BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to vehicle suspension systems, and more particularly
to a method and apparatus for selectively modifying a suspension parameter in
response to changes in the relative orientation between telescopically movable
components of a hydraulic damping device.
2. Description of Related Art
To provide means for selectively adjusting a suspension parameter
or for controlling variable leveling requirements, information regarding the relative
motion of the suspension and/or the vehicle body is required. Such information
is commonly supplied by sensors to a central electronic controller for measuring
and calculating suspension and body displacement, velocity and acceleration. This
information is used by the electronic controller to selectively control the suspension
characteristics of the vehicle to provide the desired response.
Damping devices ("dampers") are used in conjunction with automotive
suspension systems to absorb unwanted vibration (impacts, loads, etc.) which occurs
during driving. To absorb this unwanted vibration, dampers are generally connected
between the body and the suspension of the automobile. A piston is located within
the damper and is connected to the body of the automobile through a piston rod.
Because the piston valving and orifices act to restrict the flow of damping fluid
within the working chamber of the damper when the damper is compressed, the damper
is able to produce a damping force which counteracts the motion of the wheel and/or
body which would otherwise remain undamped. The greater the degree to which the
flow of damping fluid within the working chamber is restricted by the piston, the
greater the damping forces which are generated by the damper.
In selecting the amount of damping that a damper is to provide, three
vehicle performance characteristics are often considered: ride comfort, vehicle
handling and road holding ability. Ride comfort is often a function of the spring
constant of the main suspension springs of the vehicle, as well as the spring constant
of the seat, tires, and the damper. Vehicle handling is related, among other things,
to variation in the vehicle's attitude (i.e., roll, pitch and yaw). For optimum
vehicle handling and, consequently, superior body and wheel control, relatively
large damping forces are required to avoid excessively rapid variation in the vehicle's
attitude during cornering, acceleration, and deceleration. Road holding ability
is generally a function of the amount of variation in the normal load between the
tires and the ground. To optimize road holding ability, larger damping forces are
required when driving on irregular surfaces to minimize the normal load variations
so as to prevent complete loss of contact between the wheels and the ground.
To optimize ride comfort, vehicle handling, and road holding ability,
it is generally desirable to have the damping forces generated by the damper be
responsive to the frequency of the input from the road or from the body. When the
input frequency is approximately equal to a natural frequency of the body (e.g.,
approximately between 1-2 Hz), it is generally desirable to have the damper provide
relatively large damping forces (relative to critical damping) to avoid excessively
rapid variation of the vehicle's attitude during cornering, acceleration and deceleration.
When the input frequency is between 2-10 Hz (mostly from the road), it is generally
desirable to have the damper provide low damping levels so as to produce a smooth
ride and allow the wheels to follow changes in road elevation. When the input
frequency from the road is approximately equal to the natural frequency of the
automobile suspension (i.e., approximately 10-15 Hz), it is desirable on one hand
to have relatively low damping forces to provide a smooth ride, and on the other
hand provide high damping forces so as to minimize variation in tire normal load
and prevent complete loss of contact between the wheels and the ground.
Selective control of a desired suspension parameter often requires
information regarding the movement of the piston within the pressure cylinder of
the damper. This information not only identifies whether the damper is in compression
or extension, but also can provide information concerning the magnitude and frequency
of suspension motion.
Several methods are known for obtaining information regarding the
movement of the piston within the pressure cylinder. PCT Application No. PCT/US87/00615
uses a pressure sensor as well as an accelerometer to determine whether the damper
is in compression or extension, as well as to obtain information regarding the
road surface. U.K. patent application No. GB 2 177 475A and DE-U-87 02 817.4 disclose
suspension damping devices incorporating ultrasonic "sonar" wave systems for determining
positional displacement information. The positional displacement information is
obtained by determining the time from transmission of an ultrasonic wave to when
its reflected "echo" wave is received. Both references use a single transducer
acting to emit and receive the pulsed ultrasonic waves. Use of a single transducer
necessitates incorporation of costly ultrasonic wave modulation and calibration
circuitry to ensure coherent wave detection. Additionally, the transducers in both
references are mounted such that the piston acts to reflect the ultrasonic waves.
