Design and Implementation of a Prosthetic Hand

It's been a few tough weeks. But, it is time to retake this. I am not into building stuff in 3D printing or anything like this. So, it is going to be kind of hard for me to build the hand.

What i am already thinking is on applying machine learning to the hand. I can teach it to move or react to external stimulus. But, for this i will need sensors. Pressure sensors should do the trick.

For now, my journey on building this starts. I have to train the NN in my head to use solidworks good enough to replicate what i've seen on the antecedents and to copy the hand's anatomy.

1. Introduction

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.

Video 2. Flexor Tendon Anatomy

Video 3. Wrist Joint Movements

Video 4. In case you still wonder

how the fingers move

1. Introduction

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.

2. Antecedents

2.1 OpenBionics

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:

2.2 Biohand

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.

2.3 Dextra

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).

4. Discussion

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, for what i've read the devices rely on linear actuators that pull polymer strings and they just work as human tendons. The process seems to be kind of simple, pull the tendon and the finger should contract (this is called flexion in medicine).

Let's move to the hardware, i've seen that the hardware used was simple. Basic microcontrollers or microprocessors (as in Biohand) and motor drivers. This suggests that the hardware solution doesn't imply high processing requirements. Of course, limiting the scenario to a prosthesis that only enables movement without any kind of motor feedback or any complicated movement. In this sense, i think that my first designs will limit to the use of basic hardware.

Now, let's talk about the software. As in the hardware, this is simple. A short script to control the motors through their drivers. I've even seen people using Arduino for this task.

5. Conclusions

• The devices reviewed in this log depict that a functional device can be built up using basic hardware and software. The challenging part is the mechanical design, so that the prosthetic device can provide enough grasping and moving functionalities.
• The lack of knowledge about the mechanical design seems to be a problem for someone like me. I will have to read a lot to understand this topic. Perhaps, i will join forces with a mechanical or mechatronics engineer.
• There is a trade-off between functionality and cost of a prosthetic device. If we take talk about the materials that are being used for the construction of this devices then the use of rapid prototyping techniques (such as FDM) allows the development of low cost devices. If we talk about actuators, then the scenario is kind of different because actuators are not that cheap (specially linear actuators). If we talk about the embedded system that controls the device, then, the hardware and software used are incredibly cheap and workful nowadays.
• The power consumption of these devices seems to be a mistery. For what i've experienced, it will be as a function of the actuators. When working in the automation of an optical microscope i convinced myself that step motors consume a lot of current (the ones that we were using consumed a 27A battery in two hours). In this sense, i am worried about the functionality of the device when working alone. Adding a 27A battery means that the device will have to be bigger due to the size of current batteries. Nevertheless, i should not worry about this yet because my first prototypes will work next to a power source.
• The need of this kind of prosthesis seems to be in a high demand. I like this, because this will encourage many other people to build this stuff.