Light reaction mass actuator

A very light, high force reaction-mass actuator using high energy density magnetostrictive material to accelerate a mass, producing a reaction force that can be used to oppose vibration forces. The reaction-mass actuator is configured having an axially symmetric design that features a Terfenol-D rod as a core. Surrounding the rod is a magnetic coil which provides the changing magnetizing field used to actively control magnetostrictive strain of the Terfenol rod. The magnetic coil also provides a magnetic flux based on a D.C. current, for biasing the Terfenol rod. The magnetic coil, disposed about the fixed magnetostrictive rod, is incorporated into a reaction mass assembly that is suspended and sealed inside an enclosed, stiff outer shell or housing by an elastomeric material mounting configuration that facilitates an output force with suppressed harmonic content. In addition, the elastomeric mounting configuration provides sealing protection against external factors. In one embodiment, the magnetic coil is wound on a bobbin effecting a movable mass that is accelerated to produce a force on a mounting surface. The Terfenol rod is placed under axially compressive stress by a simple mechanical preloading mechanism. This increases strain in the Terfenol rod while providing a return spring for the reaction mass when it has reached its maximum stroke.

FIELD OF THE INVENTION 
The present invention relates to noise and vibration damping, and more 
particularly to damping structural vibration with high force-to-mass ratio 
actuators. 
BACKGROUND OF THE INVENTION 
Mechanical or structural vibration in mechanical systems typically can 
result in auditory noise. The structural vibration, such as from a large 
surface set in vibratory motion, substantially simultaneously sets into 
motion the acoustic medium, e.g. air or water, around the vibrating 
member. Thus, when a structure in a mechanical system is set into motion 
by a mechanical source of vibration, it in turn causes a propagation of an 
acoustical signal, i.e. noise, in the surrounding air. Furthermore, 
vibration generated in one place in a mechanical system is often 
transmitted elsewhere through transmitting structures in the mechanical 
system, and is undesirably further converted into audible or airborne 
noise by radiating structures. Accordingly, noise levels in mechanical 
systems, such as large helicopters, can be very high in part due to 
vibrations originating as structural noise, e.g. in meshing gears in the 
transmission. The low mass of mechanical systems, such as helicopter 
transmissions, accentuates the problem of structural vibration and noise. 
Resultant noise can have problematic implications in other systems in the 
aircraft, including communication systems. Typical vibration frequencies 
of helicopter transmissions and other aircraft equipment in the 1 to 2 kHz 
range can cause significant interference with the audibility of speech, 
which has many frequency components in that range. 
Past efforts to reduce structural vibration and/or noise in helicopter 
transmissions have focused on passive techniques such as: transmission 
tuning to shift the frequency of vibrations to an innocuous range; 
isolation to limit and/or preclude resonances caused by the interplay of 
vibration from different sources; and absorption to muffle or absorb noise 
by way of noise barriers. However, these approaches do not produce 
sufficient reductions in vibration and noise. 
Active feedback methods are being investigated and applied as a promising 
alternative approach to reducing narrow-band vibration and noise in 
environments where passive techniques have proven inadequate (e.g. at 
higher frequencies, for example, greater than 400 Hz). Typical active 
feedback methods require transducers or sensors to sense noise levels at 
frequencies of interest and actuators to deliver cancellation signals at 
corresponding frequencies, typically 180 degrees out of phase with the 
noise sensed. However, a lack of availability of high force-to-mass ratio 
actuators significantly limits the applicability of such active vibration 
cancellation schemes in weight sensitive applications, such as helicopters 
or other aircraft. That is, the availability of light weight actuators 
that can deliver the high forces necessary for high frequency active noise 
cancellation is very limited. In particular, of the components required in 
active noise cancellation systems (e.g. sensors, actuators, controllers 
and electronics), it is the mass of the actuator(s) that is the critical 
consideration, for example in helicopter applications. 
Reaction Mass Actuators (RMAs) are known for delivering forces for active 
noise cancellation. RMAs are typically designed and configured to be 
mounted on vibrating structures to deliver cancellation signals on the 
structures, and to substantially preclude the transmission of vibration 
through the structures. Electromagnetic voice coil RMAs are known which 
typically use springs and masses to produce a resonant system that 
achieves relatively high force densities. However, the resonant system in 
electromagnetic voice coil RMAs produces high force in a narrow band of 
frequencies, and almost no force at non-resonant frequencies. Accordingly, 
electromagnetic RMAs have a very narrow band of useful frequencies, i.e. 
they are of limited applicability in applications where a broader range of 
frequencies is involved. Further, the resonant systems in some known 
electromagnetic voice coil RMAs include solid components that engage each 
other causing "chatter" and/or harmonic noise at frequencies other than 
that of the output force. Extraneous noise generated by known voice coil 
RMAs works counter to the purpose for which the devices are generally 
employed. Additionally, disadvantageously, many known electromagnetic 
voice coil RMAs are open designs with actuators that are not readily or 
easily sealed from the elements. 
