INTRODUCTION
By-wire-steered system is an application of ‘MECHATRONICS’, which is the integration of electronic devices and mechanical systems in order to improve the performance of the system.
Recent advances in dependable embedded system technology, as well as continuing demand for improved handling and passive and active safety improvements, have led vehicle manufacturers and suppliers to actively pursue development programs in computer-controlled, by-wire subsystems. These subsystems include steer and brake-by-wire, and are composed of mechanically decoupled sets of actuators and controllers connected through multiplexed, in-vehicle computer networks. There is no mechanical page link to the driver. Steer- and brake-by-wire provide a number of packaging and assembly advantages over conventional subsystems. For instance, electromechanical brake-by-wire subsystems require no hydraulic fluid to store or load at the assembly plant and permit more modular assembly, thus reducing the number of parts to be handled during production. Steer-by-wire systems have no steering column and may also eliminate cross-car steering assemblies such as racks. Arguments for ‘by-Wire’ systems include production costs, packaging and traffic safety
The ‘by-Wire’ technology as in drive, brake and steer is gaining ground and is undoubtedly an automotive solution of the future. The arguments to support such ‘by-Wire’ systems include reduced production costs and packaging advantages and improved traffic safety (a boon for everybody involved). Emerging drive-by- wire technologies offer new possibilities for designing the steering characteristics of road vehicles. When the mechanical page link between the steering wheel and the front wheels is replaced by sensors, controllers and actuators, enormous flexibility is achieved in terms of the control device applied and in terms of the transfer function of the steering system. This offers new possibilities for optimizing the steering system for mass-produced vehicles. However, the flexibility is of even greater advantage in the area of car adjustment for drivers with physical disabilities.
A steer-by-wire system replaces the traditional mechanical linkage between the steering wheel and the road wheel actuator (e.g., a rack and pinion steering system) with an electronic connection. This allows flexibility in the packaging and modularity of the design. Since it removes the direct
Kinematic relationship between the steering and road wheels, it enables control algorithms to help enhance driver input.
The transition to purely electrical steering systems will take place step by step via systems with mechanical or hydraulic backup. 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.
‘Steer-by-Wire’ (SbW) there exists a legislation obstacle as European regulations require a mechanical connection between the steering wheel and the wheels. The column electric power steering (C-EPS) in the Opel Astra is therefore only an electric hybridization at steering level: the steering torque levels will increase when the car picks up speed. The “Dual drive” system in the Fiat Punto has an EPS with dual settings: the driver can activate the “city” mode and obtain gentler steering when parking. The main limitations of by-wire-steered system are the requirement of a 42 Volts car supply, high output alternator and new generation batteries.
The steer-by-wire principle becomes absolutely necessary when Future innovative steering functions, such as vehicle dynamic interventions, collision avoidance, individual wheel steering, tracking assistance, automatic lateral guidance, and finally autonomous driving functions have to be implemented in a system compound of various vehicle systems.
CURRENT STEERING SYSTEMS
Primary function of the steering system is to achieve angular motion of the front wheels to negotiate a turn. This is done through linkages and steering gear which convert the rotary motion of the steering wheel in to angular motion of the front road wheels.
Current steering systems can be classified in to two namely, mechanical steering systems and hydraulic power steering system.
1) MECHANICAL STEERING SYSTEM and
2) HYDRAULIC POWER STEERING SYSTEM
MECHANICAL STEERING SYSTEMS:
Conventional steering systems are based on the mechanical steering of which two variants are in use. The components of a rack-and-pinion steering and ball-and-nut steering, which is used for higher steering forces, are purely mechanical: Steering wheel, steering column, steering pinion, rack, ball-and-nut gear, tie rod. Additional components are various universal joints and bearings.
The various steering systems that are commonly used are worm and all wheel steering, cam and double roller steering, worm and nut steering, recirculating ball type steering and rack and pinion steering. Among these types most commonly used are rack and pinion steering and recirculating steering systems. Cam and double roller steering are commonly used in Ashok Leyland vehicles (steering gear ratio used=24.7:1).Recirculating ball type steering are used by Tata, Dodge/Fargo, Standard 20 vehicles.Maruti 800 cars employ rack and pinion steering. The important types of mechanical steering systems are discussed below
A) RACK AND PINION STEERING:-
Rack and pinion steering is quickly becoming the most common type of steering on cars,small trucks and SUVs.It is actually a pretty simple mechanism.A rack and pinion gear set is enclosed in a metal tube,with each end of the rack protruding from the tube.A rod,called a tie rod is connected to each end of the rack.
The pinion gear is attached to the steering shaft.When we turn the steering wheel,the gear spins,moving the rack.The tie rod at each end of the rack connects to the steering arm on the spindle.The rack and pinion gear set does two things:
It converts the rotational motion of the steering wheel into the linear motion needed to turn the wheels.
