US20070131830A1

US20070131830A1 - Variable temperature magnetic damper

Info

Publication number: US20070131830A1
Application number: US11/304,974
Authority: US
Inventors: Sarah Brennan; Allen Bronowicki
Original assignee: Northrop Grumman Corp
Current assignee: Northrop Grumman Corp; Northrop Grumman Systems Corp
Priority date: 2005-12-14
Filing date: 2005-12-14
Publication date: 2007-06-14
Also published as: WO2007070106A1

Abstract

A passive magnetic damper suitable for use as a vibration isolator coupled between two masses, one of which is subject to vibration. The damper includes at least two pairs of rare earth permanent magnets configured to provide a narrow, generally planar gap, and backing plates positioned to complete a magnetic circuit passing through the magnets and the gap. A flat conductor plate is positioned to be freely movable in the gap. Any time-varying force on the conductor plate results in the generation of eddy currents in the plate, and these in turn generate a mechanical force on the plate, which resists the time-varying force and produces a damping effect. With careful selection of materials for the magnets, the backing plates and the conductive plate, the damper is effective over a wide temperature range that includes cryogenic temperatures, and is operable over a wide range of damping levels.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to mechanical vibration damping devices and, more particularly, to damping techniques applicable to environments subject to extremes of temperature, such as in space. A common requirement for space-based structures, such as telescopes, is to provide a stable platform that is not significantly affected by vibration from various sources associated with the structure. Vibration dampers to isolate a vibrating mass from another structural element can take a variety of forms. Dampers may be generally characterized as either active or passive. Active damping typically includes a sensing element that detects vibratory movement and an actuator that is controlled to produce a damping effect. For example, the sensor and actuator may be piezoelectric transducers. Passive damping, on the other hand, is accomplished by an essentially uncontrolled mechanism installed between a vibrating mass and a structural element to be isolated from the vibration. Passive damping generates a damping force by various means; for example, from frictional effects, from the constrained movement of viscous fluids, or by using springs or elastomeric masses. Active damping has the disadvantage of higher cost and complexity, and most passive damping techniques raise issues of reliability and consistency when used in the harsh environment of space.
Although some forms of magnetic damping have been proposed in various other contexts, magnetic damping has not been seriously considered as a damping technique for space-based structures. Accordingly, there is still a need for damping device that works reliably over a wide range of temperatures, including cryogenic temperatures, and over a wide range of damping levels. The present invention fulfills this need.

SUMMARY OF THE INVENTION

The present invention resides in a magnetic damper employing principles of eddy current damping using permanent magnets. Briefly, and in general terms, the damping mechanism of the invention comprises at least two pairs of permanent magnets positioned to define a generally planar gap in which a magnetic field is generated; first and second backing plates of magnetic material positioned to complete a magnetic circuit through the magnets and the gap; and a conductor plate of conductive but non-magnetic material positioned to move freely in the gap. Vibratory movement of the conductor plate in the gap, relative to the magnets, generates eddy currents in the conductor plate, which result in generation of a damping force on the conductor plate, such that the damping force opposes and diminishes the vibratory force. The damping mechanism acts as an isolator if coupled between two masses, one of which is subject to vibration. The permanent magnets, the backing plates and conductor plate are of materials selected to provide reliable damping operation over a wide range of temperatures, including cryogenic temperatures. The structure of the mechanism may also comprise a housing of non-magnetic material, supporting the backing plates and the magnets in a unitary structure.
More specifically, in the preferred embodiment of the invention the conductor plate is of a material selected from the group consisting of copper, beryllium, and an alloy of copper and beryllium. Preferably, the alloy of copper and beryllium contains approximately 2% beryllium.
Preferably, the magnets are of rare earth materials selected from the group consisting of neodymium iron boron (NdFeB) and samarium cobalt (SmCo), and the backing plates are of a material selected for its high magnetic saturation limit, such as iron, a silicon-iron alloy, or a cobalt-iron alloy.
It will be appreciated from the foregoing that the present invention represents a significant improvement in the field of vibration damping and isolation techniques that are especially well suited for use on structures in space. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective view showing the principle of the magnetic damper of the invention.
FIG. 2 is a view similar to FIG. 1 but with one magnet removed to illustrate the relationships between magnetic flux, induced current, vibration force and damping force.
FIG. 3 depicts a mechanical model of a simple single-degree-of-freedom system including a mass subject to a time-varying force, a spring and a damping coefficient.
FIG. 4 is a simplified cross-sectional view of a magnetic damper in accordance with the invention, showing the interrelationship of magnets, backing plates and movable conductive plate.

