Chassis & Suspension
Steer-by-wire – a solution to many design challenges
In a regular steering system as we know it today in vehicles and working machines, the steering device usually consists of a steering wheel, a steering actuator, a physical link between the steering device and the actuator, and a power source. To improve active and passive safety and comfort for the driver, and for economic reasons, the industry has been working on ways to eliminate the mechanical connection between the steering device and the wheels and replace it by a purely electric or electronic solution. Steer-by-wire is a feature that promises to make such a solution possible. By approving regulation ECE R79 Revision, the UN Economic Division for Europe (UNECE) covered the use of full steer-by-wire systems on roads without mechanical backup.
Steer-by-wire is an advanced steering technique for adaptable steering and modularity which eliminates the need for a mechanical connection between the steering device and the steering wheel. With steer-by-wire, the connection is purely electric or electronic. Actuators steer the wheels and also give feedback to the driver. In today's driving world, a steering column between the steering device and the wheels is the standard.
It all started with the EU-funded X-by-Wire project with seven industrial partners from the automotive industry and two universities in 1996. Over the years, several prototypes have been built. They show impressively that steer-by-wire offers a long list of advantages such as additional comfort functions and simplification of the production process to its users as well as extensive design freedom. Steer-by-wire also has a positive impact on the environment – since no hydraulic pump is needed, no hydraulic fluids need to be disposed of and energy consumption can be optimized.
Steer-by-wire offers numerous advantages, such as selectable steering characteristics, reduction of noise and vibration from hydraulic parts, the fact that the steering unit may be placed where needed, making the driver seat position variable, and – important for designers and manufacturers – the simplification of assembly and the possibility of new modular design concepts.
The implementation of steer-by-wire in vehicles faces technical, legal and regulatory issues, and it has to be accepted by the user, that is, the driver, as a safe method of navigating a vehicle. The design of the future steering system must meet the functional and safety requirements of all authorities involved. Another major challenge is to keep the costs of steer-by-wire systems driving on public roads at a reasonable level. In order to handle these issues, several companies formed a working group within the TTA-Group, the Steer-by-Wire Working Group. One of the goals of the Steer-by-Wire Working Group is to define a reference architecture that shows that steer-by-wire is not more expensive than conventional steering systems. The challenge is to combine safety, low cost, and modularity.
Steer-by-Wire Working Group – Cooperate on Platforms, Compete on Functionality
The Steer-by-Wire Working Group was founded by TTTech and Sauer-Danfoss in 2004 with the idea of forming a community for OEMs as well as legislative bodies and component suppliers to work on harmonization and the development of standards and legislation. The intention was to develop guidelines in order to enhance reliability, interoperability, and safety for steer-by-wire in special vehicle and off-road vehicle use. The mission of the Steer-by-Wire Working Group is to define a certifiable reference architecture for a complete steer-by-wire application without mechanical backup for off-road and on-road use. In addition to TTTech and Sauer-Danfoss, several other companies are active in the working group, including Carraro, CNH, Crown, Dana, Dynapac, John Deere, Eaton, Kongsberg, Liebherr, Lord Corporation, Massey Ferguson, MTS Sensor Technologie, Ognibene, QinetiQ, SKF, STILL, TÜV Nord, TTControl and Volvo.
Manufactures and suppliers of agricultural tractors, construction machinery, and fork lifts have teamed up in a joint industry effort to develop a global guideline as base for a de facto standard. TÜV Nord has been involved from the beginning as technical supervising company. The activities of the Steer-by-Wire Working Group focus on electro-hydraulic and electro-mechanic steering systems without mechanical backup. This also includes a selection of norms and legislation to be compliant with and the derivation of functional and safety requirements with the help of experts of all members of the working group.
The question arises how such a set-up of different members of the industry does not interfere with competition. The focus of the Steer-by-Wire Working Group is on cooperation on a non-competitive level; its intention is to work on platforms and to compete on functionality at a later stage.
The Steer-by-Wire Working Group has a full agenda. One of its goals is to create a set of guidelines whose framework is oriented towards IEC 61508 and ISO 13849 which is considered state-of-the-art. In order to minimize and share cost and effort, all drafts and documents are continually reviewed by the technical supervising authority. Generic topics such as the selection of standards and directives, hazard and risk analyses for key industries (SIL as result of analysis), the definition of safety, functional and performance requirements, the creation of reference architectures as well as industry-specific topics such as the approval of the safety concept, the design specification or the implementation of steer-by-wire architecture, are addressed.
Legal Aspects of Steer-by-Wire
Mechanical steering systems are considered safe, whereas electronically controlled systems may fail without previously displaying signs of error and are therefore considered less safe. For this reason specific directives will apply to the development of steer-by-wire. The Steer-by-Wire Working Group evaluates standards and legal aspects for the implementation of steer-by-wire systems and gives guidance on how to deal with these issues.
For motor vehicles for carriage of passengers, goods and their trailers, Framework Directive 2007/46/EC applies for type approval. Among others it lists for the steering effort the Directive UNECE R79. The current ECE R79 in Revision 2 now covers the possibility of full steer-by-wire systems without mechanical backup.
For Agricultural or Forestry Tractors, especially for faster tractors above 40km/h, directive 70/311/EEC may be applied. This directive, and the more general 75/321/EEC do not cover full steer-by-wire systems, but the latest UN ECE regulations may be applied to some systems. In particular, regulation UN ECE R79 R2 may be applied, allowing full steer-by-wire systems.
For mobile machinery there exists no separate European Framework Directive. These vehicles are typically nationally type approved for on-road use. Many countries use ECE R79 R2, 70/311/ECC or 75/31/ECC concerning steering as possible basis for national approval. Therefore an independent test report on the steering system on that basis usually considerably simplifies the national type approvals, especially for those countries that also accept EC Directives and UN ECE Regulations as an alternative to the national type approval regulations.
