Electric power assisted steering (EPS) uses an electric motor to provide directional control to the driver. Most EPS systems have variable assist, which provides more assistance as the speed of a vehicle decreases and less assistance from the system during high-speed situations. This functionality requires a delicate balance of power and control that has only been available in recent years. The EPS system is replacing the hydraulic steering system and is destined to soon become mainstream among automotive manufacturers.

EPS presents a number of challenges to the system designer. These include:

• Using complex model-based control algorithms to improve motor efficiency
• Minimizing torque ripple and acoustic noise to improve the human/machine interface
• Employing fast control loops with cycle-times below 100µs to precisely measure key system parameters, such as motor currents, rotor position and supply voltage fluctuations
• Guaranteeing reliable functionality under harsh conditions with ambient temperatures up to 140°C

Freescale EPS solutions include:

• The MPC560xP family for sophisticated chassis applications enables electronic replacement for heavy hydraulic and engine driven applications to help improve fuel efficiency and reduce greenhouse gas emissions, while meeting the latest safety requirements for functional safety applications.
• 8-bit and16-bit families of microcontrollers (MCUs), such as the HC08 and 56F8000 families respectively, can be used as companion devices for implementing safety applications.
• System basis chip (SBC) solutions, such as the MC33742 family, are designed to provide fully protected fixed voltage regulators integrated with 1 Mbps CAN transceiver bus interfaces and programmable window watchdogs. The SBC can be programmed via a serial peripheral interface (SPI) with clock frequencies up to 4 MHz.
• The MC33927 field effect transistor (FET) pre-driver is designed for three-phase motor control and similar applications. Designed for 8V to 40V operating voltages, it uses SMARTMOS™ technology and contains protected FET pre-drivers with 0.5A gate drive capability. The MC33927 is programmed through an SPI port and interfaces with a 5.0 V or 3.0 V MCU via six direct input control signals and asynchronous reset, enable and interrupt signals.