Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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VIBRATION ISOLATOR
BACKGROUND OF THE INVENTION
THIS invention relates to vibration isolators for minimising the transfer of
vibration forces from a vibrating body to a body attached thereto. More
specifically, the invention relates to an inertia-type vibration isolator
having a
mechanism for dynamically tuning the isolator within a range of excitation
frequencies. The invention also relates to a method of tuning, and to a method
of operating, a vibration isolator.
For purposes of this specification, the term "vibration isolator" will be used
in
respect of isolating or at least partially isolating a first body from
vibrations in a
second body to which the first body is connected. The invention is not
directed at a vibration absorber mounted on a vibrating body to absorb or
attenuate vibrations of the vibrating body.
The dynamic stiffness of conventional elastomeric or steel spring vibration
isolators generally increases with increased frequency. This is undesirable
because vibration isolators ideally should have a relatively high static
stiffness
for supporting an isolated mass and a relatively low dynamic stiffness for
ensuring low transmission of dynamic forces.
CONFIRMATION COPY
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A number of devices utilising hydraulic mass amplification are known. For
example, US 4,236,607 discloses a spring-tuning mass vibration isolator which
employs hydraulic fluid as an absorber mass, and which relies on displacement
of the hydraulic fluid through a port between two reservoirs to generate
amplified counter-inertial forces for cancelling vibration. The dynamic
stiffness of
this device is less than the static stiffness for a particular frequency band.
A disadvantage associated with vibration isolators of the type disclosed in US
4,236,607 is that they operate at a single frequency only and hence have a
limited effective frequency band. Since the excitation frequency often is a
function of environmental or operating conditions, for instance load, these
vibration isolators cannot always cover the range of excitation frequencies
encountered.
One way of increasing the effective frequency band is to decrease damping.
However, there are physical limits on the amount of damping reduction that can
be achieved in practice. Another way of increasing the effective frequency
band
is to tune the vibration isolator by adjusting the isolation frequency to
achieve a
greater suppression band as opposed to increasing the frequency band at a
fixed isolation frequency. This can be achieved by determining the excitation
frequency and adjusting a system parameter so that the isolation and
excitation
frequencies coincide.
Although the isolation frequency is known to be very sensitive to the port
diameter, this parameter is relatively difficult to adjust in practice due to
the
incompressibility of the absorber fluid. A vibration isolation system in which
isolators are tuned by varying the dimensions of the tuning passage is
disclosed
in US 5,788,029. This patent also discloses a tuning method involving the
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application of magnetohydrodynamic force to the liquid within the tuning
passage.
It is an object of the present invention to provide an alternative inertia-
type
vibration isolator which includes a mechanism for tuning the isolator.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided a method of
tuning a vibration isolator comprising: providing a housing having spatially
opposed reservoirs; providing a port member movable relative to the housing
and providing a port interconnecting the reservoirs in fluid flow
relationship, the
reservoirs and the port providing a closed volume; providing an incompressible
fluid filling said closed volume, in which a vibrating body is connected to
one of
the housing and the port member and an isolated body is connected to the
other of the housing and the port member; rendering the incompressible fluid
in
the closed volume compliant by changing its pressure inside the closed volume;
and adjusting the compliance.
"Compliant" and "compliance" are to be interpreted as the reciprocals of
"stiff"
and "stiffness" which are used in their technical sense, i.e. necessarily
implying
resilience or elasticity.
Expressed in another way, the method may include rendering supporting of the
incompressible fluid mass in the closed volume finitely stiff and adjusting
the
stiffness. The closed volume will generally be of fixed capacity. Thus,
generally,
the housing will be rigid and the port member will be rigid.
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The method may include exposing the incompressible fluid mass in the closed
volume to a resilient force, for example bounding a portion of the closed
volume
by means of a movable partition and subjecting the partition to pressure. In
one
arrangement, the method includes exposing the fluid mass in each of the
respective reservoirs to such a resilient force. The respective resilient
forces
may be applied interdependently, preferably symmetrically.
The method may include providing a tuning chamber proximate each reservoir,
and charging a compressible fluid into the tuning chambers, preferably from a
common source of gas under pressure. Charging gas into the tuning chambers
may be via one or more flow restrictors to enable a desired pressure nominally
to be achieved within the tuning chambers and substantially to prevent
transient
pressure changes in the tuning chambers to be cancelled. Instead, charging the
gas into the tuning chambers may be via valves under the control of a control
arrangement.
Tuning the vibration isolator may be to a desired frequency, e.g. an isolation
frequency which will generally be equal to the vibration frequency of the
vibrating body, which will be referred to as the excitation frequency.
