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Stepper Motor Vs. Servo Motor — Parallax Forums

Stepper Motor Vs. Servo Motor

NWCCTVNWCCTV Posts: 3,629
edited 2012-08-06 14:21 in General Discussion
So I am almost to the point of actually doing some hardware work for my overall project. It has been a long time coming!!!! I will be implementing a robotic arm in my project and unlike many homemade ones that I have seen, this thing needs to be quite strong. It needs to (at a minimum) be able to grasp and lift a full 12-16 Oz. can of liquid. I am thinking that a Stepper motor would be my best choice, since when there is power to them but no commands being sent, they are pretty much frozen in place. Looking for the experts advice on this one.

Comments

  • Mike GMike G Posts: 2,702
    edited 2012-08-05 19:40
    16oz of water weighs about 1 lbs. or 0.45kg. Take a 1ft lever (weightless) placed perpendicular to gravity. On one end place the 16oz water load, load = [W]. On the other end place a stepper or servo, fulcrum = @.
     @-------->[W]

    At 90 degree the fulcrum, @, feels (1 lbs.)(1 ft)sin(90) of twisting force or 1ft-lbs of torque.

    Keep this in mind when designing your robotic arm.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-08-05 19:46
    There's nothing to say you can't gear the motor down quite a bit. I doubt that you need the speed on the effector end that a direct drive would give you.

    -Phil
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-08-05 20:06
    I did forget to mention that I am trying to do this with parts that I currently have on hand. I have been collecting stepper motors from printers, scenners and plotters for several years now and have quite a variety of large, medium and small Unipolar and BiPolar. Not to mention that erco keeps finding me motors I do not need but controllers that are worth what both cost!!!!
  • ercoerco Posts: 20,259
    edited 2012-08-05 20:11
    NWCCTV wrote: »
    Not to mention that erco keeps finding me motors I do not need but controllers that are worth what both cost!!!!

    Hey, I'm just the Enabler...!

    BTW, didja see they're down to $3.01 now? :)

    http://www.ebay.com/itm/5V-Stepper-Motor-28BYJ-48-With-Drive-Test-Module-Board-ULN2003-5-Line-4-Phase-/320933811659?pt=LH_DefaultDomain_0&hash=item4ab92555cb

    If bargains were drugs, I'd be a pusher!
  • MicksterMickster Posts: 2,721
    edited 2012-08-06 03:46
    NWCCTV wrote: »
    I am thinking that a Stepper motor would be my best choice, since when there is power to them but no commands being sent, they are pretty much frozen in place. Looking for the experts advice on this one.

    A closed-loop (PID) servo will not only hold position but your controller will always know the true position of the axes. Furthermore, by limiting the motor command output, you limit the motor torque which can provide compliance when clamping-up on the load. Torque-limit is also useful for "teaching" the motion path.

    Mickster
  • lardomlardom Posts: 1,659
    edited 2012-08-06 07:41
    Just my thoughts. I prefer steppers since they're designed for digital commands and the holding power is pretty good. I cannot avoid thinking about how I would design a robotic arm. I like the construction crane's use of pulleys and cables which would allow me to keep the motors on the robot base so I wouldn't add excess weight to the arm. I would also like to keep the number of motors down and I'm thinking the construction crane might offer a solution to that too.
  • prof_brainoprof_braino Posts: 4,313
    edited 2012-08-06 08:09
    Consider the stepper having a worm gear to turn the arm joint. That way the current would not have to be on once the arm is in position. Then it more a matter of joint strength rather than just motor power.
  • MicksterMickster Posts: 2,721
    edited 2012-08-06 08:24
    lardom wrote: »
    Just my thoughts. I prefer steppers since they're designed for digital commands and the holding power is pretty good.

    FYI: Industrial robots such as ABB, Fanuc, Kawasaki, Motoman, Kuka, et-al, are exclusively closed-loop servo controlled. However, I do understand that we are on a hobbyist theme here.

    Mickster
  • lardomlardom Posts: 1,659
    edited 2012-08-06 08:46
    Mickster, agreed. Encoders are an absolute requirement for industrial precision, especially heavy equipment .
  • NWCCTVNWCCTV Posts: 3,629
    edited 2012-08-06 14:21
    This is not for industrial use. It will be more for entertainment and a nice "converstion piece!!!" Aside from that, the only Servos I currently own are the Parallax Standard and Continuous rotation ones. However, I have some pretty hefty stepper motors that I have salvaged over the years and cost is always a factor. Besides, I think the misses would have my hide if I shelled out $300.00 for the one I really want!!!!
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