## 2 Structure and Working Principle
### Working Principle  
The Kapton friction layer on the rotor surface and the copper electrode layer on the stator surface form a sliding-mode triboelectric nanogenerator, which is more suitable for cyclic motion. During the rotation of the sensor's rotor within the downhole power drilling tool, friction between the stator and rotor generates electrical pulse signals from the electrode layers. The unique design of the sensor electrode geometry produces six pulses per rotor revolution. Therefore, detection of a single pulse by the subsequent microprocessor indicates a 60 degrees rotor rotation, while six pulses signify a full revolution. Consequently, rotational speed and angle are determined by counting the number of electrical pulses generated by the sensor.  
Figure 2a provides a detailed explanation of the rotational speed measurement principle. As shown in Figure 2(a-i) (initial state), the friction layer is in full contact with electrode one, generating triboelectric charges. Due to the differing electron affinities of the materials, the Kapton friction layer readily gains electrons, while the copper of electrode one readily loses them, resulting in a positive charge on the copper and an equal and opposite negative charge on the Kapton. As the rotor rotates, and as depicted in Figure 2(a-ii), the friction layer begins to separate from electrode one. During this process, the contact area between the Kapton and electrode one decreases, while the contact area with electrode two increases. The reduced attraction between the Kapton and the charge on electrode one causes charge to flow from electrode two to electrode one, inducing a current in the circuit. Further  
Figure 1: Schematic diagram of the sensor structure and working principle. (**a**) Schematic diagram of the sensor installation environment; (**b**) schematic diagram of the sensor structure; and (**c**) unfolded diagram of the sensor electrode structure.  
rotation to the state shown in Figure 2(a-iii), where the Kapton is fully separated from electrode one and in full contact with electrode two (half the distance of a single sawtooth electrode), completes the charge transfer, resulting in a peak voltage. Continued rotation to the state shown in Figure 2(a-iv) causes the Kapton to separate from electrode two and contact electrode one. This separation again reduces the attractive force, leading to a reverse charge transfer and a reverse current in the circuit. When the rotor completes a rotation across one sawtooth electrode, reaching the state in Figure 2(a-v), the Kapton is fully separated from electrode two and fully contacting electrode one, completing the reverse charge transfer and resulting in a reverse peak voltage.  
Figure 2b illustrates the correspondence between the electrodes, the output waveform, and the rotation angle. One electrical pulse is generated per sawtooth electrode traversed by the rotor. Given the six sawtooth electrodes in the design, six pulses are generated per rotor revolution, meaning each pulse corresponds to a 60 degrees rotation. However, the sensor outputs bipolar pulses with both positive and negative voltage amplitudes. Therefore, 30 degrees angular resolution is achievable by differentiating between the positive and negative amplitudes within a single pulse. Consequently, the sensor not only measures rotational speed but also provides angular measurement with a resolution of 30 degrees based on the pulse signals.