Monday, August 30, 2010

Rethinking the hand

I'm very slow at times. I became enamored of the Meka robotics hand a few months ago and tended to make my own hand project very like it in many ways. As I was completing my first finger/hand segment design, however, the way that both Andreas Maryanto's hand project and Meka's began to trouble me. I had indentified the situation I was calling the "stiff hand" without being able to work out the implications.

This morning, I waked up at about 0300 knowing full well what a stiff hand implied. If you will look at the Meka hand clip you can see what I am talking about.

Notice how the thumb can only engage the forefinger.

Here is a nice photo of the effect as well.

Looking at the the Meka hand design one could extend the range of the thumb and touch all of the fingers.  Even so, and this is the part that I kept missing, you couldn't wrap either Andreas' hand or the Meka hand around a ball and throw it.  You can grip things with a handle, like a hammer, but not a ball.

People interested in prosthetics kept telling me to locate the servos driving the fingers in the forearm like happens in a human hand.  I wanted the hand to be autonomous because of the complexity of routing the tendons contracting the fingers through the wrist joint.

I've now spent several hours trying to evolve my existing design into something that could curl the palm around a ball.  I've concluded that routing tendons through a wrist joint is perhaps not as complicated as making the palm of an autonomous hand curl around a ball ...  or a doorknob.

For inspiration, I began looking at the anatomy of the human hand.  It appears that the ability to curl the palm around an object is controlled by the thenar {attached to the thumb} and the hypothenar {attached to the little finger} muscles.

So it looks like I have at least

  • five servos for finger and thumb contraction
  • one to contract the tendon analog of the thenar and hypothenar muscles
  • one to control the lateral distance between the thumb and the palm
  • one to control the movement of the palm from side to side
  • one to control the side to side movement of the palm.
That makes a minimum of nine servos.  That's definitely going to fill up the forearm.

I wonder how you get the wrist to rotate?


  1. What you think of as wrist rotation actually happens at the elbow, and anatomists refer to two movements: pronation, which is what happens if you're lying down with your arms at your sides and you turn your hand to face the floor, and supination, the opposite movement. The forearm has two bones, the radius and the ulna. In pronation, the radius actually crosses over top of the ulna. The joint on the elbow side of the ulna is a nearly spherical joint that allows for axial rotation as well as flexion and extension.

    I greatly admire this project and your work on the RepRap, so please know that I mean no disrespect when I say: I have thought from the inception of this blog that it's obvious you don't have much anatomical knowledge, and I think this project could really benefit from some. The human hand is an amazing work of engineering, and if you want to reproduce its function it's not a bad idea to take inspiration from its form.

  2. "I have thought from the inception of this blog that it's obvious you don't have much anatomical knowledge, and I think this project could really benefit from some."

    LOL! You're right there. At the onset of the project I wasn't aiming to duplicate quite so much of the hand/arm assembly's movements. The further I get into the work, however, the more I find myself duplicating what the body does naturally.

    BTW, I figured out the part about wrist rotation being a ulna and radius thing a long time ago. Some years ago, I shattered my forearm, eight distinct breaks. I learned a lot about how it all worked with added hardware afterwards in physical therapy. :-)

    I'm trying to decide right now whether I am going to handle wrist movement like that. The joint handling is a bit tricky. :-)
    September 1, 2010 11:19 PM