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  • 1.
    Christiansson, Anna-Karin
    University West, Department of Technology.
    A unified framework for mixed continuous/discrete-time H_infinity -control2003Doctoral thesis, monograph (Other academic)
  • 2.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Engineering Science, Division of Production Engineering.
    Danielsson, Fredrik
    University West, Department of Engineering Science, Division of Production Engineering.
    Heralic, Almir
    University West, Department of Engineering Science, Division of Production Engineering.
    Ottosson, Mattias
    University West, Department of Engineering Science, Division of Production Engineering.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Production Engineering.
    Automation of a robotised metal deposition system using laser melting of wire2008In: 18th International Conference on Flexible Automation and Intelligent Manufacturing (FAIM 2008): Skövde, 30 June-2 July, 2008, p. 122-129Conference paper (Other academic)
    Abstract [en]

    This paper presents a system for full automation of free-form-fabrication of fully dense metal structures using robotized laser melting of wire. The structure is built of beads of melted wire laid side by side and layer upon layer governed by synchronized robot motion. By full automation is here meant that the process starts with a product specification of a component, and ends in a geometrically validated dense metal component fulfilling industrial material requirements. Due to the complexity of this flexible manufacturing system, a number of different disciplines are involved. This paper discusses mainly the system design, which includes how off-line programming is used for automatic generation of code and how feedback control is used for on-line adjustment of parameters based on desired building properties. To meet industrial needs, the project is carried out in a close cooperation between research and development activities in academy and industry.

  • 3.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, B.
    Chalmers University of Technology, Control Engineering Lab, Göteborg, Sweden .
    Toivonen, H.
    Åbo Akademi University, Department of Chemical Engineering, Åbo, Finland.
    Mixed continuous/discrete-time output feedback H∞ control: A unified approach1999In: European Control Conference, ECC 1999 - Conference Proceedings, 1999, p. 4077-4082Conference paper (Refereed)
    Abstract [en]

    A Riccati equation (RE) based solution to the H∞ optimal control problem for mixed continuous/discrete-time systems is presented. The results unify a number of recently penetrated H∞ control problems. In the infinite-horizon case a periodic behaviour is assumed, and it is shown how the related continuous RE with jumps can be replaced by an equivalent discrete periodic RE. A related algebraic RE, which involves the system behaviour during one period, then can be formulated and solved by standard methods. Typical applications are control of continuous-time or discrete-time periodic systems, as well as multirate and sampled-data control, including mixed continuous and sampled-data measurements. © 1999 EUCA.

  • 4.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers University of Technology, Control Engineering Lab, Göteborg, Sweden.
    Low order sampled-data H∞ control2003In: Decision and Control, 2003. Proceedings. 42nd IEEE Conference on, 2003, Vol. 3, p. 2308-2313 Vol.3Conference paper (Refereed)
    Abstract [en]

    A method for obtaining low order sampled-data H∞ controllers is presented. The method is mainly based on a parametric static feedback controller for a plant that is augmented with the controller dynamics. The design of a full-order controller is a convex problem, while the optimisation problem for lower order controllers is non convex. The proposed method starts with design of a full-order sampled-data controller using Riccati equations. Then this controller is reduced by an ordinary model reduction technique, and the reduced controller is used as an initial value for an iterative procedure using linear matrix inequalities (LMIs) in the search for an optimal controller. The matrix inequalities are in fact linear in either the Lyapunov matrix or the static controller matrix, why the solution to the non convex problem fundamentally is given by a bilinear matrix inequality (BMI). The order of the controller is reduced until the closed loop performance degrades too much. Simulations are shown for the control of a time delayed SISO-plant where the controller order can be reduced from 8th to 3rd order. Results are also shown from control of a MIMO-model of a jet engine where the reduction is successful from 15th to 4th order. It is argued that the non convexity is handled efficiently since the procedure uses a model reduction of the full-order controller as initial value.

