Tuned mass dampers and vibration isolation apparatus

A tuned mass damper is provided for use with a structure that vibrates at a first axial surge mode and a second axial surge mode, when exposed to a predetermined frequency range. The tuned mass damper includes a housing, an endcap, two masses, and two springs. The first and second masses are disposed in a housing cavity. The first tuned mass damper spring is disposed in the cavity coupling the first mass and the endcap. The second tuned mass damper spring is disposed in the cavity coupling the first and second masses. The tuned mass damper has first and second resonant frequencies in the predetermined frequency range to damp the first and second axial surge modes, respectively. The second mass resonates at an amplitude that is greater than an amplitude at which the first mass resonates, when the two masses are subjected to the second axial surge mode.

TECHNICAL FIELD

The inventive subject matter generally relates to vibration isolation apparatus and tuned mass dampers for use on vibration isolation apparatus.

BACKGROUND

A precision pointing system carrying a sensor, such as a telescope, as its payload, may be susceptible to disturbances that produce structural vibrations and, consequently, pointing errors. Such vibrations may be attributed to mechanical components or assemblies, such as reaction wheel assemblies that are used as actuators in the precision pointing system. For the most part, because these systems tend not to have significant, inherent damping, these structural vibrations may degrade system performance and even cause structural fatigue over time.

To minimize the vibrations, an isolation strut is typically used to damp the structure and isolate the payload. One type of isolation strut operates as a three-parameter vibration isolation system and includes a hollow shaft, a piston, and a main spring. The piston receives vibration from the payload and is configured to slidably move through the shaft in response to the vibration. A flange extends radially from a midsection of the piston and has a top surface that is coupled to a first sealed bellows and a bottom surface that is coupled to a second sealed bellows. Each of the bellows has a chamber that is filled with fluid. Thus, when the piston moves axially through the shaft, fluid flows from one of the bellows chambers to the other. The shaft and piston are disposed within the main spring, which provides axial stiffness to the isolation strut in general.

During system operation, the isolation strut may be subjected to a frequency that causes the main spring to resonate. In some cases, the degree of resonance is such that it interferes with the capability of the strut to damp vibrations. In other cases, the resonance may degrade the structural integrity of the spring, and the spring may become prematurely worn. To attenuate the degree of resonance, one or more elastomer pads may be contacted with or attached to the main spring. However, this configuration has drawbacks. For example, the elastomer pads may unpredictably creep when exposed to certain temperatures, and thus, may not attenuate the resonance as desired. Additionally, the elastomer pads, which are typically made of insulating material, may block electrical and/or thermal conduction thereby creating electromagnetic interference and overheating issues.

Accordingly, it is desirable to have a vibration isolation apparatus that has improved damping capabilities. In addition, it is desirable to have a vibration isolation apparatus that does not resonate significantly when subjected to predetermined frequency ranges. Furthermore, other desirable features and characteristics of the present inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.

BRIEF SUMMARY

Tuned mass dampers and vibration isolation apparatus are provided.

In an embodiment, a tuned mass damper is provided for use with a structure that vibrates at a first axial surge mode and a second axial surge mode, when exposed to a predetermined frequency range. The tuned mass damper includes a housing, an endcap, a first mass, a second mass, a first tuned mass damper spring, and a second tuned mass damper spring. The housing defines a cavity and includes an end. The endcap is disposed over the end of the housing. The first mass is disposed in the cavity, and the second mass is disposed in the cavity. The first tuned mass damper spring is disposed in the cavity coupling the first mass and the endcap. The second tuned mass damper spring is disposed in the cavity coupling the first mass and the second mass. The tuned mass damper has a first resonant frequency that is in the predetermined frequency range to damp the first axial surge mode, the tuned mass damper has a second resonant frequency that is in the predetermined frequency range to damp the second axial surge mode, and the second mass resonates at an amplitude that is greater than the amplitude at which the first mass resonates, when the first mass and the second mass are subjected to the second axial surge mode.