DE-A-3 031 980 discloses a device for measuring the distance of a
piston from the bottom of a cylinder including an ultrasonic transducer for emitting
and receiving ultrasonic waves. The processing circuitry for determining the propagation
times of the ultrasonic waves comprises a pulse generator, a timing circuit and
a processor for calculating the distance of the piston in the lifting cylinder
of an industrial truck, for example. DE-U-8 702 817 discloses an ultrasonic device
in a hydraulic actuator of a vehicle, the processing circuitry comprising a pulse
generator, a control circuit and a timing circuit for determining the propagation
time of the ultrasonic waves emitted and received by the ultrasonic transducer.
The features listed in the preamble of claim 1 can be found in this document.
It is a primary object of the present invention to provide a hydraulic
actuator of the type above referred to comprising a driving circuit which is particularly
suited for calculating the distance and/or change in distance between the piston
and the pressure cylinder in generating continuous real-time electrical signals
for selectively controlling suspension characteristics with the hydraulic actuator
which is responsive to the input frequencies of the road.
According to the invention, the object referred to is solved by the
features of claim 1. Further aspects of the invention are subject of the subclaims.
According to the invention, claim 1 features the initiation of emitting
sound waves in response to a first electrical signal from the pulse generator,
while the timing circuit includes a switch for electrically connecting the tranducer
to the oscillator of the driving circuit upon receipt of said electrical signal
from the pulse generator, the switch being further operable to electrically connect
the transducer to the timing circuit in the absence of said first electrical signal.
Thus the switches between positions to separately connect and disconnect the driving
circuit and the timing circuit to the transducer.
By measuring the time difference between the emitted ultrasonic waves
and the reflected ultrasonic waves, the distance of the piston relative to the
base valve or the end surface can be calculated by using a computer. Accordingly,
a continuous determination can therefore be generated which, as an example, can
be used by a piston control circuit to control the damping forces of the shock
absorber. Such determination can also be employed to detect the polarity (direction)
of motion of the piston.
While the preferred embodiment discloses a twin-tube shock absorber,
it is contemplated that the principles embodied in the present invention are readily
adapted to mono-tube devices and other hydraulic or pneumatic actuators having
BRIEF DESCRIPTION OF THE DRAWINGS
Various advantages of the present invention will become apparent to
one skilled in the art upon reading the following specification and by reference
to the following drawings in which:
DESCRIPTION OF THE PREFERRED EMBODIMENT
- FIG. 1 is a diagrammatic representation of the direct acting hydraulic dampers
according to the preferred embodiment of the present invention shown in operative
association with a typical automobile;
- FIG. 2 is an enlarged side elevational view, partially broken away, of the direct
acting hydraulic damper shown in FIG. 1 according to the preferred embodiment of
the present invention;
- FIG. 3 is an enlarged cross-sectional view of FIG. 2 illustrating the relative
location and operation of the ultrasonic transducer device shown in FIG. 2 according
to the preferred embodiment of the present invention;
- FIG. 4 is an enlarged cross-sectional view of the transducer mounting assembly
according to the preferred embodiment;
- FIG. 5 is a block diagram illustrating the electrical components which are used
for driving the transducer shown in FIG. 2. and
- FIG. 6 is a schematic representation illustrating the electrical circuits used
for selectively changing the desired suspension characteristics of the damper.
Referring to FIG. 1, a plurality of four hydraulic actuators defined
as dampers 20 in accordance with the preferred embodiment of the present invention
are shown. The dampers 20 are depicted in operative association with a diagrammatic
representation of a conventional automobile 22. The automobile 22 includes a rear
suspension 24 having a transversely extending rear wheel assembly 26 adapted to
support the rear wheels 28 of the automobile 22. The wheel assembly 26 is operably
connected to the automobile 22 by means of a pair of dampers 20 as well as by the
helical coil springs 30. Similarly, the automobile 22 has a front suspension system
32 including a transversely extending front wheel assembly 34 to support the front
wheels 36. The front wheel assembly 34 is connected to the automobile 22 by means
of a second pair of dampers 20 and by the helical coil springs 38. While the preferred
embodiment is described with use of coil springs, it is to be understood that any
other load-carrying device (leaf springs, air springs, hydro-pneumatic springs,
etc.) are within the scope and fair meaning of the present invention. The dampers
20 serve to damp the relative movement of the unsprung portions (i.e., the front
and rear suspensions 32 and 24) and the sprung portion (i.e., the body 39) of the
automobile 22. While the automobile 22 has been depicted as a passenger car, the
damper 20 may be used with other types of motor vehicles as well.
