• juerg posted an update 8 years, 2 months ago

    I would like to add a bit more control of the hand to keep it more horizontally (things tend to fall off Marvin’s hands). Any success stories or frustration reports about that?

    • Hello,
      I have found a pretty good solution last year. I didn’t post about it because I wanted to test it before. The hand was set up at an exhibit running the whole day for nearly two months, and the result was concluant.
      I will upload the STL files when I get a moment.
      The springs instead of being mounted directly on the cable like I tried before, are set to pull down the tendons. They only stretch when too much tension is applied to the finger. Therefore the finger is never loose and still can have extra stretching if necessary. There is no need to re-tension the tendons anymore.
      I also came up with another way of tighting the tendons on the pulley, which is much more reliable and simple. No bolts needed, just the screws as before, and drilling two small holes in the pulley.

      • It’s good to have control of the string length at the pulley, maybe your tutorials should include this update?
        For the springs – I think it’s good to keep the strings under tension – great pictures and nice cabeling! But e.g. a bit of a fragile object might still be broken because the servo won’t stop?
        I see you use your preferred HK servos in this hand. Have you modified the servos for a larger range or do you run them with 90 degrees range (which from what I see with my mg 996 would not be sufficient for fully streching and bending)?

        • The basic problem is that the you need to pull a longer distance in one direction that the other. For example the major finger needs to be pulled 27mm to close but only 17mm to open. So the servo pulleys needs to be redesigned to have two different lanes, with different diameters. I have started to experiment, but the pulleys I try now are to high. I will try to print the splines, but that requires a very good printer. The alternative is to get a set of metal gears, and use it to create the final shape.

          • @juerg, the servos won’t break parts, at least it never did on any of the hands I have assembled. In MyRobotLab you set the Min and Max for each servo, this avoids the servos to force further than necessary.
            The pulleys mounted on the HK15298, even though it rotates only 90 degrees, can actuate fully all fingers, only the Majeure finger is missing a bit of degree when closing.
            @Mats, you are correct! I suggest designing an excentric pulley would do the trick, I just didn’t try, mainly because it works fine enough for what I wanted the hand to do.

            • Thanks for posting the spring design. I think springs has to be part of the solution, since turning the wrist will pull on both sides of the servos. A different design of the pulleys may decrease the length that the springs need to flex.

            • Springs will the best/easiest way to solve the problem.

              I will use this Pully, including a spring to let the finger go down again.
              I haven’t springs at home to test, so here some different views from my drawings and a mounted pully at a servo:

              This Pully size is fine for the Index finger, when the servo makes a 180 degrees turn.
              For all other fingers, I will see if it needs to be changed in size.