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Control of Smart Structure

3. Aerospace Applications of Control of Smart Structures --Part 2

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3. Aerospace Applications of Control of Smart Structures -- Part 2

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Control of Smart Structure

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Outline • Vibration Damping – Active Damping – Passive Damping • Vibration Isolation Applications in Aircraft and Spacecraft • Active Shape Control – Space applications – Applications in fixed wing aircraft – Applications in rotary wing aircraft • Acoustic Control • Smart Skin for Aerospace Applications • A New Smart Actuator for Aerospace Applications • Health Monitoring Using Smart Materials

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s

• Passive Vibration Damping using SMAs Source: SPIE Smart Materials and Structures Conference, 2001

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s • The hysteresis between transformation to martensite and austenite in a SMA is an intrinsic material property to be used to enhance damping. • The transformation process between martensite and austenite is known for decades. Practical use of the resulting shape memory alloys (SMA) has been limited so far to switching between these two phases only and has resulted in clamps, switches or springs. • A large portion between these two phases is worth considering. • In terms of adaptive structures, where actuation comes into play, this portion of the material's constitutive behavior is unfortunately complex, especially when compared to the effort which has just been required to consider elasticplastic behavior of materials instead of elastic behavior only. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s • Considering SMA constitutive behaviour may therefore require permanent sensing of the material's condition and a complex multi-parametric control law. • This complexity may be one of the reasons why SMA reinforced composites have been discussed for around a decade now but still have not achieved a stage of true practical application. • A solution for tackling this complexity with SMAs can be to concentrate on a few key elements and to clearly describe these elements in further depth; by looking at the possibilities to enhance the damping characteristic of composite materials by integrating SMA and specifically avoiding any additional built-in sensing. • SMA’s show a clear hysteretic behavior in the higher temperature austenitic stage, which allows for energy dissipation and are thus very well suited to enhance damping. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s • To better understand this effect and potential thus requires to: – Understand and analytically describe the SMA's constitutive behaviour regarding the different parameters such as training, damping, temperature, strain rate, prestraining, etc. – Have a mechanical model which allows to perform trade-off studies regarding the material combinations to be used in a SMA reinforced composite. – Identify the actions required and to be taken to determine and manufacture promising SMA reinforced composites and composites in general with enhanced damping properties. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s • SMA constitutive behavior :The SMA constitutive behaviour can be

affected by a variety of parameters such as prestrain, strain/stress amplitude, temperature, strain rate, training, chemical composition, heat treatment or many others.

• Since the SMA is intended to enhance the damping of the SMA composite, it is worth determining where the SMA can have its highest damping contribution. Since the specific damping capacity (SDC) can be defined as the specific damping energy ED versus the specific strain energy ES, ED can be calculated from the stress-strain hysteresis measured in the experiment.

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Passive Vibration Damping-SMA’s • The optimum damping is obtained when ED is maximized and thus the hysteresis according to the applied loads fully falls into the stress-strain hysteresis of the SMA. In practical terms if one wants to take full advantage of a 0.5 % strain amplitude one has at least to prestrain the SMA up to 1.5 % in mean strain to take maximum advantage of SDC. • A similar analysis can be made when considering the influence of the strain amplitude. Here again the bigger the strain amplitude the more the SMA prestrain has to be set towards the centre of the full SMA hysteresis. (Details in the paper)

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s • SMA-Structure Coupling: To roughly understand the interaction between a host structure and an actuation device it is quite sufficient to select a relatively simple mechanical device in a first step compared to the complexity being faced in a true composite material. • Such a simple device is shown , as the schematic of a beam coupled with SMA-wires pinned to the end of the beam.

