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Top1. Introduction
Nowadays, the importance of working vehicles comfort and security is becoming always more important. These vehicles are in fact used for long time requiring a good level of comfort for the driver in the cab. One of the elements which more affects comfort and safety is the type of suspension adopted for the vehicle. In general, the kinds of suspension used for vehicles can be dived as follows:
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Passive suspension (Grott, 2010);
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Adaptive suspension (Cebon, 2000);
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Semi-active suspension (Williams, 1997);
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Active suspension (Reimpell, Stoll, & Betzler, 2001).
The passive suspensions are the most used for their simplicity and their low cost. The frequency response of this kind of suspension cannot be adjusted, since it is designed to have a compromise between comfort and a good handling. In Figure 1 it is shown a typical scheme of a passive suspension.
The adaptive suspension, instead, allows to modify the stiffness and damping properties. The commercial systems of this suspension are self-levelling and dispose of a multistage damping. The control system acts in open loop and it is activated by the driver. Semi-active suspension solution is being developed because it is a good compromise between cost and performances. The unit control is able to modify the damping characteristic in closed loop, see Figure 2.
Figure 2. Semi-active suspension model
The limits of semi-active suspension are that cannot add energy to suspension system, like the active suspension system. This system allows changing the damping thanks to orifices of variable section or thanks to magnetorheological fluids that change their viscosity depending on the electromagnetic field with which they are invested. The active suspension exercises control forces through hydraulic, pneumatic or electromechanical independent unit. The addiction of energy in suspension system allows an increase of comfort, grip and a reduction of pitching and rolling phenomena that occur respectively on curves and during traction or braking. The cost of this suspension is very high. In fact, they are usually adopted on racing and prestigious vehicles. Semi-active and active systems are very complicated. They have a particular control unit which is based on skyhook damping theory. According to this theory, the best conditions for suspension are reached when the sprung mass is also connected to an inertial reference, via a damper (Genta & Morello, 2008; Savaresi, Silani, & Bittanti S, 2005), see Figure 3.
Figure 3. Skyhook control for semi-active suspension
However, since such architecture of suspension is not achievable, the control system calculates a force exerted by the actuator, proportional to the absolute speed of the drum with respect to an inertial reference and the relative speed of the un-sprung mass with respect to the case. This technique allows having a reduction of displacement transmissibility with respect to frequency, if compared to conventional damping system.
Figure 4. Section of a hydropneumatic suspension (Citroën): (1) Filling screw; (2) The upper hemisphere; (3) Input high-pressure fluid; (4) Piston; (5) Overflow; (6) Shield rod; (7) Diaphragm; (8) Lower hemisphere; (9) The damper; (10) Cylinder; (11) Pad thrust; (12) The sealing system; (13) Protective cover dust