Some known RMAs incorporate piezoelectric or electrostrictive materials to 
effect actuation. However, such known RMAs have disadvantages in certain 
applications. Specifically, high voltages on the order of 1,000 volts are 
typically required for piezoelectric or electrostrictive actuators, 
presenting numerous issues associated with power sourcing. The power 
sourcing requirements mitigate against the use of such actuators in weight 
sensitive applications. 
Magnetostrictive materials have properties similar to piezoelectric and 
electrostrictive materials and have been used in active actuator 
applications. The magnetostrictive materials used change shape (strain) in 
the presence of magnetic fields by up to 2000 .mu.-strain (ppm) at room 
temperatures, which is twice the strain available in the best 
piezoelectric or electrostrictive materials. With magnetostrictive 
materials, achieving such large strains does not require the use of high 
voltages. 
Another advantage of magnetostrictive materials in actuator applications, 
over competing technologies such as piezoelectric, is a lack of fatigue 
phenomena. There is no known fatigue mechanism in known magnetostrictive 
materials and therefore no time or cycle dependant lifetime. This is in 
contrast with piezoelectric materials, which suffer from microfractures 
that develop during use and lead to breakdown under the high voltages 
required for implementation of piezoelectric actuators. 
Terfenol-D.RTM. magnetostrictive material, produced by ETREMA Products 
Inc., a Subsidiary of EDGE Technologies Incorporated, is a room 
temperature, high performance magnetostrictive material which increases in 
length when a magnetizing field is applied parallel to the material's 
drive axis. Magnetostrictive strain depends only on the magnitude of the 
magnetizing field, not its sign. Bi-directional actuation can be achieved 
by inclusion of a bias field such as produced by a coil or permanent 
magnets, to bias the Terfenol-D material to elongation of some degree, 
e.g. one half, of its linear range enabling actuation about a bias point. 
Some sort of mechanical prestress is generally desirable, as it has been 
determined that the best performance of Terfenol-D is achieved by 
configuring the magnetostrictive material with mechanical prestress or 
compressive preload. 
Terfenol, however, is relatively weak in tension and is known to be 
brittle. These properties can affect actuator reliability in improperly 
designed devices incorporating such material. For practical application, 
magnetostrictive actuators typically require sophisticated mechanical 
designs to ensure longevity of the actuator. Elaborate mechanical features 
designed into known magnetostrictive actuators to isolate the Terfenol rod 
from shear and tensile stresses, and to overcome weaknesses due to the 
physical properties of Terfenol, add significant cost and weight to the 
actuators. Generally, known magnetostrictive actuators have limitations 
and present significant design issues when subjected to significant loads. 
Furthermore, known Terfenol actuators have limited applicability in RMA 
applications. The mechanical design of known Terfenol actuators is such 
that they tend to be high friction devices with a significant amount of 
static friction (stiction), and damping in the actuator. In addition, they 
have a low moving mass, high actuator mass, and low power density that 
makes them unusable in most reaction mass applications. 
Terfenol magnetostrictive material has been used in actuators other than 
RMAs, such as actuators described in U.S. Pat. Nos. 5,231,887 and 
5,406,153. The mechanical configurations described therein, like other 
known magnetostrictive actuators, are not suitable for RMA applications in 
that they are high friction devices with a significant amount of static 
friction (stiction), and damping in the actuator. The designs, which are 
clearly not intended for RMA applications, include several solid 
components that will interact with each other to generate harmonic noise 
at frequencies other than that of the output force. Furthermore, in these 
illustrative implementations of magnetostrictive actuators, there is no 
concern for maximizing the mass of the actuator for the purpose of power 
density maximization. 
SUMMARY OF THE INVENTION 
The present invention provides a very light, high force reaction-mass 
actuator using high energy density magnetostrictive material in a sealed 
substantially noise or harmonic free configuration, to accelerate a mass 
producing a reaction force that can be used to oppose vibration forces. 