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Vibration Damping-SMA’s

Damping for beam model with SMA wires pinned perpendicular to the beam axis considering different distances between the SMA transformation levels (thickness of hysteresis) Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Comparison of beam tip deflections for a non reinforced and 15 vol % SMA reinforced beam with 8 MPa distance between the transformation levels

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Passive Vibration Damping-SMA’s • Conclusions: Whenever SMA is considered as a means to enhance damping in a composite, the SMA has to be prestrained such that the starting point for any further loading is well positioned within the strain ranges where transformation occurs. • Heat transfer effects are an important issue wherever large quantities of energy are dissipated, being either with high volume percentages of SMA and/or high strain rates being generated through high vibration frequencies. • Under these conditions it is essential to use models describing the constitutive behavior of SMA which also include the effect of heat transfer and are therefore thermodynamics based. • Finally the damping characteristic of a SMA is proportional to the distance in stress between the two transformation levels of the SMA as well as the applied strain. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation

• Vibration Isolation using Piezo Struts – Recent Advances Source: SPIE Smart Materials and Structures Conference, 2001

Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- VISS • The success of recent space experiments not only demonstrates the feasibility of several new technologies, but also provides a glimpse of the various future opportunities available for research and development in the smart structures area. • The currently operating Vibration Isolation, Suppression, and Steering (VISS) space experiment and the Middeck Active Control Experiment Reflight (MACE-II), as well as the upcoming Satellite Ultra-quiet Isolation Technology Experiment (SUITE) are discussed in terms of notable achievements and lessons learned over the course of their execution. • As part of a joint program with the United Kingdom to build a small experimental payload, the Space Test Research Vehicle-2 (STRV-2), The Ballistic Missile Defense Organization (BMDO) and AFRL funded a project with Honeywell, Trisys, and the Jet Propulsion Laboratory (JPL) to design, fabricate, and test the Vibration Isolation and Suppression System (VISS). Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- VISS • The system was developed in order to demonstrate the feasibility of isolating an optical system from broadband spacecraft bus disturbances by a minimum of 20 dB over 1-200 Hz, to reduce cryocooler-induced vibration of the platform by 20 dB at the first 3 cryocooler harmonics, and to provide precise fast steering (±0.30o) for the platform telescope. • VISS, was a winner of the SPIE Smart Structures Product Implementation award in 1998.

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Vibration Isolation- VISS • VISS, the first space demonstration of active vibration isolation using a hexapod Stewart platform, utilizes six hybrid isolation struts. • Passive isolation is provided by Honeywell's D-Strut, which is very compliant at the six hexapod suspension frequencies in the 2 to 5 Hz frequency range. The D-strut contains viscous fluid that is exchanged between metallic bellows through narrow orifices as the piston moves, providing damping. • Active isolation at lower frequencies is achieved through the use of a voice coil mounted in parallel to the D-strut. The active system can effectively lower the hexapod suspension frequencies by an order of magnitude so that isolation is achieved over a broader frequency range. • The active system may also be used for vibration suppression and steering functions. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- VISS • Vibration suppression is needed to counteract disturbances from noisy devices, such as cryocoolers, that are directly attached to the optical payload. The steering function enables the VISS device to be used as a precision tracking gimbal for the optical payload. Accelerometers mounted to the payload side of each strut are used as feedback sensors for all of the control functions. • The system reverts to its passive isolation performance in the event of a power failure. • The success of VISS serves as jumping off point into several new technology development opportunities. The first possibility for continuing to explore the on-orbit isolation problem is to incorporate active materials into future solutions. • Although a prior effort, the ACTEX-I2 flight experiment – developed by TRW and sponsored by AFRL and BMDO, demonstrated the use of piezoelectric patches embedded in a composite strut for vibration suppression, active materials have not yet been demonstrated in an on-orbit isolation capacity. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- VISS • The increased power density of such materials offers the possibility of power-efficient, low-weight devices that meet or exceed the performance of VISS. • The second avenue of opportunity in on-orbit isolation involves miniaturizing the system. Such an approach would offer a benefit in terms of volume, and hence cost, and offer the possibility of retrofitting an isolation solution late in a design cycle. • A third avenue exists in terms of developing tethered components, which by their very nature are isolated from the spacecraft bus. • On the path toward implementing active materials in isolation solutions, the Satellite Ultra-quiet Isolation Technology Experiment (SUITE)3 was designed and fabricated.

