Seamless mode shift control for a new Simpson planetary gearset based dual motor powertrain in electric vehicles (2022)

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Mechanism and Machine Theory

Volume 178,

December 2022

, 105056

Author links open overlay panelJinglaiWuaPersonEnvelopeXianqianHongbGuangjunFengaPersonEnvelopeYunqingZhanga

(Video) Engineering in Motion: Modeling of Moving Mechanisms in Simcenter 3D Motion

Abstract

This paper proposes a seamless mode shift control strategy for a Simpson planetary gearset based dual motor powertrain (SPGDMP) which has 6 driving modes. The dynamic model of SPGDMP is built, where the rotational inertia of transmission components, the flexibility of shafts, dynamic response of motor torque and brake pressure, and stick-slipping friction phenomenon of brake are considered. A seamless mode shift control strategy is proposed to avoid power interruption and reduce the vehicle jerk. The mode shift is decoupled to the torque transition process and speed transition process, which lowers the control difficulty. The step function is used to produce the smooth transition trajectories of torque and speed, which eliminates the large vehicle jerk induced by the unsmooth driving torque. In the speed transition process, a combination of speed feedback and torque feedforward control strategy is proposed to track the designed motor speed trajectory and the driver's input torque simultaneously. The simulation results demonstrate that the proposed mode shift control strategy can reduce the peak-to-peak value of vehicle jerk to a value below 2m/s3.

Introduction

The single motor with the one-speed transmission is the most widely used configuration of the commercialized electric vehicles (EVs) powertrain for its simplicity and low cost, but it has to trade off the dynamic and economic performance of EVs. To seek higher dynamic performance and better economic performance simultaneously, the multi-speed transmission and dual motor powertrains are being investigated more and more. They provide more alternative gears or driving modes to enlarge the range of driving torque and vehicle speed, so the dynamic performance of EVs can be improved. The multi-speed transmission can change to the gear with the lowest energy consumption according to driving conditions, which effectively improves economic performance. For the dual motor powertrain, two downsized motors are applied as a replacement of the original single motor with large power, so the torque utilization factor of the motor driving can be increased, and potentially the energy efficiency can be further improved [1]. It has been demonstrated that the dual motor powertrains have a larger potential to improve the comprehensive performance than the multi-speed transmission [2,3], e.g. the Ref. [3] shows that the dual motor powertrains can improve the energy efficiency by 6%–10% compared to single motor powertrains, and the similar conclusion is also drawn in Refs. [4,5]. Therefore, dual motor powertrains are a good choice to develop luxury or commercial EVs.

The multi-speed transmissions used in EVs include automated manual transmission (AMT), dual clutch transmission (DCT), and automatic transmission (AT). The main advantage of AMT is its high efficiency, and it has been further simplified as a clutchless AMT used in EVs. The DCT has also been used in EVs to improve their dynamic performance and economic performance. Ruan etal. [6] demonstrate that the two-speed DCT can save energy consumption. There are many types of planetary gearset based AT proposed for EVs, e.g. the single planetary gear train [7] and complex planetary gear train [8–9]. The priority of AT is its compact configuration. Through the different power coupling ways, the dual motor powertrains can be divided into torque coupling dual motor powertrain (TCDMP) [3], speed coupling dual motor powertrain (SCDMP) [10,11], and compound dual motor powertrain (CDMP) [12,13]. TCDMP is mainly based on the parallel axle structure, while SCDMP is based on the planetary gearset. The CDMP combines the priorities of TCDMP and SCDMP to realize more driving modes, so it can improve the dynamic and economic performance of EVs further, but the weakness is the complicated configuration. Hong etal. [4] propose a new Simpson planetary gearset based dual motor powertrain (SPGDMP) that is a type of CDMP. Being different from other CDMPs [12,13], the SPGDMP can provide more driving modes, including 4 single motor driving modes and 2 dual motor driving modes, so it has higher dynamic and economic performance. More detailed configuration of dual motor powertrains can be referred to [14,15].

The gear/mode shift is the key to improving the dynamic and economic performance of EVs with the multi-speed transmission or dual motor powertrains. A problem that should be solved is how to avoid power interruption and reduce vehicle jerk during the gear/mode shift. There have been some publications investigating the reduction of power interruption and vehicle jerk for different powertrains. For the clutchless AMT, the power interruption cannot be avoided, so the main target is to reduce the duration of power interruption. Mo etal. [16,17] propose a new spring-based synchronizer, called the harpoon-shift synchronizer, which can compress the duration of gear shift. Wang etal. [18] investigate several control strategies of the spring-based synchronizer to reduce the vehicle jerk during a gear shift. Lu etal. [19] cancel the friction ring in the synchronizer and control the motor to realize the synchronization of rotational speed and the adjustment of the angle phase, which further reduces the gear shift time. An anti-jerking robust controller is designed to attenuate the vibration of the powertrain and a robust speed controller is proposed to enhance the speed synchronization capability of the traction motor [20]. The Pontryagin's Minimum Principle (PMP) is employed to optimize the friction torque during the synchronization stage, and the results indicate that both the vehicle jerk and friction work are well controlled with optimized shift force [21]. Hu etal. [22] investigate the effects of the shift control parameters of the synchronizer in clutchless AMT, including the gear shift force, relative speed difference, and relative rotation angle. Although the aforementioned new synchronizers and control methods reduce the gear shift time or vehicle jerk, the power interruption cannot be avoided due to the limitation of the principle of AMT.

