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Finger Impedance Evaluation by Means of Hand Exoskeleton

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Abstract

Modulation of arm mechanical impedance is a fundamental aspect for interaction with the external environment and its regulation is essential for stability preservation during manipulation. Even though past research on human arm movements has suggested that models of human finger impedance would benefit the study of neural control mechanisms and the design of novel hand prostheses, relatively few studies have focused on finger and hand impedance. This article touches on the two main aspects of this research topic: first it introduces a mechanical refinement of a device that can be used to effectively measure finger impedance during manipulation tasks; then, it describes a pilot study aimed at identifying the inertia of the finger and the viscous and elastic properties of finger muscles. The proposed wearable exoskeleton, which has been designed to measure finger posture and impedance modulation while leaving the palm free, is capable of applying fast displacements while monitoring the interaction forces between the human finger and the robotic links. Moreover, due to the relatively small inertia of the fingers, it allows us to meet some stringent specifications, performing relatively large displacements (∼45°) before the stretch reflex intervenes (∼25 ms). The results of measurements on five subjects show that inertia, damping, and stiffness can be effectively identified and that the parameters obtained are comparable with values from previous studies.

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Notes

  1. The motors used in this device are Maxon RE 25 (∅25 mm, graphite brushes, 20 W brushed DC motor), available from http://shop.maxonmotor.com/ishop/article/article/118752.xml

  2. The encoder mounted on the motor’s shaft is a 3 channel rotary encoder. Its resolution is equal to 1000 CPT (Count per Turn).

  3. Austriamicrosystems AS5045 12-bit programmable magnetic rotary encoder.

  4. Micron Instruments semiconductor backed gauges.

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Acknowledgments

This work was partially funded by the European Commission’s Sixth Framework Programme as part of the VIACTORS project under Grant no. 231554. We want to acknowledge the RobotCub consortium for designing and maintaining the development of the electronic devices included in this project.

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Correspondence to Angelo Emanuele Fiorilla.

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Associate Editor Thurmon E. Lockhart oversaw the review of this article.

Appendix A: Exoskeleton Finger Kinematics—Denavit–Hartenberg Representation

Appendix A: Exoskeleton Finger Kinematics—Denavit–Hartenberg Representation

The kinematics of the mechanical chain that actuates each finger can be represented using the Denavit–Hartenberg convention in Table 3.

Table 3 Denavit–Hartenberg parameters for the exoskeleton finger links

L 0L 1L 2, and L 3 are the distances between the finger MCP joint and the motor shaft and the length of the three joints of the exoskeleton, while θ1, θ2, θ3, and θ4 are the angles of the motor cantilever and of the three joints of each finger of the exoskeleton (see Fig. 7).

Figure 7
figure 7

A diagram of the exoskeleton finger kinematics. The axes are arranged according to the Denavit–Hartenberg convention

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Fiorilla, A.E., Nori, F., Masia, L. et al. Finger Impedance Evaluation by Means of Hand Exoskeleton. Ann Biomed Eng 39, 2945–2954 (2011). https://doi.org/10.1007/s10439-011-0381-7

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  • DOI: https://doi.org/10.1007/s10439-011-0381-7

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