| Distributed electric wheel drive is the most promising drive type of electric vehicles in the future because of its simple transmission chain and flexible dynamic control.However,due to the limitation of rim space and unsprung mass,the multi-speed mechanical transmission or CVT is not convenient to be arranged in the electric wheel,and it is difficult to transfer the wide power demand range of the vehicle to the narrow efficient working range of the drive motor.In this paper,the dual-motor rotational speed coupling is applied into the design of the electric wheel,and a novel dual-motor coupling drive electric wheel(DCEW)is proposed to realize the electronic speed change of the electric wheel.Furthermore,a hierarchical coupling electric powertrain(HCEP)is proposed based on the novel DCEW and the torque allocation between the vehicle's front and rear axles,which provides the possibility for distributed drive electric vehicles to maintain high efficiency in a wide speed and torque range(high efficiency in a wide domain).Based on the proposed DCEW and HCEP and the design requirements of high torque density and high power density,the integration matching and optimization of the DCEW and the online energy management and mode switching control of the HCEP are researched in this article.The main research contents and conclusions are as follows:(1)The configuration,working mode,power transmission route and energy saving potential of the proposed DCEW and HCEP are studied.Firstly,the configuration of the DCEW and HCEP and the principle that the efficient working range of the vehicle is widened are described in detail.Secondly,the working functions of each sub-motor in the DCEW are divided,and the working modes and power transmission of the upper and lower coupling layers in the HCEP are analyzed.Finally,the energy saving potential of the HCEP is studied from the aspects of the proportion of the vehicle's high efficiency zone and the energy consumption under the cycle condition.The results show that under the preliminary parameter matching,the proportion of the vehicle high efficiency zone(efficiency?85%)of the HCEP reaches 52.1%,but that of the traditional powertrain equipped electric wheels slowly driven by single-motor(PSDSM)only reaches 26.2%.Under ten driving cycles,the HCEP can save energy 6.4%-11.5%compared with the PSDSM.(2)A parameter integration matching and optimization method for the novel DCEW is studied and proposed.An integrated optimization method with two optimization layers is proposed to solve the complex issue of parameter matching of the DCEW.The parameter matching of the DCEW involves multiple optimization objectives and in which the characteristic parameters and structural parameters of components are interwoven together.In the integrated optimization method,the characteristic parameters the DCEW are optimized in the outer optimization layer,while the structural parameters of the DCEW are optimized in the inner optimization layer.Firstly,the feasible characteristic parameter combination of the DCEW,i.e.,the search domain of the outer optimization layer,is determined according to the vehicle peak/common power demand and the function division of sub-motors in the DCEW.Secondly,in the inner optimization layer,the evaluation index values and the optimal structural parameters of the DCEW for each feasible characteristic parameter combination of the outer optimization layer are obtained by combining experimental design,regression analysis and multi-objective optimization.Finally,in the outer optimization layer,the optimal combination of characteristic parameters is selected from the feasible region,so as to realize the global optimization of characteristic parameters and structural parameters of the DCEW.The results show that after the integration optimization,the total volume of the core components in the DCEW decreases from3.1L to 2.7L,the total mass decreases from 13.9kg to 10.7kg,and the proportion of the vehicle efficient zone increases from 52.1%to 68.0%.(3)A online energy management strategy is studied and put forward for the HCEP.The energy management of the HCEP is complicated because the torque allocation and rotational speed allocation exist together and the latter is restricted by the former.In order to realize simple and effective online energy management of the HCEP,simplified rotational speed allocation and torque allocation methods are introduced to transform the energy management issue from optimization in three-dimensional continuous space to optimization in a set containing only six elements.Secondly,the simplified energy management issue is solved offline to obtain the optimal operating mode sequence of the HCEP.The basic decision rules of the working mode of the HCEP are extracted from the obtained optimal working mode sequence.In order to avoid frequent working modes witching,the auxiliary decision rules are built based on the basic decision rules,and the online decision logic of the working mode is further established for the HCEP.Then,according to different driving conditions,the auxiliary decision rules are optimized to increase the adaptability of the online decision logic.Finally,combined with the driving condition identification,introduced rotational speed allocation and torque allocation methods and the decision logic of the working mode,an online energy management strategy of HCEP is developed.The results show that the introduced simplified rotational speed allocation and torque allocation methods do not destroy the energy saving potential of the HCEP.The proposed online energy management strategy can not only fully guarantee the energy efficiency of the HCEP,but also greatly reduce the mode switching frequency.The recognition accuracy of the trained driving condition recognitor can reach 95.45%,which makes the proposed online energy management strategy adapt to the change of driving condition well.(4)A working mode switching control strategy of the HCEP is studied and proposed.In order to realize the fast and stable working mode switching of the HCEP,especially the simultaneous switching of working modes at multiple dynamic coupling points,the switching process among various working modes is analyzed.Control flows when only the operating mode of the lower coupling layer needs to be switched,control flows when only the operating mode of the upper coupling layer needs to be switched,and control flows when the operating mode of the upper coupling layer and that of the lower coupling layer need to be simultaneous switched are established.The causes of dynamic shock in the mode switching process are studied.And,the coordinated action method and torque compensation method are put forward to suppress these dynamic impact.The results show that the total time of mode switching is less than 0.4s when only the upper coupling layer or the lower coupling layer needs to perform mode switching.When the upper and lower coupling layers perform mode switching simultaneously,the total time of mode switching is less than 0.55s.The vehicle dynamic impact degree is withiną10m/S~3 in all mode switching processes.(5)The integrated control model of the energy management and mode switching of the HCEP is established and analyzed.The results show that under the control of the proposed online energy management strategy and mode switching strategy,the torque output by the HCEP can meet the demand of the vehicle.The HCEP can quickly and accurately track the optimal working mode and power allocation trajectory decided by the energy management strategy owe to the developed mode switching strategy. |
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