According to the invention, the reaction-mass actuator is configured having 
an axially symmetric design that features a magnetostrictive rod as a 
core. Surrounding the rod is a magnetic coil which provides the changing 
magnetizing field used to actively control magnetostrictive strain of the 
Terfenol rod. The magnetic coil also provides a magnetic flux based on a 
D.C. current, for biasing the Terfenol rod. The magnetic coil, disposed 
about the fixed magnetostrictive rod, is incorporated into a reaction mass 
assembly that is suspended and sealed inside an enclosed housing by an 
elastomeric material mounting configuration that provides an output force 
with suppressed harmonic content. 
In one embodiment of the invention, the magnetic coil is wound on a bobbin 
effecting a movable coil reaction mass that is accelerated to produce the 
output force. The magnetostrictive rod is disposed interior to the bobbin 
and placed under axially compressive stress by a simple mechanical 
preloading mechanism. This increases strain in the rod while providing a 
return spring for the reaction mass when it has reached its maximum 
stroke. The movable mass is suspended in a stiff outer shell by a 
plurality of O-rings that provide support for the moving mass and sealing 
protection against external factors, such as weather, and external shocks 
and loads. The magnetostrictive rod, which expands when it is exposed to 
the magnetic field provided by the coil, is used to accelerate the movable 
mass including the coil, producing a reaction force that is used to oppose 
vibration forces. 
Features of the invention include provision of a light weight, high force 
actuator suitable for a wide variety of vibration cancellation 
applications. A low parts count and simplicity of design result in a low 
cost actuator that has high reliability, high durability, and produces an 
output force with suppressed harmonic content. The light weight actuator 
is particularly well suited for application to vibration reduction of 
helicopter transmissions. An uncomplicated design yields a highly scalable 
device that can be produced in a wide range of force and frequency 
characteristics. Low actuator mass is facilitated by very large 
magnetostrictive strains, while a significant portion of the overall mass 
of the actuator is non-stationary, movable mass. Actuation is effected at 
a low operating voltage. High reliability is achieved through elimination 
of friction, wear, flexures, and fatigue phenomena. Additionally, 
actuators according to the invention are relatively low volume, high 
bandwidth, and mechanically simple relative to competing piezoelectric and 
electrostrictive devices.

DETAILED DESCRIPTION 
Reaction Mass Actuators ("RMA") provide force through the acceleration of a 
(reaction) mass. RMAs require only one-point mounting, and do not require 
modification for multi-degree-of-freedom systems (although multiple single 
axis actuators are required for multi-degree-of-freedom applications). 
While voice coils and piezoelectric materials have been used for RMA 
applications, use of "giant" magnetostrictive materials such as 
Terfenol-D, with strains near 2000 parts-per-million provides several 
benefits, including high specific power and near infinite fatigue life. 
Terfenol-D is excited by the application of a magnetic field in the desired 
direction of motion. Like voice coil actuators, a magnetic circuit is 
required to create magnetic flux in an appropriate location. Force is 
produced in the voice coil actuator through interaction of airgap flux 
with current carrying conductors or permanent magnets. In a 
magnetostrictively based RMA according to the invention, force is produced 
by magnetostrictive effect. Since the force produced is required to strain 
the magnetostrictive alloy as well as to do usable external work, use of 
the stored (strain) energy enables a much greater (reaction) force to be 
achieved. 
A high force to volume/mass ratio magnetostrictive reaction mass actuator 
according to the invention is illustrated in FIG. 1. The actuator operates 
as a resonant device allowing the stored strain energy to be utilized. The 
reaction-mass actuator 10 is configured having an uncomplicated axially 
symmetric design that features a Terfenol-D rod 12 as a core. A magnetic 
coil 14 is substantially symmetrically disposed on a non-magnetic bobbin 
16, formed of, for example, aluminum, plastic or non-magnetic stainless 
steel. The bobbin 16 has a hollow cylindrical portion 18 in which the 
Terfenol rod 12 is installed. 
A reaction mass 20 has an interior cavity in which a stationary drive 
assembly comprising the rod 12, bobbin 16 and coil 14 is disposed. The 
reaction mass 20 is formed of a solid piece of 303 stainless steel. In 
this illustrative embodiment, the mass 20 comprises approximately sixty 
percent of the total mass of the actuator effecting the actuator force and 
facilitating a high force to mass actuator according to the invention. 
