Page 148 - Mechatronic Systems Modelling and Simulation with HDLs

7.2 DEMONSTRATOR 1: SEMI-ACTIVE WHEEL SUSPENSION 137

The motivation for the use of semi-active wheel suspension is that driving safety
and comfort represent competing goals in the context of suspension systems. Driv-
ing safety is jeopardised because unevenness in the road triggers vibrations in the
wheel or body. The former lie in the range of around ten hertz, the latter in the range
of one hertz. In both cases it should be ensured that these resonances are sufficiently
damped. Otherwise, strong variations in the wheel load will arise. In particular, the
wheel load will fall significantly as a result of the upward movement in question.
In the extreme case individual wheels will lift. The frictional connection between
the wheels and the road, and consequently the cornering force of the wheels in
question, then falls to zero, which can have severe consequences in a bend. These
problems could be countered by basically setting the damping level high. How-
ever, this would have a negative effect upon the ride comfort, which would transmit
every unevenness in the road directly to the body and thus to the passengers. This
is particularly true for the vibrations in the range of four–eight hertz, which would
be perceived as unpleasant or at least uncomfortable by the passengers.
Whilst an electronic control of the wheel suspension was not possible the pri-
mary issue was to find a reasonable compromise between safety and comfort.
Semi-active wheel suspension is based upon the principle that the damping of road
conditions can be switched over appropriately during the journey. Problematic
driving situations requiring a high level of damping are recognised firstly by the
consideration of the vertical acceleration of the body, from which the triggering of
vibrations by the road can be deduced. In addition, driving manoeuvres that also
require increased stability are recognised, for example, sharp braking, fast driving
through bends or quick accelerator pedal movements in automatics. The resulting
pitching and rolling movements of the vehicle should be limited by higher damp-
ing. After the identification of the road condition and the driving state, the next
step is to determine the correct damping level and to set this at the shock absorber.
The implementation is based upon a digital controller that processes embedded
software, see Figure 7.1. This carries out the actual control algorithm and takes
on a whole range of additional functions such as, for example, the plausibility
testing of the sensor values to be processed, the safety concept, or the provision
of data to other components of the vehicle. In addition, there are electronics for
the signal processing, such as D/A and A/D converters, which provide the connec-
tion between the digital and analogue worlds. The actual conversion between the
physical quantities is taken care of by the acceleration sensor and the adjustable
shock absorbers. Finally, the mechanics of the suspension also has to be taken
into account.
The central component of the system is the regulateable shock absorber, which
will be considered in more detail in what follows. Fundamentally, shock absorbers
use the Stokes’ friction of a viscous liquid, e.g. hydraulic oil, in order to convert
motion energy into heat. The oil is squeezed through a narrow valve in accordance
with the movement. If we control the route of the oil in accordance with the
direction of flow, then different valve diameters and thus different damper constants
can be provided for compression and tension mode. Furthermore, several damper