Project - steerıng feel

Development and Enhancement of a Steering Feel Model for Super Lightweight SBW Vehicles

This project was funded by Marie Curie Actions—Support for training and career development of researcher (CIG) - EU FP7-PEOPLE-2011-CIG

Result In Brief

1. Arslan M.S., "A Hysteresis-based Steering Feel Model for Steer-by-Wire Systems", Mathematical Problems in Engineering, Vol. 2017, 2017.

2. Arslan M.S., "Development of a Mathematical Model for the Steering Feel in Steer by Wire Systems", International Conference On Advances In Automotive Technologies 2016 - AAT2016, İSTANBUL, TÜRKIYE, 11-14 Ekim 2016, pp.1-6.

3. Kirli A., Arslan M.S., "Steering Feel Design: The Effect of Mass Variation", IFAC-PapersOnLine, vol.49, 2016.

4. Kirli A., Arslan M.S., "Online optimized hysteresis-based steering feel model for steer-by-wire systems", Advances in Mechanical Engineering, vol.8, 2016.

5. Kırlı A., Arslan M.S., "Optimization of parameters in the hysteresis-based steering feel model for steer-by-wire systems", IFAC-PapersOnLine, vol.49, pp.129-134, 2016.

6. Karadeniz O., Arslan M.S., "Elektronik Direksiyon Sistemine Sahip Araçlar için Sürüş Hissiyatı Modeli Geliştirilmesi ve Uygulanması", TOK, Eylül 2014.

Objectives

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.

The overall aim of this research is the development of a force-feedback model for the steering system of normal, light, and super lightweight Steer-By-Wire (SBW) vehicles. The immediate goals are to explore the role of force-feedback (a.k.a., steering feel) transmitted from the front wheels to the handwheel in road vehicles and, connectedly, to emulate and enhance the steering feel in SBW systems.

More specifically, the goals are to (a) build a test rig and simulation environment, (b) perform various driving experiments in an SBW test rig to collect input/output data from/to steering system and vehicle dynamics,

(c) investigate the role of force-feedback in super lightweight vehicles so that a mathematical model for generating a proper steering feel in an SBW system can be developed, and (d) investigate the cooperation of other information, which is obtained from the vehicle dynamics, with the designed steering feel model with regard to driving performance and thus enhance the steering feel.

Driving Simulator

In this project, a test setup including the steering system of an SBW vehicle and a graphical interface 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, whose physical part consists of a steering wheel, direct drive motor, control prototyping board, motor driver, wide screen LCD monitor, driver’s seat, a body to which all of these parts are attached, and a PC, has been developed and is present in the Vehicle Dynamics Laboratory in the department of mechatronics engineering at Yildiz Technical University. As for software part of the driving simulator; an 11 degree-of-freedom dynamic model of a vehicle, a torque and position control system for steering wheel actuator, a cruise control system, a graphical environment for creating the virtual reality driving simulator, and graphical user interfaces have been developed.

The vehicle model and controllers were designed in Matlab, Simulink and the I/O connections with the controller board were made in Simulink by using Real-Time Interface software. The built code of simulation model is embedded to the controller board. The controller board is dSPACE DS1103 with which the hardware and software parts are interfaced.

In order to test the steering feel model on the SBW test rig, Hardware-In-the-Loop (HIL) simulation technique has been employed. In an HIL simulation environment, a hardware-under-test is substituted for its mathematical model in a simulation model of the whole system, and an interface between the physical system and the rest of mathematical model is designed. In this environment, the steering wheel system is the hardware-under-test. The steering system mainly consists of the steering wheel, and an AC servo direct drive motor. To present realistic results, a detailed nonlinear vehicle model is used in HIL simulations. The mathematical model is the full vehicle model, which consists of the chassis, suspension, wheel dynamics, and nonlinear tire model. The mathematical model of the vehicle, controllers and the physical system have been configured and interfaced with the physical system so as to constitute a closed-loop.

The Effect of Steering Feel on Driving

Although, the importance of steering feel has been discussed in the literature, it would be useful to show the effect of steering feel on driving by comparing the weave test results. For this case, the speed is selected as 100km/h. The effect of proposed steering feel model on the behavior of vehicle compared to the behavior of vehicle without steering feel can be seen in the figures below.

Fig.1 - Time histories of steering wheel angles.

Fig.2 - Time histories of lateral velocities.

Fig.3 - Time histories of pneumatic trails.

The change of pneumatic trail during the oscillatory movement of road wheels is shown in Fig.3. The higher values of pneumatic trail in the vehicle with no steering wheel torque feedback to driver imply the increase of slip angle, and thus, the approach of lateral tire forces to their limits. Their effects appear in the SAT of front wheels of the vehicle. High values of the pneumatic trail and the approach of SAT near zero as the steering angle increases indicate almost pure sliding of the front tires. Increase in the lateral velocity is also another undesired effect, Fig.2. Lack of steering wheel torque feedback causes the human driver make excessive movements, as shown in Fig.1, to correct the motion of vehicle. The resulting response of vehicle is similar to the response of underdamped second-order system. As can be evaluated from the results, the presence of steering wheel torque implies a restrictive movement for the driver by providing a controlled motion of vehicle.

Final Report Summary

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. 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.

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.