Shock absorber with Doppler fluid velocity sensor

A hydraulic activator operable to change a suspension characteristic in response to changes in the velocity of damping fluid flowing through a flow passage provided in flow control valving within the shock absorber. The hydraulic activator comprising a pressure cylinder forming a working chamber operable to store damping fluid and a reservoir cylinder coaxial therewith. A piston is disposed within the pressure cylinder defining an upper and lower portion of the working chamber. The activator further comprises a first transducer for emitting ultrasonic waves across the flow passage. The ultrasonic waves emitted by the first transducer are received by a second transducer. A frequency detection circuit determines the difference in frequency between the emitted and received ultrasonic waves and generates an output in response thereto. A central processor calculates the fluid velocity and causes a control circuit to change a suspension characteristic of the shock absorber.

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 determining the relative velocity between 
the telescopically movable components of a hydraulic damping device. 
2. Description of Related Art 
Damping devices ("dampers") are used in conjunction with automotive 
suspension systems to absorb unwanted vibration which occurs during 
driving To absorb this unwanted vibration, dampers are generally connected 
between the sprung mass ("body") and the unsprung mass ("wheel") of the 
automobile. A piston is located within the damper and is connected usually 
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 
dampers. Vehicle handling is related, among other things, to variation in 
the body's attitude (i.e., roll, pitch and yaw). For optimum vehicle 
handling, relatively large damping forces are required to avoid 
excessively rapid variation in the body'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 and 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. 
Various methods are known for selectively changing the damping 
characteristics of a damper in response to an input frequency from the 
road PCT Application No. PCT/US 87/00615 discloses one such method. The 
apparatus used to perform the method comprises a pressure cylinder forming 
a working chamber having first and second portions operable to store 
damping fluid. The apparatus further comprises a first valve for 
controlling the flow of damping fluid between the first and second 
portions of the working chamber during compression of the apparatus. In 
addition, the apparatus also comprises a pressure chamber in fluid 
communication with the first portion of the working chamber and the first 
valve. A solenoid is also provided for regulating the flow of damping 
fluid between the pressure chamber and the second portion of the working 
chamber. A second valve is further provided for controlling the flow of 
damping fluid between the first and second portions of the working chamber 
during rebound of the apparatus. 
When such methods are used for changing the damping characteristics of a 
damper, they often require 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 West German patent No. G 87 02 817.4 disclose suspension 
damping devices incorporating ultrasonic 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. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
method for sensing the velocity of damping fluid flowing through orifices 
provided in the telescopically movable components of a hydraulic actuator 
so as to provide information for selectively controlling the damping, 
motion, position and/or forces generated by the actuator. 
Another object of the present invention is to provide a hydraulic damper or 
actuator capable of providing fluid velocity information using ultrasonic 
sound waves. 
Another object of the present invention is to provide a method and 
apparatus for 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. 
It is a further object of the present invention to provide a method and 
apparatus for selectively controlling suspension characteristics of a 
vehicle in response to changes in the fluid velocity of damping fluid 
flowing through system piping, the piston valving and/or base valving 
using a sonar, preferably within the ultrasonic spectrum. 
A further object of the present invention is to provide a direct acting, 
telescopic, hydraulic shock absorber having a high degree of flexibility 
with respect to vehicular applications. In this regard, a related object 
of the present invention is to provide a hydraulic actuator which is 
relatively low in cost and relatively easy to maintain. 
According to the preferred embodiment of the present invention, the 
hydraulic actuator comprises a direct acting hydraulic damper having first 
transducer means for emitting sound waves and second transducer means for 
receiving sound waves. The first and second transducer means are mounted 
to opposite lateral surfaces of an orifice or flow passage, such as that 
provided in piston valving which is coaxially disposed within the pressure 
cylinder of the shock absorber. Electrical leads passing through the 
piston rod and piston connect the individual transducers to signal 
generating and processing circuitry. 
According to the method of the present invention, a wave generating circuit 
excites the first transducer means ("transmitter") so as to produce a 
constant frequency ultrasonic wave. The transmitter emits ultrasonic waves 
of a predefined frequency and duration through the damping fluid medium 
flowing through an orifice or passage during operation of the suspension 
system. The emitted ultrasonic waves are received by the second transducer 
means ("receiver") after propagating through the damping fluid. The 
receiver electrically communicates with a frequency detection circuit 
which detects the frequency of the received ultrasonic waves. 
Additionally, the wave generating circuit electrically communicates with 
the frequency detection circuit to provide a reference wave frequency 
emitted by the transmitter. 
By using the difference in frequency between the emitted ultrasonic waves 
and the received ultrasonic waves, the velocity of the fluid flowing 
through an orifice in the piston valve (or base valve) or through a 
hydraulic line connecting the damper to an accumulator can be calculated 
by using a central processor. Accordingly, a continuous velocity 
determination can therefore be generated which ca be used by a piston 
control circuit to control the damping forces of the shock absorber. Such 
fluid velocity determinations 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 present invention is readily adapted to mono-tubes, 
struts, and other hydraulic actuators having vehicular application.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 the dampers 20 and by the 
helical coil springs 38. 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 automotive 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 58 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. Pat. No. 3,771,626 
which is hereby incorporated by reference. It is contemplated, however, 
that the present invention is applicable to hydraulic actuator operable 
with or without base valves, check valve or the like. 
