Feedback Branch Research Articles

Deciphering the imprint of active galactic nucleus feedback in Seyfert galaxies. Nuclear-scale molecular gas deficits

We study the distribution of cold molecular gas in the circumunuclear disks (CND; $r pc) of a sample of 64 nearby L $=7-45 Mpc) disk galaxies ---including 45 active galactic nuclei (AGN) and 19 nonAGN--- for which high-spatial-resolution (median value $ pc) multiline CO interferometer observations have been obtained at millimeter wavelengths with the Atacama Large Millimeter Array (ALMA) and/or Plateau de Bure Interferometer (PdBI). We decipher whether or not the concentration and normalized radial distribution of cold molecular gas change as a function of X-ray luminosity in the 2--10 keV range X $) in order to analyze the imprint left by AGN feedback. We also look for similar trends in the concentration and normalized radial distribution of the hot molecular gas and in the hot-to-cold-molecular gas mass ratio in a subset of 35 galaxies using near-infrared (NIR) integral field spectroscopy data obtained for the H$_2$ 1-0 S(1) line. We find a significant turnover in the distribution of the cold molecular gas concentration as a function of X-ray luminosity with a breakpoint that divides the sample into two branches: (1) the `AGN build-up branch'' ($L_ X pm0.3 $erg s$^ $) and (2) the ``AGN feedback branch'' ($L_ X pm0.3 $erg s$^ $) . Lower-luminosity AGN and nonAGN of the AGN build-up branch show high cold molecular gas concentrations and centrally peaked radial profiles on nuclear ($r pc) scales. Higher-luminosity AGN of the AGN feedback branch show a sharp decrease in the concentration of molecular gas and flat or inverted radial profiles. The cold molecular gas concentration index ($CCI$) --- defined as the ratio of surface densities at $r pc ($ gas $) and $r pc $( gas $), namely $CCI gas gas $)--- spans a 0.63 dex range, equivalent to a factor simeq 4-5, between the galaxies lying at the high end of the AGN build-up branch and the galaxies showing the most extreme nuclear-scale molecular gas deficits in the AGN feedback branch. The concentration and radial distributions of the hot molecular gas in our sample follow qualitatively similar but less extreme trends as a function of X-ray luminosity. As a result, we find higher values of hot-to-cold molecular gas mass ratios on nuclear scales in the highest luminosity AGN sources of the AGN feedback branch. These observations confirm ---with a three times larger sample--- previous evidence found in the context of the Galaxy Activity Torus and Outflow Survey (GATOS) that the imprint of AGN feedback on the CND-scale distribution of molecular gas is more extreme in higher luminosity Seyfert galaxies of the local Universe.

A gyrator-C active inductor based tunable low noise amplifier for sub-GHz frequency range

The propose exhibits a tunable low noise amplifier (LNA), which provides the tunability at sub-GHz frequency range and hence can become choicest frequency range for 5 G applications. Common source (CS) with source degeneration inductor LNA is proposed; in which the inductor in source branch is replaced by gyrator-c active inductor which not only reduced the overall area of the circuit but can also be used to provide the feature of tunability by changing the bias voltage of CMOS based active resistor in feedback branch. Therefore, instead of having a LNA circuit for the fixed frequency, a single tunable LNA design can be used over a range of frequency from 1 GHz to 5 GHz. The proposed approach will provide the direct tuning to the circuit. The forward voltage gain achieved is greater than 10 dB, input matching parameter is less than −10 dB and the output matching parameter is less than −10 dB. The average power dissipated is 0.02 W. The results can be tuned by controlling the feedback voltage of tunable active inductor (Tunable_AI). The tunable results can be achieved up to the bandwidth of 1 GHz.

