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  1. #21
    Member mactec54's Avatar
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    Default Re: Servo sizing, determining required acceleration

    [QUOTE=jaguar36;2512794]
    Quote Originally Posted by joeavaerage View Post
    If it had solid feet on the table it would move less, but the general principle still applies, whatever acceleration you apply to your X axis will result in an acceleration
    of the machine in the opposite direction./QUOTE]

    Well usually a machine is anchored into the ground, so its only accelerating due to the flex in the frame. (And perhaps slip of the feet if its not bolted down).

    For my machine I'm assuming that the mass of the axis being accelerated is many times less than the machine mass, such that it doesn't really matter. With yours being almost 20% the mass of the machine, I can see why that would be an issue. Particularly if the machine is on casters.
    Yes, when a machine has castors for ease of moving it around, it's never going to be doing anything that requires high acceleration

    A Bridgeport CNC Knee Mill, is close to 3,000Lbs and these machines will move around with moderate acceleration. above 300IPM

    Mactec54


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    [QUOTE=mactec54;2512832]
    Quote Originally Posted by jaguar36 View Post

    Yes, when a machine has castors for ease of moving it around, it's never going to be doing anything that requires high acceleration
    If using castor I'd have fixed straight at the front and spinner rears. With a raise/lower mechanism attached to the rears and drop legs to stabilise when in place.
    Like a turntable stacker unit has on a print finishing machine.



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    Default Re: Servo sizing, determining required acceleration

    Hi,
    at 800kg the machine takes quite some effort to push around, really a two person job. With the machine tuned to max (rated) acceleration, 0.27g, the machine moves an inch or two
    back and forth. My normal settings are 0.15g, and the machine is pretty tame, and perfectly useable as is, no need for any fixed feet.

    Whether you have castors, or fixed feet, or even have the machine bolted down the accelerating axes generate an equal and opposite acceleration in the rest of the machine, at best you can decide
    how you are going to contain that force, but you can't eliminate it. The higher the acceleration and the axis mass the more thought and effort is required to stabilise the machine, no biggy,
    just something to be aware of.


    Craig



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    Default Re: Servo sizing, determining required acceleration

    Hi jag - Any resolution here? Cheers Peter



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    Default Re: Servo sizing, determining required acceleration

    Quote Originally Posted by peteeng View Post
    Hi jag - Any resolution here? Cheers Peter
    Got pulled off onto some other projects, but started looking at this some more again. I'm finding that based on the peak torque values even pretty small servo's will be more than sufficient. However that begs the question, how long can the servo put out its peak torque for? Or what duty cycle can it run its peak torque for. I've checked the Clearpath website and couldn't find any qualifiers on it. Some complicated toolpaths could mean the servo is running at peak torque for quite a bit. I'm wondering if I should be looking at the continuous rating instead.



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    Default Re: Servo sizing, determining required acceleration

    Quote Originally Posted by jaguar36 View Post
    Got pulled off onto some other projects, but started looking at this some more again. I'm finding that based on the peak torque values even pretty small servo's will be more than sufficient. However that begs the question, how long can the servo put out its peak torque for? Or what duty cycle can it run its peak torque for. I've checked the Clearpath website and couldn't find any qualifiers on it. Some complicated toolpaths could mean the servo is running at peak torque for quite a bit. I'm wondering if I should be looking at the continuous rating instead.
    You can only use the continuous rating for sizing your servo.

    There is a lot of information on this subject, it has to relate to the Servo Motor you are using as every manufacture can be slightly different, so pick one and work with that, if you don't have a Servo Motor that you are going to use, you are pretty much just wasting your time

    The maximum torque required by the motor is typically the sum of torque during acceleration, torque due to the load, and torque to overcome friction. Because maximum torque is required for only a short amount of time, it can fall outside of the motor’s continuous operating zone but must fall within the intermittent duty zone.

    1) The duration of maximum torque must fall within the time limit defined by the motor manufacturer for intermittent operation

    2) Both the maximum torque and maximum speed should fall within the intermittent zone. If either parameter lies outside the motor’s operating limits, damage to the motor could occur.

    The maximum speed of the motor is limited by voltage … or more specifically by back EMF voltage. Back EMF is generated by the motor’s rotation and opposes the applied voltage. As speed increases, so does back EMF, and at some point, the back EMF can reach or exceed the voltage supplied by the drive.

