Study On Vehicle Dynamical Model For Subjective Evaluation

In the development of the vehicle products, the full-digital simulation of objective evaluation and subjective evaluation is one of the trends of vehicle dynamical model development. The maneuvers for objective evaluation, which defined in ISO standards and others, have been simulated well, and had been used to predict vehicle performance. The driving simulator was adopted for the subjective evaluation simulation, in which it doesn't need to model the driver. However, the vehicle dynamical model used in the driving simulator cannot simulate the vehicle dynamic process very well. The paper studies the dynamic modeling with regard to the real-time simulation and subjective evaluation.The dynamic model used for subjective evaluation should simulate the dynamic process of all conditions precisely. Therefore, the vehicle model should possess certain important features, including the adaptation in all conditions, the ability to describe the process of transition between different states, modeling the dynamic process precisely and modeling complete DOFs. According to the modeling method and subsystems characteristics, some key points have to be considered in the modeling, such as decoupled dynamic subsystems, stick-slip model in the steering and wheel model, the completed steering system model, the dynamic wheel model for uneven road, the suspension model based on assembly characteristics, and the powertrain model for all conditions. This dissertation mainly focuses on the method of subsystems separating and decoupling, the suspension model based on assembly characteristics, and the powertrain model for all conditions.In order to simulate the dynamics of the steering system, the steering system model should include the following three parts:the force input of Ackerman steering mechanism, the compliance of steering system and the dry friction. It takes the rack and pinion steering system for example; achieving the steering rack connected the left and the right wheels together, then modeling stick-slip and tire elastics. This model eliminate the original two sets of forward and inverse modeling method of steering system, achieving the integration of steering system modeling. The model has the ability to resist micro interference from the input of the steering wheel and the road roughness. It can also describe the on-center characteristics and calculate the steering wheel aligning moment accurately.The dynamical wheel model is simplified into rim and rigid ring, which are connected by six spring-dampers. Rim and rigid ring are built as full wheel dynamics model. Tire slip ratios are calculated by the relative motion of wheel center and the contact patch of rigid ring. By adding the stick-slip model, the vehicle can stop when the wheels speed almost turn into stick state, so the problem that the vehicle speed couldn't decline to zero is solved. UniTire model is used to describe the friction dynamics between tire and road surface to achieve high-precision simulation of multi-condition.The same rigid body in different subsystem plays a different role in lumped mass modeling, so how to distinguish the interface between each subsystem is very important for vehicle modeling. Combined with the Vehicle Dynamics Terminology in latest version of SAE-J670-2008, it divides lumped mass in subsystems. By subsystem isolation and decoupling, the interfaces of the subsystems are demarcated in the constraint. In this way, the algebraic equations are eliminated during the modeling process, and only ODE is preferred. The concept of virtual kingpin body and spindle body are brought out to describe motion more precisely in the lumped mass modeling. Take the complex FF passenger car as example; the interfaces of the kingpin and spindle in the driving system, suspension system and steering system are isolated effectively.The suspension model is based on assembly characteristics. It includes the suspension guiding-mechanism model, suspension loading model and suspension K&C model. This suspension model can avoid the MBD method of multi links and multi-elastic components, and meet real-time simulation request. The guiding-mechanism modeling method based on roll-center and pitch-center is analyzed to obtain simplified equivalent lateral arm and longitudinal arm, so lateral and longitudinal forces between wheels and body can be transmitted. Springs, stabilizer bar and internal friction of suspension are considered in the suspension loading model, and the first-order differential equations of Fancher hysteresis model is adopted to describe suspension system-level dry friction. The key suspension K&C test data are extracted to compensate for the additional deformation caused by suspension kinematics and compliance.In order to study the dynamic process of vehicle at various speeds, a real-time powertrain is model is needed. As the clutch and transmission gear position can be switched, many combinations of conditions in P/T system occur, such as the separation phase, slipping phase and bonding phase for clutch, and neutral phase, non-neutral phase (including reverse gear) for transmission.6-phase by kinematic states and torque transfer of the clutch of transmission is discussed in this paper. Because the static and dynamic friction switches during the integration and separation process, the stick-slip clutch model is built to enable a smooth transition between different phases.When solving the rigid differential equations, the traditional multi-body dynamics model is very slow and cannot reach real-time. A new real-time simulation platform is proposed in this paper. By the isolation of subsystem and the description of equivalent constraint, the algebraic constraint equations are eliminated. Ordinary differential equations with boundary constraint and the small fixed-step integration can realize the real-time multi-body dynamics simulation. The dynamic subsystem layer, physical layer and mathematical layer are combined together to describe multi-body system and manage system simulation. At the same time the requirement of simulation management and multi-rate simulation is achieved. The multi-degree-of-freedom dynamic model built in the simulation platform has been proved effective by validation of the vehicle model.The subsystem models of the suspension and powertrain system are validated. For the suspension system model, the validations of the guiding-mechanism, loading characteristics model and K&C Correction model are carried out. The field test data of start and shift maneuver is used in the full vehicle model, validating that the P/T system can switch clutch and gear smoothly. The full vehicle simulation tests of ISO standard and China standard prove the high-precision of this model. Meanwhile, the dynamic processes of steering fight, ride and sine sweep steer are well simulated.