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Vibration Isolation- VISS • The SUITE flight hardware, consists of a hexapod assembly of six hybrid active/passive struts to provide vibration isolation and control of the platform in six degrees-of-freedom. • Each strut contains a damped mechanical flexure to provide passive isolation above 28 Hz, with a piezoelectric stack actuator in series to provide control actuation. • Unlike VISS, SUITE is not designed for steering, since its piezoelectric actuators exhibit a considerably smaller stroke than the voice coil implementation, but it has been designed with the capability to be reprogrammed to test a variety of control algorithms. • The system is roughly comparable to VISS in terms of performance and mass and occupies less volume, but requires slightly more power. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- VISS ON-ORBIT ADAPTIVE STRUCTURAL CONTROL The Middeck Active Control Experiment (MACE), flown on STS-67 in March 1995 as shown in Figure 4, was funded by NASA Langley Research Center (NASA LaRC) and jointly developed by the Massachusetts Institute of Technology (MIT) and Payload Systems to demonstrate the effectiveness of structural control in improving spacecraft stability and to assess the predictability of controller performance based on analysis and 1-g testing.

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Vibration Isolation- VISS •

The experiment was highly successful, demonstrating that structural control could be effectively accomplished using such techniques.



It also revealed several limitations of the model-based fixed-gain linear control approach, including the significant expense and time associated with developing high fidelity finite element models needed for control design, loss of robustness due to unknown or unmodeled 0-g dynamics, difficulties in handling nonlinearities, and the potential for loss of performance or instability due to time-varying dynamics or sudden failures of sensors and actuators.



These difficulties have engendered significant interest in the use of adaptive methods for controlling structures in high precision aerospace applications.



In addition, system dynamics often tend to be time varying due to either slow changes as the result of the degradation of materials and spacecraft aging or sudden failures such as the loss of a sensor or actuator.



These events are more likely with increasing complexity and service life. Adaptive methods have the potential to significantly reduce cost and increase system performance by reducing modeling, testing, and operations and maintenance requirements as well as increasing reliability.

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Vibration Isolation- VISS • By achieving this important step, opportunities exist for applying adaptive structures technology to precision deployable structures. • The Deployable Optical Telescope Test bed is a highly controlled, sparse aperture telescope requiring nanometer precision. Ground testing for such a system is an extremely difficult endeavor due to the fact that even the tiniest of vibration sources become significant in the face of such precision. • On-orbit the dynamics could change, providing the opportunity for further development of the technologies pioneered in the adaptive structures foundation built with MACE-II. • NASA’s Next Generation Space Telescope (NGST) is also a precise deployed optics structure that will face the same technological challenges. • Opportunity also exists in the field of ultra-lightweight membrane structures. Slight changes in the internal pressure of an inflatable on-orbit radar dish, for example, may lead to large-scale changes in the shape and wave dynamics of the structure, requiring adaptive control schemes. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation

• An Example of Active Vibration Isolation Source: SPIE Smart Materials and Structures Conference, 2001

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Vibration Isolation- Launch and On-orbit Isolator • The isolators are based on Honeywell’s patented three-parameter hermetically-sealed viscous D-Strut. • This isolator strut design demonstrates consistent linear damping and isolation over several orders of magnitude of input displacement and over a useful on-orbit temperature range • It supports and protects its payload during launch environments, and subsequently provides micro-inch level jitter reduction on-orbit. • An elliptical isolation.

hexapod provides six-degree-of-freedom support and

• The fluid-damped D-Strut isolation system maintains its payload optical alignment after vibration and thermal exposure. • Vibration tests at one micro-inch input and at one- tenth of an inch input show almost identical damping and isolation responses. • The 70-lb test payload was made from wood with an aluminum backbone. • The payload provided accurate mounting geometries for the six isolator struts, and precision locations for ten accelerometers and an optical cube. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Launch and On-orbit Isolator • The modified elliptical hexapod provides sixdegree-of-freedom support and isolation for both launch and on-orbit environments. • The D-Strut. isolation system maintains its payload optical alignment after vibration and thermal-cycle testing, with no stiction, low thermal pointing, and with controlled, repeatable isolation performance. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Launch and On-orbit Isolator • A rectangular box shaped optical payload weighing seventy pounds, with functions of optical communication or image collection, served as point design. • A fast steering mirror (FSM) as part of the payload would provide low-frequency disturbance correction and pointing. • The passive three-parameter D-Strut is an excellent compliment to an FSM because it provides adequate and linear damping at FSM frequencies and it can reduce vibration to the payload above the FSM bandwidth, both during launch and on-orbit. • The cg-mounted hexapod or Stewart platform, with six identical isolator struts, is a good starting design for a six-degree-of- freedom isolation system. To minimize coupling among the modes, the isolator mounting circle ideally would be centered around the payload center of gravity. • Spec required no isolator break frequencies below 20-Hz, with at least a factor of five attenuation above 100-Hz for all degrees of freedom. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Launch and On-orbit Isolator STRUT DESIGN AND TESTS OF 3-PARAMETER ISOLATOR • Flight heritage D-Strut solution uses titanium mainsprings to reduce weight, and locate the bellows in series with the spring giving a column-like D-Strut. • Frictionless blade flexures are often used as pivot mounts. • Viscous damping force is created by fluid flow through a restrictive orifice between two damping chambers, with the damping assembly hermetically sealed by metal bellows. • Three-parameter damping requires a Kb spring somewhere in series with the damper element, in addition to the Ka main spring in parallel with the damper. This particular Honeywell D-Strut. uses an internal metal helical compression spring to fine tune Kb stiffness. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Launch and On-orbit Isolator

