Isolation Mount Research Articles

Transient convection of Te doped GaSb induced by g-jitter

Gallium antimonide (GaSb) is commercially used for solar cells and thermophotovoltaic devices. It is also used as a substrate material for the epitaxial growth of heterostructures, which are used for optoelectronic applications. The semiconducting properties of the material are governed by an antisite defect that requires compensation by a donor dopant such as tellurium to provide n-type conductivity. The electronic and optical properties of Te doped GaSb are greatly influenced by the degree of Te segregation in the crystal. The latter is strongly affected by buoyancy and thermocapillary (Marangoni) driven convections. Both types of convections are associated with a temperature-induced density and surface tension gradients along the sample. During 1996 and 1997, nine microgravity experiments were conducted on the MIR space station to determine the segregation of dopants during microgravity crystal growth using the QUELD furnace on the microgravity isolation mount. The space-grown samples displayed fluctuations in [p] and hence in [Te] attributed to g-jitters events and not to Marangoni convection during crystal growth (J. Vac. Sci. Technol. A 18(2) 2000). On the numerical side, attempts were made to study the effect of convection and the g-jitter on the segregation of Te in terrestrial and in microgravity conditions. Since no data of the g-jitter were available during the experiment attempt is made to study the transient convection due to the two different types of g-jitters. In the first case, the time dependent gravity was introduced in the momentum equations. In the second case, the time dependent velocity was implemented at the boundary of the quartz sample. The full Navier–Stokes equation combined with heat and mass transfer equations were solved numerically using the finite element techniques. The results show that g-jitter and time dependent velocity applied at the quartz wall play an important role in segregation of tellurium in the melt. A detailed parametric study was performed in order to reveal the convection pattern under various conditions.

Canada's space protein crystal growth program prepares for ISS

In preparation for utilizing the International Space Station (ISS), the Canadian Space Agency's Microgravity Sciences Program (CSA MSP) is developing space hardware and supporting science in order to meet the needs of the Canadian scientific community. The utilisation philosophy includes a permanent presence on ISS on an EXPRESS rack. A Microgravity Vibration Isolation Mount (MIM) Base Unit will host simple visiting payloads in each of our research fields of materials science, protein crystal growth, and fluid science. Ground support, especially in the field of protein crystal growth (PCG), will be emphasized, in order to increase science success. Through the previous PCG missions the CSA has gained experience in this field. With our accumulated knowledge and an opportunity to utilize the Canadian allocation on ISS, we are now in a good position to fulfill the requirement of providing access to space through the use of a new, Canadian-designed apparatus built to meet our particular needs. The hardware development process, ground operations, and science support are being addressed as a single program, sharing resources at different phases of the project. The five main components of the program that will be discussed in this project are: Concept studies, Flight hardware development, Benchmark protein set, CSA PCG Mission Support Centre, and Science support.

THE VIBRATION ENVIRONMENT ON THE INTERNATIONAL SPACE STATION: ITS SIGNIFICANCE TO FLUID-BASED EXPERIMENTS

Measurements of the vibration environment on the Russian Mir space station and the US Space Shuttle are compared to the vibration levels predicted for the International Space Station (ISS) and to the vibration levels specified for isolated payload racks on the ISS. The vibration levels on the Mir and Space Shuttle are generally lower than the levels specified in the ISS isolation requirement for the isolated racks and much lower than the levels predicted for non-isolated racks. Next a summary is given of several materials science and fluid science experiments performed on the Mir and the Space Shuttle and designed specifically to examine the sensitivity from μ g to m g vibrations in the frequency range 1 Hz to above 100 Hz . These experiments were performed using the Microgravity Vibration Isolation Mount (MIM) operating in one of the three modes: isolation mode, non-isolation mode, and driven mode. Significant differences were obtained for the isolated versus the non-isolated experiments and for the driven mode experiments. This suggests that the current ISS vibratory specification, which can be achieved only through the use of isolation systems, is not conservative for many material science and fluid science experiments.

Noise radiation from a riding mower

Riding mowers are a significant contributor to community noise. This has prompted many countries to develop standards to reduce the noise emissions from riding mowers. This research will look at the individual noise sources involved with the riding mower; these are the engine, exhaust, cooling fan, the cutting blades, and the mowing deck housing. The relative contributions to the overall noise from each component were determined by isolating each component during outdoor measurements of the sound power levels. To investigate the mechanisms of the noise generated by the rotating mower blades, a test bed was built to hold the mowing deck and electric motor. The motor was isolated from the test bed via isolation mounts to provide a quiet source of power for the mower deck. Pressure sensors have been embedded into the leading edge of the blades and also at the grass exit to measure the disturbances as the blades and deck interact with each other. Results from these measurements are presented and discussed.

