Despite the increasing popularity of soft robotic research, the application of soft robots is hindered by their limited ability to change compliance and acquire force and position feedback. In this article, both the controllability of compliance and the acquisition of position feedback are achieved in soft robotic fingers in the novel design of a three-dimensional (3D)-printed multismart material substrate. The substrate is composed of shape memory polymer (SMP) and conductive elastomer thermoplastic polyurethane (TPU). The SMP material is utilized to modulate the finger stiffness through its elastic modulus change around glass transition temperature (T). The conductive TPU has two functions: one is to tune SMP temperature by producing Joule heat when electrical power is supplied and the other is to provide position feedback of the finger by utilizing the piezoresistive effect of the conductive TPU. Theoretical modeling of finger position feedback and stiffness modulation are conducted. The theoretical analysis has been experimentally validated by a prototype robotic finger built from the proposed concept.
Compared to traditional rigid robots, soft robots, primarily made of deformable, or less rigid materials, have good adaptability, conformability and safety in interacting with the environment. Although soft robots have shown great potentials for extended applications and possibilities that are impossible or difficult for rigid body robots, it is of great importance for them to have the capability of controllable stiffness modulation. Stiffness modulation allows soft robots to have reversible change between the compliant, or flexible state and the rigid state. In this paper, we summarize existing principles and methods for stiffness modulation in soft robotic development and divide them into four groups based on their working principles. Acoustic-based methods have been proposed as the potential fifth group in stiffness modulation of soft robots. Initial design proposals based on the proposed acoustic method are presented, and challenges in further development are highlighted.
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