To realize the gear shift without power interruption for the single motor powertrains, the multiple friction components have to be employed to realize the transition of power flow, e.g. the dual clutch used in DCT, and the brakes used in the AT. These powertrains can avoid power interruption from theoretical viewpoints if the motor and friction components are cooperatively controlled. The gear shift process of powertrains with clutches and brakes can be divided into the torque phase and the inertia phase. The torque phase switches the transmitted torque between clutches or brakes, while the inertia phase eliminates the speed difference between the motor and upcoming gear. There have been different control strategies to implement the gear shift, from the open loop control strategies to the optimal control strategies. Gao etal. [23] use the linear function to control the torque switch directly in the torque phase and use a PID controller to track the speed trajectory which is a third-order polynomial in the inertia phase. Walker etal. [24] use the open loop control strategy to implement the power-on and power-off gear shift in a two-speed DCT. To improve shift comfortability, a smooth gear shift control strategy of DCT is developed in [25]. To further improve shift performance, some control strategies based on the optimal control theory are proposed. Li etal. [26] propose a finite-time linear quadratic regulator (LQR) to switch the torque between clutches in the torque phase and use an integral LQR to regulate the relative speed difference between the engine and the slipping clutch during the inertia phase. The control objective is to smooth the shifting process to improve shift comfortability.

Besides the DCT, the gear shift process of AT also comprises the torque phase and inertia phase, but the torque switches between the brakes in the torque phase. Tian etal. [27] propose three strategies to control the motor torque and friction torque of brake and clutch to implement the power-on gear shift of a two-speed AT, resulting in different vehicle jerk and friction work. Wang and Meng [28] investigate the down shift control without power interruption of AT for a heavy-duty vehicle, where an adaptive sliding mode control for turbine speed is used. The result shows that the peak-to-peak value of vehicle jerk can be reduced to about 11m/s3. Wang etal. [29] investigate a smooth shift control strategy for a two-speed AT in EV by considering the vehicle jerk as an optimal control problem in the torque phase. The torque is controlled by an optimal oil pressure trajectory which is obtained by using the Radau pseudo-spectral method, and the sliding mode variable structure controller of oil pressure is designed. The simulation results demonstrate that the vehicle jerk is reduced by 60%. Roozegar etal. [8] derive the polynomial trajectory in the space of transmission angular velocity to realize the smooth gearshift of multi-speed AT in EVs. Based on the polynomial trajectory, a two-phase algorithm is developed to implement the gear shift control by using two PID controllers [30]. In [31], the linear quadratic integral controller is used to track the speed trajectory, which shows effective control under the unknown external disturbance. Mousavi etal. [32] apply the PMP to design an optimal shift controller to keep the output speed and torque constant during the gear shift while minimizing the shift time and the brake dissipation of energy for AT. Results demonstrate the ability of the transmission to exhibit a smooth shift.

For the single motor powertrain, to realize a smooth gear shift of multi-speed transmission, the motor torque and clutch/brake torque have to be cooperatively controlled in both two phases, which is actually a complicated control process and even unachievable sometimes, e.g. the clutch torque induced by the dynamic friction cannot be in the same direction as its sliding velocity. However, this phenomenon happens in the gear shift control of single motor-based AT [8], so it is actually not reasonable. Dual motor powertrains are easy to realize a smoother gear/mode shift without relying on the precise control of clutch/brake torque since the motor torque can be easily controlled and its direction can be either positive or negative. For the TCDMP, Liang etal. [33] investigate its gear shift control strategy, which uses the second motor to complement the driving torque during the gear shift. The simulation results show that the vehicle jerk can be reduced to lower than 5m/s3 through cooperative control of two motors, but the synchronizer is simplified to a friction model, which artificially improves the shift performance. To reduce jerk induced by the mode shift, Zhang etal. [34] present a stage-by-phase multivariable combination controller based on the control of position, velocity, and force of the actuators, to implement the mode shift of a dual-motor centralized and distributed coupling drive system. The simulation and experiment results demonstrate that the vehicle jerk can be reduced to lower than 10m/s3 by using the combination controller. For the SCDMP, the control strategy of mode shift is more complicated. Wu etal. [11] propose a mode shift control strategy of the planetary gear-based dual motor powertrain, where the polynomials trajectory of motors speed is formulated during the mode shift and then the motors torque is controlled by the PI controllers to track the ideal speed trajectory. The simulation results demonstrate that the vehicle jerk is extremely small, but the mode shift control strategy only depends on the control of motors without considering the influence of brakes, which leads to a deviation from the practical case. Hu etal. [13] investigate the mode shift control strategy of a CDMP, in which the maximum vehicle jerk can be reduced to 6m/s3. The mode shift control of CDMP is more complicated, because its configuration is more complicated to realize more dirving modes, which makes it need to cooperatively control more components to realize the mode shift.

