Pellet-based fused deposition modeling for the development of soft compliant robotic grippers with integrated

Fused deposition modeling (FDM) has some advantages compared to other additive manufacturing techniques, such as the in situ integration of functional components, like sensors, and recyclability of parts. However, conventional filament-based FDM techniques are limited to thermoplastic elastomers with a Shore hardness above 70 A, thus it has marginal compatibility with soft robotic structures. Due to recently emerging pellet-based FDM printer technology, the fabrication of soft grippers with low Shore hardness has become possible. In this study, styrene based thermoplastic elastomers (TPS) were used to print elastic strips and soft gripper structures down to a Shore hardness of 25 A with an integrated strain sensing element (piezoresistive sensor). Printing on a soft rather than rigid substrate affects the integration of the printed thread on the substrate, because of the softness and relaxation, during the printing softness. It was seen that integrating the sensing element on a substrate with higher Shore hardness decreased the elongation at the point of fracture and the sensitivity of the sensing element. A soft compliant gripper structure with an integrated sensing layer was printed with the TPS-based elastomers successfully, and even due to the complex deformation of the compliant gripper structure, several positions could be detected successfully. Opened and closed position of the gripper, as well as, size recognition of spools of different sizes could be monitored by the piezoresistive printed sensor layer. The most sensitive sensing performance was obtained with the TPS of the lower Shore hardness (25 A), as the value of relative change in resistance was 1, followed by the gripper of Shore hardness 65 A and a relative change in resistance of 0.51. With this study, we demonstrated that pellet-based FDM printers can be used, to print potential soft robotic structures with in-situ integrated sensor structures.与其他增材制造（additive manufacturing）技术相比，熔融沉积成型（Fused Deposition Modeling, FDM）具备多项优势，例如可原位集成传感器等功能组件，且制件可回收利用。然而，传统丝材基FDM技术仅适用于邵氏硬度（Shore hardness）高于70 A的热塑性弹性体（thermoplastic elastomers），因此与软体机器人结构（soft robotic structures）的兼容性较为有限。近年来新兴的基于粒料的FDM打印技术（pellet-based FDM printer technology），使得制备低邵氏硬度的软体夹持器成为可能。本研究采用苯乙烯基热塑性弹性体（styrene based thermoplastic elastomers, TPS）打印了弹性条（elastic strips）及软体夹持器结构（soft gripper structures），其邵氏硬度最低可达25 A，并集成了压阻式应变传感元件（strain sensing element, piezoresistive sensor）。相较于刚性基板（rigid substrate），在柔性基板上进行打印时，由于打印过程中基板的柔性与松弛特性，打印丝与基板的结合效果会受到影响。研究发现，将传感元件集成于邵氏硬度更高的基板上，会降低传感元件的断裂伸长率（elongation at the point of fracture）与灵敏度。研究团队成功采用基于TPS的弹性体打印出集成传感层的柔性柔顺夹持器结构（compliant gripper structure）；即便该柔顺夹持器发生复杂形变，仍可成功检测多个位置的状态。通过该压阻式打印传感层，可监测夹持器的开合状态以及不同尺寸线轴的尺寸识别任务。其中邵氏硬度为25 A的TPS展现出最优的传感性能，其电阻相对变化量（relative change in resistance）为1，其次为邵氏硬度65 A的夹持器，电阻相对变化量为0.51。本研究证实，基于粒料的FDM打印机可用于打印具备原位集成传感结构的潜在软体机器人结构。