DETAILED DESCRIPTION OF THE INVENTION

As shown in the drawings by way of illustration, the present invention is concerned with a passive damping mechanism utilizing eddy current damping to isolate a structural member from a vibrating mass. The invention is particularly well suited for use in a space environment in which cryogenic temperatures are encountered but is also well suited for ground testing of space structures, such as optical devices, before launch and deployment in space.
In accordance with the invention, a passive magnetic damping technique is effective over a wide range of temperatures and a wide range of damping levels. The principle of magnetic damping is used to isolate a structural member from a vibrating mass as shown in FIGS. 1 and 2. Pairs of magnets, indicated at 10 and 12 are arranged to provide a uniform gap 14, in which a conductive, but non-magnetic, plate 16 is positioned as shown diagrammatically in FIG. 1. The magnet assembly 10, 12 is rigidly attached to a vibrating mass (not shown) and the conductive plate 16 is rigidly coupled to a mechanical structure to be isolated from the vibrating mass. Alternatively, the vibrating mass may be rigidly connected to the conductive plate 16 and the mechanical structure connected to the magnet assembly 10, 12.
FIG. 2 shows the same structure as FIG. 1 but with the upper magnet component 10 removed for clarity of illustration. Moreover, the lower magnet component 12 is shown as comprising two separate magnet poles of opposite polarity. The upper magnet component 10 would, in this case, also comprise two magnet poles, with polarities opposite to the polarities of the corresponding poles in the lower magnet component 12. Therefore, one upper and lower pair of magnetic poles is shown as generating a magnetic flux density B₁in a downward direction, while the other pair of magnetic poles generates a magnetic flux density B₂in the upward direction. In most cases, these flux densities would be designed to be equal. (B₁=B₂=B) The apparatus would, of course, include additional components of magnetic material to complete the two magnetic circuits generating B₁and B₂. As indicated in FIG. 2, movement of the conductive plate 16 at a velocity v results in the generation of a loop current i in the plate, where: $i \propto \frac{v \times B}{ρ},$

- and where ρ is the resistivity of the conductor plate 16.

The loop current, known as an eddy current, generates a force on the plate 16, indicated in FIG. 2 as F_emf. The force is proportional to the velocity v and to B², the square of the flux density, and is inversely proportional to the resistivity ρ. As also indicated in FIG. 2, the force F_emfhas a direction opposite to the direction of the velocity v and the vibration force that produces in this velocity. Therefore, the force F_emfhas the effect of opposing the vibration force and isolating the two structures to which the device is connected.
A system comprising a vibrating mass 20 and a damping mechanism 22 of the same type as the present invention may be modeled as shown in FIG. 3. The vibrating mass 20 is shown as having a mass m, subject to a displacement x as a result of a time-varying force f(t). The mass is assumed to be coupled to a mechanical structure 24 by the damping mechanism 22. The connection has a stiffness characterized by a single spring of stiffness k and damping is provided by the magnetic effect discussed above, to provide a damping coefficient c_emf. The damping mechanism is symbolized by a dashpot, which is characterized as providing a damping force proportional to the relative velocity of its two components. The magnetic damping mechanism of the present invention also provides a damping force proportional to the relative velocity of its components. This linear relationship between damping force and velocity is characteristic of various forms of viscous damping.
As shown in FIG. 4, which cross-sectional view of the damping mechanism, there are three essential components: the conductor plate 16, upper and lower magnets 10 and 12, and upper and lower back plates 26 and 28, which are of magnetic material and complete the magnetic circuits depicted in the figure. The mechanism may also include a housing 30 of non-magnetic material, such as aluminum.
Selection of materials for the damping mechanism of the invention is critical to its successful operation, especially at cryogenic temperatures. First, with regard to the conductor plate 16, the preferred choice is a metal containing beryllium. An alloy of copper and beryllium has found to be most favorable. All metals exhibit lower resistivity as temperature drops, for all applications where it is desirable to have similar damping behavior in a range from 30° K to 50° K, copper beryllium is a clear choice because it exhibits constant resistivity values at temperatures less than 100° K. If an application calls for a higher level of damping, pure copper (designated C101) is a better choice. Pure beryllium may also be considered because it has a relatively flat resistivity curve in the region of interest a lower resistivity than a copper-beryllium alloy with 2% beryllium, designated C172.
For the material of the magnets themselves, rare earth magnets such as neodymium iron boron (NdFeB) and samarium cobalt (SmCo) exhibit the most attractive magnetic properties because they possess high values of residual induction B_r, a measure of the strength of the magnetic field, and equally high values of intrinsic coercive force, H_ci, which indicates a magnet's resistance to demagnetization. There is general agreement in the technical literature that SmCo is thermally stable. It is, therefore, a good candidate for the magnet material in the present invention. Neodymium iron boron (NdFeB), on the other, appears to be subject to significant degradation in magnetic field strength at cryogenic temperatures. For application to the present invention, however, a more important characteristic of the material is its remanence or residual induction after removal of any magnetizing force. Eddy current damping is not an application in which the magnetic material is cycled through various states of magnetization but rather is classified as a “DC” application in which only the residual induction of the material is relied on to generate a damping force. It turns out that NdFeB has a residual induction that is fairly constant down to 50-100° K and, for some formulations of NdFeB, to even lower temperatures. The preferred materials for the magnets have the standard designations NdFeB(53) and SmCo(27H).
In addition to selection of magnetic materials, the configuration of the magnetic circuit employed in the invention is also critical to maximizing the magnetic field strength and the resultant damping force. The horseshoe configuration shown in FIG. 4 offers a good flux path and concentrates the magnetic field in gap. It should be recognized, however, that this configuration does not produce a perfectly uniform flux distribution in the gap. Increasing magnetic field strength in the gap is the most effective way to increase the damping effect because the damping coefficient is proportional to the square of the magnetic induction. In particular, assuming a uniform flux density in the gap, the damping coefficient is given by: $c_{emf} = \frac{B_{d}^{2} L W t}{ρ},$

- where L, and W are the length and width of the magnets, Bd is the magnetic induction in the gap, t is the thickness of the conductor and ρ is its resistivity.

