Research On Capacitance-based Proximity And Tactile Electronic Skin

The development of the artificial hand has changed from simple resemblance to complex resemblance and its functions are becoming more abundant,approaching the real human hand gradually.It will play an important role in prosthetic limbs,surgical medical treatment,and family assistance.The finger surface sensing system of artificial hand undertakes the task of sensing external environment information in human-computer interaction,and is used to input control signal into the system.Therefore,the sensing technology of the artificial hand is one of the key technologies for the intelligent development.Because the surface of the finger is spatially curved,the space for the sensor is limited,and the function of a single sensor is limited,resulting in the difficulty to perceive multiple information in the limited layout space.What's more,the internal mechanical structure and electrical system of the finger will interfere with the sensing signal.As a result,there is a lack of flexible sensors for artificial hands.In view of these problems,this paper designed and developed a proximity and tactile electronic skin for the artificial hand(HIT-mini),which was developed by the State Key Laboratory of Robotics and Systems of Harbin Institute of Technology.The main work of this paper is as follows:According to the finger geometry,motion range,grip strength,control cycle and other key performance indicators,an "island-bridge" structured proximity and tactile flexible sensor based on the capacitance detection principle was designed.Through theoretical analysis and finite element simulation calculation,with the goal of improving the basic value and relative change of capacitance,the main structural parameters such as electrode configuration,electrode size,dielectric layer structure and thickness of the capacitive sensor were optimized.Aiming at the problems that the finger surface was a spatial curved surface,which caused the mechanical failure,device performance degradation and damage,a near-zero stress expansion strategy for the finger surface was designed.The expanded plane was used as the substrate of the flexible device.The second-order fractal structure was adopted for the "bridge" structure,in order to improve the large deformation ability of the device.By using photolithography,etching and secondary mold technology,the PDMS dielectric film with pyramidal microstructures was prepared.The copper flexible fractal structure electrodes were prepared by femtosecond laser processing technology.The Ecoflex film was prepared to encapsulate electrodes,which enabled electrodes to reduce strain by out-of-plane buckling deformation.The tensile performance test of the fractal structure electrode was completed,and the results showed that the fractal electrode structure can maintain good electrical conductivity under 200% extreme strain conditions.Finally,the multi-layer structure of the sensor was encapsulated by PDMS modified by oxygen plasma.The overall length of a single device was 56 mm,the width was 8.15 mm,and the thickness was 0.9 mm.A test platform for the proximity and tactile sensor was built to test the sensing performance.The results showed that the sensor can detect the proximity signal of an object within 120 mm,and the contact force detection range was 0-12 N,which completely covered the requirements of the HIT-mini artificial hand.The relative change of capacitance for proximity and tactile sensing could reach-51.7% and 20.3%,respectively.The response time of proximity sensing and tactile sensing was 31 ms and62 ms,respectively,indicating that the prepared sensor had high sensitivity and fast response capability.The sensor had different signals for objects of different conductive properties.The sensor also had good response to different conductive properties of objects approaching vertically and at different angles.Finally,the sensor was integrated and attached to the tip of the artificial hand finger,which verified the feasibility of the sensor to realize the intelligent grasping process monitoring on the artificial hand.The results of this paper will provide theoretical basis and technical support for manipulator proximity and tactile detection and its servo control system.