Air spring

An air spring has a variable interior space at a gas pressure for spring action between relatively-vibrating bodies. A control device has a sensor electrically responsive to the gas pressure for electrically operating an intake valve for admitting gas to the interior space and an outlet valve for venting gas from the interior space to atmosphere. This damps or isolates vibrations in the air spring in dependence on the control device. The control device may also respond to auxiliary transducers for the gas pressure, spring height, and level, speed, steering, braking, or acceleration of a vehicle on which the air spring is used, for example.

BACKGROUND OF THE INVENTION 
The invention relates to an air spring. 
A known air spring for use between relatively-vibrating bodies has a 
variable interior space filled with air at a mean pressure for spring 
action between the bodies, connected to a compressed-gas reservoir through 
a supply line having an intake valve, and provided with an outlet valve. A 
control device detects the pressure in the variable interior space, opens 
the outlet valve when the pressure rises above the mean pressure, and 
opens the intake valve when the pressure drops below the mean pressure to 
compensate for vibration-produced pressure variations in the interior 
space. 
For example, German patent publication DAS No. 12 82 475 discloses a 
pneumatically-damped air-suspension system in which a double-acting valve 
connects the variable interior space of an air spring to an equalizing 
space. The valve is actuated by vibration-induced pressure differentials 
between the interior space of the air spring and the equalizing space to 
provide some compensation for the over- or under-pressures in the air 
spring producing the pressure differentials. However, the damping action 
so obtained is not fully satisfactory because the pressure compensation in 
the air spring to counteract vibration-induced pressure differentials is 
actually confined within very narrow limits by essentially-identical mean 
pressures in the equalizing space and the interior space of the air 
spring. This basic situation remains the same even if the valve is 
connected to a separate damper. The isolating action therefore falls short 
of satisfying stringent requirements, except in rare cases. 
SUMMARY OF THE INVENTION 
An object of the invention is, therefore, to improve an air spring of the 
type described above to obtain better isolation of high-frequency 
vibrations than with a passive air spring, and better damping of 
vibrations in the resonance region than with known actively-compensated 
air springs. 
Another object of the invention is to permit the spring characteristics of 
an air spring to be selectively modified, for example for adapting it to 
predictable peculiarities resulting from its use in a motor vehicle. 
To these and other ends, the invention provides an air spring of the type 
described above in which a control device which responds to the pressure 
in a variable interior space of the air spring for spring action has an 
electric sensor and a drive for actuating a compressed-gas intake valve 
and an outlet valve to the variable interior space actuated by the 
electric sensor, and the outlet valve opens to a vent to the atmosphere. 
All commonly-used, electrical pressure transducers or transducer systems 
capable of detecting correctly the amplitude and phase of 
vibration-induced pressure variations superimposed on a mean pressure in 
the interior space of the air spring over a frequency range of from about 
0.5 Hz to not less than about 10 Hz frequency are suitable for the sensor 
in the invention. These include: 
foils or crystals having piezoelectric properties which, when mounted on an 
elastic or solid substrate and subjected to pressure variations, provide 
proportionate charge or voltage variations; 
bonded strain guages mounted on elastic substrates which, as 
differential-pressure elements subjected to the mean pressure of the 
interior space of the air spring on one side and the actual pressure in 
the interior space of the air spring on the other side, indicate voltage 
variations proportional to the pressure difference when a supply voltage 
is applied to them; 
bonded strain guages mounted on elastic substrates which, when subjected to 
the actual pressure of the interior space of the air spring on one side 
and a constant pressure or vacuum on the other side, change a constant 
supply voltage to a different output voltage that is proportional to any 
variation in the pressure in interior space of the the air spring; and 
pressure sensors of any electrical type having a high-pass filter in the 
output line. 
The sensor is, preferably, however, a foil having piezoelectric properties 
supported on a flexible or elastic sheet-like material flexibly exposed to 
the pressure in the interior space of the air spring only on one side. 
Pressure variations in the interior space of the air spring then give rise 
to proportionate voltages between the opposite sides of the piezoelectric 
foil sensor which also accurately reflect the direction or sign of the 
pressure variations. This sensor meets the requirements extremely well and 
is available at low cost, which is why it is preferred here. 
Sheet-like piezoelectric materials, and especially foils, have a 
predominantly polycrystalline and polarized internal molecular structure. 
Tensile or compressive forces introduced into that structure as by flexure 
of a foil result in a deformation of the crystallites forming the 
polycrystalline structure and, hence, in a disturbance of the internal 
electrical equilibrium. 