In accordance with a second aspect of the invention, there is provided a
method of operating a vibration isolator, including carrying out the method of
the first aspect continually or continuously to modulate adjustment in
response
to changes in the excitation frequency.
When the vibrating body is in the form of a pneumatic or hydraulic device or
apparatus, such as a rock drill, the method may include effecting tuning in
response to changes in the pneumatic or hydraulic pressure supply to the
device or apparatus. The method may then include using the pneumatic or
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hydraulic pressure supply as a source of pressure for pressurizing the tuning
chambers.
In accordance with a third aspect of the invention, there is provided a
vibration
isolator device including: a housing having spatially opposed reservoirs; a
port
member movable relative to the housing and providing a port interconnecting
the reservoirs in a fluid flow relationship, the reservoirs and the port
providing
an enclosed volume; and an adjustable tuning structure for rendering an
incompressible fluid contained in the enclosed volume compliant to a desired
degree by changing the pressure of the fluid inside the enclosed volume.
The adjustable tuning structure must be understood to be able to render
supporting of an incompressible fluid mass when contained in the closed
volume finitely stiff to a desired degree.
The vibration isolator device may be in the form of a vibration isolator
containing an incompressible fluid filling the closed volume.
The closed volume may be symmetric.
The adjustable tuning structure may include a tuning chamber proximate each
reservoir, a movable partition dividing each tuning chamber and the respective
reservoir, and a charging structure for charging a compressible fluid or gas
into
the respective tuning chambers at variable pressure. In one arrangement, the
movable partition is a diaphragm. Typically, the charging structure is common
to both tuning chambers to ensure equal pressures in the respective tuning
chambers. The charging structure may include at least one orifice through
which gas is charged, the orifice being adapted to allow flow of gas to
achieve a
desired nominal pressure in the tuning chambers and to prevent cancellation of
transient pressure changes in the tuning chambers. Instead, the charging
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structure may include valves for controlling the pressure in the tuning
chambers.
In accordance with a fourth aspect of the invention, there is provided an
apparatus including a vibrating body which vibrates in use, an isolated body
for
holding or mounting the apparatus, and a vibration isolator in accordance with
the invention interconnecting the vibrating body and the isolated body.
The apparatus may include a control arrangement for sensing an excitation
frequency and for controlling the charging arrangement of the vibration
isolator,
in response to said excitation frequency, to tune the vibration isolator to an
isolation frequency corresponding to said excitation frequency.
When the apparatus is in the form of a pneumatically or hydraulically driven
apparatus, the charging system may be exposed to the pressure of the
hydraulic or pneumatic supply automatically to charge the tuning chambers to a
pressure commensurate in a predetermined relation to the hydraulic or
pneumatic supply.
This invention is also applicable to a further class of devices or apparatuses
which, in operation, do not vibrate in the normal sense of the word, but which
undergo sudden movement of short duration, which may be a single
movement, or mainly a single movement, which is periodic, for example a
machine gun or the like.
More specifically, the invention provides a vibration isolator comprising: a
housing which includes housing components resiliently connected to one
another to define a first reservoir, a second reservoir and a port, the port
connecting the first reservoir to the second reservoir and being displaceable
relative to the first and second reservoirs so as to vary the volume of the
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reservoirs in response to relative motion between a vibrating body and an
isolated body along an axis of the housing; a fluid contained within the first
reservoir, the second reservoir and the port; at least one tuning diaphragm
operable on the fluid; and a second fluid pressurizable to adjust the
stiffness of
the at least one tuning diaphragm.
In a preferred embodiment, a tuning diaphragm is operable on the fluid mass in
each of the first and second reservoirs.
The tuning diaphragms may be formed from an elastomeric material, and may
form a partition between a first chamber adjacent the first reservoir, and
between a second chamber adjacent the second reservoir.
In one arrangement, the vibration isolator includes means for connecting the
first chamber and the second chamber to a source of pressurised air. In this
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embodiment, the vibration isolator may include a valve or a choke for
controlling or limiting the flow of air into or out of the first chamber and
the
second chamber.
Conveniently, the vibration isolator includes a control system for
automatically
adjusting the air pressure within the first chamber and the second chamber,
and hence the stiffness of the tuning diaphragms, in response to a change in
the excitation frequency.
Alternatively, the vibration isolator may include means for manually adjusting
the air pressure within the first and second chambers when the excitation
frequency changes.
In yet another embodiment, the vibration isolator may be designed so that the
isolation frequency coincides with the excitation frequency for all values of
supply pressure within a given range.
