Time-domain Identification Research Articles

Accuracy–simplicity trade-off for small-scale helicopter models: A comparative study based on flight data

A good model is a trade-off between simplicity of the model structure and accuracy of the prediction. Higher accuracy is generally expected from more complex models, but at the cost of higher computational burden, more complex handling and potential identifiability issues. Depending on the targeted use, assumptions and simplifications are made to find the simplest model still capturing the important phenomena. These choices are not straightforward and, to this end, the paper gives a comparison of miniature helicopter models often found in the literature. The contribution of the paper is thus twofold. A time-domain identification procedure for parametric models of miniature helicopters is first described and applied to four different models with increasing complexity. The procedure is based on flight data obtained during a manual slow-speed flight. Secondly, the accuracies of these models are evaluated and compared, which highlights the main differences and improvements brought by the differences in the aerodynamic model equations and allows the selection of a relevant model structure depending on the target application.

Frequency vs time domain identification of heritage structures

This paper discusses on the output only modal identification of historic structures. Modal identification has been carried out both in frequency and in time domain using the Frequency Domain Decomposition (FDD) and the Stochastic Subspace Iteration (SSI-data) respectively. To highlight the sensitivity of the two methods, two masonry towers under different environmental loads have been considered. In one case the dynamic excitation can be assumed as a very weak random white noise, in the other case, probably due the several external noises, the ambient noise is locally dominated by several harmonic forces. The paper highlights the challenges in the modal identification of heritage structures such as the very low weak operating response and the role of the harmonics originated by engines operating somewhere nearby the structures. Moreover, it is investigated the information trade-off between the frequency domain and the time domain identification for evaluating the modal properties of the structures in a straightforward manner.

Fractional-order model identification for state of health assessment of solid-oxide fuel cells

Solid-oxide fuel cells (SOFCs) represent a group of electrochemical conversion devices that utilise hydrogen rich fuels and are characterised by their high efficiency of energy conversion. For optimal exploitation of SOFCs, accurate and online state-of-health (SoH) assessment is of utmost importance. SoH assessment is usually performed through the frequency domain analysis by characterising the changes of the Nyqvist curves, a process also known as electrochemical impedance spectroscopy. Such an approach is time consuming and suffers from low accuracy particularly at low frequencies. Methodologies for time-domain identification of fractional-order systems offer a way of resolving these issues. Using an algebraic approach to identification, the complete fractional-order transfer function is identified from a series of several step responses. The SoH can be estimated from the identified parameters of the fractional-order transfer function either in a form of a Nyqvist curve or by directly linking particular parameter values to electrochemical processes. The proposed approach was validated on a 300 W SOFC using pure hydrogen fuel.

Modal participation in multiple input Ibrahim time domain identification

The Ibrahim time domain (ITD) identification technique was one of the first techniques formulated for multiple output modal analysis based on impulse response functions or general free decays. However, the technique has not been used much in recent decades due to the fact that the technique was originally formulated for single input systems that suffer from well-known problems in case of closely spaced modes. In this paper, a known, but more modern formulation of the ITD technique is discussed. In this formulation the technique becomes multiple input by adding some Toeplitz matrices over a set of free decays. It is shown that a special participation matrix can be defined that cancels out whenever the system matrix is estimated. The participation matrix becomes rank deficient if a mode is missing in the responses, but if any mode is present in one of the considered free decays, the participation matrix has full rank. This secures that all modes will be contained in the estimated system matrix. Finally, it is discussed how correlation functions estimated from the operational responses of structures can be used as free decays for the multiple-input ITD formulation, and the estimation errors of the identification technique are investigated in a simulation study with closely spaced modes. The simulation study shows that the multiple-input formulation provides estimates with significantly smaller errors on both mode shape and natural frequency estimates.