With particular reference to FIG. 2, the damper 20 according to a
preferred embodiment of the present invention is shown. The damper 20 comprises
an elongated tubular pressure cylinder 40 defining a damping fluid containing working
chamber 42. Disposed within the working chamber 42 is a reciprocable piston 44
that is secured to one end of an axially extending piston rod 46. The piston 44
includes a circumferential groove 48 operable to retain a piston ring 50 as is
well known in the art. The piston ring 50 is used to prevent damping fluid from
flowing between the outer periphery of the piston 44 and the inner diameter of
the cylinder 40 during movement of the piston 44. A base valve, generally designated
by the numeral 52, is located within the lower end of the pressure cylinder 40
and is used to control the flow of damping fluid between the working chamber 42
and an annular fluid reservoir 54. The annular fluid reservoir 54 is defined as
the space between the outer periphery of the cylinder 40 and the inner periphery
of a reservoir tube or cylinder 56 which is arranged centrally around the exterior
of the pressure cylinder 40. The operation of the base valve 52 may be of the type
shown and described in U.S. Patent No. 3,771,626. Additionally, the base valve
52 has a step-like upper geometry defining a first surface 51 and a second surface
53 to be detailed hereinafter. It is contemplated, however, that the present invention
is applicable to hydraulic actuator operable without base valves, check valve or
The upper and lower ends of damper 20 are provided with generally
cup-shape upper and lower end caps 58 and 60 respectively. The end caps 58 and
60 are secured to opposing ends of the reservoir tube 56 by a suitable means such
as welding. The damper 20 is shown as being provided with a dirt shield 62 which
is secured at its upper end to the upper end of the piston rod 46. Suitable end
fittings 64 are secured to the upper end of the piston rod 46 and the lower end
cap 60 for securing the damper 20 between the body and the wheel assembly of the
automobile 22. Those skilled in the art will appreciate that, upon reciprocal movement
of the piston 44, damping fluid within the pressured cylinder 40 is transferred
between the upper and lower portions of the working chamber 42, and between the
working chamber 42 and the fluid reservoir 54. By controlling the flow of damping
fluid between the upper and lower portions of the working chamber 42, the damper
20 is able to controllably dampen relative movement between the body and the wheel
of the automobile 22 so as to optimize both ride comfort and road handling ability.
The piston 44 is provided with a valve arrangement (not shown) for
controllably metering the flow of damping fluid between the upper and lower portions
of the working chamber 42 during reciprocal movement thereof. One such valve arrangement
is disclosed in PCT Application No. PCT/US87/00615. It is contemplated, however,
that the present invention may be used with other suitable valve arrangements as
well as other suitable damping devices. It is to be understood that the afore-described
structure of damper 20 is merely exemplary and that the principles of the present
invention are applicable to numerous actuator designs.
In accordance with the principles of the present invention, the damper
20 further comprises an acoustical transducer 66 acting as a transmitter/receiver
which is secured to a surface of the piston 44 opposite rod 46. The transducer
66 is used to generate ultrasonic waves having a predetermined frequency "f" in
the direction of the end wall of cylinder 40 facing transducer 66 to which base
valve 52 is secured. Ultrasonics is the name given to sound waves having a frequency
higher than to those which the human ear can respond (approximately 16KHz). The
propagation of sound waves through a relatively non-absorptive medium involves
the generation of vibrations in the elementary particles of the medium through
which the waves are passing. While the transmitter 66 may be piezoelectric device
fabricated from quartz, Barium Titanate or lead Zirconate Titanate or a magnetorestrictive
device, other suitable devices may be used. The resonant frequency "f" selected
is related to the dimensions of the piezoelectric transducer selected. When the
ultrasonic waves emitted by the transducer 66 encounter the base valve 52 (reflector),
they are reflected in a direction back toward the transducer 66. The transducer
66 receives the ultrasonic "reflected" waves which are reflected by the base valve
52 and generates an output in response thereto. For purposes of the following discussion,
the ultrasonic waves generated by transducer 66 will be referred to as the "emitted
waves", while the waves received by transducer 66 will be referred to as the "reflected
Base valve 52 is provided with a first stepped surface 51 disposed
axially above a second stepped surface 53 perpendicular to the cylindrical axis
of cylinder 40 by a predefined distance. The predefined axial displacement of surfaces
51 and 53 provide means for compensating for changes in the speed of wave propagation
through the damping fluid due to the temperature and pressure effects on viscosity.
