A General Purpose Robotic Hand Exoskeleton With Series Elastic Actuation

Abstract

This paper describes the design and control of a novel hand exoskeleton. A subcategory of upper extremity exoskeletons, hand exoskeletons have promising applications in healthcare services, industrial workplaces, virtual reality, and military. Although much progress has been made in this field, most of the existing systems are position controlled and face several design challenges, including achieving minimal size and weight, difficulty enforcing natural grasping motions, exerting sufficient grip strength, ensuring the safety of the users hand, and maintaining overall user friendliness. To address these issues, this paper proposes a novel, slim, lightweight linkage mechanism design for a hand exoskeleton with a force control paradigm enabled via a compact series elastic actuator. A detailed design overview of the proposed mechanism is provided, along with kinematic and static analyses. To validate the overall proposed hand exoskeleton system, a fully integrated prototype is developed and tested in a series of experimental trials.

Keywords: actuators and transmissions; compliant mechanisms; grasping and fixturing; multi-body dynamics and exoskelotons; wearable robots.

Fig. 1

The completed prototype hand exoskeleton (a) where the linkage mechanism is highlighted and the actuator is highlighted. The finger adduction (b) and abduction (c) capability of the exoskeleton are also shown and the motion is indicated by the arrows.

Fig. 2

The layout of the dual four-bar mechanism for a single finger. The boxes denote the first and second four-bars. The circles are the revolute joints with the filled circle denoting the revolute joints attached to the ground frame. The individual linkages are shown by the thicker lines starting from the base of the finger to the tip.

Fig. 3

The bent configuration of the finger with the design parameters l_i, d_j, where i ∈ {1, 2, 3}, j ∈ {1, 2, 3, 4}. The reference frames attached to each phalange associated linkage are denoted as x_i, y_i, where i ∈ {1, 2, 3} and the frame x₀, y₀ is the ground frame attached the glove base

Fig. 4

The design of the SEA used for the exoskeleton glove with the direction of actuation indicated by the arrow

Fig. 5

The layout of the SEA and the finger mechanism showing a user deflected configuration (black) and the uncompressed configuration (grey)

Fig. 6

The control diagram for the SEA based on a desired contact force

Fig. 7

The PCB along with the key components interfaced with the proposed hand exoskeleton

Fig. 8

The joint angles for the index finger, middle finger, and thumb as a percent of the range of motion. The optimized joint angles (small dashes) and measured joint angles (solid) are all shown.

Fig. 9

The experimental setup for the SEA force experiment showing: (a) signal conditioner, (b) microcontroller for reading force measurements, (c) load cell, (d) SEA, and (e) motor controller PCB

Fig. 10

The results of the SEA force experiment with three target forces

Fig. 11

Grasping test with various everyday objects: (a) beverage bottle (glove only), (b) magnifying lens, (c) tape, (d) cylinder-shaped metal, (e) tape (glove only), (f) pencil, (g) cleaning spray, and (h) wooden block