The word robot comes from the Polish word ‘robota’ meaning forced labour. In Russia, robota means just work, employment or operation. Funny, I’ve spent nearly two years in Russia and have probably spoken this word many many times, never really realising that it is also the root word for robot!
Anyway, this post is a photo-essay/tutorial on how I built my new robotic hand.
As you can see, it has a lot of potential as a real working hand. For productive, creative work……..
but like most other machines that mankind has built, it can be turned to more sinister applications as well…….
1. Must be built from everyday items lying around the house. No expensive stuff like 3D printing, CNC work etc etc. The only commercially available components will be SG-90 mini servo motors and of course, the amazing Arduino electronics prototyping platform!
2. Must have a working opposable thumb
3. Costs to be kept to an absolute minimum. Less than US $ 20 if possible. I think the final cost came to about US $ 18 in the end.
I used curtain wire springs (strong and very flexible), fuel pipe from my motorcycle, hot glue and thread to construct the fingers. I also used some Sugru to secure the servos to the palm. Sugru is perfect for this sort of work since it forms a strong bond and is flexible as well, taking up some of the shock of moving servos under strain.
Three pieces of curtain wire arranged as shown give flexibility to each finger while keeping its foundation rigidly attached to the palm.
Hot glue is moulded to form the finger. thread and pieces of plastic lollipop sticks add bulk to each finger joint. Notice the fuel pipe (dark yellow) in each joint that will allow the black fishing line ‘tendon’ to pass through each joint.
Here is a finger showing the detail of the hot glue, fuel piping and you can see the upper left joint has been made wider using two pieces of a lollipop stick.
I decided to fix the thumb directly onto its servo motor. this would keep the design much simpler than having the thumb operated with tendons. Here you can see how I fixed the thumb servo horn to its base. Again, hot glue, lollipop sticks and thread.
When all the fingers and the thumb were completed, I stuck them on my tabletop with blutack to see the best way in which I could position them onto the palm.
The design looks like it can grip this ball quite nicely.
Now I needed to make a palm for the hand. After many trials using all sorts of items, I finally settled on this wooden rice spoon I bought at the Sunday Market for the equivalent of about about 25 US cents. It has a nice wide, slightly curving face, a strong handle and most importantly, it was very very cheap.
After re-aligning the fingers onto the palm with blu-tack, I fixed them permanently in place using screws and a sawed off Mechano piece to hold everything securely in place. If you look closely at the base each finger, you can even see the lollipop sticks that will act as conduits for the tendons. The length of each of these lollipop conduits has been carefully chosen (trial and error method!) to allow the finger to flex correctly. A little hot glue was applied wherever needed to prevent chafing of the tendons as they were flexed.
The movement of a human thumb is very very complicated and difficult to reproduce by machines. I settled for a simple 2 axis movement that can get most things done satisfactorily.
Axis 1 (the lower servo on the right) rotates the thumb and achieves ‘opposition’
Axis 2 (the upper servo on the left) flexes the thumb
After experimenting a little (you can see the details in this video), I decided to mount the thumb rotation servo at a slight angle to the palm. The movement seemed to be much more natural in this configuration. The servos were held in place by Sugru. I initially cut that large notch in the palm to allow the thumb to rotate freely but I realised later that it wasn’t need it at all. Poor workmanship!!
This last picture shows everything in place. I fashioned a thumb guard out of thermoplastic to protect the thumb servos. I think it will also help in gripping objects as well. The other four servos have also been fixed in place, first with wood glue and then a layer of Sugru for good measure. In this picture, the tendons can be seen very clearly running along the fingers and through conduits under the thumb guard until they terminate at their respective servo horns.
Initial operations of this robotic hand can be seen in the youtube video below. Once I complete the software for the opposable thumb operation, I will do another post on thumb testing and grip testing.