A rigid housing 22 includes a baseplate portion 24 against which a portion 
of the bobbin 16 abuts in the assembled actuator. The baseplate portion 24 
includes an aperture for receiving a mounting lug 25 that includes a 
recess 27 dimensioned to receive an end portion of the magnetostrictive 
rod 12. The mounting lug 25 facilitates single point mounting of the RMA. 
The reaction mass 20 is configured to be suspended in the rigid enclosed 
housing 22 with only a contact portion 26 of the mass 20 exposed to the 
exterior of the actuator. In this illustrative embodiment two suspension 
grooves 28 are formed in the mass 20 and are dimensioned to receive 
respective elastomeric suspension members 30. In this implementation the 
elastomeric suspension members 30 comprise O-rings that fit within the 
grooves 28 but are dimensioned to have a portion extending out of the 
groove to engage inner surfaces of the housing 22 to suspend the mass 
therein. The O-rings are effective to provide sealing protection against 
external factors, such as weather, and external shocks and loads. 
A simple precompression mechanism, in the form of a wave spring 32 is 
located at a portion of the actuator 10 distal to the mounting lug 25. The 
wave spring 32 contacts portions of the reaction mass 20 and portions of a 
washer 34 that is held in place by a retaining ring 36. An interior 
portion of the reaction mass 20 engages an end of the Terfenol rod 12 so 
that the force exerted by the wave spring on the mass 20 is in turn 
exerted on the rod 12 effecting prestress thereof. The wave spring 32 is 
selected to provide sufficient prestress on the rod, as known in the art. 
Another embodiment of a high force to volume/mass ratio magnetostrictive 
reaction mass actuator according to the invention is illustrated in FIGS. 
2A, 2B and 3. The actuator is substantially similar in form and function 
to the actuator described hereinbefore with respect to FIG. 1. The 
reaction-mass actuator 10' is configured, and best illustrated in FIG. 2B 
and FIG. 3, having an axially symmetric design that similarly features a 
Terfenol-D rod 12' as a core. 
The reaction mass in this embodiment is constituted by a mass assembly 40 
comprised of a magnetic coil 14' disposed on a non-magnetic bobbin 16'. 
The bobbin 16' has a hollow cylindrical portion 18' about which the coil 
14' is substantially symmetrically wound. The magnetic coil 14' and bobbin 
16' provide the movable mass that is accelerated to produce a force on a 
mounting surface in order to effect active vibration cancellation as known 
in the art. The mass assembly 40 in this illustrative embodiment comprises 
approximately eighty percent of the total mass of the actuator effecting 
the actuator force and facilitating a high force to mass actuator 
according to the invention. 
As with the embodiment described hereinbefore, the magnetic coil 14' 
provides a changing magnetizing field, as known in the art, used to 
actively control magnetostrictive strain of the Terfenol rod 12'. In the 
present illustrative embodiment(s) of a reaction mass actuator, the 
changing magnetic field is in accordance with a mass excitation signal 
provided to the coil(s) 14, 14'. The magnetic coil(s) 14, 14' also provide 
magnetic flux for biasing the Terfenol rod, based on a D.C. biasing 
current combined with the mass excitation signal. Electronic circuitry 
(not shown), is configured as known in the art to provide both the mass 
excitation signal and the D.C. biasing current to the coil. 
Single point mounting of the actuator 10' is, again, effected by a mounting 
lug 25' disposed through a baseplate 24' which is configured to 
accommodate lead wires and electrical contacts for interconnecting the 
magnetic coil 14' to the electronic circuitry. The mounting lug 25' 
includes a recess 27' dimensioned to receive an end of the terfenol rod 
12'. The rod 12' is configured to be disposed in the recess 27' of the 
mounting lug 25', with the bobbin installed thereover so that the rod is 
contained in the hollow cylindrical portion 18' of the bobbin 16'. 
The movable mass assembly 40 is centered in a stiff outer shell 22' by a 
plurality of suspension members constituted by O-rings 30'. The O-rings 
30' are disposed in grooves 28' formed at top and bottom portions of the 
bobbin 16'. The O-rings 30' suspend the moving mass assembly 40 inside the 
stiff outer shell 22' and provide a support that is stiff radially, yet 
compliant axially. Furthermore, the O-rings provide sealing protection 
against external factors, such as weather, and external shocks and loads. 
Within approximately 0.002 inches of reaction mass translation, the effect 
of the O-rings on the system dynamics is negligible. 
