User-Independent Intent Recognition for Lower Limb Prostheses Using Depth Sensing

Objective: The intent recognizers of advanced lower limb prostheses utilize mechanical sensors on the prosthesis and/or electromyographic measurements from the residual limb. Besides the delay caused by these signals, such systems require user-specific databases to train the recognizers. In this paper, our objective is the development and validation of a user-independent intent recognition framework utilizing depth sensing. Methods: We collected a depth image dataset from 12 healthy subjects engaging in a variety of routine activities. After filtering the depth images, we extracted simple features employing a recursive strategy. The feature vectors were classified using a support vector machine. For robust activity mode switching, we implemented a voting filter scheme. Results: The model selection showed that the support vector machine classifier with no dimension reduction has the highest classification accuracy. Specifically, it reached 94.1% accuracy on the testing data from four subjects. We also observed a positive trend in the accuracy of classifiers trained with data from increasing the number of subjects. Activity mode switching using a voting filter detected 732 out of 778 activity mode transitions of the four users while initiating 70 erroneous transitions during steady-state activities. Conclusion: The intent recognizer trained on multiple subjects can be used for any other subject, providing a promising solution for supervisory control of powered lower limb prostheses. Significance: A user-independent intent recognition framework has the potential to decrease or eliminate the time required for extensive data collection regiments for intent recognizer training. This could accelerate the introduction of robotic lower limb prostheses to the market.