Xu Chen

Xu Chen
University of Washington Seattle | UW · Department of Mechanical Engineering

Professor

About

94
Publications
27,614
Reads
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658
Citations
Introduction
Xu Chen works at the Department of Mechanical Engineering, University of Washington. Dr. Chen does research in dynamic systems and controls, information fusion, additive and advanced manufacturing, robotics, autonomy, and precision mechatronics.
Additional affiliations
August 2014 - present
University of Connecticut
Position
  • Professor (Assistant)
July 2013 - May 2014
University of California, Berkeley
Position
  • Lecturer
June 2013 - July 2013
Western Digital Corporation
Position
  • Senior Research intern
Education
August 2008 - August 2013
University of California, Berkeley
Field of study
  • Mechanical Engineering
August 2004 - May 2008
Tsinghua University
Field of study
  • Mechanical Engineering

Publications

Publications (94)
Article
Full-text available
In repetitive control (RC), the enhanced servo performance at the fundamental frequency and its higher order harmonics is usually followed by undesired error amplifications at other frequencies. In this paper, we discuss a new structural configuration of the internal model in RC, wherein designers have more flexibility in the repetitive loop-shapin...
Conference Paper
Full-text available
A loop-shaping idea for dual-input-single-output (DISO) systems is presented in this paper. We explore special forms of Youla parameterization for obtaining stabilizing controllers for DISO systems, and discuss concepts to achieve reduced-complexity formulations with clearer design intuitions. The algorithm is verified by a benchmark control proble...
Conference Paper
Full-text available
We have many servo systems that require nano/micro level positioning accuracy. This requirement sets a number of interesting challenges from the viewpoint of sensing, actuation, and control algorithms. This paper considers the control aspect for precision positioning. In motion control systems at nano/micro levels, it has been noted that the contro...
Article
Full-text available
This paper presents an adaptive control scheme for identifying and rejecting unknown and/or time-varying narrow-band vibrations. We discuss an idea of safely and adaptively inverting a (possibly non-minimum phase) plant dynamics at selected frequency regions, so that structured disturbances (especially vibrations) can be estimated and canceled from...
Article
Full-text available
Many servo systems are subjected to narrow-band disturbances that generate vibrations at multiple frequencies. One example is the track-following control in a hard disk drive (HDD) system, where the airflow-excited disk and actuator vibrations introduce strong and uncertain spectral peaks to the position error signal. Such narrow-band vibrations di...
Conference Paper
Full-text available
During composites manufacturing, release coatings are applied on production tools to minimize tool-part friction and adhesive bonding. In addition to facilitating the removal of cured parts, applying release coatings reduces process-induced deformations (PIDs). The aerospace industry typically uses semi-permanent release coatings that undergo physi...
Article
Full-text available
Selective laser sintering (SLS) generates complex high-performance parts from micrometer-diameter powders. In polyamide 12 SLS, a considerable amount of expensive polyamide 12 materials remains un-joined in the additive manufacturing (AM) process. Such materials, particularly the ones near the heat-affected zones (HAZ), go through irreversible chem...
Article
Full-text available
Since its addition to additive manufacturing (AM), polyamide 12 has dominated the selective laser sintering (SLS) market, thanks to its stable thermal property and high mechanical quality. However, substantial un-sintered residue powders lead to economic losses and are burdensome to the environment. Though several works have reported the aging mech...
Preprint
Full-text available
Despite the advantages and emerging applications, broader adoption of powder bed fusion (PBF) additive manufacturing is challenged by insufficient reliability and in-process variations. Finite element modeling and control-oriented modeling have been shown to be effective for predicting and engineering part qualities in PBF. This paper first builds...
Article
Full-text available
Despite the advantages and emerging applications, broader adoption of powder bed fusion (PBF) additive manufacturing is challenged by insufficient reliability and in-process variations. Finite element modeling and control-oriented modeling have been identified fundamental for predicting and engineering part qualities in PBF. This paper first builds...
Article
Poly(ether ether ketone) (PEEK) is increasingly used in selective laser sintering (SLS) due to its superior mechanical and thermal properties and biocompatibility. It is well known that the properties of selective laser sintering (SLS) powder govern the processability and quality of the parts. The SLS printing process exposes the PEEK bed/feedstock...
