Dynamic Piezoelectric Tactile Sensor for Tissue Hardness Measurement Using Symmetrical Flexure Hinges and Anisotropic Vibration Modes

This paper presents a novel dynamic tactile sensor with a symmetrical longitudinal piezoelectric cantilever structure, which is different from the conventional transverse cantilever structure. It has a radical size of 1.1 centimeters. A flexible hinge chain and mass probe are introduced into the structure to reduce the sensor’s natural frequency to improve the estimation accuracy of tissue hardness. Double piezoelectric patches, symmetrically distributed in the sensor, are used as the actuator to generate vibration and as the sensor for resonance detection. The sensor’s basic working principle is derived mathematically, followed by simulation verification of statics analysis for safety consideration, anisotropic vibration mode analysis, harmonic response analysis, and a calibration experiment to establish fitting curves. The threshold point is set as 192 Hz/0.6 MPa in the calibration curve, and the hardness range is divided into a soft range (0–0.6 MPa) with the sensitivity of 9.75 Hz/MPa, which could realize hardness measurement of soft tissue and normal tissue, and a hard range (which exceeds 0.6 MPa) with higher frequency to realize the identification of the lesion tissue. Finally, the sensor prototype is tested on biological tissue (fresh pig liver) to verify that the calibration curve with the selected threshold is effective for identifying lesion areas and that the sensor prototype has a function of hardness measurement with a relatively stable sensitivity and lesion identification in its operating frequency range.