Evaluation of the uncertainty due to dynamic effects in linear measuring devices – Preliminary results

Systematic dynamic effects in linear measuring devices include possible amplification or attenuation, and phase shift of the spectral components of the indicated signal in respect to the original phenomenon. When this effect is non negligible, dynamic compensation should be applied. Yet in any case uncertainty on the modulus and the phase of the frequency response of the device will cause uncertainty on the final measurement results. Therefore, a simple formula for the evaluation of such uncertainty is presented, for periodic or harmonic dynamic phenomena

Passive multi-mode piezoelectric shunt damping: an approach based on matrix inequalities

Piezoelectric shunt damping is a well-known technique for suppressing vibrations in light mechanical systems. The method is based on the connection of a properly designed electrical network (shunt circuit) to a piezoelectric actuator bonded to the vibrating structure. This network can be either passive (i.e. made from resistances, capacitances and inductances) or active. When active shunts are used, possible problems related to instability of the system can raise. This paper addresses a new approach for designing shunt electrical circuits allowing to damp more than one mechanical mode of the structure at the same time with a single piezoelectric actuator. Moreover, the method assures to design passive shunt impedances, thus avoiding instability problems. Starting from a state space description of the electro-mechanical system, the definition of the shunt circuit is achieved using an approach based on matrix inequalities, which allows to design shunt circuits with different goals by expressing the desired target as a single or a system of matrix inequalities

Vibration control by means of piezoelectric actuators shunted with LR impedances: Performance and robustness analysis

This paper deals with passive monomodal vibration control by shunting piezoelectric actuators to electric impedances constituting the series of a resistance and an inductance. Although this kind of vibration attenuation strategy has long been employed, there are still unsolved problems; particularly, this kind of control does suffer from issues relative to robustness because the features of the electric impedance cannot be adapted to changes of the system. This work investigates different algorithms that can be employed to optimise the values of the electric components of the shunt impedance. Some of these algorithms derive from the theory of the tuned mass dampers. First a performance analysis is provided, comparing the attenuation achievable with these algorithms. Then, an analysis and comparison of the same algorithms in terms of robustness are carried out. The approach adopted herein allows identifying the algorithm capable of providing the highest degree of robustness and explains the solutions that can be employed to resolve some of the issues concerning the practical implementation of this control technique. The analytical and numerical results presented in the paper have been validated experimentally by means of a proper test setup

Evaluation of the uncertainty due to dynamic effects in linear measuring devices

The evaluation of the uncertainty due to systematic dynamic effects is addressed. When high dynamic performance is required, they should be compensated, by solving the associated inverse dynamic problem. When instead they are considered compatible with the target uncertainty, they may be simply included in the uncertainty budget. Furthermore, even in the case of dynamic compensation, a residual uncertainty remains, due to the imperfect compensation, and should be evaluated. Therefore, simple formulas are presented here, applicable to many classes of dynamic phenomena, including periodic, harmonic, transitory impulsive and stochastic stationary ones

Short-Training Damage Detection Method for Axially Loaded Beams Subject to Seasonal Thermal Variations

Vibration-based damage features are widely adopted in the field of structural health monitoring (SHM), and particularly in the monitoring of axially loaded beams, due to their high sensitivity to damage-related changes in structural properties. However, changes in environmental and operating conditions often cause damage feature variations which can mask any possible change due to damage, thus strongly affecting the effectiveness of the monitoring strategy. Most of the approaches proposed to tackle this problem rely on the availability of a wide training dataset, accounting for the most part of the damage feature variability due to environmental and operating conditions. These approaches are reliable when a complete training set is available, and this represents a significant limitation in applications where only a short training set can be used. This often occurs when SHM systems aim at monitoring the health state of an already existing and possibly already damaged structure (e.g., tie-rods in historical buildings), or for systems which can undergo rapid deterioration. To overcome this limit, this work proposes a new damage index not affected by environmental conditions and able to properly detect system damages, even in case of short training set. The proposed index is based on the principal component analysis (PCA) of vibration-based damage features. PCA is shown to allow for a simple filtering procedure of the operating and environmental effects on the damage feature, thus avoiding any dependence on the extent of the training set. The proposed index effectiveness is shown through both simulated and experimental case studies related to an axially loaded beam-like structure, and it is compared with a Mahalanobis square distance-based index, as a reference. The obtained results highlight the capability of the proposed index in filtering out the temperature effects on a multivariate damage feature composed of eigenfrequencies, in case of both short and long training set. Moreover, the proposed PCA-based strategy is shown to outperform the benchmark one, both in terms of temperature dependency and damage sensitivity