The backing plates 26 and 28 also have an influence on the performance of the magnetic damper. Because the magnets have relatively high flux, it is necessary to select backing plates with a high saturation limit. All magnetic materials exhibit a characteristic saturation level on a curve plotting magnetic flux (B) against applied magnetic force (H), referred to as the B-H curve. At the saturation limit of a material, no significant increase in magnetic flux is obtained no matter how much the applied magnetic force is increased. The requirement for a high saturation limit eliminates nickel-iron alloys from consideration; they are good for shielding because of their extremely high permeability values, but have relatively low saturation limits of around 0.8 tesla (0.8 T).
Both pure iron and high-resistivity silicon-iron (SiFe) have similar saturation limits of around 1.7 T, although SiFe is more costly. Cobalt-iron alloys, such as the one sold under the designation Hiperco50A, offer a much higher saturation limit (2.4 T), but at a higher cost than iron-based materials. Cobalt-iron alloys, however, offer about a one-third weight saving compared to the iron-based materials. Therefore, cobalt-alloys are preferred materials for space applications of the magnetic damper. Although permeability of these materials can vary significantly with temperature, their saturation limits are relatively stable with temperature. Therefore, the choice of backing plate materials will not have a significant influence on the gap magnetic induction as temperature varies.
It will be appreciated from the foregoing that the present invention provides a significant advance in the field of damping and isolation devices, particularly for use in a space environment. In particular, the invention provides a passive damping mechanism that is operable over a wide temperature range, including cryogenic temperatures, and is operable over a wide range of damping levels. In this regard, the damping mechanism of the invention has been demonstrated to operate over damping levels from 1 mm down to 0.4 μm.
It will also be appreciated that although specific examples of structures and materials embodying the invention have been discussed by way of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the accompanying claims.

Claims

1. A magnetic damping mechanism for use over a wide temperature range that includes cryogenic temperatures, the damping mechanism comprising:

at least two pairs of permanent magnets positioned to define a generally planar gap in which a magnetic field is generated;

first and second backing plates of magnetic material positioned to complete a magnetic circuit through the magnets and the gap; and

a conductor plate of conductive but non-magnetic material positioned to move freely in the gap;

wherein vibratory movement of the conductor plate in the gap, relative to the magnets, generates eddy currents in the conductor plate, which result in generation of a damping force on the conductor plate, such that the damping force opposes and diminishes the vibratory force, whereby the damping mechanism acts as an isolator if coupled between two masses, one of which is subject to vibration;

and wherein the permanent magnets, the backing plates and conductor plate are of materials selected to provide reliable damping operation over a wide range of temperatures.

2. A magnetic damping mechanism as defined in claim 1, and further comprising:

a housing of non-magnetic material, supporting the backing plates and the magnets in a unitary structure.

3. A magnetic damping mechanism as defined in claim 1, wherein:

the conductor plate is of a material selected from the group consisting of copper, beryllium, and an alloy of copper and beryllium.

4. A magnetic damping mechanism as defined in claim 3, wherein:

the alloy of copper and beryllium contains approximately 2% beryllium.

5. A magnetic damping mechanism as defined in claim 1, wherein:

the magnets are of rare earth materials selected from the group consisting of neodymium iron boron (NdFeB) and samarium cobalt (SmCo).

6. A magnetic damping mechanism as defined in claim 1, wherein:

the backing plates are of a material selected for its high magnetic saturation limit.

7. A magnetic damping mechanism as defined in claim 6, wherein:

the backing plates are of a material selected from the group consisting of iron, silicon-iron alloys, and cobalt-iron alloys.

8. A magnetic damping mechanism for use over a wide temperature range that includes cryogenic temperatures, the damping mechanism comprising:

at least two pairs of permanent rare earth magnets positioned to define a generally planar gap in which a magnetic field is generated, the magnet materials being selected from the group consisting of neodymium iron boron (NdFeB) and samarium cobalt (SmCo);

first and second backing plates of magnetic material positioned to complete a magnetic circuit through the magnets and the gap, the backing plate materials being selected from the group consisting of iron, silicon iron alloys, and cobalt-iron alloys; and

a conductor plate of conductive but non-magnetic material positioned to move freely in the gap, the conductor plate material being selected from the group consisting of copper, beryllium, and an alloy of copper and beryllium;

wherein vibratory movement of the conductor plate in the gap, relative to the magnets, generates eddy currents in the conductor plate, which result in generation of a damping force on the conductor plate, such that the damping force opposes and diminishes the vibratory force, whereby the damping mechanism acts as an isolator if coupled between two masses, one of which is subject to vibration.