Power Matching and Energy Efficiency Improvement of Hydraulic Excavator Driven with Speed and Displacement Variable Power Source
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Mobile machinery energy efficiency and emission pollution are the national and worldwide issues. This paper contributes in solving these problems by applying a speed variable power source. Unfortunately, almost all of the speed variable systems have the dynamic response problem when the motor starts with full load or heavy load. To address this problem, a hydraulic accumulator is used to balance the load of the power source for assisting starting of the motor and a matching method combined with speed and displacement control of the pump is proposed to improve the energy efficiency and dynamic performance simultaneously under different working conditions. Also, the power source/valve combined control strategy of an independent metering system is designed to realize flow matching of the whole system. Firstly, a test system is established to study the dynamic performance and energy efficiency of the speed variable power source with an auxiliary accumulator. Working performance and energy consumption of the power source under different rotating speeds and different loads are studied. And then, the hydraulic excavator test rig with the proposed system is constructed. Furthermore, the working performance of the excavator with the speed-fixed and speed-variable strategy are studied comparatively. Results show that, compared with fixed-speed strategy, the electric power consumption during the idle period and partial load condition can be reduced about 2.05 kW and 1.37 kW. The energy efficiency of speed variable power source is about 40%‒71%, which is higher than that of the fixed-speed power source by 3%–10%.
KeywordsHydraulic excavator Power matching Speed variable Energy efficiency
As the main construction machinery, hydraulic excavator is widely used in earthwork construction field, due to their small size-to-power ratio, compact structure and big actuation forces. In the excavator, a displacement variable pump driven by a diesel engine is normally used as power source, and a set of four-sides spool valves are used to control the actuators. In such systems, one or two pumps supplied oil to more than four actuators and the flow distributing by throttling caused large energy consumption.
Ref.  shows that the energy efficiency of the pump is about 87%, the efficiency of hydraulic control system is only 30%, the efficiency of the mechanical system is about 90%, thus the energy consumption for working is only 23% of the engine output. Besides, the average energy efficiency of the engine is only about 35%, the energy efficiency of the whole machine is very low and there is serious emission pollution [2, 3]. This problem not only exists in hydraulic excavators, but also the common problems of other engineering equipment. Thus, improving the energy efficiency and reducing emission has been the research focus in this field.
Considering the energy transfer chain, there are two ways to improve the energy efficiency of hydraulic excavator: one is to improve load-engine matching performance, another is to reduce the throttling loss.
In the conventional excavator system, the operating point of its engine should meet the needs of load power, which varies widely when the excavator works . Thus, the engine often works under partial load condition, during which the energy efficiency is low. Many research efforts on these issues of the excavator have been undertaken. A conventional way to improve the efficiency of the diesel engine is adjusting the engine speed by the operator according to load condition . Yang proposed a diesel engine cylinder deactivation technology by cutting off the fuel supply to one or two of the engine cylinder . However, these two ways can only improve the economy of diesel engine by mechanical stepped speed changing system and it is only effectively in light load. In order to global improve the engine efficiency, the hybrid technology, in which a hydraulic motor combined with accumulator or electric motor/generator is used as the auxiliary power unit, is proposed and it can downsize the engine power also [7, 8]. Kim et al.  studied this hybrid system and proposed an algorithm based on the equivalent fuel minimization strategy (ECMS) to analyze the behavior of the co-state of the optimal control problem. Lin et al. [10, 11] proposed a boom potential energy regeneration system for a hybrid hydraulic excavator. The test results showed that an estimated 45% of the total potential energy could be regenerated. Shen et al. [12, 13] studied a hydraulic hybrid excavator based on common pressure rail in which the throttling loss was eliminated completely.
In mobile fluid power systems, several actuators are connected to one common pump. The pressure level and flow demand of one actuator will normally vary considerably during a duty cycle. One obvious way of meeting the demand of energy efficiency is to make use of load sensing, negative control and positive control technology [14, 15, 16]. However, in these systems, a four-side spool valve is used to control the actuator, which causes large throttling loss. Some theses attempt to deal with this problem by using an independent metering in and metering out system for controlling the excavator hydraulic actuators. And this is also one of the research hotspots in valve controlled systems [17, 18]. Sitte et al.  introduced the structure of the independent metering system, and given an examination for mobile applications. Xu et al. and Quan et al. studied the pump/valves coordinate control of the independent metering system for excavator [20, 21, 22]. Shi et al.  proposed a velocity and position control system based on independent metering system for a hydraulic excavator.
