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A general-purpose method based on the implementation of the asymmetric Pound–Drever–Hall (PDH) technique is proposed to measure the carrier-envelope offset (CEO) frequency of a mode-locked laser using an external optical cavity. By analyzing the synchronously demodulated signal of the spectrally filtered cavity reflection when the optical resonator is locked to the mode-locked laser, a discriminating signal depending on the relative frequency offset between the mode-locked and optical cavity comb-like spectra is obtained. For a given geometry and group delay dispersion (GDD) of the cavity parameters (i.e., a known cavity mode offset), this signal can be used to retrieve the laser CEO. This approach turns out to be advantageous in terms of setup complexity with respect to other well-known techniques that rely on non-linear frequency generation, such as f–2f interferometers. In addition, this method can be used to precisely determine the laser–cavity spectral coupling, which is an important topic in cavity-enhanced spectroscopy and non-linear optics applications. After the theoretical description of the generalized asymmetric PDH signal, an experimental validation of the proposed method is reported using an Er-doped fiber frequency comb source centered at 1,550 nm, with a repetition rate of 250 MHz, locked to a linear optical cavity with a 1 GHz free spectral range. The theoretical effect of the GDD is confirmed experimentally using different cavity configurations. Moreover, the comparison with the CEO frequency values measured using an f–2f interferometer demonstrates the feasibility of the proposed method.

Sensing technologies using optical fibers have been studied and applied since the 1970s in oil and gas, industrial, medical, aerospace, and civil areas. Detecting ultrasound acoustic waves through fiber-optic hydrophone (FOH) sensors can be one solution for continuous measurement of volumes inside production tanks used by these industries. This work presents an FOH system composed of two optical fiber coils made with commercial single mode fiber (SMF) working in the sensor head of a Michelson's interferometer (MI) supported by an active stabilization mechanism that drives another optical coil wound around a piezoelectric actuator (PZT) in the reference arm to mitigate external mechanical and thermal noise from the environment. A 1000 mL glass graduated cylinder filled with water is used as a test tank, inside which the sensor head and an ultrasound source are placed. For detection, amplitudes and phases are measured, and machine learning algorithms predict their respective liquid volumes. The acoustic waves create patterns electronically detected with resolution of 1 mL and sensitivity of 340 mrad/mL and 70 mvolts/mL. The nonlinear behavior of both measurands requires classification, distance metrics, and regression algorithms to define an adequate model. The results show the system can determine liquid volumes with an accuracy of 99.4% using a k-nearest neighbors (k-NN) classification with one neighbor and Manhattan's distance. Moreover, Gaussian process regression using rational quadratic metrics presented a root mean squared error (RMSE) of 0.211 mL. [ABSTRACT FROM AUTHOR]

Background/Aim: The high rate of root resorption resulting from tooth replantation represents a serious clinical problem. In order to prevent ankylosis and replacement resorption, the contemporary literature highlights the importance of using a flexible stabilization for traumatized teeth. For this purpose, orthodontic devices may be promising for obtaining a better prognosis and periodontal repair. The aim of this study was to evaluate the effect of an active splinting protocol with controlled force in dog's teeth following replantation. Material and Methods: Sixty premolar roots from three dogs were used. They were submitted to endodontic treatment, hemisected, atraumatically extracted and subsequently replanted. They were divided into four groups: Passive Stabilization (n = 20)‐after 20 min in a dry medium; Active Stabilization (n = 20)‐after 20 min in a dry medium; Negative control (n = 10)‐immediate replantation and passive Stabilization; and Positive control (n = 10)‐90 min of extra‐alveolar time and passive Stabilization. The samples were collected and submitted to histologic processing. They were then evaluated for the count of inflammatory cells, expression of neurotrophin 4, osteoclasts, apoptotic cells and collagen fibres. The results were submitted to ANOVA or Kruskal‐Wallis statistical tests followed by Tukey or Dunn post‐tests (α = 5%). Results: Passive Stabilization with orthodontic brackets without traction used after replantation had the highest number of inflammatory cells (p =.0122), osteoclasts (p =.0013) and percentage of collagen fibres in the periodontal ligament (p <.0001) when compared to Active Stabilization with orthodontic brackets applying amild tensile force. Neurotrophin 4 had no statistically significant difference (p =.05), regardless of the treatment. The apoptotic cells count revealed statistical differences (p <.0001) between Active Stabilization (189.70 ± 47.99) and Positive Control (198.90 ± 88.92) when compared to Passive Stabilization (21.19 ± 32.94). Conclusion: The active splinting protocol using orthodontic appliances generating a light and controlled force favoured periodontal ligament repair of replanted teeth. [ABSTRACT FROM AUTHOR]

