Marc Thomas

With a view to minimising the amplitude and duration of vibrations of a flexible beam, an active control system comprising a displacement sensor, digital compensation and a mechanical excitation system is studied. The results of the low-pass filter and the state feedback control laws are presented. The low-pass filter has been used to control the first vibrational mode and an

The aim of this study is to evaluate linemen's perception of hardness during the climbing of different wood species and treatment combinations in order to determine if psychophysical measurements could be used to evaluate “climbability” of wood treated utility poles. Twenty-four (24) ...

The issue of climbability has been raised on several occasions for more than a decade in North America. Presently, climbability is estimated from the pole hardness measured by the Pilodyn measurements (6 J). However, the use of Pilodyn measurements to discriminate the pole hardness value is criticized by climbers, who claim that the Pilodyn hardness measurement is affected by species-treatment combinations and that it does not reflect gaff penetration or climbability. Furthermore, climbability evaluations have been conducted in which test poles were climbed by linemen, and corresponding subjective ratings were recorded. However, the ability of psychophysical measurements to accurately discriminate close hardness pole values and to differentiate species-treatment combinations at specific hardness levels have not yet been fully documented. The aim of this study is to evaluate the psychophysical perception of linemen and the mechanical measurements of gaff penetration and gaff impact d...

A method is developed for the automatic identification of the spectrum and modal parameters of an operational modal analysis using multi sensors. A multivariate autoregressive model is presented, and its parameters are estimated by least squares via the implementation of QR factorization. A noise-independent minimum model order, from which all available physical modes may be identified, is developed. This so-called optimal model order is selected from the convergence of a global order-wise signal-to-noise ratio index. At this model order or higher, the modes are classified based on a decreasing damped modal signal-to-noise (DMSN) criterion. This decreasing order classification allows for easy identification of all the physical modes. A significant change in the DMSN index enables the determination of the number of physical modes in a specific frequency range, and thus, an automatic procedure for identifying the modal parameters can be developed to discriminate harmonic and natural frequencies from spurious ones. Furthermore, a multispectral matrix can be constructed from selected frequencies by introducing a powered amplification factor, which provides a smooth, balanced, noise-free spectrum with all main peaks. The proposed method has been performed on simulated multi-degree-of-freedom systems, on a laboratory test bench, and on an industrial operating high power hydro-electric generator offering the potential for automatic operational modal analysis and structural health monitoring.

This paper presents improvements of a multivariable autoregressive (AR) model for applications in operational modal analysis considering simultaneously the temporal response data of multi-channel measurements. The parameters are estimated by using the least squares method via the implementation of the QR factorization. A new noise rate-based factor called the Noise rate Order Factor (NOF) is introduced for use in the effective selection of model order and noise rate estimation. For the selection of structural modes, an orderwise criterion called the Order Modal Assurance Criterion (OMAC) is used, based on the correlation of mode shapes computed from two successive orders. Specifically, the algorithm is updated with respect to model order from a small value to produce a cost-effective computation. Furthermore, the confidence intervals of each natural frequency, damping ratio and mode shapes are also computed and evaluated with respect to model order and noise rate. This method is thus very effective for identifying the modal parameters in case of ambient vibrations dealing with modern output-only modal analysis. Simulations and discussions on a steel plate structure are presented, and the experimental results show good agreement with the finite element analysis.

Diagnostic of machine using the time- frequency representation of vibration is becoming widely used by many companies and experts. In recent years, the preventive maintenance is no more desirable and in fact it has evolved into the prediction maintenance. The current methods of machine diagnosis can not provide detailed diagnostics and condition prediction. This paper proposes the application of Short-

This paper presents recent advances in the Structural Health Monitoring (SHM) project within the Consortium for Research and Innovation in Aerospace in Québec (CRIAQ). This project aims at developing technologies and a SHM system for potentially reducing the high costs associated with periodic prescribed inspections of aircraft structures and components. This paper presents an overview of models and strategies developed

La détection des vibrations est cruciale en usinage, et leur surveillance est primordiale pour assurer une bonne qualité de surface, pour améliorer la productivité et pour préserver à la fois les pièces fabriquées et les machines de production. Ce papier présente les bases d’une surveillance du chatter en usinage à grande vitesse, fondé sur une analyse des efforts de coupe et de la vitesse angulaire instantanée de la broche dans le domaine angulaire. Une opération de rainurage dans un alliage d’aluminium sert de base pour des essais expérimentaux. Les efforts de coupe et la vitesse angulaire de la broche sont acquis de manière synchrone. La vitesse est obtenue à partir du codeur installé par défaut sur le moteur de la broche. Ce même codeur est utilisé comme base pour l’échantillonnage simultané des signaux. La méthode d’acquisition est décrite dans cet article. L’analyse des données expérimentales confirme la pertinence des effort s de coupe pour la détection de chatter en usinage,...

La détection des vibrations est cruciale en usinage, et leur surveillance est primordiale pour assurer une bonne qualité de surface, pour améliorer la productivité et pour préserver à la fois les pièces fabriquées et les machines de production. Ce papier présente les bases d’une surveillance du chatter en usinage à grande vitesse, fondé sur une analyse des efforts de coupe et de la vitesse angulaire instantanée de la broche dans le domaine angulaire. Une opération de rainurage dans un alliage d’aluminium sert de base pour des essais expérimentaux. Les efforts de coupe et la vitesse angulaire de la broche sont acquis de manière synchrone. La vitesse est obtenue à partir du codeur installé par défaut sur le moteur de la broche. Ce même codeur est utilisé comme base pour l’échantillonnage simultané des signaux. La méthode d’acquisition est décrite dans cet article. L’analyse des données expérimentales confirme la pertinence des effort s de coupe pour la détection de chatter en usinage,...

ABSTRACT The detection of chatter is crucial in the machining process and its monitoring is a key issue to ensure a better surface quality, to increase productivity and to protect both the machine and the workpiece. An investigation of chatter monitoring in the high speed machining process on the basis of the analysis of cutting forces and instantaneous angular speeds (IAS) in the angular domain is presented in this paper. Experimental cutting tests were carried out on slot milling operation of aluminum alloy. Our experimental set-up allows for, on the one hand, the acquisition of the angular cutting force and, on the other hand, the measurement of IAS. The latter is computed from the signal delivered by the standard encoder mounted on the spindle motors. The same signal is used as a base for synchronous angular sampling. The acquisition methodology is described and discussed in this paper. Data analysis confirms the relevance of cutting forces for chatter monitoring and the good correlation between cutting forces and IAS of the spindle. Two chatter indicators are developed in the angular domain from the periodic and residual parts of angular speed and cutting force signals. They are used successfully for the detection of the chatter phenomenon. The indicator based on IAS is very convenient because it does not require additional sensors and can be realized without increasing the cost of the manufacturing system.