Novel toggle-rate based energy-efficient scheme for heavy load real-time IM-DD OFDM-PON with ONU LLID identification in time-domain using amplitude decision

A novel low complexity and energy-efficient scheme by controlling the toggle-rate of ONU with time-domain amplitude identification is proposed for a heavy load downlink in an intensity-modulation and direct-detection orthogonal frequency division multiplexing passive optical network (IM-DD OFDM-PON). In a conventional OFDM-PON downlink, all ONUs have to perform demodulation for all the OFDM frames in a broadcast way no matter whether the frames are targeted to or not, which causes a huge energy waste. However, in our scheme, the optical network unit (ONU) logical link identifications (LLIDs) are inserted into each downlink OFDM frame in time-domain at the optical line terminal (OLT) side. At the ONU side, the LLID is obtained with a low complexity and high precision amplitude identification method. The ONU sets the toggle-rate of demodulation module to zero when the frames are not targeted to, which avoids unnecessary digital signal processing (DSP) energy consumption. Compared with the sleep-mode methods consisting of clock recovery and synchronization, toggle-rate shows its advantage in fast changing, which is more suitable for the heavy load scenarios. Moreover, for the first time to our knowledge, the characteristics of the proposed scheme are investigated in a real-time IM-DD OFDM system, which performs well at the received optical power as low as -21dBm. The experimental results show that 25.1% energy consumption can be saved in the receiver compared to the conventional configurations.

Identification of structural stiffness and excitation forces in time domain using noncontact vision-based displacement measurement

The emerging noncontact vision-based displacement sensor system offers a promising alternative to the conventional sensors for quantitative structural integrity assessment. Significant advantages of the noncontact vision-based sensor include its low cost, ease of operation, and flexibility to extract structural displacement responses at multiple points. This study aims to link the measured displacement data to the quantification of the structural health condition, by validating the feasibility of simultaneous identification of structural stiffness and unknown excitation forces in time domain using output-only vision-based displacement measurement. Numerical analysis are first carried out to investigate the accuracy, convergence and robustness of identified results to different noise levels, sensor numbers, and initial estimates of structural parameters. Then, experiment on a laboratory scaled beam structure is conducted. Results show that the global stiffness of the beam specimen as well as external hammer excitation forces can be successfully and accurately identified from displacement measurement at two points using one camera. The proposed output-only time-domain identification procedure utilizing vision-based displacement measurement represents a low-cost method for either periodic or long-term bridge performance assessment.

Research on the dynamic characteristics of a turbine flow meter

In the hardware-in-loop simulation of aero-engine control system where the real fuel regulator is engaged, it's crucial to measure the real-time flow rate. In view of this, a flow meter with high precision and fast response is important. In this paper, modeling and experiments are conducted to verify the dynamic characteristics of a turbine flow meter (TFM). For the modeling part, driving torque and resistance torques are analyzed to derive the kinetic equation of TFM. Simulation with the kinetic equation shows good dynamic performance of TFM. In experiments, a workbench is designed to generate step-type flow and sine-type flow for identification in time domain and frequency domain. Results show that the settling time for TFM is no more than 100ms and its band-with is over 4.61Hz. Compared with the settling time of a main fuel valve and the band-width of a main fuel control loop, that is, 1.2s and 2Hz respectively, TFM is considered to be adaptive to measure the fuel of aero-engine.

A simplified time-domain design and implementation of cascaded PI-sliding mode controller for dc–dc converters used in off-grid photovoltaic applications with field test results

A time-domain design methodology for voltage regulation control of dc–dc boost and buck-boost converters based on a multi-loop controller with PI regulator for the outer loop and an inner loop with sliding mode current controller has been developed for renewable energy applications such as photovoltaic (PV)-fed dc–dc converters. This paper proposes a new method for the design of PI regulators in such multi-loop control scheme. The proposed design presents a simple analytical method for selecting controller gains and has been validated by simulation as well as hardware implementation. Also, this paper presents an illustrative example based on the proposed design for the voltage regulation control of PV-fed boost converters for off-grid applications. The simulation results for varying irradiation, temperature and load along with stability analysis have been presented in this paper. The proposed controller is implemented in hardware for a 1.1 kW PV-array-fed boost converter. Performance analysis based on field test results using real-time weather data validates the proposed design. Therefore the proposed controller could be considered as an attractive solution for off-grid renewable energy applications like PV- or fuel-cell-fed dc–dc converter, where the variations are stochastic in nature.

Simplified design of proportional-integral-derivative (PID) controller to give a time domain specification for high order processes

An efficient simplified method is proposed for the time domain design of industrial proportional-integral-derivative (PID) controllers and lead-lag compensators for high order single input single output (SISO) systems. The proposed analytical method requires no trial error steps for a lead-lag compensator design in the time domain by using the root-locus method. A practical PID controller design method was obtained based on the corresponding lead-lag compensator to give a required time-domain specification. Simulation studies were carried out to illustrate the control performance of the controllers by the proposed method. The proposed PID controller and lead-lag compensator directly satisfied time domain control specifications such as damping ratio, maximum overshoot, settling time and steady sate error without trial and error steps. The suggested algorithm can easily be integrated with a toolbox in commercial software such as Matlab.

