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Thread: Servos or Steppers?

  1. #1
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    Question Servos or Steppers?

    I need to build a robot arm for my robot butler project. At first I though a simple arm with basic cheap servos would work but it turns out it wont. The nicest servo powered arm I could locate for under $500 could only lift 13oz. For a robot butler that isn't very much. The $448 dollar price tag with out controller also put me off. Now my question is dose anybody know where to get a cheap robot arm capable of lifting 1lb or more. Or at least cheap servos with extremely high torque. Also as the title say I was wondering if I should switch from servos to stepper motors. Stepper motors seem more powerful and cheaper however I wonder how hard they are to control with a computer. Also where can I get a cheap stepper motor robot arm. Or at leas a stepper motor controller that can control six or seven stepper motors.
    Dalton Caughell

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    Re: Servos or Steppers?

    The general rule is: you can do it the expensive way or the hard way. "cheap servos with extremely high torque" do not exist, unless maybe you are building them in your basement... and then you have to question, what is your time worth, and are they really cheap anymore? People tend to complain about the cost of the bioloid kit, but given that I make quite a bit of money each hour I work, it's worth it to me to have an awesome supply of servos/brackets that I can build just about anything from in an evening. Generally, you will get what you pay for (if you do it the hard way, you'll be paying with time, which really is just money anyways). I used to spend lots of time designing things I could have bought, to save a few dollars. Then I realized: you just can't compete with China, if you can buy a module that does what you want, do it, you're gonna waste months building a knock-off... and in the end, you might very well have spent just as much money.

    Stepper motors aren't as widely used in hobby robotics (they are quite prominent in certain parts of industry), typically people use them because they got them for next to nothing, and they insist on using them. Most controllers are likely to be either home-brew or industrial..

    If you really insist on using cheap servos... you'll probably have to crack open your physics book, or one of those "501 mechanical thingamabobs" books to come up with a set of mechanical advantage devices for your arm... and of course, that means it will be REALLY slow.... since cheap servos really are slow to begin with.

    -Fergs

  3. #3

    Re: Servos or Steppers?

    I agree with Fergs...

    Minimum lifting force is 16oz. So a 5" arm requires 5*16oz or 80oz-in. This does not include link and motor weights. You should have enough information to make a decision.

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    Re: Servos or Steppers?

    Hey guys, what if...

    ...he bought 2 Phidgets Stepper motor controllers (2 x 58.30 = $116.6):
    http://www.trossenrobotics.com/phidg...ontroller.aspx

    ...and for the stepper motors got these (6 x 22.50 = $135)
    http://www.trossenrobotics.com/store...DC-7-5DEG.aspx

    With a holding torque of almost 190 oz-in each, would it work if the arm was short? (that puts it at $250 + shipping). Of course there's the materials into building the brakets too. Even if it could be done I bet it would be slow.

    Take a look at the image here:
    http://www.lynxmotion.com/Product.as...CategoryID=133

    There is a balancing spring that I'm guessing helps balance the weight distribution and helps reduce the torque required for the lifting servo. Is that right?

    As Ferg pointed out there is the "time is money" aspect. Something I run into is all the unforseen costs. What if you don't have the right sized drill bit? If you can't find it by itself at $4 then you'll have to buy it in a kit for $10. What about any extra wire or connectors you find you need later? All this stuff adds up.

    Dunno, that's my 2 cents. And that's all I have left after building my robot (if it can even be called a robot with no arms).

    -NB
    Last edited by nbdeveloper; 06-11-2009 at 10:15 AM. Reason: pointing out my "robot" doesn't have arms
    I don't know what I'm doing but I like it. - NB

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    Re: Servos or Steppers?

    Quote Originally Posted by nbdeveloper View Post
    Hey guys, what if...

    ...he bought 2 Phidgets Stepper motor controllers (2 x 58.30 = $116.6):
    http://www.trossenrobotics.com/phidg...ontroller.aspx

    ...and for the stepper motors got these (6 x 22.50 = $135)
    http://www.trossenrobotics.com/store...DC-7-5DEG.aspx

    With a holding torque of almost 190 oz-in each, would it work if the arm was short? (that puts it at $250 + shipping). Of course there's the materials into building the brakets too. Even if it could be done I bet it would be slow.

    Take a look at the image here:
    http://www.lynxmotion.com/Product.as...CategoryID=133

    There is a balancing spring that I'm guessing helps balance the weight distribution and helps reduce the torque required for the lifting servo. Is that right?

