Performance of the Transmission Parking Mechanism of a Battery Electric Vehicle Simulated with Adams Software
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The electric parking mechanism is studied for an electrically controlled two-speed auto transmission that is being developed for electric vehicles. Safety requirements include low-speed safe parking, reliable self-lock and the avoidance of abnormal parking. A dynamic model of the parking mechanism is established and analyzed using Adams software. Finally, failure of the parking mechanism due to wear is observed in bench testing and compared with experimental results after optimization.
KeywordsBattery electric vehicle Two-speed auto transmission Parking mechanism Safety performance
The battery electric vehicle (BEV) is the main object of development of new energy vehicles in China. The vehicle is also an important energy-saving and emission–reduction means of China’s 13th Five-Year Plan. The BEV has been rapidly developed in recent years within the framework of the Chinese government’s support and preferential policies. In terms of improving the efficiency and decreasing the speed of the motor of the electric drive system, there has been a development trend of replacing a single machine reducer with high-performance two-stage decelerator in the transmission of some advanced vehicles. At the same time, major automobile corporations are more concerned about the safety of the BEV. More and more BEVs are equipped with parking mechanisms for greater safety and standardization.
The role of the parking mechanism is to park vehicle reliably. When the automatic transmission is shifted to the park (P) position, the vehicle drive wheel is locked through locking of the shaft or differential to prevent the vehicle from moving on flat or angled ground [1, 2, 3, 4].
Parking mechanisms can be classified as those of a mechanical drive, electrohydraulic drive and electric drive. The mechanical drive was widely used in early automatic transmission and continuously variable transmission. It relies on interconnected mechanical parts transmitting the force from the shift lever to the parking pawl to complete the parking lock action by locking the ratchet [1, 5]. The electrohydraulic parking mechanism has no mechanical connections between the parking pawl and shifting lever. The electrohydraulic system completely determines whether the parking pawl is engaged or disengaged. It usually depends on the position of the shift lever or other safety factors, such as the driver’s door being open, the transmission being in a working state or the ignition key being pulled out. The electric parking mechanism has no mechanical structure or hydraulic circuit between the shift lever and the actuator and controls the execution motor to complete the parking lock action via the shift lever position signal. The electric parking mechanism allows a more flexible internal structure of the transmission and easier control of the driver’s shift force.
In the non-working state, the automatic transmission is in the P position most of the time. Reliability is an important aspect of the performance of the transmission [6, 7] and has attracted the attention of researchers. Because parking on a ramp relates to the safety of the vehicle and the parking mechanism is a complex nonlinear multi-body system , it is an important consideration in the design of automatic transmission.
2 Structure of the Parking Mechanism
The parking mechanism is driven by the motor to complete the lock and takeoff action. The mechanical execution part is composed of a ratchet, pawl, slider, actuating rod, guide pins, press block and other components. The fork of the motor pushes the actuating rod back and forth through the sliding block, and the actuating rod then presses the block between the pawl and guide pins until the parking action is complete. The whole mechanism relies on the actuating rod spring realizing flexible engagement between the pawl and ratchet, and disengagement is completed by the torsion spring. It is seen that the electric drive greatly simplifies the parking mechanism relative to the situations of the mechanical drive and electrohydraulic drive.
3 Design and Calculation of the Parking Mechanism
3.1 Rolling Distance in Ramp Parking
3.2 Critical Parking Speed
In theory, whether the pawl and ratchet can engage is determined as follows. The parking process begins when the pawl teeth come into contact with the ratchet teeth. It can be engaged when the root of the pawl fillet contact with the root of the ratchet fillet . The rotation angle is shown on this basis in Fig. 4. In this process, the angle of the ratchet is denoted \(\Delta \varphi \), and the angle of the pawl is denoted \(\gamma \). There is smooth engagement if the pawl rotation time is less than the ratchet rotation time. The parking function cannot otherwise be achieved by the parking mechanism. The critical parking speed is obtained when the two times are equal.
The rotation time of the pawl can be adjusted using the spring force of the actuating rod. Usually, the spring force for which the pawl can engage smoothly with the ratchet is first calculated and then further modified using the simulation results.