High spatial resolution modal identification of a stadium suspension roof: Assessment of the estimates uncertainty and of modal contributions

In Operational Modal Analysis (OMA) engineers are usually confronted with the challenge of extracting as much information as possible from the data collected in ambient vibration tests in order to characterize the modal behaviour of the tested structures under environmental and operational conditions. There exist several tools for this purpose that combined can be used to estimate valuable information regarding modal behaviour of the tested structures from the output vibration responses measured in these tests. In this context, this paper describes the strategies and techniques employed, as well as the results obtained from the OMA performed to identify the modal properties of a football stadium suspension roof. The main focus of the paper is the employment of state-of-the-art identification techniques, in time and frequency domain, in order to estimate the modal parameters of the roof structure with high spatial resolution for the mode shapes of vibration together with their uncertainties bounds, as well as to assess the contribution of the identified modes to the measured responses under different environmental and operational conditions. A particular challenge of the presented example is the need to simultaneously process a large number of time series (15 datasets with 12 sensors – 180 time series). At the end of the paper, the estimates provided by the ambient vibration test are compared with the ones obtained with a permanent monitoring system operating during one year, in order to evaluate the representativeness of the results provided by a single ambient vibration test.

Time-domain output error system identification of iced aircraft aerodynamics

A \(\Delta\)-model approach to characterize changed aircraft dynamics due to icing is analytically derived and its parameters are estimated. The model extension is formulated as a separate module in the aircraft flight mechanics simulation and can be used in existing simulation models. The application of the output error method in time domain allows to satisfactorily match model outputs and measurements as well as calculated aerodynamic derivatives. Using available flight data of a light business jet with different ice accumulation cases provided by EMBRAER, model parameters of the \(\Delta\)-model are determined and interpreted pertaining to the aerodynamic degradation caused by icing. The results show good promise that the combination of modeling approach and estimation technique could be applicable to extend existing simulator models providing a basis for pilot training under icing conditions.

空间一维结构分布动载荷时域识别方法

A time domain identification method for distributed dynamic loads on 1D spatial structures was proposed. Based on the idea of spatial and temporal divisions, the formula and process for load time history identification were derived. The load identification program was compiled by means of MATLAB, and the identification method was verified through 2 examples: a simply supported beam subjected to distributed dynamic loads and a power line subjected to stochastic wind loads. Moreover, the effect of noise on the load identification was investigated through numerical simulation. It is shown that the distributed loads can be accurately identified for linear problems and approximately identified for weakly nonlinear problems with the presented method, which provides an efficient way to identify distributed loads on engineering structures.

Improved absorption efficiency of silicon (Si) solar cells through Resonant Waveguide Gratings (RWGs) – A hybrid design of RWG and Si solar cell

This research work focuses on the improved performance of a Si solar cell by increasing solar absorption. The silicon solar cell is designed with a hybrid photonic structure of Resonant Waveguide Gratings (RWGs) to enhance absorption efficiency. The Waveguide Gratings have been designed in Fourier Model Method (FMM) using regression algorithm. The subsequent hybrid design of RWG with Si solar cell has been carried out using Finite Difference Time Domain (FDTD) design tool. The grating corrugated layer is designed in a fused silica (SiO2) material and coated with a high index amorphous TiO2 material. One of the possible realizable grating designed parameters are: grating period d=316nm, refractive index of TiO2, nt=2.385, refractive index of fused silica ns=1.450, duty cycle=65%, structure line width w=205nm, resonance wavelength λr=632.8nm (Helium-Neon laser), angle of incidence θi=18°, and grating grooved height hg=120nm, whereas the designed parameters for Si solar cell are: refractive index of Si N=3.882+i(0.019) at resonance wavelength λr=632.8nm, cell thickness T=500nm. The employment of optimized grating parameters in simulations leads to efficient hybrid device performance.