Biomechanics in crunch assisted walking

Crutch-assisted walking is very common among patients with a temporary or permanent impairment affecting lower limb biomechanics. Correct crutches’ handling is the way to avoid undesired side effects in lower limbs recovery or, in chronic users, upper limbs joints diseases. Active exoskeletons for spinal cord injured patients are commonly crutch assisted. In such cases, in which upper limbs must be preserved, specific training in crutch use is mandatory. A walking test setup was prepared to monitor healthy volunteers during crunch use as a first step. Measurements were performed by using both a motion capture system and instrumented crutches measuring load distribution. In this paper, we present preliminary tests results based on different subjects - having a variety of anthropometrical characteristics - during walking with parallel or alternate crutches, the so-called three and two-points strategies. Tests results present inter and intra subject variabilities and, as a first goal, influencing factors affecting crutch loads have been identified. In the future we aim to address crutch use errors that could lead to delayed recovery or upper limbs suffering in patients, giving valuable information to physicians and therapists to improve user’s training

Biomechanics in crutch assisted walking

Crutch-assisted walking is very common among patients with a temporary or permanent impairment affecting lower limb biomechanics. Correct crutches' handling is the way to avoid undesired side effects in lower limbs recovery or, in chronic users, upper limbs joints diseases. Active exoskeletons for spinal cord injured patients are commonly crutch assisted. In such cases, in which upper limbs must be preserved, specific training in crutch use is mandatory. A walking test setup was prepared to monitor healthy volunteers during crunch use as a first step. Measurements were performed by using both a motion capture system and instrumented crutches measuring load distribution. In this paper, we present preliminary tests results based on different subjects - having a variety of anthropometrical characteristics - during walking with parallel or alternate crutches, the so-called three and two-points strategies. Tests results present inter and intra subject variabilities and, as a first goal, influencing factors affecting crutch loads have been identified. In the future we aim to address crutch use errors that could lead to delayed recovery or upper limbs suffering in patients, giving valuable information to physicians and therapists to improve user's training

Fatigue Assessment of Moorings for Floating Offshore Wind Turbines by Advanced Spectral Analysis Methods

The fatigue assessment of mooring lines for floating offshore wind turbines represents a challenging issue not only for the reliable design of the stationkeeping system but also for the economic impact on the installation and maintenance costs over the entire lifetime of the offshore wind farm. After a brief review about the state-of-art, the nonlinear time-domain hydrodynamic model of floating offshore wind turbines moored by chain cables is discussed. Subsequently, the assessment of the fatigue damage in the mooring lines is outlined, focusing on the combined-spectrum approach. The relevant fatigue parameters, due to the low-and wave-frequency components of the stress process, are estimated by two different methods. The former is based on the time-domain analysis of the filtered stress process time history. The latter, instead, is based on the spectral analysis of the stress process by two advanced methods, namely the Welch and Thomson ones. Subsequently, a benchmark study is performed, assuming as reference floating offshore wind turbine the OC4-DeepCWind semisubmersible platform, equipped with the 5 MW NREL wind turbine. The cumulative fatigue damage is determined for eight load conditions, including both power production and parked wind turbine situations. A comparative analysis between time-domain and spectral analysis methods is also performed. Current results clearly show that the endorsement of advanced spectral analysis methods can be helpful to improve the reliability of the fatigue life assessment of mooring lines

Vibration control with piezoelectric elements: The indirect measurement of the modal capacitance and coupling factor

The knowledge of the modal capacitance and electro-mechanical coupling factor is essential for a proper design of systems with embedded piezoelectric transducers and materials. In light of this, this paper presents two indirect methods for measuring the piezoelectric modal capacitance and a method to estimate the modal electro-mechanical coupling factor. All methods rely on simple vibration measurements of the structure with the piezoelectric transducer connected to a proper shunt impedance, thus avoiding measurements of piezoelectric current and voltage by expensive equipment. For the modal electro-mechanical coupling factor, the proposed method guarantees reduced uncertainty compared to traditional experimental estimation procedures. Upon introduction of the underlying theory, the paper experimentally demonstrates the reliability and effectiveness of the methods by comparison with well-established procedures