It provides a gear reduction ,making it easier to turn the wheels
Some cars have variable ratio steering, which uses a rack and pinion gear set that has a different tooth pitch (number of teeths per inch)in the centre than it has on the outside. This makes the car respond quickly when starting a turn and also reduces effort near the wheel’s turning limit.
POWER RACK AND PINION STEERING:-
When the rack and pinion is used in power steering system, the rack has a slightly different design.
Part of the rack contains a cylinder with a piston in the middle.The piston is connected to the rack.There are two fluid ports,one on either side of the piston.Supplying higher pressure fluid to one side of the piston forces the piston to move,which in turn moves the rack,providing the power assistance.
B) RECIRCULATING BALL STEERING:-
Recirculating ball steering used in many and SUVs today. The linkage that turns the wheels is slightly different than on a rack and pinion system.
The recirculating ball steering gear contains a worm gear. We can imagine the gear in two parts. The first part is a block of metal with a threaded hole in it. The block has a greater teeth cut into the outside of it; which engages a gear that moves the pitman arm. The steering wheel connects to a threaded rod,similar to a bolt that sticks into the hole in the block.When the steering wheel turns ,it turns the bolt.Instead of twisting further into the block,the way a regular bolt would ,this bolt is held fixed so that when it spins,it moves the block which moves the gear that turns the wheels.
Instead of the bolt directly engaging the threads in the block, all of the threads are filled with ball bearings that recirculate through the gear as it turns.The balls actually serve two purposes;first ,they reduce friction and wear in the gear,second ,they reduce loosing of the steering wheel.
The recirculating ball steering also can be used in power steering systems.In such a case assistance is provided by supplying higher pressure fluid to one side of the block.
HYDRAULIC POWER STEERING SYSTEM:–
The hydraulic power steering system today is the most used steering system. It is based on the components of the mechanical steering system, in addition there is a hydraulic system, usually consisting of hydro pump with V-belt drive, hydraulic lines, oil reservoir and steering valve. The essential new function of this power steering is the hydraulic support of the steering movement, so that the driver’s steering-wheel effort is reduced. Therefore in the event of failure, the loss of steering boost arises as a new safety aspect in comparison to purely manual steering.
This can be caused by a leakage of the hydraulic system or by a hydro pump failure.
In the above figure Hydraulic power steering is shown. Since by design the manual steering system is further Available, in case of a failure the steering function is further available and the driver can adapt himself by the usually slowly rising steering-wheel effort in good time to the missing steering boost. In the regulation ECE R79 concerning the approval of vehicles with regard to steering equipment the maximum permitted steering control effort with a failure in the steering equipment is stipulated. In the context of the vehicle approval the appropriate investigations for the failure modes of the hydraulic power steering are performed and the compliance with the prescribed steering efforts is checked.
There are a couple of key components in power steering in addition to the rack-and-pinion or recirculating-ball mechanism.Pump, rotary valve, pulley and v-belt are some among such components.The pump usually used is a rotary vane pump due to its excellent volumetric efficiency,high pressure capability,reduced noise emission,low power consumption,long life,light weight and cost efficiency.
NEW STEERING SYSTEMS
New steering systems use more and more electrical components. This is mainly because of two reasons: Energy saving and installation simplification by modular design.
ELECTRO-HYDRAULIC POWER STEERING SYSTEM:–
The electro-hydraulic power steering system is based on the rack-and-pinion hydraulic power steering and its essential new feature is an electrically driven hydraulic pump, which substitutes the steering pump driven so far by the vehicle engine. Additional new components compared to the hydraulic power steering are: Electric motor, electronic control unit and an optional sensor for steering velocity. The pressure supply unit integrates electric motor and electronic as well as hydraulic pump and oil reservoir–
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 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.
Electrical power steering 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 activities as well as too strong steering boosts are 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 page 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(steering ratio is the ratio of how far we turn the steering wheel to how far the steered wheels turn) 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.
Principle illustration steer-by-wire
First, steering wheel module and steering module are implemented with familiar components of mechanical and electrical steering systems, like: Steering wheel, gearbox, electrical motors, and 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.
SYSTEM STRUCTURES OF SAFE ELECTRICAL STEERING SYSTEMS
a)SYSTEM STRUCTURE OF ELECTRICAL POWER STEERING :–
In an electrical power steering system the steering torque initiated by the driver 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.
