Hydroline Powersteering B.V.

Gildenveld 1

3892 DC

Zeewolde

The Netherlands

+31 36 - 538 72 80

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People nowadays are so used to power steering that they would probably have a hard time parking and making certain maneuvers without it. The first vehicles didn't have it and some former communist brands, such as Dacia or Lada, lacked power steering until quite recently. Assisted steering is not such a big deal at high speeds, but it becomes vital for stationary vehicles or for low speeds.

The first power steering patent goes back to 1932 in the United States, while some 22 years later another inventor files a similar patent, only this time in Canada. The first vehicle to incorporate the system was a 1951 Chrysler Imperial, their power steering being called Hydraguide.

Hydraulic Power Systems

As most units today, the Hydraguide was also fully hydraulic, which means that the pressure was provided by a pump powered by the engine. The heart of the hydraulic system is a double-acting cylinder which is controlled by valves operated by the steering wheel. The cylinder exerts force on the steering gear, which acts upon the steering axis. The force applied to the steering wheels is directly proportional to the force applied by the driver to the steering wheels.

1951 Chrysler Imperial

One of the problems early hydraulic designs had was that the hydraulic assistance was constant, regardless of the speeds vehicles traveled at. As previously mentioned, the amount of assistance is considerably smaller at high speeds than at low speeds.

The first manufactured that tried to change this was Citroen with the DIRAVI - DIRection A Rappel AsserVI – system, also known as Varipower. Maximum power was provided at low speeds and it was reduced as the velocity increased. The first model to incorporate this system was the Citroen SM, in 1970.

The regular hydraulic system that provides assistance when steering the wheels is supplemented by a second one which tries to bring back the steering wheel to the initial position. The DIRAVI requires little physical effort, but even with the second hydraulic system aiming to give steering a more real feel, it still doesn't provide enough feedback to the driver.

Electric Power Systems

With the stringent need for lower fuel consumption, manufacturers came up with another power steering system that relies completely on electricity. The electric power steering replaces the car's engine with an electric motor to drive the steering mechanism. The motor is attached to the steering rack via gear mechanism, while a microprocessor controls steering dynamics and driver effort.

The key component of the electric power systems is the steering sensor located on the input shaft where it enters the gearbox housing. This sensor converts steering torque input and rotation speed into signals that are fed into the microprocessor, which controls the motor.

Electric power systems are said to have a 1-MPG advantage in fuel efficiency compared to hydraulic systems, which involve a pump that is constantly working. Opponents of the electric power systems say that they don't provide enough feedback to the driver, but this can be a very subjective matter

Electric Power Steering Scheme

Electro-Hydraulic Power Systems

Perhaps the best compromise between fully hydraulic systems and fully electric systems is the electro-hydraulic one, also known as a hybrid. The force to steer the wheels is provided by a hydraulic system, but this is fed by an electric motor and not by the engine itself. The pump driving the hydraulic system can be run at low speed or shut off completely to save energy.

Here's how BMW describes their electro-hydraulic system, called Servotronic.

"The Servotronic control unit adjusts the amount of steering assistance to suit the speed of the vehicle. An electromagnetic valve accurately controls the amount of force applied by the steering hydraulics, enabling outstandingly precise steering that suits your current driving situation. Conventional power steering systems, by contrast, regulate the power steering in relation to the engine's RPM.

The electromechanical steering system EPS uses an electric motor to achieve the same effect. Thanks to Servotronic, driving along narrow streets or parking becomes easier as it requires minimum effort to turn the steering wheel. The power assist progressively decreases when the vehicle speeds up, ensuring greater stability, improved precision and smoother vehicle behaviour."