FRAME
Albert Tai
The exoskeleton frame attaches to the wearer’s arm through a series of straps with internal padding to conform to the user’s shape. The shoulder and elbow rigidly define the axis that the exoskeleton arm’s linkages pivot about. The dimensions of the frame’s linkages were first determined from a conglomerate of male and female average arm dimensions (approximately 13” from the shoulder joint to elbow joint and 10” from the elbow to the wrist) and then fine-tuned with a physical prototype model which was worn for extended periods of time in order to assess the comfort of the frame. Because the elbow joint is not directly attached to the shoulder joint, the frame can be adjusted to fit upper arm lengths 7.5” and longer and forearm lengths 9” and longer. The Frame consist of the Back Mount, Forearm Frame, Elbow Joint, and Shoulder Joint.
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SHOULDER JOINT
The shoulder joint allows for two degrees of freedom of motion, however only one of these degrees (raising the arm up and down) is powered. This limitation, primarily imposed by the weight of an extra motor and the scope of the project. These active and passive degrees of freedom were selected by considering which degrees of freedom were most essential for lifting. The shoulder joint consists of an outer “slider” race (which the upper arm and forearm linkages are attached to) and an inner “rail” race (which is mounted to the user’s back). These races are constrained to each other via steel balls which sit in grooves and sockets in the races. The V-shaped groove in the rail was chosen in order to minimize contact area, and thuspossible sources of friction, between the two shoulder races. For motor control feedback,a rotary encoder in series with the non-powered axis detects the angle at which the armis passively abducted from the body, while an electronic three-axis gyroscope (AdafruitLSM6DS33) detects the angle at which the arm is actively raised. Steel end stops boltedonto the shoulder frame assembly allow the arm to be abducted 45 degrees relative to theneutral position, while the grooved rail allows for the arm to be raised 90 degrees.
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ELBOW JOINT
The elbow joint allows for one degree of freedom. It is sized for a bicep approximately 3.5 inches in diameter and a forearm approximately 3 inches in diameter at its maximum girth. As previously stated, the elbow joint is not rigidly connected to the shoulder joint to accommodate a wide variety of upper arm lengths. A rotary encoder mounted in series with the elbow joint axis detects the angle at which the forearm is actuated. Steel end stops bolted onto the elbow frame assembly allow the elbow to be bent 75 degrees relative to the neutral position.
FOREARM FRAME
A hinged frame locked by two latches encloses the user’s forearm. The forearm frame can accommodate a wrist of approximately 2.5 inches in diameter at its maximum girth. A 1”pulley mounted on the forearm frame aids in reducing the angle of the elbow-flexing bowden cable relative to the cable sheath, which aids in power transmission and reduces friction between the cable and sheath.
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BACK MOUNT
The arm’s shoulder joint is mounted to a back harness which can be strapped to the user’s back. This back mount consists of three plates (separated to conform to the contours of the user’s back) which the shoulder and elbow motor assemblies are attached to. These three plates are mounted onto a pad, with straps, so that the harness can be worn like a backpack. A waterproof electronics enclosure, which will contain the micro controller and drivers used to control the motors will also be mounted to one of the back mount plates.
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