Note: Descriptions are shown in the official language in which they were submitted.
CA 02485830 1994-12-O1
Title: VIBRATION ISOLATION SYSTEM
TECHNICAL FIELD
The present invention relates to ~ system which ~ainimizes
the transfer of vibration forces and moments from a vibrating
body to a body attached thereto.
BACKGROUND ART
Vibration in helicopters causes many undesirable effects.
These include: crew fatigue, resulting in decreased
proficiency unacceptable passenger discomfort; decreased
component reliability, resulting in increased operating costs;
and, in many cases, limited maximum cruising speed.
The main rotor-transmission assembly (the "pylon") is a
major source of helicopter vibration. In operation, the rotor
causes pylon vibration in all six degrees of freedom; that is,
vertical, lateral, and longitudinal forces, and roll, pitch,
and yaw moments. The predominant pylon vibration harmonic
occurs at the blade passage frequency (thae "b/rev frequency" ) ,
which is equal to the number of rotor blades times the angular
velocity of the rotor.
Early pylon mounting systems resulted in fuselage
vibration levels which exceeded 0.5 g. Next, isolation systems
using focal isolation mounts were generally able to limit b/rev
fuselage vibration at cruise airspeeds to about 0.158, but
vibration at transition airspeeds (approximately 0 to 25 knots)
exceeded that level.
During the 1970's, the U. S . military reduced the b/rev
vibration standard to 0.05 g at cruise airspeeds. Several
pylon isolation systems which included one or more
antiresonant, force-cancelling devices were developed in an
attempt to meet that standard. Each of the force-cancelling
devices included a spring and a mechanically-amplified tuning
mass to partially or completely cancel pylon b/rev vibration in
one degree of freedom. Systems combining one or more such
force-cancelling devices with focal mounts were able to effeet
at least partial isolation in up to five degrees of freedom.
However, such systems were complex, expensive, required
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CA 02485830 1994-12-O1
considerable space, and imposed a weight penalty which varied
from 2-3% of helicopter design weight; and none of the systems
was able to meet the 0.05 g standard.
U.S. Patent No. 4,236,607 (Halwes et al.) discloses a
spring-tuning mass vibration isolatcar in which force
cancellation is accomplished by hydraulically amplifying the
inertia of a liquid tuning mass. The Halwes Liquid Inertial
Vibration Eliminator ("LIVE") isolator 1 is shown schematically
in cross section in Fig. 1. A layer of low-damped rubber 3 is
bonded between the outer surface of a piston 5 and the inner
surface of a cylinder 7. The rubber 3 acts as an elastomeric
spring and as a liquid seal. Upper and lower end caps 9, 1l,
respectively, are secured to the ends of the cylinder 5 to
prevent fluid leakage from the interior thereof, thereby
forming two chambers 13, 15. The chambers 13, 15 are connected
by a tuning passage 17 in the piston 5, and the chambers 13, 15
and the tuning passage 1.7 are filled with a high-density,
incompressible, low-viscosity fluid such as, for example,
liquid mercury.
A vibrating body 19 is attached to the upper end cap 9.
-The vibrating body oscillates in the direction indicated by
arrow 21. A body 23 to be isolated from the vibration (the
"isolated body") of the vibrating body l9 is connected to the
piston 5 by means of a bracket 25 and lugs 27. The oscillatory
force produced by the vibrating body 19 in the direction 21
causes relative motion between the piston 5 and the cylinder 7.
That relative motion creates an oscillatory reaction force due
to strain in the rubber spring 3. At the same dyne, the
volumes of the chambers 13, 15 are alternately increased and
decreased, and the liquid contained in the chambers 13 ,15 and
the tuning passage 17 is pumped back and forth between the
chambers 13, 15 through the tuning passage 17. The inertial
mass of the liquid in the tuning passage 17 (the "tuning mass")
is amplified by the ratio of the effective cross-sectional area
of the piston 5 and rubber spring 3 (t.he "effective piston
area") to the cross-sectional area of the tuning passage 17
(the "tuning passage area"). The inertial force created by
2
CA 02485830 1994-12-O1
acceleration of the tuning mass is out of phase with the
reaction force of the rubber spring 3. In a system with no
damping, at some frequency (the "isolation frequency"), the
inertial force becomes equal and opposite to the spring force,
complete force cancellation occurs, and no vibration is
transferred to the isolated body 23. In a system having
damping, complete force cancellation does not occur, but
minimum transfer of vibration to the isolated body occurs at
the isolation frequency. The isolation frequency for the
vibrating body-LIVE isolator-isolated body system, ft, is
calculated as follows:
__ _1 k
2Tt R(R-I)L ~ ~' ~'hele
k = the spring rate of the rubber spring;
R - the ratio of the effective piston area to the tuning
passage area;
L = the length of the tuning passage;
A = the tuning passage area; and
p = the mass density of the liquid.
