Additional future requirements for automobiles such as improved vehicle dynamics control. Enhanced comfort, increased safety and compact packaging are met by modern electrical steering systems. Based on these requirements the new functionality is realized by various additional electrical components for measuring, signal processing and actuator control. However, the reliability of these new systems has to meet the standard of today’s automotive steering products.
To achieve the demands of the respective components (e. g. sensors, bus systems, electronic control units, power units, actuators) the systems have to be fault-tolerant. * Electrical power steering system (mechanical system with electrical boosting) * Steer-by-wire system with hydraulic back-up and * Full steer-by-wire system The paper presents solutions for these systems and discusses the various advantages and disadvantages, respectively. Furthermore strategies for failure detection, failure localization and failure treatment are presented.
Finally the various specifications for the components used are discussed. 1. INTRODUCTION In this paper the technical solutions and safety aspects of various electrical steering systems are described. For car manufacturers and end customers the use of electrical steering systems offers many advantages concerning flexibility, enhancement of familiar steering functions and the introduction of innovative steering functions. New steering functions, which are even coupled with automatic steering interventions, call for an adaptation of regulations concerning the approval of steering equipment.
Development and production of the next generations of electrical steering systems up to purely electrical steering systems create high safety demands on components and systems. Reliable and safe electrical steering systems can be realized by using appropriate safety techniques for these new systems and their components combined with the know-how of safety relevant vehicle systems. At the same time the transition to purely electrical steering systems will take place step by step via systems with mechanical or hydraulic backup.
ELECTRIC POWER STEERING SYSTEM The electric power steering system combines a mechanical steering system with an electronically controlled electric motor to a dry power steering. The hydraulic system, which so far delivered the steering boost, is substituted by an electrical system. For this, a torque sensor measures the steering wheel torque and an electronic control unit calculates the necessary servo torque. This is delivered by an electric motor in such a way that the desired torque curve at the steering wheel is created.
Depending on the necessary steering forces the electric motor engages by a worm gear at the steering column or at the pinion and for high forces directly at the rack by a ball-and-nut gear. In figure 4 the pinion-solution is represented, which is intended for middle class vehicles. The components involved in the electrical power steering are besides the mechanical steering components: Electric motor, electronic control unit, power electronics, steering wheel torque sensor and CAN data bus to other systems.
The electrical power steering system offers large benefits compared to the hydraulic power steering. Apart from about 80% lower energy consumption the omission of the hydraulic fluid increases the environmental compatibility. The electrical power steering is delivered to the car manufacturer as a complete system module ready-to install. The adaptation of the servo power assistance to certain vehicle types as well as the modification of the control strategy dependent on different parameters and vehicle sizes is easily and rapidly feasible.
From the safety point of view as with the other power steering systems due to failures in electrical components, again the steering boost can be impaired, here by faults of components of the electrical servo system. The steering system’s unintentional self-activity as well as too strong steering boosts is to be concerned as new potential safety critical effects, which must be avoided by appropriate countermeasures. CONCLUSION This paper presents various types of electrical steering systems and their afety aspects. The electro hydraulic power steering does no longer operate the hydraulic pump via a V-belt drive from the internal combustion engine. Rather, an electric motor is used, yielding energy savings and flexibility of installation. Electrical power steering pursues this trend and offers additional advantages since no hydraulic system is required.