A Linear Magnetic Soft Actuator with Image-based Force Sensing for Mechanical Characterization of Soft Materials

Soft actuator plays a critical role in soft robotics. Mechanical characterization of the soft materials serves as a must-do process for accurate modelling, simulation and control of soft actuators. Uniaxial tensile test, as a fundamental testing modality, of fragile biological materials like hydrogel used in soft robotics, biomechanical systems and tissue engineering remains a challenge, because of their poor mechanical properties (kPa ~ MPa), sample gripping difficulties, low force range and hence inaccuracy of force sensing caused by friction of mechanical guidance. In this paper, a linear soft actuator with magnetic fixture and actuation is developed to address the mentioned problems. This device offers a loading range of 0.1 mN to 0.1 N and a corresponding stretching range of a few millimeters. A USB-microscope is employed for remote detection of displacement and force based on image processing and a magnetic-simulation-based lookup table. Maxwell model of viscoelastic material is adopted for system modelling and a closed-loop PI controller is designed here for steady force and displacement outputs. Finally, elastic, creep and relaxation tests are implemented on hydrogel and silicone materials. The reasonable results prove the feasibility of this measurement approach. This device stands out among various mechanical testing equipments due to its contactless, friction-free, low-cost and accurate operational modalities.