## 2 Structure and Principle of the Semi-Active Energy Regenerative Suspension with an EHA  
The basic structure of the semi-active energy regenerative suspension system with an EHA is shown in Figure 1. The system is mainly composed of a spiral spring, a hydraulic cylinder, a hydraulic motor, a brushless DC motor, a digital signal processor controller, a composite energy recovery device, and a digital potentiometer and the corresponding sensor. The hydraulic cylinder uses a double pole double-acting symmetrical hydraulic cylinder; the hydraulic motor uses a gear motor that can carry out positive inversion; and the brushless DC generator uses a permanent magnet brushless DC generator [16]. When the vehicle is running, the hydraulic cylinder moves with the body vibration and pushes hydraulic oil to drive the hydraulic motor to rotate, the output shaft of the hydraulic motor drives the coaxial brushless DC generator by coupling, and the generated electrical energy is recovered and stored by a composite energy recovery device to realize energy regeneration in the EHA energy regenerative suspension. At the same time, the sensor sends a signal of the vehicle's operation condition  
Figure 1: The structure of the semi-active energy regenerative suspension with an electro-hydraulic actuator (EHA). DC: direct current. to the controller. According to the control strategy, the controller changes the external load resistance value of the brushless DC motor by adjusting the value of the digital potentiometer. Therefore, the electromagnetic torque of the motor can be changed, and the damping force of the hydraulic cylinder can be controlled to realize the control function of the EHA semi-active suspension. The composite energy recovery device is shown in Figure 2. The device is mainly composed of a three-phase rectifier, a filter circuit, boost module 1, boost module 2, MOSFET A, MOSFET B, a voltage equalizing circuit, a super capacitor group, a voltage sensor, a DSP controller, a voltage stabilizing circuit, a diode, and a storage battery. Among them, the super capacitor group consists of four capacitors from the 2.7 V 100 F super capacitor series, in which the voltage is 10.8 V, the capacity is 25 F, and the optimal charging voltage is 9-10 V. Boost module 1 boosts to 9 V for super capacitor charging. Referring to the vehicle battery, the lead-acid battery used is 12 V 8 ampere-hour, in which the optimal charging voltage is 14.4 \(\pm\) 0.2 V. Booster module 2 boosts to 14.4 V for charging the battery after the voltage stabilizing circuit. In order to prevent the battery from over-charging the super capacitor, a unidirectional conducting diode is connected in series between the voltage stabilizing circuit and the storage battery. The specific working process of the composite energy recovery device is as follows. The voltage sensor connected in parallel at both ends of the super capacitor group is detected in real-time by the super capacitor group's voltage signal, which is collected in the DSP controller by the A/D sampling module of the DSP controller. When the voltage at both ends of the super capacitor group is detected to reach 4 V, the DSP controller outputs two pulse width modulation signals. One output, pulse width modulation 1, is high and it drives the MOSFET switch A to open. The other output, pulse width modulation 2, is low and it drives the MOSFET switch B to close. At this moment, the super capacitor group is in charge status and the three-phase alternating current generated by the brushless DC motor through a three-phase rectifier bridge and a filter circuit becomes a stable DC voltage that is boosted to 9 V by boost module 1. The super capacitor group is steadily charged by 9 V's voltage through the MOSFET switch A and the equalizing circuit. When the voltage at both ends of the super capacitor group is detected to reach 9 V, the DSP controller outputs two pulse width modulation signals. One output, pulse width modulation 2, is high and it drives the MOSFET switch B to open.