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  1. #61

    Re: Servo or Stepper for CNC?

    Absolutely, the on-shelf product difference are exactly as you described. These differences are largely due to design legacy and not actual limitations, but they exist nonetheless.

    I would say with total certainty, as a motor designer, I'd rather design a servo than a stepper. It's simpler and you usually get paid more . That said, you could almost certainly do a fair bit of refinement on steppers if you cared to, there just isn't enough of a market to justify it. Keeping in mind that while you can get within 5-10% of real on cheap to free magnetic FEA software, you can't get better until you shell out >$10k monthly or large annual leasing. With something that's pennies of margin on the dollar vs high margin 'servos', there is no reason a motor designer would start aggressively optimizing legacy architectures.

    The control world is different, they can be prototyped faster and simulated with good accuracy, tuned, and the availability of chips is astounding (the global shortages of today notwithstanding). So there is a lot more pennies to be had in improving control systems rather than the motor hardware.

    With the higher margins, new materials and manufacturing methods, and new/old architectures being made practical, the simpler but more flexible synchronous 3-phase motor is much preferred by motor designers (and their wallets). There is also a greater control challenge there because it's a floating rather than indexing positioning problem, on 3 phases, with at least 2 common winding interconnect schemes (though it's possible to run independent phases like a 2-phase stepper, using 2-wires per phase). The motor also needs to be a consistent and shape conformal waveform or your controller has to be highly integrated via tuning or design. For example, a generic controller can spin a trapezoidal or a sinusoidal motor fine, but a sinusoidal focused controller will do worse on the trapezoidal motor and better on the sinusoidal. They may have nearly identical hardware, but different firmware or even control strategies. One is a nearly square current device and one is a sinusoidal device! Think that sinusoidal sounds easier than a square-ish current? Well, low resistance and inductance means that you need tight current control with a very high speed PID loop, actively tracking that sinusoid while every on cycle makes the current want to jump significantly and every off makes it want to drop. It's a game of fast reactions and high (current) stakes!

    But again, when deciding on your purchases you need to go by the specs, the actual performance at the shaft will not be so noticeably different between the two types except in extreme cases, for a stepper or a servo with the same ratings.

  2. #62
    Join Date
    Jul 2018
    Posts
    6249

    Re: Servo or Stepper for CNC?

    Hi Strawb - You mention driving an ac servo with a stepper controller open loop. Does it still have a flat torque curve when you do this? If so I'm interested in doing this... Peter

  3. #63
    Quote Originally Posted by peteeng View Post
    Hi Strawb - You mention driving an ac servo with a stepper controller open loop. Does it still have a flat torque curve when you do this? If so I'm interested in doing this... Peter
    Yeah, you would need 3 stepper drivers and a 6 lead AC servo (not connected as a Wye or Delta). It would work... but your control would be garbage without lots of programming.

    My point, which I realized after the edit option closed for me, was actually just that it would turn the motor functionally, not that it would on any way be desirable or plug and play.

    Before people scream about no six lead servos, that's totally a thing when a motor is designed to be connected as Delta or Wye before final installation for voltage selection. Delta is a low voltage and Wye and high voltage, in for an otherwise identical windings. And no, not all motors can be either Delta or Wye. Many slot/pole arrangements are trapezoidal as Delta and Sinusoidal as Wye. If they are sinusoidal as Delta they will be sinusoidal as Wye. Some are sinusoidal as Wye and garbagezoidal as Delta.

    Also as another note since I couldn't edit that original post, just cause the motor will spin under control doesn't mean it will spin optimally under control. [Big explanation summarized as follows:] There is a reason not to drive a three phase motor designed for interlinked phases with isolated phase architectures (ie stepper). But it will work, poorly. Gods of the internet forgive me for opening that can of worms on my first post in this forum via my mobile device.

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