  • 5.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers University of Technology, Control Engineering Lab, SE-41 2 96 Goteborg, Sweden .
    Toivonen, H.
    Åbo Akademi University, Department of Chemical Engineering, FIN-20500 Abo, Finland .
    Continuous-, discrete- and sampled-data- H∞ˆž control - a unified framework2000In: IEEE Proceedings: Conference on Decision and Control (CDC), ISSN 0191-2216, Vol. 2, p. 1936-1941Article in journal (Refereed)
    Abstract [en]

    We present a unified and general framework for H∞-control in both continuous time, discrete time and combinations of these. The general result is a hybrid continuous-/discrete-time H∞ˆž-controller. Using a compact hybrid notation, the work shows a close relationship between the continuous-and discrete-time solutions. In fact, the pure continuous and discrete time equations may be obtained as two similar interpretations of the general result. There are no assumptions made on certain system matrices being zero or normalised, e.g. D11 = 0. The method is Riccati equation (RE) based, and it is shown how the continuous REs can be "lifted" into discrete ones reflecting the system behaviour during the period. Typical applications are control of continuous-time or discrete-time periodic systems, as well as multirate and sampled-data control, including mixed continuous and sampled-data measurements.

  • 6.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers University of Technology, Control Engineering Lab, Göteborg, Sweden.
    Toivonen, H.T.
    Åbo Akademi University, Department of Chemical Engineering, Åbo, Finland..
    Sampled-data H∞ˆž-control for time-varying hybrid systems2001In: Dynamics of continuous, discrete and impulsive systems, ISSN 1201-3390, E-ISSN 1918-2538, Dynamics of Continuous, Discrete and Impulsive Systems Series B: Applications and Algorithms, ISSN 1492-8760, Vol. 8, no 4b, p. 427-444Article in journal (Refereed)
    Abstract [en]

    This paper presents sampled-data H∞ˆž-control of linear mixed continuous-time and discrete-time systems, including a mix of continuous-time and discrete-time performance signals and disturbances. However, neither continuous-time control signals nor continuous-time measurements are included. The sampling may well be multirate, i.e. different signals may be sampled by different rates, as long as a periodic pattern is repeated over the period, consisting of a number of sampling steps. In fact, the results are applicable even to arbitrary linear time-varying systems. The sampled-data system is discretised such that the continuous-time performance is reflected in the discretised or "lifted" system. This lifted system is combined with discrete-time updates at the sampling instants to achieve the sampled-data controller. The "hold-states" case is presented as a special application, when there is also a hold circuit at the controller output. The continuous-time Riccati equations with discrete-time updates then can be replaced by one discrete-time static feedback and filter Riccati equation respectively, which also reflect the intersample behaviour. Furthermore, convergence results between the discretised Riccati solutions and the corresponding continuous-time solutions are shown, when the sampling interval decreases towards zero. Simulations are shown when the results are applied to the control of a MIMO jet-engine model.

  • 7.
    Christiansson, Anna-Karin
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Ottosson, Mattias
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Sensor and control system for metal deposition using robotised laser welding.2006In: 16th International Conference on Computer Technology in Welding and Manufacturing: Kiev, Ukraine, 2006, p. 328-332Conference paper (Refereed)
  • 8.
    De Backer, Jeroen
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Bolmsjö, Gunnar
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Temperature control of robotic friction stir welding using the thermoelectric effect2014In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 70, no 1-4, p. 375-383Article in journal (Refereed)
    Abstract [en]

    Friction stir welding (FSW) of non-linear joints receives an increasing interest from several industrial sectors like automotive, urban transport and aerospace. A force-controlled robot is particularly suitable for welding complex geometries in lightweight alloys. However, complex geometries including three-dimensional joints, non-constant thicknesses and heat sinks such as clamps cause varying heat dissipation in the welded product. This will lead to changes in the process temperature and hence an unstable FSW process with varying mechanical properties. Furthermore, overheating can lead to a meltdown, causing the tool to sink down into the workpiece. This paper describes a temperature controller that modifies the spindle speed to maintain a constant welding temperature. A newly developed temperature measurement method is used which is able to measure the average tool temperature without the need for thermocouples inside the tool. The method is used to control both the plunging and welding operation. The developments presented here are applied to a robotic FSW system and can be directly implemented in a production setting.

