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  • Robotic arm help

    Discussion in 'The main mechanical design forum' started by Dritech, Feb 17, 2013.

    1. Dritech

      Dritech New Member

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      Hi all,

      I am going to build a robotic arm for my end of year school project. Below is a diagram showing the approximate dimensions of the arm.
      [​IMG] Now I have some question about the motors' torque and their position:

      1) Is it better to use levers (as shown in the attached sketch) instead of installing the motors at each axis/joint ??

      2) If using lever is better, whet are the advantages? which type of lever is better 1st class or 3rd class, and why?

      3) If a stud/threaded rod is used as shown in the attached diagram, will that be efficient? will it reduce the torque?

      4) If I use a motor with 15kg-cm of torque as motor A and use a motor with 10kg-cm of torque as motor B, will the arm be able to lift at least 2kg?? or do I need to use motors with higher torque?


      Any help will be appreciated.
      Thanks in advance.

      Robotic arm sketch:
      17022013047.jpg
       
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    3. g3engineer

      g3engineer New Member

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      1) yes it will be "better" to use a lever arm because the torque required from the motor would be less.
      2) reduced torque from the motor; do a free body diagram to get the specific values
      3) the threaded stud does not "reduce the torque" required, but it does have advantages. Do an internet search on the term "overloading" and you will see what I mean. It basically mean that the screw willnot "unwind" under load. Remember you will not be using a simple machine screw (1/4-20 UNC or a M5, etc) you would use a lead screw or a ball screw or similar. Of course, if you are going to build this you might use a machine screw to save money, but that is up to you.
      4) do a free body diagram. Research lead screws. The pitch will determine the output torque given a specific input torque. The mechanical advantage you will end up with all depends on the geometry... specifically the length of your moment arms. Do the math; it is quite a simple problem to solve, but it does not have a unique solution until you enter the physical values into the equations.


      Hope that helps
       
    4. joshuafswain

      joshuafswain New Member

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      Just looking at your sketch, if you want to use a lever configuration you will need to use counter weights at the ends of the lever as the mechanical advantage of the load will increase as the robot extends its reach. Doing this will relieve stress on the motors and make the robot less likely to tip over.
       
    5. jayakumarjkl

      jayakumarjkl New Member

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      better to use hydraulic controls

      I do not know much about robots but I can say this use a single hydraulic pump .1 or 2 control valves and two actuators either linear as in your case or rotary.
       
    6. fintan

      fintan New Member

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      What you propose in your sketch is theoretically possible but I'm afraid in practice won't work without a whole bunch of expensive machine components.

      Normally your motor and stud arrangement would look something like this:
      http://machinedesign.com/article/designing-motion-control-systems-with-electric-cylinders-0125

      Pins are used to anchor the ends to structure you must fabricate very accurately.

      An electrical control system would be required or the machine will likely smash itself to bits the moment power is applied.

      Maybe a purely mechanical system using ropes and pulleys rather than motors and lead screws may be more realistic?
       
    7. Dana

      Dana Well-Known Member

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      1. Using a lever as you show is not a bad way to do it. If you mount the motor directly at the pivot, it would have to be a high ratio geared motor to get the same force as you would get with the lever and a lead screw... and all of the connections would have to be very rigid and withstand the full torque.

      2. I had to google 1st class, 3rd class lever. It may be a term used in school but not in the real world. However, a 3rd class lever would require more force from the motor.

      3. What you term "threaded rod" is in this context generally referred to as a "lead screw". Lead screws always have some inefficiency, but they do allow accurate control and high force for a given input torque. The exact ratio of output force to input torque depends on the pitch of the screw; finer pitch gives more force for a given torque. Normally you wouldn't use an ordinary screw thread like on a nut and bolt, but a thread type optimized for this kind of application. Acme threads are probably the oldest, most common, least expensive, and least efficient, but there are many other types; some are termed "high helix" lead screws, then there are fancier (more efficient, more expensive) things like ball screws and roller screws. As in everything, you get what you pay for.

      4. Without dimensions it's impossible to say. It depends on the length of the arms and levers, the pitch and efficiency of the lead screw. You will need to calculate it. Draw the free body diagram(s) as mentioned above. Start from the load, its weight and how far you have to move it, and work backwards, working out the moment at each pivot, the force at the other end of the lever, then the torque required at the screw to produce that force... but that's your homework assignment.
       

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