Sensing technologies using optical fibers have been studied and applied since the 1970s in oil and gas, industrial, medical, aerospace, and civil areas. Detecting ultrasound acoustic waves through fiber-optic hydrophone (FOH) sensors can be one solution for continuous measurement of volumes inside production tanks used by these industries. This work presents an FOH system composed of two optical fiber coils made with commercial single mode fiber (SMF) working in the sensor head of a Michelson’s interferometer (MI) supported by an active stabilization mechanism that drives another optical coil wound around a piezoelectric actuator (PZT) in the reference arm to mitigate external mechanical and thermal noise from the environment. A 1000 mL glass graduated cylinder filled with water is used as a test tank, inside which the sensor head and an ultrasound source are placed. For detection, amplitudes and phases are measured, and machine learning algorithms predict their respective liquid volumes. The acoustic waves create patterns electronically detected with resolution of 1 mL and sensitivity of 340 mrad/mL and 70 mvolts/mL. The nonlinear behavior of both measurands requires classification, distance metrics, and regression algorithms to define an adequate model. The results show the system can determine liquid volumes with an accuracy of 99.4% using a k-nearest neighbors (k-NN) classification with one neighbor and Manhattan’s distance. Moreover, Gaussian process regression using rational quadratic metrics presented a root mean squared error (RMSE) of 0.211 mL.

The advancement in technology has seen a rapid increase in the use of unmanned aerial vehicles for various applications. These unmanned aerial vehicles are often equipped with the imaging platform like a camera. During the unmanned aerial vehicle flight, the camera is subjected to vibrations which hamper the quality of the captured images/videos. The high-frequency vibrations from the unmanned aerial vehicle are transmitted to the camera. Conventionally, passive rubber mounts are used to isolate the camera from the drone vibrations. The passive mounts are able to provide reduction in response near the resonance. However, this comes at the cost of amplification of response at the higher frequency. This article proposes an active vibration isolation system which exhibits improved performance at the higher frequencies than the conventional system. The active isolation system consists of a contact-less voice coil actuator supported by four springs. Experiments are carried out to study the effect of vibrations on the quality of images captured. The characterization of drone vibrations is also carried out by recording the acceleration during different flight modes. The performance of the proposed isolation system is experimentally validated on a real drone camera subjected to the recorded drone acceleration spectrum. The isolation system is found to perform better than the conventional rubber mounts and is able to reduce the vibrations to a factor of one-fourth. It can be effectively used to improve the image acquisition quality of the unmanned aerial vehicles. [ABSTRACT FROM AUTHOR]

A two-stage electric vehicle (EV) battery charger typically consists of an ac–dc converter cascaded with a dc–dc converter. In such a cascaded system, maintaining stability at the intermediate dc link is imperative for reliable operation of the battery charger under different operating modes. This paper addresses the intermediate dc-link stability challenges that exist in a bidirectional two-stage grid connected single-phase battery charger. It is delineated that the small-signal load-dependent resistance of the ac–dc converter plays a crucial role in determining the stability of the bidirectional battery charger. A virtual-resistor-based active damping control strategy that does not require any additional sensors is explored for the ac–dc converter to stabilize the cascaded system under all operating modes irrespective of the power flow direction. Experimental results on a grid-connected single-phase battery charger hardware prototype are presented to validate the proposed models and showcase the improvement in the dc-link stability due to the virtual-resistance-based active damping approach. [ABSTRACT FROM AUTHOR]

Multi-terminal AC/DC interconnection will be an important form of future distribution networks. In a multi-terminal AC/DC system, if scheduled power for the AC/DC converter exceeds limits this may result in instability of the DC network. In order to overcome these limitations and avoid an unstable situation during coordinated control, this paper proposes a general active stabilization method for a low-voltage multi-terminal AC/DC hybrid system. First, the typical coordinated control modes for a hybrid system are analyzed. Second, a multi-level active stabilization controller, using the Lyapunov method, is introduced, and a feedback law allowing large signal stability is proposed. Finally, a system simulation model is further established, and the proposed active stabilization method is tested and verified. Study results show that only low stabilizing power with a slight influence on the DC network dynamic can improve the system's stability and ensure stable system voltage. [ABSTRACT FROM AUTHOR]