As the isolation effect of the LIVE isolator is dependent
on the inertial effect caused by pumping the liquid, the LIVE
isolator is effective only along an axis which is perpendicular
to, and passes through the~geometric center of, the effective
piston area (the "operating axis"). Thus, when a vibration
force is applied to a LIVE isolator 1 along other than the
isolator's operating axis, only the component of the vibration
force along the operating axis is~isolated.
Curve 27 in Fig. 2 shows a plot of the frequency response
of the isolated body 25 along the operating axis of the LIVE
oscillator 1. The curve 27 is for a system having
approximately one percent critical damping. Line 29 represents
the response of the isolated body 25 along the operating axis
for an equivalent rigid-body system. As can be seen, a
relatively narrow isolation "notch" 30 in the curve 27 occurs
in the vicinity of the isolation frequency, fI (also known as
the antiresonance frequency). Maximum isolation (minimum
3
CA 02485830 1994-12-O1
isolated body response) is 99% at the isolation frequency; that
is, only 1% of the vibrating body force is transferred to the
isolated body at the isolation frequency.
The Halwes LIVE isolator 1 is rugged, compact,
lightweight, self-contained, and provides excellent vibration
isolation along its operating axis for vibration near the
isolation frequency of the vibrating body-LIVE isolator
isolated body system.
Other inertial isolators are known in the art. See, for
example, U.S. Patent No. 4,811,919 (3ones) and U.S. Patent No.
5,1'74,552 (Hodgson et al.), each of which show inertial
isolators having an external tuning passage. However, in each
inertial isolator other that the Halwes LIVE isolator, the
configuration of the tuning passage is such that it presents a
greater resistance to liquid flow than in the LIVE isolator.
That flow resistance increases the damping of the isolator,
which decreases the isolator's effectiveness.
D.R. Halwes, "Total Main Rotor Isolation System," Bell
Helicopter Textron Inc. (1981) and D.R. Halwes, "Controlling
the Dynamic Environanent During NOE Flight," Bell Helicopter
Textron Inc. (1985) describes a six degree of freedom ("6 DOF")
helicopter pylon isolation system. The system uses six LIVE
isolator links to attach the pylon to the fuselage and to
isolate b/rev vibration in all six degrees of freedom.
Although the 6 DOF system provides b/rev isolation that is
superior to that of previous systems, it has several
shortcomings. First, to ensure that each LIVE isolator is
exposed only to forces along its operating axis, each LIVE
isolator link is pinned at each of its ends by means of
elastomeric bearings. In addition to being expensive, the
elastomeric bearings introduce additional damping into the
pylon-LIVE isolator-fuselage system, which decreases the
effectiveness of the isolators.
Second, since the effectiveness of a LIVE isolator
decreases markedly as isolation frequency varies from the
vibration frequency, relatively minor changes in the 6 DOF
system which effect the isolation frequency produce a
4
CA 02485830 1994-12-O1
relatively large decrease in the system's effectiveness: Such
changes include: changes in the isolators and elastomeric
bearings due to aging; changes in the properties of the rubber
springs, elastomeric bearings, and liquid due to the
temperature variations; and variations between the system's
isolators due to manufacturing tolerances. In addition, a
helicopter's main rotor is often operated at other than its
nominal rotational speed, resulting in a mismatch between the
main rotor b/rev frequency and the 6 DOF system's isolation
frequency, which decreases the isolator;a' effectiveness: A
means for adjusting the 6 DOF system's isolators during
operation to compensate for such changes and to match the
system's isolation freguency to the main rotor b/rev frequency
would allow the system to provide optimal b/rev isolation at
1 all times.
The isolation frequency of a system which includes a LIVE
or other inertial isolator can be changed by changing the
length or the cross-sectional area of the isolator's inertial
passage. For example, see U.S. Patent No. 4,969,632 (Hodgson
et al.) and U.S. Patent No. 4,641,808 (Flower). However, the
prior art means for effecting such changes significantly
increase isolator damping, which decreases the isoiatoros
effectiveness. A means for changing isolator inertial passage
length or cross-sectional area without significantly increasing
damping would allow the system's isolation frequency to be
changed while maintaining high isolator effectiveness.
