Progettazione di un sistema Steer by Wire e sperimentazione tramite banco Hardware in the Loop

“By-wire” systems are characterized by the lack of mechanical link between the control device and the actuator, being it hydraulic or mechanical. The connection is replaced by a mechatronic system, equipped with redundant sensors and actuators [1]. SBW consists in two electro-mechanical actuators that steer the vehicle wheels and generate a resistance torque against the rotation of the steering wheel during cornering maneuvers. Each actuator is characterized by the presence of an electric motor, the first is connected to the pinion of the steering system, the other is connected to the steering wheel. The electric motor connected to the pinion is controlled in position and the command signal is generated by a PID controller. Reference position depends only on steering wheel angle and steering ratio. The torque control of the second electric motor is more difficult; the reference torque value has to be realistic in order to grant the proper feeling to the driver. The steering column is removed, as well as every hydraulic component that is part of the classic servo-assisted steering system. The logic that controls the system needs the information provided by two position sensors: one encoder is devoted to measure the steering wheel angular position, one linear position transducer measures the position of the rack [2]. The connection between the control units of the two sub-systems is realized through a communication bus. The aim of SBW technology is to completely remove as many mechanical components (steering shaft, column, gear reduction mechanism, etc.) as possible, thus simplifying the car interior design, increasing the driver safety in case of frontal crash. Moreover, active safety and handling are increased thanks to the variable ratio between steering wheel angle and tyres’ steering angle, determined by a control logic [3]. SbW system’s steering wheel has to generate a resistant torque which adds to the friction one. Such torque must be felt as natural by the average driver and carry information about vehicle dynamic condition. The paper describes the design of a SBW prototype, obtained from a classical steering system (see Fig.13) [4] [5]. A Hardware-in-the-Loop (HIL) test bench is also described: it is realized in order to test feedback torque generation and steering wheel efficiency influence on vehicle behavior. Actual steering wheel angle is used as input for a ten degrees of freedom model that calculates the vehicle dynamics, defining the steered wheels motion and the resistance torque at the pinion (see Fig.10). An analog signal is generated in order to control the steering wheel electric motor torque. Experimental tests are performed to optimize the feedback torque control. The reference torque is then calculated only using signals coming from vehicle on board sensors. Control logic is finally improved in order to stiffen the steering system unnaturally when lateral acceleration is about to reach adherence limit and prevent further steering action (see Fig.14).