EPS system steers automakers toward higher mileage
By KIRK BELL For Sun-Times Media January 4, 2013 2:18PM
By 2025 the automakers’ corporate average fuel economy has to reach a government-mandated 54.5 miles per gallon. Automakers have to reach several benchmarks along the way, with increasing fuel economy standards every year.
As we approach those standards, automakers are employing every technology they can muster to make cars go easier on gas. One of those technologies is electric assist power steering, which replaces the traditional hydraulic assist power steering.
Instead of a hydraulic pump on the power steering rack, EPS uses an electric motor to reduce the effort needed to turn the wheels. While a hydraulic pump is always powered by the vehicle’s engine, an electric motor is powered about 2 percent of the time, thus reducing drag on the engine. According to automotive supplier ZF, EPS saves one mile per gallon in a vehicle that gets 22 mpg versus a car with hydraulic power steering. That also translates into reduced output of carbon dioxide.
The vehicle’s computer can control the electric motor to do several things not possible with hydraulic assist power steering. EPS is used to tailor steering feel and comfort, control the steering completely in certain situations, and take part in various safety features, which can include lane departure warning and lane keeping assist.
Lane departure warning uses cameras to read the lane lines, and when it detects that the vehicle is crossing those lines without the use of a turn signal, it sends a warning to the driver that can be audible, visual or a pulse through the steering wheel. The last example is made possible by EPS. In a lane departure prevention system, EPS allows the computer to turn the wheel slightly to nudge the vehicle back into its lane. Though it hasn’t been used thus far, in the future EPS could actually steer a vehicle to avoid an accident. Current accident avoidance systems sound warnings and apply brakes; steering could be next.
EPS is also used to make steering more comfortable. Crown compensation is an example. In this case, the vehicle’s computer uses EPS to make small corrections to keep the vehicle from veering off path due to the crown of the road. The same goes for disturbance rejection, which corrects the steering angle if it is knocked off path by ruts or bumps.
EPS can also compensate for torque steer, a tendency for the wheel to pull to one side under hard acceleration in a front-wheel-drive vehicle, and it can smooth out steering wheel shudder caused by tires that are out of balance. A more extreme example is the automatic parking systems made popular by Lexus and Ford. These systems work with other vehicle systems, including cameras and other sensors, to steer the vehicle into a parking spot without the driver touching the steering wheel.
Finally, steering feel with EPS is easily tunable, which allows for various driving modes such as sport and comfort that vary the assist to make the steering feel heavier or lighter. Automotive journalists have decried the lack of steering feel in many EPS-equipped new cars, including BMWs, Porsches and Lexuses. However, we’ve also driven vehicles with great EPS systems, such as the Scion FR-S, the Subaru BRZ and even the Ford F-150. Our feeling is that automotive engineers had many years to learn how to tune hydraulic assist systems; they will get better at tuning EPS in the future.
That future is sure to include much more use of EPS. Not only does it save gas, but as we’ve discovered, it allows automakers much greater control of their steering systems.
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