Isolation Design for Fully Flexible Systems

It is commonly suggested that vibration isolators should be designed with a low-frequency mount, that is, the isolator frequency is much less than the first mode of the system being isolated. This is a direct result of the single-degree-of-freedom (SDOF) isolator model which states that the best performing isolator is the one that has the lowest natural frequency. When flexibility is included in the isolator design, the low-frequency mount design still performs best in an overall sense. However, with system flexibility being modeled, the isolator can now be designed between the modes of the flexible system. These types of isolator mounts tend to have a coupling effect, that is, the frequencies around the isolator mount have such a strong interaction between each other that when isolator damping is present, multiple system modes are attenuated. Therefore, this paper fully explores the choice of isolator mount frequency and isolator placement for fully flexible systems. Results show that a low-frequency mount design considering base and equipment flexibility, will have the first natural frequency shift by as much as 15.6%. For a mid-frequency mount design, the shift of the first three modes can be as high as 34.9%, 26.6% and 11.3%, respectively. Also, when base and system flexibility is considered, isolator placement becomes a critical issue. There can be as much as 16% difference in the first mode for a low-frequency mount design and as high as 25% difference for a mid-frequency mount design.

Structural acoustic noise modeling of a supercavitating high-speed vehicle using statistical energy analysis

The simulation of propagation of structureborne vibratory and acoustic energy within a supercavitating high-speed underwater vehicle is studied herein. The supercavitating vehicle is rocket propelled and rides entirely within a cavity in order to reduce drag and allow for higher underwater speeds. A ventilation system exists within the vehicle to ensure the cavity completely encloses the vehicle. With the exception of isolated control surfaces, the only wetted portion of the vehicle is the cavitating sonar array at the vehicle nose. This paper focuses on the use of statistical energy analysis (SEA) to quantify and parametrize the high-frequency transmission of vibro-acoustic energy within the complex, multiply connected marine vehicle, as well as including contributions from the reverberant cavity and seawater acoustic paths. Additionally, an examination of the many potential noise sources is made, including a discussion on handling the effects of surface backscattering and volume reverberation within an SEA framework. Numerical case studies are performed to exercise the model in order to predict the effects of specific design modifications such as structural design variations, isolation mounting scenarios, etc.

Optimal Design of a Vibration Isolation Mount Using Physical Programming

Vibration isolation tables find application in diverse production and research environments. The structure of a table is such that a designer is forced to compromise among design metrics such as transmissibility, force-disturbance rejection, and controller effort. As both structural and controller parameters impact these design metrics, we chose to perform simultaneous control-structure integrated design (CSID) to optimize the overall performance. We employed a new model that enhances physical insight and used physical programming as the optimization framework. We explored several design scenarios and effectively uncovered the appropriate compromises among the competing objectives. The results highlight the utility of the new model in this design context and the usefulness of physical programming in performing simultaneous CSID.

Vibration isolation of two elastic structures using active compliance at the isolation mounts

A mathematical model of the vibration isolation of two elastically connected structures (via isolation mounts) employing active springs is presented. Two elastic structures are connected by an arbitrary number of springs, each made active by the inclusion of a piezoelectric pad at one of the spring and structure interfaces. Single-input–single-output feedback controllers at each spring support seek to minimize the force transmitted from one structure to the other. Solutions are based upon the formalism of Lagrange’s equations using the normal modes of the uncoupled structures as the generalized coordinates. The constraints by the springs and supports are enforced by Lagrange multipliers. A 2-D numerical example of two elastic beams is presented. One beam, driven by a harmonic disturbance force, is coupled to the second elastically supported beam through three active springs. A beam-relative displacement sensor colocated at each spring is the input to an arbitrary feedback filter which can incorporate practical amplifier characteristics and actuator dynamics. Results of the numerical simulation of the feedback control case are discussed.

