by Siddharth Pachahara, Clemson Automotive Engineering Graduate Student
Basic Description
Traditionally, when the driver gives a steering angle input it is directly transferred to the wheels using a mechanically predetermined steering ratio. Power steering systems assist the driver by adding torque to the input thus reducing driver effort, and these systems can be hydraulic or electric motor driven. Active steering systems use electronic controls to adjust the steering ratio depending on the driving situation.
The figure below describes the basic layout of a typical column assist active steering system. In the equation at the bottom of the figure, δs is the steering wheel angle, δm is the electric motor angle, δp is the pinion angle, id is the ratio between steering wheel and pinion and im is ratio between motor shaft and pinion. A combination of planetary and worm gears is the “heart” of the system, it is made up of three concentrically arranged parts. The system is capable of generating additional steering angle in a fraction of a second.
How the system works
When the vehicle is traveling at low speeds (up to 100 km/h) the motor adds an angle, thus reducing the overall steering ratio. For higher velocities (greater than 100 km/h) the motor will subtract an angle, thus increasing the steering ratio.
For example, if driver turns the steering wheel 100 degrees, while traveling at 30 km/h the actuator might add an angle of 25 degrees making the resultant angle as 125 degrees thus increasing driving comfort. While traveling at 150 km/h the actuator might subtract 25 degrees making the resultant angle 75 degrees providing a smaller road wheel angle and increasing vehicle stability.
As far as ASS manufacturers are concerned, only ZF offers a complete package solution, all other suppliers either provide the electric motor or the gear set or just the control unit. The video below demonstrates a CarSim simulation where two cars with and without active steering are compared. The green car (equipped with active steering) has improved agility and stability as compared to the blue one while performing a double lane change maneuver at 110 km/h.
Advanced versions of active steering control include four wheel active steer systems (4WAS) and the steer-by-wire systems. Four wheel active steering systems provide the ability to turn all four wheels further improving the vehicle's nimbleness and stability. Currently 4WAS is available in Infinity, Nissan, Renault and Porsche cars. Steer-by-wire was introduced by Infinity as an option on their Q50 model. It took infinity almost 10 years to develop and tune their steer-by-wire system to provide an appropriate “steering feel”. Steer-by-wire systems give electronic systems greater flexibility to enhance a vehicle's performance and stability, while also promising to reduce weight and improve reliability by eliminating complex mechanical linkages.
