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  1. #1

    Default steppers vs servos

    I understand the basic differences between a servo and a stepper, but in a CNC application, what benefits do servos give?

    I notice that a 750w servo has quite a modest torque (2.4Nm), so you clearly need quite beefy PSU's and drivers.

    Obviously the servo is closed loop so there are no missed steps, but theoretically you can have encoder equipped steppers to address that issue. (not sure how well that works in practice?)

    I assume that servo positioning accuracy can be better, as the encoder will be much better than the 200 steps per rev on a stepper and there's no loss of torque compared to a micro-stepped stepper.

    Are servos worth the extra costs, assuming you don't have C3 grade ballscrews???

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  2. #2
    Member ger21's Avatar
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    Default Re: steppers vs servos

    Steppers have a fixed resolution of 200 steps/rev, and lose torque as rpm's increase. In order to move at high speeds with steppers, you need to spin them slower, where they have more torque. This reduces you're already low resolution even further, as the motor spins fewer times per distance traveled. A good rule of thumb with steppers, to maximize performance, is to get about 0.5-1" of travel per mtor revolution. You can usually count on 1/2 step accuracy, so you'll end up with 400-800 step/inch accuracy this way.

    Servos, on the other hand, have far figher resolution, and much more torque at higher rpm.

    I'm using 400w servos for my X and Y axis, and 570oz steppers for my Z axis on the machine I'm building.
    Physically, they are very similar in size.
    But that's where the similarity ends.

    The 400W AC servo has about 175oz of continuous torque, but about 500oz of peak torque. And these torque ratings are at 3000 rpm, where as the steppers torque rating is at 0RPM.
    You can use this peak torque rating during acceleration. And furthermore, you'll typically use a belt reduction with a servo, which increases the available power even further.

    Say you are using a rack and pinion.
    With a stepper, you'd usually use a 3:1 reduction, and have about 1" of movement per motor revolution.
    So, at 1000 rpm, you'd be moving at 1000 ipm. At 1000 rpm, a 570oz stepper would have about 150-200oz of torque available. With the 3:1 reduction, you'd have about 500-600oz of torque at your pinion.

    Now take a servo, and use a 10:1 reduction. For simplicity, let's just call it 9:1. So now, at 1000ipm, the servo is spinning at 3000 rpm. At 3000rpm, you have 500oz of peak torque to accelerate. with your 10:1 reduction, you now have 5000 oz of torque.

    So, with a 400w ac servo, during acceleration, you can have 8-10x more power than a stepper that's physically the same size.

    With AC servos, you can run them directly from 240V AC voltage, so there's no power supply needed.

    The downside to servos, is their cost.
    My 570oz stepper, and Leadshine AM882 drive were about $125.

    A 400w servo and drive, plus cables, is closer to $500-$700, depending on brand. And with servos, you may need to use a planetary gearbox for reduction. High quality, low backlash gearboxes can get very expensive. Chinese versions start at about $250 for low backlash models. If you're using ballscrews, you can usually just get away with a belt reduction.


    Servos accel at high speed performance. Steppers are a better choice in lower speed applications, due to their much lower cost, and simplicity.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

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

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


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steppers vs servos

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