Research Papers

Performance Evaluation of a Planar 3DOF Robotic Exoskeleton for Motor Assessment

[+] Author and Article Information
Stephen J. Ball

Department of Electrical and Computer Engineering, Queen’s University, Kingston, ON, K7L 3N6, Canadasball@biomed.queensu.ca

Ian E. Brown

Department of Anatomy and Cell Biology, Centre for Neuroscience Studies, Queen’s University, Kingston, ON, K7L 3N6, Canadaianbrown@biomed.queensu.ca

Stephen H. Scott1

Department of Anatomy and Cell Biology, and Department of Medicine, Centre for Neuroscience Studies, Queen’s University, Kingston, ON, K7L 3N6, Canadasteve@biomed.queensu.ca


Corresponding author.

J. Med. Devices 3(2), 021002 (May 27, 2009) (12 pages) doi:10.1115/1.3131727 History: Received November 13, 2008; Revised April 04, 2009; Published May 27, 2009

A new planar robotic exoskeleton for upper-limb motor assessment has been developed. It provides independent control of a user’s shoulder, elbow, and wrist joints in the horizontal plane. The lightweight backdriveable robot is based on a novel cable-driven curved track and carriage system that enables the entire mechanism to be located underneath the user’s arm. It has been designed to extend the assessment capabilities of an existing planar robotic exoskeleton. This paper presents the design and performance of the new robot.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

CAD drawings of Planar MEDARM: (a) setup with a user, (b) top view, and (c) side view. Actuation cables are not shown for clarity.

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Figure 2

Design (a) offers no way to maintain cable length when adjusting the link lengths, provides support only at the shoulder joint axis, and interferes with the user’s body. (b) The passive triangular linkages permit adjustment of the limb segment lengths (in gray with arrows) without the cables falling off. (c) The curved track and carriage underneath the arm ensure that the exoskeleton and the user’s limb are well-supported. The virtual four-bar linkage (dashed lines) that allows all equipment to be moved away from the user. (d) A CAD drawing of the final mechanism is shown without cables for clarity. The center of mass of the exoskeleton is indicated.

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Figure 3

Top view schematics of (a) Planar MEDARM and (b) KINARM. Planar MEDARM provides planar 3DOF motion (shoulder, elbow, and wrist), and is driven by a joint that is offset from the shoulder joint axis and that is part of a virtual four-bar linkage. KINARM provides planar 2DOF motion (shoulder and elbow) and is driven directly through the shoulder joint axis, where the elbow is driven by a four-bar linkage (parallelogram). The shoulder joint axis is the only support point for KINARM, while Planar MEDARM is supported by its curved track.

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Figure 4

A labeled schematic of the virtual four-bar linkage. θo=−20 deg, lo=0.125 m, lsd=0.20 m, and lg=0.25 m. lu is the length of the user’s upper arm, and is a fixed value between 0.26m and 0.37m. θsd is the shoulder driving joint angle, θs is the shoulder joint angle, and θg1 is the guide pulley angle. τsd is the torque applied at the shoulder driving joint, and τs is the torque that appears at the shoulder joint. Positive angle is defined as counterclockwise from the x-axis.

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Figure 5

(a) A side view of the exoskeleton showing the segment length adjustments for the exoskeleton. The labels are as follows: UL—upper-arm length, FL—forearm length, UT—upper-arm trough, FT—forearm trough, and H=handle. (b) A close-up view of the carriage (opposite side view dashed rectangle in (a)), showing the quick-release clamp for the upper-arm length adjustment.

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Figure 6

(a) A CAD drawing and (b) a simplified planar schematic representation of the original cable routing structure. Each of the four cables is denoted by a different line type. Symbols s, ξ, r, τ, and θ represent cable displacement, cable force, pulley radius, joint torque, and joint angle, respectively.

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Figure 7

A photo of the fully constructed Planar MEDARM prototype in its original configuration. The view was chosen to match the CAD view from Fig. 1.

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Figure 8

(a) Planar MEDARM’s virtual display includes a basic structure that holds a semitransparent mirror and a set of LED lights above the workspace. (b) The mirror is placed equidistant from the working plane of the robot and the LEDs so that the user can see their limb and an image of the LED in the same plane. This system allows unrestricted access to the workspace. (c) The visual display system was used to present the subject with a reaching task which involved making movements from the center target (black circle) to one of the eight outlying targets (hollow circle, 10 cm distance).

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Figure 9

Planar schematic representation of (a) the alternate cable routing structure of system c and (b) the 2DOF structure of system e. In (a), there are four cables at the shoulder, three at the elbow, and two at the wrist. In (b), there are three cables at the shoulder, two at the elbow, and zero at the wrist (free to rotate).

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Figure 10

Overall (a) joint friction, (b) inertia as seen by the joints, and (c) compliance at the joints for all variants of the prototype. In (a), the dark and light gray bars for the shoulder joint indicate the total measured friction with the original and new carriage designs, respectively. In (b), the dashed lines represent the estimated inertia of limbs corresponding to the minimum and maximum size of Planar MEDARM. The bottom and top of the bars indicate the estimated inertia of the exoskeleton when set up for the smallest and largest arms, respectively. In (c), the total height of the bars indicates the total measured compliance of the exoskeleton. The light portion of the bars indicates the estimated contribution of the cables to the measured compliance.

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Figure 11

Results of the vertical compliance test, comparing Planar MEDARM with KINARM

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Figure 12

Results of a basic reaching experiment using the Planar MEDARM. (a) The hand path traces for all trials are plotted. (b) The joint angle profile for one of the trials to target 1.

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Figure 13

Results of the viscous load stability tests, comparing Planar MEDARM with KINARM. The highest velocity feedback gain that can be applied to each joint for each configuration using the velocity signal provided by (a) the motor encoders and (b) the secondary encoders.




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