Electronic Power Steering

Basic Description

Driven by the need for greater fuel efficiency, electronic power steering or electric power-assisted steering (EPS) uses an electric motor to provide directional control to the driver. Drivers are also looking for improvements in handling at high vehicle speeds as well as little effort needed to steer a vehicle at low speeds or when parking. Unlike the traditional hydraulic power steering (HPS) system, EPS can handle both tasks and it eliminates HPS components such as its pump, hoses, fluid, drive belt, and pulley.

Most EPS systems have variable assist, which provides more assistance as the speed of a vehicle decreases and less assistance at higher speeds. This variable assist utility has only in recent years been available due to the necessary balance of control and power now capable with current electronic control unit (ECU) microchips, software, and electric motors. EPS systems are used in both low-end and high-end vehicles. The current trend shows that EPS will be the dominate steering system among automotive manufacturers soon, by replacing HPS systems. This is due to their higher reliability, higher assistance force, and gradually decreasing cost.

How the system works:

• The EPS electronic control unit (ECU) calculates the assisting power needed based on the steering wheel position and the vehicle’s speed.
• To adjust the steering force, the EPS motor rotates the steering gear based on the assisting power needed.

There are essentially four forms of EPS, based on the position of the assist motor. They are the column assist type (C-EPS), the pinion assist type (P-EPS), the direct drive type (D-EPS) and the rack assist type (R-EPS). In the P-EPS system , the power assist unit is connected to the steering gear's pinion shaft and because the assist unit is not located in the passenger compartment it eliminates interior noise. The P-EPS system works well in small cars. The D-EPS system has low inertia and friction because it's steering gear and assist unit are a single unit. The R-EPS type has the assist unit connected to the steering gear. R-EPS systems can be used on mid to full sized vehicles due to its very low inertia from high reduction gear ratios. The C-EPS type has a power assist unit, torque sensor, and controller connected to the steering column.

Unlike a hydraulic power steering system that continuously drives a hydraulic pump, the efficiency advantage of an EPS system is that it powers the EPS motor only when necessary. This results in reduced vehicle fuel consumption compared to the same vehicle with an HPS system. These systems can be tuned by simply modifying the software controlling the ECU. This provides a unique and cost effective opportunity to adjust the steering "feel" to suit the automotive model class. Additional advantages of EPS is its ability to compensate for one-sided forces such as a flat tire. It also corrects steering in emergency maneuvers in conjunction with the electronic stability control. EPS system can be quickly adapted to different cars by changing their software.

The next step in electronic steering is to remove the remaining mechanical components and to convert to pure electronic steering, which has been named steer-by-wire (SBW). This functions by transmitting digital signals to one or more remote electric motors instead of a rack and pinion assembly, which in turn steer the vehicle. SBW systems have recently been widely studied by both academic research labs and automobile manufacturers due to their ability to cost-effectively and accurately simulate a driving scenario using advanced programs to then write software to control the system.

Sensors

Steering wheel torque sensor, steering wheel position sensor, wheel speed sensor

Actuators

Electric motor

Data Communications

CAN bus data communication between ESP and engine controller

Manufacturers

Bosch, Delphi, Denso, Koyo, NSK, TRW, ZF

For More Information

[1] Electronic Power Steering, Freescale web site.

[2] Electronic Power Steering Overview, indiacar.com.

[3] Electric Power Steering, www.aa1car.com.

[4] Electric Power Steering, www.nsxprime.com.

[5] BMW Electric power steering EPS, YouTube, Nov. 2008.

[6] Hyundai Power Steering (MDPS), YouTube, July, 2009.

[7] Electric Power Steering: Swapping Hoses and Pumps for Electric Motors and Electronic Control, Julian Edgar, Autospeed.com, July 23, 2008.

[8] Research Analysis: A Review of Electric Power Steering Systems, Just-auto.com, August 6, 2007.