When most people think of steering, they think of the front wheels and tires doing all the work. In reality, the rear wheels aid the process, and some car makers have taken this concept a step farther and added more extensively capabilities to the rear axle to aid overall vehicle steering performance and make higher cornering limits easier for drivers to achieve while simultaneously easing the steering task.
One innovator in the rear-steering area is Nissan. Its engineers have been developing rear-steering systems for improved vehicle dynamics for more than 20 years. In 1985, the company introduced the HICAS (High Capacity Active Steering) four-wheel steering. Since then, the company’s steering engineering philosophy has evolved from same-phase transient to phase-reversal control, then to model-following control—and rear-steering-angle actuator technology advanced from open hydraulic control to higher-accuracy position servo control using a compact and lightweight motor.
All of this engineering effort has culminated in what the company calls the world’s first four-wheel active steer (4WAS) system. The system integrates rear- and front-wheel active-steering capabilities for the latest Nissan Skyline and Infiniti G35. The goal of company engineers was to achieve dynamic performance characteristics traditionally regarded as contradictory: quick, sports car-like reflexes at low to medium speeds combined with vehicle stability in the high-speed range. This was accomplished by increasing steady-state yaw-rate gain at low to medium speeds and reducing it in the high-speed range.
Steering both the front and rear wheels actively means that there are two control inputs, making it possible to optimize both yawing and lateral motion. For example, the front wheel steering angle can be increased in relation to steering input in the medium-speed range while simultaneously steering the rear wheels in the same direction. As a result, both yawing and the lateral g response are improved and vehicle sideslip is reduced.
The 4WAS system uses front and rear steering actuators and two electronic control units, one for each axle. Model-following control, a technique used in aircraft, was adopted to give control targets—in the form of a reference model that possesses the desired dynamic characteristics—and have the actual vehicle response follow the model. Nissan engineers have used this control method since the electric-powered Super HICAS four-wheel steering system was adopted on the Skyline GT-R (R34 model) in 1995.
Nissan engineers say that 4WAS is far superior to traditional two-wheel (front) steering (2WS) and even rear active steering (RAS), one example being in vehicle performance when traveling at 95 km/h (59 mph) and executing a 3.75-m (12.3-ft) lane-change maneuver.
The driver’s steering workload for a 2WS vehicle is the largest and showed the quickest input of all three specifications. The yaw rate and lateral acceleration is the greatest among all the three steering types, and they showed a pronounced phase delay in relation to driver steering action. While the maximum steering workload was nearly the same as for 2WS, RAS required less steering action and the phase delays of both yaw rate and lateral acceleration were reduced. A driver of a 4WAS vehicle requires the least steering workload and action, yaw rate and lateral acceleration the lowest of the three specifications, and the phase delay is also markedly reduced.
Because delay in steering response with 4WAS is much smaller than with 2WS, the driver could steer without having to predict the vehicle’s behavior in advance.
Nissan engineers are so encouraged by the benefits of 4WAS that they plan to make continuous improvements and refinements to the system for future vehicle applications.
Information for this article was provided by Takeshi Katayama, Yoshiki Yasuno, Tomoaki Oida, Masayuki Sao, Masayuki Imamura, Nagatoshi Seki, and Yasuharu Satou of Nissan's Vehicle Test Department.