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A Positive Flow Control System for Electric Excavators Based on Variable Speed Control

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A Positive Flow Control System for Electric Excavators Based on Variable Speed Control applied sciences Article A Positive Flow Control System for Electric Excavators Based on Variable Speed Control Shengjie Fu, Zhongshen Li , Tianliang Lin *, Qihuai Chen and Haoling Ren College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China; [email protected] (S.F.); [email protected] (Z.L.); [email protected] (Q.C.); [email protected] (H.R.) * Correspondence: [email protected] Received: 11 June 2020; Accepted: 13 July 2020; Published: 14 July 2020 Abstract: Energy conservation and emission reduction of construction machinery are the focus of current research. The traditional excavator, whose hydraulic pump is driven by the engine, has high fuel consumption and emissions. Furthermore, it is difficult to match the working point of the engine to that of the hydraulic pump. Current pure electric drive technology has the advantages of zero pollution and low noise, and the motor used has the advantages of fast response and a wide speed range. Based on the characteristics of the pure electric drive technology, a positive flow system based on variable speed constant displacement instead of a variable displacement pump for pure electric construction machinery is put forward to realize the flow-matching of the whole machine. The basic structure and working principle were introduced. The control process was analyzed. The controllability and energy saving of the proposed system were tested through simulation and experimental analysis. The research results showed that the controllability of the proposed positive flow system was comparable to that of the traditional throttling speed-regulating control system. The energy-saving efficiency of the proposed positive flow system is increased by 35.2% compared to that of the tradition control system. To further exploit the strong overload capacity of electric motors of electric construction machinery and solve the insufficient power under sudden load, research on constant power control will be carried out in the future. Keywords: construction machinery; energy saving; positive flow system (PFS); throttling speed-regulating control system (TSCS); 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. Appl. Sci. 2020, 10, 4826; doi:10.3390/app10144826 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4826 2 of 13 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]. Lin 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]. Bender 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]. Han 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. The remainder of this paper is organized as followings: Section2 briefly introduces the structure scheme and working principle of the proposed PFS based on motor variable speed control. The control rules and control strategies of the PFS are introduced in Section3. The controllability and energy-saving performance of the proposed system are analyzed in Sections4 and5 by simulation and experiment. Conclusions are given in Section6. 2. Structure and Principle To further improve the energy saving and controllability of hydraulic excavators, a PFS based on variable speed control for pure electric excavators is proposed, as shown in Figure1. The power of the motor is supplied by the power network, which minimally increases the installation cost, but can reduce the use cost to the user. In the proposed PFS, the output flow of the hydraulic pump is adjusted by changing the motor speed to meet the requirement of the load. Therefore, the energy saving of the hydraulic system can be realized by reducing the flow loss. The working process of the proposed system is as follows: when the electronic control handle receives a signal and leaves the middle position, it outputs the control signal to the motor frequency converter and to the pilot pressure-reducing valve through the controller. The multiway valve works at a certain opening according to the control signal and outputs a certain pressure and flow rate to the actuator. The sensor detects the pressure signal of the actuator and sends it to the controller. The controller outputs the control signal to the frequency converter to control the motor speed to make the output flow of the hydraulic pump match the requirement of the load. In this process, all the output flow of the hydraulic pump enters the actuator. Accordingly, the relief valve, in parallel with the main pump, which is quantitative, is used as a safety valve and there is no overflow loss. When the handle is returned to the middle position, the hydraulic pump outputs flow directly back to the tank through the multiway valve. The proposed system has the following advantages: (1) There is no Appl. Sci. 2020, 10, 4826 3 of 13 overflow loss and throttling loss; therefore, the efficiency is high. (2) Due to the fast response of the Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 motor, the pressure can be quickly established to overcome the load when the handle leaves the middle middleposition. position. (3) Since (3) the Since motor the hasmotor high has effi highciency efficien in acy wide in a speed wide range,speed range, it can makeit can themake main the pump main pumphave a have wide a range wide ofrange output of output and adapt and toadapt different to different working working conditions. conditions. Figure 1. PrinciplePrinciple diagram diagram of of the the positive positive flow flow system system (PFS) (PFS) of of an an electric electric excavator excavator based based on on variable variable speed control. 3. Control Control Strategy Strategy Figure2 2 is is the the control control structure structure diagram diagram of theof the PFS PFS based based on theon variablethe variable speed speed control control for a purefor a pureelectric electric excavator. excavator. It includes It includes the following the following three parts: three (1) parts: Control (1) unit,Control which unit, consists which of consists an electronic of an electroniccontrol handle control and ahandle controller. and (2)a controller. Volumetric speed(2) Volumetric regulation, speed which regulation, consists of awhich frequency consists converter, of a frequencya motor, and converter, a main pump. a motor, (3) and Throttling a main speed-regulating, pump. (3) Throttling which speed-regulating, includes a pilot pressure-reducingwhich includes a pilotvalve pressure-reducing and a multiway valve.
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