Soft Inductive Tactile Sensor Using Flow-Channel Enclosing Liquid Metal

There are structural challenges in increasing the softness of conventional soft tactile sensors because rigid electrical elements have to be installed around the sensing areas, which should be compressive, stretchable, and durable. To solve these issues, we propose an inductive tactile sensor whose silicone-rubber body has only two liquid-metal reservoirs connected by an elongated flow channel. When one reservoir is placed around the sensing area, another one can be placed at a non-sensing area. Furthermore, in this structure, touch can be detected by monitoring the inflow and outflow of the liquid-metal in the latter reservoir by using a separately placed coil circuit based on the eddy-current effect. The proposed method requires no direct electrical connections with liquid metal in the reservoirs or flow channels. This means that the sensor body has no inhibitor that reduces its compressibility and stretchability, and that deteriorates its durability. The experimental results demonstrated that larger reservoir diameters provided larger sensitivity and higher signal-to-noise ratio of approximately 65 dB. Additionally, we observed that the bending of the body does not affect the sensor response as much as gravity. Therefore, we conclude that our sensor has structural advantages for tactile-sensor installation, especially in soft actuators, because our completely soft sensor does not experience reduction or deterioration in its functionality owing to its softness.