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In road vehicles, the steering action is being performed through the mechanical link between the wheels and the steering wheel. When the road wheels are steered, a reaction torque, which is mainly based on the self-aligning torque (SAT), occurs, and the driver experiences the steering feel. Since the steering feel has a significant effect on the handling quality and safe driving, it is recognized as an essential feedback to the driver.
A test setup including the steering system of an SBW vehicle and a graphical interface, which both constitutes a hardware-in-the-loop simulation system, for driving experiments have been developed. By doing so, the steering feel models can be tested in driving tests. For this purpose, a driving simulator has been developed.
After investigating the approaches in the current literature to the design of steering wheel, we have understood that the most important characteristic influencing the steering dynamics is the steering wheel angle and the steering wheel torque.
In this direction, firstly, two different steering feel model have been studied: i) a PD controller model representing stiffness and damping characteristics, and ii) a SAT based controller, which holds rich information about vehicle dynamics. In the driving simulator, the models have been tested to apply torque to the driver during some standard maneuvering tests. It has been understood from the results of this study that the model only having spring and damper dynamics cannot reflect the characteristics of steering. The driving task becomes difficult or impossible for higher speeds. On the other hand, SAT based model requires estimation of many parameters inherent to the tire dynamics.
Based on the studies of steering feel in the literature, the most important and main characteristic influencing the steering feel is evident in the relation between steering wheel angle and steering wheel torque feedback. In real driving tests, this relation was identified as a hysteresis curve. In the light of this fact, the design of a steering feel by using a hysteresis model was proposed. The motivation behind using a hysteresis model is that the hysteresis-based steering feel model can pattern the hysteresis characteristic appearing in the conventional steering systems.
In this study, the Bouc-Wen hysteresis model has been selected to model the steering feel. Its mathematical simplicity and suitability for modeling the mechanical systems have been the factors in selecting the Bouc-Wen model. The Bouc-Wen hysteresis model itself cannot describe the steering dynamics of a vehicle fully. Therefore, the model has been tailored for use in steer-by-wire systems.
Following the proposal of the model, we proposed an online optimization model for hysteresis-based steering feel model. The performance criteria have been selected as: the vehicle’s path following performance, presenting the ability of following a given path; the physical workload, determining the required effort for driving; and the lateral acceleration, indicating the comfortable and safe driving. In addition to these, on-center handling performance has also been investigated. The suggested online method provides a better performance compared to other studied models.
Lastly, we have shown that the weight of vehicle has an effect on the steering feel. The parameters of the steering feel model set for a certain weight of vehicle are not suitable for another vehicle with a different weight. The proposed adaptive model can work very well for both different weights and the variation of load during driving.
To test the performance of the proposed models, the hardware-in-the-loop (HIL) simulation approach in the test environment, which has been developed for this project, has been used. To perform simulations, mainly, two case studies based on standard test procedures have been presented: The weave test and double lane-change test. The performance of a vehicle with hysteresis-based steering feel model has been compared with the same vehicle with SAT-based model.
The results of tests have shown the effectiveness of the proposed steering feel models. The hysteresis-based model can be designed more easily compared to other models due to its design flexibility in wider ranges and forms. Since the steering related motion of road wheels or their interaction with the road is reflected to the model indirectly, even though the pneumatic trail is used in the model, this proposed model is not directly affected by the interference of active steering control commands. In conclusion, the research shows that hysteresis-based steering feel model provides a realistic and informative steering feel to enable a comfortable and safe driving, and the model would be a useful tool by providing the flexibility and the ease of design and tuning.
The studies on the control of steer-by-wire systems have been yielding quite satisfactory results; however, the steering feel still remains an open problem. To the best knowledge of the fellow, there is no similar research or patent work in the literature. By studying the hysteresis-based model, two important contributions can be mentioned. First, the elimination of mechanical connection in the steering system poses an essential question: What would be the form and magnitude of steering wheel torque compared to the one in conventional vehicles? There is no requirement or evidence that they both must be the same. In the search of answer, the hysteresis-based model would be a useful tool by providing the flexibility and the ease of design and tuning. Second, the hysteresis-based model can pattern the hysteresis characteristic appearing in the conventional steering systems. Hence, such a model appears as a useful tool in the enhancement of steering feel, as well.
Being awarded to the EU grant has been evaluated very positively and been considered as one of the most effective point in accepting the researcher to the assistant professorship in the department of mechatronics engineering at the host institution. The publications produced from this project would be highly effective in the renewal of job contract.
By using the reintegration grant and to be able to conduct research for this project, the researcher has established the Vehicle Dynamics Laboratory in the department of mechatronics engineering at Yildiz Technical University. All equipment and necessary expenses are covered by the grant of this project.
There are ongoing studies, which have been originated from this project. Based on the experience gained in this project, in order to advance the developed models and search more models of steering feel, a project proposal as an extension of this project has been submitted to The Scientific and Technological Research Council of Turkey.
Steer-by-wire system is applicable not just for passenger vehicles but for any rubber wheeled vehicles, electrical forklifts and tractors. A steering feel model is suitable for all systems in which steering feel is indispensable for providing the feedback torque while steering. On the other hand, driving simulators require steering feel model. An appropriate and widely accepted steering model is highly needed in the automotive tests, video games, and education. Although the demand to SBW systems is low and limited in classical automobile industry, it is clear that the SBW systems are preferred in electrical vehicles. It appears that the models developed in this project would find a wide application area.