• Two tests were performed on each completed strut to determine its stiffness, damping, and isolation characteristics. A complex mechanical impedance test measured the static stiffness, dynamic stiffness, and damping coefficient of each strut. • Peak displacement during impedance testing was 1 to 5-thousandths of an inch. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Launch and On-orbit Isolator • Shock testing, launch-level random vibration, and launch sine vibration were also conducted. • The system was also subjected to thermal cycling. Functional transmissibility tests were performed before, midway, and after launch environments, at 0.25-g and 2.5-g sine input levels. • In summary, a single passive isolation system can perform well both during launch and during on-orbit flight.

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Vibration Isolation- Miniature Isolation system • A Miniature Vibration Isolator Source: SPIE Smart Materials and Structures Conference, 2001

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Vibration Isolation- Miniature Isolation system • Introduction: Space-based optical sensors demand maneuvers with exacting motion. They can stand very little unsettling movement such as vibration or jitter and still perform optimally. •

Major sources of jitter for most satellites include momentum/reaction wheels, solar array drive mechanisms, and specialized devices with a moving or rotating mass such as cryogenic coolers.



As optical satellites evolve their structures are becoming larger and more flexible. This makes their payloads more susceptible to jitter than ever before.



The Miniature Vibration Isolation System (MVIS) can be utilized at either or both locations, with minimal envelope and mass impact.



The MVIS program was sponsored by the U.S Air Force Research Laboratory to develop technology that performed isolation functions and flight demonstrate a reliable, application-flexible, low-cost miniature alternative to the larger Vibration Isolation Steering, and Suppression system.

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Vibration Isolation- Miniature Isolation system

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Vibration Isolation- Miniature Isolation system • MVIS SPECIFICATION: MVIS system active stage attenuates low frequency jitter, and the passive stage removes high frequency disturbances. • The active stage strut level requirements stem from the worst case disturbance. Worst case disturbance values were determined to be around 2000 micro-inches (with the exception of Space Station). The active stage was designed using these performance goals while utilizing minimal real estate. ¾ A few observations can be made about the performance requirements for MVIS. • Displacement-sensitive Space Station Applications will be left to the larger size, longer-stroke, VISS hybrid technology or soft passive isolation, for now. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system • MVIS did not remove all of the payload displacement. This is because a significant portion of the disturbance for the cases studied was in 1-Hz to 3-Hz frequency band, and MVIS is designed for disturbances above that range, with at least20dB of attenuation from 5 to 200-Hz. • Because most optical systems have a fast-steering mirror with DC to at least 5-Hz bandwidth, and because many force-sensitive payloads are insensitive to low-frequency inputs, MVIS will effectively isolate most on-orbit precision payloads. • Ground testing (with simple offload), and low-frequency payload reactions and on-board disturbances, can be accommodated by increasing the passive D-StrutTM stroke capability using heritage designs. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system • Comparative performance shows that for payloads requiring large strokes VISS is an optimal solution whereas MVIS becomes a more attractive solution for payloads requiring less stroke.

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Vibration Isolation- Miniature Isolation system MVIS SYSTEM ARCHITECTURE • Best candidate hardware architecture was a deterministic hexapod, or Stewart Platform, mount because six identical elements could be utilized, lowering cost and maximizing 6-degree-of-freedom isolation performance.

