## 4 Electro-Hydraulic Speed Smoothness Control and Energy Recovery for Heavy Vehicles
Sliding mode control method is used to design the wheel slip controller, and the control input is calculated by the estimated braking forces and brake disk-pad friction coefficient. The performance and effectiveness of proposed vehicle speed smooth control system are proved in hardware in-the-loop simulator (HILS) experiments. For the hydrostatic driving system of closed-type pump-control motor applied in the tunnel segment truck, [PERSON] and [PERSON] (2017) designed a hydraulic retarder continuous braking device embedded into the hydraulic drive system. The devices are constituted of constant pressure pump, proportional relief valve, and other electro-hydraulic components. And it makes up for the problem of brake overheating caused by braking for a long time, which easily leads to brake failure and safety accident. The architecture schematic of close pump-controlled motor hydraulic system is shown in Figure 10.  
Figure 9: Control scheme of three-layer speed smooth control. [PERSON] et al. [78] analyzed the influence of displacement with a hydraulic pump on braking performance for hydraulic driving vehicles under long down-slope, and proposed the principle that hydraulic system and anti-drag braking ability of engine should be exerted reasonably. For the research on eddy current retarder of heavy-duty vehicle, [PERSON] et al. [79] optimized the design of eddy current retarder based on the robust optimization theory. [PERSON] [80] proposed a new system structure scheme, which combines the hydraulic motor-generator energy recovery unit and series throttling valve. Furthermore, a stepwise optimization design method with structural parameters of stator and rotor is proposed. Energy recovery of heavy engineering vehicles under sustained negative load can not only improve energy efficiency, but also extend the life of vehicle components. Moreover, it is of great significance for the requirements for energy saving and emission reduction. In order to regenerate vehicle vibration into electric power and adjust the required damping according to the road condition, [PERSON] et al. [81] proposed an active control approach to recover energy from damped and regenerative dampers. In addition, an electromagnetic energy harvesting damper with 12 independently controlled slot winding sensors (SWT) is designed. [PERSON] et al. [82] presented a hydraulic energy regeneration system based on closed-loop hydraulic transmission device, and a hydraulic accumulator is used as an energy storage system to recover kinetic energy without any reversal of fluid flow. Moreover, a hierarchical control system is established, and adaptive fuzzy sliding mode control is designed for speed control of secondary device. At present, the technology has been applied to the hydraulic system of excavators. Energy recovery usually uses two types: no energy storage devices and energy storage devices. The former mainly reuses the cycle energy through energy regeneration systems, but the energy utilization is limited, which restricts its application [83]. The latter mainly includes mechanical, hydraulic recovery, electrical recovery, and hybrid. Through mechanical recycling technology, the recovery energy is converted to kinetic energy of flywheel. And the energy is released when needed, which plays the role of auxiliary oil supply. Although the technology has been applied in hydraulic system, the controllable performance is not ideal. Hydraulic recovery technology mainly uses the accumulator to realize the recovery of system pressure and potential energy. The technology is relatively mature, and is more suitable for occasions with changeable working conditions. Caterpillar et al. [84] used accumulator to recover the large chamber pressure of hydraulic brake and redundant power of pump, and the effect energy saving is improved well. Domestic universities are also actively studying hydraulic energy recovery technology, such as Zhejiang University, Central South University, Jilin University, etc.