Spring devices – Vehicle – Comprising compressible fluid
Reexamination Certificate
2001-05-24
2003-01-21
Schwartz, Christopher P. (Department: 3613)
Spring devices
Vehicle
Comprising compressible fluid
C267S064240
Reexamination Certificate
active
06508460
ABSTRACT:
BACKGROUND OF THE INVENTION
When utilizing air springs in passenger cars, a largest possible air volume is to be used to obtain optimal suspension comfort. Mostly, there is insufficient space at the wheel because of chassis components such as a longitudinal control arm, brake and drive shaft. For this reason, this large air volume is subdivided into an air spring volume and an ancillary volume (see
FIG. 1
a
). The ancillary volume can then be accommodated at a location in the vicinity such as in the engine compartment, in the longitudinal support, in the trunk, et cetera. Both volumes are then connected by a line having a cross section which is of such a dimension that an air exchange can take place very rapidly and without significant pressure loss. If the vehicle travels on cobblestones, for example, then the air spring contracts and expands in correspondence to the road speed at a high frequency. Each spring contraction operation and each spring expansion operation is associated with an air exchange which may not be hindered because the suspension comfort would otherwise be reduced.
A high suspension comfort means a reduced spring stiffness. In accordance with the above, this is achieved with a large air spring volume. It is, however, a disadvantage that the steering becomes loose. Likewise, for a low spring stiffness, the driving performance changes when braking, when accelerating, and in travel through a curve as well as with rapid avoidance maneuvers. This change in driving performance is in the direction of instability which is unwanted because driving safety is thereby affected.
In order to resolve this conflict between comfortable air spring design and stability of the driving performance, the above-described line is provided with a valve, which can be blocked (see
FIG. 1
b
). During normal driving conditions, the valve is open and is open in such a manner that the valve presents no significant hindrance for the air exchange between the air spring and the ancillary volume. If the vehicle is now braked, accelerated or driven in a curve or is compelled to execute a rapid defensive maneuver, then the valve is abruptly closed by a control apparatus which can detect the driving state by means of sensors. Thus, the air spring and the ancillary volumes are separated from each other with the consequence that only the air spring volume is available for the suspension operation. The spring stiffness is therefore higher and the vehicle has a more stable driving performance.
The valve is again opened as soon as the control apparatus detects that none of the above-described driving conditions is present any longer. This opening operation has to be carried out in such a manner that a pressure difference between the air spring volume and the ancillary volume, which has possibly formed in the meantime, can be slowly compensated so that there is therefore no sudden drop or upward bucking of the vehicle. Only when the pressure compensation is complete can the valve again be completely opened.
Valves for this task are known. These valves are mostly realized in the assembly of trucks as precontrol valves. A small electromagnetic valve switches a larger pneumatically actuated valve (FIG.
2
). The alternative is an electromagnetic actuation of the valve. In the manufacture of passenger cars, no corresponding compressed air source having sufficient power is present in order to switch the pneumatically actuated valve. For this reason, only the electromagnetic actuation remains (FIG.
3
).
If one wants to continuously adjust the cross section to be cleared by the valve in order to, for example, obtain specific spring frequencies or to make possible the described slow pressure compensation, then the valve must operate as independently as possible of the existing pressure differences in the system and its inherent friction forces. Forces caused by pressure differences should operate so that they mutually cancel each other and therefore have no influence on the switching or adjusting operation of the valve. Friction forces should be as small as possible and have a constant level. If these requirements are satisfied, then a specific valve setting is assigned to each specific current level supplied to the electromagnet. A continuous clearance of the cross section is thereby provided.
In order to be as independent of pressure as possible, the pressure relief principle shown in
FIG. 4
is suitable. A complete pressure relief is, however, not possible (for example, for a star nozzle and a round nozzle), even for the principle illustrated. The reason for this is that a plate membrane would be necessary for pressure relief. The plate membrane, however, has an effective diameter Dw which changes in dependence upon service life (because of stretching) and, in addition, is dependent upon axial and radial built-in tolerances. A roll membrane is not suitable for this purpose because this membrane is turned inside out with a pressure reversal and would thereby be destroyed. This also applies to the plate membrane even though this membrane is somewhat less sensitive.
The same problems (non-constant active diameter and inversion) result also when a slider valve is provided with a plate membrane or roll membrane (FIG.
5
). If the slide valve is provided with a seal (
FIGS. 6
a
and
6
b
), then this seal is burdened with wear and leakage. Likewise, friction forces must be overcome when switching and controlling and these forces change in dependence upon pressure. This can go so far that the friction force is greater than the electromagnetic force and the valve can therefore not switch. A reliably switching valve or a valve wherein each specific current level is assigned to a specific valve position is therefore not realizable therewith.
It is conceivable to utilize a slider valve with a seal (
FIGS. 6
a
and
6
b
) wherein only small pressure differences occur. In the area of passenger car air spring systems, large pressure differences however occur, which are caused by rapid spring contraction and expansion, so that, at the present time, a use is only possible under reduced requirements.
Furthermore, an electromagnet is required for valve actuation which has a large number of turns with low electrical resistance and therefore has a large valve mass, large structural space and incurs high costs. Also, the armature of the valve would be accelerated toward the valve seat when switching on the actuator current. As a consequence of the large electromagnetic force, which must be made available, large speeds could occur so that large decelerations would become effective when striking the valve seat, that is, the armature generates a noise when striking the valve seat which can be in the nature of a hammer bolt.
In air springs systems for trucks, valves exist for hammer-like closure and slow opening on the basis of pneumatic actuation.
In passenger car air springs, magnetic valves are known which can be adapted to the larger line cross section. Additionally, a pressure relief is provided in order to reduce the acting forces. However, all these solutions are burdened with friction and therefore do not permit a trouble-free adjustment and control. In the manufacture of trucks, the valves are pneumatically actuated because the pneumatic has a high energy density. The high energy consumption (pressurized air escapes) is not significant there. Likewise, the switching noise is also of not much consequence.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a motor vehicle air spring wherein the valve used has the following advantageous characteristics, namely:
a) continuous adjustability;
b) independence of existing pressure differences;
c) low leakage;
d) very short reaction time;
e) stable performance in the presence of flow forces;
f) low mass;
g) low friction;
h) adequate service life;
i) small structure;
j) little electrical energy consumed;
k) finely metered continuous opening possible;
l) the complete cross section cleared without throttling;
m) cost effective; and,
n) no disturbing noises.
Th
Continental Aktiengesellschaft
Ottesen Walter
Schwartz Christopher P.
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