Transactions of the Canadian Society for Mechanical Engineering

Volume 31 (2007), Issue 1

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The increased requirement to operate military platforms and aerospace structures beyond their designed life imposes heavy maintenance and inspection burden on aircraft operators and owners. In-service structural health monitoring is potentially a cost-effective approach by which service usage information can be obtained and knowledgeable decisions can be made. Advanced sensor technology, such as optical fibres, are expected to provide existing and future aircraft with added intelligence and functionality, reduced weight and cost, enhanced robustness and performance. This paper furthers the understanding of technical and practical issues related to full implementation of a fibre optic sensor based structural health monitoring system for aerospace and military platforms. It also reports experimental findings on the use of fibre Bragg grating sensors for measurement of parameters relevant to aircraft structural monitoring and smart structures; with an emphasis on the suitability of multifunctional fibre optic sensor system. Experimental evaluations revealed that Bragg grating sensors correlate well with conventional sensors technology for temperature, stain, crack growth and cure monitoring and were insensitive to pressures up to 300 psi. These sensors were determined to have minimum impact on the structural integrity when embedded parallel to host fibres into composite laminates. Recommendations on the implementation and integration of these sensors into a structural health monitoring system are also provided.

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Smart structures are seen as an enabling technology for designing innovative control actuation systems for future missiles. In this study, the feasibility of employing shape memory alloy (SMA)-actuated micro-flow effectors to control the vortex shedding behaviour that produces side forces on slender body missiles is examined. Supersonic wind tunnel tests were performed on a slender finless missile equipped with static micro-flow effectors on a conical nose to determine suitable configurations that could generate significant side forces. Shape memory alloy actuators for the flow effector were developed using numerical techniques and validated experimentally. Matching the force-displacement characteristics of the SMA actuator to the micro-flow effector force-displacement requirement was accomplished by a compliant transmission mechanism. The dynamic performance of the micro-flow effector was assured with a two-step variable structure control law. Closed-loop test results showed that the control law was capable of providing effective displacement control up to 1.0 Hz.

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Helicopters are susceptible to high vibratory loads, excessive noise levels and poor flight stability compared to fixed-wing aircraft. The multidisciplinary nature of helicopter structures offers many opportunities for the innovative smart structure technology to improve helicopter performance. This paper provides a review of smart structures research at the National Research Council Canada for helicopter vibration and cabin noise control applications. The patented Smart Spring approach is developed to vary the blade impedance properties adaptively to reduce the vibratory hub loads transmitted to the fuselage by vibration reduction at the source. A smart gearbox strut and active structural acoustic control technologies are investigated to suppress the vibration and tonal gear meshing noise into the cabin either by modifying the vibration load transmission path, or weakening the coupling between exterior and cabin acoustic fields. Two adaptive seat mount concepts are proposed to reduce the vibration of the aircrew directly to improve ride quality of the vehicle.

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A new lightweight planar robot is designed to achieve high acceleration and execute high-speed pick and place tasks. However, due to the lightweight structure of the system, unwanted structural vibrations are induced during motion of the platform. This work focuses on the investigation of the characteristics of this structural vibration utilizing distributed Lead Zirconate Titanate (PZT) transducers. Experimental Modal Analysis (EMA) tests were performed on an experimental three degree of freedom planar parallel robot with one flexible linkage, for cases in which the robot is stationary and in motion. For both cases, configuration dependency of link structural vibration is investigated by performing EMA in different configurations using different combinations of transducers. It is observed that FRFs obtained for the case in which the robot is in motion have more complex frequency compositions and more significant configuration-dependency than those in which the robot is stationary. However, the two groups of analyses exhibit two nearly identical most-significant modes. Based on this observation, the transfer function from motor inputs to linkage vibration is simplified permitting an linear active vibration controller to be used.