Although many technologies have been made to improve the energy efficiency of the hydraulic excavator, such as hybrid technology. However, the energy efficiency of the engine is not higher than 40% and the emissions can not be eliminated completely. This paper proposes an advanced construction machine configuration named electric excavator, in which a frequency conversion electric motor is used to drive a displacement variable pump. As a certain operation point can be supplied by different combinations of driving speed and pump displacement, intelligent control strategies can address major issues like energy efficiency, process dynamics and noise level in hydraulic excavators [24, 25, 26]. Yan et al.  studied the noise of the speed and displacement variable power source. Roosen et al.  studied the energy optimization of hydraulic pump system, and the design process was presented by using the “ Parker-Drive-Creator” software. In this paper, a control method based on the segmented speed and continuous displacement control of the pump is proposed to improve the energy efficiency and dynamic performance simultaneously under different working conditions. And also, an accumulator is introduced to the pump inlet port to further improve the system dynamic response performance when the electric motor starts with full load or heavy load. Furthermore, an independent metering in and metering out system is used to reduce the throttling loss.
The paper is organized as follows. Section 2 presents the structure and principle of the electric excavator system with independent metering in and metering out system. The dynamic performance and energy consumption characteristic of power source which consists of speed variable electric motor and displacement variable pump based on mathematical model and experiment results in Section 3. Section 4 concentrates on the controller design. Section 5 provides the experimental results. Finally, conclusions are drawn in Section 6.
2 Working Principle of Electro-hydraulic Excavator System
As shown in Figure 1, a speed variable inverter motor is used to drive an electric displacement and pressure variable pump for controlling the flow rate and pressure of the system. And,an independent metering in and metering out system is used to distribute the flow to the actuators. And also, the boom cylinder, arm cylinder and swing motor are controlled by the independent metering in and metering out system, and bucket cylinder is controlled by a traditional three-position four-way proportional valve. And, in the system, some displacement sensors are installed in the actuators to detect the displacement and velocity, and some pressure sensors to detect the pressure inside the actuators and pump port, the power sensor and rotating speed sensor on the motor to detect the electric power and rotating speed.
The proposed system works as below: the controller takes in the inputs of joystick and the measured quantities such as pressures, powers, displacements, flow and speed of the motor. And then the controller analysis the demand automatically and output the voltage signals directly applied to the converter motor, pump and valves according to the set strategy.
The core of the research work is to improve the energy efficiency of the power source and to reduce the throttling loss of the hydraulic system, under the premise of ensuring dynamic characteristics. In the following, these two sections are described in detail.
3 Dynamic Response and Energy Efficiency of Electro-hydraulic Power Source
The degree of freedom of speed and displacement variable pumps can be used to adjust the operation points of the electric drive and the hydraulic pump to maximize the overall energy efficiency. And the process dynamics is determined by the electric motor and pump.
3.1 Dynamic Response
As shown in Figure 2, the electronic proportional pressure and flow pump is driven by an inverter motor. An electric proportional relief valve is used to load the system. And an accumulator is introduced to the system for auxiliary starting and braking. The pump can suction oil from the tank through the check valve or from the accumulator through the solenoid valve. The pump outlet and accumulator oil port are equipped with pressure sensors to detect the pressure of system. The pump outlet is equipped with flow meter also. In order to detect the electric power input of inverter motor, electric power meter is installed before and after the inverter. The ds1103 is used for controlling and signals acquisition.
3.1.1 Dynamic Response of Electro-hydraulic Power Source
Speed variable power source often starts with full load or heavy load, and sometimes its dynamic response may not meet the system requirements. With the system shown in Figure 2, the dynamic characteristics of the power source under different load condition are studied. The flow output of the pump is detected to characterize the dynamic response.
