- The vehicle suspension system is responsible for driving comfort and safety as the suspension caries the vehicle body and transmits all forces between the body and the road.
- In order to positively influence these properties, semi-active and/or active components are introduced. These enable the suspension system to adapt to various driving conditions.
- By adding a variable damper and/or spring, driving comfort and safety are considerably improved compared to suspension setups with fixed properties.
This strategy requires that the control behavior of these components is known and that laws on how to adapt the free parameters depending on the driving excitations are known.
- This also requires the identification and fault detection of the involved components resulting in a mechatronic design.
• Vehicle Suspension System
- The vehicle suspension system consists of wishbones, the spring, and the shock absorber to transmit and also filter all forces between the body and road. - The spring carries the body mass and isolates the body from road disturbances and thus contributes to drive comfort.
The damper contributes to both driving safety and comfort. Its task is the damping of body and wheel oscillations, where the avoidance of wheel oscillations directly refers to drive safety, as a non-bouncing wheel is the condition for transferring road-contact forces.
• Driving safety is the result of a harmonious suspension design in terms of wheel suspension, springing, steering, and braking, and is reflected in an optimal dynamic behavior of the vehicle. Tire load variation is an indicator for the road contact and can be used for determining a
quantitative value for safety.
- Driving Comfort
• Driving comfort results from keeping the physiological stress that the vehicle occupants are subjected to by vibrations, noise, and climatic conditions down to as low a level as possible. The acceleration of the body is an obvious quantity for the motion and vibration of the car body and can be used for determining a quantitative value for driving comfort.
In order to improve the ride quality, it is necessary to isolate the body, also called the sprung mass, from the road disturbances and to decrease the resonance peak of the sprung mass near 1 Hz, which is known to be a sensitive frequency to the human body.
- In order to improve the ride stability, it is important to keep the tire in contact with the road surface and therefore to decrease the resonance peak near 10 Hz, which is the resonance frequency of the wheel, also called the unsprung mass.
- For a given suspension spring, the better isolation of the sprung mass from road disturbances can be achieved with a soft damping by allowing a larger suspension deflection.
- However, better road contact can be achieved with a hard damping preventing unnecessary suspension deflections.
- Therefore, the ride quality and the drive stability are two conflicting criteria, as shown below.
As can be seen from the diagram, the fixed setting of a passive suspension system is always a compromise between comfort and safety for any given input set of road conditions and a specific stress.
- Semi-active / active suspension systems try to solve or at least reduce this conflict.
- The mechanism of semi-active suspension systems is the adaptation of the damping and/or stiffness of the spring to the actual demands.
- Active suspension systems in contrast provide an extra force input in addition to possible existing passive systems and therefore need much more energyThe figure also clarifies the dependency of a vehicle suspension setup on parameter changes as a result of temperature, deflection, and wear and tear. These changes must be taken into account when designing
a controller for an active or semi-active suspension to avoid unnecessary performance loss.
- In order to prevent this, a robust or an adaptive controller has to be implemented. The adaptive controller results in a parameter-adaptive suspension system that refers to a control system which adapts its behavior to the changing settings of the system to be controlled and its signals.
- Suspension systems are classified as passive, semi-active, active and various in-between systems.
- Typical features are the required energy and the characteristic frequency of the actuator.
This diagram points out the conflict that automotive manufacturers face in their endeavor to improve drive safety and comfort as high-performing suspension systems can only be achieved by high-energy demand and mostly expansive and complex actuation systems.