The predominant type of power steering from the 1950s through the early 2000s was hydraulic power steering. Hydraulic power steering, as the name suggests, uses hydraulic fluid that is pressurized by an engine-driven pump. Although it has served the automotive world well over these 50 years, this type of system has some disadvantages: Energy is wasted as the pump runs continuously even when the vehicle is going straight and no assistance is needed. Also, the hydraulic fluid needs to be replaced regularly, and if one of the hydraulic lines leaks, not only does it get dirty, but power assistance is lost. However, it is still possible to drive a car without the power steering working.
Electric Power Steering (EPS) is standard on new cars. A full-metal steering shaft runs from the steering wheel to the steering rack, which steers the tires, but the rest is high-tech. EPS uses an electric motor that draws power from the vehicle's electrical system to provide steering assistance. This electric motor can be mounted directly on the steering rack (this arrangement is more expensive and is more common on sports and luxury cars) or mounted on the steering column. Sensors detect the torque or force the driver is applying to the steering wheel and a computer decides how much assistance to add.
In most systems, the computer changes steering effort based on vehicle speed: at parking speed, the steering is light and easy to turn, while at highway speed, the effort increases, giving the driver a sense of greater stability and control.
The advantages of electrical support are manifold: It reduces fuel consumption by a few percent, since the Electric motor only consumes energy when it is necessary; eliminates the aforementioned hydraulic fluid maintenance; and also enables a variety of functions. Any driver assistance or convenience functions that involve turning the wheels without driver intervention are activated by the electric power steering. Features such as lane departure warning, automatic parking and lane changing, and the ability to guide the car around obstacles leverage the EPS' ability to steer itself when needed.
Between the hydraulic and electric power steering, there is a hybrid of the two systems referred to as electro-hydraulic. It works like a hydraulic assist system, except that the hydraulic pressure is generated by an electric motor instead of the engine driving the pump. This eliminates the previously mentioned wasted energy complaint, but it doesn't enable all the features that are possible with electric power steering. Currently, only a few vehicles, including some heavy-duty trucks, use this system.