Fabrication of a Soft Robotic Gripper With Integrated Strain Sensing Elements Using Multi-Material Additive Manufacturing

Abstract

With the purpose of making soft robotic structures with embedded sensors, additive manufacturing techniques like fused deposition modeling (FDM) are popular. Thermoplastic polyurethane (TPU) filaments, with and without conductive fillers, are now commercially available. However, conventional FDM still has some limitations because of the marginal compatibility with soft materials. Material selection criteria for the available material options for FDM have not been established. In this study, an open-source soft robotic gripper design has been used to evaluate the FDM printing of TPU structures with integrated strain sensing elements in order to provide some guidelines for the material selection when an elastomer and a soft piezoresistive sensor are combined. Such soft grippers, with integrated strain sensing elements, were successfully printed using a multi-material FDM 3D printer. Characterization of the integrated piezoresistive sensor function, using dynamic tensile testing, revealed that the sensors exhibited good linearity up to 30% strain, which was sufficient for the deformation range of the selected gripper structure. Grippers produced using four different TPU materials were used to investigate the effect of the Shore hardness of the TPU on the piezoresistive sensor properties. The results indicated that the in situ printed strain sensing elements on the soft gripper were able to detect the deformation of the structure when the tentacles of the gripper were open or closed. The sensor signal could differentiate between the picking of small or big objects and when an obstacle prevented the tentacles from opening. Interestingly, the sensors embedded in the tentacles exhibited good reproducibility and linearity, and the sensitivity of the sensor response changed with the Shore hardness of the gripper. Correlation between TPU Shore hardness, used for the gripper body and sensitivity of the integrated in situ strain sensing elements, showed that material selection affects the sensor signal significantly.

Use of modified deep eutectic solvent as an additional chemical in a flexible conductive natural rubber sensor for motion analysis

“…Both dynamic and quasi-static tests were performed between 0-50 and 50-100% strain levels. Further details on the dynamic and quasi-static analysis of soft sensor materials are reported elsewhere [30][31][32][33][34].…”

Thermoplastic elastomer composite strips with damage detection capabilities for self-healing elastomers

“…In soft robotics, tendon or pneumatic bending actuators are often used 11,24,35,36 . Due to the limited amount of self-healing material, the performance of the SECs was investigated under bending conditions on a 3D printed TPU hinge (Fig.…”

“…Piezoresistive sensors can be used to detect damage and monitor when the healing process has been successfully achieved 8,9,10 . Moreover, the integration of a piezoresistive sensor allows to determine the actual position of the robot limbs 10,11 and closed-loop control of robotic movements 12,13 .…”

Thermoplastic elastomer composite strips with damage detection capabilities for self-healing elastomers

“…In soft robotics, tendon or pneumatic bending actuators are often used [11,28,43,44]. Due to the limited amount of self-healing material, the performance of the SECs was investigated under bending conditions on a 3D printed TPU hinge (Fig.…”

“…Piezoresistive sensors can be used to detect damage and monitor when the healing process has been successfully achieved [8][9][10]. Moreover, the integration of a piezoresistive sensor allows to determine the actual position of the robot limbs [10,11] and closed-loop control of robotic movements [12,13].…”