No description available.

No description available.

Thesis (M. S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: Aleksandra Vinogradov. Includes bibliographical references (leaf 130).

No description available.

Presently, there is no standard testing procedure for piezoelectric actuators. It is then very difficult for a very specific given application to design the most efficient actuator in terms of blocked force, displacement, power consumption, weight, cost, etc. Piezoelectric actuator suppliers would like to have the possibility to fully characterize their actuators to be able to guide their customers on selection of the most suitable actuator based on their utilization. However, this is not an easy goal to reach since performance of a given actuator depends on the specific dynamic conditions under which it is applied. In order to characterize an actuator, it is therefore necessary to recreate similar conditions to those experienced in the real application. Because of the infinite variety of possible applications for piezoelectric actuators, physically recreating those conditions could take an enormous amount of time, means and money. The aim of the research is then to develop the technology required in order to test an actuator under a various range of dynamic load conditions using a single automated test set-up. To do so, a second actuator will be used with a suitable sensing apparatus (impedance head) and an active control system. Using data from the sensing apparatus (force and velocity signals), the active control system will drive the second actuator to recreate any load condition the first actuator would be supposed to experience in a real application. <i>[Vita removed May 14, 2012. Gmc]</i> / Master of Science

A structural health monitoring technique, developed at the Center for Intelligent Material Systems and Structures, employs piezoelectric (PZT) materials for tracking the structural impedance to qualitatively identify damage. The mechanical impedance of a structure is a function of the structure's mass, stiffness, damping, and structural boundary conditions. Changes in any of the above-mentioned properties lead to a change in the mechanical impedance of the structure and a change in the impedance pattern of the structure. The mechanical impedance of a structure can be measured by coupling the electrical and mechanical impedances via PZT patches. Therefore any change in the mechanical impedance leads to a change in the electrical impedance of the PZT bonded to the structure of interest. However, change of the electrical impedance can also occur due to changes in temperature. Piezoelectric materials have been known to have temperature dependency regarding their basic properties, such as the dielectric constant and the piezoelectric coefficient. In this thesis, this temperature dependency will be investigated. The motivation of this work is linked to the impedance-based nondestructive evaluation (NDE) technique which employs PZT sensors for tracking changes in the structural impedance, by measuring the electrical impedance, to qualitatively identify damage. However, for this NDE technique to be successful in all types of environments, it must be insensitive to temperature variations. As mentioned earlier, piezoelectric materials have strong temperature dependency and a temperature compensation procedure is necessary. Therefore, two software correction techniques were developed to eliminate the effects of temperature in the electrical impedance measurements of PZT sensors. (NDE) technique which employs PZT sensors for tracking changes in the structural impedance, by measuring the electrical impedance, to qualitatively identify damage. However, for this NDE technique to be successful in all types of environments, it must be insensitive to temperature variations. As mentioned earlier, piezoelectric materials have strong temperature dependency and a temperature compensation procedure is necessary. Therefore, two software correction techniques were developed to eliminate the effects of temperature in the electrical impedance measurements of PZT sensors. The second correction technique is based on the sensor output. Through experimental investigation, it was found that temperature will have the effect of shifting the electrical impedance magnitude of the piezoelectric sensor, while leaving the impedance phase unaffected. To characterize the temperature effects in PZT materials, a temperature coefficient which is independent of frequency has defined. Finally, based on the defined temperature coefficient, a simple temperature compensation technique has been implemented successfully, eliminating the effects of temperature on PZT sensors while not eliminating the effects of temperature on the structure. / Master of Science

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Bernard Dov Adelstein. / M.S.