Figure 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.
b)SYSTEM STRUCTURE OF STEER-BY-WIRE SYSTEM WITH HYDRAULIC BACKUP –
The steer-by-wire system with hydraulic backup is shown in Figure. It represents advantageous combination of the functional possibilities of electrical steer-by-wire systems with the high reliability of proven hydraulic elements. 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. The control unit in Figure is presented as a central control unit. 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 life time. The pressure within the backup level is continuously monitored by a pressure or displacement sensor. 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 page link between steering wheel and the vehicle wheels. On the hydraulic backup level vehicle dynamic intervention is no longer possible.
c)SYSTEM STRUCTURE OF THE PURELY ELECTRICAL STEER-BY-WIRE SYSTEM –
Figure shows the structure of a purely electrical steering system. 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 tearing 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 page 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. 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.
ADVANTAGES OF STEER-BY-WIRE SYSTEMS
1) Reduction in weight of the automobile
2) Less constrains on the driver interface
3) Makes the driving easy
4) Low manufacturing cost
5) Overall safety is improved
6) It is an environmentally friendly technology
7) No problems of leakage of the power steering fluid
8) Energy will be consumed only during steering
9) Steering assistance is available even when the engine is not running
10) Less force is required for steering
11) Steering comfort is improved
12) Reduction of injury risk in an accident by missing steering column
13) Steering ratio can be adjusted by suitable softwares
14) More space available inside the car
15) Less power consumption
16) Steering wheel return torques can be adjusted variably by using softwares
17) Steering behaviour (under steering, over steering, self steering properties) can be matched by softwares
FILO CONCEPT CAR
The Filo concept car was designed by Bertone, a leading automotive design house, and SKF, the automotive industry’s leading supplier of bearing and sealing solutions. FILO is a by-wire-steered concept car. It is steered ,braked and accelerated utilizing by-wire-technology.
With the Filo, driver input (steering, acceleration, braking and gear shifting) is translated into electrical signals that go by wire to electromechanical control units. Electric motors take over the conventional mechanical or hydraulic functions with integrated sensors and measurement systems determining position, force and displacement. The electronic logic between the actuators and driver controls offers vehicle designers an easier way of making the vehicle's "feel" programmable to suit different drivers.
The Filo concept uses “drive by wire” technology engineered by SKF, enabling a radical re-evaluation of the man-machine interface and the architectural of a car’s interior. STILE BERTONE has created a living space inside a car devoid of the traditional constraints imposed on position and freedom of movement.
Mechatronics, the combination of mechanical devices under intelligent, electronic control, is at the heart of the Filo. It is these techniques that allow much of the interior redefinition, allowing for a reassessment of how a driver interacts with the vehicle and environment in which it is being driven.
Stile Bertone utilizes “drive by wire” technology to re-design the interior architecture of the automobile and to create a living space devoid of the traditional constraints imposed on position and freedom of movement.
Finally freed of the bulky and potentially dangerous presence of the steering column, the designers have been able to consider a dashboard free of obstructions and constraints, a large free surface, simple, clean, relaxing. A central binnacle houses the information and driving equipment, while the air outlets disappear and the air conditioning is diffused evenly. The waved floor offers various possibilities for the driver to rest his feet, no longer restricted by the presence of pedals. Both front and rear seats have the appearance of lounge sofas, with the rear positioned higher than that of the front to give greater forward visibility for the passengers. Entrance into the car is made easier due to the absence of the central pillar. A much closer page link between the environment of the home and the car is inevitable.
Bertone strongly believe that by utilising fly-by-wire technology, the overall safety and level of function will be improved. It will be increasingly possible to integrate the comfort and luxuries of the home with the automobile. Safety is improved in collision and there is a reduction of industrial investment to change from right to left-hand drive. By-wire is also an environmentally friendly technology, dispensing with hydraulic oil for the braking and power steering systems. The “feel” for the driver’s controls from the mechatronic actuation systems is provided by closed-loop sensor and feedback system. The car is a true concept in not only appearance but, perhaps more importantly, in function. Bertone have shown with the Filo that use of technology already proven in other industries - such as aerospace - can not only functionally improve the vehicle but also increase safety. Progress in this field now depends upon the relevant legislators - such as the EU and US Government - to make provision for fly-by-wire technology and to ensure high standards of manufacture when this happens. FILO drive-by-wire technology, bertone developed jointly with SKF, has made it possible to radically rethink the man-car interface and the car's interior architecture. No more pedals, therefore: steering, accelerator, brakes, gearbox and clutch are all controlled by wire. And inside, STILE BERTONE has created living space unhampered by the usual restrictions to freedom of position and movement.
CONCLUSION
This paper presented various types of electrical steering systems and their system structure starting from current and new steering systems. 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. It had been stated that redundant fail-safe structures for electric and electronic components are to be established due to the fact that no mechanical or hydraulic connection between steering wheel and vehicle wheels are available. 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.
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