Stacked Rigid and Compliant Dielectric Structures for Increasing Force Range in Soft Capacitive Sensors

Soft force sensors have a myriad of applications including soft robotics, medical devices, and industrial environments. Compared to mechanoreceptors in the skin, emerging technologies are still poor in the combination of sensitivity, directional distinction, and dynamic range. In this work a capacitive sensing approach is presented that employs a mechanically structured dielectric to enable a combination of high sensitivity at low forces that correspond to light touch, and an ability to operate at high forces needed for tight gripping. The sensor uses the change in capacitance resulting from the movement of four electrodes on the top surface of the dielectric relative to a bottom electrode to determine normal and shear displacements, and corresponding forces. A soft silicone elastomer (Shore Hardness 00-30) is used to form the body and conductive elements of the sensor. Initial work using a dielectric with a pillar structure showed high sensitivity at pressures of up to 76 kPa, but signal saturation occurred at higher loading as pillars are flattened and contact one another at large strains. To increase force range while maintaining sensitivity, a two-stage dielectric architecture is used to create a structure which is highly compliant to normal and shear forces at small forces, while maintaining reasonable sensitivity at high normal forces. Several shapes are explored for the purposes of enhancing low-force response, combining a softer upper architecture such as pyramids with stiffer lower pillars. A molding and bonding process is used to fabricate these stacked sensors. Four primary designs are presented in this work with different combinations of pyramid and square structures. The selection of structures is informed by literature on pyramidal sensors, empirical design, and simulations. COMSOL is used to predict sensor response. These sensors are characterized in a three-axis characterization setup for low-force, and a single-axis characterization setup for high-force. The combination of inverted pyramids in the high sensitivity layer and square pillars in the high force layer achieves a balance of high sensitivity while maintaining a wide dynamic range and a relatively flat, skin-like upper surface. This sensor functions in a normal force range of 0.05 N - 50 N. At lower forces (0-20 N) shear displacements are measured, with a sensitivity of 265 fF/N (1.35 fF/Pa), enabling detection of a displacement of 23 mm, corresponding to a force of 60 mN, and a shear stress of 89 Pa. The normal force sensitivity is 54 fF/N (0.28 fF/kPa), down to a normal force of 13 mN. This resolution is similar to that of human mechanoreceptors at frequencies of less than 20 Hz. At large loads (25 N – 50 N), there is some hysteresis and loss of sensitivity, but a change of 15 fF/N (0.08 fF/kPa) enabling force discrimination. Multi-stage dielectric structures consisting of two different pillar shapes allow for capacitive sensors to have a large force range, and yet high sensitivity to light touch, by combining shapes of varying compliance in series. Force range and sensitivity have potential to adapt to higher levels by using stiffer elastomers. Lateral spatial resolution in these sensors is several millimeters – significantly lower than in skin – but is resolvable by making the sensing skin thinner to increase resolution.