Active or adaptive suspension is an automotive technology that controls the vertical movement of the wheels with an onboard system rather than the movement being determined entirely by the road surface. The system virtually eliminates body roll and pitch variation in many driving situations including cornering, accelerating, and braking.
This technology allows 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 suspension.
Active suspensions, the first to be introduced, use separate actuators which can exert an independent force on the suspension to improve the riding characteristics. The drawbacks of this design (at least today) are high cost, added complication/mass of the apparatus, and the need for rather frequent maintenance on some implementations. Maintenance can be problematic, since only a factory-authorized dealer will have the tools and mechanics with knowledge of the system, and some problems can be difficult to diagnose.
Michelin's Active Wheel incorporates an in-wheel electrical suspension motor that controls torque distribution, traction, turning maneuvers, pitch, roll and suspension damping for that wheel, in addition to an in-wheel electric traction motor.
Hydraulically actuated suspensions are controlled with the use of hydraulic servomechanisms. The hydraulic pressure to the servos is supplied by a high pressure radial piston hydraulic pump. Sensors continually monitor body movement and vehicle ride level, constantly supplying the computer with new data.
As the computer receives and processes data, it operates the hydraulic servos, mounted beside each wheel. Almost instantly, the servo-regulated suspension generates counter forces to body lean, dive, and squat during driving maneuvers.
In practice, the system has always incorporated the desirable self-leveling suspension and height adjustable suspension features, with the latter now tied to vehicle speed for improved aerodynamic performance, as the vehicle lowers itself at high speed.
This type of active suspension uses linear electromagnetic motors attached to each wheel. It provides extremely fast response, and allows regeneration of power consumed by utilizing the motors as generators. This nearly surmounts the issues of slow response times and high power consumption of hydraulic systems. It has only recently come to light as a proof of concept model from the Bose company, the founder of which has been working on exotic suspensions for many years while he worked as an MIT professor.
Semi-active systems can only change the viscous damping coefficient of the shock absorber, and do not add energy to the suspension system. Though limited in their intervention (for example, the control force can never have different direction than that of the current speed of the suspension), semi-active suspensions are less expensive to design and consume far less energy. In recent times, research in semi-active suspensions has continued to advance with respect to their capabilities, narrowing the gap between semi-active and fully active suspension systems.
Advantages of an Active Suspension System
In an active suspension system, the passive force elements are replaced or assisted by active force elements. These elements are able to produce a force when required and act independent of the suspension condition. Therefore, the trade-off between ride comforts, suspension travel and wheel load variations can be better resolved. Furthermore, an active suspension system can be used in order to eliminate body roll during cornering. As a consequence, the wheels can be oriented optimally with respect to the road both in case of encountering a bump and during cornering. The mentioned trade-off disappears, and also an anti-roll bar is not needed anymore. Thanks to this system, the complicated and space consuming suspension links can be replaced with a compact and simple trailing arm suspension. The compact suspension system allows for designing a smaller and lower car without affecting its interior space. This leads to lower air drag. Also, static load variations can be taken care of. In case of an active suspension system with variable spring stiffness, this stiffness can be adjusted proportionally to the change of mass. As a consequence, the natural vertical frequency of the car’s body will not change and can be chosen at a frequency which is less uncomfortable for the human body. In case of a passive suspension system, a significant portion of the available suspension travel is used to take care of static load variations and of body roll caused by cornering. The active suspension system can take care of these variations by adjusting its stiffness. Therefore, a lower initial stiffness can be used which is favorable for the comfort level.
An active suspension system introduces the possibility to adjust the suspension setup to the type of driving situation and to individualize the handling characteristics and comfort level of the vehicle. The short trip to the supermarket may well be a bit bumpy, but the long boring drive to office should rather be more comfortable. Thanks to this, a possible passenger is able to serenely check his or her e-mail, schedule a meeting or glance through the minutes of yesterday’s meeting without getting carsick. Because every single person is different, everybody puts different demands on the suspension characteristics. Because the car may be used by different persons every now and then, the suspension of the car should be adjustable such that a broad spectrum of handling-, steering- and comfort characteristics can be achieved. One could think of a system which allows the driver to switch between pre-programmed suspension conditions, from soft to hard and several conditions in between, or a manually programmed condition. Everyone can then drive the car he or she prefers. Thanks to the development of a large number of Advanced Driver Assistance Systems (ADAS), the active safety of future cars will increase tremendously. Nevertheless, the suspension should always guarantee maximal grip on the road surface to take care of unexpected hazardous situations.
The price of energy is ever increasing and thus an important issue in designing a car is its energy consumption. An active suspension system provides a lot of possibilities to improve the ride but often consumes a large amount of energy. It is an enormous challenge to make it energy efficient. An addition may be to implement dampers which are capable of converting their dissipated power into electrical power. In Appendix II simulations are performed in order to determine the amount of energy that is dissipated by the dampers on different road classes. Results show that, except in case of poor road quality, only a small amount of energy is dissipated by the dampers. Therefore, this option is not further investigated in this report.
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