This disturbance can be measured between the surfaces of the sheet-like 
material in the form of a voltage impulse when electrically-conductive 
layers are applied to the surfaces, for example as vapor-deposited metal 
coatings. The direction of the voltage impulse depends on whether a 
tensile or compressive stress, in this case form a pressure increase or 
decrease in the interior space of the air spring, has been exerted on the 
sheet-like material. The amplitude of the impulse is a function of the 
magnitude of the deformation, and its fall time a function of the 
cessation of the deformation, of the electrical capacitance of the 
piezoelectric sensor structure, and of the magnitude of the electrical 
resistance of the materials between the conductive layers. Pressure 
variations in the interior of an air spring ranging from about 0.1 Hz to 
not less than about 20 Hz which reach a magnitude of at least about 0.01 
bar can be correctly detected. Many vibrations introduced into the 
suspension system of a motor vehicle meet these criteria and, therefore, 
can be so detected. By the use of the air spring of the invention, 
therefore, they can be compensated in an unexpected, hitherto unavailable 
manner. 
With a view to achieving good responsiveness, it has been found 
particularly advantageous for the piezoelectric sensor to be a foil of 
polyvinylidene fluoride or a copolymer thereof with opposed, 
electrically-conductive outer surfaces for actuating the control device. 
Whenever possible, the foil should have a thickness ranging from about 5 
microns to about 500 microns with vapor-deposited aluminum layers on 
opposed, outer surfaces. 
The voltage impulse generated by such a piezoelectric foil is dependent 
specifically on the degree of foil deformation produced by the pressure 
variation. For this, it has proved advantageous for the foil to bear on a 
layer of a resilient material on the side opposite that exposed to the 
interior space of the air spring. In such cases, the foil may also be 
sealed along its periphery to the air spring and exposed centrally thereof 
to its interior space. 
A charge- or voltage-controlled amplifier is, preferably, connected between 
the drive actuating the intake and outlet valves and the piezoelectric 
sensor for response thereto as the control device. The control device may, 
optionally, also have auxiliary devices for modifying the actuation of the 
drive in response to the piezoelectric sensor, for example, a phase 
shifter and/or another amplifier. 
Preferably, such auxiliary devices are responsive to auxiliary sensors, for 
example, one sensing the level, speed, steering- and/or braking-effort of 
a motor vehicle on which the air spring is used. The spring 
characteristics of the air spring can thus be adapted automatically to 
these or other operating conditions of the motor vehicle. 
The intake and outlet valves may be combined into a valve block and 
provided with a common drive. This design meets the usual technical 
requirements and can be manufactured at particularly low cost. 
However, significantly higher changeover frequencies can be if the intake 
and outlet valves are provided with separate drives. Such a design offers 
considerable advantages, especially with respect to the isolation 
characteristics secured. 
In the simplest case, the valves may be digitally controlled, but this has 
not proved particularly satisfactory with respect to the isolation 
characteristics obtained. Designs amenable to proportional control are, 
therefore, preferred for use with the present invention. 
Slow to static changes in the sprung mass, for example, those having a 
frequency of less than 0.1 Hz, can be detected by an electrical 
displacement transducer, for example a potentiometer with an output 
low-pass filter, and substantially compensated by responsive variation of 
the cross-sectional areas of the intake and outlet valves or their open 
times. Depending on the conditions of use, a preset level generally can be 
maintained in this way to deviation of not more than .+-.3 mm. 
The principal advantage of the air spring in accordance with the invention 
is that it reliably provides both good isolation of high-frequency 
vibrations and good damping of low-frequency vibrations substantially 
independently of variations in the supply pressure and spring-supported 
mass. The air spring of the invention therefore is extremely well suited 
for a wide range of applications in the manufacture of vehicles, for 
example in wheel suspensions or in the suspension of driver's cabs and/or 
seats.

FIG. 1 shows an air spring at 1 connecting the frame F of a motor vehicle 
to the axle A of a wheel supporting the vehicle frame. The air spring has 
a hollow cylinder 1a which is closed at one, top end secured to the frame 
F and open at the other, bottom end for receiving a cylindrical piston 1b 
connected to the axle A. The hollow cylinder and piston are made axially 
movable relative to each other and sealed with respect to each other by a 
rolling diaphragm 1c therebetween. Relative axial displacement of the 
cylinder and piston is, therefore, not accompanied by appreciable 
mechanical damping. 
