This paper addresses the problem of optimal tuning of a tuned mass damper (TMD) attached to a complex structure that is dynamically excited by its base. It proposes new analytical formulae which are based on the reduction of the multiple degree of freedom (MDOF) model of the host-structure into an equivalent single degree of freedom (SDOF) model. As it has been recognized in the literature that the traditional single mode approximation used to perform this reduction is not valid for base-excited systems, we propose an improved version that leads to the definition of two mass ratios instead of one in the traditional approach. Taking into account this new mass ratio, the equal peak method is used to derive analytically the optimal values of stiffness and damping of the TMD for a given mass ratio of the device. The introduction of a second mass ratio leads to the existence of two sets of equations for the optimal parameters, depending on the relative values of the two mass ratios. It is shown, however, that only the first set of equations is of practical use. The application of these new tuning rules is illustrated using a MDOF model of a high-rise building. It demonstrates the efficiency of the approach when the first mode of vibration is targeted. When higher modes are of interest, modal interactions are important, which cause a slight to moderate unbalance of the peaks.

This paper presents an exact H ∞ tuning methodology for a positive position feedback (PPF) controller applied to a single-degree-of-freedom (SDOF) system. To this end, an equivalence between the closed-loop receptances of a PPF controller and a resistive–inductive shunt with a negative capacitance is put forward, which, in turn, enables us to adopt the existing shunt tuning rule in the active control case. The resulting tuning procedure is demonstrated using two numerical examples, namely, an SDOF system and a finite element model of a cantilever beam. Based on the results obtained on the cantilever beam, it is shown that the influence of higher-frequency modes cannot be neglected to obtain effective vibration damping. The design procedure proposed for the PPF controller is then extended to this case and validated using an experimental cantilever beam.

Generating reduced state-space models of base-excited structures is of great help in control engineering but is not straightforward to implement in practice, and this problem is addressed in this work. Methods to impose non-homogeneous boundary displacements or accelerations are reviewed, and a novel relative acceleration method is proposed to treat the case of imposed displacements. Various construction approaches for state-space models having prescribed displacements or accelerations as input and including a static correction term are then developed. The theoretical developments are eventually illustrated with structures of increasing complexity, namely, a bar, a beam, and a multi-story building model.

Integral-force-feedback (IFF) is a popular control law in active vibration damping of mechanical system when a force sensor is collocated with a force actuator. While it is simple, robust to resonance uncertainty and stable for any feedback gains, its efficiency is limited by system's parameters and in particular the stiffness ratio between the structure and the actuator. Therefore, the control authority decreases at high frequency resonances or when the actuator is weakly coupled to the structure. It has been shown that the use of double integrator with a real zero, named α-controller, can improve the control authority of a target mode. However, this technique like IFF cannot be easily implemented in practice because of low frequency saturation issue induced by significantly amplifying the low frequency content during the integration process. This paper proposes a new control law, named resonant-force-feedback (RFF), based on a second order low pass filter to damp a target mode resonance. Through the mechanical analogy of the proposed system, RFF can be seen as an active realization of an inerter-spring-damper (ISD) system. In addition, the parameters of RFF are optimized based on two methods, that is, maximum damping criterion and H∞ optimization which consists in minimizing the settling time of the impulse response and the peak amplitude in the frequency domain, respectively. It is shown that RFF always provides a higher control authority of a target mode in comparison to IFF for a given stiffness ratio and in particular when the stiffness ratio is low. Despite the fact that the performance of the system, in terms of the closed-loop damping ratio or the amplitude reduction, obtained by RFF is very close to that of α-controller, RFF requires less control effort in comparison to α-controller. The stability of the proposed system is also assessed in terms of the gain margin and the phase margin although the system is unconditionally stable. Moreover, the robustness of the designed RFF is compared to that of IFF under stiffness uncertainty. Although IFF can tolerate a higher level of uncertainty, the performance of RFF is superior to that of IFF for almost 50% of changes in the stiffness of the primary system.

