Mechanical Electrogoniometer

Task

Design an affordable electrogoniometer that could be used on multiple joints with an accuracy of +- 2 degrees

For

Human Control and Dynamics Lab

Box Title

Fall 2014 – Spring 2015

Relevant Skills

Mechanical Design
Creo 3.0
Vicon Motion Capture
Arduino

About Project

Traditional electrogoniometers are suffer from various issues ranging from cost, to bulkiness, to accuracy. As part of my independent study through the Human Dynamics and Control Lab led by Dr. Elizabeth Hsiao Wecksler, I was tasked to develop an electrogoniometer that was not burdened by the above problems. The ultimate goal was to design a device that could be used to support the graduate student research in the lab. The applications ranged from measuring angle of the contralateral angle of a subject using the lab’s portable pneumatic ankle foot orthosis (PPAFO), to measuring wrist angles of subjects using redesigned lofstrand crutches. The device would allow testing to occur outside of the lab, in varying environments.

Design and Engineering

The device was developed using principles explained by Roduit et al. For a pair of strings with 1 anchored end and 1 free sliding end separated by a fixed distance, the angle formed by the ends of the pairs is directly related the difference in displacement of the free ends between the string pair.

The difference in displacements were measured using linear trimming potentiometers. The analog signals were converted to digital signals through an Arduino. These were logged into an SD card at a rate of 100hz.

The mechanical design comprised of 3D printed housings for the anchored ends of bronze cables and for the linear trimming potentiometers. The potentiometers were spring loaded to ensure they returned to their resting position when the device was not bent. To keep the distance between the two strings fixed, a flexible dual channel tube was 3D printed in rubber-like material. Initially, a medical Foley catheter was used. However, this proved to have created too much friction to allow the smooth sliding of the bronze cables

Testing and Validation

The device was verified against a manual goniometer and using Vicon motion capture systems.

In static testing, the system proved capable of measuring rotation in a single plane well if there was no torsional twisting in the device. Used in the appropriate manner, the device had about a 5 degree maximum error in its maximum range of -55 to +55 degrees. From -50 to +50 degrees, the device had a maximum error of 2.5 degrees. This put it within the 2 degree error of commercial systems that are more expensive.

In dynamic testing the system showed promise in accurately tracking angles with a systemic error being introduced. This might have been due to measurement inaccuracies that can be addressed in future tests.