Building a Better Robotic Hand

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.

Everything is in place

Robotic Hand Version 2.

As you can see, it has a lot of potential as a real working hand. For productive, creative work……..

Holding a Glue Gun

Holding a Glue Gun

but like most other machines that mankind has built, it can be turned to more sinister applications as well…….

The beginnings of Skynet!!

The beginnings of Skynet!!

Objectives.

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.

Construction.

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.

Junk from around the house

Junk from around the house. Curtain springs, fuel pipe, hot glue

Curtain wire springs are flexible and strong

Curtain wire springs are flexible and strong

Three pieces of curtain wire arranged as shown give flexibility to each finger while keeping its foundation rigidly attached to the palm.

Finger - Step 1. Form the finger

Finger – Step 1. Form the finger

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.

Plastic pipe from a motorcycle's fuel line

Building the fingers

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.

Hot glue, plastic pipe, thread and a little careful moulding

Hot glue, plastic pipe, thread and lollipop handles

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.

Thumb servo horn construction

Thumb servo horn construction

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 fingers stcuk to the table with bluetack look like they can hold onto something

The fingers stuck to the table with blu-tack look like they can hold onto something

The design looks like it can grip this ball quite nicely.

This ball looks like it could be gripped nicely

Ball gripping test

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.

I settled on a wooden spoon from the Sunday Market.

I settled on a wooden spoon from the Sunday Market.

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.

Tendons pass through the fingers and finally to the servo motors.

Tendons pass through the fingers and finally to the servo motors.

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

Thumb servos joined together

Thumb servos joined together

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!!

Modified positioning of thumb servos

Modified positioning of thumb servos

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.

Everything is in place

Everything is in place

Testing

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.

Advertisements

2 thoughts on “Building a Better Robotic Hand

  1. Pingback: DIY Haptic Control Glove | Bayesian Adventures

  2. Pingback: Haptic Controlled Robot Hand | Bayesian Adventures

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s