An occupational exposure to hand-arm vibration can cause a complex of neurological, vascular and musculo-skeletal disturbances, known as the 'vibration syndrome'. However, the underlying pathophysiological mechanisms are not at all clear. Early signs of an incipient vibration syndrome are often intermittent disturbances in the cutaneous sensibility of the fingers, i.e. numbness and/or tactile paresthesias. At later stages, a vasoconstrictive phenomenon appears, usually as episodes of finger blanching. When using a vibratory tool, all mechanical energy entering the body has to be transmitted through, or absorbed by, the glabrous skin in contact with the handle. Therefore, the aims of this study was to investigate: (i) mechanical responses of the skin to vibrations, (ii) the response properties of cutaneous mechanoreceptors to vibrations, and (iii) influences of vibration exposure on touch perception. It was found by measuring the mechanical point impedance (0.02-10 kHz) that the skin is easy to make vibrate within the range of 80 to 200 Hz. Within or close to this range are the dominant frequencies of many vibratory tools. Thus, strong mechanical loads, such as compressive and/or tensile strain, can appear in the skin which, in turn, may induce temporary or permanent injuries. Recordings of impulses in single mechanoreceptive afferents, while the skin as exposed to vibrations, were obtained using needle electrodes inserted into the median nerve. The 4 types of mechanoreceptive afferents (FA I, FA II, SA I, and SA II) in the glabrous skin exhibited different response characteristics to vibrations. The FA I units were most easily excited at vibratory frequencies between ca 8 and 64 Hz and the FA II units between ca 64 and 400 Hz. The SA units were most sensitive at lower frequencies. At high stimulus amplitudes, such as may occur while using vibratory tools, a considerable overlap existed between the frequency ranges at which the units were exited. Evidence was also provided, that mechanical skin stimuli produced by edges of a vibrating object, compared to flat surfaces, more vigorously excited the FA I and particularly the SA I units. Thus, a marked edge enhancement, essential for tactile gnosis and precision manipulation, seems to exist already within the peripheral nervous system. Acure impairment of tactile sensibility caused by vibrations, proved to be due to a reduced sensitivity of the mechanoreceptive afferents. A loss of manual dexterity a*vi an increased risk for accidents may therefore appear, both during and after a vibration exposure. Percussive tools, high speed drills and ultrasonic devices are known to generate mechanical energy at frequencies above 1 kHz, i.e. frequencies usually not felt. At these frequencies, it is known that most of the energy, entering the body, is absorbed by the skin. Therefore, it was investigated whether a long-term exposure to high-frequency vibration may have a detrimental effect on the cutaneous sensitivity. One group of dentists and one of therapists, professionally exposed to high-frequency vibrations, were studied with regard to vibrotactile thresholds in their hands. The study showed that deleterious effects on tactile sensibility, at local exposure to high frequency vibration, can not be excluded. / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1985, härtill 6 uppsatser</p> / digitalisering@umu

abstract: The human ankle is a critical joint required for mobility and stability of the body during static and dynamic activity. The absence of necessary torque output by the ankle due to neurological disorder or near-fatal injury can severely restrict locomotion and cause an inability to perform daily tasks. Physical Human-Robot Interaction (pHRI) has explored the potential of controlled actuators to positively impact human joints and partly restoring the required torque and stability at the joint to perform a task. However, a trade-off between agility and stability of the control technique of these devices can reduce the complete utilization of the performance to create a desirable impact on human joints. This research focuses on two control techniques of an Active Ankle Foot Orthosis (AFO) namely, Variable Stiffness (VS) and Variable Damping (VD) controllers to modulate ankle during walking. The VS controller is active during the stance phase and is used to restore the ankle trajectory of healthy participants that has been altered by adding a dead-weight of 2 Kgs. The VD controller is active during the terminal stance and early-swing phase and provides augmentative force during push-off that results in increased propulsion and stabilizes the ankle based on user-intuitions. Both controllers have a positive impact on Medial Gastrocnemius (GAS) muscle and Soleus (SOL) muscle which are powerful plantar - flexors critical to propulsion and kinematic properties during walking. The VS controller has recorded an 8.18% decrease in GAS and an 9.63 % decrease in SOL muscle activity during the stance phase amongst participants while decreasing mean ankle position error by 22.28 % and peak ankle position error by 17.43%. The VD controller demonstrated a 7.59 % decrease in GAS muscle and a 10.15 % decrease in SOL muscle activity during push-off amongst the participants while increasing the range-of-motion (ROM) by 7.84 %. Comprehensively, the study has shown a positive impact on ankle trajectory and the corresponding muscle effort at respective stages of the controller activity. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2020

