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Component question - Pivoting robot arm?

Discussion in 'General Electronics Discussion' started by icydash, Mar 22, 2015.

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  1. icydash

    icydash

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    May 22, 2013
    Hey all,

    I'm working on a robot and want to have an arm that can move in the X and Y axis and be controlled by a microcontroller. Do you know of any electronic pivots (like an electrically controlled ball-and-socket joint?) or something like that which would work for this? Of course, if it can also move in the Z axis, even better.

    Thanks!
     
  2. TehMaxwell

    TehMaxwell

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    Mar 21, 2015
    What your asking is rather complicated, to replicate the motion of a human joint is something that takes a pretty complex mechanism. I made an arm myself this year and found that it's much easier to do what is commonly done in industry and use five main pivot points and a claw. You have a pivot in the base that rotates the whole assembly, then a "shoulder section which raises and lowers the arm, then an "elbow" section that raises and lowers the arm further up, then a "wrist y" which does the same as the shoulder and elbow, then a "wrist x" which twists the arm and then the claw. This achieves all the axis of rotation you require in a much simpler format. If you want something simpler you could remove the "elbow" and "wrist y" and it would still do x, y and z.

    Then you just need to think about how much force your going to put into the pivots, or in other terms how heavy an item you want to lift.A bigger stronger arm would use stepper motors. But if you want to build a desktop one then go for servos.

    Here's a link to my build video, it shows pretty much the whole process of putting it together.



    The important thing to think about is how versatile you want the arm to be. Generally the more joints it has the more complex maneuvers it can do!

    Hope this helps,

    Sam
     
  3. icydash

    icydash

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    May 22, 2013
    Great, thank you Sam! I very much appreciate it. I'm not going for anything crazy complicated. One servo in the base to give me 1 axis of movement, and one servo in the arm to give me another axis of movement, is probably sufficient. If I want the arm to be able to lift objects up to, say 50 lbs, what would you recommend?
     
  4. BobK

    BobK

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    1,686
    Jan 5, 2010
    50 pounds? Really? How much does the robot weigh?

    Bob
     
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  5. TehMaxwell

    TehMaxwell

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    Mar 21, 2015
    What are you going to be lifting? 50lbs is a lot of weight (about 22.5 kg in my money), it can be done but you would need a heavier duty machine to do it. It would have to be built out of metal, as the thickness of wood required would be ridiculous and not worth putting all of the time into the project. Possibly aluminium, although that may just bend under the weight. You will have to use stepper motors, as Servo's just aren't designed to cope with that kind of weight.

    Remember you are lifting the weight of the arm as well, so you have to figure that into your calculations for the torque of the motor.
     
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  6. icydash

    icydash

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    May 22, 2013
    Probably just pushing around rocks and tree branches and stuff. It's just for fun, but figured it'd be cool if it could help me with my yard work outside =) I haven't built the rest of the robot yet, was going to work backwards to see what i'd need for the arm.
     
  7. TehMaxwell

    TehMaxwell

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    Mar 21, 2015
    Well for that sort of heavy duty work your going to need stepper motors. They can cope with more weight and as they use bearings are much more sturdy than servo's are. This will mean more expenditure though as you will need stepper drivers and a program that can interpret inputs and driver the steppers accordingly.If you know a language then this could be done.

    Ultimately it's simpler with servo's and at a push with some big heavy duty ones you could possibly play around in the yard.

    Hope this helps,

    Sam
     
  8. BobK

    BobK

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    Jan 5, 2010
    Lifting 55 pounds at 1 foot per second would take 1/10 of a Horsepower or 75W. Allowing for electrical and mechanical losses, it would take about 150W motor to do this. So we are not talking about servos or small stepper motors.

    Bob
     
  9. icydash

    icydash

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    May 22, 2013
    Ok thanks for all the info!

    If instead I just wanted to do 20 lbs, what do you think I'd need to use?
     
  10. TehMaxwell

    TehMaxwell

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    Mar 21, 2015
    You could possibly use steppers and gear up the system too increase the torque output. This would decrease the rotation speed, but that helps you control the arm better anyway so a win win really. Still has to be steppers or a motor though, servo's are not built for that kind of thing.

    Even with some bigger ones as I mentioned previously, you may only be able to lift a small branch, if that. Also the expenditure would be high, as you would need metal geared servo's to stop the gears stripping and ones with high torque. From what it sounds like you need steppers for this project, as they can cope with a lot heavier weight than other things can.
     
  11. icydash

    icydash

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    May 22, 2013
    Great, thanks! I appreciate all the help. Slowing the rotation speed is fine; doesn't have to be quick movements. I'll look into steppers.
     
  12. TehMaxwell

    TehMaxwell

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    Mar 21, 2015
    Steppers aren't too complicated, especially if you are willing to learn a bit of g code. You can use an open source program called GRBL with some steppers and drivers to run the arm. You could then just write a program that sent raw gcode commands or even make a control that sent the commands. Or you could type it directly into the serial port. Infact if you really wanted to you could map the stepper movement to a joystick!

    Hope I've helped and if you have any more questions don't hesitate to ask!
     
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