Future Electrical Steering System - Seminar Report

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

Ø  Electrical power steering system (mechanical system with electrical boosting)

Ø  Steer-by-wire system with hydraulic back-up and

Ø  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.

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.

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.

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 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.

First, steering wheel module and steering module are implemented with familiar components of mechanical and electrical steering systems, like: Steering wheel, gearbox, electrical motors, rack. The operational principle is, however, in principle open for more futuristic designs like side stick operation on the driver’s side and single wheel steering on the wheel side. While in systems with mechanical connection in the case of electrical errors only the steering boost is concerned, corresponding measures must be taken with steer-by wire systems that in case of any electrical failure steering control is always guaranteed.

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.

SYSTEM   STRUCTURE  OF  ELECTRICAL  POWER  STEERING

Functional description

In an electrical power steering system the steering torque initiated by the driver Fig. 4.1 is measured by a steering wheel torque sensor and is fed into an electronic control unit. The latter then calculates along with the driving speed a reference torque for the steering motor, which, however, can optionally also depend on the steering angle and steering angle velocity. By means of the calculated reference torque the currents of the steering motor are actuated. Fig. 4.1 shows the pinion-type realization, where at the pinion the electrical torque is superimposed to the torque initiated by the driver. In further versions both torques can be superimposed either on the steering column or on the rack. In case of a failing electrical component of this steering system the non-boosted mechanical intervention by the driver is maintained.

Safety features

The system’s fail-safe behaviour concerning electrical faults is accomplished by detecting and evaluating all electrical failures. In case of major electrical faults the electrical power steering system is switched off. Sensor failures or failures in the electronic control unit might be considered as an example, resulting in an unintentional self-activity of the steering or in a too strong steering boost. Risks of that kind are avoided by an effective monitoring strategy where failures are detected on time and the power steering system is switched-off. One detection method for this constitutes checking sensor signals and motor currents for plausible system conditions on a second path.

SYSTEM STRUCTURE OF STEER-BY-WIRE SYSTEM WITH HYDRAULIC BACKUP

Functional description

The steer-by-wire system with hydraulic backup is shown in Fig.4.2. The system consists of components at the steering wheel and at the vehicle wheel level, an electronic control unit and a hydraulic backup. Steering wheel motor and sensors for the steering wheel angle and the steering wheel torque are arranged at the steering wheel. These components identify the driver’s desire and reproduce the return forces to the steering wheel, which are transferred to the steering wheel by conventional steering systems. These feedback forces are important to gain a safe feeling while driving. At the vehicle wheels side the system consists of an electric motor directing the mechanically coupled wheels via a gear and a rack, and of sensors to measure angles and torques. The electronic control unit registers periodically all sensor values, processes them via efficient control algorithms and supplies the control signals to actuate the motors. Via a serial data bus, the electronic control unit communicates with a vehicle guidance unit, which coordinates the superior steering interventions, e.g. to improve vehicle dynamics. This unit at the same time constitutes an interface to the driver information system, and to additional control units for engine and brakes.  It can also be divided into two modules arranged close to the steering wheel and steering motor, and connected to a data bus system for communication. A closed hydraulic unit, consisting of a hydraulic pump at the steering wheel and a plunger on the vehicle wheel level, constitutes the backup. Both sides of these components are connected with each other by hydraulic lines. During normal operation the plunger is bypassed. In case of failure, the fail-safe switching valve actuated by the electronic control unit will close the bypass. Thus, via the hydraulic backup, the steering actuator can be operated by means of the steering wheel. Without electric current, the switching valve must be closed. In case of failure of the 42V vehicle electrical system thus the hydraulic bypass is automatically closed and the backup safely activated. If the steering wheel motor can still be controlled during backup operation it can be adequately actuated to support power steering. The increased pressure needed to operate the hydraulic backup is provided by means of a small pressure reservoir with check valve. This pressure accumulator compensates the leakage, which occurs during the vehicle lifetime. The pressure within the backup level is continuously monitored by a pressure or displacement sensor.

Safety features

The system’s fail-safe behaviour concerning electrical faults is accomplished by detecting and evaluating all electrical failures. According to the respective importance of the fault the functionality of the system is reduced. In case of major electrical faults the electrical steering system is completely switched off and the switching valve is safely actuated, establishing a firm hydraulic link between steering wheel and the vehicle wheels. On the hydraulic backup level vehicle dynamic intervention is no longer possible.

 SYSTEM STRUCTURE OF THE PURELY ELECTRICAL STEER-BY-WIRE SYSTEM

 Functional description

 The reduced safety by omitting steering column and hydraulic backup is compensated by higher demands on the safety structure of electrical and electronic components. Again, the system consists of components at the steering wheel, on the vehicle wheel level, and it comprises a control unit and a 42V vehicle electrical system. In this case this must be implemented as a safe 42V vehicle electrical system containing additional elements for the diagnosis of charge condition, as well as for the disconnection of batteries.

The steering wheel motor and sensors indicating steering wheel angle and steering wheel torque are arranged at the steering wheel. These components identify the driver’s desire and reproduce the return forces transferred to the steering wheel. For a safe acquisition of steering wheel position two redundant steering angle sensors are used. Power stage and power supply for the steering wheel motor are likewise redundant. In order to exert a return force on the steering wheel in case of a defective steering motor a torsion spring is available to generate the return torque. Optionally a second steering wheel motor can be used in order to redundantly generate the return torque. On the vehicle wheel level the system is equipped with a redundant set of electric motors and redundant sensors measuring angles and torques.

The electronic control unit is designed fail-safe in terms of redundant power supply, signal processing and power actuation. Sensor values are identified periodically and redundantly, further processed via matched control algorithms and the calculated actuation signals are supplied to the two steering motors as well as the steering wheel motor. As to the link between the electronic control unit and the vehicle guidance unit as well as dividing the functions of these components to the decentralized units the explanations are in accordance with what has been described earlier referring to the steer-by-wire system with hydraulic backup.

Safety features

Failure tolerance is required in these areas: sensors, electronics, actuators, vehicle electrical system and data transmission. This is accomplished by appropriate redundant structures. The fail-safe behaviour against electric faults is to be ensured by a complete detection and locating of all electric failures. Locating a defective channel during signal detection or signal processing requires majority decisions. The needed redundancy is achieved either by hardware components or by including additional processing variables of the same kind. The defective channel is then switched-off consequently. In spite of electrical faults both steerability and vehicle dynamic interventions are ensured on account of the redundant system structure.

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.