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Vibration Isolation- Miniature Isolation system HYBRID D-STRUTTM ELEMENT • The Hybrid D-Strut element used for the MVIS system is truly unique and incorporates several synergistic features to efficiently perform its required functions. • This is a piezoelectric based actuator just over one inch long that strokes as much as a nine-inch long piezoelectric actuator considered current state of the art (.003 inches). • The actuator is combined with the time proven D-Strut in such a manner as to behave like a kinematic linkage to be used in deterministic hexapod mounts.

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Vibration Isolation- Miniature Isolation system • Figure illustrates how MVIS performs its vibration isolation function. Base disturbances are sensed and rejected by the active stage over a prescribed frequency bandwidth. • The passive stage damps isolator resonance and removes high frequency disturbance components easing the active stage performance burden. • The launch lock system shunts the hybrid isolator during launch and is removed once the system is system is called into operation. • The one-dimensional schematic of Figure can be easily expanded to a full 6 degree of freedom system by configuring 6 hybrid elements into a hexapod or Stewart Platform. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system • Passive D-Strut sizing involves determining the necessary bellows static and volumetric stiffness (series stiffness with damper) and also optimizing the damping coefficient. • Aspect ratio and number of convolutes determines bellows stiffness, while damping coefficient is a function of flow restriction geometry and fluid viscosity. • Dynamic relationships between the three parameters, Ka, Kb, and Ca of the D-Strut are presented in Figure . • The Bode plot shown in Figure can be generated for a strut element by fixing one end of the strut to ground and vibrating the opposite end of the strut over the desired frequency band. • Measurements of the elements force, displacement and phasing relationships over this frequency range provides the information needed to produce the dynamic stiffness plot. • By strategically placing the isolator parameters excellent damping at resonance, and a second-order roll-off after resonance can be achieved. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system

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Vibration Isolation- Miniature Isolation system HYBRID STRUT PERFORMANCE • Two hybrid struts were developed and the hybrid element was made up of an active and passive stage. • The active stage accomplishes actuation using a proprietary piezoelectric based actuator. The active stage is complemented by the addition of a serially mounted miniature D-Strut. • This combination allows the hybrid element to behave as a kinematic link improving the linearity and predictability of the MVIS hexapod. • The addition of an integrally mounted miniature accelerometer completes the element and provides the feedback sense necessary for active control. • Each completed element weighs 0.2 lb. The Hybrid Strut element measures just over 1 inch in diameter and 2 inches long making it easily packaged within most payload envelopes. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system

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Vibration Isolation- Miniature Isolation system Active Stage Performance: • An open-loop test was conducted to determine if the actuator section would perform both statically and dynamically. The MVIS strut was designed for an active stroke of 0.003-inch (+/-0.0015). • A Micro-sense non-contacting displacement probe was used to measure displacement. • The actuator is located where the red wire enters the base of the MVIS strut. A stand and cylindrical adapter (the black wire crosses the adapter) are below the MVIS active element. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system • The results of static testing were encouraging in that 0.003-inch extension was measured repeatedly. • The dynamic test required that a sine sweep be put into the driver. The dynamic sweep was performed, and a frequency response from DC to approximately 1500-Hz was obtained. • The response is flat to about 600-Hz, where the actuator resonance occurs. This usable bandwidth is well beyond what is necessary for MVIS application (< 200 Hz.). Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system Passive Bipod Transmissibility • The two struts are each at an angle of 45-degrees. This geometry has the property that the bipod has the same axial stiffness as each single strut.. The bipod also requires more fixturing and structure, so that fixture resonances at high frequencies are more likely. This strut was for the frequency and damping (peaking) of the first lateral mode, at somewhere above 80-Hz..

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Vibration Isolation- Miniature Isolation system • Above 700 Hertz, the high frequency attenuation is shown to be affected by fixture or structural resonances, at a noise floor 25 times lower than the input. • There is also some noise on the slope at 200 Hertz, which may be the shaker/stiffening bar resonance (the bipod setup is not as stiff as the single-strut setup). • There are no apparent lateral modes of the struts near the expected 80 Hertz. The 200-Hertz noise could be well-damped lateral modes, but the consensus is that lateral modes are probably at a lower frequency, but are so well damped that they don’t show at all. • This is exceptional isolation performance, and the complete hexapod of six struts should perform almost exactly like the demonstrated bipod curves. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Vibration Isolation- Miniature Isolation system CONCLUSIONS • The passive strut transmissibility test results are excellent. • The active stage transfer function has more than adequate (> 3 times bandwidth) to perform the MVIS isolation function. • The bipod tests are much better than expected, since the lateral modes internal to the strut element were very well damped and therefore non-existent. Passive performance of the MVIS D-Strut. system is excellent in every respect. • The outstanding performance of the active stage of the hybrid strut hardware has great application beyond just high performance isolation systems. The D-Strut portion of the Hybrid shows excellent damping and roll-off characteristics. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Whole-Spacecraft Vibration Isolation (Passive) Source: CSA Engineering