The stepped surfaces 51 and 53 produce separate reflected waves, the trailing wave
referred to as the "echo wave", both of which are received by transducer 66. The
echo wave lags behind the reflected wave. While commonly used damping fluids have
sufficiently low attenuation properties up to about 3 MHz, compensation means are
still preferably used. The predetermined distance between surfaces 51 and 53 thereby
provide a known reference distance. The relationship between the predefined axial
distance and the echo wave permits adjustments for viscosity changes.
When the piston 44 is stationary relative to the base valve 52, the
time for the reflected ultrasonic waves to reach the transducer 66 will be constant.
However, when the piston 44 is moving in a direction toward the cylinder end to
which base valve 52 is secured, the time between transmission and receipt of the
reflected sound wave will be shorter. In contrast, when the piston 44 is moving
in a direction away from the base valve 52, the time for the reflected ultrasonic
wave to reach the transducer 66 will be longer. By continuous, repetitive measurement
of such time differences, the displacement or distance of the piston 44 relative
to base valve 52 can be determined. Such information can, in turn, be used to determine
related data such as velocity, acceleration, body frequency and jerk.
In operation, the preferable wave control system employed is the pulse-echo
method. This method involves the transmission of a pulsed acoustic wave followed
by determination of the time taken for the first echo from the reflection at the
base valve 52 to return to transducer 66. Figure 5 shows a block diagram illustrating
this control method. The carrier or resonant frequency "f" is chosen according
to the mechanical characteristics of the transducer 66 and of the damping fluid.
When the piezoelectric crystal 68 of the transducer 66 is excited
by a sinusoidal voltage input, a finite time is required for it to reach an equilibrium
state. Similarly, a finite time is taken for the crystal 68 to stop vibrating once
the electrical excitation has been removed. In order to use a single transducer
66 to emit and receive the ultrasonic sound waves, the crystal 68 must be capable
of attenuating its oscillation after transmission of the pulse "f" in order to
prevent overlap of the transmitted and echo pulses. Consequently, the transducer
should have a small modulation pulse width thereby permitting operation when there
is a minimum distance between the transducer 66 and base valve 52. The control
strategy is also able to discriminate between a first reflected wave and subsequent
reflected waves due to the time between transmitted pulses being set longer than
the acoustical damping time of the fluid.
To provide means for driving the transducer 66, a wave generating
circuit 72 is provided. The wave generating circuit 72 is electrically connected
to the transducer 66 so as to enable the transducer 66 to produce emitted ultrasonic
waves of a predetermined carrier frequency "f". Preferably, the wave generating
circuit 72 causes the emitted ultrasonic waves generated by the transducer 66 to
be of continuous form so as to enable efficient, continuous, real-time determinations
of the relative distance between the piston 44 and the base valve 52. The wave
generating circuit 72 comprises an oscillator 74, and a pulse generator or modulator
76. It is contemplated that other types of wave generating circuits known in the
art may be readily substituted heretofore.
To provide means for detecting the reception of the reflected wave
or pulse by transducer 66, a amplification circuit 90 is provided. Amplification
circuit 90 is electrically connected to transducer 66 so as to amplify the reflected
signal and determine the largest peak amplitude thereof.
To provide means for measuring the time difference between the reflected
ultrasonic waves and the emitted ultrasonic waves, the shock absorber 20 further
comprises a timing circuit 100. As shown in Figures 5 and 6, the timing circuit
100 receives a signal from the wave generating circuit 72 to "start" the timing
circuit 100. Timing circuit 100 further receives a signal from the amplification
circuit 90 to "stop" the timing circuit 100 upon receipt by the transducer 66 of
the reflected pulse wave.
Specifically, oscillator 74 generates a carrier frequency "f". Pulse
modulator 76 provides the pulsed signal which will be timed from transmission to
reception by transducer 66. On the rising edge of the modulated pulse, switch 82
is moved to position "A" and timing circuit 100 is started at time T&sub1;. On
the falling edge of the modulated pulse, switch 82 moves to position "B" so as
to be ready to receive the reflected wave pulse. The reflected pulse is received
by an amplification circuit 90 for amplifying and detecting the peak amplitude.
