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servo vs. stepper — Parallax Forums

servo vs. stepper

kelvin jameskelvin james Posts: 531
edited 2004-12-06 22:23 in BASIC Stamp
I suppose this is more of a robotics issue, but since this is the "hot spot" on the forum, i figured i would post it here. I see a lot of new people getting into robotics that have believed that servos are the only ones used for this application. Though steppers are somewhat bigger and demand more power than servos, i personally find them much easier to work with and more accurate for positioning. There are no need for modifications, no encoders for position feedback, and unless you want to race, the speed is substantial for most applications. And they are low cost, but you do need a driver or controller. I also realize at slow speeds the servo is smoother, but how much of a factor is that? A stepper, when reaching a position will hold there and be able to return to that exact position, where as a servo won't, which to me, is the ideal behind robotics. Just thought this might make a good debate , and if someone can convince me to go to servos, i would like to hear it. kelvin

Comments

  • lunarknightlunarknight Posts: 13
    edited 2004-12-06 18:32
    My project is to create a humanoid robot (which requires about 27 servo's). I'm going to control them using two parallax servo controllers that can control 16 servo's each.

    I'm not really sure where to find 16+ stepper motor controller... or even 16+ motor controller. I tried googling for stepper motors and had a really hard time finding any stores that sell them at all. Parallax sells one, but it is not as cheap as a small servo. Do you know any stepper motor stores? And how would you control 27 of them?

    - LK
  • allanlane5allanlane5 Posts: 3,815
    edited 2004-12-06 19:21
    Nice things about servos:
    1. They are cheap. Less than $20 (and some less than $10), they come with their own comparator, DC motor, and H-bridge.

    2. They are easy to control. Give them a PULSOUT of between 1 and 2 mSec, go do something else, then refresh every 20 to 50 mSec.

    3. They work with a single-tasking BS2. The BS2 can be doing something else besides driving the control signals for that 20 to 50 mSec. A BS2 can control 4 servo's very easily, where controlling 4 steppers may be harder.

    4. A Servo takes a single control line to go forward, reverse, or stop.

    Drawbacks:

    1. They are limited in power
    2. You get either 180 degree motion with repeatability, or 360 degree motion with speed control. You can't get 360 degree WITH position control.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-12-06 22:23
    I think that each one has their merits...And appropriate applications...One thing a stepper motor can do better is get to a destination faster...Old Floppy & Hard Drives used them to step the head.· A servo would be too slow.· But if for some reason the servo was obstructed from making it's destination, then the position would no longer be accurate.· A servo would keep trying to get to it's destination position.· The servo would lose it's position (In steps).· That why the old Commodore Floppy Drives would step the head all the way in one direction to reset the home position.·

    I do think servos are more versatile though...They seem to have more uses in the hobby world anyway...

    But I wouldn't say servos are useless...Not by a long-shot.



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