Electronics architecture for electric power steering systems
The introduction of new safety standard ISO 26262 (“Road vehicles – Functional safety”) increases the emphasis on the demonstration of robustness, with compliance made more important in an environment where zero defect is standard. Starting in 2011, all developments of new cars have to be aligned to this standard. This, along with sharp increases in the number of electronic functions, new communications protocols and the introduction of standardized Autosar software, increases the amount of memory and processing speed that steering systems require.
The conventional approach to cost-effective manufacture is to increase the volumes of standardized units, increasing component purchasing power and supporting investment in the most efficient production processes. Minimal component variation also drives up quality throughout the supply chain and makes control of stock levels more robust.
However, the increasing diversity of the automotive market inevitably means that, in many applications, a standardized system will carry features required only by certain customers, resulting in over-specification of the product and making cost-effective design more difficult. The alternative strategy, developed by Nexteer, is a modular approach based on building block architecture.
The ECU is divided into a series of separate elements, such as the CPU section, power section, sensor section and relay section. The block architecture eliminates the repetition of basic design work for each new application, building up the system instead with proven, optimized elements. This approach results in detailed knowledge and high levels of expertise in the individual blocks, their capabilities, limitations and how best to optimize them. Design effort can instead focus on cost reduction, system performance increases and design improvement.
Nexteer uses common circuits, configured in a custom board layout for each application. Its components are standard, commercially available and automotive-grade. The printed circuit board can be built on almost any electronics assembly line with appropriate fixturing and controls. Packaging and final assembly takes place in-house at a Nexteer plant.
In application, the building block strategy means that all vehicles produced by a manufacturer, from a city car to a large SUV, can share most of the ECU hardware and logic with just different power sections to satisfy the differing steering rack load requirements. Alternatively, vehicles in the same market sector from different manufacturers can be supplied with the same ECU hardware with only different diagnostic and CAN or FlexRay layer sections to suit the vehicle manufacturer’s individual requirements.
Compatibility between the blocks means that although Nexteer uses three different motor sensor technologies, each can be used with the same ECU, torque sensor and position sensor. The company has already introduced solutions that eliminate the steering angle sensor, which is needed for functions such as stability control, active lighting and other Advanced Driver Assistance Systems (ADAS). The new architecture will be able to broadcast absolute steering position based just on the EPS motor sensor.
The block architecture can also take advantage of multiple voltage sources, so if a high power output to the rack is required, the power section can run at 24v while the logic sections continue to use 12v. The variety of voltages used in hybrid electric vehicles is also readily accommodated.
The architecture is scalable to suit the demands of different vehicle categories. At the entry level, a simplified control platform offers a low-cost system for economy cars and emerging markets, with the flexibility to add more sophisticated features for future upgrades. For premium platforms, the modular architecture allows a more flexible approach to features and customization, making a wider range of options possible and providing a development path into the future.
In the medium term the architecture could also provide a basis for Nexteer’s electronics designers to integrate further additional functions into the steering controller. An example of this could be the integration of the vehicle immobilizer function into the column-mounted EPS system. By combining the immobilizer coil and wiring into a column EPS unit and the reader/exciter circuitry into the EPS controller, the system package size is reduced and the lower parts count contributes to improved cost and reliability in the combined system. Security authorization is achieved through the EPS processor with codes sent to the PCM (Nexteer Control Unit) via CAN or FlexRay communication.
Pressure on package space leads to other examples of component integration: Nexteer is developing a compact motor position sensing system by replacing the separate sense board with components soldered directly onto the controller and using a miniaturized sense magnet.
As steering systems continue to integrate more fully with other systems to improve vehicle safety, driver satisfaction and occupant comfort, having the right electronic architecture onboard will be key. Modern EPS technology already provides functions that improve the driving experience and even the user’s perception of the vehicle characteristics. This trend is likely to accelerate as increased processing speeds and greater memory capacity combine with faster, more robust communication protocols.
About the author:
Betram Möller is EPS Technology Manager at Nexteer Automotive.
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