A new cooperative braking control strategy (CBCS) is proposed for a parallel hybrid electric vehicle (HEV) with both a regenerative braking system and an antilock braking system (ABS) to achieve improved braking performance and energy regeneration. The braking system of the vehicle is based on a new method of HEV braking torque distribution that makes the antilock braking system work together with the regenerative braking system harmoniously. In the cooperative braking control strategy, a sliding mode controller (SMC) for ABS is designed to maintain the wheel slip within an optimal range by adjusting the hydraulic braking torque continuously; to reduce the chattering in SMC, a boundary-layer method with moderate tuning of a saturation function is also investigated; based on the wheel slip ratio, battery state of charge (SOC), and the motor speed, a fuzzy logic control strategy (FLC) is applied to adjust the regenerative braking torque dynamically. In order to evaluate the performance of the cooperative braking control strategy, the braking system model of a hybrid electric vehicle is built in MATLAB/SIMULINK. It is found from the simulation that the cooperative braking control strategy suggested in this paper provides satisfactory braking performance, passenger comfort, and high regenerative efficiency.
In the hybrid electric vehicle, regenerative braking takes place by transforming the mechanical energy into electric energy via a generator, the electric energy is stored in the energy storing device such as battery or supercapacitor, and the stored energy is recycled to propel the vehicle via a motor. Energy regeneration during braking is an effective approach to improve vehicle efficiency, especially for vehicles in heavy stop and go traffic [
Generally, in order to ensure appropriate braking performance and energy regeneration, most HEVs are equipped with both an antilock braking system and a regenerative braking system. As one of the most popular active systems of vehicles, the antilock braking system (ABS) helps the driver to maintain control of the vehicle during emergency braking or braking on a slippery road by preventing wheel lockup; it has dramatically improved vehicle stability during braking. The regenerative braking system works together with the antilock braking system for the following reasons: (a) the regenerative braking torque is not large enough to cover the required braking torque; (b) the regenerative braking cannot be used for many reasons such as a high state of charge (SOC) or high temperature of the battery to increase the battery life. In these cases, the antilock braking system works to supply the required braking torque. Therefore, the cooperative control strategy between the antilock braking system and the regenerative braking system is an important issue to research on HEV.
As for the cooperative control strategy, few investigations have been reported. Present research mainly focuses on two different braking aspects. One is the regeneration efficiency of various types of regenerative braking systems for the electric vehicle (EV) and HEV [
Simulation is a crucial step of research and development nowadays. In order to test and verify the cooperative braking control strategy and to evaluate the control effect, simulation is required. To carry out the simulation, appropriate models including those of the vehicle (overall dynamics), the tire, the electric motor, the battery, and the hydraulic system were built.
Since only a straight-line braking maneuver is considered, the vehicle model takes into account only longitudinal movement. For a straight-line braking motion, that is, the movement does not include lateral and vertical motions, and the effect of the slight vibration of suspension system, yaw movement can be neglected, as the effect is very small. Therefore, a typical three degrees of freedom vehicle dynamic model, which possesses the fundamental characteristics of an actual system and sufficient accuracy, is used in this paper. These 3-DOF include the longitudinal velocity, the front-wheel angular speed, and the rear-wheel angular speed of the vehicle. In the hybrid electric vehicle, the regenerative braking torque works often only on the front axle of the vehicle; considering this and the normal force transfers, by making use of Newton’s second law and law of rotation, the vehicle dynamic model is built; the dynamic equations are therefore derived as follows:
The tire model is a very essential part in vehicle simulation research. The Pacejka nonlinear tire model “Magic Formula” is a widely used tire model in automotive industry, which possesses a high fitting accuracy and was first proposed by Pacejka in 1991. In this paper, Pacejka’s nonlinear tire model “Magic Formula” is used. In the tire model of the mathematical descriptions, a set of trigonometric formulas is used to describe the mathematical relation of the athletic parameters of tire. The magic formula tire model is presented by
Slip and the friction coefficient relationship based on Pacejka’s nonlinear tire model.
The motor is used for tractive effort when the battery is discharged and is used as a generator when the battery is charged, in other words in the regenerative braking mode. Based on the test date, an experimental motor model of the mathematical descriptions of the 20 kW PMSM motor is used in this paper, which emphasizes the mathematical relation between inputs and outputs and neglects the complicated physical and electrodynamic movements in electricmotor. The efficiency and torque characteristics of the electric motor are shown in Figure
Efficiency characteristics of the electric motor.
