Wiring angle - an issue of which joint bends first.

For discussion of hand designs.
Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Thu Apr 30, 2015 11:25 am

I brought this issue up during a recent R&D hangout, but I felt it deserved to be fleshed out a bit.

One problem that I have with our hands is that they bend the fingertips long before they bend at the palm. It's not just a little bit, if you hold a fingertip and try to keep it straight as you bend the hand, it will fight you. This is because there's no leverage on the phalanges. All of the force on the wire passes through the phalanges to the tie-off point on the fingertip.

This causes a problem for all of the fingers, but it is most obvious on the thumb. Here is a hand trying to hold a cup.

01 holding a cup.JPG
01 holding a cup.JPG (74.21 KiB) Viewed 1900 times


It can barely get around it. If you squeeze harder, you expect to get a better grip. Instead, the thumb curls inward, forcing the cup out of your grip entirely.

02 a tighter grip pushes the cup out.JPG
02 a tighter grip pushes the cup out.JPG (63.05 KiB) Viewed 1900 times


There are multiple issues here. One is that for an item this large, you don't really want the fingertip to bend at all. Another is that the gripper box provides adaptive grip between the fingers, but it won't redistribute the load on the thumb, even when you might want it to bend a different amount than the fingers.

The issue that I'm looking at now is that the fingertips and thumbtip have a massive amount of leverage pulling them down before the joint at the palm moves at all. Skip suggested that the angle of the wires could help with this problem. To test this, I made a little spacer that I could add to the hand.

03 A little spacer bar added in.JPG
03 A little spacer bar added in.JPG (71.21 KiB) Viewed 1900 times


With this, the wire is pulling on the phalanges from the bottom, instead of the top. That's the direction that we want in order to force it to go down. There are little feat on the sides of my spacer to hold it onto the palm, but the bar that the wire goes around is flush with the bottom of the hand. This is equivalent to adding a little bar to the palm, it's just much easier to print, and I can add or remove it easily for testing.

The first test was to see if the palm joint bends any sooner with the spacer than without. Here it is without the spacer:

04 bending without the spacer.JPG
04 bending without the spacer.JPG (67.79 KiB) Viewed 1900 times


and with it:

05 with spacer, no difference.JPG
05 with spacer, no difference.JPG (63.06 KiB) Viewed 1900 times


There's no difference. When I pulled on the fingertip as I bent the hand, it felt a little easier to keep it straight, but not a whole lot. Bending the fingers around objects, even smaller objects, wasn't improved at all. The fingertip bends first, then stays that way.

The next improvised test was to flip the spacer over, massively increasing the angle of the wire against the phalanges.

06 extreme spacer helps.JPG
06 extreme spacer helps.JPG (65.38 KiB) Viewed 1900 times


I can't show you any pictures of bending the hand using this arrangement because holding the spacer in place and using the hand took too many hands. However, this finally got some results. It needed an angle of about 45 degrees to get enough leverage against the phalanges to pull it down.

Getting that much angle into a palm will be tricky. Now that I've tested it, I think a combination of two things will get enough angle. 1) Printing out a palm with a little bar between the palm and the phalanges. The wire can loop around this and pull down on the phalanges. 2) Changing the geometry of the phalanges. Instead of having an open groove at the bottom of the base, I'm going to partially close it off. Not all the way, I've tried that before because I liked the look of it, and it makes getting wires through the groove difficult. However, closing if off about halfway up the channel should give the wire something to pull against much closer to the palm. The combination of those two things should give us the 45 degree angle that will close the phalanges much sooner.

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Fri May 01, 2015 10:15 am

Well, that didn't go as I imagined.

Here's what I tested.

DSCF3304.JPG
DSCF3304.JPG (55.8 KiB) Viewed 1890 times


As you can see I built a little bar into the palm, right under where the thumb is attached. I cut that joint off the model so that I didn't have to print a whole palm to test it. This works pretty well, I get a finger sticking out of a little plate. I just pull the string at the back, and the finger closes.

I also closed off the gap on the bottom of the phalanges. The idea was that the angle would be increased, forcing the phalanges down sooner.

The idea didn't work. In fact, it backfired. The angle forces the wire to bind against that rod, making it harder for the phalanges to close, not easier. Just to be sure I tried another version where the phalanges was completely closed off. That was even worse.

So I decided to try it with a regular phalanges, but keep the wire routed around the little bar.

DSCF3307.JPG
DSCF3307.JPG (40.32 KiB) Viewed 1890 times


That actually provides decent results.

DSCF3305.JPG
DSCF3305.JPG (35.24 KiB) Viewed 1890 times


It's not as good as I'd like, but the phalanges is closing sooner. I really wish I could get the phalanges to do all the closing until it hits resistance, then close the tip, but this might be the best I can do.

Next I'm going to work on the fingers. They might not work as well. The thumb is actually raised above the flat of the palm a bit so it can sit at an angle. The fingers are more flush. So I might not be able to get a good enough angle. I'm going to try lengthening the palm just a little bit to give a little more room for a bar. 1 or 2 milimeters. We'll see if that helps or hurts.

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Fri May 01, 2015 2:53 pm

I managed to take a good shot comparing the regular wiring to the wiring with more leverage.

DSCF3310.JPG
DSCF3310.JPG (45.79 KiB) Viewed 1889 times

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Fri May 01, 2015 4:14 pm

I think I've got it!

DSCF3312.JPG
DSCF3312.JPG (53.17 KiB) Viewed 1888 times


Trouble and I both had the same idea, using slightly stronger dental elastic bands on the fingertip than on the phalanges. I didn't think that would be nearly enough, since there's a lot of leverage working against it.

Well, I got new rubber bands today, slightly longer rubber bands than what I already had. So I tried it on my test hand. It did help a little, although not entirely. So I put the two different sizes on my test finger, the one that has the bar for more leverage, and it worked! It looks like the combination of more leverage and slightly stronger bands on the phalangi works!

The fingertip stays straight until the phalanges hits something, then it starts turning too. It's only a small difference, so when I let up the pressure, both joints unfold together. That's exactly the behavior I was looking for.

I'm printing out a test rig for the fingers now. They might not get as much leverage as well as the thumb, but they should get some improvement. Once I get the details worked out, I'll make a whole hand with this system.

I think this is going to provide a very noticeable improvement in the usability of the hand.

droomurray
Posts: 69
Joined: Mon Feb 16, 2015 4:04 pm

Re: Wiring angle - an issue of which joint bends first.

Postby droomurray » Fri May 01, 2015 7:15 pm

Jason,

Good stuff, so is that the lighter band on the end of the finger and the thicker one near the palm ?

Drew.

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Fri May 01, 2015 11:48 pm

Other way around. The slightly shorter band is on the fingertip. That pulls it tighter, keeping it straight while the joint at the palm is bending.

Jason M Bryant
Posts: 198
Joined: Mon Feb 16, 2015 1:59 am

Re: Wiring angle - an issue of which joint bends first.

Postby Jason M Bryant » Tue May 12, 2015 8:56 am

Here's the video that I posted on the Google Group.

https://youtu.be/LAvAC101WXU


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