loop
    stepper system offered. Your explanation that FOC can be used to advantage with respect to DC efficiency is not evident with his
    hardware. Even if it were it still does not address the loss of torque at speed issue.

    I hasten to add that the same high pole count allows a stepper to significantly out perform a servo at low speed in terms
    of torque. Thus a stepper has a much higher torque density compared to a servo of the same size, at least up to speeds of
    about 500 rpm. Over about 1000 rpm servos hold a distinct torque advantage given comparable size.

    Given the premium that closed loop steppers attract (cf open loop stepper/drivers) relative to the modest advantages gained I concluded
    that they are not worth the money. With a slightly greater expenditure genuine AC servos can be had.

    Craig

  14. #14
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    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by joeavaerage View Post
    Hi,
    my only experience with closed loop steppers comes as a result helping a friend with setting his ones up. They were constant
    current types.

    I am intrigued about your description of a stepper being a high pole count AC machine. I concede that the analogy is good.

    The Achilles heel of any stepper is the loss of torque at speed. This was the problem for which my friend purchased close loop steppers,
    it didn't work.

    We concluded that the marked increase in resolution and loss of position detection were the only real advantages that a closed loop
    stepper system offered. Your explanation that FOC can be used to advantage with respect to DC efficiency is not evident with his
    hardware. Even if it were it still does not address the loss of torque at speed issue

    I don't believe you can get a higher torque density from a stepper motor than a servo. what the stepper motor has going for it is it is a hybrid switched reluctance motor. basically the stator coils are pushing on the magnet and pulling on the iron at the same time. to get costs down, hybrid car manufactures are using 8 or 10 pole hybrid switched reluctance motors, but the magnets are buried under the poles of the rotor and they face the stator poles, where as stepper motors have several additional leakage flux paths compared to all other motor designs due to their unique geometry, but they are intrinsically a hybrid switched reluctance motor which can be driven with FOC, however, you have to have a fairly intelligent controller to achieve that and certainly not all of them do.

    I haven't taken any servos apart; but if you know with certainty that the permanent magnets are glued to the surface of the rotor, then such a servo does not benefit from the 30% increase in torque possible by switching to a switched reluctance design (which requires a fairly complex rotor stamping and minimum 2 times as many magnets.

    Surface mounted neodymium magnets could still get an air gap flux density as high as 1T perhaps (especially if you fit a halback array of magnets in the rotor), but i don't believe the geometry of a stepper motor allows for anything higher than maybe .6-.8T average flux density.

    stepper motors due to the leakage flux have high inductance which hinders shoving enough current into the motor at rpm, but due to the high frequencies involved they also have high eddy current and hysteries losses which also hinder torque production.

  15. #15
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    Re: Closed loop hybrid stepper motors.

    Hi Eldon_Joh,

    I don't believe you can get a higher torque density from a stepper motor than a servo.
    I disagree, if you look at the torque/speed curves I posted previously you will see a 34 size stepper 95mm length
    produce a very impressive 3.5Nm, albeit at low speed. A similar sized servo might manage two thirds of that.
    I have attached a torque/speed curve for a 34 size 750W stepper. Note that it has about 2/3 the rated torque
    of an equivalent stepper but because of it higher rated speed is a very much more powerful unit.
    This indicates that the torque density of the stepper is higher than the servo, but over a few hundred rpm the servo
    wins hands down.

    Steppers are good at low speeds and are favourably priced for amateur use. Servos are still however the choice
    for high speed operation despite the extra cost.

    Craig
    Attached Thumbnails Attached Thumbnails servo.png  

  16. #16
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    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by joeavaerage View Post
    Hi Eldon_Joh,



    I disagree, if you look at the torque/speed curves I posted previously you will see a 34 size stepper 95mm length
    produce a very impressive 3.5Nm, albeit at low speed. A similar sized servo might manage two thirds of that.
    i believe that, but part of that optimization also is a result of the industry using servos for high rpm and steppers for low rpm. the servo is limited by copper losses in the stator and the moment you can cool it off it should outperform the stepper. the stepper is limited by the magnetic performance of the rotor: that's why they don't have intermittent torque ratings. (though some might)

    in some cases the intermittant torque rating of a servo is 3 or 4 times higher than the continuous duty torque. if you don't know this: torque follows flux density. copper losses follow flux density squared. for a servo with a peak torque 3 times higher than the continous torque: the heat dumped into the copper is 9 times what it is during continuous duty!

    anyhow, hybrid cars are literally using stepper motors. they just call them hybrid switched reluctance motors. they also only have 8 or 10 poles and the geometry is fundamentally different than the stepper motor. but it would be no surprise to me if some servo manufacturers are using buried permanent magnets and their torque density is 30% higher than the competition's surface mounted permanent magnet servos.