L'utilisation de plus en plus fréquente de matériau composite, qui combine une raideur importante pour une faible masse, afin d'alléger les structures aéronautiques entraîne la dégradation des performances d'isolation acoustique aux bruits extérieurs. La plupart du temps, ces nuisances sonores sont réduites par l'installation de matériaux isolants. Ces méthodes, dites passives, deviennent inefficaces aux basses fréquences et il est possible de mettre en place un contrôle actif au moyen de transducteurs électromécaniques. Dans le but de réduire la puissance acoustique transmise à travers la double paroi aéronautique dans la cabine, des unités de contrôle composées d'un actionneur et d'un capteur colocalisé dual sont réparties sur le panneau intérieur afin d'en modifier la vibration. Cette stratégie de contrôle actif vibroacoustique permet, pour des perturbations primaires harmoniques, d'imposer localement une impédance mécanique virtuelle à la structure, au moyen d'un contrôleur décentralisé. Cependant, sans communication entre les unités, le contrôle peut difficilement minimiser un critère global comme la puissance acoustique rayonnée. Afin de calculer les impédances mécaniques virtuelles qui garantissent la minimisation de la puissance acoustique rayonnée par la structure, une approche en deux étapes est considérée : (1) la matrice diagonale des impédances mécaniques virtuelles optimales est calculée à partir de mesures acoustiques ou vibratoires de la perturbation primaire et des transferts avec les actionneurs secondaires, (2) l'objectif exprimé en terme d'impédances mécaniques virtuelles est atteint grâce à un contrôle en temps réel. Une attention particulière est portée à la comparaison de cette approche avec une stratégie classique d'amortissement actif réalisée par un contrôle par rétroaction sur la vitesse de la structure, où l'impédance mécanique virtuelle alors imposée est un réel positif. Le calcul optimal réalisé à l'issue de la première étape se faisant pour une perturbation primaire donnée, la robustesse de la méthode aux variations de la perturbation primaire est un aspect également développé dans cette étude. Des résultats théoriques et expérimentaux sont comparés dans le cas académique d'une plaque mince d'aluminium simplement appuyée et soumise à une onde plane incidente. Enfin, la méthode est appliquée au panneau intérieur d'une double paroi aéronautique, à savoir une structure courbée, en matériau composite, et composée d'un hublot. Contrairement à la majorité des études qui considèrent l'implantation d'impédances virtuelles dissipatives, il apparaît que dans certains cas, le contrôle optimal requiert l'injection d'énergie des unités à la structure. / Composite materials are widely used in the aeronautic industry for their low mass/stiffness ratio. However, this property tends to reduce the acoustic transmission loss, particularly at low frequencies. At these frequencies, active control is an effective mean of controlling sound transmission. Among the various approaches, Active Structural Acoustic Control (ASAC) has received considerable attention because transducers can be integrated to the structure. In order to reduce the acoustic power radiated by a flexible panel, dual colocated actuator sensor pairs are used to modify its vibration. The control strategy implemented for harmonic disturbances leads to locally impose a virtual mechanical impedance to the structure, using a decentralized controller. This virtual mechanical impedance is computed in order to minimise the radiated acoustic power. The challenging problem is then to find the local control to impose on each independent devices that minimizes the global acoustic radiation of the structure. The proposed approach consists in two steps : (1) the matrix of optimal virtual mechanical impedance is calculated by measuring the primary disturbance and the transfer functions between actuators and structural / acoustic sensors, (2) the virtual mechanical impedance objective is achieved using a real-time integral controller. Special focus is put on the discussion about such control approach versus a classical active damping strategy were the virtual mechanical impedance is defined as real positive. Considering that optimal control is computed during the first step for a given primary disturbance, the robustness of the method to variations of the primary disturbance between step 1 and step 2 is discussed. Theoretical and experimental results are compared in the case of a simply supported thin aluminum plate and a primary disturbance under the form of an incident plane wave. Then, the method is implemented on a curved composite aircraft panel comprising a window. Unlike most of previous studies where dissipative virtual mechanical impedance are imposed, it clearly appears that optimal control can require energy injection from the control units into the structure.