Electric power steering offers greater vehicle safety by adapting variable steering ratios to human needs, filtering drivetrain influences and even adjusting active steering torque in critical situations. In addition, it can make cars lighter and more fuel efficient when compared to those using hydraulic steering systems.

The central electronic elements of today's power steering systems are modern 32-bit microcontrollers (MCUs). Only high-performance MCUs can provide sufficient computing power and specialized peripherals for complex motor control functions. Since power steering is a safety-critical function, it also requires new MCU elements that support the functional safety of the overall system.

This article provides an overview of the latest generation of Power Architecture' based MCUs and describes how they are used in power steering applications. New innovative elements, such as the cross-triggering unit for motor control and the fault collection unit for monitoring and reporting safety critical signals, are explained.

Introduction

During the last decade, advanced chassis control functions have become main technology drivers for active safety systems in vehicles, and electric power steering (EPS) nicely combines vehicle safety with higher fuel efficiency. With the first systems entering the market in the mid 1990s, purely electronic steering systems have migrated to almost every segment of the vehicle market.

EPS in modern cars can significantly reduce fuel consumption when compared to cars using hydraulic solutions. Industry studies have shown that EPS can save up to 85 percent of the energy normally needed to steer a vehicle with conventional hydraulic systems. The result is fuel consumption reductions of up to 0.3 liters per 100 kilometers driven. EPS is so efficient because the system is only activated when steering support is really needed. As a result, a permanent engine load is not required.

EPS systems also can help ensure safer driving. The steering torque is adapted to the vehicle's speed and optimized for different driving situations. For example, during low-speed driving maneuvers, such as parking, EPS provides a higher level of assistance than it does at higher speeds, when electronic power assist is gradually reduced to enable more direct steering and better feedback from the road.

By integrating sensors and network connectivity, EPS can further enhance its safety characteristics through improved dynamic control and warning functions:

• Improved vehicle dynamics control:

— Helping and guiding the driver with additional steering torque in over-steer situations

— Reducing stopping distance by coordinating with the electronic stability control (ESC) system on roadways with differing friction levels

•Warning functions

— Generating a slight counter-steering torque in order to prevent the vehicle from unintentionally drifting out of its lane System overview