Article
Full-text available
Following rapidly and precisely a moving target has become the core functionality in robotic systems for transportation, manufacturing, and medical devices. Among existing targeting following methods, vision-based tracking continues to thrive as one of the most popular, and is the closest method to human perception. However, the low sampling rate a...
Article
Full-text available
Polyamide 12 is one of the most extensively used semi-crystalline polymer materials to date in selective laser sintering (SLS) additive manufacturing, or SLS 3D printing. In this powder-based direct digital manufacturing process, a substantial amount of expensive materials remains un-sintered, recyclable, and reusable. Recently, understanding the m...
Preprint
Full-text available
Despite the advantages and emerging applications, broader adoption of powder bed fusion (PBF) additive manufacturing is challenged by insufficient reliability and in-process variations. Finite element modeling and control-oriented modeling have been identified fundamental for predicting and engineering part qualities in PBF. This paper first builds...
Article
Full-text available
A high-precision additive manufacturing process, powder bed fusion (PBF) has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While finite element modeling and closed-loop control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm...
Article
Full-text available
Capable of building high-quality, complex parts directly from digital models, selective laser sintering (SLS) additive manufacture (AM) is a core method of agile manufacturing. Polyamide 12 is the most commonly and successfully used polymer powders to date in SLS due to the conforming thermal behaviors of this thermoplastic polymer. State of the ar...
Preprint
Full-text available
A high-precision additive manufacturing process, powder bed fusion (PBF) has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While finite element modeling and closed-loop control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm...
Conference Paper
Full-text available
Given the rapid transformation of the aerospace sector in the last decade, teaching practices should prepare engineers to face fast-paced industries that are dealing with exceedingly complex problems. Now more than ever we need engineers who are capable of working and communicating effectively within large and multidisciplinary groups, considering...
Conference Paper
Full-text available
Selective laser sintering (SLS) technology produces a substantial amount of un-sintered polyamide 12 powders after the manufacturing process. Failure to recycle and reuse these aged powders not only leads to economic losses but also is environmentally unfriendly. This is particularly problematic for powder particles close to the heat-affected zones...
Conference Paper
Full-text available
Powder bed fusion (PBF) additive manufacturing has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While high-fidelity finite element modeling and feedback control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm are developed...
Preprint
Full-text available
Stably inverting a dynamic system model is fundamental to subsequent servo designs. Current inversion techniques have provided effective model matching for feedforward controls. However, when the inverse models are to be implemented in feedback systems, additional considerations are demanded for assuring causality, robustness, and stability under c...
Article
Full-text available
Although laser-based additive manufacturing (AM) has enabled unprecedented fabrication of complex parts directly from digital models, broader adoption of the technology remains challenged by insufficient reliability and in-process variations. In pursuit of assuring quality in the selective laser sintering (SLS) AM, this paper builds a modeling and...
Preprint
Although laser-based additive manufacturing (AM) has enabled unprecedented fabrication of complex parts directly from digital models, broader adoption of the technology remains challenged by insufficient reliability and in-process variations. In pursuit of assuring quality in the selective laser sintering (SLS) AM, this paper builds a modeling and...
Conference Paper
Full-text available
Although visual feedback has enabled a wide range of robotic capabilities such as autonomous navigation and robotic surgery, low sampling rate and time delays of visual outputs continue to hinder real-time applications. When partial knowledge of the target dynamics is available, however, we show the potential of significant performance gain in visi...
Article
Full-text available
This paper considers the real-time recovery of a fast time series by using sparsely sampled measurements from sensors whose sampling speeds are prohibitively slow originally. Specifically, when the fast signal is an autoregressive process, we propose an online information recovery algorithm that reconstructs the dense underlying temporal dynamics f...
Article
Full-text available
Closed-loop disturbance rejection without sacrificing overall system performance is a fundamental issue in a wide range of applications from precision motion control, active noise cancellation, to advanced manufacturing. The core of rejecting band-limited disturbances is the shaping of feedback loops to actively and flexibly respond to different di...
Article
Full-text available
The demand on sustainable operations of large-scale wind turbines necessitates the concurrent advancement of power regulation and load mitigation through blade pitch control. Traditional collective pitch control (CPC) mechanisms can only deal with symmetric disturbances. The advent of individual pitch control (IPC) provides new opportunities to mit...
Preprint
Stably inverting a dynamic system model is fundamental to subsequent servo designs. Current inversion techniques have provided effective model matching for feedforward controls. However, when the inverse models are to be implemented in feedback systems, additional considerations are demanded for assuring causality, robustness, and stability under c...