    Mactec54


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    Default Re: Servo sizing, determining required acceleration

    Quote Originally Posted by mactec54 View Post
    if you don't have a Servo Motor that you are going to use, you are pretty much just wasting your time
    This whole post has been about choosing the appropriate servo.

    Quote Originally Posted by mactec54 View Post

    1) The duration of maximum torque must fall within the time limit defined by the motor manufacturer for intermittent operation

    Right, so that was the question I asked. What is that time limit? I just found on Yaskawa's site that they say its for 3 seconds, presumably Technik's is in the same ballpark. Then the question is what is the downtime after that? aka, what is the duty cycle? I could assume that if you run at 300% of the continuous torque rating for 3 seconds, then you need to run at less than 1/3 the continuous torque rating for 3 seconds to prevent overheating. However I imagine its probably not a simple linear relationship like that.



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    Default Re: Servo sizing, determining required acceleration

    Hi Jag - Do not count on overdriving the servo. Size the servo based on its continuous spec. Peak torques as you have found are only available for seconds and the thermal constants are quite big ie it takes a long time to get rid of the heat a short time to heat it up. If you size the motor depending on peaks you will cook the motor. If you want to dig into thermal managment just ask the supplier for the mechanical and thermodynamic data for the motor they will supply the design curves. But as stated this is not a good way to go for a cnc machine.

    I think the main issue in sizing a motor is defining a realistic acceleration for the machine. The main torque requirement is driven by this number. Cut back the accel and small motors can be used, high accels require very big motors. Hobby machines run very well at 0.1g commercial machine 0.5-1.0g and very fast machine's 3g plus... Here's a spreadsheet to help Peter

    Attached Files Attached Files


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    Default Re: Servo sizing, determining required acceleration

    Quote Originally Posted by jaguar36 View Post
    This whole post has been about choosing the appropriate servo.



    Right, so that was the question I asked. What is that time limit? I just found on Yaskawa's site that they say its for 3 seconds, presumably Technik's is in the same ballpark. Then the question is what is the downtime after that? aka, what is the duty cycle? I could assume that if you run at 300% of the continuous torque rating for 3 seconds, then you need to run at less than 1/3 the continuous torque rating for 3 seconds to prevent overheating. However I imagine its probably not a simple linear relationship like that.
    You would never even consider comparing Yaskawa to Technik's

    You can't work out a Servo Motor size unless you know what work it is going to be doing

    Mactec54


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    Default Re: Servo sizing, determining required acceleration

    I dug into the documentation and it looks like the thing to do is to calculate the RMS of the torque profile and compare that to the continuous duty rating. That should be easy enough to do since I already generated the torque/acceleration profile generated for a couple different toolpaths as I previously mentioned to find the sweet spot for acceleration levels.

    It also sounds like not exceeding the recommended the allowable load moment of inertia is important as well. I'm curious how 'real' VMCs handle this. Looking at a HAAS VF1, and the servo it uses, with a full table load of 3000lbs its going to be way exceeding that allowable. Interestingly Clearpath seems to have much higher allowables (100x motor inertia) vs Mitsubishi or Yaskawa (5-15x).



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    Default Re: Servo sizing, determining required acceleration

    Hi,
    Clearpath are in the business of selling servos to people how don't know anything about servos....and it looks like they are doing a good job.

    You are so taken with the peak torques and all that rubbish, when in fact all you need to know is the continuous rated torque.....thats what you design to.
    The overload is nice to have but you should not be designing to require it.

    Interestingly Clearpath seems to have much higher allowables (100x motor inertia) vs Mitsubishi or Yaskawa (5-15x).
    More rubbish,. A servo system becomes increasing hard to tune for stability the higher the inertia ratio. Almost all manufacturers do fine up to 15:1 but beyond that you need
    notch filters and specialised feedforward additions. 20:1 to 25:1 is obtainable but not easy .....100:1 is just fanciful.

    To use the inertia ratio as a figure of merit is a good idea, but then you must be able to calculate the inertia equation.

    Looking at a HAAS VF1, and the servo it uses, with a full table load of 3000lbs its going to be way exceeding that allowable.
    This suggest that you don't know how the calculation shakes out. More often than not the inertia is dominated NOT by the linear axis mass but by the ROTATING components, namely
    the servo armature and the ballscrew. As an example my new build mill has 115kg cast iron axis beds, and allowing for a vice and the workpiece I assumed a total linear axis mass of 150kg.
    I used 32mm diameter 5mm pitch ground ballscrews and Delta B2 750W servos.

