Gravity-Balanced Arm Support With Energy-Free Adjustment

[+] Author and Article Information
Wouter D. van Dorsser, Boudewijn M. Wisse

 InteSpring B.V., Rotterdamseweg 145, 2628 AL Delft, The Netherlands

Rogier Barents

Faculty of Mechanical, Maritime and Materials Engineering, Department of Biomechanical Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands; and  InteSpring B.V., Rotterdamseweg 145, 2628 AL Delft, The Netherlands

Just L. Herder

Faculty of Mechanical, Maritime and Materials Engineering, Department of Biomechanical Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlandsj.l.herder@3me.tudelft.nl

J. Med. Devices 1(2), 151-158 (Jan 29, 2007) (8 pages) doi:10.1115/1.2736400 History: Received April 01, 2006; Revised January 29, 2007

People with neuromuscular diseases have very limited muscle force. Many of them rely on mobile arm supports to move their arms. Most of these supports incorporate gravity balancers, i.e., spring-loaded mechanisms that achieve a constant total potential energy, thus eliminating any preferred position. The springs and the mechanism topology and dimensions are designed to exactly or approximately balance the weight of the user’s arm. Quasistatically, the mechanism, once statically balanced, can thus be moved virtually without operating energy. In case of change of effective arm weight, e.g., due to picking up an object or putting on a coat, the support mechanism should ideally be readjusted. In all available support mechanisms, this adjustment is associated with considerable mechanical effort, while clearly this application would benefit greatly from an energy-free adjustment. This paper will present an arm support that includes a novel design concept to adjust spring-based static balancers with no need for external energy. This concept will be explained, and several variants will be shown. Subsequently, the application of this concept in a mobile arm support will be described in detail, including preliminary clinical trial results.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Basic 1DOF gravity equilibrator: Mass m is perfectly statically balanced by using a zero-free-length spring in the arrangement shown

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Figure 2

Energy-free adjustment in a gravity equilibrator according to the simultaneous displacement concept: An auxiliary link CD is used to adjust distances a and r at the same time, thus changing the balancing settings without changing the spring energy. This version has an adjustment angle of 90deg.

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Figure 3

Other embodiment variants of gravity balancers using the simultaneous displacement concept: (a) Adjustment position at φ=180deg and (b) adjustment position at φ=0deg in which case the link CD (see Fig. 2) can reduce to a point

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Figure 4

Support force Fm (equal to mg) as a function of the adjustment of r for the versions with adjustment angles of φ=0deg, φ=90deg, and φ=180deg, respectively (spring stiffness k=1 and link length L=1000mm), where it is assumed that all three versions have the same maximum extension of the spring (in this example Δsmax=L) so as to obtain equal maximum support force

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Figure 5

Energy conservation in the long term explained: (a) Balancer picking up payload, (b) and moving it to a higher level, (c) adjusting to zero load and releasing the payload, (d) moving back down, (e) picking up next payload and adjusting to it, and (f) lifting second payload. From the length reduction of the spring between (a) and (e), it can be observed that this process is not energy free.

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Figure 6

Diagram of the SD concept demonstrator showing the adjustment mechanism in combination with the pulley-and-string mechanism used to emulate zero-free-length spring behavior. Pin P locks the vertical slider B during normal operation. If the moving link is placed in the vertical position, then slider A on the moving link connects with the vertical slider and presses on pin P to release the latter. Pin Q can then be retracted to release slider A, and the sliders can be moved as one block to adjust the SD balancer (see also Fig. 7).

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Figure 7

Photograph of the concept demonstrator, in a view corresponding with the diagram shown in Fig. 6. The inset shows more detail on the engagement of the sliders, the automatic release of pin P, and the manually operated pin Q, which is also used to move the sliders together to adjust the SD balancer.

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Figure 8

Commercially available nonpowered arm support Focal TOP HELP: (a) In the lowest position (θ=90deg), (b) in the highest position (θ=40deg), and (c) in use (θ≈45deg)

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Figure 9

Two possible versions of the SD adjustment mechanism, each shown with auxiliary link CD in the extreme positions: (a) range of motion of link CD approximately 0deg<ψ<45deg, and (b) range of motion of link CD approximately 45deg<ψ<90deg

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Figure 10

Simultaneous displacement concept as applied in the mobile arm support, which can be adjusted effortlessly at θ=90deg by adjusting handle H, which causes movement of link CD

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Figure 11

Schematic representation of the final prototype design, showing pulley and string arrangement

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Figure 12

Final prototype design, cross section of 3D computer model. The inset shows a photograph of the side view of the prototype for comparison. As compared to Fig. 1, the mechanism (link OE) has been extended with a parallelogram, so as to keep the end support in a horizontal orientation at all times.

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Figure 13

Impression of operating handle with additional slots 8–21

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Figure 14

Photographs of locking mechanism: (a) side view and (b) bottom view

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Figure 15

Locking mechanism, schematic representation of transverse cross section. Left: before engagement, pin Q locks slider A to link OE; Center: first contact; Right: adjustment position (ϕ=θ=π∕2), pin Q is forced inwards by wedge C and releases slider A from link OE.

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Figure 16

Measurement of support force Fm generated by the SD balancer in a laboratory setting. Note that the preexisting rubber springs (left figure) are not used.

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Figure 17

Support force versus handle position. Black diamonds: calculated values for the range available in the prototype; Gray diamonds: calculated values for the extension of the φ=90deg line in Fig. 4, showing that the force increments between the equidistant adjustment steps are smaller and much more evenly distributed than in the present prototype, especially from slot 12 through 21; white dots: Measured values as determined in the laboratory setting of Fig. 1.

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Figure 18

User with the prototype during exploratory field testing



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