Conference Paper
Full-text available
Modulating the closed-loop transmission of energy in a wide frequency band without sacrificing overall system performance is a fundamental issue in a wide range of applications from precision control, active noise cancellation, to energy guiding. This paper introduces a loop-shaping approach to create such wideband closed-loop behaviors, with a par...
Conference Paper
This paper considers the real-time recovery of a fast time series (e.g., updated every T seconds) by using sparsely sampled measurements from two sensors whose sampling intervals are much larger than T (e.g., MT and NT, where M and N are integers). Specifically, when the fast signal is an autoregressive process, we propose an online information rec...
Article
Full-text available
A fundamental problem arises in feedback control when the system is subject to fast disturbances but can only get slowly updated sensor feedback. The problem is particularly challenging when the disturbances have frequency components near or beyond the sensor's Nyquist sampling frequency. Such difficulties occur to selective laser sintering, an add...
Conference Paper
Full-text available
This paper studies control approaches to advance the quality of repetitive energy deposition in powder bed fusion (PBF) additive manufacturing. A key pattern in the nascent manufacturing process, the repetitive scanning of the laser or electron beam can be fundamentally improved by repetitive control (RC) algorithms. An intrinsic limitation, howeve...
Preprint
Full-text available
This paper considers the real-time recovery of a fast time series (e.g., updated every T seconds) by using sparsely sampled measurements from two sensors whose sampling intervals are much larger than T (e.g., MT and NT, where M and N are integers). Specifically, when the fast signal is an autoregressive process, we propose an online information rec...
Conference Paper
Full-text available
This paper studies repetitive control (RC) algorithms to advance the quality of repetitive energy deposition in laser-based additive manufacturing (AM). An intrinsic limitation appears in discrete-time RC when the period of the exogenous signal is not an integer multiple of the sampling time. Such a challenge hampers high-performance applications o...
Article
A discrete-time backstepping control algorithm is proposed for reference tracking of systems affected by both broadband disturbances at low frequencies and narrow band disturbances at high frequencies. A discrete time DOB, which is constructed based on infinite impulse response filters is applied to compensate for narrow band disturbances at high f...
Conference Paper
Full-text available
A robust individual pitch control strategy is presented to deal with periodic load disturbances on wind turbines under operating point variation. The asymmetric loads are mainly caused by the tower shadow and wind shear effects. Multi-blade coordinate (MBC) transformation is utilized to model the turbine dynamics under various operating points. The...
Article
Full-text available
This paper discusses fractional-order repetitive control (RC) to advance the quality of periodic energy deposition in laser-based additive manufacturing (AM). It addresses an intrinsic RC limitation when the exogenous signal frequency cannot divide the sampling frequency of the sensor, e.g., in imaging-based control of fast laser-material interacti...
Article
Full-text available
A fundamental challenge in sampled-data control arises when a continuous-time plant is subject to disturbances that possess significant frequency components beyond the Nyquist frequency of the feedback sensor. Such intrinsic difficulties create formidable barriers for fast high-performance controls in modern and emerging technologies such as additi...
Article
Full-text available
Many servo systems require micro/nano-level positioning accuracy. This requirement sets a number of challenges from the viewpoint of sensing, actuation, and control algorithms. This article considers control algorithms for precision positioning. We examine how prior knowledge about the parameterization of control structure and the disturbance spect...
Conference Paper
Full-text available
Selective laser sintering (SLS) is an additive manufacturing (AM) process that builds 3-dimensional (3D) parts by scanning a laser beam over powder materials in a layerwise fashion. Due to its capability of processing a broad range of materials, the rapidly developing SLS has attracted wide research attention. The increasing demands on part quality...
Conference Paper
Nowadays, despite the emerging adoption of the solid state drives, hard disk drives (HDDs) are still used extensively as cost-effective and reliable solutions for data storage. In addition to its usual application in desktops and laptops, HDDs become the primary storage medium for the data centers. The track following control task in HDD requires t...
Conference Paper
Full-text available
In many laser processing applications, galvanometer scanners are integrated into a larger system that creates external disturbances to the actual laser-material interaction. To reject such disturbance with feedback-based control schemes, the sampling of the output needs to be fast enough to capture all major frequency components of the disturbance....
