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Essentially every cobot I've worked with uses BLDCs instead of stepper motors in the joints. When searching for answers as to why I see a lot of posts that suggest one or the other, but nothing explains the consistent trend in the industry. The closest I've come to an answer is that stepper motors are worse with dynamic loads, but I don't understand why. Steppers have much higher precision due to their detant torque, so it seems like they should win out.

If anyone could help point me in the right direction I would greatly appreciate it.

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  1. Size for size, steppers are wimpy compared to brushless (feel free to look at some catalogs and check; be sure to compare both torque and maximum RPM, because a motor can always be geared).
  2. Without fancy electronics, steppers are run open-loop. When they get lost, their average torque goes to zero. I would not want that close to my face if it happened.
    1. With fancy electronics, a stepper may as well be a brushless. A wimpy brushless.
  3. As long as position feedback is used and it's not limited by sticktion, a brushless can attain precision proportional to the accuracy of the feedback.
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  • $\begingroup$ This is a great and concise answer! I'm currently having a discussion with someone and they're claiming that a stepper driven system is going to be more precise due to the step increments. Do you think that the superior precision of a steppers steps are negligible compared to the precision of a brushless with feedback? $\endgroup$ – Ernest Wright Jun 11 '19 at 0:41
  • $\begingroup$ A brushless with feedback can be as or more precise. But a poorly done stepper may be better than a poorly done brushless. "Well done" means that the mechanical, electrical and control systems design is done well by a coordinated team. $\endgroup$ – TimWescott Jun 11 '19 at 0:45
  • $\begingroup$ I think you've answered my question very well. I appreciate your input! If you don't mind, do you have any suggested resources (websites, papers, books, or anything really) for what constitutes a "well done" brushless controller? $\endgroup$ – Ernest Wright Jun 11 '19 at 0:58
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Well, there are a few reasons to that. Servos have a closed-loop mechanism, with an encoder or another device giving accurate feedback on the actual motor path. Steppers provide no feedback; control is through movement in incremental steps. Servos normally boast consistent torque output over a wide speed range; copes well with short-term power bursts (e.g., due to rapid acceleration).
At low RPM, steppers performs better, while, closer to the maximum limit, torque is degrading; can provide holding torque in the power-off state. There are also some other reasons for that trend. You can better idea of the difference in that article Steppers for a cobot

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