17-01-2013, 10:54 AM
Future Electrical Steering Systems
1Future Electrical.doc (Size: 147 KB / Downloads: 44)
ABSTRACT
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
And/or fail-silent. The realization of the derived safety structures requires both expertise and experience in design and mass production of safety relevant electrical systems. Beside system safety and system availability the redundant electrical systems also have to meet economic and market requirements. Within this scope the paper discusses three different realizations of electrical steering systems
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 are easily and rapidly feasible.
FUTURE STEERING SYSTEMS
The main feature of future steering systems is the missing direct mechanical link between steering wheel and steered wheels. With such a steer-by-wire steering system Fig. 3.1 the missing steering column’s function must be reproduced in both directions of action. In forward direction the angle set by the driver at the steering wheel is measured by a steering angle sensor and transferred with the suitable steering ratio to the wheels. In reverse direction the steering torque occurring at the wheels is picked up via a torque sensor and attenuated respectively, modified fed back to the driver as a counter torque on the steering wheel.
ADVANTAGES OF STEER-BY-WIRE SYSTEMS
Steer-by-wire is a universal actuator for automatic steering intervention. – For vehicle dynamic steering intervention a steering angle actuator is needed which does not affect the steering wheel while rapidly correcting the vehicle wheels. On the other hand, a torque actuator will be needed for automatic lateral guidance interference and future steering systems of autonomous driving, thus imparting a superimposed torque onto the steering wheel and letting the driver with that know the intended direction, evaluated by the lateral guidance control system.
Steer-by-wire meets both requirements ideally. Along with "drive by wire” and "brake by wire“it provides the condition to materialize vehicle dynamics and comfort oriented automatic controls in one system.
CONCLUSION
This paper presents various types of electrical steering systems and their safety 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. A steer-by-wire system with hydraulic backup and a purely electrical system were discussed.
Future innovative steering functions, such as vehicle dynamic interventions, collision avoidance, individual wheel steering, tracking assistance, automatic lateral guidance, and finally autonomous driving functions will be implemented in a system compound of various vehicle systems. Future steering systems will thus have to be integrated into a system compound, in terms of interfaces and functions. The steer-by-wire principle becomes absolutely necessary when those innovative functions are to be achieved. The transition to purely electrical steering systems will proceed step by step, both for safety reasons and acceptance by the customer. The path will lead from electrical power steering via a steer-by-wire system with a hydraulic or mechanical backup towards purely electrical steer-by-wire systems.