Third, the 6 DOF system is relatively space-consuming. A
more compact system would save both space and weight.
5
CA 02485830 2006-08-11
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f~abl~ d~ai~cia &~i7~~i~~~~ f ~a~ any ~r
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i~at~a~r~ ~t~ m~.sl<f.~~;~~ ~ v~~~~~~~tn t~»a»i~i~f;
~tli~crp~C~~'~ pyl~~ ~t~ ~t~ g~~t~e~qa~ a~~n.
Tt~~ ~~rt~~l~ s~ul~~~~~cii,~o~~~~t~r~ ~a~~ cc~~ri~~ ~t~s ~ia~o~i~-
t~ ~f.»e~~.~~;~~~~~ ~it~ ~~ix ~r~~i
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~~h r~in~~lc-ai~i~ i~v~,at~>~ 3~ tined .>~~ ma~n~ raf ~~ ~ti.~,~,i
~~r>u~abi~ ~i~~v~ ~hif~t ~ p~~ct ~f r.is~aii >~t ir,
isa~ax~to~~' s ~~,f.~~t p~~a~~w,~ g~~s * 1 ~~~r~-~ lcot~r ~ ~ ~~t ir~~r ~.tt>r
a~~s~h
a ra~~k aid g~.~t.~.~r~ ~~a~ ~ri~r ~~~~r~~s arid rot.~r~~~ th.~ ~~.na~~,
t~a~~i~g ~~~~i~g es~ ~~ing~ ~~.:yf the iia~i~a>e~
frequency of the ~fra~.~a-~x~.~ i~l~~c~r.
"1".kt~ i.~~al~'~esr e~~~e~ Pa~~c~~~.~~ ~r~,b~at~s~~, ~,o~.~~"~n ~.rt ~~..~
~i~ ~r~gre~>f ~~~.
In accordance with a first broad aspect of the
present invention, there is provided in a hydraulic
inertial vibration isolator having a tuning passage, a
magnetically influencable liquid and a generator which
generates an alternating magnetic field which acts
upon said liquid in said tuning passage, thereby
allowing the isolation frequency of said isolator to
be adjusted and allowing energy to be added to the
liquid to compensate for damping or other energy
losses.
In accordance with a second broad aspect of the
present invention, there is provided a vibration
isolation system comprising a plurality of multi-axis
hydraulic inertial vibration isolators connected
between a vibrating body and an isolated body.
6
DOCSMTL: 2165859\1
CA 02485830 1994-12-O1
BRIEF DESORIPTION OF DRAWINGS
An embodiment of the invention will be described, by way
of example only, with reference to the accompanying drawings,
in which:
Fig. I is a side sectional view of the prior art Halves
LIVE isolator;
Fig. 2 is a plot of the freguency response of an isolated
body along the effective axis of a LIVE isolator;
Fig. 3 is a sectional side view of a tunable dual-axis
LIVE isolator in accordance with the present invention;
Fig. 4 is a sectional view of the isolator taken through
plane 4-4 in Fig. 3;
Fig. 5 is an enlarged sectional side view of the vertical
tuning passage of the isolator of Figs. 3 and 4;
Fig. 6 a sectional view of the sleeve-actuation means
taken through plane 6-6 in Fig. 5;
Fig. 7 is a sectional side view of an alternate means for
tuning the isolator;
Fig. 8 is a sectional view of the alternate tuning means
taken through plane 8-8 in Fig. 7;
Fig. 9 is a top view of the alternate tuning weans of
Figs. 7 and 8, shown with the radially movable member in its
most inward position;
Fig. 10 is a top view of the alternate tuning means of
Figs. 7 and 8, shown with the radially mavable member at its
most outward position;
Fig. i1 is a sectional side view of another alternate
means for tuning the isolator;
Fig. 12 is a sectional view of the other alternate means
of Fig. 11 taken through plane 12-1~;
Fig. 13 is a side view of a vibration isolation system for
a helicopter pylon which includes four tunable dual-axis
isolators;
Fig. I4 is a top view of the vibration isolation system of
Fig. 13;
Fig. 15 is a top view of a vibration isolation system for
a helicopter pylon which includes three tunable dual-axis
7
S
CA 02485830 1994-12-O1
isolators;
Fig.. 16 is a top view of a five-degree-of-freedom
vibration isolation system for a helicopter pylon which
includes three tunable dual-axis isolators;
Fig. 17 is a diagrammatic representation of the cruciform
member and rubber springs of the isolators shown in Figs. 3 and
4;
Fig. 18 is a diagrammatic perspective view of the
vibration isolation system of Figs. 13 and 14 showing the
system°s vertical spring rates;
Fig. 19 is a diagrammatic top view of the vibration
isolation system of Figs. 13 and 14 showing the systems spring
rates in the horizontal plane; and
Fig. 20 is a block diagram of the control system for the
helicopter pylon vibration isolation system.
