Al Ferri

Effect of Planetary Gear Carrier-Plate Cracks on Vibration Spectrum

This paper examines the problem of identifying cracks in planetary gear systems through the use of vibration sensors on the stationary gearbox housing. In particular, the effect of unequal spacing of planet gears relative to the rotating carrier plate on various frequency components in the vibration spectra is studied. The mathematical analysis is validated with experimental data comparing the vibration signature of helicopter transmissions operating either normally or with damage, leading to shifts in the planet gear positions. The theory presented is able to explain certain features and trends in the measured vibration signals of healthy and faulty transmissions. The characterization offered may serve as a means of detecting damage in planetary gear systems.

Adaptive Length IIR Filters Implemented with Imprecise Computing

This paper explores the use of imprecise computing to implement adaptive length IIR filters in order to meet realtime constraints. An adaptive-length filter can provide a resource manager with alternatives that can be used if resources become scarce. This paper discusses the use of standard IIR filters used in controls applications, such as a Butterworth filter, and how this filter can be implemented in prioritized stages that can be skipped if necessary. The paper also examines the use of filter banks with different size filters. In both cases, transition management methods such as bumpless transfer are investigated to mitigate the transients induced when switching between filters.

Adhesive and Mechanical Properties of Carbon Nanotube Probes Contacting Chemically-Treated Surfaces

Carbon nanotubes are useful in a variety of measurement applications. In the case of Atomic Force Microscopes (AFMs), carbon nanotubes can be affixed to the tip of the AFM cantilever to improve image resolution and enable images of surfaces with deep crevices and trench structures. In this paper, the mechanical response of long, straight, small walled carbon nanotubes (SWNTs) under compressive and tensile load is examined with an atomic force microscope. Multi-dimensional force spectroscopy (MDFS) is used to simultaneously measure the cantilever resonant frequency, deflection, and scanner motion. The acquired force curves reveal that the SWNT buckles shortly after contact is initiated. As the scanner continues to rise and then reverses direction, the SWNT undergoes a number of adhesion/sticking episodes, buckling, and slip events. The bulk properties of the nanotube are estimated by measuring the shift in natural frequency during tension. Finally, the carbon nanotube is modeled as an elastica in order to predict the post-buckled shape of the SWNT. By comparing the model results with MDFS results, the static coefficient of friction between the SWNT and a variety of surfaces is estimated. The study suggests that MDFS has a wide applicability for studying the mechanical and adhesive properties of various nanotubes, nanorods and nanofibers.Copyright

Application of Anytime Control to an Inherently Unstable Physical System

This paper extends the use of closed-loop anytime control to systems that are inherently unstable in the open-loop. Previous work has shown that anytime control is very effective in compensating for occasional missed deadlines in the computer processor. When misses occur, the control law is truncated or partially executed. However, the previous work assumed that the open-loop system was stable. In this paper, the anytime strategy is applied to an inverted pendulum system. An LQR controller with estimated state feedback is designed and decomposed into two stages. Both stages are implemented most of the time, but in a small percentage of time, only the first stage is applied, with the resulting closed-loop system being unstable for short periods of time. The statistical performance of the closed-loop system is studied using Monte-Carlo simulations. It is seen that, on average, the closed-loop performance is very close to that of the full-order controller as long as the miss rate is relatively small. However, the variance of the response shows much higher dependence on the miss rate, suggesting that the response becomes more unpredictable. At a critical value of miss rate, the closed-loop system is unstable. The critical miss rate found through simulation is seen to correlate well with the results of a deterministic stability analysis. The statistics on the settling time are also studied, and shown to grow longer as the miss rate increases. The transient behavior of the system is studied for a range of initial conditions.Copyright

Vibration Isolation From Harmonic Disturbances Through Use of Internally Rotating Masses

This paper considers the use of a chain of translating carts or housings having internally rotating eccentric masses in order to accomplish vibration isolation. First a single degree-of-freedom system is harmonically excited to uncover the qualitative behavior of each rotating mass. The simple model is then expanded into a chain of housings, containing rotating eccentric masses, which are interconnected with springs. The internal rotating eccentric masses are damped along their circular pathway by means of linear viscous damping. Due to the lack of elastic or gravitational constraint on the rotating eccentric masses, they provide a nonlinear inertial coupling to their housings. Previous research has shown that such systems are capable of reducing shock or impulsive loading by converting some of the translational kinetic energy into rotational kinetic energy of the internal masses. This paper examines the potential for vibration isolation of a chain of such systems subjected to persistent, harmonic excitation. It is seen that the dynamics of these systems is very complicated, but that trends are observed which have implications for practical isolation systems. Using simulation studies, tradeoffs are examined between displacement and transmitted force for a range of physical parameter values.Copyright