An Active Damping Method With Improved Capacitor Current Feedback

Abstract Capacitor-Current Feedback Active Damping (CCFAD) is widely employed in LCL grid-tied inverters. However, the delay introduced by digital control can alter the traditional active damping characteristics, with the equivalent virtual resistance having a positive/negative crossover frequency of f s/6. The LCL filter’s resonant frequency may change due to the extensive variation in grid impedance under weak grid conditions. When the resonant frequency of the LCL filter exceeds f s/6, the stability of the system is severely compromised. A better approach is to add a lead compensation element to the current feedback branch of traditional CCFAD in response to these issues. This compensates for the phase lag caused by control delay, effectively extending the active damping region to (0, f R), f R ∈ (f s/3, f s/2), thereby enhancing system stability and robustness. The proposed method’s effectiveness has been confirmed by theoretical analysis and simulation on the MATLAB/Simulink platform, which utilizes a single-phase LCL grid-tied inverter model.

Near-optimal Multiplier-Less Broadband Noise Shaping Filters

Noise shaping (NS) filters reduce the quantization noise power in one or more frequency band(s) while amplifying it in other bands. Narrow-band noise shaping filters are state of the art in audio signal processing, analog-digital and digital-analog conversion, direct digital synthesis, and other applications. However, it is much more difficult to design broadband NS filters. Since NS filters are used in feedback branches, they must therefore be designed direct path free which imposes a constraint on the filter coefficients. This constraint leads to prohibitive large filter coefficients employing state of the art filter design techniques.This paper investigates the theoretical bound for NS filters and shows results about a novel design method for broadband FIR and IIR noise shaping filters and its multiplier-less hardware implementation. The method employed is a purely numerical approximation technique and leads to filter designs close to the discussed theoretical bound. The quantization of the filter coefficients is performed by a Canonical Signed Digit (CSD) representation of the coefficients. Two alternative architectures for the implementation of the filters are discussed. The design technique and the CSD quantization are realized in a MATLAB toolbox. The filters were moreover implemented in VHDL.

BiDFDC-Net: a dense connection network based on bi-directional feedback for skin image segmentation.

Accurate segmentation of skin lesions in dermoscopic images plays an important role in improving the survival rate of patients. However, due to the blurred boundaries of pigment regions, the diversity of lesion features, and the mutations and metastases of diseased cells, the effectiveness and robustness of skin image segmentation algorithms are still a challenging subject. For this reason, we proposed a bi-directional feedback dense connection network framework (called BiDFDC-Net), which can perform skin lesions accurately. Firstly, under the framework of U-Net, we integrated the edge modules into each layer of the encoder which can solve the problem of gradient vanishing and network information loss caused by network deepening. Then, each layer of our model takes input from the previous layer and passes its feature map to the densely connected network of subsequent layers to achieve information interaction and enhance feature propagation and reuse. Finally, in the decoder stage, a two-branch module was used to feed the dense feedback branch and the ordinary feedback branch back to the same layer of coding, to realize the fusion of multi-scale features and multi-level context information. By testing on the two datasets of ISIC-2018 and PH2, the accuracy on the two datasets was given by 93.51% and 94.58%, respectively.

Security-Enhanced Electro-Optic Mutual Injection Secure Communication Scheme With Time-Delay Signature Suppressing

In this article, we propose and numerically demonstrate a novel dual-phase modulated time-delay signature (TDS) concealed communication system. The proposed system addresses the fatal disadvantage of conventional electro-optic chaotic systems in that TDS is vulnerable to detection. Combined with the dispersive devices ahead, which significantly expands the complexity of the signal, the scheme perturbs the transmitted data with the mutual injection of the feedforward and feedback branches in the phase, strengthening the nonlinear effect of the system considerably. In addition, the key space of the system against illegal users’ exhaustive attack within reasonable parameters reaches 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> , which is 14 orders of magnitude larger than that of the classical electro-optic system. A confidential 32 G/s OOK signal was transmitted over 100 km and successfully recovered. Numerical results demonstrate that the chaotic system can tolerate minor parameter mismatches which are controllable in practice, proving that the system can be used for secure communication.