8
CA 02485830 1994-12-O1
BEST MODE FOR CARRYING OUT T~iE INVENTION
Figs. 3 and 4 are sectional views of a tunable dual-axis
LIVE isolator 31 embodying the present invention. The dual-
axis isolator 31 comprises a vertical single-axis isolator 33
and a horizontal single-axis isolator 35 having their operating
axes 37, 39, respectively, disposed substantially orthogonally
to each other. A cruciform member 41 serves as. pistons for
both the single-axis isolators 33, 35. A rubber spring 43 is
bonded to an outer surface of an upper portion 45 of the
cruciform member 41 and an inner surface of an .upper cap 47,
thereby forming an upper chamber 49. Another rubber spring 51
is bonded to an outer surface of a lawer portion 53 of the
cruciform member 41 and to an inner surface of a lower cap 55,
farming a lower chamber 57. Similarly, a rubber spring 59, a
-5 left portion 61 of the cruciform member 41, and a left cap 63
form a left chamber 65p and a rubber spring 67, a right portion
69 of the cruciform member 41, and a right cap 7I form a right
chamber 73.
It will be appreciated that the upper and lower caps 47,
55 must be restrained from moving relative to each other for
the vertical isolator 33 to function as a LIVE isolator. When
so restrained, the caps 47, 55 perform the same functions as
the cylinder 7 and, end caps 9, 11 shown in Fig. 1. Similarly,
the left and right caps 63, 71 must be restrained from moving
relative to each other for the horizontal oscillator 35 to
function as a LIVE isolator. As described below in connection
with Figs. 13 and 14, the caps 47, 55, 63, 71 are so restrained
by structures which are not shown in Figs. 3 and 4.
The dual-axis isolators of this embodiment of the
invention are essentially two single-axis LIVE isolators
attached together With their operating axes at an angle to each
other. Preferably the pistons of the single-axis isolators are
attached together, but it will be appreciated that other parts
of the isolators can be attached, such as the piston of one
isolator to the caps of the other isolator, or the caps of one
isolator to the caps of the other isolator. Although the
operating axes of the single-axis isolators are preferably
9
CA 02485830 1994-12-O1
perpendicular to each other, this is not required. So long as
the axes are at some angle to each other, i.e., not parallel,
the dual-axis isolator will function to isolate vibration in
two axes. Finally, while the isolators used in this embodiment
of the invention are Halwes LIVE isolators, it will be
appreciated that any inertial isolator can be used. Further,
it will be apparent to those skilled :in the art that three
single-axis isolators can be connected together with the
operating axis of each isolator at an angle to the operating
to axes of the two other isolators, thereby forming a three-axis
isolator.
A vertical tuning port 75 through the cruciform member 41
connects the upper and lower chambers 49, 57. The tuning port
75 and chambers 49, 57 are filled with liquid through a
removable plug 77 in the upper cap 47. Similarly, a horizontal
tuning post 79 through the cruciform member 41 connects the
, left and right chambers 63, 71. The tuning port 79 and
chambers 63, 71 are filled with liquid through a removable plug
81 in the right cap 71.
20 To minimize damping, the rubber springs 43, 51, 59, 67 are
constructed of low-damped, broad temperature rubber. In
addition, the inlets to the tuning ports 75, 79 are carefully
designed to ensure smooth flow of the liquid.
Although mercury is an ideal fluid for a LIVE isolator in
25 some respects, it has two serious shortcomings: it is toxic
and it is corrosive. For that reason, this embodiment of the
invention uses a low-viscosity, incompressible,
environmentally-safe lipoid. Although the liquid is not as
dense as mercury, resulting in a somewhat larger isolator 31,
30 the liquid is neither toxic nor corrosive.