As shown in Figure 3(a), under rated current of the motor, with the increase of the load pressure, the starting time of the electro-hydraulic power source becomes longer, but the braking time becomes shorter. When the load pressures are 0 MPa and 18 MPa, the starting times are about 0.58 s and 1.43 s, and braking times are about 2.62 s and 0.66 s. As shown in Figure 3(b), when the maximum starting current is set to 1.5 times of the rated current, the starting time increases with the load pressure, and the braking time decreases. It can be concluded that, under the 1.5 times of rated starting current, the increasing of the load pressure has less influence on the dynamic characteristics. And the starting time can meet the requirement of the excavator.
3.1.2 Dynamic Characteristics of Pump
The pump used in the system is an electrohydraulic axial piston variable displacement pump whose pressure and flow can be continuously adjusted. And also, benefitted from the feedback of the actual swivel angle and pressure, the controller can be designed to meet flow, pressure and power demand in proportion. The acquired actual values are processed in the amplifier and compared with the given command values.
3.2 Energy Efficiency of the Electro-hydraulic Power Source
During the working process of electro-hydraulic power source, the energy conversion process of power source system is about frequency converter-electric motor-pump. The energy efficiency of the power source changing with the speed, pressure and flow. The main purpose of this paper is to design an efficient electro-hydraulic power source for mobile machine to improve its energy efficiency.
3.2.1 Electric Motor
3.2.2 Axial Piston Pump
It can be concluded that the larger the output flow rate of the pump, the higher the efficiency of the pump, for a certain pump. In addition, the shape of the flow contours and energy efficiency contours are basically the same. Thus, we can conclude that the changing the speed or displacement have less influence on the overall efficiency under a certain flow rate requirement condition. However, for the hydraulic pump, its efficiency is affected by pressure, temperature, rotating speed, and so on. Thus, it is difficult to forecast relationship of these parameters with the energy efficiency of the pump. According to the Ref. , the loss of displacement-controlled pump presented is similar to the theoretical analysis in this paper. Thus, the theoretical data can be used for guiding the control strategy design in this paper.
3.2.3 Electro-hydraulic Power Source
As shown in Figure 6, the energy efficiency of the electro-hydraulic power source increases with the load power increasing. And, according to the tested results, When the load power is given, we can find an efficient speed set value. For example, when the load power is about 5 kW, the energy efficiencies under 300 r/min, 600 r/min, 900 r/min, 1200 r/min and 1500 r/min, are 49.0%, 62.0%, 62.6%, 57.8% and 51.8% respectively. Thus, in the practical application, the power source efficiency can be improved by detecting the load power to change the speed of the power source.
3.3 Dynamic Response Improvement of the Power Source
4 Control Strategy of Electric Excavator
4.1 Flow Calculation Module
4.2 Power Source Module
As shown in Figure 10, according to qd, the speed of the motor nset1 is calculated when the displacement of the pump is set as 80% rated displacement; and then according to qd and the pump’s outlet pressure pp, the load power Pp can be calculated out, the table look up method is used to determine the speed nset2 under which the electro-hydraulic power source works under high efficiency; and then the maximum value of these two speeds are chosen to control the motor.
4.3 Flow Distribution Module
As shown in Figure 11, the control model can be divided into open circuit pump control, metering out control, metering in control, regeneration control and metering in and metering out control, according to the direction of the load and velocity. When the actuator works under resistance load, the open circuit pump control is selected to reduce throttling loss by fully opening the control valves. When the actuator works under over-running load, the metering out control and regeneration control are selected. And when there are more than one actuators working simultaneously, the metering in control method is used to control the actuator which works under low load, and also when this actuator works under over-running load, the metering in and metering out control method is used.
Take cylinder extension as an example, firstly, the load condition of this cylinder can be determined by the cylinder pressures, and then the control model and ratio coefficients kA and kB can be identified, at last, the inlet and outlet flow rates are calculated to control the valves based on the basis of feedback of pressure difference.