Passive Isolation for payloads (PIP)

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Passive Isolation for payloads (PIP) Source: CSA Engineering

CSA Engineering has developed a vibration isolation system for use on a medium launch vehicle (Delta II class) which will reduce structureborne lateral loads imposed on a spacecraft from launch vehicles. Whole-spacecraft isolation is a challenging problem requiring a great deal of system-level and detail design engineering. The concept was to incorporate an isolation system into the payload attach fitting (PAF), which is the structure that connects the spacecraft to the launch vehicle. This program used actual models of spacecraft and launch vehicles. Selected launch environment loads were used in the design trade studies. At the conclusion of the design phase, complete coupled-loads analyses were also performed by McDonnell Douglas Aerospace to verify the performance of the isolation system. Full-scale prototype hardware (69 inches in diameter) was fabricated and tested to verify the analytical models. The isolated payload attach fitting was one-for-one replacement for the original, and flight version will weigh only 5% more than a conventional fitting. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Passive Isolation for payloads (PIP)

Source: CSA Engineering Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Active Shape Control-Materials and Actuators • Review of Materials and Actuators for Shape Control Source: SPIE Smart Materials and Structures Conference, 2001

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Active Shape Control-Materials and Actuators •

Electro active materials and hybrid components have been initially developed for the needs of the navy detection (sonars), the medical imaging and low speed sub micro positioning.



There was also an idea to use them as actuators, with a notion of mechanical power capability, for dynamic shape control of structure.



This activity, strongly encouraged from its start by the helicopter community, also initiated some interest on the fixed wing for the control of vortex moving on small lift surfaces.



The advanced concepts of self-adaptive control of helicopters rotor blades taking benefit of the so called smart materials have been in fact strongly sensed to solve the following problems of increasing difficulty : improvement of the aerodynamic efficiency by pushing away the stall limits, reduction of the vibrations to improve cabin comfort and gun shots accuracy, and reduction of noises caused by particularly intense vortex blades interferences during the descent flight .



These electrically driven materials, needing little space and having a short response time, are attractive for the manufacture of reduced scale demonstrators, in order to ascertain the merits of some new concepts by quantifying the gaps reached in the three domains.

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Active Shape Control-Materials and Actuators • At full scale, these devices are sensed to be also able to advantageously replace classical hydraulic or electromagnetic solutions thanks to clearly higher volume and mass, admissible force densities and running frequencies. • These materials present some drawbacks: a weak tensile strength requiring to pre-stress them in the case of the presence of alternate efforts, a shock brittleness, a high electrical capacitance, favorable to a noticeable heating in dynamic motion with the risk of loosing completely if partially the piezoelectric properties, and the reduction of life duration. • Powers can be developed with a weak electromechanical efficiency (lower than ten per cent in the best cases) are on the other hand very depending on material nature, geometry, sizes, polarization and acting modes, integration inside the structure and driving electronics whose output impedance is most often not adapted to the high capacitance loads to supply and whose current output is too much limited. • In any case to guarantee an optimal efficiency, their choice must be made in function of the power to transmit and their setting has to be done close to structure. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Active Shape Control-Materials and Actuators MATERIALS •

Except the restrictive considerations of mass, bandwidth, hysteresis and environmental stability , the performances in terms of maximal free deformation and of mass energy density are important parameters for the choice of materials.



The ultimate and decisive criteria are related to the toughness and damage tolerance capability.



The deformation of parts induces high stress levels which are necessarily supported by the active material. So the polymeric piezoelectric films, too flexible are quite unsuited.



Piezocomposites are dedicated to applications requiring a complex and rapid shaping of the surfaces but the deformations which can be generated are small and the carried efforts are low.



