Characterisation of torsional actuation in highly twisted yarns and fibres

Abstract

Highly twisted oriented polymer fibres and carbon nanotube yarns show large scale torsional actuation from volume expansion that can be induced, for example, thermally or by electrochemical charging. When formed into spring-like coils, the torsional actuation within the fibre or yarn generates powerful tensile actuation per muscle weight. For further development of these coil actuators and for the practical application of torsional actuators, it is important to standardise methods for characterising both the torsional stroke (rotation) and torque generated. By analogy with tensile actuators, we here introduce a method to measure both the free stroke and blocked torque in a one-end-tethered fibre. In addition, the torsional actuation can be measured when operating against an externally applied torque (isotonic) and actuation against a return spring fibre (variable torque). A theoretical treatment of torsional actuation was formulated using torsion mechanics and evaluated using a commercially available highly-oriented polyamide fibre. Good agreement between experimental measurements and calculated values was obtained. The analysis allows the prediction of torsional stroke under any external loading condition based on the fundamental characteristics of the actuator: free stroke and stiffness. AbstractHighly twisted oriented polymer fibres and carbon nanotube yarns show large scale torsional actuation from volume expansion that can be induced, for example, thermally or by electrochemical charging. When formed into spring-like coils, the torsional actuation within the fibre or yarn generates powerful tensile actuation per muscle weight. For further development of these coil actuators and for the practical application of torsional actuators, it is important to standardise methods for characterising both the torsional stroke (rotation) and torque generated. By analogy with tensile actuators, we here introduce a method to measure both the free stroke and blocked torque in a one-end-tethered fibre. In addition, the torsional actuation can be measured when operating against an externally applied torque (isotonic) and actuation against a return spring fibre (variable torque). A theoretical treatment of torsional actuation was formulated using torsion mechanics and evaluated using a commercially available highly-oriented polyamide fibre. Good agreement between experimental measurements and calculated values was obtained. The analysis allows the prediction of torsional stroke under any external loading condition based on the fundamental characteristics of the actuator: free stroke and stiffness.

High performance and tunable artificial muscle based on two-way shape memory polymer

“…32 The twisting then coiling manipulation of precursor polymeric bers gives rise to muscles with outstanding performances, including energy densities as high as 2.63 kJ kg À1 , power densities over 5.3 kW kg À1 and strokes up to 49%. Most recently, a number of studies have been conducted on this class of muscles, including testing and characterization methods, [33][34][35] mathematical modeling, [22][23][24][25] and applications such as robotics, 31 morphing airplanes and vehicles, 28 and self-healing systems. 29,30 It is revealed that the negative coefficient of thermal expansion (NCTE) in the ber axial direction plays a critical role in muscle actuation.…”

Stretchable artificial muscles from coiled polymer fibers

“…For example, heating monofilament nylon-6 fibers of different diameters from 26 o C -62 o C caused the fiber diameter to increase by ~2.5% [22] and a simultaneous reduction in modulus of ~10% [23]. As shown by equation (3), these two effects exactly cancel when coil geometries do not change so that the coil stiffness remains constant.…”

“…Full recovery of the untwisting occurred when the volume expansion was reversed by discharging, but only when the yarn was connected to a non-actuating 'return spring' fiber. The effects of this and other testing configurations on the torsional actuation stroke and torque have been recently elaborated [23]. Forming composite yarns by impregnating the porous carbon nanotube or niobium nanowire twisted yarns with a guest material, such as paraffin wax, enabled the preparation of totally dry, electrolyte-free torsional muscles (Fig.…”

“…Experimental studies have shown that thermal expansion in oriented polymer fibers can be highly anisotropic [23,27]. Typically, oriented polymers show small expansions and sometimes contractions in the length direction on heating along with diameter direction expansion of significantly larger magnitude [27].…”

Self-Sensing of Deflection, Force, and Temperature for Joule-Heated Twisted and Coiled Polymer Muscles via Electrical Impedance

“…Training of the muscle is usually seen as repeating the actuation cycle in the setup a number of times before performing the actual experiment [5], [20], [21]. We let muscles undergo a number of cycles of heating and cooling, from room temperature to the maximum actuation temperature, in the intended setup.…”