# A Positive Flow Control System for Electric Excavators Based on Variable Speed Control
## 1 Introduction  
Construction machinery consumes much energy and has great significance to energy conservation and emissions reduction. Various energy-saving forms have been proposed, such as hybrid power technology, which can improve fuel efficiency to some extent, but still depend on the engine [1]. There are some problems with the engine, such as low energy conversion rate, large noise, high vibration, and bad pollution discharge. With the development of power electronics technology, frequency control technology, and battery-based energy storage technology, pure electric drive technology is now widely used. The application of pure electric drive technology in construction machinery can reduce the emissions and noise, which are the shortcomings of traditional diesel and hybrid motors. Pure electric drive technology has the following advantages: (1) Electric energy, which is clean and efficient, is used. It can truly achieve zero emissions and operate pollution-free, and can meet the goal of energy conservation, emissions reduction, and sustainable development [2]. (2) The motor is used as the power source, which can improve the energy conversion rate of the power source. (3) The motor has good speed regulation, which can help to realize the power matching of the system, reducing energy loss and improving the controllability and energy-saving [3; 4]. (4) The motor has good short-term overload capacity and can be applied to large burst load conditions. To improve the energy utilization of the construction machinery, many research institutions and scholars have devoted themselves to studying the energy conservation of hydraulic systems. Some technologies, such as the negative flow control system [5, 6], positive flow system (PFS) [7, 8], load sensitive control system [9, 10], and load port independent control system [11, 12] have been successively proposed and developed. Their purpose is to reduce overflow loss and throttling loss of the system through flow matching to improve the energy utilization. PFS has no bypass oil return and no oil return loss. Furthermore, there is no pressure compensator, which can add throttling loss. Compared with other hydraulic energy saving technologies, PFS can reach the best energy saving effect. However, in a PFS, when the actuator needs to work, it takes the pump a long time to set up pressure to overcome the load [13]. [PERSON] et al. studied the automatic idle control characteristics of a hydraulic excavator based on a PFS driven by pure electricity, but they did not discuss the characteristics of the PFS [14]. [PERSON] et al. introduced a predictive operator modeling of a virtual prototype of a hydraulic excavator using a PFS and analyzed the influence of different driver factors on the working cycle time and capacity consumption, but they did not investigate the performance of the PFS [15]. [PERSON] put forward an energy-saving technology of a fully controlled positive flow excavator, and analyzed the energy saving property of the system. A variable pump was used in the proposed system [16]. At present, the research on the PFS of construction machinery is mainly focused on the variable pump driven by the engine, and there is less research on quantitative pumps driven by the variable speed control of the motor, which has excellent speed regulation performance. Using motor frequency control to substitute the engine is dependent on the performance of both the motor and quantitative pump. To achieve as high energy savings as possible, optimization of the hydraulic system and parameter matching between the pump and the motor should be carried out. The frequency control technology is adapted to the hydraulic excavator by adjusting the motor speed to achieve flow matching between the hydraulic pump and the load. A PFS based on variable speed control is proposed through optimizing the parameters of the hydraulic system and the motor. Simulation and experimental analysis are used to verify the feasibility and efficiency of the proposed PFS based on variable speed.