The automotive industry is currently undergoing a paradigm shift. One such example in the next generation steering is the Steer-by-Wire (SbW) technology. SbW comes with a lot of advantages but one of the big challenges is to provide the driver with a realistic steering feel. More precisely, steering feel can be defined as the relationships between the steering wheel torque, the steering wheel angle and the dynamics of the vehicle. Accordingly, the first contribution of this work will be to present transfer functions between these quantities that resemble those observed in traditional steering systems. The steering feel/feedback is then achieved by an electric motor which can be controlled by different control strategies. In this thesis three different control strategies are investigated. The first straightforward strategy is called open loop since there is no feedback controller in the system. The second strategy is torque feedback control and the third strategy is angle feedback control. All three systems are evaluated in terms of reference tracking, stability, robustness and sensitivity. Here reference tracking is defined as tracking a desired transfer function. The desired transfer function is denoted as the reference generator. When fulfilling the requirements the analysis shows that the torque feedback system has a better reference tracking than the other evaluated systems. It is also concluded that the open loop system has a compromised reference tracking compared to the torque and angle feedback systems. Since the SbW technology is still an undergoing area of research within the automotive sector this work can be used as a basis for choice of control strategy for steering feedback systems and also as a guideline for future hardware choices.

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Road feedback is an essential part of the driving experience, representing a connection betweenthe driver and the vehicle motion. Road feedback in a steer-by-wire system must berecreated and transferred to the driver through a feedback motor mounted on the steeringwheel. This project aimed to implement a function in the feedback motor control with thepurpose of giving the driver road feedback. The function should enhance the drivers trustand confidence in the steering system. Specifically, the function should provide the driverwith a feeling of being connected to the vehicle. A pre-study on the subject of steering feel was made as well as an interview study, whichresulted in a list of hypotheses. The hypotheses became a basis for generating conceptideas, together with measurement data of vehicle network signals for different drivingscenarios. Two different functions were then modelled and implemented in MathWorksSimulink. Function 1 models force components acting on the front road wheels in longitudinal,lateral and vertical direction. The forces result in a torque contribution fromeach dimension that acts around the steering axis and represents a reaction in the steeringsystem due to road disturbances. The torque is then translated to a steering wheel torque.Function 2 strives to capture road surface roughness through the high frequency informationin the steering rack motor torque. Three different road surfaces were studied; smoothasphalt, rough asphalt and gravel road. A test rig was used in order to verify the behaviour of the functions. The final step of theproject was to implement the functions in a test vehicle, where they could be further tunedand evaluated. The force component models of Function 1 captured different types of roadfeedback which were evaluated separately by timing, authenticity and desirability. Theresult of this evaluation was positive considering them separately. When the models werecombined it resulted in an unwanted behaviour. Function 2 gave torque feedback that feltauthentic and natural, especially for the gravel road case. However, it was more difficult todistinguish different asphalt types. The conclusion was that neither Function 1 nor 2 couldbe approved as finished functions, however both are considered as interesting concepts forfurther development. Fully steer-by-wire steering transmissions have now been approved for usage. This meansthat one of the remaining challenges for implementing steer-by-wire cars on today’s marketis the subjective views from the customers. Function 1 and 2 could be a way to providethe drivers with trust towards the steering, and at the same time, enhance the drivingexperience. / <p>The presentation was held at ACAS at Linköping University. The presentation included a summary and learnings from the project. A short demonstration was performed using a test-rig consisting of a steering wheel, a feedback motor and a control unit. </p>