Article
Full-text available
Plug-in or add-on control is integral for high-performance control in modern precision systems. Despite the capability of greatly enhancing the steady-state performance, add-on compensation can introduce output discontinuity and significant transient response. Motivated by the vast application and the practical importance of add-on control designs,...
Article
Full-text available
This paper provides an optimization-based approach to assure the strict positive real (SPR) condition in a set of recursive parameter adaptation algorithms (PAA). The developed algorithms and tools enable a multiobjective formulation of the SPR problem, creating new controls of the stability and parameter convergence in PAAs. In addition to assurin...
Article
Full-text available
Time-varying unknown wind disturbances influence significantly the dynamics of wind turbines. In this research, we formulate a disturbance observer (DOB) structure that is added to a proportional-integral-derivative (PID) feedback controller, aiming at asymptotically rejecting disturbances to wind turbines at above-rated wind speeds. Specifically,...
Article
Full-text available
Modern hard disk drive (HDD) systems are subjected to various external disturbances. One particular category, defined as wide-band disturbances, can generate vibrations with their energy highly concentrated at several frequency bands. Such vibrations are commonly time-varying and strongly environment/product-dependent; and the wide spectral peaks c...
Conference Paper
Full-text available
Disturbance observer based (DOB) control has been implemented in motion control to reject unknown or time-varying disturbances. In this research, an internal model-based disturbance observer (DOB) design combined with a PID type feedback controller is formulated for wind turbine speed and power regulation. The DOB controller facilitates model-based...
Article
Full-text available
A fundamental challenge in digital control arises when the controlled plant is subjected to a fast disturbance dynamics but is only equipped with a relatively slow sensor. Such intrinsic difficulties are, however, commonly encountered in many novel applications such as laser- and electron-beam-based additive manufacturing, human–machine interaction...
Conference Paper
Full-text available
A fundamental challenge in digital and sampled-data control arises when the continuous-time plant is subject to fast disturbances that possess significant frequency components beyond Nyquist frequency. Such intrinsic difficulties are more and more encountered in modern manufacturing applications, where the measurement speed of the sensor is physica...
Conference Paper
Full-text available
A fundamental challenge in digital and sampled-data control arises when the controlled plant is subjected to fast disturbances with, however, a slow or limited sensor measurement. Such intrinsic difficulties are commonly encountered in many novel applications such as advanced and additive manufacturing, human-machine interaction, etc. This paper in...
Conference Paper
Full-text available
An adaptive regulation scheme using the Internal Model Principle and an IIR Youla-Kucera controller parametrization is proposed for the suppression of multiple unknown and time varying narrow band disturbances. This development has been motivated on one hand by the need of simplifying the design of the central controller for direct adaptive regulat...
Conference Paper
In this research, a disturbance observer based (DOB) control scheme is illustrated to reject the unknown low frequency disturbances to wind turbines. Specifically, we aim at maintaining the constant output power but achieving better generator speed regulation when the wind turbine is operated at time-varying and turbulent wind field. The disturbanc...
Article
A direct adaptive regulation scheme using a FIR Youla-Kučera Filter has been proposed for solving the EJC Benchmark [4] for rejection of multiple unknown and time-varying narrow-band disturbances. Despite the excellent results this approach requires a careful design of the central controller in terms of selection of some of the assigned closed-loop...
Article
Full-text available
This paper discusses a discrete-time loop shaping algorithm for servo enhancement at multiple wide frequency bands. Such design considerations are motivated by a large class of practical control problems such as vibration rejection, active noise control, and periodical reference tracking; as well as recent novel challenges that demand new design in...
Conference Paper
Full-text available
We present a discrete-time output feedback nonlinear control algorithm for reference tracking under both broad-band disturbances at low frequencies and narrow-band disturbances at high frequencies. A discrete-time nonlinear damping backstepping controller with an extended state observer is proposed to track the desired output and to compensate for...
Conference Paper
Full-text available
In this paper, a networked-based rehabilitation system is introduced for lower-extremity tele-rehabilitation. In order to enable high-level motion planning of the rehabilitation robot in real-time for enhanced safety and appropriate human-robot interactions, a time series model is proposed to capture the kinematics of knee joint rotations. A major...
Article
Full-text available
A controller design is presented for directly shaping the sensitivity function in the feedback control of dual-input single-output (DISO) systems. We provide special forms of discrete-time Youla-Kucera (YK) parameterization to obtain stabilizing controllers for DISO systems, and discuss concepts to achieve a reduced-complexity formulation. The prop...