Design And Control Of A Servo Force-aided Electromechanical Actuator For Dry DCTs

Dry clutches,taking advantages of high efficiency and simple structure,are widely used to engage and disengage rotating components of modern automatic transmissions in vehicles.The actuation system is a key control component of clutches and has a potential for performance improvement.Compared to a conventional hydraulic actuation system,an electromechanical system has many advantages,including better energy-efficiency,control precision and robustness.However,the torque/force density it offers is lower than that from a hydraulic system.As a consequence,the size required makes an electromechanical actuator difficult to be packaged.Therefore,it is necessary for a DCT actuation system to output a large force with a small-force actuator.This paper proposed a new motor-driven servo force-aided actuator for dual clutch transmission of dry clutches.The actuator can provide aiding force in the clutch operation,and the aiding force value increases with the increase of the clutch travelling,which reduces the implementation of the motor torque requirements,but also effectively reduce the power requirements.In this paper,the design,dynamic response and control strategy of the proposed actuation system for clutch to clutch shift are studied.In contrast to the conventional preloaded spring actuator,the proposed actuation system in this paper is composed of DC motor,preload spring,lever and so on,in which the elastic element is arranged vertically with the actuator movement direction.And the direction of preload spring force changes with the movement of the actuator.Therefore the actuator has the “servo force-aided” characteristics.That is,the aiding force is zero at the initial position and increases gradually along with the clutch travel,which reduces the demand of the motor torque and power requirements.In this paper,a simulation model is developed for the proposed servo force-aided actuator,which is used to analyze and evaluate its performance.The simulation results are verified by the experimental results.And the characteristics of torque-saving and power-saving are analyzed.The results from quasi-steady experiments show the proposed actuator reduces the motor torque up to 23.9%,the motor power-loss up to 38.7%,and the motor output power up to 21.9% compared with the ordinary leverbased actuator.As a whole,the energy consumption during a usual clutch engagement is reduced by 30.1%.The servo characteristic means that the function of the preload force is like adding torsional stiffness to the lever instead of applying a large step input.Through this method the unsatisfactory dynamic response of the step input can be avoided.Generally,The clutch of DCTs are normally open clutch,whose negative stiffness area of diaphragm spring corresponds to the clutch slip state.Therefore,it is necessary to ensure the stability of the whole system in the negative stiffness area of the diaphragm spring.In this paper,the stability of the servo force-aided actuator is analyzed,and it is concluded that the preload spring force and the negative stiffness of the diaphragm spring are the main factors leading to the instability of the system.The clutch actuator must take the initiative to adjust the deformation of the diaphragm spring in order to make the system stable and get good dynamic performance.The actuator model is further integrated into a Dual Clutch Transmission(DCT)and driveline system to demonstrate the effectiveness of the concept in shifting control.Clutch slip control of the inertia phase during the clutch-to-clutch shift process is the key to ensure good drive quality in DCTs.In this paper,a novel adaptive internal model control(AIMC)strategy is designed for the inertia phase of the clutch to clutch gearshift process,which reduces the tracking error of the clutch slip reference and reduces the vehicle jerk.The internal model control includes feed forward loop and feedback loop.The former is obtained by inversing the nominal mode while the latter is obtained from the difference between the output of the plant and the nominal model.The control structure is intuitive and it is the ideal control method in the tracking control.In this paper,the lead compensation is designed to modify the reference curve in order to deal with the hysteresis problems caused by filtering functions in IMC control.The disturbance feed forward compensator is designed to reduce the tracking error caused by the engine torque.Aiming at the problem of nonlinearity of clutch normal force is to clutch travelling and the time variation of the friction coefficient,the adaptive control is designed to identify the key parameters of the clutch,and the robustness of the controller is improved.Compared with the traditional internal model control,the proposed AIMC reduces the speed tracking error by 45% and the vehicle jerk by 25%.This paper also studies the effects of various reference curves,different engine torques,and different friction coefficients.When the controller model parameters are not accurate,the adaptive control can identify the parameters and improve the robustness of the controller.Compared with no adaptive internal model control,the speed tracking error is reduced by 35% and the vehicle jerk is reduced by 20 %.Compared with the traditional PID closed-loop control,the proposed AIMC can be adaptable to different engine torque,different reference curves and different friction coefficients.Therefore the proposed AIMC is more robust and can obtain better tracking effect and shift qualityIn summary,this paper focuses on the design and control of the servo force-aided actuator.The main conclusions are as follows: 1)The actuators proposed in this paper have the characteristics of servo force-aided features,and the aiding force value increases with the increase of the clutch travelling.Simulation and experiment results verify its torque saving characteristics and power saving features.2)The instability problem of the negative stiffness region of the diaphragm spring is studied,and the stability of the servo force-aided actuation system is analyzed.3)A novel adaptive internal model control method is designed to reduce the tracking error of clutch slip speed reference during inertia phase and the gear quality is improved.