| With the increasing requirements of modern agriculture for environmental protection,the demand for green agricultural machinery equipment is more and more urgent,making electric tractors have become a research hotspot.Among them,the extended-range electric tractor has attracted much attention in the demand of medium and large horsepower.It has a unique advantage as an intermediate product,which not only makes up for the lack of endurance of pure electric tractors,but also reduces pollution emissions compared with traditional tractors.In order to better achieve energy conservation and environmental protection,the research on energy management strategies for extended-range electric tractors has become particularly important.In addition,as one of the important agricultural tasks of tractors,the operation characteristics of rotary tillage are different from other agricultural operations(such as ploughing and transportation).To sum up,a medium-size horsepower extended-range electric rotary-tilling tractor is taken as a research object in this paper,on the basis of the analysis of the driving characteristics and power flow characteristics,the powertrain system model of the extended-range electric rotary-tilling tractor is established,and the rotary tillage cycle model is designed to provide the simulation environment.Then,three different structures including single-motor drive structure,dual-motor coupling drive structure,and dual-motor independent drive structure are studied by means of energy management strategy based on Dynamic Programming(DP),respectively.The main research contents of this paper are as follows:Firstly,taking a wheeled rotary-tilling tractor as the research object,the driving formula of the rotary-tilling tractor is established by analyzing the dynamics under the rotary tillage operation in the dry field.After that,the power flows of the transmission system for the three structures including the single-motor drive structure,dual-motor coupling drive structure,and dual-motor independent drive structure are analyzed.The results show that a closed-loop system of pure mechanical energy will be formed in the transmission system of the single-motor drive structure;in the transmission system of the dual-motor coupling drive structure,there are two closed-loop systems,one is a closed-loop system of pure mechanical energy,the other is a closed-loop system of “mechanical energy-electrical energy-mechanical energy”;in the dual-motor independent drive structure,there is no closed-loop system of pure mechanical energy through the transmission system,but energy conversion will occur by forming a closed-loop system of “mechanical energy-electrical energy-mechanical energy”.Furthermore,taking the shaft of rotary tiller blades as the object,the kinematics analysis of the rotary tiller is carried out.Based on the principle of power balance,the relationship between the power required by the drive motor and the return power under three powertrains is derived respectively,which provides a theoretical basis for the research on energy management control strategies.Secondly,according to the rotary tillage operation characteristics of the extended-range electric rotary-tilling tractor,a novel backward model method for rotary tillage operation is proposed,specifically,the driving speed information and rotary tillage torque are used as inputs in the model.Afterward,a simulator for the extended-range electric tractor is designed including the Genset model,drive motor model,battery pack model,transmission system(reducer,differential)model,wheel-soil system model,and engine accessory and auxiliary load system model,which provides a simulation platform for the research on energy management strategies.Thirdly,based on the small pure rotary-tilling tractor with a dual-motor independent powertrain which was fabricated by the research team,rotary tillage experiments in the field are carried out to obtain the power values of the two motors(one is to drive the rotary-tilling tractor forward,the other is to drive the Power Take-off(PTO)shaft).The test results show that under the tillage depths of 5 cm and 13 cm,the ratio of the power demand of the PTO motor to the total power demand of the whole machine is 65%～75% and80%～90%,respectively,which is similar to the traditional tractor under the rotary tillage operation,indicating that the obtained experimental data can be used to simulate single-motor drive structure and dual-motor drive structure.On this basis,a design method based on the combination of actual data and empirical formulas is proposed to establish a rotary tillage operation cycle model for a medium-size rotary-tilling tractor,which gives an environment of operation cycles to study energy management strategies of extended-range hybrid electric rotary-tilling tractors.Then,to deal with the energy management problem of the extended-range electric rotary-tilling tractor,the general formula of mathematics is defined.Combined with the characteristics of the rotary tillage operation,an energy management strategy based on DP is proposed.Under the established cycle models of rotary tillage with the depth of 5 cm and the depth of 13 cm,simulation experiments are carried out on the single-motor drive structure,the double-motor coupled drive structure,and the double-motor independent drive structure,respectively.The results show that the reduction power generated by the horizontal component of the rotary tillage resistance always exists,but the feedback power does not necessarily exist;the cycle utilization rate of the pure mechanical feedback power in the transmission system is much larger than the cycle of the feedback power through the mechanical energy-electrical energy-mechanical energy conversion utilization.Finally,taking the dual-motor independent drive structure as an example,a new type of rapid prototype test platform is built to carry out the rotary tillage operation test with the depth of 5 cm,and the bench test results are compared with the simulation test results.The results show that the simulation curves are in good agreement with the bench test curves,and the simulation model can describe the changes of the power of each motor,the power of the Genset,and the SOC of the power battery pack.The fuel consumptions of the simulation test and the bench test are 4065.5 g and 3994.7 g,respectively,and the relative error is about 1.77%,which verifies the rationality and accuracy of the established simulation model for the extended-range electric rotary-tilling tractor. |
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