Recent Developments in Impedance-Based Tactile Sensors: A Review

In the past decades, tactile sensors have been well developed and utilized in many fields, including intelligent robots, human–computer interaction, health care, and recreational equipment. Great efforts have been made to enhance the performance of tactile sensors, i.e., the resolution, response time, capability to sensing multiple parameters, power consumption, and weight. Among different sensing principles, impedance-based tactile sensors could provide a cost-effective, fast-response solution for tactile sensing, as compared with ultrasonic or optical sensors. Especially, the combination of functional materials and impedance measurement techniques has made it possible to achieve similar functions as human skins, regarding the capabilities of multiple parameters and distributed sensing. This article presents a comprehensive review of impedance-based tactile sensors, with an emphasis on their principle, material, structure, and applications. The emerging applications include the artificial tactile feedback system, gesture and/or emotion recognition, surgical robots, health monitoring, musical instruments, and so on. In general, the sensors could be integrated or embedded into the tactile materials. These sensors could be categorized into four different kinds according to sensing principle, i.e., piezoresistive, capacitive, piezoelectric, and triboelectric. To achieve better spatial resolution, it usually requires a group of sensors to be embedded in the tactile materials. The structure of a sensor array has been categorized into three types based on the connection pattern, i.e., the individual array sensors, mesh sensors, and tomographic sensors. Trends and future development of impedance-based tactile sensors, sensor structures, data processing methods, and a variety of established and potential applications are also discussed.