Soft Three-Axis Capacitive Force Sensor for Robotic E-Skin on Curved Surfaces

Many soft sensors have emerged to meet the needs of modern technologies, such as soft robotics, where high conformability and sensor adaptability are required. Little attention has been paid to how these soft sensors perform when used in a curved geometry, such as on the forearm or finger of the robot arm. We explore the effect of indentation by a flat object at radii of curvature as small as 10-mm—corresponding to a wide finger. The three-axis capacitive force sensor is composed of Ecoflex 00-30 pillars forming the dielectric and patterned carbon black/Ecoflex electrodes, mounted on a flexible printed circuit board for bottom connections. As the sensor radius of curvature decreases, the sensor increases in perceived sensitivity to applied force, with normal force sensitivity increasing from 0.27% change in capacitance per Newton (1.4%kPa
⁻¹
) at 100-mm radii to 0.47%N
⁻¹
(2.4%kPa
⁻¹
) at 10-mm radii, a 58% increase. Shear sensitivity in the noncurved direction increases by 155%, whereas in the circumferential direction, the sensitivity increases by 366%. This increase in sensitivity is due to the increase in local pressure due to decreasing surface area contact, especially in the circumferential direction when the sensor becomes greater than 25% of the radius of curvature. A similar trend is expected for grasping of rounded objects, where higher curvature will lead to a concentration of stress, with sharp edges being the extreme case. The sensor is applied to a robotic hand to assess normal and shear forces when gripping and lifting objects.