A variable-impedance piezoelectric tactile sensor with tunable sensing performance for tissue hardness sensing in robotic tumor palpation

This paper presents a novel piezoelectric tactile sensor for tissue hardness sensing, featuring online tunable sensing performance (resonant frequency, sensitivity, measurement range) through varying its mechanical impedance. The variable-impedance mechanism is achieved with a unique double-cantilever structure, which also enables the sensor to operate at a low frequency suitable for testing biological tissues. A simultaneous actuation and sensing technique is used which allows detecting resonant frequency from electrical impedance of the piezoelectric actuator without using additional motion sensors, therefore simplifies the sensor's structure and instrumentation. The basic working principle and the variable-impedance method for performance tuning are explained mathematically and verified by numerical and experimental studies. The test results on a prototype show that it has an operating frequency (<300 Hz) much better than those in the literature. With the variable-impedance mechanism, sensitivity improvement ratios of 1.7, 2.6 and 3.8 times are achieved in three test ranges. And measurement range improvement ratio of 6.2 times is achieved. Ex vivo experiment confirms its effectiveness in detecting embedded lump. The proposed sensor can find wide applications in robotics, biomedical engineering, aerospace, etc, and may eventually inspire the designs of a new class of tactile sensors with online performance tuning capability.