The rising edge of the final reflected pulse triggers timing circuit 100 to stop
timing at time T&sub2;. The timing circuit 100 is not activated again until subsequent
reflected pulses due to ancillary reflections have attenuated. As a non-limiting
example, a relatively low resonance frequency "f" is centered at about 2.2 MHz.
The 2.2 MHz modulated signal was generated by the pulse generator 76. The modulated
pulse width is approximately 1.25 microseconds and is repeated every 440 microseconds.
This provided a sampling rate of approximately 2.3 KHz. It is to be understood,
however, that other suitable means for measuring the time difference between emitted
and reflected and echo pulses may be used.
To provide means for calculating the relative distance between the
piston 44 and the base valve 52, a central electronic processor ("computer") 110
is used. The computer 110 uses the output from the timing circuit 100 to calculate
the relative distance between the moveable piston 44 and the end of the cylinder
40 or the base valve 52. After the distance between the piston 44 with respect
to the base valve 52 has been determined by the computer 110, the computer 110
generates an output in response to the distance calculation which can be used in
various suspension control algorithms requiring relative positional distance input.
One such control scheme is to deliver the distance calculation to a piston control
circuit 120. The piston control circuit 120 then acts to change the damping characteristics
of the piston 44 to obtain the desired road handling characteristics.
Such distance determinations can also be employed to provide positional,
directional and/or force information applicable to controlling active or dynamic
leveling actuators instead of, or in addition to, damping control.
In order to increase the sensitivity of the receiver circuitry it
is preferable that transducer 66 be able to drive into a high impedance when it
is acting as a receiver. However, the circuitry should provide low output impedance
when the transducer is being driven as an emitter. Using commonly available electronics,
the impedance level can be switched from low to high between the time of transmission
Referring now to Figure 4, the transducer 66 is illustrated. Transducer
66 is mounted between a first mounting plate 130 and a second mounting plate 132.
The transducer is generally centrally aligned over the base valve 52 to provide
optimum reflective characteristics. Preferably the mounting plates 130 and 132
are fabricated from aluminum. Electrical connections (not shown) are made with
transducer 66 on a first lower surface 134 of the piezoelectric crystal 68. A tinned
copper ring 136 is soldered to the peripheral surface of the opposite top surface
138 of the crystal 68 to ensure sufficient electrical connections to first mounting
plate 130. A layer of neoprene rubber 140 is used between the mounting plates 130
and 132 to electrically insulate the lower surface face of transducer 66 from the
second mounting plate 132.
Preferably the electrical connections to the driving circuitry are
made through a hollow central bore of rod 46 using screened cable. It is contemplated
that any means for securably mounting the transducer to the end of the piston 44
which provides the requisite sensitivity is within the scope of this invention.
Further, any transducer assembly providing the requisite electrical connections
and alignment relative to the reflective member is suitable.
Base valve 52 is provided with a step-like upper first surface 51
and second lower surface 53. This provides known "fixed" distance reflecting surfaces
for calibrating ultrasonic distance "h" measurements where changes in the speed
of wave propagation is varied due to temperature and pressure changes of the damping
fluid. Such a step-like base valve 52 provides two distinct reflected pulses. Changes
in the time between the reception of the two reflected pulses (previously referred
to as the reflected and echo waves) provides a reference for "fine-tuning" the
control circuit to compensate for such variation. Alternatively, a thermocouple
may be positioned within the shock absorber 20 to determine temperature fluctuations.
Such information would permit use of look-up tables by the central processor 110
to compensate the temperature dependence of the velocity of sound through the fluid.
Also, it is contemplated that the present invention may comprise various methods,
currently utilized in sonar systems, for adjusting the relative distance determination
"h" to compensate for changes in the speed of wave propagation due to the temperature
and viscosity of the damping medium. Such compensation can be incorporated into
the computer software based on known characteristic of the damping fluid.
While it is apparent that the preferred embodiment illustrated herein
is well calculated to fill the objects stated above, it will be appreciated that
the present invention is susceptible to modification, variation and change without
departing from the scope of the invention. For example, it is contemplated that
the timing circuit 100, the computer 110 and wave generating circuit 72 may all
be located either internal or external with respect to the damper. If located externally
of the damper 20, a single computer 110 may be used to calculate the relative distance
and control any suspension parameter (leveling, damping, springing, etc.) for each
of the dampers in the vehicle suspension. Likewise, it is contemplated that acoustical
waves outside the ultrasonic spectrum could also be used with the present invention
if suitable for the particular application.