The vertical and horizontal tuning passages 75, 79 each
includes an axially-extendable, slidable sleeve 83, 85,
respectively, for tuning the respective isolator 33 or 35 to a
range of isolation frequencies. The slidable sleeves 83, 85
35 are shown in their fully-extended positions in Figs: 3 and 4.
Refer now to Figs. 5 and 6. The vertical sleeve 83 is
slidably disposed within a vertical orifice 8'7 in the cruciform
CA 02485830 1994-12-O1
member 41. Thus, the vertical passage 75 is formed by the
inner surface of the sleeve 83 and a portion of the inner
surface of the orifice 87. When the sleeve is fully retracted,
its upper end 89 is disposed within a, complimentary-shaped
recess 91 in an upper surface 93 of the crtaciform member 41.
To minimize damping, the upper end 89 and the lower end 95
of the sleeve 83 are shaped to ensure smooth fluid flow. In
addition, to minimize the effect on isolation frequency due to
the difference between the inside diameters of the sleeve 83
and the orifice 87, the wall thickness of the sleeve 83 is as
thin as possible, consistent with adequate strength.
An axially-extending rack gear 97 is attached to an outer
surface of the sleeve 83 by suitable means, such as by bonding
or conventional fasteners. Alternately, the rack gear 97 may
be an integral portion of the sleeve 83. A suitably-shaped and
suitably-dimensioned void 99 is provided in the inner surface
. of the orifice to allow axial movement of the rack gear 97 to
the extent required to move the sleeve 83 between its fully-
extended and fully-retracted positions" A pinion gear 101
engages the rack gear 99. The pinion gear is connected to an
output shaft 103 of a small electric motor 105. When the motor
105 rotates the pinion 101, the rack gear 97 moves axially,
extending or retracting the sleeve 83. While an electric motor
is used in this embodiment, any suitable rotary actuator, such
as one powered by hydraulic or pneumatic pressure, may be used.
Further, this feature of the invention is not limited to
positioning the sleeve 83 by means of the described rotary
actuator-gear system. Any suitable means for extending and
retracting the sleeve 83, such as a linear actuator, is
included within the scope of the invention.
The electric motor 105 and, thus, the position of the
sleeve 83, is controlled by a control system which responds to
vibration. The control system is discussed below in connection
with Fig. 20.
The slidable sleeve 85 included in the horizontal tuning
passage 79 is actuated by another electric motor 107 (Fig. 4)
in the same manner as described above.
11
CA 02485830 1994-12-O1
As the isolation frequency along the operating axis of a
LIVE isolator is inversely related to the square root of the
volume of the isolator's tuning passage, extending or
retracting a sleeve 83 or 85 decreases or increases,
respectively, the isolation frequency of the respective single-
axis isolator 33, 35 ~ Thus, the sleeves 83, 85 provide a means
to tune the vertical and horizontal isolators 33, 35 to a range
of isolation frequencies.
Figs. 7-10 illustrate an alternate means f~r tuning the
I0 single-axis isolators 33, 35 shown in Fig. 3 and 4. For
clarity, only the piston of the vertical isolator 33 is shown.
The inner surface of a radially movable member 109 forms
a portion of the wall of the vertical passage 75. The
adjustable member 109 is slidably disposed within a vertical
channel I10 in the vertical piston 33. An upper extension 111
of the adjustable member 109 is slidably disposed within an
upper horizontal recess 112 in the upper surface of the
vertical piston 33. Similarly, a lower extension 1I3 of the
adjustable member 109 is slidably disposed in a lower
20 horizontal recess 114.
A threaded output shaft 116 of an electric motor 117
engages a threaded blind orifice 118 in the movable member 109.
When the motor 117 is operated, the output shaft 116 cooperates
with the blind orifice 118 to move the movable member inwardly
25 or outwardly relative to the vertical axis of the passage 75.
Figs. 9 and 10 show the movable member 109 in its most inward
and most outward positions, respectively.
An orifice 115 in the upper horizontal extension 111
allows liquid to enter and exit the portion of the channel I10
30 not occupied by the movable member 1019. Thus, the volume
available for liquid within the vertical isolator 33 remains
substantially constant as the movable member is moved inwardly
and outwardly. It will be appreciated that the orifice 115 can
be located in the lower horizontal extension 113, rather than
35 in the upper horizontal extension 111. F3owever, to preclude
transfer of liquid between the upper and lower reservoirs 13,
through the vertical channel 110, only one of the horizontal
I2
CA 02485830 1994-12-O1
extensions 111, 113 includes an orifice: 115:
Recall the equation for isolation frequency:
1 ~ _
2ft R(R-1)L A p' ''here
k = the spring rate of the rubber spring;
R - the ratio of the effective piston area to the tuning
passage area;
L = the length of the tuning passage;
A = the tuning passage area; and
p = the mass density of the liquid.