Area averaging sensors for vibro-acoustic control

NRL has been conducting acoustic control research for underwater and in-air sound and vibration applications. In many of these studies, the success of the control system depends substantially on the ability of the sensors to measure the physical quantities of interest accurately while ignoring extraneous contributions. For example, the sensors developed for use in the Acoustic Boundary Control (ABC) research were required to monitor accurately the normal component of the average acoustic field on the tile surface, while being insensitive to transverse and high wave number components. The extremely low noise figures required in this study forced the development of special-purpose acoustic pressure and velocity sensors which now are finding use in other applications, including vibration isolation mounts.

The Effect of Vibration Isolator Properties and Structural Stiffness on Isolator Performance

The structural stiffness above and below a vibration isolation mount has an important effect on isolator performance. Knowledge of these as well as isolator properties is necessary to be able to predict the insertion loss of an isolator and the transmitted power to the foundation. A set of generalized mobility equa tions is used to analyze both the single-connection point and multiple-connection points cases. Experimental methods for determining the four-pole parameters, which describe the frequency-dependent dynamic proper ties of a vibration isolator, are discussed, and the test facility developed by the authors for the measurement of four-pole parameters is described. Test results for a commercially available isolator are presented. The importance of the structural dynamic properties above and below the isolator is illustrated by calculating the isolator effectiveness for different foundation properties.

Recent Measurements of Experiment Sensitivity to G-Jitter and Their Significance to ISS Facility Development

AbstractRecent experimental measurements of various microgravity experiments have been taken on the Mir and Space Shuttle under different conditions of microgravity using the Microgravity-vibration Isolation Mount (MIM). The results to date show a clear difference when the experimental measurements are taken from g-levels offered by the Mir and the Space Shuttle (non isolated) to g-levels offered by MIM (isolated) which have been reduced by two orders of magnitude. Concern for the International Space Station (ISS) experimental facilities arises when the quality of microgravity on the Mir and Space Shuttle (non isolated), which is believed to be not good enough, has been measured to be better than the ISS Requirement established by NASA for isolated racks, which will be significantly better than those racks not isolated.

DECOUPLING ACTUATORS WITHH∞ROBUST CONTROL FOR ACTIVELY ISOLATING VIBRATING MACHINES

A feedback controller designed to minimise the vibration transmitted to a flexible beam from a vibrating machine via two active isolation mounts is described. The state–space equation for the mechanical structure is obtained by performing modal analysis on the associated conservative structure.H∞control methods are used to derive a decoupled (diagonal) controller which provides robustness against unmodeled high-order frequency dynamics and optimises the performance in terms of the frequency response shaping of the modal accelerations. The performances of the decoupled controller were found to be similar to those of the coupled (global) one.

A control system for a microgravity isolation mount

This paper describes the development of a control system and associated technology for a microgravity isolation mount (MGIM)-a device to isolate sensitive scientific payloads on orbiting spacecraft from ambient vibration. The construction of the MGIM, and the specifications which it must satisfy, are briefly described; the control system which achieves this is described in more detail. A model for the MGIM is derived which includes the effect of asymmetric distribution of the payload mass. It is shown that this induces angular motion of the payload and violation of the tolerable microgravity level, unless counteracting control forces are produced. The technology required to support the concept is also described and the results of vibration tests conducted on a full-scale laboratory version of the MGIM are presented.

Performance of a Smart Vibration Isolator for Precision Spacecraft Instruments

This paper describes the configuration and measured performance of an hybrid, active-passive vibration isolation mount designed to mitigate vibratory disturbances on sensitive spacecraft instruments. The device, called the Active Vibration Canceler (AVC), consists of a passive isolation stage using composite leaf springs faced with shape-memory metal sheet material to provide load-adaptive damping, and an active, force-canceling stage using a motion-amplified electrostrictive stack actuator to augment attenuation of transmitted vibrations in the 10-120 Hz band. The paper discusses the features of the AVC design and active control architecture, and presents results of experiments characterizing the broadband performance of the device. Performance measurements show that the AVC provides up to 40 dB attenuation of base disturbances at 100 Hz, and that the electrostrictive active stage provides in excess of 10 dB attenuation over the passive isolator in the range 10-100 Hz. The measurements indicate that hybrid devices of this type can be effective in reducing transmitted vibrations to sensitive instruments, and that the incorporation of active elements can significantly enhance attenuation of broadband vibrations in isolation applications.

