Study on gear shift control based on multivariable MPC for optimization of shift performance of a transmission vehicle

Abstract

When shifting gears, the gear shift time and the Maximum Variation of the Output Shaft Torque (MVOT) should be properly adjusted which are in the trade-off relationship. At this time, the gear shift time is related to the clutch friction energy loss and vehicle drivability, and MVOT is related to the vehicle jerk. In this study, gear shift performance is related with the gear shift time and MVOT, and it is defined as the ratio of the gear shift time and MVOT.On the other hand, the gear shift time and MVOT can be adjusted by controlling the clutch slip speed and output shaft torque, and the clutch slip speed and output shaft torque are again controlled by power source torque and clutch transmitted torque.The structure of gear shift control is largely divided into a target trajectory generator, an upper-level controller, and a lower-level controller. The target trajectory generator of gear shifting refers to the controller step that generates the target clutch slip speed and output shaft torque according to purposes(fast shifting, smooth shifting, etc.). The upper-level controller refers to the control step that generates target power source torque and clutch torque when target clutch slip speed and output shaft torque are given. The lower-level controller refers to the control step that tracking-controls the target power source torque and clutch torque generated by the upper-level controller. In addition, In this study, the integrated upper-level controller means a controller including the target trajectory generator and upper-level controller.In this study, the lower-level controller and integrated upper-level controller are addressed among the gear shift controllers, and only the control of the clutch torque excluding the power source torque is dealt with in the lower-level controller.In production vehicles, clutch torque sensors cannot be installed due to cost and space issues. Therefore, the clutch torque is controlled generally in a feedforward manner using a clutch torque model. Here, the clutch torque model refers to the relationship between the control input of a clutch actuator and the clutch torque, and in the case of a dry clutch mainly covered in this study, the actuator control input refers to actuator motor position. However, the scale and touch point of the clutch torque model are changed depending on clutch temperature and clutch wear. Therefore, the uncertainty of the clutch torque model should be properly compensated when controlling the clutch torque.In most previous studies on the clutch torque control, it was assumed that the touch point of the clutch torque model was known when controlling the clutch torque, and only the scale of the clutch torque model was compensated. However, few studies have been conducted in which the scale and touch point were compensated together in the clutch torque control. Therefore, in the study on the lower-level controller, a clutch torque control method is newly proposed in which the scale and touch point are compensated together in the clutch torque control.In addition, in order to adjust the gear shift time and MVOT during gear shifting, Multi-Input-Multi-Ouput(MIMO) control should be performed, which controls the clutch slip speed and output shaft torque using the power source torque and clutch torque. However, in a conventional way, the gear shift time and MVOT have been adjusted by using several trajectory pairs of the power source torque and clutch torque generated heuristically through long time tuning. However, the tuning load is very large to generate the trajectory of the power source torque and clutch torque in all situations through tuning. Therefore, a MIMO control method is needed which is capable of adjusting the gear shift time and MVOT using a few tuning parameters.In the study on the integrated upper-level controller, a gear shift controller is proposed which generates target power source torque and clutch torque based on MIMO Model Predictive Control(MPC) and can adjust the gear shift time and MVOT with only one tuning parameter.The lower-level controller and integrated upper-level controller of gear shifting proposed in this study are verified with a test bench equipped with a production dry dual-clutch transmission.