【Prosthetic Hands】14 Third Generation - Hardware Design

Q1: Where should the active DoFs be? ...more importantly, How many active DoFs do I need?

In the second generation, I reduced the number of actuators to fit the larger, more powerful actuators inside a shorter forearm. From a practical perspective, the reduction was indeed reasonable, and here is the reason. The number of motions that a prosthetic hand can perform in real-life situation is limited by the number of available control signals, which is the number of recognition classes in this case. Therefore, instead of merely pursuing the motion diversity of the prosthetic hand, it is more practical that we find the minimum number of active DoFs that a hand needs to accomplish most daily grasping tasks.

I referred to the experiment results [1] and decided to give my third generation prosthetic hand 6 active DoFs. Here I synchronized the motion the ring and little fingers because in most of the grasping motion, the independent motion of little finger is very rare. In addition, I reserved the active DoF of the index PIP joint because I found that for some motions in daily life, such as typing on keyboard or scrolling on smartphone, this active DoF is necessary. 

Q2: Now I have got the numbers of the actuators. Which actuators are more suitable for my need? How should I fit them inside the small hand?

The problem of the actuators used in the first generation was their strength, whereas the actuators used in the second generation were too large. Fortunately, since the actuators I used in the previous generations were very cheap (<10 USD each), it was quite easy for me to find small actuators with enough output torque. I utilized many pieces of PTFE tube, whose self‑lubricating feature could significantly help me overcome the resistance of the wire channels. These tubes made it possible for me to design the hand more compactly because they allow slight twist in the wire path.

The final design and the dimension are shown below.

Here you can see a handler, this handler is for the able-bodied people to hold the device for experiments. In addition, there is an empty place at the bottom of the hand (roughly 4 cm). This space is for the implementation of the prosthesis socket or extra sensors. 

I will write another article about the circuit design and the micro-controller I used. In the next article, I am going to talk about the EMG acquisition, feature extraction and the classification. (I think I might have to separate the content into three article...) Thank you for reading this article! Feel free to leave your comments.

[1] I. M. Bullock, J. Z. Zheng, S. De La Rosa, C. Guertler and A. M. Dollar, "Grasp Frequency and Usage in Daily Household and Machine Shop Tasks," in IEEE Transactions on Haptics, vol. 6, no. 3, pp. 296-308, July-Sept. 2013, doi: 10.1109/TOH.2013.6.