Vibration isolation mounting system

A system is disclosed for mounting a vibration producing device onto a spacecraft structure and also for isolating the vibration forces thereof from the structure. The system includes a mount on which the device is securely mounted and inner and outer rings. The rings and mount are concentrically positioned. The system includes a base (secured to the structure) and a set of links which are interconnected by a set of torsion bars which allow and resist relative rotational movement therebetween. The set of links are also rotatably connected to a set of brackets which are rigidly connected to the outer ring. Damped leaf springs interconnect the inner and outer rings and the mount allow relative translational movement therebetween in X and Y directions. The links, brackets and base are interconnected and configured so that they allow and resist translational movement of the device in the Z direction so that in combination with the springs they provide absorption of vibrational energy produced by the device in all three dimensions while providing rotational stiffness about all three axes to prevent undesired rotational motions.

Effect of intrinsic damping on vibration transmissibility of nickel-titanium shape memory alloy springs

A research study was undertaken to measure the transmissibility of nickel-titanium (Ni-Ti) shape memory alloy (SMA) springs and to compare the results to corresponding data on steel and IN-CONEL springs. It was motivated by interest in an effective metal alternative to rubber-based machinery isolation mounts, with possible active control features. Ni-Ti was used due to its well-known properties of shape memory and high intrinsic damping. Acceleration transmissibility was measured on a spring-mass system. Due to the distributed mass in the spring coils, standing waves occurred at high frequencies. However, due to the high intrinsic damping in Ni-Ti, the standing wave resonance peaks were as much as 20 dB lower than corresponding peaks in steel and 1NCONEL springs. Thus, the capability of Ni-Ti springs for high frequency acoustic isolation is significantly better than that of steel or INCONEL. Also, it is judged that the Ni-Ti material could be used in a variety of other isolation mount designs with a high likelihood for further improvement in passive isolation properties. In addition, it may be possible to use the shape memory effect (SME) in active control concepts.

Shock and vibration isolation mounting device having suspension vibration dampening

A mounting device providing a mechanically stable, spring-biased, yieldable suspension for printed circuit boards, and other vibration and shock sensitive objects, with respect to an article, and also providing frictional dissipation of the energy of shock and/or vibration into heat so that suspension vibration is rapidly dampened. The mounting device according to the present invention is composed generally of a pair of T-brackets, each T-bracket has a base and a perpendicularly oriented flange medially located on the base whereby each opposite end of the printed circuit board is connected with a respective flange of the T-brackets; two pairs of S-springs, each S-spring having a pseudo "S" profile characterized by an elongatedly curved section terminating at a distal end, an acutely curved section terminating at a near end, and a moderately curved section located therebetween, wherein the distal end of each S-spring of each pair of the S-springs is connected with a respective surface of two opposing surfaces of the article and wherein the apical of the acutely curved section of each S-spring of each pair of S-springs resiliently biases against a respective base; and a pair of V-springs, each having a pseudo "V" profile characterized by an apex and a pair of feet, wherein the apex of each V-spring is connected with a respective base and the feet thereof are resiliently biased against a respective adjacent article surface.

Mechanical design and simulation of a microgravity isolation mount for Columbus

A microgravity isolation mount (MGIM) designed to provide active vibration isolation for microgravity payloads is described. The required microgravity level is achieved by adopting a noncontact strategy, whereby the payload floats inside its enclosure and its position is controlled by magnetic actuators. The design concentrates on the requirements of materials science payloads and provides a detailed description of the locking mechanisms used to release and capture the payload during microgravity periods. A nonlinear model used to simulate payload release shows that a successful release depends on the severity of the input vibration level.

Vibration isolation support mounting system

A continuously active pressure accumulator power transfer system for a vehicle or the like comprises an engine, a pump driven by the engine, a main pressure accumulator maintained at a substantially constant fluid pressure and fluid volume by the pump during operation, a fluid motor for propelling the vehicle which is supplied with driving fluid pressure from the main pressure accumulator and a second pressure accumulator at a lower pressure then the main accumulator for supplying fluid to at least one vibration isolation mount.

Hybrid type vibration isolation apparatus

A hybrid type vibration isolation mounting apparatus for supporting a first member at a second member, and including a passive vibration absorbing device, such as a typical rubberized engine mount, located between the first and second members, and an active vibration actuator for applying counter vibration isolation forces in parallel in response to the detected relative vibration of the first and second members, wherein the actuator is located on a support mount frame attached to the second member. The support mount frame provides a structure for the actuator force application, with continuous transfer of static loads between the first and second member being provided by the passive device. In its preferred embodiment, the apparatus is utilized in conjunction with vehicle engine mounts wherein the first member comprises the engine and the second member comprises the vehicle frame.