  • 9.
    De Backer, Jeroen
    et al.
    University West, Department of Engineering Science, Division of Automation Systems.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation Systems.
    Surface Quality and Strength in Robotic Friction Stir Welding of Thin Automotive Aluminium Alloys2011In: The 4th International Swedish Production Symposium / [ed] Jan-Eric Ståhl, The Swedish Production Academy , 2011, p. 554-562Conference paper (Refereed)
    Abstract [en]

    Friction Stir Welding (FSW) is a novel method for joining materials without using consumablesand without melting the materials. It uses a rotating tool that creates frictionalheat and mixes the materials mechanically together. Robotic application of FSW allowsthree-dimensional welding of light-weight metals in e.g. the automotive industry. TheStiRoLight project is driven by Saab Automobile AB and performed at University Westfor investigation of robotic FSW of three-dimensional welding seams. It aims to introduceFSW in the automotive production line. This paper describes the effect of penetrationdepth of the FSW tool during force controlled robotic welding of thin (< 2 mm) aluminium inoverlap configuration. The influence of pin length on strength of welded aluminium sheetsis investigated using tensile and peel tests. The main limiting factor for penetration depthis the surface quality on the backside of the weld, which often is important in automotiveapplications. Further, the roughness of the plates on the backside is measured and relatedto pin length and backing bar properties. This paper shows a relation between penetrationdepth and tensile strength, and suggests an optimal pin length to guarantee a good weldquality while maintaining an acceptable surface quality. The influence of sheet thicknesstolerance is also discussed. Knowledge is fed back to designers and manufacturingengineers to facilitate for use in production with guaranteed product quality.

  • 10.
    De Backer, Jeroen
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Process and Product Development.
    Oqueka, Jens
    University West, Department of Engineering Science.
    Bolmsjö, Gunnar
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Investigation of path compensation methods for robotic friction stir welding2012In: Industrial robot, ISSN 0143-991X, E-ISSN 1758-5791, Vol. 39, no 6, p. 601-608Article in journal (Refereed)
    Abstract [en]

    Purpose – Friction stir welding (FSW) is a novel method for joining materials without using consumables and without melting the materials. The purpose of this paper is to present the state of the art in robotic FSW and outline important steps for its implementation in industry and specifically the automotive industry.

    Design/methodology/approach – This study focuses on the robot deflections during FSW, by relating process forces to the deviations from the programmed robot path and to the strength of the obtained joint. A robot adapted for the FSW process has been used in the experimental study. Two sensor-based methods are implemented to determine path deviations during test runs and the resulting welds were examined with respect to tensile strength and path deviation.

    Findings – It can be concluded that deflections must be compensated for in high strengths alloys. Several strategies can be applied including online sensing or compensation of the deflection in the robot program. The welding process was proven to be insensitive for small deviations and the presented path compensation methods are sufficient to obtain a strong and defect-free welding joint.

    Originality/value – This paper demonstrates the effect of FSW process forces on the robot, which is not found in literature. This is expected to contribute to the use of robots for FSW. The experiments were performed in a demonstrator facility which clearly showed the possibility of applying robotic FSW as a flexible industrial manufacturing process.

  • 11.
    De Backer, Jeroen
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Soron, Mikael
    ESAB Welding AB .
    Ilar, Torbjörn
    University West, Department of Engineering Science, Division of Production Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Process and Product Development.
    Friction stir welding with robot for light vehicle design2010In: Proceedings from the 8th International Friction Stir Welding Symposium: Timmendorfer Strand, Germany 18-20 May 2010, The Welding Institute , 2010Conference paper (Other academic)
    Abstract [en]

    Reducing weight is one of the enablers to design more environmentally friendly vehicles. Friction Stir Welding (FSW) supports low weight design through its capability to join different combinations of light weight materials, e.g. different aluminium alloys, but also through its possibilities in producing continuous joints. StiRoLight is a recently started project for robotised FSW for joining of light weight materials emphasising on the vehicle industry, an industry with a long-time experience of robotic welding. The first task involves investigation of force feedback for maintaining the desired contact force. Another important aspect in robotised FSW is the compliance of the robot, which may result in deviations from the pre-programmed path as a result of the high process forces experienced during the welding operation. The further exploration of three-dimensional FSW seams and definition of the process windows will be part of further research within this project.

  • 12.
    Elefante, Arianna
    et al.
    University of Bari, Physics Department, Via Amendola 173, 70126 Bari, Italy.
    Nilsen, Morgan
    University West, Department of Engineering Science, Division of Production System.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Production System.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Production System.
    Maggipinto, Tommaso
    University of Bari, Physics Department, Via Amendola 173, 70126 Bari, Italy.
    Ancona, Antonio
    University West, Department of Engineering Science, Division of Production System. IFN-CNR Institute for Photonics and Nanotechnologies, Physics Department, Via Amendola 173, 70126 Bari, Italy.
    Detecting beam offsets in laser welding of closed-square-butt joints by wavelet analysis of an optical process signal2019In: Optics & Laser Technology, ISSN 0030-3992, Vol. 109, p. 178-185Article in journal (Refereed)
    Abstract [en]