Time domain identification and ranking of noise sources in a pneumatic nail gun

This paper investigates the noise sources in a pneumatic nail gun. The study combines two complementary experimental approaches. The first uses simultaneous data observation, with sound, acceleration and air pressure signals simultaneously recorded in conjunction with a nail gun motion high speed video. This strategy allows the identification of the physical processes involved in the operation of the machine at different time instants, as well as the associated noise generation mechanisms. However, since multiple noise sources radiate at the same time, this observation technique is not sufficient for noise source identification and ranking. Thus, a second approach introduces a selective wrapping procedure, and the strategy assures a reliable classification of the noise sources. The investigation considers the following noise origins: the air exhaust, the machine body and the workpiece/worktable.In the standardized setup, the workpiece is placed in a sandbox to minimize its noise contribution. Since the final efficiency of this setup has never been established, the study evaluates the sandbox efficiency and compares it with a more realistic test arrangement, where the workpiece is placed on a worktable. The analysis shows that the sandbox setup does not sufficiently attenuate the workpiece noise. With the worktable setup, the workpiece/worktable noise source appears as the main contributor to the total emitted noise, while the air exhaust and the machine body noise are ranked as the second and third sources, respectively. With the sandbox setup, the workpiece noise is reduced, but remains the dominant source with the air exhaust, in equal measure.

Optimal sensor placement for time-domain identification using a wavelet-based genetic algorithm

This paper presents a wavelet-based genetic algorithm strategy for optimal sensor placement (OSP) effective for time-domain structural identification. Initially, the GA-based fitness evaluation is significantly improved by using adaptive wavelet functions. Later, a multi-species decimal GA coding system is modified to be suitable for an efficient search around the local optima. In this regard, a local operation of mutation is introduced in addition with regeneration and reintroduction operators. It is concluded that different characteristics of applied force influence the features of structural responses, and therefore the accuracy of time-domain structural identification is directly affected. Thus, the reliable OSP strategy prior to the time-domain identification will be achieved by those methods dealing with minimizing the distance of simulated responses for the entire system and condensed system considering the force effects. The numerical and experimental verification on the effectiveness of the proposed strategy demonstrates the considerably high computational performance of the proposed OSP strategy, in terms of computational cost and the accuracy of identification. It is deduced that the robustness of the proposed OSP algorithm lies in the precise and fast fitness evaluation at larger sampling rates which result in the optimum evaluation of the GA-based exploration and exploitation phases towards the global optimum solution.

Algebraic Riccati equation based Q and R matrices selection algorithm for optimal LQR applied to tracking control of 3rd order magnetic levitation system

AbstractThis paper presents an analytical approach for solving the weighting matrices selection problem of a linear quadratic regulator (LQR) for the trajectory tracking application of a magnetic levitation system. One of the challenging problems in the design of LQR for tracking applications is the choice of Q and R matrices. Conventionally, the weights of a LQR controller are chosen based on a trial and error approach to determine the optimum state feedback controller gains. However, it is often time consuming and tedious to tune the controller gains via a trial and error method. To address this problem, by utilizing the relation between the algebraic Riccati equation (ARE) and the Lagrangian optimization principle, an analytical methodology for selecting the elements of Q and R matrices has been formulated. The novelty of the methodology is the emphasis on the synthesis of time domain design specifications for the formulation of the cost function of LQR, which directly translates the system requirement into a cost function so that the optimal performance can be obtained via a systematic approach. The efficacy of the proposed methodology is tested on the benchmark Quanser magnetic levitation system and a detailed simulation and experimental results are presented. Experimental results prove that the proposed methodology not only provides a systematic way of selecting the weighting matrices but also significantly improves the tracking performance of the system.

Vibration-based structural health monitoring for offshore wind turbines - Experimental validation of stochastic subspace algorithms

