Finally got round to building some body parts for my android. Hands are the hardest by far.
They are visible so need to look the part , joke shop android flesh hand or sci fi B move robot just sucks big time style-wise.
Gloves are ok , or goodlooking terminator style works well. Hobby kit robo claws are limited on functionality & servos are to bulky and noisy. Which ever way you go you seem to be stuck between a rock and a hard place.
Great Demo of a draw string hand. It don't get any easier than this +1 to who ever originated this ideaRobot Hand
Anyway , after a days hard craft and hours researching on the net this is what Ive knocked up so far.
I decided on using pull string, the pullers for the hand will be mounted in near the elbow. Plenty of room for powerful motors/servos/why and it keeps the weight off the end of the armBIG TIP
, the pull chords have to pass exactly through the wrist joint. (that is the joint between the arm and the wrist) a few mm either side and as the fingers close it will make the articulated wrist snap up or down resulting in instant spasticicato concertino
Big plus with this method is the hands close fast, i mean real fast
faster than a human hand could. Down side is the torque is pretty low. Interestingly the last bit of the finger(finger tip joint) is near impossible to prize open when its curled up, needing at about 10kg of force until the pull on the chord is released. shame it doesn't have much closing strength. This lack of grip is big problem with almost every hand design you can think off.
Its possible to close the finger with 1 cord (resulting in death lock grip of the finger tip as just mentioned) however the next two parts unfurl pretty easy and lack closing grip...but not speed !
so im thing of using 3 chords for each finger, 1 running past all 3 segments for speed , 1 running past the two closest to the wrist to give the second joint some strength & 1 for the first joint to give that some strength too.
The pic shows the little finger. My android is gonna be close to 7ft tall as the volume available (and work room ) goes up quickly when you increase the size. It will also help with dexterity due to the long reach, and halve the visual processing time, you get a realy clean view with good perspective lines for judging distance when you are looking down on things. It also helps to see over people heads , and get a fix on walls and objects further away. If you ever been at a concert or in a realy busy room you know how frustrating it is to see or know whats going on.
Im gonna have to come up with something a bit special at the elbow joint, i will have to include wrist rotation in the first few inches of the forearm (whole forearm will rotate) I hate the bulky wrist look you often see with , this design has a very slim wrist and teh joint allows for almost 180 degrees up and down flip of the wrist for very realistic hand movements.
Human hand Movement = approx
wrist + 60 - 80
1st finger bone +30 -90
2nd finger bone -0 + 120
3rd finger bone -5 +90
One thing that was a real pain was the stop pins. Pins that stop the fingers or wrist bending to far backwards or forwards. Hence all the screws. The chords are attached to the lower stop pins, nor perfect but works well enough. I barley have a a mm to spare in each direction, was a minor miracle it works at all. drilling done by eye and feel not a ruler in sight lol
The only thing a single pull
chord design cant do is hold the fingers straight at 90 degrees to the wrist. all finger movement is a curl.
The forearm to finger tip is 22 inches , prolly be adding another 2inches at least with the elbow joint on & another 2 inches with the big finger (little finger shown in the pick below)