A Novel Soft Actuator for the Musculoskeletal System

Abstract

actuator. In addition, large mechanical power output, good tensile actuation, and dynamic response are essential requirements for this application.Twisted and coiled polymer (TCP) muscle from fishing line emerged as a promising actuator as shown in Haines et al.; [6] it can deliver a large strain of 49%, lift loads over 100 times heavier than those lifted by a human muscle of the same length and weight, generate 5.3 kW of mechanical work per kilogram of muscle weight, and undergo over 1 million life cycles. The fundamental mechanism in twisted and coiled actuator is the thermally induced fiber untwist in the coiled structure, which allows for both torsional and tensile actuation. [7,8] TCP muscle from fishing line can be actuated via hydrothermal actuation, but Joule heating is a convenient method because it eliminates a complicated actuator system like hot water reservoir and fluid pump needed for hydrothermal actuation. [9] However, as an inherently nonconductive material, actuation of TCP muscle from fishing line via electrothermal actuation is still a challenge. Different approaches have been proposed to add electrical conductors to artificial muscles, for instance, wrapping flexible carbon nanotube sheets, [6] coating nylon fishing line with conductive silver paint, [10,11] and twisting metal wire and fishing line simultaneously. [12][13][14][15] Metal wires used include copper, nickel-titanium shape memory alloy, iron, and nichrome, ranging from a diameter of 140 to 200 µm. Our unique approach is to wrap the resistance wire around the twisted fiber. This novel fabrication method can yield a better actuator for several reasons. First, it will enable the actuation of TCP muscle from fishing line (nonconductive) via electrothermal actuation (Joule heating). Second, avoiding twist to resistance wire enables the use of a very thin resistance wire of a diameter of 80 µm (tensile strength 690 MPa). If twist is inserted into the resistance wire, the thin resistance wire would break during the fabrication procedure. This is one of the main reasons why we avoid twisting the resistance wire. Third, the fabrication process using a very thin resistance wire will have negligible mechanical effect on the actuation. Lastly, the resistance of TCP muscle will no longer change significantly with the increase of temperature. Hereinafter, we refer to this actuator as TCP NC FL muscle. Counterparts in nature serve as the best resource for inspiration and biomimicry for the design of an artificial MS. Given the unique biological significance of the ball-and-socket joint in a variety of species, we propose an artificial MS based on the ball-and-socket joint, which can serve as a biomimetic building block that can then be cascaded in various fashions to createThe musculoskeletal system (MS) is essential for the movements of biological systems. Inspired by this natural structure, several attempts are made to create a synthetic MS. However, one of the challenges in developing an artificial MS for biomimetic robots is the lack of a hi...

A Modeling of Twisted and Coiled Polymer Artificial Muscles Based on Elastic Rod Theory

“…where C is spring index which is the ratio of mean coil diameter D2 to the fiber diameter d and P = −F. According to Equation (20), the maximum tensile force F can be expressed as…”

“…These modules produce maximum bending angle of 40 • in any direction, and reversible radius of curvature reaching 0.23 mm −1 [18]. Integrated systems have been proposed, including musculoskeletal system [19,20], robotic hand [21,22], power generation systems [23,24], and robotic skin, in which the TCP muscles are embedded within the soft silicone skin as linear actuators, allowing the skin to generate desired movements without noise [25].…”

Closed-Loop Control Through Self-Sensing of a Joule-Heated Twisted and Coiled Polymer Muscle

“…In this case, closed-loop control reduces to the interaction between the temperature model in (1) and the control law in (7). A potential source of instability is the saturation in (9). Separating the nonlinearity from the dynamics allows for stability analysis via describing functions.…”

“…tions: robotic fingers, [4][5][6] joints, [7][8][9] orthoses, 10,11 complete robots, 12,13 or being embedded in a silicon manipulator, 14 silicon skin for robotic facial expressions, 15 or a selfadjusting sports bra. 16 Systems that benefit most from TCPMs are typically lightweight and inexpensive, and should function in versatile environments.…”

Double Helix Actuators

“…The recent discovery that high‐performance actuator materials can be made by simply twisting and coiling ordinary polymer fibers has created much interest in their properties and applications . Heating the twist‐oriented polymer fibers induces a partial untwisting due to their anisotropic thermal volume expansion .…”