Typically, the housing comprises a pair of outer housing components which
partly define the first and second reservoirs, and an inner housing component
which defines the port, the inner housing component being resiliently mounted
to the outer housing components for reciprocal motion relative to the first
and
second reservoirs.
The resilient connection between the inner housing component and the outer
housing components may comprise an elastomeric spring, a steel spring or a
pneumatic spring.
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Ideally, the fluid mass comprises a dense, incompressible fluid with a
relatively high surface tension and a relatively low viscosity, for example
liquid
mercury. In this case, the rigid components of the housing may be formed
from stainless steel or may be coated with a protective coating to resist the
corrosive effect of mercury. Other, lower density liquids may also be used
where increases in the dimensions of the vibration isoiator are acceptable.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only,
with
reference to the accompanying drawings in which:
Figure 1 shows, diagrammatically, a cross-sectional view of a vibration
isolator according to the present invention;
Figure 2 is a graph illustrating a roll-off line fitted through the minimum
transmissibility points on a set of transmissibility curves obtained
by varying the diaphragm stiffness; and
Figure 3 is a graph illustrating the isolation frequency as a function of the
supply pressure.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 of the drawings illustrates a vibration isolator 10 according to the
present invention. The isolator 10 is designed for connection between a
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vibrating body (not shown), oscillating along the line 12, and a body 14 to be
isolated. The vibration isolator 10 includes a housing 16 composed of outer
housing components 18 and 20 and an inner housing component 22.
In the illustrated embodiment, the housing components 18 and 20 are spaced
from one another by rigid spacers 24, and the inner housing component 22 is
resiliently connected to the outer housing components with elastomeric,
primary
springs 26 and 28. These springs serve to transfer loads and to form a seal
between the inner housing component 22 and the outer housing components
18 and 20. It will be appreciated that the springs 26 and 28 need not be
formed
from an elastomeric material and that with suitable modifications the sealing
function and the load-transfer function could be separated, in which case
these
springs could be steel springs or pneumatic springs.
The inner and outer housing components are seen in Figure 1 to define a first
reservoir 30, a second reservoir 32 and a port 34 connecting the first
reservoir
to the second reservoir. The reservoirs 30 and 32 and the port 34 contain a
fluid mass which preferably is a dense, incompressible liquid having a
relatively
high surface tension and a relatively low viscosity, for example liquid
mercury.
A diaphragm 36 defines an outer limit to the reservoir 30 and separates this
reservoir from an adjacent chamber 38. Similarly, a diaphragm 40 defines an
outer limit to the reservoir 32 and separates this reservoir from an adjacent
chamber 42. The chambers 38 and 42 are connected to a supply of pressurised
air (not shown) via conduits 46 and a pressure control valve 48, and serve as
air springs for the reservoirs 30 and 32, in use. A Choke in the form of an
orifice
50 limits the flow rate of air into and out of the chamber 38, and a choke
(which
is
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enlarged for clarity in Figure 1) in the form of an orifice 52 limits the flow
rate of
air into and out of the chamber 42.
In practice, the inner housing component 22 is connected to a vibrating body
(not shown) and the outer housing component 18 is connected to an isolated
body which is designated with the reference numeral 14 in Figure 1. It should
be
appreciated that this arrangement could be reversed so that the vibrating body
is connected to the outer housing component 18 and the isolated body is
connected to the inner housing component 22.
The application of vibrationary forces to the inner housing component 22
causes
relative motion between this component and the outer housing components 18
and 20. The volumes of the reservoirs 30 and 32 are alternately increased and
decreased as the port 34 reciprocates relative to these reservoirs, and this
causes the liquid mass to be pumped back and forth through the port 34.
Without the liquid mass, the isolated body 14 is simply suspended by the
primary springs 26 and 28. However, the inertia of the liquid mass modifies
the
dynamic stiffness of the isolator 10 to be less than the static stiffness
thereof
over a particular frequency band.
The purpose of the diaphragms 36 and 40 is to allow for tuning of the
vibration
isolator 10 to the excitation frequency. This is achieved by modifying the
continuity equation for the device which is represented as:
Aby+ 1 Ab(u-y)= -An (x-Y)+(A6-Aa)Y+AaxB
[b_Oi0
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where:
XB represents the displacement of the fluid in the port;
db represents the diameter of the reservoir;
da represents the diameter at the entrance to the port;
Aa represents the area defined by the diameter of the port;
Ab represents the area defined by the diameter of the reservoir;
Ao represents the area defined by the diameter at the entrance to
the port;
x represents the displacement of the port;
y represents the displacement of the reservoir; and
u represents the displacement at the centre of the diaphragm.
