## 4 Electro-Hydraulic Speed Smoothness Control and Energy Recovery for Heavy Vehicles  
Under the flat road and climbing conditions, the displacement pump of the walking system for heavy-duty vehicles is driven by the engine. The speed regulation of hydraulic motor is mainly controlled by displacement regulation for variable displacement pump. However, if there are no necessary retarding measures under long down-slope, the speed of vehicle will continue to increase under the influence of gravity. Traditional braking methods use the travelling brake and parking brake on each motor to maintain the speed of vehicle. However, when the transport vehicle is downhill for a long distance, using a long-lasting brake to control the speed will cause overheating on the brake pads, which results in brake failure and safety accidents. Therefore, in order to ensure the function of speed regulation when going downhill and at a constant speed, an auxiliary continuous braking scheme is needed. Current major solutions of retarded braking for heavy vehicle include hydraulic auxiliary brake, eddy-current auxiliary brake, engine auxiliary brake, etc. In view of electro-hydraulic retarding control strategy and energy recovery and reutilization for heavy vehicles, [PERSON] et al. [69] proposed a nonlinear control method based on adaptive sliding mode control, which solves the problem of engine torque control in regenerative mode. Experimental results show that the controller has better performance, such as robustness, convergence of tracking error, and anti-interference characteristics. Aiming at the problem of high idle speed and high fuel consumption with the engine in the long-distance downhill condition, [PERSON] et al. [70] designed an anti-drag feedback electric drive system and raised a three-layer speed smooth control strategy. The upper control algorithm determines the driving state of vehicle in accordance to the influence factors, such as drag state confirmation value, vehicle speed and engine speed. The middle controller is in charge of starting and running control, and simultaneously realizes the tracking of rotor frequency and voltage to reduce the impact of starting current. The lower controller deals with the driver's brake pedal and operational intent, where normal exit and emergency braking intention are judged according to the brake pedal opening and its change rate. Moreover, the output anti drag speed is calculated by brake pedal opening and bus voltage. The control scheme of proposed three-layer speed smooth control is shown in Figure 9.  
Aiming at the problem of speed regulation and energy recovery in hydraulic drive system for construction machinery, for instance hydraulic excavator under intermittent negative load, [PERSON] et al. (2017); [PERSON] et al. (2017) proposed a control scheme of energy regeneration based on a hydraulic motor-generator, which achieves satisfactory control effect in the experiment. In the reference ([PERSON] et al., 2017), an engine brake mechanism design for diesel engine application with decompression effect is presented, with which the emissions are drastically reduced, even eliminated. Considering the problem of insufficient braking torque with eddy current brakes (ECB) at low speeds, [PERSON] et al. (2017) studied new methods of improving the generation capacity of braking torque with ECB. Aiming at the shortcomings of heavy duty vehicle in retardation braking, [PERSON] et al. (2017) proposed a design scheme of hydraulic auxiliary retarder, which mainly consists of hydraulic pump/motor element, accumulator and relief valve, etc. [PERSON] et al. (2016) proposed a speed smooth control system of vehicle, which composes of the braking monitor, wheel slip controller, and target slip assignment algorithm. The extended Kalman filter is used to estimate tire braking force at each wheel, the brake disk-pad friction coefficient, and the lateral tire forces.