The upper and lower ends of the damper 20 are provided with generally 
cup-shape upper and lower end caps 58 and 60 respectively. The end caps 58 
and 80 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 having bushings 65 confined 
therein 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 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 which is 
hereby incorporated by reference. It is contemplated, however, that the 
present invention may be used with other suitable valve arrangements as 
well as other suitable damping devices. 
In accordance with the principles of the first preferred embodiment, the 
damper 20 further comprises an acoustical transmitter 66 and a receiver 
68, both of which are secured to opposite lateral surfaces of orifice 45 
which extends axially through piston 44. More particularly, transmitter 66 
and receiver 68 are mounted in recessed passages 69. (See FIGS. 3 and 4). 
The transmitter 66 is used to generate ultrasonic waves having a 
predetermined resonant frequency f.sub.1 in a direction generally 
transverse to the flow of fluid through orifice 45. Ultrasonics is the 
name given to sound waves having a frequency higher than those to which 
the human ear can respond (approximately 16 KHz). The propagation of sound 
waves through a relatively non-absorptive mediam (damping fluid) involves 
the generation of vibrations in the elementary particles of the medium 
(damping fluid) through which the waves are propagating. While the 
transmitter 66 may be piezoelectric or magnetorestrictive device, other 
suitable devices may be used. 
When the ultrasonic waves emitted by the transmitter 66 encounters the 
fluid flowing through orifice 45, their wavelength and frequency are 
modified proportionately to the fluid velocity. The receiver 68 is used to 
receive the ultrasonic waves of frequency f.sub.2 and generate an output 
in response thereto. While the receiver 68 may be a piezoelectric or 
magnetorestrictive device, other suitable devices may be used. 
For purposes of the following discussion, the ultrasonic waves generated by 
the transducer will be referred to as the "emitted" ultrasonic waves, 
while the ultrasonic waves received by the receiver 68 will be referred to 
as the "received" ultrasonic wave. When the piston 44 is stationary 
(static) with respect to the base valve 52, the received ultrasonic waves 
will have the same frequency as the emitted ultrasonic waves. When the 
piston 44 is stationary no flow is occurring and therefore the frequency 
of the transmitted wave f.sub.1 will not be modified. As such, f.sub.2 
should substantially equal f.sub.1. However, when the piston 44 is moving 
in a direction toward or away from the base valve 52, the frequency of the 
received ultrasonic waves will be different than the frequency of the 
emitted ultrasonic waves. In short, the wavelength of the received 
ultrasonic waves propagating through flowing fluid will be modified 
proportionately to the fluid velocity. This phenomenon is referred to as 
the Doppler Effect. As such the Doppler effect can be utilized in 
flowmeter devices to directly measure fluid velocity. 
By using the Doppler Effect, the velocity of the damping fluid flowing 
through either the piston valving, base valving or any system passage may 
be determined. In this regard, the velocity of the fluid flowing within 
the system may be calculated according to either of the following 
equations: 
##EQU1## 
where: 
f.sub.1 =frequency of the "emitted" ultrasonic waves generated by the 
transmitter 66. 
f.sub.2 =frequency of the "received" ultrasonic waves received by the 
receiver 68. 
f.sub.b ="beating frequency" defined as f.sub.2 -f.sub.1 
v=velocity of the fluid flowing through valving orifice 45. 
c=velocity of wave propagation through the damping fluid. 
The velocity of wave propagation "c" is largely dependent on the 
characteristics of the damping medium through which the waves propagate. 
While the preferred embodiment of the present invention uses hydraulic 
damping fluid as the damping medium, it is contemplated that the present 
invention can be readily adaptable to other suitable damping mediums as 
well. 
To provide means for driving the transmitter 66, a wave generating circuit 
72 is provided. The wave generating circuit 72 is electrically connected 
to the transmitter 66 so as to enable the transmitter 66 to produce 
emitted ultrasonic waves of a predetermined frequency f.sub.1. When the 
piezoelectric crystal (not shown) of the transmitter 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 to 
stop vibrating once the electrical excitation has been removed. 
Consequently, the transmitter 66 should have a small modulation pulse 
width thereby permitting operation when there is a maximum velocity of 
fluid flowing through orifice 45. The control strategy is also able to 
discriminate between a first received wave and subsequent echoes due to 
the time between transmitted pulses being set longer than the acoustical 
damping time of the fluid. Preferably, the wave generating circuit 72 
causes the emitted ultrasonic waves generated by the transmitter 66 to be 
of continuous form so as to enable efficient, continuous, real-time 
determinations of the fluid velocity flowing within damper 20 during 
movement of the vehicle suspension. While the wave generating circuit 72 
may be an oscillator, other types of wave generating circuits such as 
pulse wave generators may be used. 