In summary, the dual motor powertrains have a priority to reduce the vehicle jerk in mode shift due to the good controllability of motors torque compared to the single motor powertrains. However, the aforementioned control methods for dual motor powertrains do not take full advantage of their potential to reduce the vehicle jerk in a large extent, e.g. most of them still produce relatively large vehicle jerk [13,34]. Some of them have smaller vehicle jerk [11,33], but their dynamic models are simplified without considering the influence of the synchronizer or brake, which makes the gear/mode shift performance in simulation too idealized. In the author's viewpoint, the shift performance of dual motor powertrains has not been fully exploited, and the vehicle jerk can be reduced further even after the complicated brake model is considered. This paper tries to design a decoupled mode shift control strategy that reduces the vehicle jerk to achieve the seamless mode shift performance by using a simple control logic. The mode shift of dual motor powertrains needs to realize the torque transition between motors and brakes, the speed transition between motors, as well as the accurate track of the driver's expected driving torque to reduce the vehicle jerk. Therefore, we need to cooperatively control the motors and brakes to realize the three targets simultaneously, but how formulating the simple and executable mode shift control logic is difficult. This paper investigates the mode shift control strategy based on the SPGDMP, which is a type of CDMP with 6 driving modes. A decoupled control structure of torque transition and speed transition is designed to lower the control difficulty. The step function is used to produce the trajectories of torque transition between motors and brakes, which avoids the unsmooth change of torque. The combination of speed feedback control and torque feedforward control is proposed to realize the speed transition between motors, so the driver's expected driving torque can be accurately tracked. Section2 provides the configuration and constructs a lever model of the SPGDMP. The dynamic modeling of SPGDMP is presented in Section3, including the transmission model, motor torque model, and brake torque model. The mode shift control strategy to realize the seamless mode shift performance is proposed in Section4. Section5 gives the simulation results of the proposed mode shift control strategy, and some conclusions are shown in the last section.

Section snippets

Modes of SPGDMP

The SPGDMP is proposed in Ref. [4], but here we adjust the configuration by changing the output component from the 2nd carrier to the 2nd ring gear. This configuration change is mainly used to adjust the transmission ratio since we find the new configuration may have higher energy efficiency than the original configuration, but it does not affect the function of the original SPGDMP. The main scope of this paper is to investigate the mode shift control strategy, so we will not expand the

Dynamic model of transmission

To investigate the control strategy of gear shift, the dynamic model of the SPGDMP should be built. Considering the shaft as a torsional spring and damper, the multibody dynamic model of the transmission can be expressed in Fig.4.

Here we only consider the vehicle in straight driving condition, so the two wheels have the same speed and then they are simplified as one wheel. In the figure, the symbol I denotes the rotational inertia of transmission components and the subscript denotes the

Seamless mode shift control strategy

Theoretically, each mode can shift to any other mode, so there are 30 different mode shifts considering SPGDMP has 6 driving modes. Considering the topology of power flow of type Ⅰ and type II is different, the mode shifts can be classified into three categories, which are the mode shifts within type Ⅰ (C1), mode shifts within type II (C2), and mode shifts between type Ⅰ and type II (C3), shown as Table3.

For the mode shifts in C1, they only change the status of two motors and brakes B1 and B2,

Simulation results

The driver's expected driving torque is designed to track a given smooth speed trajectory shown in Fig.14, where the vehicle accelerates from 0km/h to 80km/h between 4s and 35s. We will simulate the proposed mode shift control strategy at different vehicle speeds, i.e. the start time of mode shift is set as t0=30s, 25s, 20s, and 15s, respectively.

Conclusions

This paper investigates the seamless mode shift control strategy for a new dual motor powertrain, termed SPGDMP. The SPGDMP has 6 driving modes which are realized by changing the status of two driving motors and three brakes. Four typical mode shifts are simulated under the accelerating condition of the vehicle. The simulation results show that during the mode shift the peak-to-peak values of vehicle jerk is less than 2m/s3, which is extremely small compared to the results shown in other Refs.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This research is supported in part by Fundamental Research Funds for the Central Universities (12100/5003100116).

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