The Study On Distributed Hybrid Braking System Based On EMB

The distributed electric vehicle with four in-wheel motors attracts enormous attentions due to its simple and reliable transmission structure,the high transmission efficiency,and the good dynamics controllability.Pure regenerative braking can't apply large braking force because of the limitation of characteristics of in-wheel motor and charging power of battery.Therefore,the hybrid braking system combining regenerative and friction braking force is required.As a brake-by-wire system,electromechanical brake(EMB)has the capability to satisfy the sense of brake pedal and to continuously and accurately adjust the mechanical friction force.Therefore the hybrid braking system with EMB exhibits high braking energy recovery rate,good braking safety and comfort.Unfortunately,so far,there is no EMB available,and in the substantial repeated calculations is usually necessary to design EMB,and the theory and method for hybrid braking force control still remains to be improved.The aim of the thesis is to investigate the design and development of EMB actuators,the four-wheel braking force distribution,the failure control of EMB,the braking force distribution of hybrid braking force and ABS control.And then the integrated control of distributed hybrid braking system was realized.(1)Within the size limitation for in-wheel installation,an EMB actuator with the desired braking performance was designed and developed for a distributed electric vehicle modified based on “SWIFT”.The output torque of the frameless torque motor is transformed to the clamping force through the planetary gear and ball screw.Parking brake is performed by equipping a V-shape unidirectional clutch at the output end of the planet carrier.The design procedure of the EMB actuator was proposed,and the matching design,structure design,intensity check,system dynamics modeling and performance analysis based on ADAMS,and production of a prototype of the actuator were completed accordingly.(2)In the case of EMB actuator,we proposed an optimal selection of motor,planetary gear and ball screw in two-stage electromechanical systems.The inputs to the method are component data of candidate motors,ball screws,and planetary gears,which ensure the availability of matching results.The constraints are the dynamic resistance load,size limitation,operating range of the motor and performance requirements of ball screw and planetary gear.Additionally,all feasible combinations of motor,planetary gear and ball screw can be achieved.Finally,the multi-objective optimal combination was obtained through the multi-objective function which incorporates actuator weight,acceleration at starting stage,air gap closing time during braking,and equivalent continuous output torque of motor.The efficiency of matching is improved profoundly.At the same time,it avoids the need of explicitly formulating and solving multi-objective nonlinear optimization problems whose solution is critical.(3)The simulation and experiment platform of EMB has been established.And the step response characteristics,frequency response characteristics,and system friction performance of EMB were analyzed.The result indicates that the EMB characteristics meet the target requirements.The frictions which could influence gap closing time mainly come from ball screw and motor,while the frictions which would affect the capability to clamp are from ball screw,motor and thread bearing,respectively.With the clamping force increasing,the influence of the frictions from ball screw and thread bearing becomes more significantly,while the impact of friction from motor greatly reduced.(4)Based on the modularization methodology,the integration of four-wheel braking force distribution,ABS control and braking failure control has been completed for the distributed braking system of pure EMB.To improve the braking stability for braking-in-turn maneuver,taking into account the load transfer and lateral force requirement,a sequential distribution method,which distributes the front-rear braking force prior to inner-outer braking force,was applied.For front-rear braking force distribution,giving priority to meeting the lateral force demand,the remained friction force between tire and road can be used for longitudinal braking force.The inner-outer braking force was distributed proportional to the vertical force.To reduce the development time,an improved ABS logic threshold control algorithm was proposed for EMB braking system.Furthermore,braking force redistribution based on regulation was suggested for the failure of one EMB wheel and the failures of two EMB wheels at different sides.The simulation platform was established,and the simulation results show that:with the strategy for distribution of four-wheel braking force proposed in this paper,the braking stability could be maintained during braking-in-turn maneuver,and the ground adhesion could be fully utilized.With the improved logic threshold ABS control,effective control could be obtained on uniform road with the various adhesions(i.e.with high,medium and low adhesions)and docking road and during braking-in-turn maneuver.For the failure of one EMB wheel and the failures of two EMB wheels at different sides,with the integrated control strategy about braking force redistribution based on regulation and ABS,the requirements for low and medium braking strength can be satisfied,wheel lock can be prevented,and unexpected active yaw torqued also can be reduced.(5)With hierarchical control methodology,integrated control of hybrid braking system with four-wheel braking force distribution,ABS control,braking failure control and hybrid braking force coordination control has been completed.The strategy of hybrid braking force coordination control was proposed and verified by test results of composite braking performance of electric wheel,which takes some factors,specifically braking regulations,braking energy recovery rate and braking comfort,into account.For the conventional braking,the regenerative braking force is dominant and mechanical friction braking from EMB is the supplement to the regenerative braking.When ABS is activated,the strategy to adjust hybrid braking force will be determined according to the ration of the regenerative braking torque to hybrid braking torque.Consequently,the problems,such as delay and over-control caused by the activation of two kinds of braking forces at a given time,can be avoided.Finally,through simulation analysis,it was verified that on the high,medium and low adhesion,with the ABS control of hybrid brake system,the recovered braking energy are1.70e5 J,2.59e5 J and 4.29e5 J respectively.Compared with pure EMB ABS control,the braking distance is shortened by 2.28 m,1.79 m and 3.2m.