Def fining
R = ~, Wheze
5
the effective piston area,
allows the isolation frequency to be expressed as follows:
f1 = _I Ic - _1 k~
2'~ C BaA~BA) .L A p ~2~ ( A2 - B) L P
Thus , decreas inch
the cross-sectional area, A, of the vertical passage 75 by
moving the movable member 109 inwardly eiecreases the isolation
frequency of the vertical isolator 3.3; and increasing the
cross-sectional area by moving the movable member 109 outwardly
increases the isolation frequency. As a similar structure is
provided for the horizontal isolator 3:5 (Figs. 3 and 4), the
vertical and horizontal isolators 33, 35 can be tuned to a
range of isolation frequencies.
Figs. 1l and 12 illustrate another alternate means for
tuning the single-axis isolators 33, 55~ (Figs. 3 and 4). For
clarity, only the piston of the vertica:~ isolator 33 is shown..
An electromagnetic coil llfi is disposed around the tuning
passage 75. When an electrical voltage is applied across
terminals 117, I18, current flows in the coil 116, thereby
generating a magnetic field in the: tuning passage 75..
13
CA 02485830 1994-12-O1
Depending on the polarity of the voltage applied to the
terminals 117, 118, the direction of the magnetic field is one
of the directions indicated by the two-~headed arrow 119. In
this embodiment, the liquid filling the passage 75 and the
upper and lower chambers 49, 53 (Fig. 3) is an electrolytic
solution (i.e., an ionized liquid, such as saline salution) or
an electrically-conducting liquid metal such as mercury.
Magnetohydrodynamic ( "MfiD'° ) force acts 'upon the liquid in the
tuning passage 75 when the coil 116 is activated. The
magnitude and direction of the MHD force: is dependent upon the
magnitude and direction of the current flowing through the coil
116.
Applying an alternating voltage to the terminals 117, 118
produces an alternating MFiD force. The r~FiD force can be phased
'S to coincide with the displacement of the liquid in the tuning
passage 75, thereby causing an apparent decrease in the
liquid s mass. Referring again to the equation for isolation
frequency, it will be appreciated that a decrease in the
apparent mass of the liquid in the tuninc; passage 75 results in.
an increased isolation frequency. Likewise, phasing the I~~iD
force to coincide with the acceleration of the liquid in the
tuning passage causes an apparent increase in the liquid s
mass, which decreases the isolation frequency. The amount of
increase or decrease in the isolation frequency depends on the
magnitude of the current flowing through the coil 116.
Alternately, the PgiD force can be phased to coincide with.
the velocity of the liquid in the tuning passage 75, thereby
adding kinetic energy t~ the liguid i:o compensate for the:
energy dissipated due to viscous damping losses. As a result,
the depth of the isolation notch 20 (Fig. 2j increases,
providing greater isolation.
Figs. 13 and 14 illustrate a vibration isolation system
for a helicopter pylon (the main rotor-transmission assembly]
which includes four tunable dual-axis isolators 127-133. Each
isolator 127-133 is identical to the isolator 31 described in
connection with Figs. 3-6.
A main rotor transmission I21 receives power from one or
14
CA 02485830 1994-12-O1
more engines (not shown) through a drive shaft 123. The
transmission 121 transfers power to a main rotor (not shown)
through a mast 125.
The dual-axis isolators 127-3.33 are attached to the
transmission 12I by four means of four brackets 135-141,
respectively. The upper and lower caps 47, 55 (Fig. 1) of each
isolator 127-133 is attached to a respective vertical bracket
135-i41, and the brackets 135-141 are attached to the
transmission 121. The brackets 135-141 also restrain the upper
and lower caps 47, 55 from moving relative to each other,
thereby allowing them to function in the manner of the cylinder
7 in Fag. 1. The isolators 127-133 are attached to an upper
deck 143 of a helicopter fuselage (not shown) by means of four
stands 145-151. The left and right caps ~ 3 , 71 ( Figs . 3 and 4 )
of each isolator 127-133 is attached to a respective stand 1.45
I51, and the stands 145-1,51 are attached to the upper deck 143
It will be appreciated that att<~ching the upper and lower
caps 47, 55 to the ugper deck 143 and attaching the left and
right caps 63, 71 to the transmission 121 is equivalent to the
2t~ configuration described above in connection with Figs. 13 and
15. In either configuration, pylon vertical shears, roll
moments, and pitch moments are isohated from the fuselage: by
the vertical isolator portions 33 (Figs. 3 and 4) of the dual-
axis isolators i27-133; and pylon longitudinal shears, lateral
shears, and yaw moments are isolatedl from the fuselage by the
horizontal isolator portions 35 of the isolators 127-1:33.