Time-Delay Signature Concealment in a Security-Enhanced Optical System With Dual-Loop Electro-Optic Self- Feedback Phase Encryption

In this paper, a novel phase encryption scheme based on a dual-loop electro-optic self-feedback structure is proposed for time-delay signature (TDS) concealment. As for a conventional single-loop feedback structure, the TDS is extremely vulnerable to exposure in the common link, resulting in a fatal weakness in the entire security system, whereas, the introduction of an additional feedback branch, brings about the mutual dynamics between the phase feedbacks, and effectively solves the problem. The modulation depth and dispersion values, which are two crucial variables affecting TDS concealment, are investigated in detail. In principle, the scheme is verified to have better robustness, more security, and can supply large key space. Error-free transmission of high-speed signals is possible. Thanks to the aforementioned benefits, the dual-loop electro-optic structure proposed could inspire fresh ideas for secure optical communication systems in the future.

An Estimated δ-Based Iterative Block Decision Feedback Equalization in SC-FDE System

We provide a novel nonlinear frequency domain equalization algorithm for the frequency domain equalization of an SC-FDE system by improving the classical iterative block decision feedback equalization (IBDFE) algorithm and applying δ estimation to the improved algorithm. The improvement of the IBDFE algorithm is carried out by replacing the ZF equalization in the feedback branch with the MMSE equalization and eliminating the iteration of the correlation factor, thus reducing the noise error and the computational complexity of the original algorithm. δ estimation can estimate residual inter-symbol interference in the signal after MMSE equalization and reject it, thus further improving the equalization accuracy. The simulation results show that the performance of the novel algorithm is better than that of the IBDFE algorithm with similar complexity, or the complexity of the novel algorithm is lower than that of the IBDFE algorithm with similar performance.

An Active Damping Control Strategy for Grid-Tied Type-4 Wind Turbine Generator Based on Energy Feedback

In a grid-tied type-4 wind turbine generator (WTG) system, the frequency of sub/super-synchronous oscillation varies in a wide range, making it difficult to suppress the oscillation. In view of this problem, an active damping control strategy based on energy feedback is proposed. First, the energy balance equation of stored energy, dissipation energy, and interaction energy in a grid-tied type-4 WTG system is established. And the mathematical relationship between the increasing/decreasing trend of stored energy and the divergence/convergence of oscillation, no matter how the oscillation frequency varies, is derived. Then, by calculating the contribution degrees of different energy feedback branches to the variation rate of stored energy, the key energy feedback branches that cause the stored energy to increase are selected. And then, with the variation rate of stored energy being made negative as the control objective, this article derives a reactive current compensation function and constructs an additional energy feedback branch to eliminate the effect of key energy feedback branches, thus realizing active damping suppression of oscillations in a broad frequency band. Finally, the real-time digital simulator based hardware-in-loop test platform with a practical controller of a real 2.5-MW type-4 WTG and experimental platform are both built on which the proposed strategy is verified. Simulation results show that the proposed method can realize effective damping control of oscillations in a broad frequency range. In addition, this article is accompanied by a video demonstrating hardware-in-loop test results.

A New Approach for Solving the False Balance of a Closed-Loop Fluxgate Current Transducer

The closed-loop fluxgate current transducer (CFCT) is the only current sensor whose measurement accuracy can reach a level of the parts per million (ppm) at present. However, the false balance, which makes the CFCT unable to work, caused by an excessive offset current is the inherent flaw of the CFCT. The high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">di/dt</i> (rate of current change) of the primary current is the main factor resulting in the offset current overload. Therefore, the CFCT is currently only applied for steady-state direct-current measurements. In this article, an approach is proposed to eliminate the false balance of the CFCT in the dynamic process by adding a feedforward branch, which provides a follow-up bias point for the CFCT to make the CFCT always work in a small-signal range, where a risk of magnetic saturation does not arise. The zero-flux state of the CFCT still only depends on the feedback branch, so its high measurement accuracy is maintained. The experiments prove that the CFCT can measure a flat-top pulsed current with an accuracy of the ppm level by this method.

