By : Rohini H M USN : 2VX11LVS19

 This system includes sensors for measuring vehicle speed; steering input; relative displacement of the wheel assembly and car body/chassis; lateral acceleration; and yaw rate.  The outputs are electrical signals to the shock absorber/strut actuators and to the motor/compressor that pressurizes the pneumatic springs.  The actuators can be solenoid-operated (switched) orifices or motor- driven variable orifices or electromagnets for RH fluid-type variable viscosity struts.  The control system typically is in the form of a microcontroller or microprocessor-based digital controller. The inputs from each sensor are sampled, converted to digital format, and stored in memory.

 The body acceleration measurement can be used to evaluate ride quality. The controller does this by computing a weighted average of the spectrum of the acceleration.  The relative body/wheel motion can be used to estimate tire normal force, and damping is then adjusted to try to optimize this normal force.  The yaw rate sensor provides data which in relationship to vehicle speed and steering input measurements can be used to evaluate cornering performance.  Under program control in accordance with the control strategy, the electronic control system generates output electrical signals to the various actuators.

 The important inputs to the vehicle suspension control system come from road roughness induced forces and inertial forces, steering inputs, and vehicle speed.  When driving along a nominally straight road with small steering inputs, the road input is dominant. In this case, the control is based on the spectral content (frequency region) of the relative motion.  Whenever the weighted amplitude of the spectrum near the peak frequencies exceeds a threshold, damping is increased, yielding a firmer ride and improved handling. Otherwise, damping is kept low (soft suspension).

 If in addition the vehicle is equipped with an accelerometer (usually located in the car body near the center of gravity) and with motor-driven variable-aperture shock absorbers, then an additional control strategy is possible.  Whenever a relatively large steering input is sensed, such as during a cornering maneuver, then the control strategy switches to the smaller aperture, yielding a “stiffer” suspension and improved handling.  In particular, the combination of cornering on a relatively rough road calls for damping that optimizes tire normal force, thereby maximizing cornering forces.

 The steering effort required of the driver to overcome restoring torque generally decreases with vehicle speed and increases with steering angle. Traditionally, the steering effort required by the driver has been reduced by incorporating a hydraulic power steering system in the vehicle.  Whenever there is a steering input from the driver, hydraulic pressure from an engine-driven pump is applied to a hydraulic cylinder that boosts the steering effort of the driver.  Typically, the effort available from the pump increases with engine speed (i.e., with vehicle speed), whereas the required effort decreases. It would be desirable to reduce steering boost as vehicle speed increases.  An electronically controlled power steering system adjusts steering boost adaptively to driving conditions.

 An alternative power steering scheme utilizes a special electric motor to provide the boost required instead of the hydraulic boost.  Electric boost power steering has several advantages over traditional hydraulic power steering.  Electronic control of electric boost systems is straightforward and can be accomplished without any energy conversion from electrical power to mechanical actuation.  Moreover, electronic control offers very sophisticated adaptive control in which the system can adapt to the driving environment.

 In the 4WS equipped vehicles, the front wheels are directly linked mechanically to the steering wheel, as in traditional vehicles. There is a power steering boost for the front wheels as in a standard two-wheel steering system. The rear wheels are steered under the control of a microcontroller via an actuator.  The front wheels are steered to a steering angle δf by the driver’s steering wheel input. A sensor (S) measures the steering angle and another sensor (U) gives the vehicle speed. The microcontroller (C) determines the desired rear steering angle δr under program control as a function of speed and front steering angle.

 For speeds below 10 mph, the rear steering angle is in the opposite direction to the front steering angle. This control strategy has the effect of decreasing the car’s turning radius by as much as 30% from the value it has for front wheel steering only.  At intermediate speeds (e.g., 11mph < U < 30mph), the steering might be front wheel only. At higher speeds (including highway cruise), the front and rear wheels are steered in the same direction.  In this strategy, the rear wheels turn in the opposite direction to the front wheels for a very short period (on the order of one second) and then turn in the same direction as the front wheels.  Notice that the 4WS strategy yields a lane change in a shorter distance and avoids the overshoot common in a standard-steering vehicle.

 When front and rear wheels turn in the same direction, the angle between the car and trailer axes is less than it is for front wheel steering only.  The reduction in this angle means that the lateral force applied to the rear wheels by the trailer in curves is less than that for front wheel only steering.  This lateral force reduction improves the stability of the car or truck/trailer combination relative to front steering only.