The evaluation of transmission zeros is of great importance for the control engineering applications. The structures equipped with piezoelectric patches are complex to model and usually require finite element approaches supplemented by model reduction. This study rigorously investigates the influence of mesh size, model reduction, boundary conditions (free and clamped), and sensor/actuator configuration (collocated and non-collocated) on the evaluation of transmission zeros of the piezoelectric structures. The numerical illustrations are presented for a thin rectangular plate equipped with a single pair of piezoelectric voltage sensor/ voltage actuator. Through the examples considered in this study, a link is presented between the static response (or static deflected shape) and the transmission zeros of the piezoelectric structures. This interesting observation forms the basis of: (i) a local mesh refinement strategy for computationally efficient estimation of the transmission zeros and (ii) a physical interpretation of the pole-zero pattern in the case of piezoelectric structures. The physical interpretation developed in this study helps in qualitatively explaining the pole-zero patterns observed for different configurations. It is also shown that this understanding of the relation between the static deformed shape and the transmission zeros can be used by the practitioners to modify the pole-zero pattern through a careful selection of the orientation and the size of the piezoelectric patches.

In this paper, we propose to monitor the evolution of damage in freeze-thaw cycles tests using embedded ultrasonic piezoelectric transducers. The transducers are used in emitter-receiver pairs. Concrete specimens were subjected to 70 freeze-thaw cycles of 12 h in temperature ranges from −20 ℃ to +17 ℃ and the monitoring system automatically and continuously recorded ultrasonic signals every 5 min. With such rich information, it is possible to monitor the difference between recorded signals during freezing and thawing phases, as well as transition times between these two phases. Several indicators were extracted from the recorded signals to evaluate the level of degradation of the concrete, including the traditional wave velocity and the first wave amplitude of the recorded signals as well as Coda wave parameters. The results show that the first wave amplitude is a more sensitive indicator of early freeze-thaw damage than the traditionally used wave velocity. Coda wave interferometry (CWI) also proves to be an interesting and complementary indicator to better understand the freeze-thaw damage mechanism in concrete.

This study deals with the optimisation of pendulum tuned mass damper parameters for different types of excitations and responses of the host structure to which it is attached. The study considers force excitation and base excitation with different types of output quantities to be minimised on the host structure. It also considers both harmonic motion with H ∞ optimisation of the different transfer functions and random white noise excitation where the variance of the output signal is minimised, leading to H2 optimisation. Although a lot of work has been done on optimisation of tuned mass dampers, there exists in the literature only a few solutions for optimisation of the pendulum tuned mass dampers not covering all possible types of loads and output quantities. The analogy between the mass spring tuned mass damper and pendulum tuned mass damper presented in this study allows to use all the tuning rules developed for tuned mass dampers in the case of pendulum tuned mass dampers. In addition, the existing tuning rules for tuned mass dampers are extended to cases which were not previously solved in the literature for H2 optimisation and validated by comparing with numerical optimisation. Finally, a discussion is presented where the different tuning rules are compared, and the performance degradation is assessed when the wrong tuning rule is used. This is representative of the case where, for example, both wind and earthquake excitation exist on the structure, and the pendulum tuned mass damper is tuned for just wind excitation.

This work presents a novel vibration damping approach for bladed structures. Piezoelectric transducers bonded to a structure can be used simultaneously as actuators and sensors to mitigate the vibrations of their host. This can be achieved by connecting a transducer to a digital vibration absorber composed of a voltage sensor, a digital processing unit and a current injector. The digital vibration absorber thereby emulates a piezoelectric shunt. In this study, this technique is applied to bladed structures featuring small modal damping and closely spaced resonance frequencies grouped in mode families. A strategy exploiting the high modal density is presented. Effective vibration mitigation is experimentally demonstrated on multiple mode families simultaneously.

The construction site is a hazardous place. The dynamic, complex interaction between workers, machinery, and the environment leads to dangerous risks. In response to such risks, the goal is to fulfill the zero accidents philosophy, which requires the development of safety skills among workers and the provision of tools for risk prevention. In pursuit of that vision, this work studies collective protective equipment (CPE). Traditional methodologies propose visual inspections using checklists, the effectiveness of which depends on the quality of the inspection by the safety advisor (SA). This paper analyses the traditional process of safety inspections in building projects: the traditional methods, main pain points, and bottlenecks are identified, along with the key performance indicators (KPIs) needed to complete these processes correctly. Because of this, a methodology that digitises the CPE inspection process is proposed. Augmented reality (AR) is used as a 3D viewer with an intuitive interface for the SA, and, accordingly, functional requirements are detailed and different information layers and user interfaces for AR applications are proposed. In addition, the workflow and KPIs are shown. To demonstrate the feasibility of the proposal, a proof of concept is developed and evaluated. The relevance of this work lies in providing background for the use of AR in safety inspection processes on construction sites and in offering methodological recommendations for the development and evaluation of these applications.