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Macro-micro systems allow high-resolution positioning over greater ranges of operation that would be achievable with precision positioning systems. Piezoceramic actuators have established themselves as the principle technology for commercial micro-positioning applications, and the trend in research is to push the limits of resolution down to the nanometer and sub-nanometer scales. Other smart materials offer the potential for lightweight, continuous actuation over small ranges, and hence may be useful in micro-positioning applications. This work focuses on the potential for SMA actuators to enable low-cost micro-positioning. Compared to piezos, SMA offer longer range and lower actuation voltages, enabling lower-cost drive electronics and removing the need for costly precision mechanical amplification stages. A prototype single-axis macro-micro positioning system is described, with a macro range of 200 mm and relative positioning precision of better than 5 µm. The micro stage is driven by an NM70 SMA actuator from NanoMuscle. Macro and micro stages are modelled and controllers developed, and experimental system performance is evaluated. The success of the system provides an inexpensive platform for the study of macro-micro positioning issues such as stage coupling, friction, and drive flexibility, as well as for the position control of SMA.

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Piezo-positioning mechanisms are often used in high-precision positioning applications. Due to their materials, nonlinear hysteretic behavior is commonly observed in such mechanisms and can be described by a LuGre model. In this paper, we develop two robust adaptive backstepping control algorithms for piezo-positioning mechanisms. In the first scheme, we take the structure of the LuGre model into account in the controller design, if the parameters of the model are known. A nonlinear observer is designed to estimate the hysteresis force. In the second scheme, there is no apriori information required from these parameters and thus they can be allowed totally uncertain. In this case, the LuGre model is divided into two parts. While the unknown parameters of one part are incorporated with unknown system parameters for estimation, the effect of the other part is treated as a bounded disturbance. An update law is used to estimate the bound involving this partial hysteresis effect and the external load. For both schemes, it is shown that not only global stability is guaranteed by the proposed controller, but also both transient and asymptotic performances are quantified as explicit functions of the design parameters so that designers can tune the design parameters in an explicit way to obtain the required closed loop behavior.

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The applications of the piezoelectric Polyvinylidene Fluoride, PVDF, integrated with the beams, plates, and membranes, performing as sensor, actuator or combination have been received considerable attention in the recent years. However, not much work has been reported on the influence of the PVDF's orthotropic behavior, particularly the effect of the orientation of the PVDF film in the host structure, on the performance of the system. In the present study, the effect of the piezoelectric PVDF film orientation on the output voltage, the actuation force, and the dynamic response of the integrated structures has been studied using the finite element method. In the sensory mode, the difference between the output voltages obtained from the biaxial piezoelectric PVDF film and uniaxial one, when the orientation of the film varies from 0 to 90 degree, is investigated. In each case the proportion contributions of the involved piezoelectric coefficients including d₃₁, d₃₂ and J₃₃ are studied. Alternatively, in the actuation mode, the effect of orthotropic behavior of the actuator on the nodal displacements has been taken into consideration. The influence of the material orthotropic property of the transducer on the free undamped response of the system is also investigated. Moreover an effective Young's modulus and effective Poisson ratio for the uniaxial PVDF film has been introduced using an optimization procedure to minimize the error caused by isotropic assumption of uniaxial PVDF film.

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A hysteretic operator is proposed to set up an expanded input space so as to transform the multi-valued mapping of hysteresis to a one-to-one mapping so that the neural networks can be applied to model of the behavior of hysteresis. Based on the proposed neural modeling strategy for hysteresis, a pseudo control scheme is developed to handle the control of nonlinear dynamic systems with hysteresis. A neural estimator is constructed to predict the system residual so that it avoids constructing the inverse model of hysteresis. Thus, the control strategy can be used for the case where the output of hysteresis is immeasurable directly. Then, the corresponding adaptive control strategy is presented. The application of the novel modeling approach to hysteresis in a piezoelectric actuator is illustrated. Then a numerical example of using the proposed control strategy for a nonlinear system with hysteresis is presented.