    Robotized laser beam welding of closed-square-butt joints is sensitive to the positioning of the laser beam with respect to the joint since even a small offset may result in a detrimental lack of sidewall fusion. An evaluation of a system using a photodiode aligned coaxial to the processing laser beam confirms the ability to detect variations of the process conditions, such as when there is an evolution of an offset between the laser beam and the joint. Welding with different robot trajectories and with the processing laser operating in both continuous and pulsed mode provided data for this evaluation. The detection method uses wavelet analysis of the photodetector signal that carries information of the process condition revealed by the plasma plume optical emissions during welding. This experimental data have been evaluated offline. The results show the potential of this detection method that is clearly beneficial for the development of a system for welding joint tracking.

  • 13.
    Ericsson, Mikael
    et al.
    University West, Department of Engineering Science, Division of Automation Systems.
    Ryberg, Anders
    Nilsson, Jim
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers.
    Off-Line Simulation of Advanced Stereo Vision Welding Applications2010In: Machine Vision and Applications, ISSN 0932-8092, E-ISSN 1432-1769Article in journal (Refereed)
  • 14.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Automatic detection of material phase transitions from spectroscopic data2013In: Proceedings of the IECON 2013: 39th Annual Conference of the IEEE Industrial Electronics Society, IEEE, 2013, p. 2384-2389Conference paper (Refereed)
    Abstract [en]

    When using a temperaturemeasurementmethod which utilizes spectral information for measuring the temperature of varying emissivity measurands, there is a need for a temperature reference at some point in time. In this work, such a reference is created from the spectral radiance data already used by the temperature measurement method. A method of using knowledge of the measurand material's phase transitions and spectral radiance data as a temperature reference is presented. Through automatical identification of phase transitions from radiance spectra employing signal processing, the temperature is known at a certain instance in time, just like required by the temperature measurement method. Three methods for automatic identification of material phase transitions from spectroscopic data are examined and evaluated. The methods are, based on derivatives, steady-state identification and cross correlation respectively. They are introduced and evaluated using experimental data collected from a solidifying copper sample. All methods proved to identify the phase transitions correctly. The addition of automatic phase transition identification supplements the existing temperature measurement method such that it becomes a stand alone, reference free method for measuring the true absolute temperature of a measurand with varying emissivity.

  • 15.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation Systems.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation Systems.
    Heralic, Almir
    GKN Aerospace.
    Automation of a laser welding system for additive manufacturing2015In: Proceedings of the 2015 IEEE International Conference on Automation Science and Engineering / [ed] Kazuhiro Saitou, Univ. of Michigan, IEEE, 2015, p. 900-905Conference paper (Refereed)
    Abstract [en]

    This paper presents the benefits and challenges ofusing a standard robotised laser welding cell for additive manufacturing(AM). Additive manufacturing, sometimes denoted3D-printing or rapid prototyping, has lately met strong interestin several areas of society, and a variety of technologies andmaterials have been in focus. The current paper summarisesautomation efforts for AM of advanced aero engine componentsusing high power laser with welding optics as power source formelting metal wire and using an industrial robot for obtaininga 3-dimensional feature shape. The challenges are related to theprocess itself encountering high and repeated temperatures withmelting and solidification of the metal as the main players. Themajor research solutions discussed in this paper are relatedto automation issues for obtaining a stable process and tohave control of the temperatures and temperature changes thatthe metals encounter during the process. The solutions aresuccessfully implemented in an industrial laser welding cell.

  • 16.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Heralic, Almir
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Manufacturing Processes. University West, Department of Engineering Science, Division of Production System. Chalmers.
    Resistance based iterative learning control of additive manufacturing with wire2015In: Mechatronics (Oxford), ISSN 0957-4158, E-ISSN 1873-4006, Vol. 31, p. 116-123Article in journal (Refereed)
    Abstract [en]

    This paper presents successful feed forward control of additive manufacturing of fully dense metallic components. The study is a refinement of former control solutions of the process, providing more robust and industrially acceptable measurement techniques. The system uses a solid state laser that melts metal wire, which in turn is deposited and solidified to build the desired solid feature on a substrate. The process is inherently subjected to disturbances that might hinder consecutive layers to be deposited appropriately. The control action is a modified wire feed rate depending on the surface of the deposited former layer, in this case measured as a resistance. The resistance of the wire stick-out and the weld pool has shown to give an accurate measure of the process stability, and a solution is proposed on how to measure it. By controlling the wire feed rate based on the resistance measure, the next layer surface can be made more even. A second order iterative learning control algorithm is used for determining the wire feed rate, and the solution is implemented and validated in an industrial setting for building a single bead wall in titanium alloy. A comparison is made between a controlled and an uncontrolled situation when a relevant disturbance is introduced throughout all layers. The controller proves to successfully mitigate these disturbances and maintain stable deposition while the uncontrolled deposition fails.