Thus, either configuration of the described i-solation system
provides vibration isolation in all six degrees of freedom.
It will be appreciated that a minimum of three dual-axis
isolators 3i (Figs. 3 and 4) are required to provide an
isolation system that is statically stable in all six degrees
of freedom. Such a system is shown schematically in Fig. I5:
The three dual-axis isolators 153-157 and associated moun~:ing
structure (not shown] provide a system which is statically
stable in all six degrees of freedom and which provides
isolation in all six degrees of freedom.
The isolation system illustrated in Fig. 16 provides
CA 02485830 1994-12-O1
isolation in five degrees of freedom. The effective axes
(indicated by arrows 16?) of the horizontal isolator portions
35 (Figs. 3 and 4) of the dual-axis isolators 161-165 are
aligned with transmission radii 169. Thus, vibration force
applied to the isolators 161-165 due to a yaw moment 1?1 is
perpendicular to the operating axes of the vertical and
horizontal isolator portions 33, 35, respectively, of the
isolators 161-165. Therefore, the yaw moment will not be
isolated.
The isolation frequency for a pylon-isolator-fuselage
system may be calculated as follows:
_ 1
2n R$(R8-1)m$' s''here
', 5 k$ = effective spring rate of the
system parallel to the vibration force being isolated;
Rs = the ratio of the effective piston area to the tuning
passage area for those isolators whose operating axes are
parallel to a component of the vibration force being
2a isolated;
mg = the sum of the tuning masses para7llel to a component of
the vibration force being isolated (the "effective tuning
masses").
For each single-axis isolator portion of each dual-axis
25 isolator, the effective tuning mass may be calculated as
follows:
m = L A p cos 8, where
L - the length of the tuning passage of the single=axis
isolator; A - the tuning passage area of the single-axis
30 isolator;
p = the mass density of the liquid; and
9 - the angle between the vibration force and the operating
axis
of the single-axis isolator.
35 If the present system included only one isolator, and the:
vibration force were acting along the: isolator s operating
axis, the preceding two equations would yield the equation foz-
16
CA 02485830 1994-12-O1
isolation frequency disclosed in Halwes. Unlike the system
described in Halwes, however, the present invention includes
more than one isolator and is exposed to vibration in all six
degrees of freedom. Therefore, it is necessary to calculate
the effective spring rates and tuning ma~,sses for each degree of
freedom.
Fig. 17 is a diagrammatic representation of the cruciform
member 41 and the rubber springs 43, 51, 59, 67 described above:
in connection with Figs. 3 and 4. Due to the geometry of the
rubber springs and the inherent properties of the rubber used
in their construction, the spring rates of.the springs depend
on the direction in which the springs az~e strained . The spring
rates of each spring are denoted k1,, k2, and k3. In thi
embodiment, the value of k~ is approximately one-half the value
5 of k2. Were it not for a series of shims 191 which are molded
into each rubber spring, k2 would be equal to k3. Due to the
shims 191, the value of k3 is approximately 30 times the value
of k2.
The spring rates k1, k2, and k3 of each rubber spring 43,
51, 59, 67 effect the effective spring rates of the cruciform
member 41. The effective spring rates kV, kT and kROf the
cruciform member 41 are calculated as follows:
1
kv ._ kr ~ I + 1
2ka 2k3
and
kR = I 1 1 = k~ .
2kz 2kz
When calculating effective spring
rates and effective tuning masses, a transmission mount plane
193 (Figs. 18 and 19j provides a convenient coordinate system.
The operating axes 37, 39 tFig. 3j of each dual-axis isolator
intersect. The transmission mount plane 193 is the plane which
passes through the operating-axes intersection of each of the
dual-axis isolators 127-133 (Figs. Z1 and 12j.
