Fotis Kopsaftopoulos

Fotis Kopsaftopoulos
Rensselaer Polytechnic Institute | RPI · Department of Mechanical, Aerospace and Nuclear Engineering

PhD

About

96
Publications
35,418
Reads
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1,213
Citations
Introduction
Fotis Kopsaftopoulos currently works at the Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute.
Additional affiliations
April 2013 - September 2017
Stanford University
Position
  • Fellow
February 2012 - February 2013
University of Patras
Position
  • PostDoc Position
October 2004 - December 2011
University of Patras
Position
  • Research Assistant

Publications

Publications (96)
Article
Full-text available
In this work, a novel data-based stochastic global identification framework is introduced for air vehicles operating under varying flight states and uncertainty. In this context, the term global refers to the identification of a model that is capable of representing the system dynamics under any admissible flight state based on data recorded from s...
Conference Paper
This work presents the investigation and critical assessment, within the framework of Dynamic Data Driven Applications Systems (DDDAS), of two probabilistic state awareness approaches for fly-by-feel aerial vehicles based on (i) stochastic adaptive time-dependent time series models and (ii) Bayesian learning via homoscedastic and het-eroscedastic G...
Conference Paper
Safety envelopes are meant to determine under which conditions and state space regions a probabilistic property of a data-driven system can be asserted with high confidence. Dynamic data-driven applications systems (DDDAS) can make use of safety envelopes to be cognizant of the formal warranties derived from their models and assumptions. An example...
Preprint
Full-text available
Damage detection in active-sensing, guided-waves-based Structural Health Monitoring (SHM) has evolved through multiple eras of development during the past decades. Nevertheless, there still exists a number of challenges facing the current state-of-the-art approaches, including low damage sensitivity, lack of robustness to uncertainties, need for us...
Conference Paper
Active-sensing acousto-ultrasound structural health monitoring (SHM) constitutes an important family of methods for both metallic and composite structures. However, the presence of varying operational and environmental conditions in the real world can significantly affect their accuracy and robustness in the face of uncertainty. In this context , s...
Article
In the near future, Structural Health Monitoring (SHM) technologies will be capable of overcoming the drawbacks in the current maintenance and life-cycle management paradigms, namely, cost, increased downtime, less-than-optimal safety management paradigm and the limited applicability of fully-autonomous operations. One of the most challenging tasks...
Preprint
Full-text available
In this work, a probabilistic damage detection and identification scheme using stochastic time series models in the context of acousto-ultrasound guided wave-based SHM is proposed, and its performance is assessed experimentally. In order to simplify the damage detection and identification process, model parameters are modified based on the singular...
Preprint
Full-text available
Modern-day civil, mechanical, and aeronautical structures are transitioning towards a continuous, online, and automated maintenance paradigm in order to ensure increased safety and reliability. The field of structural health monitoring (SHM) is playing a key role in this respect and active sensing acousto-ultrasound guided-wave based SHM techniques...
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-2083.vid A novel machine-learning probabilistic framework for online rotor fault detection, identification, and quantification in multicopters via strain signals is introduced. The framework performs robustly under varying flight states, i.e. forward velocity and gross weight configurations, a...
Article
Full-text available
In the context of active-sensing guided-wave-based acousto-ultrasound structural health monitoring, environmental and operational variability poses a considerable challenge in the damage diagnosis process as they may mask the presence of damage. In this work, the stochastic nature of guided wave propagation due to the small temperature variation, n...
Article
This paper presents the introduction, investigation, and critical assessment of three data-driven methods for rotor failure detection and identification in a multicopter. These methods are based on aircraft attitude signals obtained from forward flight under turbulence and uncertainty. The knowledge-based method exploits the system rigid-body dynam...
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2021-3182.vid In this study, a data-driven approach to identify the flight state of a bio-inspired self-sensing wing is proposed. The flight state is characterized by the angle of attack of the wing, and must be inferred from vibrational signals recorded on the surface of the wing. Various transfor...