    As Ferg pointed out there is the "time is money" aspect. Something I run into is all the unforseen costs. What if you don't have the right sized drill bit? If you can't find it by itself at $4 then you'll have to buy it in a kit for $10. What about any extra wire or connectors you find you need later? All this stuff adds up.

    Dunno, that's my 2 cents. And that's all I have left after building my robot (if it can even be called a robot with no arms).

    -NB
    You would still need to add a feedback loop.... tune it so as not to oscillate, etc. Steppers may skip/jump if they are under heavy load...

    -Fergs

  6. #6

    Re: Servos or Steppers?

    Quote Originally Posted by nbdeveloper View Post

    ...and for the stepper motors got these (6 x 22.50 = $135)
    http://www.trossenrobotics.com/store...DC-7-5DEG.aspx


    -NB
    That stepper does have 190 as a holding torque, but is that oz-in or gm-cm? Steppers are of this size are weak and would believe this one to have low moving torque. Note that "holding torque" is greater than the dynamic torque in steppers.

    Teaching arms like Rhino arm or Scorbot use DC gearmotors with encoders. Manufacturing arms can use BLDC motors on harmonic drives with encoders. Steppers this size are adequate for cheap printers.
    Last edited by robologist; 06-11-2009 at 11:51 PM. Reason: clarifying statements

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    Re: Servos or Steppers?

    From what I hear stepper motors won't work is this true? I am willing to put in tons of time but I would prefer to spend under $500. Nbdeveloper's plan sounded like it would work well but whats holding torque? Dose that mean it can lift 190oz or dose that mean it can only hold 190oz. I think a good idea would be to mix steppers and servos. How hard is it to create this feedback loop? Also how precision are steppers. Are they like servos where you can just give them a degree to go to or do they require more advanced programming. Sorry for all the questions but most of my experience is with servos and basic motors.
    Dalton Caughell

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    Re: Servos or Steppers?

    [disclaimer] I don't know much about steppers, never worked with them, only read about them...a little. The list I made was more exploratory than a solution. [/disclaimer]

    The holding torque is a measure of the amount of force the motor can exert on the output shaft if all the windings are on. I don't believe you would get the same torque when it steps (even a full step vs a half step). I found a really good PDF on steppers over on solarobotics:

    http://www.solarbotics.net/library/p...f/motorbas.pdf

    I was going to suggest a particular section, but pretty much, just read the whole thing. Based on the PDF if a 190 oz-in (Trossen doesn't say whether its oz-in vs gm-cm) and a full step only uses 2 poles in a 4 pole winding then that makes it half the holding torque. I think. Also, looking back at the product after reading the PDF...that stepper has a 7.5 degree step. Seems kind of high, but I guess it depends on how long the reach is on your arm. 7.5 degrees can telescope kind of big on a 12 inch long arm.

    I learned a few things about steppers today too. They are "open loop" because there is no feedback telling you thier position. You can keep track of thier position by keeping track of thier input signals (to step). As Inxfery pointed out you would need a feedback loop to "tune" the control of the stepper. The PDF points out the potential for a stepper to oscilate. Fergs also pointed out that the stepper can slip so you'd lose your position by only counting the input. That's why you would need an optical encoder...which requires more hardware/stuff.

    [NB's robot noob perspective on encoders:]

    So you got this disk thing that has different colors (back and white) mounted on your motor. Then there's an extra bit of hardware (optical encoder) that "looks" at the disk as its spinning around with your motor and "tells" you where its at. Even if the stepper slips, the optical encoder keeps track of the position because it tracks the change directly from the ouput shaft by recognizing the change from white to black on the disk.

    This video might help:

    http://forums.trossenrobotics.com/tu...computer-2283/

    If it was up to me, I'd stick with servos, as expensive as they are. Just save up for what you need, or buy them a piece at a time. From a noob perspective there seems to be more information and support for them as opposed to steppers.
    I don't know what I'm doing but I like it. - NB

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    Re: Servos or Steppers?

    Thanks for the detailed responses! I think Ill stick with servos too. I think I might use these to help up the torque. http://www.servocity.com/html/spg425..._rotation.html
    Dalton Caughell

  10. #10

    Re: Servos or Steppers?

    Quote Originally Posted by nbdeveloper View Post
    [disclaimer] I found a really good PDF on steppers over on solarobotics:

    http://www.solarbotics.net/library/p...f/motorbas.pdf
    Kinda like the same one I linked above?

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