Under operating conditions, however, the variable interior space of the air 
spring enclosed by the hollow cylinder, piston and rolling diaphragm is 
filled with air or another suitable gas or gas mixture under sufficient 
pressure relative to the mass of the vehicle on the frame to space the 
piston from the top end of the cylinder springingly. The relative, 
spring-acting movement of the piston and cylinder, for example when the 
wheel hits a bump in the road over which the vehicle is moving, then 
varies the gas pressure in the interior space of the air spring. 
The cylinder has an opening 1d through which the pressure of the air in its 
interior space acts upon a control device at 2. The control device has a 
piezoelectric, foil sensor 6 across the opening 1d to be exposed to the 
air pressure on one side. For this, a two-part housing 4 made of a 
nonconductive plastic encloses the foil 6 on the cylinder 1a about the 
opening 1d. The foil 6 is made of polyvinylidene fluoride having a 
vapor-deposited metallic surface coating on each side. The side of the 
foil opposite that exposed to the pressure in the interior space of the 
cylinder is flexibly supported on a resilient layer 5 on the housing. The 
foil is circularly bounded and covered along its edge on both sides by 
electrically-conductive, circular rings 7 which are provided with contact 
pins 8. The latter are connected through electrical conductors 9 and 10 to 
valve-actuating magnetic coils 3a,3b of a combined intake/outlet valve at 
3 via an interposed voltage amplifier 11 which matches the voltage 
impulses generated by foil deformation to the level required for moving 
the valve with the magnetic coils. 
The combined intake/outlet valve 3 is provided with three gas connections. 
On one side, it communicates with the interior space of the cylinder 
through a line 12; on the other side, it communicates with a 
compressed-gas reservoir 13a through a supply line 13 and with the 
atmosphere through a vent line 14 in dependence upon the position of the 
valve. 
An auxiliary, displacement transducer 15 indicated schematically as a 
potentiometer is disposed between the piston and cylinder parts of the air 
spring 1 and connected to the amplifier 11 to modify of the voltage 
therefrom for actuating the intake/outlet valve such that a constant, 
relative mean position of the piston and cylinder is maintained regardless 
of the load, the supply pressure, or the frequency of vibrations 
introduced. 
The principle of operation of the air spring is as follows: 
Starting from a mean pressure in the variable interior space of the 
cylinder, piston and rolling diaphragm, it will be assumed that the wheel 
passes over a bump in the road, thus causing compression of the air 
spring, i.e. relative movement of the piston and cylinder together. This 
reduces the volume of the interior space and, hence, produces a pressure 
rise in the interior space. 
The pressure rise deforms piezoelectric foil 6 via the opening 1d and 
resilient backing 5 and the foil, therefore, releases a voltage impulse 
which corresponds in magnitude and direction (sign) to the pressure 
variation. The voltage impulse is amplified by amplifier 11 as required to 
actuate the outlet valve 3 to connect vent line 14 to line 12 to the 
interior space of the cylinder. This allows compressed air to escape from 
the interior space of the cylinder, thus producing a drop in the pressure 
in the interior space of the cylinder toward the mean pressure which is 
accompanied by the recovery from deformation of piezoelectric foil 6 and, 
hence, by the decaying of the voltage impulse and, thus, closing of the 
outlet valve. 
If, instead, the wheel entered a pothole which caused the air spring to 
relax, i.e. separation of the piston and cylinder, the pressure in the 
interior space of the cylinder would be reduced from the mean pressure 
initially present. In this case, the piezoelectric foil 6 is deformed in 
the opposite direction, which gives rise to an oppositely directed voltage 
impulse. After amplification in amplifier 11, this results in actuating 
the valve to connect the supply line 13 to line 12 to the interior space 
of the cylinder, with compressed air then rushing from the reservoir 13a 
to the interior space of the cylinder until the original, mean pressure is 
restored to relieve the film deformation and, thus, close the valve. 
In both cases, the mean pressure in the interior space of the cylinder is 
set by the auxiliary, displacement transducer 15. It sends a signal to the 
amplifier 11 in dependence on the spacing of the piston and cylinder to 
actuate the valve 3 similarly until the pressure, i.e. the mean pressure, 
so produced provides a predetermined spacing. The mean pressure is, 
therefore, a function of the load mass of the vehicle on the frame. 
The auxiliary transducer 15 therefore also functions as a level transducer 
for the vehicle. If the frame F of the vehicle were to tilt from a load 
shift or braking, for example, the relative position of the cylinder 1a on 
the frame and piston 1b on the wheel axle A would change and the 
transducer 15 respond thereto to change the mean pressure to reset the 
preset position as before. An absolute level sensor such as a pendulum 
actuated potentiometer (not shown), for example, could also be used in 
another embodiment (not shown) for the same and further results. 