The Terfenol rod 12' is placed under axially compressive stress by a simple 
mechanical preloading or prestress mechanism including a wave spring 32'. 
Preloading increases strain in the Terfenol rod. The spring 32' also 
provides a return spring for the reaction mass when it has reached its 
maximum stroke. In this embodiment, a top portion of the hollow 
cylindrical portion 18' of the bobbin 16' has an interior thread that 
receives a positioning screw 42. With the reaction mass assembly 40 
suspended in the shell or housing 22', the positioning screw 42 is used to 
set the height or axial position of the mass assembly 40 in the shell 22'. 
As a portion of the bobbin 16', comprising part of the mass assembly 40, 
pushes against the preload spring 32', adjustment of the positioning screw 
in effect adjusts the amount of spring force exerted against the mass 
assembly 40, i.e. setting the baseline preload as a function of the 
position of the mass 40 as determined by the screw 42. 
In this illustrative embodiment, heat generated in the coil is removed 
through conduction and convection through air to the baseplate and shell. 
In higher power application(s), the sealed shell 32 can be filled with 
Fluorinert, a dielectric cooling fluid produced by 3M. Such two-phase 
cooling allows extremely high power levels and high temperature operating 
environments. 
Magnetostrictive materials, such as Terfenol, are well suited for 
application in the reaction mass actuator(s) described hereinbefore, due 
to their inherent high force and bandwidth. The illustrative Terfenol 
based reaction-mass actuators are 2.125 inches long and 1.5 inches in 
diameter. They produces more than .+-.30 lbf at 700 Hz and weighs 0.6 lbm. 
However, it should be appreciated that a family of reaction mass actuators 
could be developed based on the design illustrated herein. For example, 
reduction of the actuator force given the same Terfenol rod length would 
allow the reduction of the reaction mass weight by half, assuming the 
Terfenol rod cross-sectional area were reduced by half to maintain the 
same resonant frequency. 
Further, it should be appreciated that the illustrative design described 
hereinbefore allows for the variation of bias field in order to reduce 
power consumption when full force output is not required. Moderately 
higher actuator power consumption occurs at full force output due to 
increased coil heating. Weight reduction requirements, however, mitigate 
against the use of permanent magnet biasing to reduce power consumption, 
unless the permanent magnets are packaged onto the moving assembly. The 
weight penalty for including permanent magnet biasing can be limited, for 
example, by including cylindrical magnets mounted on the moving bobbin and 
surrounding the windings. The operation of such an actuator is 
functionally identical to that of the actuator described hereinbefore. 
Thus, it should be appreciated that although a single coil is described in 
the actuator according to the invention as both providing the moving mass 
and effecting the electromagnetic force for actuation of the Terfenol rod, 
while an electronic implementation effects both a bias current and an 
actuation signal delivered through the single coil, the bias field or some 
portion thereof can be alternatively provided by implementing a permanent 
magnet biasing scheme while maintaining the actuator signal coil as a 
substantial portion of the moving mass. That is, permanent magnet biasing 
can be implemented in conjunction with or as an alternative to DC current 
applied to the coil to produce the needed DC field for biasing the 
terfenol rod. 
Although a simple preload mechanism in the form of a wave spring is 
implemented in the illustrative embodiments described herein, it should be 
appreciated that other preload mechanisms could be implemented, such as 
mechanical biasing means including various types of springs, elastomers or 
compressive materials, adjustable threaded members, flexures or the like. 
Although the reaction mass described herein is "suspended" in the outer 
shell by means of two suspension grooves formed in the mass and 
dimensioned to receive respective elastomeric suspension members in the 
form of O-rings, it should be appreciated that an alternative number of 
suspension grooves and elastomeric suspension members, i.e greater than 
two, can be implemented, and further, other suspension members or support 
a mechanisms can be incorporated in the reaction mass actuator according 
to the invention to effect the substantially symmetrical design, including 
various other bearing surfaces, alternative geometrically dimensioned 
elastomeric mounts, flexures or the like. 
While the reaction-mass actuator described herein is discussed as a high 
force-density actuator for active noise and vibration cancellation in 
helicopter gear boxes, it should be appreciated that the reaction mass 
actuator according to the invention can be alternatively implemented in 
other applications, including applications where a high force-to-mass 
ratio is critical, such as other aerospace or land-based applications 
including applications for removal of engine noise, gear whine, vibration 
from rotating machines, or the like. 
Although the invention has been shown and described with respect to 
exemplary embodiments thereof, various other changes, omissions and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.