In an embodiment, by way of example only, a vibration isolation apparatus includes a main spring and a tuned mass damper. The main spring is capable of vibrating at a first axial surge mode when exposed to a first frequency and a second axial surge mode when exposed to a second frequency and has a length measured from a first end to a second end. The tuned mass damper is coupled to the main spring and includes a housing, an endcap, a first mass, a second mass, a first tuned mass damper spring, and a second tuned mass damper spring. The housing defines a cavity and includes an end. The endcap covers the end of the housing. The first mass is disposed in the cavity, and the second mass is disposed in the cavity. The first tuned mass damper spring is disposed in the cavity coupling the first mass and the endcap, and the second tuned mass damper spring is disposed in the cavity coupling the first mass and the second mass. The tuned mass damper has a first resonant frequency within the predetermined frequency range to damp the first axial surge mode, the tuned mass damper has a second resonant frequency within the predetermined frequency range to damp the second axial surge mode, and the tuned mass damper is disposed at an axial location on the main spring that is located between a first axial position at about 37.5% of the length of the main spring and a second axial position at about 62.5% of the length of the main spring.

DETAILED DESCRIPTION

The following detailed description of the inventive subject matter is merely exemplary in nature and is not intended to limit the inventive subject matter or the application and uses of the inventive subject matter. While the isolation struts are discussed with reference to exemplary embodiments, any one of numerous other embodiments of a vibration isolation apparatus having a main spring may be implemented as well. For example, it will be appreciated that the inventive subject matter may be implemented in any vibration isolation apparatus that includes a main spring. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

FIGS. 1 and 2show a side view and a cross-sectional view of a vibration isolation apparatus100, in accordance with an embodiment. In one embodiment, the vibration isolation apparatus100may include a pivot102, a support104, and a vibration isolation apparatus106extending therebetween. The pivot102is adapted to receive vibratory motion from a non-illustrated payload, which may be a telescope, a reaction wheel assembly or any other device or apparatus that may vibrate and that may be damped. The support104attaches the vibration isolation apparatus106to a non-illustrated base. In an embodiment, the support104may be integrally formed as part of the vibration isolation apparatus106. In another embodiment, the support104may be separately formed and coupled to the vibration isolation apparatus106. Although the support104is illustrated inFIG. 1as being disposed on an opposite end of the vibration isolation apparatus106from the pivot102, it will be appreciated that the support104alternatively may be formed on or coupled to any other suitable part of the vibration isolation apparatus106.

As shown in more detail inFIG. 2, the vibration isolation apparatus106includes a main spring116, a shaft108, a piston110, first and second bellows112,114, and at least one tuned mass dampers118,120, in an embodiment. The main spring116is generally cylindrical and houses at least a portion of the shaft108, piston110, and bellows112,114therein. In an embodiment, the main spring116includes a first opening138at one end140, and a second opening142at an opposite end144. The end140on which the first opening138is formed is coupled to the pivot102via an attachment cap146, while the end144on which the second opening142is formed is used to attach the main spring116to the support104. In an embodiment, the attachment cap146and the support104enclose the shaft108, piston110, and bellows112,114within the main spring116.

As alluded to above, the shaft108is adapted to cooperate with the piston110and the bellows112,114to damp a portion of the motion. In accordance with an embodiment, the shaft108is generally cylindrical and has an inner surface122and ends126,128. The inner surface122defines a passage124within which the piston110and bellows112,114are disposed. A first end126of the shaft108may be at least partially covered with a first endcap130, and a second end128of the shaft108may be at least partially covered by a second endcap132. In an embodiment, the endcaps130,132each include openings134,136.

To maintain the piston110in position relative to the shaft108, the piston110is attached to the first endcap130via the first bellows112and to the second endcap132via the second bellows114. Fluid (not shown), such as a viscous liquid or a gas, fills the passage124to provide damping when the piston110receives vibration from the pivot102. The vibration isolation apparatus106may also include a temperature compensator133to compensate for thermal expansion that may be experienced by the fluid. In an embodiment, the temperature compensator133may include a bellows that attaches the piston110to the support104.

During operation, the main spring116may resonate with multiple axial surge modes when vibrated within a predetermined frequency range. An axial surge mode is a frequency which causes the main spring116to vibrate axially. The predetermined frequency range may be a range that includes one or more resonant frequencies of the main spring116. The main spring116may resonate with a first axial surge mode when vibrated at a first frequency within the predetermined frequency range, a second axial surge mode when vibrated at a second frequency within the predetermined frequency range, and so on. In some cases, the resonance of the main spring116may interfere with damping capabilities of the vibration isolation apparatus100.