Learning Enriched Features for Image Super Resolution

In recent years, significant progress has been made in image super-resolution (SR) methods based on convolutional neural networks. However, most of them do not fully utilize multi-scale feature correspondence in the image SR process, resulting in blurred and artifact detail restoration, especially for SR tasks with larger scaling factors (i.e. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 4$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 8$ </tex-math></inline-formula> ). A multi-scale feature-enhanced SR network (MFENet) is proposed to solve the problems mentioned above. Specifically, the super-resolution method is enhanced by utilizing multi-scale feature correspondence in two ways. On the one hand, we propose an effective multi-scale non-local attention module and it can fully exploit a large number of low-level features at different scales of a single low-resolution (LR) feature map by exhaustively evaluating the correlations in local feature, in-scale non-local feature, and cross-scale non-local feature. On the other hand, the mapping from high-resolution (HR) to LR images is explored by a feedback branch we introduced, that provides an additional constraint for images reconstruction. It can penalize incorrect SR predictions to achieve better SR performance. Experimental results on 5 benchmark datasets show that our method outperforms state-of-the-art methods in terms of both subjective visual quality and objective quantitative metrics.

Joint blind image deblurring and super-resolution via double-branch projection feedback network

Unexpected situations, such as object movement and camera shaking, cause interference and produce blurry, degraded images. These degraded images can adversely affect visual processing tasks, such as target detection and facial recognition. In addition, the super-resolution (SR) of a single image is a basic task to improve the image quality in visual applications. However, due to the existence of artifacts, it is difficult to generate clear SR images from rough and degraded images. In this work, we propose a double-branch projection feedback network that can generate clear high-resolution (HR) images from severely blurred low-resolution (LR) images. One is a deep feature projection feedback branch that can obtain texture-rich SR feature maps and reduce the accumulative errors with depth, and a channel adjustment and multi-scale feature fusion mechanism are proposed to better obtain the mapping relation between LR and HR images. The other is an autoencoder deblurring module, which is used to get deblurred features of an input image. Moreover, a mapping associated feature block is applied to match non-local feature information in the feature fusion module. Extensive experiments show that the network is effective and performs better than the state-of-the-art methods in single image deblurring and SR.

On the Tuning of Fractional Order Resonant Controllers for a Voltage Source Converter in a Weak AC Grid Context

This paper proposes a method for tuning the fractional exponent of different types of fractional order resonant controllers for a voltage source converter in a weak AC grid context. The main objective is to ensure the stability of the controlled system in a weak AC grid environment and to achieve an adequate dynamic response under disturbances. Therefore, six commonly used integer order proportional resonant (PR) control structures are selected from the literature and compared with each other according to their frequency behaviour. Afterwards, a rational approximation for the fractional order term is selected based on continuous fraction expansion technique. The inclusion of a fractional exponent in each integer order PR structure generates the fractional order proportional resonant (FPR) control transfer functions. Once the FPR controllers have been obtained, their closed-loop responses are tested via eigenvalue trajectory analysis. For each FPR control structure, a range of the fractional exponent that ensures stability is obtained. The conclusions of eigenvalue trajectory analysis are tested by implementing the FPR control structures in an specific application consisting in a modular multi-level converter (MMC) connected to a weak AC grid with adjustable short-circuit ratio. By means of time-domain simulations, not only the previous eigenvalue analyses are validated, but also new tuning criteria are given for the fractional exponent in combination with other control parameters, such as the damping frequency and the inductance of the complementary feedback branch. Moreover, a sensitivity analysis of the tuning criteria is carried out for other sizes of the AC filter inductance.

