Tech Stuff

Electronic power steering

1. EPS control unit. 2. EPS motor and resolver. 3. Reduction gear. 4. Torque sensor.
<p>1. EPS control unit. 2. EPS motor and resolver. 3. Reduction gear. 4. Torque sensor.</p>

About the author: Leon serves as one of the leading technical editors for Mitchell 1. He is a graduate of Universal Technical Institute and has previously worked for Aamco Transmissions and as a mobile service technician. He holds 609 Certification and specializes in automotive diagnostics.

As gas prices seem to never drop to a comfortable price, it’s important for vehicles to be as fuel efficient as possible. That is where an electronic power steering (EPS) system comes in handy.

By eliminating the power steering pump which can use up to 10 hp under load, an EPS system provides up to a 2% increase in fuel economy over the conventional system. Another benefit of having an electric steering system is that it eliminates the use of hoses and fluid, therefore eliminating power steering leaks as well as a reduction in weight.

Electronic power steering systems are becoming widely popular among auto manufacturers due to the fact that they provide a more refined feel that can be adjusted as needed.

The EPS system consists of four major components: the EPS control module which collects data from the EPS components and sends out the required information; the EPS motor, its speed strength and direction controlled by the EPS Control Unit; the reduction gear, which inputs the power assist to the steering rack assembly; and torque sensor, which monitors the driver’s input and the EPS system’s mechanical output.

The EPS is powered by a permanent magnet alternating current motor and is not dependent on the engine for its power source so steering feel is not affected when the engine is shut off. The torque sensor itself has two independent coils of wire. One of the coils determines if a right hand turn is being made, the other coil determines if a left is being made. The signal is then sent from the EPS module to the appropriate coil, which will assist the vehicle in steering.

How electronic power steering works

A hybrid type of electronic power steering has been in place for some time now, but that included the use of an electric motor to drive a hydraulic pump.

The new version of EPS is all electronic. The system works by incorporating information with the EPS control unit, EPS motor, reduction gear and torque sensor.

An EPS system works by using a pinion gear assist that provides the power assist by rotating the pinion gear. The reduction gear is press fitted onto a set of splines on the pinion shaft and delivers the assist to the rack gear instead of pushing on the rack gear as in a hydraulic system.

The steering gear itself is a manual rack with an electric motor mounted on the steering column or the rack. When the driver turns the wheel, a steering sensor detects the position and rate of rotation of the steering wheel. This information along with input from a torque sensor mounted in the steering shaft is sent to the power steering control module. The system also uses other inputs from vehicle speed sensors and the traction control system which are factored in to determine how much steering assist is required. The control module then tells the motor to rotate the required amount.

Attached to the motor is the motor resolver sensor, which measures the rotation of the motor and sends the data to the EPS control module.

Different surfaces will require different amounts of steering assist. For example, a vehicle traveling on pavement will require much less steering assist than a vehicle traveling on sand or snow. With the EPS system working with other sensors, it can much more easily provide the required assistance for any kind of terrain and vehicle speed.

Electric power steering modes

• Normal mode — Left and right assist is provided in response to inputs and vehicle speed. During normal operation power assist levels will decrease as the vehicle speed increases.

• Assist limitation — Will occur if there is a problem with the information going back to the EPS control module, overheating of the EPS module or a malfunction with the controller area network.

• Assist off — System is turned off if there is a problem with any of the major EPS components.

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