As it has changed gradually from more electric aircraft (MEA) to all electric aircraft, the stabilization of the on-board dc power system is a hot research topic in recent years. In such an application, small and light systems are required, and thus, the optimization of passive LC filter is a significant issue. It is known that reduction of the LC filter capacitor will cause instability when the constant power load is employed. There is always a compromise between stability margin and system size. A practical method using the current perturbation was proposed in order to stabilize the on-board power system. The main objective is to suppress the instability and improve the stability margin, while keeping the benefits of the low weight and volume. The mathematical model of the system is analyzed, and the simulations and experimentations are performed to confirm the validity and effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper describes the results of a six-year project aiming at designing and constructing a flapping twin-wing robot of the size of hummingbird (Colibri in French) capable of hovering. Our prototype has a total mass of 22 g, a wing span of 21 cm and a flapping frequency of 22 Hz; it is actively stabilized in pitch and roll by changing the wing camber with a mechanism known as wing twist modulation. The proposed design of wing twist modulation effectively alters the mean lift vector with respect to the center of gravity by reorganization of the airflow. This mechanism is modulated by an onboard control board which calculates the corrective feedback control signals through a closed-loop PD controller in order to stabilize the robot. Currently, there is no control on the yaw axis which is passively stable, and the vertical position is controlled manually by tuning the flapping frequency. The paper describes the recent evolution of the various sub-systems: the wings, the flapping mechanism, the generation of control torques, the avionics and the PD control. The robot has demonstrated successful hovering flights with an on-board battery for the flight autonomy of 15-20 s. [ABSTRACT FROM AUTHOR]

Multi-terminal AC/DC interconnection will be an important form of future distribution networks. In a multi-terminal AC/DC system, if scheduled power for the AC/DC converter exceeds limits this may result in instability of the DC network. In order to overcome these limitations and avoid an unstable situation during coordinated control, this paper proposes a general active stabilization method for a low-voltage multi-terminal AC/DC hybrid system. First, the typical coordinated control modes for a hybrid system are analyzed. Second, a multi-level active stabilization controller, using the Lyapunov method, is introduced, and a feedback law allowing large signal stability is proposed. Finally, a system simulation model is further established, and the proposed active stabilization method is tested and verified. Study results show that only low stabilizing power with a slight influence on the DC network dynamic can improve the system’s stability and ensure stable system voltage.

The virtual impedance concept is increasingly used for the control of power electronic systems. Generally, the virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. This paper presents an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters. The control output impedance shaping attained by the virtual impedances is generalized first using the impedance-based models. Different virtual impedances and their implementation issues are then discussed. A number of practical examples are demonstrated to illustrate the feasibility of virtual impedances. Emerging applications and future trends of virtual impedances in power electronic systems conclude this paper. [ABSTRACT FROM AUTHOR]

DC microgrids are increasingly used in transportation systems such as more electric aircraft. Such applications require small and light systems, and thus, the optimization of passive elements like dc-bus capacitor and filtering inductor is an important issue. It is known that the reduction of dc-bus capacitance may lead to instability of the dc microgrid when tightly controlled loads are employed. To overcome this risk, a seductive solution is to implement a centralized stabilization system in the dc microgrid. Nevertheless, a centralized stabilizer requires a lot of sensors and particularly those providing the load input voltages. The latter is often far from the stabilizer, and fast data transmission lines must be provided. In this paper, an observer is developed for estimating the load input voltages, thus removing the required voltage sensors. Its convergence as well as the stability of the whole system is proved. The validity of the proposed method is confirmed by simulations and experimentations. [ABSTRACT FROM AUTHOR]

In this paper, we describe the extension of our fiber-optic frequency transfer system to the time transfer capability. In contrast to standard two-way transfer schemes which offer only comparisons of two distant clocks, our system displays distribution functionality, reproducing the time and frequency signals of the reference clock in the remote location. By using active compensation of the fiber delay fluctuations, we obtained a time deviation of 0.3 ps (for time transfer) and an Allan deviation of 1.2 × 10-17 (for frequency transfer) at 105-s averaging. The experiments presented were carried out using a 60-km-long fiber loop, forming a part of the real urban network around Kraków. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a simple model of the final focus of a linear particle collider. Adopting an integrated approach, several control strategies are tested to stabilize the mechanical parts, and control the beam. One of the key features of the model is that it has been updated using vibration spectra measured in the CMS experimental area of the LHC. Using this model, it has been possible to estimate objectively the performances of a final focus system, compare and propose new solutions to improve the mechanical design. [Copyright &y& Elsevier]

We study the application of electromagnetic actuators for the active stabilization of the Leipholz column. The cases of the compressive and tensional load of the column placed in air and in water are considered. The partial differential equation of the column is discretized by Galerkin's procedure, and the stability of the obtained control system is evaluated by the eigenvalues of its linearization. Four different methods of active stabilization are investigated. They incorporate control systems based on feedback proportional to the transverse displacement of the column, its velocity and the current in the electromagnets. Conditions in which these strategies are effective in securing safe operation of the column are discussed in detail. [ABSTRACT FROM AUTHOR]