Input-output linearized spacecraft formation control via differential drag using relative orbital elements

The derivation, implementation, and performance assessment of a differential drag based algorithm, designed for spacecraft formation control using a relative orbital elements formulation, is presented. In contrast to similar approaches found in literature, the underlying motion dynamics model used to derive the control law includes J 2 perturbation as well as atmospheric drag perturbation based on a nonlinear density model. The basic idea of the control algorithm is to algebraically transform the nonlinear plant into a linear substitute. The resulting control law can be interpreted as combined feedforward and feedback PID control, applied to the linear substitute model. Motion dynamics and perturbations are accounted for in a feedforward branch while a PID feedback branch undertakes the cancellation of errors caused by model uncertainties and unmodeled physics. For linearized plant models, the computation of a feedforward branch is straight forward from the state transition matrix. In the nonlinear case proposed in this paper, the inclusion of a feedforward signal is achieved using the input-output linearization technique, yielding a nonlinear and globally valid control law. Sample numerical simulations are presented to support the validity of the controller and to demonstrate its performance compared to a simple PID controller. Simulation results show enhanced tracking performance of the mean along-track separation reference signal and reduced control effort. Moreover, input–output linearization enables the control of a large set of operating points using one set of control parameters.

A distributed interpolative sigma-delta interface for fluxgate sensor

This paper presents a sigma-delta interface for fluxgate sensor based on a single-loop high-order modulation. A feedforward summation loop structure can reduce the linearity overload in integrators and the swing of integration signals. A Local feedback branch is proposed to realize the zero of noise transfer function (NTF) optimization and enhance noise shaping performance within signal band. The interface is fabricated in a standard 0.5 [Formula: see text]m CMOS process and the active circuit area is about 13 mm2. The chip consumes 120 mW from a 10 V supply with a sampling clock of 250 k Hz. The average noise floor of the digital fluxgate is about −115 dBV/Hz[Formula: see text] over a 100 Hz bandwidth.

New method to analyse and optimise thermoacoustic power generators for the recovery of residual energy

The analysis of thermoacoustic engines and the equations that describe the internal physical phenomena are far from simple. Previous studies on energetic characterisation of thermoacoustic engines are based only on the determination of the active acoustic power flow distribution. In this paper, another variable, known as the reactive acoustic power, is additionally estimated and studied. This article proposes a simple method which is based on the combination of both active and reactive acoustic power flow for the evaluation and optimisation of thermoacoustic Stirling engines. In addition the paper illustrates the method using a thermoacoustic Stirling engine demonstrator design which is able to fit three different feedback branches. The results show that the amount of reactive acoustic power supplied towards the core branch differs depending on the specifications of the selected feedback branch. Besides, the amount of reactive acoustic power distributed towards the core branch is a good indicator of the grade of traveling-wave phasing. An improvement of 16.4% in the active acoustic power towards the extraction branch is achieved and besides, the amplification of the active acoustic power through the core branch has increased by 12%. The method can serve as an effective tool to study and optimise thermoacoustic devices.

Compensation of Chromatic Dispersion and Nonlinear Phase Noise Using Iterative Soft Decision Feedback Equalizer for Coherent Optical FBMC/OQAM Systems

In this article, we propose an iterative soft decision feedback (ISDF) equalizer to compensate for the chromatic dispersion and nonlinear phase noise effects in 30-Gbaud coherent optical filter bank multicarrier with off-set quadrature amplitude modulation (FBMC/OQAM) system. Conventional optical FBMC/OQAM system usually uses a 1-tap zero-forcing equalizer per subcarrier to combat the inter-symbol interference (ISI) caused by the chromatic dispersion (CD) in optical fiber. However, residual ISI and inter-carrier interference (ICI) still remain in the system and have significant impact on the system performance. To overcome this problem, a new ISDF N-tap frequency sampling (FS) equalizer is proposed to jointly mitigate the residual ISI and ICI which permits a better trade-off between performance and complexity. Simulation results show that, the proposed technique is capable of suppressing the interference more effectively than using only 1-tap equalizer. The feedback branch of the proposed ISDF equalizer can be exploited further to compensate for the nonlinear effect of the optical fiber. With compensation of linear and nonlinear impairments the proposed technique acts as a backpropagation method for optical FBMC/OQAM system.