Preprint
Full-text available
In the near future, Structural Health Monitoring (SHM) technologies will be capable of overcoming the drawbacks in the current maintenance and life-cycle management paradigms, namely: cost, increased downtime, less-than-optimal safety management paradigm and the limited applicability of fully-autonomous operations. In the context of SHM, one of the...
Article
Full-text available
Damage detection in active-sensing, guided-waves-based structural health monitoring (SHM) has evolved through multiple eras of development during the past decades. Nevertheless, there still exist a number of challenges facing the current state-of-the-art approaches, both in the industry as well as in research and development, including low damage s...
Article
Full-text available
This paper presents the introduction, investigation, and critical assessment of three data-driven statistical residual-based time series methods for rotor fault detection and identification (FDI) in multicopters. A concise overview of statistical residual-based FDI methods is provided based on scalar (univariate) and vector (multivariate) stochasti...
Conference Paper
In this work, a probabilistic Structural Health Monitoring (SHM) framework integrating physics-based and data-based models for active-sensing, guided-wave SHM under varying damage and loading states is proposed. Physics-based models from the family of load-compensation algorithms are used to generate/synthesize baseline signals under varying loads...
Conference Paper
In today’s world, engineering structures are subject to many sources of uncertainty, from varying operational and environmental conditions to complex damage evolution patterns. As such, in the context of active-sensing guided-wave Structural Health Monitoring (SHM), these uncertainties must be initially accounted for, where probabilistic SHM method...
Conference Paper
A robust framework for fault detection and identification of rotor degradation in multicopters while effectively rejecting the effects of gusts is introduced. The rotor fault detection and identification methods employed in this study are based on excitation-response signals of the aircraft under ambient turbulence to distinguish between an aircraf...
Conference Paper
This work introduces the use of "global" stochastic models to detect and identify rotor failures in multicopters under different operating conditions, turbulence, and uncertainty. The identification of an extended class of time-series models known as Vector-dependent Functionally Pooled AutoRegressive models, which are characterized by parameters t...
Conference Paper
This work presents formal progress envelopes applied to flight systems for distinctly classifying a system's state space into regions where a formal proof of progress for a distributed algorithm holds or does not hold. It also presents an approach for runtime integration of formal methods in the dynamic data-driven applications systems (DDDAS) arch...
Article
In this paper, we propose a two–step procedure for the automated detection of micro-earthquakes, using single-station, three-component passive seismic data. The first step consists of the computation of an appropriate characteristic function, along with an energy-based thresholding scheme, in order to attain an initial discrimination of the seismic...
Conference Paper
The complex dynamics of rotorcraft structures under varying operational and environmental conditions demand the development of accurate and robust-to-uncertainties structural health monitoring (SHM) approaches. The inherent uncertainty within monitoring data makes it difficult for conventional methods to accurately and robustly detect and quantify...
Article
This work presents the introduction and experimental investigation of an active-sensing acousto-ultrasound structural health monitoring approach for damage size quantification based on piezoelectric sensors/actuators mounted on multiple seemingly identical structural components. The objective of this work is to determine how reliable the damage dia...
Conference Paper
Full-text available
Methodologies dealing with the detection of micro-earthquakes and the accurate estimation of body waves' arrival time constitute, during the last decades, a topic of ongoing research. The extraction and efficient analysis of the useful information from the continuous recordings is of great importance, since it is a prerequisite for reliable interpr...
Conference Paper
Full-text available
This work developed and evaluated an acousto-ultrasonic system that measures battery state of charge and, in particular, its state of health using a built-in, low-profile piezoelectric transducer system. A diagnostic method was proposed that relates changes in the guided wave signals to the charge, discharge, and aging processes, via electrochemica...
Conference Paper
Full-text available
This paper presents the assessment of a stochastic data-driven global modelling framework for data-driven state awareness under varying flight (airspeed) and structural (damage) states. The proposed framework utilizes stochastic Vector-dependent Func- tionally Pooled (VFP) models that compound data pertaining to different flight states to develop a...
Conference Paper
Full-text available
Dynamic data-driven application systems [1, 2] (DDDAS) allow for unprecedented self-healing and self-diagnostic behavior across a broad swathe of domains. The useful- ness of these systems is offset against their inherent complexity, and therefore fragility to specification or implementation error. Further, DDDAS techniques are often applied in saf...