To provide means for detecting and measuring the change in frequency 
between the emitted ultrasonic waves f.sub.1 and the received ultrasonic 
waves f.sub.2, the damper 20 further comprises a frequency detection 
circuit 80. As shown in FIGS. 3 and 4, the frequency detection circuit 80 
receive the output f.sub.1 from the wave generating circuit 72 as well as 
f.sub.2 from the receiver 68. The frequency detection circuit 80 
determines the change in frequency by adding or .-+.superimposing" the 
outputs from the wave generating circuit 72 and the receiver 68. 
Superimposition of the outputs corresponding to the emitted ultrasonic 
waves and the received ultrasonic waves is a frequency detection method 
commonly utilized in Doppler radar systems. As shown in equation (2), the 
superimposed "beat" frequency f.sub.b is linearly proportional to the 
fluid velocity v, so as to permit continuous detection of changes thereof. 
It is to be understood, however, that other suitable means for measuring 
the difference between the frequency of the reflected ultrasonic waves and 
the emitted ultrasonic waves may be used. 
Upon determination of the "beat" frequency, the velocity of the fluid 
flowing through piston valve orifice 45 is calculated in the manner 
discussed above. Referring to FIG. 4, means for calculating the velocity 
of the fluid comprising a central electronic processor ("computer") 90 is 
disclosed. The computer 90 uses the output from the frequency detection 
circuit 80 to calculate the fluid velocity. After the velocity of the 
fluid has been determined by the computer 90, the computer 90 generates an 
output in response to the velocity calculation which may be used in 
various algorithms requiring information on fluid velocity. One such 
control scheme is to deliver the velocity calculation to a piston control 
circuit 100. The piston control circuit 100 then acts to change the 
damping characteristics of the piston 44 to obtain the desired road 
handling characteristics. While the preferred embodiment discloses the use 
of a piston control circuit to selectively vary damping characteristics, 
it is to be understood that other suspension control circuitry associated 
with suspension actuators are readily adaptable. 
Velocity determinations can also be employed to provide positional and/or 
force information applicable to controlling active or dynamic leveling 
actuators instead of, or in addition to, damping control. 
It is contemplated that the present invention may comprise various methods, 
currently utilized in doppler systems, for adjusting the velocity 
determination "v" to compensate for frequency changes due to variations in 
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. Generally, such velocity correction methods include 
means for compensating for changes in the speed of wave propagation 
through the damping fluid due to the temperature and pressure effects on 
viscosity. One non-limiting example would include the application of a 
thermocouple device positioned within damper 20 to measure temperature 
variations. Such information would permit use of "look-up" tables by 
computer 90 to compensate for the temperature dependence of "c". While 
commonly used damping fluids have sufficiently low attenuation properties 
of up to about a frequency of 3 MHz, compensation means are still 
preferably used. 
In reference to FIG. 5, a second embodiment of the present invention is 
disclosed. Specifically, shock absorber 200 comprises an acoustical 
transducer 202 acting as a transmitter/receiver which is secured to a 
lower surface 204 of end cap 60 substantially below, and in axial 
alignment with, base valve 52. The transducer 202 emits an ultrasonic 
sound wave of frequency f.sub.1 in the direction of a lower surface of 
base valve 52. The emitted sound waves of frequency f.sub.1 are 
back-scattered by elementary particles or air cavitation bubbles flowing 
with the fluid through base valve 52. When the piston 44 is stationary 
relative to the base valve 52, the frequency f.sub.2 of the received waves 
reaching the transducer 202 will be substantially similar to f.sub.1. As 
previously described, when the ultrasonic means of frequency f.sub.1 
encounters fluid flowing through an orifice in base valve 52, their 
frequency and wavelength are modified proportionately to the fluid 
velocity. 
In order to use a single transducer 202 to emit and receive the ultrasonic 
sound waves, the transducer 202 must be capable of attentuating sound wave 
oscillations after transmission of wave f.sub.1 in order to prevent 
overlap of the emitted and received pulses. Consequently the transducer 
should have a small modulation pulse width. However, it should be 
appreciated that an individual transmitter and receiver, aligned in 
adjacent orientation coulbe be employed. Likewise, while transducer 202 is 
shown centrally positioned relative to base valve 52 it is contemplated 
that the transducer can be positioned in any orientation which generates 
adequate sound wave reflections. 
Preferably, total fluid flow through piston valve orifice 45 or base valve 
52 should be measured. However, if the fluid flow paths in the piston 
valving of the base valve are symmetrical and substantially identical, it 
is possible to measure fluid velocity flowing through only one such 
orifice. Likewise, transducers in separate orifices for rebound and 
compression motion may be necessary if the piston valving warrants such 
structure. 
While it is apparent that the preferred embodiments illustrated herein are 
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 frequency detection circuit 80, the computer 90 
and wave generating circuit 70 may all be located either internal or 
external with respect to the damper. If located externally of the damper 
20, a single computer 90 may be used to calculate the fluid velocity and 
control any suspension parameter (leveling, damping, springing, etc.) for 
each of the dampers in the vehicle suspension. Likewise, it is 
contemplated that the present invention can be employed in any passage or 
orifice through which fluid velocity changes correspond to a desired 
suspension characteristic.