Semi-active suspension

Active suspension is a type of automotive suspension that controls the vertical movement of the wheels relative to the chassis or vehicle body with an onboard system, rather than in passive suspension where the movement is being determined entirely by the road surface; see Skyhook theory. Active suspensions can be generally divided into two classes: pure active suspensions, and adaptive/semi-active suspensions. While adaptive suspensions only vary shock absorber firmness to match changing road or dynamic conditions, active suspensions use some type of actuator to raise and lower the chassis independently at each wheel.

These technologies allow car manufacturers to achieve a greater degree of ride quality and car handling by keeping the tires perpendicular to the road in corners, allowing better traction and control. An onboard computer detects body movement from sensors throughout the vehicle and, using data calculated by opportune control techniques, controls the action of the active and semi-active suspensions. The system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating, and braking.

Skyhook theory is the idea these systems are targeting - the vehicle maintains a stable posture as if suspended by an imaginary hook in the sky, unaffected by road conditions.

Since there is no imaginary line or hook in the sky, so instead, an actuator will be operated. The imaginary line (acceleration = 0) is calculated based on the value provided by an acceleration sensor installed on the top of the vehicle (Fig. 3). Since the dynamic elements are only made up of the linear spring and the linear damper, no complicated calculations are necessary.

A vehicle contacts the ground through the spring and damper in a normal spring damper suspension, as in Fig. 1. To achieve the same sustainability in the Skyhook theory, the vehicle must contact the ground through the spring, and the imaginary line with the damper, as in Fig. 2. Theoretically, in a case where the coefficient of the damper reaches an infinite value, the vehicle will be in a state where it is completely fixed to the imaginary line, thus the vehicle will not shake.

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