To attenuate the multiple axial surge modes, one or more tuned mass dampers118,120are mounted to the main spring116, in an embodiment. The tuned mass dampers118,120are each adapted to attenuate multiple axial surge modes, in an embodiment. In other embodiments, one or more of the tuned mass dampers118,120may be further adapted to attenuate lateral surge modes. Although two tuned mass dampers118,120are included in the depicted embodiment, a single tuned mass damper may be included in one embodiment. In another embodiment, more than two tuned mass dampers may alternatively be included. The number of tuned mass dampers included on the main spring116may depend on the particular frequencies of the axial surge modes and/or lateral surge modes to be damped, and/or the particular dimensions of the main spring116.

FIG. 3is a cross-sectional view of a tuned mass damper300, in accordance with an embodiment. The tuned mass damper300includes a housing302that is generally cylindrical and that defines a cavity304. According to an embodiment, a first mass306, a second mass308, a first tuned mass damper spring310, a second tuned mass damper spring312, and a fluid (not shown) are disposed in the cavity304. The first and second masses306,308are ring-shaped and have inner diameter surfaces316,318that form openings. In an embodiment, the first tuned mass damper spring310couples the first mass306to an endcap324, and the second tuned mass damper spring312couples the first mass306to the second mass308. The endcap324is disposed over an end of the cylindrical housing302and seals the cavity304. In accordance with an embodiment, a shaft326protrudes from the endcap324to extend through the openings of each mass306,308. In an embodiment, the endcap324may include an aperture328that is formed along its length to provide communication from the cavity304to a temperature compensation chamber330formed in the endcap324. The fluid fills the cavity304, the aperture328, and the temperature compensation chamber330, and surrounds the masses306,308and the springs310,312. A temperature compensator bellows332may be disposed within the endcap324in communication with the temperature compensation chamber330, and may be adjusted to compensate for thermal expansion of the fluid. Optionally, a flange334may extend outwardly from the cylindrical housing302for coupling the tuned mass damper300to a main spring (e.g., main spring116ofFIG. 2).

The tuned mass damper300is configured to damp multiple axial surge modes. In this regard, the masses306,308combined with springs310,312may be configured to act as a single mass that resonates at a particular frequency to thereby damp a first axial surge mode of a structure (e.g., the main spring116) that may be experienced by the structure at a frequency in a first frequency range. According to another embodiment, the masses306,308combined with springs310,312may be configured to act as a single mass that resonates at a particular frequency to thereby damp a second axial surge mode of a structure (e.g., the main spring116) that may be experienced by the structure at a frequency in a second frequency range. In yet another embodiment, the second mass308(or the mass having an end coupled to a tuned mass damper spring312and unconstrained end) in combination with the spring coupled adjacent thereto (e.g., spring312), may be configured such that it resonates with a particular frequency at an amplitude to damp the second axial surge mode of the structure, where the amplitude is more than the amplitude at which the first mass306resonates when the two masses306,308are subjected to the frequency in the second frequency range. In another embodiment, the frequencies in the first and second frequency ranges at which the first and second masses306,308may damp may be less than the frequencies in the first and second frequency ranges at which the structure may experience the first and the second axial surge modes, respectively. For example, the frequencies in the first and second frequency ranges at which the first and second masses306,308may damp may be in a range of about 5% to about 10% lower than the frequencies in the first and second frequency ranges at which the structure may experience the first and the second axial surge modes, respectively.

Generally, to design the tuned mass damper300to resonate in the manner described above, desired frequencies to be damped by the tuned mass damper300are first selected. In an embodiment, two desired frequencies may be selected; however in other embodiments, more may be selected. According to an embodiment, the desired frequencies of the tuned mass damper300may coincide with the first two frequencies (e.g., axial surge modes) of the structure to be isolated (e.g., main spring116) and may be obtained by appropriate selection of the weights of the first and second mass306,308and the stiffness of the first and the second tuned mass damper springs310,312. In an embodiment, Equation 1 may be used as part of the sizing selection process.

k1=stiffness of the spring connecting the first mass and ground (e.g., first tuned mass damper spring310);

k2=stiffness of the spring connecting the first mass and the second mass (e.g., second tuned mass damper spring312); and

M1, M2, k1 and k2 have consistent mass and stiffness units.

In other embodiments, more exact analyses which include viscous damping, may be employed.