An experimental battery model of the mathematical descriptions is built, which focuses on the mathematical relation between the charge resistance and the state of charge (SOC). The original capacity of Ni-MH battery is 80 Ah and the nominal voltage is 336 V. In the charging or discharging process, the inner resistance of the battery changes depending on the battery’s SOC. The characteristics of SOC and the charging resistance of the battery being tested are shown in Figure
Charging resistance characteristics of the Ni-MH battery.
According to the theory of hydrokinetics and lab tests, the ABS hydraulic system model is described as pressure increasing, pressure holding, and pressure dumping processes, which also uses the mathematical descriptions and neglects the complicated physical process and movement. The theory diagram of the hydraulic pressure increase/decrease is shown in Figure
Increasing and decreasing pressure theory diagram of the ABS hydraulic braking system.
For hybrid electric vehicles, under the effect of regenerative braking torque and hydraulic braking torques, the braking control system must make the two braking torques work together harmoniously to assure the braking safety of vehicle and maintain comfortable sense for driver. In this paper, the cooperative braking control strategy is divided into two parts: the first part is used to adjust the antilock braking torque in the conventional hydraulic braking system using a sliding mode controller that is based on the target slip ratio to control the braking pressure increase, holding, and decrease; the second part is used to adjust the regenerative braking torque from the electric motor using a fuzzy logic control strategy that is based on the target slip ratio, battery state of charge (SOC), and the motor speed to adjust the regenerative braking torque dynamically. In the cooperative braking control strategy, the wheel slip ratio is the control common variable; the regenerative braking torque and the hydraulic braking toque should be adjusted (increased or decreased) harmoniously based on the changes of slip ratio to assure the braking stability of vehicle.
In the control strategy, the required braking torque of driver
The design procedure of sliding mode control methodology consists of two main steps: first, a sliding surface that models the desired closed-loop performance is chosen, and then, the control law, such that the system state trajectories are forced toward the sliding surface, is derived [
Substituting (
A fuzzy logic control strategy is used to adjust the regenerative braking torque dynamically. There are three inputs in the fuzzy logic controller and the three inputs are the difference (
Block diagram of the fuzzy logic controller for the regenerative braking system.
Based on the simulation analysis, the
Membership functions for the inputs and outputs of the fuzzy logic controller.
Assuming that there are
Figure
Output surface of the fuzzy inference regenerative braking system.
In order to evaluate the performance of the cooperative braking control strategy, the simulation is implemented in MATLAB/SIMULINK. The simulation parameters of main components of the vehicle are listed in Table
Vehicle parameters.
PMSM | Power M/G | 20/13 KW |
---|---|---|
Battery | Capacity | 80 Ah |
Nominal voltage | 336 V | |
| ||
Vehicle | Vehicle mass |
1320 kg |
Tire radius |
0.272 m | |
|
1.1 kg·m2 | |
|
0.015 | |
|
8.9, 1.6, 1, 0.5 | |
|
2.4, 0.5, 0.9, 1.4 (m) | |
CVT gear ratio | 0.451~2.462 |
First, the vehicle was brought to a steady longitudinal velocity of 30 m/s along a straight path. Then, the ABS and the regenerative braking were applied on the front wheels simultaneously. To increase stability and manoeuvrability of the vehicle and to decrease the stopping distance during emergency braking, the CBCS is implemented to maintain the target slip ratio value of 0.2. The simulation results are shown in Figures
Comparison of (a) vehicle speed and wheel speed and (b) slip ratio for conventional control and SMC control.
(a) Regenerative braking torque and hydraulic braking torque and (b) battery SOC for CBCS during emergency braking by ABS and regenerative braking.
The wheel speed and slip ratio with conventional control and SMC control are compared in Figures
Figures
Simulation results are shown in Figure
Simulation results for the NEDC.
A new cooperative braking control strategy (CBCS) for a parallel hybrid electric vehicle is proposed in this paper. The CBCS combines a sliding mode controller and a fuzzy logic control strategy to ensure the vehicle’s longitudinal braking performance, which keeps the wheels from being locked and regenerates more energy effectively. The simulation shows that the model of the HEV’s braking system and the cooperative braking control strategy developed in this paper are right. It is also found from the simulation that the cooperative braking control strategy suggested in this paper provides satisfactory braking performance, passenger comfort, and high regenerative efficiency. Although the simulation results have a certain guiding significance for real applications, the deviation between the simulation model and the braking system of actual vehicle is inevitable. Hence, the real vehicle test and a hardware-in-the-loop simulation with cosimulations between MATLAB/SIMULINK and dSPACE will be researched to validate the effectiveness of the proposed cooperative braking control strategy in the future.
The paper is supported by NSFC (no. 51105074) and Foundation of State Key Laboratory of Mechanical Transmission (SKLMT-KFKT-201206).