The efficiency of wind turbines (WT) is primarily reflected in their ability to generate electricity at any time. Downtimes of WTs due to conventional inspections are cost-intensive and undesirable for investors. For this reason, there is a need for structural health monitoring (SHM) systems, to enable service and maintenance on demand and to increase the inspection intervals. In general, monitoring increases the cost effectiveness of WTs. This publication concentrates on the application of two vibration-based SHM algorithms for stability and structural change monitoring of offshore WTs. Only data driven, output-only algorithms based on stochastic subspace identification (SSI) in time domain are considered. The centerpiece of this paper deals with the rough mathematical description of the dynamic behavior of offshore WTs and with the basic presentation of stochastic subspace-based algorithms and their application to these structures. Due to the early stage of the industrial application of SHM on offshore WT on the one side and the required confidentiality to the plant manufacturer and operator on the other side, up to now it is not possible to analyze different isolated structural damages resp. changes in a systematic manner, directly by means of in-situ measurement and to make these acknowledgements publicly available. For this reason, the sensitivity of the methods for monitoring purposes are demonstrated through their application on long time measurements from a 1:10 large scale test rig of an offshore WT under different conditions: undamaged, different levels of loosened bolt connections between tower parts, different levels of fouling, scouring and structure inclination. The limitation and further requirements for the approaches and their applicability on real foundations are discussed along the paper.

Time Domain Optimization of Filters Used in a Loudspeaker Array for Personal Audio

This paper describes a time domain design method for calculating the coefficients of FIR filters used to drive a loudspeaker array for personal audio. A motivating application is to boost the television audio in a certain spatial region with the aim of increasing the speech intelligibility of the hearing impaired. As the array is of small size, superdirective beamforming is applied to increase the low and mid frequency directional performance of the radiator. The filters for such arrays have previously been designed one frequency at a time, leading to non-causal filters that require a modeling delay for real time implementation. By posing the filter optimization in the time domain, the filter responses can be causally constrained, and the optimization is performed once for all frequencies. This paper considers the performance of such filters by carrying out off-line simulations, firstly using the impulse responses of point sources in the free field, and then with the measured responses of a loudspeaker array in an anechoic chamber. The simulation results show how the time domain optimization allows the creation of filters with either a low order or a low modeling delay.

Robust frequency-domain identification of parametric radiation force models for a floating wind turbine

This paper mainly concerns the hydrodynamic responses prediction of a floating wind turbine. A new frequency-domain identification approach is first proposed to fit a parametric model (state space model) to approximate the convolution term in the Cummins equation of the floating wind turbine. This frequency-domain identification approach consists of finding the vector of parameters that give the best least squares fitting to the frequency responses, and a damped Gauss–Newton algorithm is first proposed to solve this non-linear least squares problem. The damped Gauss–Newton algorithm consists of calculating an increment vector based on the information of residuals and Jacobian values, finding a “fraction” factor by using a line search algorithm, then updating the values of the vector of parameters until the norm of the gradient vector is less than a specific tolerance value. Various kinds of hydrodynamic radiation forces (moments) and motion responses of this floating wind turbine have been calculated. For comparison purpose, the calculation results based on a time-domain identification approach for fitting the state space model are also included in this paper. This time-domain identification approach is based on finding a parametric transfer function of an auto-regressive moving-average (ARMA) filter based on prescribed time domain responses by using Prony׳s method. The accuracy and efficiency of our proposed frequency-domain identification approach have been convincingly substantiated through the comparisons of the calculation results in this article.

Optimal Proportional–Integral–Derivative Control of Time-Delay Systems Using the Differential Quadrature Method

This paper presents an accurate and computationally efficient time-domain design method for the proportional–integral–derivative (PID) control of first-order and second-order plants in the presence of discrete time delays. As time delays would generally deteriorate the achievable performance of the PID controllers, their effects should be thoroughly considered in the controller design and parameter tuning process. This paper is thereby motivated to propose a time-domain semi-analytical method for the parameter tuning and stability analysis of PID controllers of the time-delay systems. To facilitate this development, the transfer functions of the investigated plants associated with the PID controllers are first rewritten as linear periodic delayed differential equations (DDEs) in state-space form. Then, the differential quadrature method (DQM) is adopted to estimate the time derivative of the state-space function at each sampling grid point within a duration of the time delay by the weighted linear sum of the function values over the whole sampling grid points. In this way, the DDEs in the time-delay duration are discretized as a series of algebraic equations, and the transition matrix can be obtained by combining these discretized algebraic equations. Thereafter, the stability boundary can be determined and the optimal control gains are obtained by minimizing the largest absolute eigenvalue of the transition matrix. As the minimum problems are commonly solved by the gradient descent approaches, the analytical form of the gradient of the largest absolute eigenvalue of transition matrix with respect to the control gains is explicitly presented. Finally, extensive numeric examples are provided, and the proposed DQM is proven to be an accurate and computationally efficient way to tune the optimal control gains and estimate the stability region in the control gain space.