Low Creep 3D-Printed Piezoresistive Force Sensor for Structural Integration

Attempts in research to equip robotic end-effectors with tactile sensing facilitate an advanced environment perception and provide the means for dexterous interaction. Sensing at the fingertips can be realized using force sensors. In this work, we present an additively manufactured universal force-sensor offering structural integration to accomplish fast adaptation to application-specific needs. The piezoresistive sensor consists of commercially available conductive polylactic acid (PLA). Its geometry is based on rigid PLA spring elements to overcome the inherent limitations of elastomers. A curved shape increases the length of the deformation element, thus, the sensitivity, while retaining the flexibility necessary to allow for a displacement-induced change of the electrical resistance. The sensor features an additional integrated spring, which enables the adaptation of the mechanical stiffness and therefore of the measurement range. We use thread-forming screws to achieve a robust and enduring electrical connection between wires and the conductive polymer. The characterization of the sensor takes place in a universal testing machine with an applied load up to 5 N. The resistance measured gives a nearly linear characteristic and is proportional to the displacement. We obtain a sensitivity of 6.5 Ohm/N and a relative change of resistance of 6%. Low creep (0.12%) during phases with constant load reveals an advanced geometry-induced mechanical behavior. Thus, our printed piezoresistive PLA sensor demonstrates the suitability of conductive rigid materials for their tailored application as force sensors in robotics.