The steer-by-wire development project reported on in this paper disclosed the following: The rack drive type electric power steering technology can be utilized for the steer-by-wire front axle actuator subsystem. However, in order to exploit the advantages of steer-by-wire with respect to steering response time and speed dependent steering ratio the electric drive has to provide more output torque margin and higher output power than a comparable electric assist for a fixed ratio steering system. Realistic front axle actuator performance requirements therefore call for a 42 Volts power supply or the means to boost the voltage of the 14 Volts board net. If the feedback torque generation is supposed to simulate steep torque gradients and a mechanical lock stop feel with a decent torque level, the design considerations for the electric drive of the hand wheel force feedback actuator subsystem inevitably lead to the worm gearbox solution which is standard for column drive type electric power steering. The requirements regarding the bandwidth of the feedback torque control loop are significantly higher than for an electric assist system. The achievable bandwidth of the torque control loop depends on the resonance frequency of the electric drive, the performance of the underlaying motor phase current control and last but not least on the available computing power of the HWA microcontroller. The fail safe steer-by-wire prototype system built from such subsystems and fitted to the test vehicle meets the expectations regarding function and performance. Provided that the 14 Volts power supply issue can be solved, the application of a fail safe steer-by-wire system is feasible in the near future, at least from a technical point of view. The actuator technologies utilized for the fail safe steerby- wire prototype system can be applied to the fault tolerant architecture as well. An acceptable integrity level of the fault tolerant steer-by-wire system requires major redundancies within the actuators subsystems. Even with more intelligent solutions than twin electric drives the requirement for a redundant power supply will remain. Thus the expenses for a fault tolerant steer-by-wire system can only be justified, if the application not only provides advantages regarding function and performance of the steering system itself, but due to the missing mechanical link between the steering handle and the front axle becomes a key enabling technology for a future vehicle concept.
Development of a fault tolerant steer-by-wire steering system
Entwicklung eines fehlertoleranten Steer-by-Wire-Systems
2004
8 Seiten, 10 Bilder, 8 Quellen
Conference paper
English
Development of a fault tolerant steer-by-wire steering system
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