Piezoelectric fibers and even ribbons are promising on the two aspects of capable maximal deformation and mass energy density with the complementary benefits of complex shape warping, possible control of the anisotropy and the possibility to cover large surfaces with a relative safety.



Ceramic embedded in resin becomes less brittle.

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Active Shape Control-Materials and Actuators • Most ceramics used are piezoelectric because of their high availability in various forms and their easy handling. For actuation, only bonded bulk ceramics and stacks have been considered. Under the voltage and electrical field, soft ceramics have higher displacements. Their Curie temperature is low. •

Hard ceramics lengthen quite less but can be submitted to highest fields as well in the polarization as in the opposite directions, that gives them again a bit of competiveness.

• Hard ceramics are also more favorable to dynamic operation because their smaller capacitance necessitates a lower current feed. • The resulting heating is lower and allows a secure working towards a Curie temperature yet higher than soft ceramics one. • It is currently admitted that piezoelectric properties are not deeply affected up to a temperature equal to half the Curie temperature and that it is possible to operate according to the opposite side up to half the polarization field. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Active Shape Control-Materials and Actuators • Electrostrictive ceramics present at ambient temperature, deformations slighter higher than those of piezoelectric materials (from 10 to 20% highest). • This information is to be taken with care because it depends on the thickness and the sintering conditions of the product. • Bulk electrostrictive ceramics are in fact very delicate to elaborate at an industrial scale and properties are very sensitive to temperature, particularly around 0°C, and to the frequency of actuation. • Another advantage lies in the absence of polarity.

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Active Shape Control-Materials and Actuators • • •

• • • •

Stripped Stack Actuators: These actuators working according to d33 direction are supposed to produce the highest energies. Two families have to be distinguished: low voltage (100 to 200, and even 300 or 400 V) and high voltage (1000 to 1500V) types. Whereas no particular constraint of use exists in this domain for demonstrators which are tested in wind tunnel or on ground, only the first kind are suitable onboard the aircraft for obvious flight safety reasons. Low voltage actuators are obtained by the bonded assembly of elementary stacks of 20 to 40 co-fired layers, having for most of them a 80 to 120 µm thickness according to materials involved. The force-displacement curves look like the torque-speed characteristics of continuous current torque-motors, the maximal allowable effort or blocking force at inner position. The majority of them consists of soft ceramics because a long stroke is often looked for. Co-firing of very thin layers is a very delicate operation and is still more difficult to overcome for hard ceramics. If the strokes of these latter are smaller ,blocking forces and admissible stresses are generally higher.

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Active Shape Control-Materials and Actuators • In standard, low voltage stacks size doesn’t exceed a 10x10 mm2 section. • High voltage stacks are directly obtained by bonding of elements having a thickness between 0.5 to 2 mm. These stacks have generally a circular section in order to avoid the development of electric arcs which could originate from straight corners. • Diameters of actuators already assembled reach 75 mm. For a same total height and electrical field, as there are less bonded joints, the displacement is greater for low voltage than for high voltage stacks. • Nevertheless, because of a larger section and thus a best behavior to buckling, the total height of high voltage stacks can be increased and allow greater strokes. The slenderness ratio of active layers can reach 5 to 6.

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Active Shape Control-Materials and Actuators • Compared to piezoelectric materials, electrostrictive materials have very high capacitances and loss factors and changes significantly with frequency and temperature. • Other types of actuators such as moonies, cymbals, thunders and bi or multi-morphs presenting intermediate forcedisplacement diagrams show gradually larger runs and develop low forces.

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Active Shape Control-Materials and Actuators