Implantation of complex dc-power network is one of the main research topics in more electric aircraft (MEA). In such applications, small and light systems are required and so optimization of passive elements, such as dc-bus capacitance and filtering inductance, is an important issue. It is known that the reduction of dc-bus capacitance may lead to instability of an MVdc network. So, if no stabilizer is used, the risk of instability must be considered, while designing the system passive elements. In this paper, we will first study the small signal stability of an MVdc network composed of three loads: an inverter supplying a permanent magnet synchronous motor, a dc/dc converter feeding a resistive load, and a supercapacitor controlled by a bidirectional dc/dc converter. Then, we will propose a large-signal-stabilizing system to ensure global stability by generating proper stabilizing power references for the whole system. The contribution of the loads to network stability is adjustable. The validity of the proposed method will be confirmed by experimentations. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an experimental validation of a control strategy capable of both stabilizing and positioning the heavy electromagnets of future particle colliders. The originality of the approach is to use the same active mounts to perform both tasks, with a nanometer precision. In a previous paper, the concept has been studied numerically, and validated on a scaled single degree of freedom (d.o.f.) test bench. In this paper, it is extended to a two d.o.f. test bench, constituted of a heavy mass mounted on two active legs. Firstly, the model is described and the performances are discussed numerically. Secondly, experimental results are presented, and found to correlate well with the model, and comply with the requirements. Finally, the experimental results are combined with a simplified model of the beam-based feedback to evaluate the jitter of the beam. It is found that, at the scale of a single quadrupole, the mechanical stabilization of the quadrupoles reduces the vertical beam jitter by a factor 10. [Copyright &y& Elsevier]

Abstract: Electrophysiological recordings from a single or population of neurons are currently the standard method for investigating neural mechanisms with high spatio-temporal resolution. It is often difficult or even impossible to obtain stable recordings because of brain movements generated by the cardiac and respiratory functions and/or motor activity. An alternative approach to extensive surgical procedures aimed to reduce these movements would be to develop a control system capable of compensating the relative movement between the recording site and the electrode. As a first step towards such a system, an accurate method capable of measuring brain micromotion, preferably in 3D, in a non-invasive manner is required. A wide variety of technical solutions exist for displacement measurement. However, increased sensitivity in the measurement is often accompanied by strict limitations to sensor handling, implementation and external environment. In addition, majority of the current methods are limited to measurement along only one axis. We present a novel, minimally invasive, 3D displacement sensor with displacement resolution exceeding 70nm along each axis. The sensor is based on optoelectronic detection of movements of a spring-like element with three degrees of freedom. It is remarkably compact with needle-like probe and can be packaged to withstand considerable mishandling, which allow easy implementation to existing measurement systems. We quantify the sensor performance and demonstrate its capabilities with an in vivo measurement of blowfly brain micromotion in a preparation commonly used for electrophysiology. [Copyright &y& Elsevier]

Abstract: This paper presents the design and characterization of a fiber Fabry–Perot interferometer (FFPI) acoustic wave detector with its Q point being stabilized actively. The relationship between the reflectivity of the F–P cavity facets and cavity length was theoretically analyzed, and high visibility of 100% was realized by optimized design of the F–P cavity. To prevent the drifting of the Q point, a new stabilization method by actively feedback controlling of the diode laser is proposed and demonstrated, indicating the method is simple and easy operating. Measurement shows that good tracing of Q point was effectively realized. [Copyright &y& Elsevier]

Off-pump Coronary Artery Bypass Grafting (CABG) is still a technically difficult procedure. The mechanical stabilizers used for local suppression of the heart excursion have been demonstrated to exhibit significant residual motion, which could lead to a lack of accuracy in performing the surgical task, particularly when using a minimally invasive surgery (MIS) approach. We therefore propose a novel active stabilizer to compensate for the residual motion whose architecture is compatible with MIS. An experimental evaluation of a commercially available totally endoscopic stabilizer is first presented to demonstrate the unsatisfactory behavior of this device. Then, the interaction between the heart and a mechanical stabilizer is assessed in vivo using an animal model. Finally, the principle of active stabilization, based on the high-speed vision-based control of a piezo-actuated compliant mechanism, is presented, along with in vivo experimental results obtained using a prototype to demonstrate its efficiency. [ABSTRACT FROM AUTHOR]

Previous results concerning instability of the dc link in inverter drives fed from a dc grid or via a rectifier are extended. It is shown that rectifier-inverter drives equipped with small (film) dc-link capacitors may need active stabilization. The impact of limited bandwidth and switching frequency in the inverter-motor current control loop is considered, and recommendations for selection of the dc-link capacitor, the switching frequency, and the dc-link stabilization control law in relation to each other are given. This control law is incorporated in a field-weakening (to enhance voltage sag ride-through) current controller for which design recommendations are presented. [ABSTRACT FROM AUTHOR]