In the second of a two-part article, our mechanical engineer covers the basics of electric power steering.
MOTORCAR manufacturers have been moving away from hydraulically assisted steering. Initially, the belt-driven pump was replaced by an electric motor-driven one. This reduced weight and engine power consumption because there was no longer a need for the crankshaft to drive a pump. But the oil tank and hydraulic piping network remained. In the most modern version of power-assisted steering, there is no hydraulic oil pressure to speak of. Assistance is provided solely by an electric motor mounted on the steering column or steering rack, depending on the car’s make and model.
Logically, the direct mechanical coupling between the motor and the steering mechanism should mean lucid response and undiminished feel. The reality, however, is not so simple. The electric motor receives electronic signals from the steering wheel and has to convert these into proportional electrical inputs to the motor. This is a FEATURE TECH STORY SHREEJIT CHANGAROTH complicated aff air that requires clever software management. It has taken a while for the technology to be developed properly, but it hasn’t taken as long as the industry’s first consumer applications of hydraulic power-assisted steering.
Today’s purely electrical assistance for steering systems is said to have attained the responsiveness, precision and sensation of the best traditional hydraulic power steering systems. In fact, according to my Torque colleague, Dr Andre Lam, the electric steering of Porsche’s newest 911 GT3 RS is inexplicably superior to the hydraulic steering of his own previous-generation 911 GT3 RS. Perhaps the most fascinating evolution in steering control is Nissan’s steer-by-wire Direct Adaptive Steering (DAS), installed in the Infiniti Q50. Contrary to its name, the system has taken away any direct link between the steering wheel and the front axle.
An electronic control unit, with two backup ECUs, measures a plethora of dynamic parameters and reacts in milliseconds to activate the high-speed electric motor that continuously traverses the steering rack. This is “Playstation” in real life, on the road. What the driver does with the steering is interpreted by the electronic sensors and the information is sent to the ECU. In response, the ECU commands the steering motor to drive the steering rack in the desired direction and at the intended speed. That is how it works in theory, but DAS is far more sophisticated in practice.
Since there is no direct mechanical connection, the ECU is able to control the wheel angle independent of the steering wheel. Therefore, road inconsistencies that would require the driver to make minor corrections with the wheel are handled by the steering motor, resulting in significantly reduced eff ort by the person behind the wheel. Serious surface irregularities that cause tramlining, or strong crosswinds, or varying inclinations, these influence the direction treaded by the front tyres. DAS is able to counter these by making precise corrections on the go, while at the same time providing the driver with the necessary feel and feedback.
Theoretically, the system could be designed to take charge in extreme situations to correct understeer or oversteer – vehicular behaviour that the average driver is probably incapable of correcting. So, what happens if DAS fails? Well, the “safety” is an actual mechanical link, and the steer-by-wire mode requires a clutch to decouple the steering shaft before full electronic control can take over. Nissan/Infiniti’s Direct Adaptive Steering is the first of its kind and the latest development in electric power steering.