    The inertia calculation shows that 80% is in the ballscrew, 12.5% in the armature of the servo and ONLY 7.5% in the 150kg axis mass. You'd swear the the momentum would be because of the great big chunk of
    cast iron moving back and forth.....but no....it forms only a small fraction of the overall momentum. So despite the huge table mass of the Hass, its still highly probable that the momentum is dominated
    by rotating components.

    I can help with the inertia calculation if you wish.

    Many servo manufacturers have design tools, but I'd have to recommend Yaskawa, certainly the most comprehensive and free to download. Clearpath are leading you up the garden path.

    Craig



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    Default Re: Servo sizing, determining required acceleration

    @joeaverage Great info on inertia ratio, there is almost zero discussion of this on the online/hobbiest forums, and I think a lot of disappointment after builds don't meet expectations. I don't know the math, maybe you can add to this:

    A servo engineer explained to me that the "control authority" (my words, not his) of the servo goes up at something like a x^3 rate with increased gear ratio. So a screw that is driven by a 1:1 coupler with have 8x less control authority than a screw with 2:1 coupler. This comes down to the increased available torque and the increased # of resolution steps for the servo do its algorithmic magic. This translates hugely into stiffness/accel/cut quality/MRR/etc.

    Also, what max rapids can you achieve with the 3205 ground screw you have? What length of screw? And fixed-simple/floating bearing config? I'm trying to spec a ball screw for 1650mm of travel, 40kg moving mass, .25g accel min (.5g would be great, rest of machine weighs 800kg, bolted to ground, boxed steel construction). Did you find the online calculators to be accurate with respect to critical speed? (no one lists what mounting accuracy requirements are needed to achieve these numbers, so I have no idea how to add a fudge/safety factor).

    Thanks!



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    Default Re: Servo sizing, determining required acceleration

    Hi,
    I have 32mm diameter 5mm pitch ballscrews direct coupled to 750W servos.

    The rated speed of the servos is 3000rpm, and the max speed is 5000rpm. I have tuned to 5000 rpm for G0 rapids of 25m/min.
    The acceleration at rated torque (2.4Nm) is 0.27g and 0.81g at overload torque (7.1Nm).


    In truth this is more than I require. My machine weighs 800kg and is on big castors and at 0.25g it lurches around quite a bit. I've dialled it down to 0.15g and wedged it in a corner
    at its fine. I've dialled down the max speed to 15m/min, and often really only use half of that. My machine is not for production, and I find 0.15g and 15m/min perfectly
    adequate.

    The screws are 650mm length, so the critical speed is very high, much faster than my servos anyway...so I haven't bothered with the critical speed calculation, your
    longer screws may be more of a problem.

    Craig



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    Default Re: Servo sizing, determining required acceleration

    Quote Originally Posted by double0jimb0 View Post
    @joeaverage Great info on inertia ratio, there is almost zero discussion of this on the online/hobbiest forums, and I think a lot of disappointment after builds don't meet expectations. I don't know the math, maybe you can add to this:

    A servo engineer explained to me that the "control authority" (my words, not his) of the servo goes up at something like a x^3 rate with increased gear ratio. So a screw that is driven by a 1:1 coupler with have 8x less control authority than a screw with 2:1 coupler. This comes down to the increased available torque and the increased # of resolution steps for the servo do its algorithmic magic. This translates hugely into stiffness/accel/cut quality/MRR/etc.

    Also, what max rapids can you achieve with the 3205 ground screw you have? What length of screw? And fixed-simple/floating bearing config? I'm trying to spec a ball screw for 1650mm of travel, 40kg moving mass, .25g accel min (.5g would be great, rest of machine weighs 800kg, bolted to ground, boxed steel construction). Did you find the online calculators to be accurate with respect to critical speed? (no one lists what mounting accuracy requirements are needed to achieve these numbers, so I have no idea how to add a fudge/safety factor).

    Thanks!
    Reflected inertia changes by the square of the gear ratio.

    Look up "reflected inertia"

    This affects the inertia that the servo motor "sees" / the inertia ratio

    Inertia ratio is important for control stability.
    Too high a ratio and the servo will not perform as well / longer settling times / resonance.

    But gearboxes / belt reductions often have downsides. Backlash, flex etc.

    (I am just a hobbyist, not an engineer, so my understanding is limited)



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Servo sizing, determining required acceleration

Servo sizing, determining required acceleration