I finally found an excuse to build a 3D printed hand. Luckily, a final project from my university demands me to build one of these. I am going to log everything on Hackaday because this page is awesome.
Research suggests that +30 million people require a prosthetic counting the regions of Latin America, Africa and Asia.
I am not going to describe all the anatomy of the hand because it has a lot of names that describe each of the bones and muscles. I think that the key elements to understand are the muscles, ligaments and joints because understanding them allow the analogy with a mechanical system. Just what i was looking to comprehend.
So, i am linking the following videos that depict with detail the anatomy of the hand, the kinematics of the hand and the finger movement anatomy.
The Hackaday page has lots of projects that describe how to make prosthesis. Actually, there is a wide set of projects. Some that replicate a human hand, others that replicate a human leg and even some exoskeletons to enhance human movement. So, in order to design my own prosthesis i am going to make a summary of the "Open Source" devices i consider to be the best.
I think this the best project in the "Open Source" bionic hand field. The novel design they have modeled allows adaptive grips, great movility and very low cost. They have a great tutorial on how to build his device.
They basically use linear actuators to pull tendons (made of string) and, thus, move the fingers. Here's a video that shows their device:
Biohand seems to be a very accurate design. As his video show:
The thesis demonstrates that hacking commercial RC servomotors can be achieved and with his design it is possible to create linear actuators. Therefore, with those tools he was able to simulate tendons that move fingers in a human way. His design also included a robotic thumb. It was built using a servomotor to simulate abduction/adduction and the same linear actuar to simulate the flexion/extension of the finger.
The cost of the device is 400$, very cheap for such a functional prototype. As i read in the thesis and can be demonstrated by his videos, the user has enough functionality to perform some graspings and movements. This would allow the user the ability to perform some daily tasks such as getting dressed, eating, drinking and opening doors.
Dextra is an amazing device also hosted here on Hackaday. This is a very representative video:
The design uses linear actuators to pull a tendon made of string. As he claims, this design would allow the user to perform adaptive grips. What i like about this design is that it is more compact and seems to require less power. The problem with Biohand was the use of rigid phalanges which limits the biomechanics of the device. Apart from that, the mechanical design is almost the same as the other prototypes.
3. Devices' Summary
After reading about the previous models, i decided to make a comparative table to summarize the following features: material, technique of construction, grasping functionality, movement functionality, DOFs and cost. The grasping and movement features will be scored using three parameters (low, medium, high).
Technique of Construction
Degrees of Freedom
Okay, so i checked all the devices that i found to be the most useful. I think they all present a trade-off between cost and functionality. Therefore, this feature gives me a good intuition about what i should do.
For what i've seen, there is missing information about the human hand anatomy. I think that making a log about this is necessary to summarize useful information that can be used for the design. It's just like biomimetics.
Other important feature i consider important is weight, i really liked Biohand 's discussion about the weight and size of current prosthesis. I agree with him in the fact that actual prosthesis might be heavy for users as a friend that has one claims. I think that user comfort is important, but it is better to sacrifice comfort for functionality. Eventually materials will become lighter, therefore, it's better to trade this feature. About the size, i think that personalized designs according to human proportions are adequate. That means 0.6% of the total mass of the body for men and 0.5% of the total mass of the body for women.
Now, i want to talk about the mechanical designs that i've seen in all the devices. Unfortunately, i am not a mechatronics or a mechanical engineer. In this sense, i have almost no experience with mechanisms or actuators. Nonetheless,...