Conference Paper
Full-text available
Motivated by the supreme flight skills of birds, a new concept called “fly-by-feel” (FBF) has been proposed to develop the next generation of intelligent aircrafts. To achieve this goal, Stanford Structures and Composites Lab (SACL) has developed a smart wing which embeds a multifunctional sensor network on the surface layup of the wing [1]. By lev...
Conference Paper
Full-text available
One of the major challenges the structural health monitoring (SHM) community is facing today is related to enabling accurate and robust damage detection and quantification under varying operating and environmental conditions with a limited number of sensing data sets. In particular, state-of-the-art active-sensing SHM technologies face significant...
Conference Paper
Full-text available
The main objective of this work is the investigation and numerical assessment of random (stochastic) broadband high-frequency actuation for active-sensing structural health monitoring (SHM) via stochastic time-series models, and the comparison with traditional deterministic tone-burst actuation under varying temperature. The main hy- pothesis exami...
Conference Paper
Full-text available
Guided-wave-based acousto-ultrasound structural health monitoring (SHM) methods have attracted the interest of the SHM community as guided waves can travel long distances without significant dissipation and are capable of detecting small damage sizes of several types. However, when subject to changing environmental and operational conditions (EOC),...
Conference Paper
Full-text available
Existing Structural Health Monitoring (SHM) techniques generally depend on deterministic parameters in order to detect, localize, and quantify damage. This limits the applicability of such systems in real-life situations, where stochastic, time-varying structural response, as well as complex damage types immersed in operational/environmental uncert...
Conference Paper
Full-text available
This work introduces the use of statistical time series methods to detect rotor failures in multicopters. A concise overview of the development of various time series models using scalar or vector signals, statistics, and fault detection methods is provided. The fault detection methods employed in this study are based on parametric time series repr...
Article
This work proposes and analyzes a structurally-integrated lithium-ion battery concept. The multifunctional energy storage composite (MESC) structures developed here encapsulate lithium-ion battery materials inside high-strength carbon-fiber composites and use interlocking polymer rivets to stabilize the electrode layer stack mechanically. These riv...
Article
Full-text available
The vibration of a wing structure in the air reflects coupled aerodynamic–mechanical responses under varying flight states that are defined by the angle of attack and airspeed. It is of great challenge to identify the flight state from the complex vibration signals. In this paper, a novel one-dimension convolutional neural network (CNN) is develope...
Article
Full-text available
The 11th International Workshop on Structural Health Monitoring (IWSHM) was held at Stanford University, CA, on 12–14 September 2017. A total of 401 scientific contributions were selected and included in the Workshop Proceedings. This Special Issue of Structural Health Monitoring (SHM) was arranged by selecting high-quality representative papers fr...
Preprint
Full-text available
This work proposes and analyzes a structurally-integrated lithium-ion battery concept. The multifunctional energy storage composite (MESC) structures developed here encapsulate lithium-ion battery materials inside high-strength carbon-fiber composites and use interlocking polymer rivets to stabilize the electrode layer stack mechanically. These riv...
Conference Paper
Full-text available
This paper presents a novel guided-wave-based framework for estimating state of charge (SoC) and state of health (SoH) of lithium-ion (Li-ion) batteries. The guided waves are propagated and sensed using low-profile, surface-mounted piezoelectric transducers on off-the-shelf Li-ion pouch cells. Special emphasis is given to the development of an effi...
Article
This work presents the feasibility of monitoring state of charge (SoC) and state of health (SoH) of lithium-ion pouch batteries with acousto-ultrasonic guided waves. The guided waves are propagated and sensed using low-profile, built-in piezoelectric disc transducers that can be retrofitted onto off-the-shelf batteries. Both experimental and analyt...
Article
Full-text available
In this work, a data-driven approach for identifying the flight state of a self-sensing wing structure with an embedded multi-functional sensing network is proposed. The flight state is characterized by the structural vibration signals recorded from a series of wind tunnel experiments under varying angles of attack and airspeeds. A large feature po...