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Active Shape Control-Materials and Actuators Actuators in Pre Stressed Casings • The linear actuators incorporate quasi exclusively soft PZT ceramics because for their primary purpose (micro positioning). • Polytec P.I proposes a large array of low voltage (100V) and high voltage (1000V) actuators. The stacks of these actuators are contained in a protective cylindrical stainless housing and are pre-stressed during assembly by a stack of ‘Belleville’ washers. • This prestress allows to work in the two directions but often dissymmetrical and more in compression than in tension. • The effective mechanical power available for the external work is then reduced compared to a pure induced strain stack by the internal work absorbed by the whole elastic element (i.e washers and housing). • Low voltage elliptic amplified actuators developed for micro positioning present large strokes while keeping quite satisfactory force transmission capability. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Active Shape Control-Materials and Actuators • Flap Deflection: The control of the deflection of a flap located on a rotor blade, based on a 1/3rd scale model , has been studied by addressing several principles using commercial actuators coupled to innovative linkages or the bending of beams realized by the action of couples of bonded electroactive ceramics. • Flap Deflection by Actuators: The first system uses a linear P.I actuator, centered on the pitch axis of the blade and coupled to a roller screw which clutches the axis to the flap through a fork. • Another variant of the motion transforming device consists of a lever arm with an orthogonal angle reverse which have some judicious flexible knee-joints. • The resistant hinge moment, representative of the action of the aerodynamic moment which applies on the flap and which is assumed to be linear with the deflection angle, is done by several sets of two stiff calibrated flexible strips. • The drawback of this first solution is the great inertia of moving parts and friction in the bearings.

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Active Shape Control-Materials and Actuators

• The second demonstrator, which was investigated and can be included in a OA312 airfoil with 140 mm chord, starts from a standard elliptic APA230 actuator (stroke : 210µm for 180 V) from Cedrat Recherche, coupled to a innovative kinematics for driving the flap . • This short and direct concept, limiting the number of moving parts, without play and friction sources, and driven by an adequate electronics has allowed to improve the performance. Department of Mechanical Engineering Dr. G. Song, Associate Professor

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Active Shape Control-Materials and Actuators • Flap Deflection by Beam Bending: The deflection of the flap is driven by a long beam bended by two piezoelectric actuators bonded on each side of the beam and electrically supplied according opposite phases. • The ceramics feeders are connected in such a way that the upper ceramic lengthens while the lower ceramic shortens. The beam takes a circular bending shape. • Experimental deflection of 19µm is measured.

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Active Shape Control-Materials and Actuators • •

• •

Active Twist: Related to twisting of the tip sections of helicopter rotor blade. Two methods were studied : the actuation by thick ceramics bonded in the inner side of the blade skin and the actuation by active fiber or ribbons active plies embedded inside the other plies of the blade skin. The main drawback of the first technique lies in the brittleness of thick ceramics and the resulting enhanced risk of supply failure. Active Twist by Bulk Ceramics: Actuators, crossed at 90°, and at ± 45° with respect to the longitudinal axis of the plate, are power supplied in phases. This configuration provides a pure torsion with a slight bending. The spacing between two pairs of elements is such as there is no encroachment and so no interference between their respective action.

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Active Shape Control-Materials and Actuators • Active Twist by Active Fiber or Ribbons Composites (AFC or ARC) • Fibers (ribbons) foreseen to manufacture these laminar actuators and generate a noticeable work have a diameter of 125 µm (a 125 x 500 µm section). Primer actuators with fibers necessitate the introduction of PZT powder to increase the dielectric coefficient and of fiberglass to stiffen the ply 20. The electrical field favoring the d33 elongation mode was applied.

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Active Shape Control-Materials and Actuators • • •

The new idea consists of embedding ribbons instead of fibers and to use the d31 as well the d33 mode which needs lower voltages. The use of ribbons, enables having round corners to avoid electrical arcs and also to reduce the cracking sensitivity. It also removes a more dense material in favor of a higher d33, a higher stiffness exempting from the use of additional fiberglass, a better piezoelectric coupling exempting from the introduction of PZT powder in the resin, a higher force generation capability and a better energy efficiency. Another benefits is easier implementation.

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Active Shape Control-Materials and Actuators •







Swelling of aerodynamic profile: The aim is to modify the aerodynamic profile of a relatively flexible wing model destined to low speed wing tunnel testing by a combining controlled increase of the relative thickness and of the twist of the wing tip. An experiment inflating the small length of elliptic constant section (long axis : 300 mm, short axis : 53 mm, spanwise 100mm) made up of 1.4mm of fiberglass fabric plies 21 was conducted. A 0.8 mm Nitinol wire, which is simple effect treated and whose transition temperature have been measured (As=54°C, Af=79°C, Ms=40°C and Mf=28°C), is set in the martensitic state and preloaded. A voltage of 1.86 V and 3 A were applied for a 45 s to modify the thickness as shown.

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