FIG. 1 also shows another auxiliary transducer, a speed sensor, which is a 
generator 27 connected to the piston 1b and responsive to the speed of 
rotation of the axle A of the wheel to generate a proportionate voltage. 
This is supplied to terminal 28 of amplifier 11 to actuate valve 3 in 
relation to the speed. This allows more or less pressure response in the 
cylinder in proportion to vehicle speed as may be desired in the design of 
the vehicle suspension. 
FIG. 2 shows a portion of the control device at 2' of another embodiment. 
In it, a housing 4' holds a resilient member 5' across the opening 1d' 
into the variable interior space of the cylinder 1a', as before, but in 
this embodiment, a strain guage 6' is on the side of the resilient element 
exposed to the pressure of the interior space by the opening and the other 
side of the resilient element is exposed to a constant or substantially 
constant gas pressure, in this case from the reservoir 13a' via pipe 13b. 
The strain guage 6' is connected to the amplifier 11 (FIG. 1) for 
operation as before. 
FIG. 3 shows a portion of another embodiment having a vibration sensor 20, 
a vertical accelerometer, for example, on the frame F' of the vehicle to 
sense periodic vibrations as may be produced in the frame by the engine 
(not shown) of the vehicle, for example. The vibration sensor provides a 
signal corresponding to the vibrations to a phase shifter 22 which changes 
its phase to compensate for the phase lag in actuating the valve 3 (FIG. 
1) and changing the pressure in the interior space of the cylinder 1a 
(FIG. 1) correspondingly. In this way, the pressure in the cylinder is 
changed to isolate the vibrations, i.e. reduced for compressive vibrations 
and vice versa as the vibrations occur. Such isolation has more utility 
when the air spring is used with a vehicle seat, for example, rather than 
the wheel shown in FIG. 1, but the principle is the same in either case. 
Opposite pressure compensation is possible, too, of course, and would damp 
vibrations as may be desired for large, slow vibrations on starting the 
engine, for example. 
The phase-shifted vibration signals from phase shifter 11, or the other 
auxiliary transducers before or hereafter described, may not have the same 
amplitude as those from the piezoelectric or strain guage sensors 6,6' of 
FIGS. 1 and 2. The amplifier 11 (FIG. 1) may not, therefore, respond 
sufficiently or properly to the phase shifter or other auxiliary 
transducer signals for actuating the valve 3 properly. Accordingly another 
amplifier 24 is provided to adjust these signals, in this case the 
phase-shifted vibration signals from phase shifter 22, into correspondence 
with those from the other sensor 6 or 6' for amplifier 11 to actuate valve 
3 properly. The signals from the amplifier are, therefore, provided to 
terminal 26 on amplifier 11 (FIG. 1) for its control like auxiliary 
transducer 15 (FIG. 1). 
FIG. 4 very schematically shows other auxiliary transducers of still 
another embodiment. One is a potentiometer 30 responsive to rotation of a 
steering wheel 32 for the vehicle (shown by frame F"). In this way, the 
air spring suspension of opposite front wheels of the vehicle can be 
changed to compensate for the dipping acceleration of turning the vehicle, 
for example. Braking acceleration dip can be similarly compensated for by 
adjusting the air spring suspension of both front wheels in response to 
another potentiometer auxiliary transducer 34 connected to brake pedal 36. 
This may also be done for engine-driven acceleration via the accelerator 
pedal (not shown) or both brake and accelerator accelerations may be 
responsive to a horizontally responsive accelerometer 38 on the vehicle 
frame F". All of these auxiliary transducers are, therefore, appropriately 
connected to a terminal 28' on amplifier 11 (FIG. 1) for controlling the 
air spring suspension as desired by its design. 
In these other embodiments, therefore, the amplifier 11 functions also as 
an auxiliary controller responsive to the height of the motor vehicle 
above the ground, the traveling speed of the motor vehicle, the 
instantaneous steering position of the motor vehicle and/or the 
instantaneously effective accelerative or braking forces on the motor 
vehicle. Apart from the level control, the specific settings of auxiliary 
transducers are preferably made in a test drive of the vehicle and will 
result in an appreciable improvement of riding comfort and driving safety. 
It will be appreciated that the instant specification and claims are set 
forth by way of illustration and not of limitation, and that various 
changes and modifications may be made without departing from the spirit 
and scope of the present invention.