Conference Paper
Full-text available
In this work, an experimental study of a novel data-driven fly-by-feel state awareness method is presented. A non-parametric probabilistic approach for stall detection is investigated and assessed via a series of wind tunnel experiments. The method is based on the statistical analysis of the recorded signals and subsequent statistical hypothesis te...
Article
Full-text available
Monitoring the bondline integrity of adhesively bonded joints is one of the most critical concerns in the design of aircraft structures to date. Due to the lack of confidence on the integrity of the bondline both during fabrication and service, the industry standards and regulations require assembling the primary airframe structure using the ineffi...
Conference Paper
Full-text available
With the development of micro-fabrication techniques, multi-functional sensor networks have been created with many micro-sensor nodes for various functions. An UAV wing is integrated with a stretchable sensor network including piezoelectric transducer, strain gauge and resistance temperature detector embedded near the wing surface and this novel de...
Conference Paper
Full-text available
With the development of micro-fabrication techniques, multi-functional sensor networks have been created with many micro-sensor nodes for various functions. An UAV wing is integrated with a stretchable sensor network including piezoelectric transducer, strain gauge and resistance temperature detector embedded near the wing surface and this novel de...
Conference Paper
Full-text available
This work presents a novel scalable and field deployable framework for estimating battery state of charge (SoC) based on pitch-catch guided wave propagation using low-profile built-in piezoelectric transducers. A preliminary study is performed through experiments using piezoelectric disc transducers mounted on commercial lithium-ion (Li-ion) pouch...
Conference Paper
Full-text available
Recent work on multifunctional materials has demonstrated that high-strength composites could be integrated with active Li-ion battery material to create high strength and high energy density storage structures that could meet the transportation requirements on mobility and energy storage density. However, the performance and the design of the mult...
Conference Paper
Full-text available
Assessing the reliability of damage quantification is a critical and necessary process for the evaluation of Nondestructive Evaluation (NDE) or Structural Health Monitoring (SHM) techniques. When it comes to NDE techniques, appropriate processes have been matured and established for asserting the reliability of damage quantification. However, such...
Conference Paper
Assessing the reliability of damage quantification is a critical and necessary process for the evaluation of Nondestructive Evaluation (NDE) or Structural Health Monitoring (SHM) techniques. When it comes to NDE techniques, appropriate processes have been matured and established for asserting the reliability of damage quantification. However, such...
Chapter
The monitoring and subsequent management of structural health plays an essential role in composite materials and structures. Structural health monitoring (SHM) is an emerging technology which combines advanced sensor technologies with intelligent algorithms to interrogate the “health” condition of structures in real time or whenever necessary. Bene...
Conference Paper
Full-text available
This work presents a novel scalable and field-deployable framework for monitoring lithium-ion (Li-ion) battery state of charge (SoC) and state of health (SoH), based on ultrasonic guided waves using low-profile built-in piezoelectric transducers. The feasibility of this technique is demonstrated through experiments using surface-mounted piezoelectr...
Article
Piezoelectric transducers have applications from ultrasonic structural health monitoring to micro-electromechanical systems. Small physical size coupled with large actuation is desirable in many applications, requiring unique transducer designs to take advantage of the material properties. Screen-printed piezoceramics were developed as a means of m...
Article
Full-text available
This article describes how piezoelectric transducers embedded in the adhesive bond-line of lap-joints can be used to effectively monitor structural integrity. Various lap-joint coupons with embedded piezoelectric transducers were manufactured with and without artificial contamination at the bond-line and tested statically and cyclically. A novel sc...
Conference Paper
Full-text available
This work presents the development of the first-generation Multifunctional Energy Storage (MES) Composites-a multifunctional structural battery which embeds li-ion battery materials into high-strength composites together with in-situ networks of sensors and actuators. MES Composites not only can supply electrical power but also serve as structural...
Conference Paper
Full-text available
We propose the concept of Multifunctional-Energy-Storage Composites (MES Composites) which highlights a unique integration technique for embedding lithium-ion battery materials in structural carbon-fiber-reinforced-polymers (CFRP). Unlike standard lithium-ion pouch cells, the MES Composites maximizes material utilization by using CFRP facesheets to...