1. Electronic four-wheel steering Introduction
By steering a vehicle’s rear wheels as well as its front wheels, a four-wheel steering system, either mechanical or electrical, offers improved handling stability and other benefits. The 4WS/E-4WS system used by Honda performs two distinct operations: in-phase steering, whereby the rear wheels are turned in the same direction as the front wheels, and counter-phase steering, whereby the rear wheels are turned in the opposite direction. The Honda 4WS/E-4WS system is effective in the following situations:
During lane-change maneuvers at high speed, the system performs in-phase steering. This operation enables the vehicle to move in a crab-like manner rather than in a curved path. As a result, the vehicle is more stable and controllable.
High-speed Straight-line Operation
Even when travelling in a straight line at high speed, a vehicle’s driver frequently needs to make small steering corrections to maintain his/her desired course. With the Honda 4WS/E-4WS system, in-phase steering of the rear wheels minimizes these corrective steering inputs.
Side Winds and Other Disturbances
When a vehicle is subjected to side winds, bumpy road surfaces, or other external disturbances, the driver needs to make steering corrections to maintain his/her desired course. The Honda 4WS/E-4WS system enables the driver to make these corrective steering inputs without causing significant changes in the vehicle’s body attitude.
Slippery Road Surfaces
During steering operations on snowy, icy, muddy, and other low-friction surfaces, steering of the rear wheels suppresses sideways drift of the vehicle’s rear end. As a result, the vehicle’s direction is easier to control.
On gentle curves, in-phase steering of the rear wheels improves the vehicle’s stability.
During a parking maneuver, a vehicle’s driver typically turns the steering wheel through a large angle to achieve a small turning radius. By performing counter-phase steering of the rear wheels, the Honda 4WS/E-4WS system realizes a smaller turning radius than is possible with a two-wheel steering (2WS) system. As a result, the vehicle is easier to maneuver into garages and other parking spaces.
On a crossroads or other junction where roads intersect at 90 degrees or tighter angles, counter-phase steering of the rear wheels causes the front and rear wheels to follow more-or-less the same path. As a result, the vehicle can be turned more tightly as it negotiates the junction. With a 2WS system, the vehicle would need to follow a relatively curved path.
On narrow roads with tight bends, counter-phase steering of the rear wheels minimizes the vehicle’s turning radius, thereby reducing side-to-side rotation of the steering wheel and making the vehicle easier to maneuver.
By minimizing the vehicle’s turning radius, counter-phase steering of the rear wheels enables U-turns to be performed easily on narrow roads.
2. Electronic Four-wheel steering Major Components
The major components of the E-4WS system are as follows:
1 Front main steering angle sensor
2 4WS control unit
3 Rear sub steering angle sensor
4 Rear main steering angle sensor
5 Rear wheel speed sensors (left and right)
6 Rear steering actuator
7 Front sub steering angle sensor
8 Vehicle speed sensor (VSS)
3. Basic Function of Electronic Four-wheel steering
An E-4WS system uses electronic circuitry to control rear-wheel steering operations. It determines and achieves the optimum rear-wheel steering phase and angle in accordance with the vehicle speed, the degree of steering wheel rotation, and the speed of steering wheel rotation. Simply stated, the system performs counter-phase steering of the rear-wheels at low speeds and in-phase steering of the rear-wheels at high speeds. As a result, the system improves stability during lane-change maneuvers at high speed and minimizes the vehicle’s turning radius at low speeds.
1 Vehicle speed sensor
2 Front main steering angle sensor
3 Rear wheel speed sensor
4 Rear sub steering angle sensor
5 Rear steering actuator
6 Rear main steering angle sensor
7 4WS control unit
8 Front sub steering angle sensor
The 4WS control unit senses the vehicle speed using signals from the vehicle speed sensor, and it senses the steering direction, the degree of steering wheel rotation, and the speed of steering wheel rotation using signals from the front main steering angle sensor and front sub steering angle sensor. In accordance with these data, the 4WS control unit determines the optimum rear-wheel steering angle and phase and achieves them by issuing signals to the rear steering actuator.
During steering operations, the 4WS control unit determines the actual amount and direction of rear steering actuator movement using signals from the rear main steering angle sensor and rear sub steering angle sensor. Also, the 4WS control unit checks for discrepancies between the desired and actual rear-wheel steering angles using signals from the left and right rear wheel speed sensors (difference between left and right rear wheel speeds). This feedback arrangement enables highly precise control of rear-wheel steering operations.
Steering of Rear Wheels
The E-4WS control unit alters the rear-wheel steering characteristics in accordance with the vehicle speed as shown in the following illustration.
Also, the E-4WS control unit alters the rear-wheel steering characteristics in accordance with the speed of steering wheel rotation as shown in the following illustration.
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