In essence, the stiffness of the diaphragms 36 and 40 will control the amount
of
fluid displaced, and the dynamic characteristics of displacement such as
velocity
and acceleration, through the port 34, which in turn will vary the isolation
frequency. In the illustrated embodiment, the stiffness of the diaphragms 36
and
40 is a function of the air pressure within the chambers 38 and 42, and the
stiffness of the elastomeric material forming the diaphragms. If a large
tuning
band and low damping is to be achieved, the diaphragm stiffness must be
minimised.
The orifices 50 and 52 allow air into or out of the chambers 38 and 42 at low
frequencies and minimise this airflow at high frequencies. This allows for
tuning
of the isolator 10 since the pressure changes within each chamber 38 and 42
due to the flexing of the diaphragms 36 and 40 at a given excitation frequency
will occur too quickly to allow substantial airflow across the chokes, but the
average pressure in the chambers 38 and 42 will eventually equal the average
pressure in the conduit 46.
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If a small orifice diameter is selected, the tuning rate will be compromised.
Accordingly, applications requiring rapid tuning will include a more
sophisticated
arrangement, such as a solenoid vaive for sealing the chambers 38 and 42 from
the conduit 46 during steady state operation and allowing relatively high flow
rates across the valve during pressure adjustments.
The tuning may be effected automatically by a control system which determines
the excitation frequency and automatically adjusts the stiffness of the
diaphragms 36 and 40 by increasing or decreasing the air pressure within the
chambers 38 and 42, thereby to ensure that the isolation frequency and the
excitation frequency coincide. This adjustment could also be effected
manually.
In experimental tests, a vibration isolator according to the present invention
with
a liquid mass of water and a glycol additive was connected to a Zonic
hydraulic
actuator and loaded with masses in 10kg increments. The acceleration was
measured on the actuator and on the isolated mass with 100mV/g PCB ICP
accelerometers. A 5Hz bandpass filter was applied to the input and output
signals to ensure single frequency transmissibility measurements, and the
input
accelerometer provided feedback to ensure a constant acceleration input from
10Hz to 70Hz. The tests produced transmissibility curves for which the lowest
recorded coherence was 0.98.
Figure 2 illustrates the transmissibility curve set for a 47.5kg isolated
mass. The
points of minimum transmissibility are connected with a linear curve fit, as
shown. The broken line represents a corresponding curve for a conventional
vibration isolator. The slope of the curves for the various values of isolated
mass
used in the tests are summarised in the Table below:
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Roll-off slopes for tested isolated masses
Mass [kg] Roll-off slope [dB/decade]
17.5 -94.7
27.5 -95.9
37.5 -93.0
47.5 -89.3
For the four isolated masses used, the diaphragm stiffness was found to be the
same and consistently increasing with pressure.
Figure 3 shows the isolation frequency as a function of supply pressure. This
graph shows that the isolation frequency could be changed by 12 Hz. The
relationship between pressure and isolation frequency is given by:
f = -0.280ps2 + 3.787ps +22.473
where:
pS represents the supply pressure in bars; and
f represents the isolation frequency.
The roots of this equation can be used to tune the vibration isolator in a
simple
arrangement where the excitation frequency is measured and the supply
pressure is adjusted to find the closest isolation frequency.
The experimental set-up achieved roll-off values more than twice that of a
conventional isolator over a 12 Hz frequency band.
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It will be understood that the isolator 10 may be applied in a wide range of
different applications. For example, the vibration isolator may be used on
vibrating machinery, such as vibrating mixers and separators, vibration
transport
and processing machines, or vibration crushers; rotating machinery including
turbocompressors and turbopumps; reciprocating machinery; pneumatic hand
tools; and impact machinery such as compactors. The vibration isolator may
also be used in devices undergoing a single, or mainly a single, sudden jerk
which is periodic such as in a machine gun.
A major advantage of the vibration isolator 10 is that it is effective in
isolating
vibration over a relatively broad excitation frequency band by the continual,
dynamic tuning of the device to an isolation frequency corresponding to a
varying excitation frequency. It will also be appreciated that the static
deflection
is constant regardless of the pressure within the chambers 38 and 42.
Furthermore, the isolator 10 can achieve lower transmissibility than a
conventional isolator over a limited frequency band. It will also be
understood
that the vibration isolator according to the present invention can be
manufactured as a relatively small, robust and lightweight unit which is easy
to
handle.