Referring now to Figs. 18 and 19, the effective spring
17
CA 02485830 1994-12-O1
rates of the isolator system (Figs. 13 wind 14) parallel to the
z axis (vertical relative to the transmission horizontal mount
plane 193), the x axis (longitudinal), and the y axis (lateral)
are:
kZ=4k~,;
and
kr = ky = 4 (kR + kT) cc~s 45 .
Since mount-plane roll and pitch moments resolve to forces
along the isolators° vertical operating axes, and mount plane:
yaw moments resolve to forces along the: isolators' horizontal.
operating axes,
,5 kpitch - knoll °~ 4 ky:
and
kyaw °' 4 kT.
Since the mount-plane vertical forces and the forces due
to mount-plane roll and pitch moments act along the dual-axis
isolators' vertical operating axes (and perpendicular to the
isolators' horizontal axes), only the vertical tuning masses
affect the system's isolation frequency in the mount-plane
vertical, roll, and pitch degrees c~f freedom; and, when
calculating the effective tuning masses for the vertical, roll,,
and pitch degrees of freedom, a = 0° . Since mount-plane forces
along the x and y axes act at an angle of 45° to the dual-axis
isolators' horizontal operating axes (and perpendicular to the
isolators' vertical axes), only the horizontal tuning masse;
affect the system°s isolation frequency in the longitudinal and
lateral degrees of freedom, and B = 45°. Finally, since the
forces due to mount-plane yaw moments act along the dual-axis
isolators' horizontal axes hand perpendicular to the isolators'
vertical axes), only the horizontal tinning masses affect the
system's isolation frequency in the yaw degree of freedom, and
~ . ~o.
Fig. 20 is a block diagram of the control system included
18
CA 02485830 1994-12-O1
in this embodiment of the invention to tune the dual-axis
isolators to achieve optimal isolation of pylon b/rev vibration
from the helicopter's fuselage. A sensor array 201 for sensing
fuselage vibration provides a series of signals representing
the vibration to a.controller 203. In response to the
vibration sensor signals, the mu:lti-input/multi-output
controller 203 generates signals which are routed to the
isolator actuators 205 (the electric motors 105, 107 in Fig.
3), which act to tune the system°s isolators. Tuning the
isolators changes the physical characteristics of the pylon-
isolator-fuselage system 207, which changes the vibration
sensed by the sensor array 201.
In this embodiment, the vibration-sensing array comprises
six accelerometers disposed to sense vibration in all six
'5 degrees of freedom. Alternately, an array of other suitable
sensors, such as strain gages, could be used. Because an
important object of the present invention is to provide the:
helicopter°s passengers with a vibration-free ride, the sensor
array 201 is located a.n the helicopter°s passenger compartment.
The controller 203, responds to the signals from ths:
sensor array 201 to develop output signals which cause
appropriate isolator actuators 205 to move the corresponding
isolator tuning sleeves 83, 85 (Figs. 3-6), or the alternate
tluning means described in connection with Fags. 7-12, thereby
changing the system°s isolation frequency in one or more
degrees of freedom. Such changes change the characteristics of
the pylon-isolator-fuselage system 207, resulting in an
increase or a decrease in vibration in the corresponding
degrees of freedom, which vibration is transmitted from the
main rotor transmission to the fuselage and, thus, to the
sensor array 201. As can seen, the control system is a closed~-
loop feedback system.
If a change in an isolation frequency results in an
increase in the vibration in the corresponding degrees of
freedom, that increase is sensed by the sensor array 201. As
a result, the controller 203 acts to tune the corresponding
system isolation frequency in the opposite direction. If the
19
CA 02485830 1994-12-O1
original change in isolation frequency decreases the vibration
in the corresponding degrees of freedom, the controller
continues to adjust the appropriate isolator tuning sleeves in
the same direction until vibration in the corresponding degrees
of freedom reaches a predetermined value or begins to increase.
Discontinuing tuning at a predetenained value minimizes
controller "hunting."
It will be appreciated that the sensors 201 respond to
vibration from all sources, not just that originating in the
pylon. As a result, the controller 203 i~.unes the isolators to
minimize vibration at the passengers' seats, whatever the
source of that vibration.
While a preferred embodiment of the invention has been
shown and described, it will be apparent to those skilled in
this art that various modifications may be made to this
embodiment without departing from the spirit of the present
invention. For that reason, the scope of the invention is set
forth in the following claims.
