Development of novel capacitive sensors using functional materials

Abstract

In this paper, studies on the development of novel practical capacitive-type devices using the functional materials (e.g., Liquid metal and AgNW) are presented. First, a novel capacitive-type touch sensor that uses liquid metal (LM) droplets and simultaneously has a large dynamic range, high sensitivity at relatively good spatial resolution is introduced. The sensor uses the changes in capacitance caused by the overlap area between the LM droplet and a flat-bottom electrode to improve its dynamic range and sensitivity to capacitance. A total of 36 sensing elements with a spatial resolution of 2 mm and arranged in a 6 × 6 array are successfully fabricated using micromachining techniques. The performances of the fabricated device in terms of its dynamic range, sensitivity, repeatability, and response time are analyzed by one-cell experiments. The fabricated device has a large dynamic range (∼100 pF) and high sensitivity (∼147pF/N). In addition, multi-touch tests using a finger and acryl (=acrylic) stamps are successfully detected by the fabricated device. Second, an interesting design for a capacitive touch sensing mechanism using LM droplets to maintain the advantage of the floating electrode (robustness) and simultaneously improve the dynamic range of the device is introduced. LM has electric conductivity, thus it can be used as a floating electrode, and it simultaneously has the deformability of liquid, which does not suffer from fatigue. Therefore, the robustness of the device can be improved. The sensor uses changes in capacitance caused by the overlap area between the LM droplet and a pair of flat-bottom electrodes to improve its dynamic range. To verify the performance of the sensor, a total of 36 sensing elements with a spatial resolution of 2 mm and arranged in a 6 × 6 array were successfully fabricated using micromachining techniques. The performance of the fabricated device was analyzed by one-cell and multi-touch tests. The device has a large dynamic range (∼40 pF). In addition, using the merits of the device, we applied our concept to an end-effector. Third, a design method to develop a bending-insensitive capacitive-type touch sensor for soft robotics applications is reported. To achieve bending-insensitive properties and a shielding effect to block the parasitic capacitance, the sensor is composed of a AgNW-based stretchable top electrode layer, micro-structured dielectric layer, and film-type bottom electrode layer. The stretchability of the top electrode (i.e., tension or compression according to bending) and the microstructures (i.e., shape, pitch, Young’s modulus, and subdivision of the structure), which affect the performance (i.e., initial value and sensitivity) of the sensor in bending situations, are analyzed using mathematical modeling and bending tests based on two perspectives. The analysis is conducted in both bending directions, that is, inward and outward, and the differences in the performance in both directions are analyzed. The fabricated sensor is capable of measuring a wide range of pressures (up to 700 kPa) with a sensitivity of ~0.14 MPa-1. The sensitivity can be controlled by hardly changing the initial value during bending depending on various applications. The sensor can be successfully applied to soft robotics applications. Finally, utilizing the advantages of a LM (i.e., mercury) and its electro-mechanical properties (i.e., high density, high surface tension, and high electrical conductivity), a novel capacitive-type two-axis accelerometer is proposed. The device using a liquid type proof mass (i.e., LM droplet) located in a cone-shaped guiding channel. In the device, the Laplace pressure induced by the guiding channel and LM droplet acts as a spring due to the high surface tension of LM. To accurately set the spring constant in the device, a 2D mathematical model is established. Through the mathematical model, how the shape of the channel affects the sensitivity of the device is analyzed. Despite measuring the two-axis accelerations using a single proof mass, the accelerometer shows less than 1% cross-axis sensitivity for both the x- and y-axes. The accelerometer also demonstrates a similar output as a reference accelerometer for a randomly applied acceleration. Due to the interesting nature of the liquid-type proof mass, even if the proof mass is destroyed, its function is recovered by simply shaking the accelerometer. Finally, within the 15,000-cycle test, the accelerometer shows a 1.4% change in output. Finally, the device is applied to a maze escape game application for verification.