In order to design a tuned mass damper having particular characteristics, several conditions may be established prior to calculating the values for M1, M2, k1, and k2. For example, a system (e.g., system100) may have a desired total weight limitation. Thus, in an embodiment, the total mass of the two masses of a tuned mass damper (e.g., first mass306and second mass308) may be set to about 5% of the mass of a structure having a vibration to be damped (e.g., main spring116). In another example in which space limitations may exist within the tuned mass damper (e.g., tuned mass damper300), the tuned mass damper may be desired to include a larger mass (e.g., first mass306) and a smaller mass (e.g., second mass308), where the larger mass may have a mass that is about 50% larger than the smaller mass. In still another example, the tuned mass damper may be designed to have a certain acceptable tolerance. For instance, the two mass damper springs may be selected such that the tuned mass damper reduces the magnitude of vibration at the first and second axial surge modes by about 5% to about 10%.

Although the first mass306is depicted as being larger than the second mass308, other embodiments may include the first mass306as being smaller than the second mass308. No matter the particular configuration, one mass (either first mass306or second mass308) may be smaller or larger than the other mass to tune the tuned mass damper300to frequencies that damp the first and second axial surge modes. Moreover, although two masses306,308are shown in the tuned mass damper300, more masses may alternatively be included. In particular, in embodiments in which more than two axial surge modes are experienced by the structure having a vibration to be damped (e.g., main spring116), more than two masses may be included and each mass may be selected to damp one or more of the axial surge modes. For example, in embodiments in which three axial surge modes are experienced by the structure having a vibration to be damped, three masses could be included in the tuned mass damper.

In any case, by connecting the endcap324, the first tuned mass damper spring310, the first mass306, the second tuned mass damper spring312, and the second mass308in series with each other and by including fluid in the tuned mass damper, a damped two degree-of-freedom oscillator is formed. Additionally, the first two resonant frequencies of the tuned mass damper can be tuned to match desired frequencies by adjusting various design parameters, such as mass, spring stiffness, and damping values.

In addition to damping axial surge modes, the tuned mass damper300may be configured to damp a lateral spring surge mode, in accordance with another embodiment. The lateral spring surge mode may be a frequency at which the tuned mass damper300(or a component, e.g. main spring116) vibrates laterally and may result from operation of the tuned mass damper300itself. To provide a damping mechanism for a lateral spring surge mode, a gap between one or both of the inner diameter surfaces316,318of the masses306,308and an outer surface of the shaft326may be particularly sized. For example, to provide an increased stiffness, the gap between the inner diameter surfaces316,318of one or both of the masses306,308and the outer diameter surface of shaft326may be decreased. Likewise, to provide decreased stiffness, the gap between the inner diameter surfaces316,318of one or both of the masses306,308and the outer diameter surface of shaft326may be increased.

To further optimize the ability of the tuned mass damper300to damp particular axial surge modes of a structure having a vibration to be damped (e.g., main spring116), the tuned mass damper300may be disposed at particular positions on the structure. For example, returning toFIG. 2, in an embodiment, the tuned mass dampers118,120may be located at axial positions that are between two locations, where the first axial position160is located in a range of about 30% to 45% of the length of the main spring116when the length is measured from the end144to the end130with a first axial position160located at about 37.5% of the length of the main spring116, being preferred, in an embodiment. The second axial position162is located in a range of about 55% to about 70% of the length of the main spring when the length is measured from the end144to the end130, with the second axial position162located at about 62.5% of the length of the main spring116, being preferred, in an embodiment. However, it will be appreciated that the particular axial positioning of the tuned mass dampers118,120may depend on the particular axial surge modes to be damped, the particular dimensions and configuration of the main spring116, the size and configuration of the payload and/or base, and/or other factors. In an embodiment, one or both of the tuned mass dampers118,120are directly attached to the main spring116. In another embodiment, a flange154may be mounted directly to the main spring116, and the tuned mass dampers118,120are attached to the flange154, as shown inFIG. 2.

Although shown attached to an outer surface of the main spring116, the tuned mass dampers118,120may alternatively be attached to an inner surface.FIG. 4illustrates a cross-sectional view of a vibration isolation apparatus406including a main spring416having an inner surface458. Tuned mass dampers418,420are attached to the inner surface458. To maintain damping capabilities in such an embodiment, the tuned mass dampers418,420are disposed such that they do not contact any of the components that are contained within the main spring416.

A vibration isolation apparatus has been provided that exhibits improved damping capabilities over conventional vibration isolation apparatus. In particular, tuned mass dampers are included on the improved vibration isolation apparatus that are each capable of damping multiple axial surge modes. In some embodiments, each tuned mass damper is further adapted to damp lateral surge modes. By allowing a single tuned mass damper to damp multiple surge modes, fewer components, and hence, less weight and space, may be occupied by the vibration isolation apparatus.