A novel model of avalanche current generation in the GaN HEMT equivalent circuit

We consider the large-signal equivalent circuit of a field-effect transistor (FET) with characteristics and parameters calculated using a two-dimensional (2D) quasi-hydrodynamic model while taking into account the electron velocity overshoot. In accordance with this equivalent circuit when applied to an AlGaN/GaN high-electron-mobility transistor (HEMT), the avalanche current (in the feedback branch) is zero at all operating points. However, this contradicts the significant difference seen between the results of time-domain simulations of a radiofrequency (RF) amplifier in which the transistor is simulated using a 2D model that does versus does not include the avalanche multiplication of the charge carriers. We propose a new model for the avalanche current generation, based on such simulations including the avalanche multiplication and on the theory of impact-ionization avalanche transit-time (IMPATT) diodes. This model allows the synthesis of amplifiers with high power-added efficiency (PAE).

Energy and Efficiency Evaluation of Feedback Branch Design in Thermoacoustic Stirling-Like Engines

Stirling-like thermoacoustic generators are external combustion engines that provide useful acoustic power in the absence of moving parts with high reliability and respect for the environment. The study of these systems involves a great complexity since the parameters that describe them, besides being numerous, present a high degree of coupling between them. This implies a great difficulty in characterizing the effects of any parametric variation on the performance of these devices. Due to the huge amount of data to analyze, the experiments and simulations required to address the problem involve high investments in time and resources, sometimes unaffordable. This article presents, how a sensitivity analysis applying the response surface methodology can be applied to optimize the feedback branch of a thermoacoustic Stirling-like engine. The proposed study is made by evaluating the comparative relevance of seven design variables. The dimensional reduction process identifies three significant factors: the frequency of operation, the internal diameter of compliance, and the inertance. Subsequently, the Response Surface Methodology is applied to assess the interaction effects of these three design parameters on the efficiency of the thermoacoustic engine, and an improvement of 6% has been achieved. The enhanced values given by the response surface methodology are validated using the DeltaEC software.

Cascade Control of Antagonistic VSA-An Engineering Control Approach to a Bioinspired Robot Actuator.

A cascade control structure for the simultaneous position and stiffness control of antagonistic tendon-driven variable stiffness actuators (VSAs) implemented in a laboratory setup is presented in the paper. Cascade control has the ability to accelerate, additionally stabilize, and reduce oscillations, which are all extremely important in systems such as a tendon-driven compliant actuators with elastic transmission. Inner-loop controllers are closed in terms of motor positions, and outer-loop controllers in terms of actuator position and estimated stiffness. The dominant dynamics of the system (position and stiffness), composed of the mechanical part and inner loops, are identified by a closed-loop auto-regressive with exogenous input (ARX) model. The outer-loop controllers are tuned on the basis of experimentally identified transfer functions of the system in several nominal operating points for different stiffness values. After the system is identified, a controller bank is generated in which a pair of actuator position and stiffness controllers correspond to a nominal operating point and covers the area surrounding the nominal point for which it is designed. The controllers used are integral-proportional differential (I-PD) and integral-proportional (I-P) controllers, which are a variation of the PID and PI controllers with dislocated proportional and derivative gains from a direct to feedback branch that result to no overshoot for even fast reference changes (i.e., step signal), which is essential for preventing tendon slackening (meeting the pulling constraint). Analytical formulas for controller tuning based on only one parameter, λ, are also presented. Since position and stiffness loops are decoupled, it is possible to change λ for both loops independently and adjust their performance separately according to the needs. Also, the controller structure secures the smooth response without overshooting step reference or step disturbance signal, which make practical implementation possible. After all the controllers were designed, the cascade control structure for simultaneous position and stiffness control was successfully evaluated in a laboratory setup. Thus, the presented control approach is simple to implement, but with a performance that ensures a pulling constraint for tendon-driven actuators as a foundation for bioinspired antagonistic VSAs.