Shock and Vibration Isolation Using Internally Rotating Masses

This paper considers the use of a chain of springs and masses to reduce the transmission of shock and vibration through the system. The masses are equipped with internally rotating masses that absorb some of the axial vibration into internal kinetic energy of the masses. The internal masses have viscous damping, but no elastic or gravitational restraint. Previous research has shown that a single cart system attached to a vibrating structure can help mitigate shock through targeted energy transfer. This paper examines the potential for shock isolation provided by a chain of such systems. Through numerical simulations, tradeoffs are examined between displacement and transmitted force.Copyright

Shock Isolation Through Translational-to-Rotational Energy Transference

The design of isolation mounts is of critical importance in the protection of structures and sensitive equipment from damage or failure. Simultaneous protection from both shock and vibration is particularly challenging because of the broadband nature of the input signal and because of the deleterious effect of damping on high-frequency isolation. Prior work by the authors has shown that chains of translating mass/spring elements can act as a “mechanical filter” for input disturbances. However, in finite-length chains, wave reflections can result in secondary pulses that hit the structure and can diminish the effectiveness of the isolator. In this paper, a new type of isolator is developed that converts translational input forces into a combination of translational and rotational motion. If designed correctly, the rotational motion can be managed so that it does not result in additional forces transmitted to the structure. In effect, the isolator is able to trap some of the input energy into rotational vibration, preventing it from reaching the structure. Parametric simulation studies are conducted as various system parameters are varied.Copyright

Shock Isolation in Finite-Length Dimer Chains With Linear, Cubic and Hertzian Spring Interactions

A numerical investigation to mitigate the effects of shock in finite 1:1 dimer chains is performed. Dimer chains consist of alternating light and heavy masses. Changing the mass ratio has provided interesting results in previous research. In particular, in the case of Hertzian contacts with zero-preload, certain mass ratios have revealed minimal levels of transmitted force. This paper examines this phenomena from the perspective of utilizing it in practical isolation systems. The zero-preload Hertzian contact case is contrasted with chains connected by linear or cubic springs. Through numerical simulations, tradeoffs are examined between displacement and transmitted force.Copyright

Improvement of Convergence Properties for Ritz Formulations of Acousto-Elasticity Problems

Many applications require the analysis of structures with cavities filled with fluids or gases. In many cases the fluid domains can be ignored, and we can safely assume that the structure’s in vacuo properties apply. Sometimes, however, resonances in the fluid can couple with structural resonances to yield acousto-elastic modes of response. The most popular approach to these problems is to describe the trapped fluid in terms of a finite number of eigenmodes of a geometrically identical cavity with rigid boundaries. Then, the structural and fluid domains are coupled by matching the pressure across the wetted surface. However, for light structures with embedded cavities of relatively heavy fluids such as water, this technique may not be satisfactory because the interior rigid-cavity modes are poor candidates to satisfy the “natural boundary conditions” that exist at the fluid-structure boundary. This paper explores the use of an expanded set of Ritz functions for the fluid domain, to include a number of functions that explicitly allow for motion along the wetted surface. The method is applied to a two-dimensional rectangular acoustic cavity, with rigid boundaries on all sides except for a flexible membrane on the top surface. Through comparisons with the “exact solution,” it is shown that the solution using the expanded set of Ritz functions converges more quickly than do solutions employing rigid cavity modes. The convergence trends of the first few natural frequencies are computed for a number of different physical and geometric system properties.Copyright

Passive, Transitioning Mounts for Simultaneous Shock and Vibration Isolation

A design strategy to simultaneously mitigate the effects of both shock and vibration is introduced. The proposed isolation mount is a passive, transitioning mount and consists of sliding friction elements in series connection with springs and dampers. A linear and a displacement dependent viscous damper are considered, while linear, hardening and softening springs, are considered. The isolation mount’s response is determined by numerical simulation. For a single-degree-of-freedom system, the tradeoff curve for a half-sine velocity input is determined, as is the nonlinear transmissibility for harmonic excitation. The method is found to achieve satisfactory isolation against shock events as well as persistent harmonic inputs. The suggested mount configuration was also found to have good performance against a ‘combined’ input with both resonant and transient content.Copyright