Fig. 3.

Signals decoupling for stiffness recognition. a Decoupling process of the measured signals. b Scatter diagram of the peak-peak voltage versus different stiffness. c Voltage of different objects versus different stiffness under known displacement. d Time interval between the positive peak value and the negative one under different displacement for Object 9. e Fitting relationship between time interval and displacement. f Young’s Modulus fitting curve generated from peak-peak voltage and time interval. g Young’s modulus measurement comparison between TENG and the commercial sensor. h Comparison between our sensor and other works