Flexible tactile sensor for the grasping control of robot fingers

Grasping an object of unknown weight is the most important demand for a robot. However, the control of the grasping force needs the detection of slippage to prevent the object falling from the robot´s fingers. A novel flexible tactile sensor consists of piezoelectric and piezoresistive transducers for detecting incidents of slippage and contact force, respectively. The sensor structure is composed of two patterned flexible printed circuits (FPC) sandwiching a polymer piezoelectric film (PVDF), a foil strain gauge laminated on the bottom FPC, a bump-like structure attached on the top FPC corresponding to the PVDF area and the PDMS packaging material. As the tactile sensor is of a small size and intrinsically flexible, the sensor can easily conform to the robot finger surface as the feedback force sensor. In this study, numerical results verified two opposite voltage outputs from the PVDF film as lateral force acting on the sensor surface due to the bending effect of the bump-like structure. In addition, the sensitivity of the contact force is also enhanced due to the stress concentration of the bump-like structure. According to the experimental results, the tactile sensor can detect a contact force from 0.1 N to 10 N. However, the output voltages showed a two-stage linear behavior. The sensitivities of the contact force are 0.0313 mV/N and 0.0057 mV/N for the force ranges of 0.1~4 N and 5~20 N, respectively. Furthermore, the sensitivity of the periodic lateral force is 10.5 mV/N at 1 N for normal force clamped on the object. For the feedback of the contact force and to avoid incidents of slippage, readout circuits and a display program were also developed for the integration of the sensor with robotic control. In general, we have successfully demonstrated a flexible tactile sensor for the detection of the contact force and incidents of slippage, which could provide more precise control over grasping objects for a robot.