## 1 Introduction
[11] presented an energy management algorithm based on the IMV configuration using a single-edge meter-out control valve (one proportional valve) to enhance the energy efficiency and reduce investment costs of the hydraulic excavator. The coordinated control between the pump and valve was designed with two-level control for the valve (flow/pressure control) and three-level control for the pump (pressure/flow/hybrid control) for motion characteristic tracking and saving energy on the system. This proposed algorithm showed that the energy saving efficiency reached up to 28% compared with the load-sensing system. Especially, by fully opening the meter-in valve and only controlling the meter-out valve can reduce the energy loss of the system. Nevertheless, multiple control strategies for valves and pumps cause complexity in system control. Moreover, open-loop control at flow control mode for the pump can make a lack of flow in the cylinder due to the leakage of the pipes and the valves. Generally, the above studies have reduced the number of proportional valves and provided a reasonable control strategy for the IMV system, but the load factor has not been considered, especially influence of the variable load during the operation of the system. In our previous studies, the investigation of NIMV system in the boom excavator showed that this system was efficient in terms of energy saving and investment costs. With a configuration that uses three EHPV, the NIMV system reduced one proportional valve and the check valve on the CIMV while ensuring the working performance across the metering modes. Moreover, applying the VFPB control for the meter-out valve and position feedback control (PFC) for the pump helped the system to operate stably, increase tracking precision, and save up to 6.5% energy compared with the CIMV system [31; 32]. However, this study was only focused on the case of a fixed load condition. In fact, the cylinder was always under the influence of a variable load during operation, this was effect to the accuracy and the energy consumption of the NIMV system. To solve the above problem, this paper proposes a novel control method for the NIMV system. The proposed control method is the cooperation between the pump and valve control for the boom system under variable load. The main contributions of the article are presented as follows:1. The velocity-load feedforward and position feedback (VLFPB) control method is applied for meter-out valve control, and the velocity feedforward and position feedback (VFPB) methods are used for the speed control of the pump for the purpose of enhancing the tracking precision of the cylinder under variable load;
2. Depending on the working conditions, a switching mode based on the load and the velocity of the cylinder is presented to select the suitable mode to improve the energy saving of the system. Moreover, the co-simulation model by using the LMS AMESim and MATLAB software is built to demonstrate the effectiveness of the proposed control method. The simulation results of the proposed method are compared with the previous study based on two criteria, namely energy consumption and tracking precision. In detail, the displacement error of the boom cylinder can be reduced by 3.6% compared with the previous study. In addition, energy consumption is reduced by 6.4%. The rest of this paper is organized as follows: Section 2 presents the metering modes of the NIMV system. Control strategy that includes valve control, pump control and switching mode of the proposed control method is provided in Section 3. Meanwhile, the co-simulation and result are discussed in Section 4. Moreover, the conclusions are shown in Section 5.