Steer-by-wire variable ratio: A study about speed & position dependent ratios

Abstract

This thesis aims to explore the field of Steer-By-Wire and rationalise how a steering ratio can vary depending on the speed of the vehicle and the position of the steering wheel while providing a good combination of mobility and stability. The thesis also investigates how a phase delay, which occurs between the input and output signal, affects the vehicles steerability. To answer these questions, a test bench and a test vehicle had to be set up where various experiment could be done. The tests done on the bench and in the vehicle involved researching different function and how they affect the steerability. The experimental process is separated into two parts in this report, where one part is for a speed dependent solution and the other part is for a vehicle speed and angular position dependent ratio with a fixed end lock. It is worth noting that the speed dependent ratio is based on the speed of the vehicle, while a speed and position dependent ratio is based on the vehicle speed and the steering angle. There were two main speed dependent functions that were tested, one exponential and one quadratic. There was only one function tested for the speed and position dependent ratio. The testing process for these functions involved changing different parameter values until a satisfying result was obtained. When a parameter was changed, the system was tested on a designated testing track to observe how it affects the vehicle. It was observed after conducting these tests that the factor with the biggest impact on the speed dependent function was the maximum steer ratio and when the minimum ratio was reached. However, the factor with the most impact for the vehicle speed and angular position dependent ratio was the position of the end lock. The result of these tests showed that a smaller end lock angle made the vehicle more unstable at higher speed but provided high mobility while a bigger end lock angle made the vehicle more stable, but a decrease of mobility was noted. It was observed that when a more direct ratio was used it increased the phase delay between the steering wheel and the power steering system. This happens because a more direct ratio creates a greater difference between the input signal, the requested value and output signal, the actual value, which causes problems with the rack regulator. This made the vehicle difficult to control around corners and the signal had magnitude losses when rapid movements were done with the steering wheel. However, this delay can be reduced significantly by replacing the hardware with a more precise power steering system that can handle more rapid movements.