A novel piezoelectric energy generator embedded in vehicle brake pads and excited by magnetic repulsion is developed. The generators are mounted on the backing plate of the brake pad through the perforated friction layer. Slotted brake rotor with embedded magnets is equipped to ensure the braking performance of the vehicle. During the braking process, dynamic magnetic repulsion will be generated when the overlapping area of the embedded magnets in the brake pad and brake rotor is changing. The magnetic repulsion is generated when two magnets are close to each other, and the force is proportionally changing with the overlapping area of the two magnets. As a result of repulsion between the magnets, the piezoelectric stack will experience compressive forces, creating an electrical charge for generating energy. To illustrate the voltage generation, a mathematical model with experimental verification is established to calculate the electric charge and output voltage considering the charge dissipation. The energy harvesting process is evaluated by simulating the transient charging of the storage capacitor through the diode bridge, which was experimentally validated in literature. The influences of the dimensional and material properties of the piezoelectric stack, the vehicle speed, the magnetic repulsion, the diameter of the magnetic actuator, the capacitance of the storage capacitor and the distance between rotor center to the actuator on the root mean square (RMS) of the charging power are discussed. A total RMS power of 0.0710 W can be achieved with thirty-six generators embedded in both the inner and the outer brake pads within one brake caliper using APC850 (PZT4) material, and a total RMS power of 1.1226 W can be achieved using PMN-PT-B (PT=0.3-0.33) material at 120 km/h speed of the vehicle. This novel generator will be useful for efficient and practical energy harvesting applications during vehicle braking process.

Research works have been conducted on transverse and longitudinal mode piezoelectric energy generation to collect energy from ambient vibrations. However, the inconsistency with the frequency of the energy source and low output power density remain problems for high energy output. In this work, we propose a shear mode piezoelectric energy generator, which utilizes the friction-induced vibration and high shear mode piezoelectric coefficient to improve the energy output. A piezoelectric coupled friction-induced vibration mathematical model is developed to accurately calculate the dynamic vibration response and voltage output. The dynamic voltage response is validated by experiment, and it proves the possibility of continuous friction-induced high-frequency vibration. The energy generation process is evaluated by transient charging simulation of a storage capacitor through an iteration process, which was experimentally validated in the literature. Parameter studies have been conducted to investigate the influences of the piezoelectric patch dimensional parameters, vibration system parameters, friction model parameters, methods of electrical connections, and different piezoelectric materials on the energy generation performance to provide guidance for better design. Under ideal experiment conditions with proper parameters, a volume of 6.25×10^(-8) m3 PZT4 piezoelectric material indicates RMS charging power density of 5.38×10^3 Wm-3 and 4.70×10^3 Wm-3 with electrically in parallel and electrically in series, respectively. While using the same amount of material and structural setup, the single crystal PMN-PT piezoelectric material shows RMS charging power density of 2.72×10^4 Wm-3 and 2.58×10^4 Wm-3 with electrically in parallel and electrically in series, correspondingly. These promising results demonstrate that close to W-level RMS charging power output may be realized by structure optimization of energy generator design and incorporating multiple generators together for operation. Possible incorporation into vehicle braking systems can be considered to utilize the wasted friction energy, and it may offer an energy supply for low-power wireless devices.

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