Mobility Solutions

Electric Power Steering facilitates networked driving

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  • June 17, 2013
  • Mobility Solutions
  • Press releases

press release

The usage of electric power steering for passenger cars, like the ZF Servolectric® developed by ZF Lenksysteme GmbH, provides not only consummate ride comfort and unrivaled precision, but also allows a host of assistance systems to be integrated and supported. These assistance systems help enhance driving safety and comfort. The way in which data is transferred to the steering system is crucial in terms of the interaction between these networked systems. These driving dynamics parameters must also be coordinated and prioritized in relation to the steering assistance required by the driver.

Electric power steering system advantages
The increasing uptake of electromechanical steering gears (electric power steering, EPS) in passenger cars goes hand in hand with enhanced safety, ride comfort, and driving precision. The usage of electric motors to provide power assistance enables, unlike hydraulic steering systems, the power assistance to be easily controlled not only depending on the driver's requirements but also by overlaying additional torque. In the case of the electric power steering system, a sensor detects the driver's steering torque and forwards it to the electric motor via a control unit; the electric motor then transmits the power assistance to the steered wheels via the steering rack. A key advantage in this respect is that the power assistance torque can not only be calculated as a function of the steering torque on the steering wheel, but a host of additional state variables in the vehicle can be easily taken into account using software. With three different versions, the ZF Servolectric covers all vehicle classes from supermini cars to mid-size and full-size models as well as light commercial vehicles. Depending on the installation space, performance of the vehicle electric system, and required steering torque, the servo unit is mounted on the steering column, a second
pinion, or in parallel to the steering rack.

Assistance systems: "Driver thinks – car steers"
In modern volume-production applications of electromechanical steering systems, a host of assistance functions are already linked to the steering or enabled in the first place through the EPS. Speed-sensitive power steering is one example of a driver assistance function that increases comfort as part of the enhanced steering functions. Another option for increasing comfort, which has already been implemented in volume-production vehicles, entails personalizing the parameters that are responsible for the steering feel, i.e. mapping a range of setup variants that facilitate, for instance, a sporty and a comfort-oriented driving style. The enhanced steering functions also include algorithms which are used to ensure directional stability. Thus a crosswind or an inclined road surface, for example, may produce torque on the steering wheel which can be offset using suitable algorithms.
In conjunction with other systems, the use of electromechanical steering also supports new functions, such as automatic parking. Separate control units are used to measure potential parking spaces and to calculate the trajectory. The driver still operates the accelerator and brake pedal, while the steering movements are performed automatically by the steering system. Lane departure warning systems as well as enhanced lane keeping functions developed on the basis of these systems are also available for volume-production models. These interventions now essentially occur via interfaces provided by the Servolectric; the interfaces are used by networked control units. In addition to the actual requirements placed on the functionality of the assistance functions, statutory provisions also result in additional requirements.
Other assistance functions are used to provide the driver with suitable steering recommendations in order to respond correctly in critical driving situations. These come into their own e.g. during oversteer or under µ-split braking. The electronic stability program (ESP) requests additional steering torque which provides the driver with suitable assistance. These interventions are limited so that the driver is able to correct the steering at all times.

Future visions
In addition to the assistance functions already available for volume-production models, numerous enhanced and new steering assistance functions are now under development. Many of the current assistance functions are based on information from other control units. If, however, these functions are integrated into the steering, the assistance systems can be substantially improved by integrating internal steering data as well. At present, for instance, a steering recommendation is issued on the basis of ESP signals. Functions are currently being developed which themselves optimally assess the driving situation directly on the steering control unit, thus facilitating or improving functions such as µ-split braking, oversteer/understeer compensation, crosswind compensation, or rollover prevention. In addition to optimizing existing assistance systems, a host of new functions are under development. The evasive maneuver assistance function is an assistance system that would enhance safety. Based on camera and radar information coupled with conventional driving dynamics sensors, an evasive trajectory can be calculated in the case of an imminent collision, and assistance is also provided where the driver prompts a suitable response.
Enhanced lane assist functions are also being developed. Based on information from camera systems, the time before the vehicle leaves the lane can be calculated in the steering using information such as lane camber, distance to the lane markings, and position in the lane, and a tiered warning concept can be implemented accordingly. The driver is suitably alerted to correct his steering intervention by applying additional torque to the steering system. Similarly, a collision warning can also be implemented using additional information on objects in the blind spot.
Another area of development involves enhancing current lane assist functions to include information from navigation systems. This information extends the anticipation horizon by including information on radii of curvature outside the camera's range, data on speed limits, or lane constrictions to specify further lane assistance.

Assistance functions are also being developed that make it easier to reverse with a trailer. Based on sensors that measure the articulation angle, the driver could specify the target articulation angle by means of an additional device, such as a touch screen. The steering then automatically sets the necessary wheel steering angle. A vehicle with a trailer can therefore be easily reversed.

The steering is increasingly being networked. Complex assistance systems require suitable strategies to coordinate and prioritize the individual requirements. ZF Lenksysteme has developed a "FunctionCoordinator" to this end, which coordinates and prioritizes the various target values on several levels. It has to meet the most stringent safety requirements. The "FunctionCoordinator" links disparate assistance functions, networked or internally, while complying with all safety, coordination, and prioritization requirements. The individual requirements are managed and controlled centrally, thus allowing functions to be added easily and all requirements managed safely.

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ZF Lenksysteme GmbH, a 50:50 joint venture of Robert Bosch GmbH and ZF Friedrichshafen AG, is a specialist and technological leader in the field of steering equipment and has a workforce of more than 13.000 at 18 locations in eight countries. In 2013, the Group generated revenues of 4.1 billion euros.

PI8153 - June 17, 2013

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