February 19, 2019

Motorized Prosthetic Hand


Enabling the Future (e-NABLE), a global network of volunteers, was founded to design and 3D-print prosthetics for people in need all over the world. These prosthetics are often purely mechanical and doesn’t contain any electric parts at all, such as in our project The Hand for Diego.

The purpose of this project was to take a new approach to the design and construction of low-cost motorized prosthetic hands. By distancing all the electronic components from the hand, including the motor, the project aimed to create a device compatible with all current designs of the e-NABLE community. This project was carried out as a Bachelor thesis in mechatronics, for the Department of Machine Design at KTH Royal Institute of Technology, in collaboration with Assistive Technology at KTH.


To conceptualize the idea a demonstrator was constructed and tested. A muscle sensor was used allowing the user to control the hand by tightening his or her muscles. The distance between the electronic components and the prosthetic hand measured approximately one and a half meters and enough force was transferred to close the hand with an adequate grip strength. The final product could never be tested on a person with a real hand loss.













The entire demonstrator. Note that the electronics isn’t connected correctly in this picture. For a full circuit scheme and additional information see Bachelor thesis report.

When the muscle sensor detects enough muscle contractility the hand changes its state from open to closed or vice versa. In the picture, all electronics used in the final demonstrator has been numbered.
1) Power supply
2) Motor
3) Micro switch
4) Motor driver
5) Microcontroller
6) Muscle sensor












The control box.

A control box, preferably placed in a backpack, was built consisting of two floors. Floor 1 contained a bobbin, attached on the motor shaft, which rolled a fishing line. The fishing line was connected to the Bowden cable through a clamp grip. To close the hand the fishing line is rolled up on the bobbin.











The micro switch is open when the hand is closed and closed when the hand is open.

In order to keep track of the hands current position a micro switch was installed. The micro switch sends a signal to the micro controller when the hand is entirely open.

The Process










All parts for the arm and the control box were 3D printed.










A set of motorized tensioner system prototypes were created to test the
concept. The prototype seen in the picture used a stepper motor. This motor was too
weak and slow in order to work properly.












Different whippletree configurations were tested to distribute the force from
the motor, along the Bowden cable, between each finger correctly. The configuration
in the picture, with two whippletrees, was later replaced with a single whippletree due
to lower resistance in the system.






A dummy arm was constructed to illustrate how a real arm could be placed.


Bachelor thesis report