An Ecoflex-based Capacitive 3D Force Sensing System And Its Tactile Sensing Application

Force haptic is an important way for robots to acquire data that can help them to perceive contact force information and perform a variety of complex tasks.Current researches on single-dimensional contact force sensors have made great progress.However,considering the complex and uneven surface of most objects,it is difficult to meet the haptic sensing requirements of intelligent robots by measuring only singledimensional forces.Therefore,it is necessary to study the three-dimensional(3D)force information at the contact surfaces between robots and objects to analyze surface properties(e.g.material,roughness,etc.)of the objects.This thesis investigates a flexible haptic sensing system based on the Ecoflex capacitive 3D force.The system consists of two parts:a flexible three-dimensional force sensor and an information acquisition system.In particular,the flexible 3D force sensor consists of a parallel plate capacitive array and an Ecoflex elastic dielectric layer,where the pressure-sensitive deformation of the dielectric layer is translated into a change in the capacitance values.The experimental data is used to fit a theoretical equation to decouple the magnitude and direction of the spatial force,thus achieving the three-dimensional force detection.Compared to conventional 3D force sensors,this thesis introduces air cavities in the solid Ecoflex and investigates the effect of the number of air cavities on the sensitivity of the sensor from both simulation and experiment.The results show that the presence of air cavities greatly improves the sensitivity of the sensor.For the information acquisition system,this thesis uses a low drift,high signal-to-noise ratio AC amplifier to convert the sensor array capacitance into voltage and exploits the rapid charging and discharging of the capacitor to achieve accurate capacitance measurements.To verify the feasibility of a 3D force sensing system in the field of haptic perception,this thesis uses the system to measure the coefficient of friction of different material surfaces(e.g.sandpaper,fabric,table surface)and the spatial 3D force distribution during grasping.The results of this thesis show that by introducing a 25(5×5)air cavity array into the flexible 3D force sensor,the normal force sensitivity of the sensor is improved by a factor of 4.0 compared to that of a solid sensor.In addition,the hysteresis error of the sensor is 1.6%,demonstrating a low hysteresis.After applying and releasing a force of 2.8 N for 50 times,the sensor still returns to the initial capacitance value,demonstrating the good repeatability of the sensor.In addition,the sensor has a fast response time(261 ms)and recovery time(255 ms).Moreover,the signal acquisition system measures capacitance in the range of 0～3 pF with a sensitivity of 0.914 V/pF.The system noise is within 0.002 pF and the measurement error does not exceed 2.3‰.The information acquisition system has wide measurement range,high measurement sensitivity,low system noise,and low measurement error.