This paper studies the possibility of extending the already proved link between the pole-zero distance and the maximum reachable damping ratio in single input single output (SISO) systems to multiple inputs multiple outputs (MIMO) ones. This extension is shown to be possible when the considered system presents specific properties: (i) it is equipped with collocated transducers with small authority, (ii) the system has a small modal density in the frequency band of interest and (iii) a low authority control law is used. It is indeed demonstrated that when these three conditions are satisfied, the analytical development of the closed-loop poles convergence is equivalent to the one observed with SISO cases, except that the anti-resonances are replaced by the transmission zeros (TZs). Consequently, it is concluded that the maximum reachable damping ratio is directly proportional to the pole-transmission zero distance for such MIMO systems. This conclusion is demonstrated with two numerical examples (a cantilever beam and a simply supported plate) and experimentally validated on a cantilever beam where all the studied systems are equipped with two collocated pairs of piezoelectric patches.

This paper shows the complementarity of two nonlinear ultrasonic imaging methods to characterize closed macro cracks in concrete. A time reversal mirror is used to locally probe and image the nonlinearity of a cracked region. Two nonlinear parameters are extracted to map the cracked region. The image obtained using the first parameter relates the harmonic generation due to the contact and frictional behaviors at the crack lips, correlated with vibro-thermography imaging of the same crack. The image obtained using the second parameter is based on the conditioning of the material induced by distributed micro cracks arising from the fracture process zone. These results show a great potential for characterization of fracture processes in concrete, with the possibility to uncouple the effects of the crack itself from surrounding distributed micro damage.

This paper presents the results of a long-term concrete monitoring campaign in an underground tunnel in Brussels. The system consists of several pairs of embedded ultrasonic piezoelectric transducers arranged in a pitch-catch configuration which have been placed in the concrete ceiling of the tunnel in areas where old concrete was demolished and then repaired. The monitoring system is fully automated and sends the recorded signals to a cloud-based system in our university where they are post-processed to extract indicators of structural changes in the monitored regions, and send automated email reports. A first period of six months is studied, during which the monitored areas have been repaired with skim mortar. The post-processing of the measured signals allows to identify clearly the time of repair in each zone and the evolution of the hardening process of the repair mortar. A second monitoring period of one year is then studied where it is found that despite the proposed improvement to the time stretching technique used to filter out the effects of changing environmental conditions, our indicators are still showing variations in periods when the temperature is very high in the tunnel. A method based on observed statistical correlations between the indicators computed in the different regions is then proposed and shown to be very efficient to remove the remaining variability and make the system very robust to environmental changes. Extreme value statistics is also presented as a tool to establish relevant thresholds for alarm-triggering with a very low level of potential false alarms. With all these developments, the monitoring system can automatically detect structural changes in the tunnel in real time while being robust to unavoidable changes in the environmental conditions in the tunnel.

A new and computationally efficient criterion is developed concerning the optimal placement of a collocated sensor-actuator pair for active control of structural vibrations with a low authority controller. The basic idea behind the proposed criterion is based on the maximization of the pole-zero distance in open-loop which has a direct link with the maximum achievable damping ratio once in closed-loop. Unlike poles, the determination of transmission zeros is computationally tedious because it depends upon the placement of the sensor-actuator pair. Therefore, a new approximation to reliably estimate the transmission zeros is also introduced which remarkably enhances the computational efficiency of the proposed optimization criterion. The effectiveness of the proposed criterion is demonstrated on a cantilever beam and a simply supported plate and is also compared with two other widely used criteria: the Gramian controllability and the spatial H-2 norm. It is shown that the proposed criterion is pertinent and ensures higher damping values for the cantilever beam compared to the other two criteria. Nonetheless, the criterion requires adaptations to improve its reliability for structure with large modal density such as a simply supported plate. (C) 2021 Elsevier Ltd. All rights reserved.