  17. #17
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    Re: Closed loop hybrid stepper motors.

    Hi Eldon_Joh,
    you are correct there are servo manufacturers who use buried permanent magnets in the rotors. While there is a loss
    in the flux density by comparison to surface mounted magnets they can withstand higher speeds because the rather brittle
    rare earth magnets can be contained and therefor not subject to fracture.

    I believe Baldor has a range of servos using that idea.

    A consequence of buried magnets is that the rotor can take on, to a limited degree, the characteristics of a induction rotor
    and find use in spindle motors.

    Many other manufacturers retain surface mounted magnets first by gluing them to the rotor and then enclosing them in a thin
    stainless shell. I understand that the shell is 'shrunk' onto the rotor and constitutes a significant manufacturing procedure that
    few publish.

    I should certainly like to see some of the recent advances you have alluded to in the design of 'steppers' for automotive use,
    again I suppose those who know are not publishing!

    For CNC purposes I have found that steppers are entirely adequate for my home made mill but would be to slow for a production setup.
    I do not find the case for closed loop steppers as opposed to open loop steppers to be convincing. If I were tempted to or otherwise required
    closed loop performance I would go for AC servos and not mess around with closed loop steppers at all.

    I follow your argument regarding the relationship of copper loss to peak torque, surprise, surprise, physics is the same in New Zealand as it is in your
    home country!

    Craig

  18. #18
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    Re: Closed loop hybrid stepper motors.

    Quote Originally Posted by joeavaerage View Post
    Hi Eldon_Joh,
    you are correct there are servo manufacturers who use buried permanent magnets in the rotors. While there is a loss
    in the flux density by comparison to surface mounted magnets they can withstand higher speeds because the rather brittle
    rare earth magnets can be contained and therefor not subject to fracture.

    I believe Baldor has a range of servos using that idea.g
    not the same things. see here
    https://youtu.be/xt-nfcwv6Kc?t=2685

    i can't find the rest of the guy's lectures on youtube at the moment.

  19. #19
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    Re: Closed loop hybrid stepper motors.

    Hi,
    that is an excellent link. Its great to see where motor technology is going. Thanks for sharing it.

    Craig

  20. #20
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    Re: Closed loop hybrid stepper motors.

    Hi,
    clearly BMW have invested millions in research and development of that motor and as the lecture has shown other manufacturers have
    done likewise with their designs. I noted also quite a few of the innovations are patented.

    I guess the question I have is will those ideas find application in servos and/or steppers? Certainly licencing the patented ideas will add to the cost
    of servos to use those ideas. Is there sufficient performance gain relative to the cost of persuing it?

    I note for instance that Allen Bradley claim air gap flux densities in excess of 1.2 Tesla (cf the BMW motor's 1 Tesla) with surface mounted magnets.
    While a motor of that description would be a poor contender for automotive traction purposes it has resulted in a good servo motor design.

    In automotive use high torque density measured in Nm per kg is sought after. The same measure might be applied to servos but note that torque density
    measured as Nm per amp is often used as a figure of merit.

    Low voltage (60V or so) tend to have torque constants of 0.1 Nm/A, 200V servos in the region of 0.8-0.9 Nm/A and 400V servos in the range 1- 1.2 Nm/A.
    Of course it is also possible to increase the torque constant by increasing the turns and therefore reduce the top speed for a given voltage. Thus for torque
    constants to have value as a comparative tool the compared servos must be the same voltage and speed class.

    Another area of difference between servos and automotive traction motors is field weakening. There is use made of field weakening in servos in production
    CNC machines but seems to be regarded as a useful technique rather than design necessity. Automotive traction motors like the BMW motor require and rely
    on field weakening to achieve high speeds and efforts to maximize that potential are part of the design process.

    No doubt there will be some cross fertilization but the differences in application may limit it.

    Craig

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