Reflex: A Closed-Loop Tactile Feedback System for Use in Upper Limb Prosthesis Grip Control
Osborn, Luke Emmett
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Tactile sensing provides valuable insight to the environment in which we interact with. Upper limb amputees lack the sensations that generates the necessary information to stably grasp the wide variety of objects we interact with on a daily basis. Utilizing tactile sensing to provide feedback to a prosthetic hand provides a mechanism for replacing the grip control functionality of the mechanoreceptors found in human skin. Novel customizable, low cost tactile sensors for monitoring the dynamics of an object grasped by a prosthetic hand are developed and presented as part of this thesis. The response of sensors placed on a prosthetic hand provides information regarding the state of a grasped object, particularly contact and slip. The sensors are made up of various textile materials, including stretchable interfacing layers and conductive traces. Essentially a force sensitive resistor, each sensor is shaped into stretchable cu ff that can be placed around the finger of a prosthetic hand. An outer rubber layer on the sensor provides compliance, which is found to enhance grasping performance with a prosthesis. Two control algorithms were developed as part of the closed-loop tactile feedback system, called Reflex, to enhance grasping functionality with a prosthesis. A Contact Detection strategy uses force information to effectively reduce the user's electromyography (EMG) signals, which are used to control the prosthesis. Essentially, the goal of this strategy is to help a user grab fragile objects without breaking them. A second strategy, Slip Prevention, uses the derivative of a force signal to detect slip of a grasped object. Instances of slip trigger electrical pulses sent from the prosthesis control unit to close the hand in an effort to prevent additional slip. The Reflex system, comprised of two control strategies along with flexible textile based force sensors on the fingers of a prosthesis, was shown to improve the grasping functionality of a prosthesis under normal use conditions. Able body participants were used to test the system. Results show the sensors' ability to greatly enhance grasping fragile objects while also helping prevent object slip. The compliant nature of the sensors enables users to more confidently pick up and move small,fragile objects, such as foam peanuts and crackers. Without sensors and tactile feedback, users had a higher likelihood of breaking objects while grabbing them. The addition of sensors reduced this failure rate, and the failure rate was reduced even further with the implementation of control algorithms running in real-time. The slip prevention strategy was also shown to help reduce the amount of object movement after a grasp is initiated, although the most benefit comes from the compliant nature of the sensors. Reflex is the first closed-loop tactile feedback system with multiple control strategies that can be used on a prosthetic hand to enhance grasping functionality. The system allows one to switch between Contact Detection or Slip Prevention control strategies, giving the user the ability to use each control as needed. Feedback from the textile sensors directly to the prosthesis control unit provides valuable information regarding grasping forces. This research aims to help improve prosthetic technology so that one day amputees will feel as if their device is a natural extension of their body.