Steering Feedback Torque Definition and Generation in a Steer by Wire System 2008-01-0498
Steer by wire (SbW) system is examined, considering the positive effects of the lack of direct mechanical connection between steering wheel and rack. 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.
System prototype is obtained from a classical steering system. Steering wheel is linked to a brushless 12V DC current electric motor designed to develop resistance torque, after steering column is removed, triple stadium planetary gear is necessary to increase the torque output.
A hardware in the loop test bench is realized in order to test feedback torque generation and steering wheel efficiency influence on vehicle behaviour. Steering wheel is fixed to the bench and its rotation acquired by an optic encoder. Steering wheel angle is used as input for a ten degrees of freedom vehicle model through an acquisition data board. The same board is used to generate an analog signal capable of controlling electric motor torque.
A classical steering system is modelled using a multi-body software. Suspension elastokinematics is considered to obtain steering system characteristics and steering torque during several manoeuvres. A second steering model is developed with Simulink software and validated through the previous one, in order to be merged with former vehicle model and used in real time simulation with steering test bench.
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.