5 Experiment and Results Analysis
5.1 Experiment Rig
5.2 Experiment Results
5.2.1 Boom Cylinder
18.104.22.168 High velocity
As shown in Figures 13 and 14, the demand flow is set as 50 L/min. At the time of 1.4 s, the operator gives the extending velocity demand signal by operating the joystick. The electric motor and pump work under the set signals. And at the time of 4.2 s, the operator releases the joystick, the electric motor works under a speed of 300 r/min, and the displacement of the pump is set as its minimum. At the time of 7.6 s, the operator gives the retraction signal, and the boom cylinder retracts under a regeneration model.
As shown in Figure 13, under the same flow rates, the speed and swivel of the pump are about 1500 r/min and 46% with the constant speed strategy, and which are about 900 r/min and 77% with the new designed coordination method. From the pressure and displacement curves, it can be seen that the dynamic performances are approximately equal.
As shown in Figure 14, when the boom cylinder extends out, the electric power consumptions with the two methods are almost the same, which is about 14.84 kW, and the energy efficiency of the power source is about 67.3%.
However, when the pump does not output flow, the power consumption is about 2.67 kW with the constant speed strategy, and which is only 0.62 kW with the new designed method. It can be obtained that the electric power consumption at idle can be effectively reduced.
22.214.171.124 Medium velocity
As shown in Figures 15 and 16, the demand flow is set as 30 L/min. At the time of 1.7 s, the operator gives the extending velocity demand signal by operating the joystick. The electric motor and pump work under the set signals. And at the time of 6 s, the operator releases the joystick, the electric motor works under a speed of 300 r/min, and the displacement of the pump is set as its minimum.
As shown in Figure 15, under the same flow rates, the speed and swivel of the pump are about 1500 r/min and 29% with the constant speed strategy, and which are about 600 r/min and 71% with the new designed coordination method. From the pressure and displacement curves, it can be seen that the dynamic performances are approximately equal.
As shown in Figure 16, when the boom cylinder extends out, the electric power consumptions with the constant speed method is 9.12 kW, which is 7.75 kW with the new coordination method, and the energy efficiencies of the power source are about 57.6% and 67.8%. It can be obtained that the electric power consumption under partial load can be effectively reduced.
5.2.2 The Whole Machine Working
When the boom cylinder extends out, the arm cylinder extends out and retracts in, the bucket cylinder extends out and retracts in, the energy efficiency of the power source with the constant speed method are as follows 67.3%, 48.3%, 61.2%, 39.5% and 58.2%, and which are about 70.3%, 52.5%, 71.5%, 43.2% and 62.6% with the new coordination method.
During most working conditions, the lowest and highest energy efficiency of the power source driven by an electric motor are 43% and 71%, which is significantly higher than the power source with a fuel engine. And also, using the new coordination method, the energy efficiency can be increased about 3%‒10%.
During the idle period, the motor speed is set as 300 r/min and the power source output a constant pressure about 1.5 MPa, the power consumption is about 0.62 kW with the new coordination method, and which is about 2.67 kW with the constant speed method.
When the electric motor starts with full load or heavy load, the dynamic response of the power source is relatively slow. However, the dynamic response of the system can be improved to meet the needs of use with the segmented speed control and continuous displacement control and an accumulator.
LG was in charge of the whole trial and wrote the manuscript; LQ guided the writing of the manuscript. XZ, ZD and JY assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.
Lei Ge, born in 1987, is currently a postdoctoral at Taiyuan University of Technology, China. He received his PhD degree from Taiyuan University of Technology, China. His research interests include power matching of electro-hydraulic power source, energy saving of mobile machinery.
Long Quan, born in 1959, is currently a professor at Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education, Taiyuan University of Technology, China. His research interests include electro-hydraulic control system and hydraulic excavator.
Xiaogang Zhang, born in 1964, is currently an associate professor at Taiyuan University of Technology, China.
Zhixin Dong, born in 1984, is currently a PhD candidate at Taiyuan University of Technology, China.
Jing Yang, born in 1972, is currently an associate professor at Taiyuan University of Technology, China.
The authors declare that they have no competing interests.
Supported by National Natural Science Foundation of China (Grant Nos. 51575374, U1510206).
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