In this paper, a modified active tuned inerter damper concept which is more suitable for practical applications is proposed. The proposed device is composed of a pair of collocated reactive actuator and force sensor. A second-order low-pass filter and a proportional term are combined to form the controller. The equivalent mechanical model of the controller's components is derived in order to better interpret the coupled system. The second-order low-pass filter is mechanically equivalent to a pure mechanical network which comprises an inerter, a spring and a damper connected in parallel. The proportional term mechanically represents a spring which is connected in series with the inherent actuator spring. Simple regressions are derived based on the H-infinity optimisation criterion wherein the optimal feedback gains are calculated to minimise the maximal response of the driving-point receptance of the system. The numerical study is also experimentally validated. The obtained results are found to correspond well with the theoretical developments.

Positive position feedback is an attractive control law for the control of plants having no high frequency roll-off. The tuning of the parameters of the positive position feedback to obtain the desired closed-loop performance is quite challenging. This paper presents a technique to design the positive position feedback controller with the optimal damping. The technique is demonstrated on a single degree-of-freedom system. The poles of the positive position feedback are tuned using the method of maximum damping, which states that the maximum damping is achieved when both closed-loop poles of the system are merged. The parameters of the positive position feedback are dependent on the desired target damping in the closed-loop system. However, arbitrary choice of target damping results in high response at the frequencies lower than the tuning frequency. The optimal value of the target damping is obtained by minimizing the [Formula: see text] norm of the closed-loop transfer function of the system. The influence of the various parameters of the positive position feedback on the closed-loop response of the system is also studied. Finally, the experiments are conducted to verify the effectiveness of the proposed technique.

A robust design of tuned mass dampers (TMDs) must consider the influence of uncertainties associated with structural parameters to avoid detuning and malfunctioning. The existing robust equal-peak approach [1] accounts for uncertainties in stiffness and damping of single degree-of-freedom (SDOF) host structures but may lead to a high computational cost and sub-optimal design in practical cases where host structures are represented by a large number of degrees-of-freedom and also exhibit some damping. To overcome these limitations, a numerical optimisation technique is proposed which has the advantage of a low computational cost and generalization to any type of model of the host structure, including damping. Additionally, to account for the fact that the bounds of the uncertainty intervals are never known exactly for real-life conditions, this study employs fuzzy numbers to represent the structural uncertainties. The proposed approach is illustrated in case of an existing footbridge located in Durbuy (Belgium).

The goal of this study is to demonstrate the capability to actively isolate a platform from seismic vibrations using an interferometric inertial sensor. To this purpose, a homemade high-resolution interferometric uni-axial inertial sensor (in the vertical direction) is developed and then integrated to a single-degree-of-freedom (SDOF) seismic isolation system. A theoretical study is firstly performed in order to better understand the dynamics of the system. It is found that the sensor suffers from a tilt-vertical coupling due to the influence of the gravity force if the sensor is not perfectly aligned. By taking this tilt coupling into account, a dedicated controller is designed, seeking a large vibration isolation in the frequency band of interest. Experiments are then conducted for validating the theoretical analysis and examining the vibration isolation performance. It shows a reduction of the transmitted motion of up to 60 dB in a frequency range from 0.1 Hz to 10 Hz. (C) 2020 Elsevier Ltd. All rights reserved.

This paper investigates the potential of using an active control system to mitigate broadband vibrations of bladed structures. Piezoelectric patches are used for both sensing the motion and actuating the control force. To maximize control authority, the size and location of piezoelectric patches are optimized based on maximizing the strain energy. Two new designs of active control law, which are inspired by force-feedback configuration, are developed for the plant having alternating pole-zero pairs in the frequency response function (FRF). Numerical simulations are performed to assess the performance of the designed control system in terms of the closed-loop damping of the system with high modal density. The parameters of the control law are tuned based on maximizing the minimum damping of the first family of modes. Experimental tests are performed to validate the numerical design.

This work proposes a method to synthesize an electrical network which, when coupled to a complex periodic or nearly-periodic structure through an array of piezoelectric transducers, provides multimodal vibration mitigation. The structure is decomposed into multiple substructures and a reduced-order model is built for each of them. From these models, it is possible to synthesize a network with simple algebraic transformations. The link between these transformations and electromechanical modal coupling is derived, and conditions are given in order to guarantee the passivity of the electrical network. The proposed approach is illustrated on a bladed rail, for which damping of one or multiple families of blade modes is demonstrated.