## 1 Introduction  
Various types of heavy construction machinery and engineering vehicles [1] are playing an increasingly important role in the construction of large-scale projects, such as high-speed railway, highways, large bridges, as well as the construction of underground tunnels for coal mining. The working condition under sustained negative load is common in electro-hydraulic loading control system [2]. Then, the traveling drive system, hoist system and swing system of construction machineries and engineering vehicles are inseparable from the electro-hydraulic power mechanism under sustained negative load. As the crucial technology of above systems, the control technology of electro-hydraulic power mechanism has greatly influenced the control performance for construction machinery and engineering vehicles. Therefore, systematic study on advanced regulation mechanism and control strategy for electro-hydraulic power mechanism with negative load is of great value, which enhances integrated control performance for overall electro-hydraulic mechanism. Sustained negative load, also known as over-running load, often occurs in long down-slope condition in traveling system of mobile machines, descending condition under heavy load in electro-hydraulic hoist systems of hoisting cranes, swinging and boom sticks conditions in excavators, and negative load condition of electro-hydraulic loading systems. Negative load operation is inevitable and standard working condition to a completed task cycle. The characteristics of negative load in electro-hydraulic power system is that the load force's direction is in accordance with the orientation of load movement, and the function of load force is changed from opposing motion to driving motion. In fact, the running speed of moving parts will continuously increase due to the increase of driving force incorporating the force from negative load. But the speed may even exceed the actuating components' maximum running speed which can be provided by flow rate of the hydraulic pump. Moreover, the dithering phenomenon in the process of system running can easily cause safety accidents. Aiming at the method of traditional flow rate regulation for electro-hydraulic control system with sustained negative load, two control modes are usually used. One mode is the valve-control mode, which uses dual-port mechanical linkage control valve to realize the throttling regulation. The other mode is the pump-control mode, which uses proportional or servo variable displacement pump to achieve the volume regulation. The former adopts inlet/outlet mechanical coupling metering, and a large valve-restriction pressure difference is needed for balancing the negative load. And the latter commonly adopts the balance valve in return-oil line to form the back-pressure for speed smoothness control. In the former case, because of the throttling and coupling control with dual-port mechanical linkage control valve, it is difficult to achieve superior control effect on speed control and smooth regulation. For the latter, since the balance valve can't be actively regulated, the satisfactory control effect also can't be obtained. Under sustained negative load, the operating points of electro-hydraulic control valve for motion control system are in quadrants II and IV, and pressure flow coefficient of the valve is reduced, then control characteristics are deteriorated when the orifice is small. The coupling control with flow rate and load pressure in traditional electro-hydraulic mechanism results in a large valve-restriction pressure difference in inlet/outlet oil loop, then a good coordination between speed control and speed-smooth regulation cannot be realized. In references [3_A_], the hydraulic balance method and the stability of balance valve under negative load are studied, and relevant hydraulic balance scheme is proposed.