Fascicular module of nylon twisted actuators with large force and variable stiffness

[Display omitted] •Nylon twisted and coiled actuators are connected in parallel, forming fascicular module to increase the load capacity.•The contraction length of the fascicular module is increased by 2∼23 times compared with single nylon twisted and coiled actuator fiber.•Standard mechanical interface was designed to promote its application in soft-rigid devices.•A variable stiffness rotary joint was realized by a pair of antagonistic fascicular modules. In terms of stunning characteristics of ultra-high power density, large stroke and low hysteresis, nylon twisted actuators are ideal actuators for fusion applications of soft robots and conventional rigid-body robots. However, most of previous studies only used single ply of fiber in the devices like prosthesis hands, leading to inadequate force output. Moreover, the stiffness adjustability which is critical to perform dexterous and safe actions has been neglected. In this work, a fascicular module consisting of nylon twisted actuators in parallel connection was proposed and a structural configuration method aiming to realize variable stiffness function was presented and corroborated. The contraction length at the temperature of 120℃ was from 2.22 % to 9.40 % with a fascicular module while it was from 1.29 % to 6.07 % with a single fiber. Under a load of 300 g the contraction length was 0.85%–9.40% with a fascicular module while it was from 0.04%–6.07% with a single fiber. A stiffness variation range of 60.2 % from 1.498 N mm° to 2.402 N mm° was achieved. The enhanced load capacity and standard interface make it more feasible to utilize nylon twisted actuators in bionic robots, rehabilitation devices, etc. In addition, variable stiffness characteristics ensure that soft-rigid devices function in a more reliable and safer way.