Besides regulatory requirements from the type approval procedure, the manufacturer of steer-by-wire systems also needs to consider liability aspects including the electronic control and communication system. Standards such as IEC 61508 are used to avoid and control any systematic design faults and to deal with random hardware faults.
Steer-by-Wire Solutions Forum
Steer-by-Wire Working Group members benefit from forums where members present their solutions. The Steer-by-Wire Solutions Forums are usually one-day events held once a year where suppliers have the possibility to promote their steer-by-wire offerings and related offerings. Not only active Working Group members but also OEMs that are not active in the group may join and benefit from the Solutions Forum. The latest Steer-by-Wire Solutions Forum has taken place in April 2010 in Munich during the BAUMA 2010.
Below, some suppliers are presented as well as and their solutions.
MTS Sensor Technologie is a well-known supplier of position sensors for mobile vehicles used in the construction, agricultural, and material handling markets. The Temposonics® M-Series sensors are typically embedded inside a hydraulic cylinder and are especially suited for the rugged environment in steer-by-wire applications found in off-highway equipment. In a steering system composed of a control device and a steering cylinder actuated via a proportional valve, the sensor reliably detects the precise position of the steering cylinder and respectively the steering angle of the wheel.
Ognibene has developed a fully-functional steer-by-wire driving control unit for agricultural tractors. The architecture was designed to fulfill all legal requirements. It features redundancy to tolerate individual errors. Ognibene's steer-by-wire system was tested very thoroughly and successfully. Failures were injected in order to verify that the redundant design could satisfy the new R79 regulation for steering systems. Significant advantages of Ognibene's steer-by-wire systems include easier layout of the vehicle, faster installation of the steering system and better automation of the steering function. In addition, the installation is more flexible, and so is its on-field behavior. The steering of the vehicle can be parameterized according to on-field conditions, and the steering behavior can change with the state of the vehicle.
Ognibene collaborated with TTControl to design the bus architecture based on the Time-Triggered Protocol (TTP®). A distributed and redundant real-time system was chosen for the architecture. Functional units are grouped in sub-systems, one or several electronic control units are attached to one sub-system.
TTControl offers electronic control systems for off-highway vehicles such as construction machinery, forklifts, cranes, construction and agricultural machinery, and snow groomers. The company has a variety of products suitable for steer-by-wire ranging from high-end to low-end solutions. The top models of the portfolio have been certified by TÜV Nord according to the IEC 61508 standard and reach safety levels up to SIL 3.
Sauer-Danfoss' new OSPE electro-hydraulic steering unit incorporates a range of innovative features designed to meet stringent new safety legislation. Featuring proven OSP steering technology and an integrated electrohydraulic steering valve, the OSPE steering unit helps simplify hydraulic system architecture. The high level of integration minimizes the need for additional components and provides OEMs with a complete package – designed, developed, and tested for optimal performance.
For many years, SKF has been supplying mechatronic solutions to the off-highway market for a variety of applications. The range of products includes basic sensor units with incremental sensors for electric motor control, units with absolute sensors as position or feedback sensors and more complex systems such as parking brake actuators including software. SKF offers mechatronic products, which fully support the hybrid technology trend. As one of the first companies to develop a viable steer-by-wire solution, SKF has demonstrated steer-by-wire technology through various concept vehicle designs for both highway and off-road applications.
Advantages and Drivers of Steer-by-Wire
Steer-by-wire offers a vast number of advantages for the automotive and off-highway industry. For the vehicle manufacturer, the interior design is simplified, and the omission of the steering column results in maximal design freedom. With this new independence, completely new designs and aesthetics can be realized. Alternate steering concepts such as joy sticks may be implemented. The most important benefits of steer-by-wire are:
Selectable steering characteristics. With steer-by-wire, more advanced support systems for parking maneuvers, especially helpful for trucks or other large vehicles, are possible. With enhanced vehicle control and new driver assistance functions, the scope of the vehicle can be broadened and therefore productivity significantly increased. It is, for example, thinkable to generate a high steering angle with only a small deviation of the steering device. At low speeds, this would improve the handling of working machinery and tractors.
Reduction of noise and vibrations coming from hydraulic parts (electric power steering). In a typical mechanical steering system the driver’s input is transmitted by a steering shaft through some type of gear reduction mechanism to generate steering motion at the front wheels. Power steering assistance is a standard feature. Hydraulic power steering uses hydraulic pressure supplied by an engine-driven pump. Power steering amplifies and supplements the torque applied by the driver, consequently less steering effort is required. The introduction of electric power steering eliminates the need for a hydraulic pump. Electric power steering is more efficient than hydraulic power steering since the electric power steering motor only needs to provide assistance whenever the steering wheel is turned, whereas the hydraulic pump has to run constantly. This results in a considerable reduction of noise and vibration.
Steering unit can be placed where it is needed. With steer-by-wire, the steering unit may be placed anywhere in the vehicle. The driver will also enjoy better visibility.
Variable driver seat position. The driver seat position will be more flexible, more ergonomic and therefore more comfortable for the driver.
Simplification of assembly by using modules. Steer-by-wire is not only beneficial for the driver – the life of the manufacturers will become much easier, too. Assembly of vehicles will be simplified by the possibility of using modules for wheel actuators, control systems and cockpits. Engine and gearbox placement will no longer present a problem since there will be no need for a steering column; the steering mechanism can be designed and installed as a modular unit.
Markus Plankensteiner is Director Marketing for TTTech Computertechnik AG and coordinator of the TTA Group. He can be contacted under firstname.lastname@example.org
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