Research On Key Problems Of Regenerative Braking For Electric Vehicles

Regenerative braking is an important way to improve energy efficiency in electric vehicles. Thereare many key unresolved problems in the application of regenerative braking in electric vehicles (EV).The following three questions are studied in the dissertation. The first is how to improve the brakingstability and increase the recovery of the braking energy of a rear-drive series hybrid or pure electricurban bus using parallel regenerative braking strategy. The second is how to maintain a constant speeddownhill using cooperative braking realized by a service-brake and regenerative brake system for aDC motor driven electric bus (DCMEB). The third is how to make the drive wheels of an EV drivenby in-wheel Brushless Direct Current Motors (BLDCM) retain the anti-skid braking function whileonly performing regenerative braking actions on low adhesion coefficient roads (such as ice-orsnow-covered road).With aims of improving the parallel regenerative braking strategy for a rear-drive series hybrid orpure electric urban bus, a novel parallel regenerative braking strategy with a simple technique and asmall additional cost is proposed. According to this new strategy, Braking force distribution ratio of amechanical braking system during electro-mechanical parallel braking is adjusted to maximum withinEuropean Regulation13(ECE R13) permissible range by inertia proportional valve, and the amountof the regenerative braking force is corresponding increased. On the other hand, with a considerationon engineering practices of an electric urban bus, the braking intensity is determined between0.1and0.3to ensure the braking stability in this condition. To verify the effect of the new strategy, taking aseries hybrid electric bus simulation model in ADVISOR2002as an example, this model isredeveloped to a rear-wheel-drive series hybrid electric bus simulation model. Simulation results withthis strategy show a significant improvement of this bus braking energy reclaiming performance underthe typical urban driving cycles conditions.To use regenerative braking to act as an auxiliary brake to maintain the constant speed of aDCMEB on downhill, based on the feature of double-loop control structure of the control method forelectric vehicle traction motor and the variable structural characteristics of PWM Control System forDC Motor, a double-manifold variable structure control method to control regenerative braking isproposed for the bus cruising downhill. The impact of lead-acid batteries charge acceptance abilityover a long charging period on the regenerative braking force of a driving motor is analysed. Dynamicmodel of the bus on long downhill is established. A double-manifold variable structure controller is designed for the DCMEB on long downhill. The simulation results show that the control systemmaintains enough stability and strong robustness. It may be achieved for the bus to maintain aconstant speed downhill only by regenerative braking on a smaller slope(such as i<2%). But thedynamic process is very slow. When deceleration or constant speed is desire on a larger slope(such asi>2%), only by electro-mechanical parallel braking can the bus track the target speed precisely andquickly.A novel method of anti-skid braking is proposed to control the duty cycle of the PWM powerconverter to retain the anti-skid braking function of an EV driven by BLDCM while only performingregenerative braking actions on ice-or snow-covered road. An enable condition is designed forregenerative and mechanical braking to coordinately active. A single wheel vehicle dynamics model isderived. A double-loop control model of regenerative ABS (anti-skid braking system) is established.On this basis, a simulation model of regenerative ABS is established in MATLAB/SIMULINKenvironment. The simulation results show that slip rate can be quickly stabilized at the target slip rateon whether ice-or snow-covered road in spite of the existence of the modeling error caused by themodel uncertainty. By comparing the VSC controller and the traditional PI controller to control thewheel slip rate, the simulation results show that the system has stronger robustness and disturbancerejection capability with the former controller. The comparative analysis of the regenerative ABS usedsingle and double switching modulation scheme, respectively, shows the advantages of a higherquality control, a shorter braking distance and more energy recovery for the latter.Finally, a semi-physical test bench of regenerative ABS of an EV driven by BLDCM is designedand produced based on the principle of BLDCM regenerative braking under single-tube modulationmode. The engineering feasibility of regenerative braking as anti-skid braking system on the lowfriction coefficient road as ice-covered road is verified for this EV.