3.3 Self-Locking Performance
3.4 Prevent Abnormal Parking
4 Simulation Analysis of the Parking Mechanism
Simulation parameter settings
Damping (N s/mm)
Penetration depth (mm)
Stiction transition Vel. (mm/s)
Friction transition Vel. (mm/s)
4.1 Simulation of the Critical Vehicle Speed
Using the dynamics simulation environment that Adams provides, the initial state and the actions to be performed are set for the established simulation model. An angular acceleration is applied to the ratchet using the STEP function [17, 18, 19]. A sensor is set to stop acceleration when the ratchet speed is equivalent to 6 km/h, the speed at which the vehicle travels. The parking action is simulated by applying a drive to the fork of the motor. When the pawl is engaged with the ratchet, the ratchet stops and the simulation ends. The step in the pawl velocity curve shows the critical vehicle speed.
4.2 Self-Locking Performance Simulation
4.3 Simulation of the Disengagement Performance
When the vehicle stops on the ramp and the transmission is in the P position, the vehicle will sometimes move a short distance. The main reason for the nonzero rolling distance is that when the transmission is shifted to the P position, the pawl is generally on top of the ratchet wheel and will engage completely after the ratchet wheel rotates a certain angle. The vehicle stops moving when the ratchet wheel and pawl come into contact and produce a torque that prevents movement of the vehicle. In some cases, it is the contact force that prevents the parking mechanism from disengaging smoothly. The aim of the simulation of the disengagement performance is to verify whether the transmission can exit the P position smoothly when needed.
Specifically, the simulation verifies whether the pawl successfully disengages from the ratchet when the vehicle is parked on a 30% slope with a full load (GVM) after the press block and actuating rod are removed.
4.4 Simulation of the Parking Effect on the Motion State
Simulation results presented in Fig. 17 show that the impact load is higher at speeds below 10 km/h and above 60 km/h. In the low-speed stage, the coinciding area of the movement track between the pawl and ratchet is relatively large, owing to the low ratchet speed. The pawl is deep into the ratchet, and the part that comes into contact changes from the fillet of the pawl to the plane above it. The impact arm is therefore shorter and the impact force higher. In mid- and low-speed stages, with an increase in the ratcheting speed, the coinciding area of the movement track decreases rapidly, which means that the area of the pawl in the ratchet is greatly reduced. The impact force is thus lower. In the high-speed stage, an increase in speed increases the impact force. This phenomenon is more obvious above 30 km/h. The above peak impact load is in the design safety range with a safety factor of 2.
5 Experimental Optimization of the Parking Mechanism
Another developed automatic transmission parking mechanism is taken as an example. In the experiments, the parking mechanism initially runs well. However, there are disengagement difficulties after a long period of use and wear. The bench test program is designed to verify the durability and reliability of the parking mechanism. In the program, the slope is simulated by different loads, and ramp tests are completed in turn. The durability test is conducted for a 30% equivalent slope. Adjacent tests are kept at regular intervals.
Figures 19 and 20 show that, after the durability test, much engaging and disengaging have worn the contact rounds of the ratchet. Correspondingly, wear and scratches appear on the front and rear contact surfaces of the pawl. It is seen that the partial grinding of the ratchet wheel is serious on the left side of Fig. 18. The reason is that the stiffness of the cantilever shaft of the pawl is insufficient and deformation appears along the direction of the force. This phenomenon is eliminated in the subsequent improvement.
Generally, the larger the radius of the ratchet, the lower the contact force and the less the wear, and the greater the room for the improved design of the parking mechanism. However, under the premise that the overall size of the transmission has been determined, the ratchet radius is not allowed to increase. Therefore, the improvement direction of the program is an extension of the contact arm of the pawl, increase in the contact fillet radius, change in the friction coefficient, change in the tooth contact angle and replacement of the material.
After a series of tests, the effective and economical solution is found to be an increase in the length of the pawl contact arm, strengthening of the surface for the ratchet, no change to the friction coefficient, and a change from wire contact to surface contact by adjusting the relative positions of the ratchet and pawl. To ensure the critical vehicle speed, the contact fillets are maintained at R1.5 and R1. Figures 21 and 22 show that the wear condition of the ratchet and pawl improves greatly, which ensures normal use of the parking mechanism.
An electric drive, compared with a mechanical drive or electrohydraulic drive, greatly simplifies the parking mechanism.
Safety requirements are met, and relevant performance indicators are determined through reasonable and systematic design and calculation.
A simulation model is established using multi-body dynamics software, and the correctness of theoretical calculation is verified.
A common problem faced by parking mechanisms wear failure is reproduced in experiments, and a feasible solution is presented.
This paper provides a reference for the design and development of the parking mechanism.
The authors acknowledge financial support from the Science and Technology Research Youth Fund Project of Hebei Colleges and Universities (QN2016197).
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