  • 17.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Heralic, Almir
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers.
    Resistance measurements for control of laser metal wire deposition2014In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 54, no March, p. 62-67Article in journal (Refereed)
    Abstract [en]

    A method for controlling robotized laser metal wire deposition online by electrical resistance metering is proposed. The concept of measuring the combined resistance of the wire and the weld pool is introduced and evaluated for automatic control purposes. Droplet formation, detachment of the wire from the weld pool and stubbing can be hard to avoid during processing due to the sensitive process and short reaction times needed for making on-line adjustments. The implemented system shows a possible route for automatic control of the process wherein such problems can be avoided automatically. The method proves to successfully adjust the distance between the tool and the workpiece through controlling the robot height position, thus increasing stability of the laser metal wire deposition process.

  • 18.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Kristoffersen, Hans
    Swerea IVF AB, Box 104, SE-431 22 Mölndal, Sweden.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Temperature Monitoring of Induction Hardening Using Spectral Pyrometry2014In: Proceedings of the 6th International Swedish Production Symposium 2014 / [ed] Stahre, Johan, Johansson, Björn & Björkman, Mats, 2014, p. 1-7Conference paper (Refereed)
    Abstract [en]

    In this study, a recently developed multispectral temperature measurement method is applied for temperature monitoring of induction hardening of steel. An industry-like induction heating process is used for evaluating the method and an automatic calibration procedure is presented. Thermocouples and a conventional pyrometer are used for comparison, showing that the multispectral method gives more accurate results than the conventional pyrometer. These results confirm that the multispectral method is well suited for accurate, non-contacting temperature measurements for induction hardening processes. Enabling measurements which have previously not been possible. This enables fast selection of process parameters which can improve productivity.

  • 19.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Emissivity estimation for high temperature radiation pyrometry on Ti–6Al–4V2013In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 46, no 2, p. 871-880Article in journal (Refereed)
    Abstract [en]

    The paper demonstrates a versatile procedure suitable for industrial implementation of temperature measurement on a hot titanium alloy. The driving force has been the need for an accurate temperature measurement during additive manufacturing using laser welding technology where Ti–6Al–4V-wire is melted. The challenges consider both industrial constraints and the varying emissivity of the surface. Measurements makes use of a narrow bandwidth spot radiation pyrometer and a calibration procedure for estimation of the surface temperature through spectral emissivity estimation. The theoretical results are validated through experiments. A number of difficulties in radiation temperature measurements for metals with varying surface properties are discussed; especially the case of surface oxidation. The uncertainty in temperature reading due to the uncertainty in the emissivity estimate is established along with a model that qualitatively describes surface oxidation. The procedure is expected to be useful for several manufacturing applications where it is important to control high temperatures.

  • 20.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Process and Product Development.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers.
    Emissivity compensated spectral pyrometry for varying emissivity metallic measurands2014In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 25, no 2, p. 025010-Article in journal (Refereed)
    Abstract [en]

    A novel method for converting electromagnetic spectral radiance information into emperature measurements is presented. It allows for varying spectral emissivity of the metallic measurand during the course of the measurement. Such variations are due to e.g. thermal oxidation or temperature dependent emissivity. Based on the assumption that emissivity changes with time and temperature in a continuous manner, it is further assumed that an emissivity estimate at one sample instance can be derived from the estimated emissivity found at the previous samples together with updated spectral information. This leads to successive recalculations of spectral emissivity together with corresponding temperature values. The proposed algorithm has been proven to give accurate temperature estimates from a measurement based on data captured by a standard UV-Vis spectrophotometer even for an oxidizing Ti-6Al-4V specimen in a temperature range between 900K and 1400K. The method however, is not limited to these wavelength- or temperature-ranges.

  • 21.
    Hagqvist, Petter
    et al.
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Process and Product Development.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Chalmers.
    Emissivity compensated spectral pyrometry-algorithm and sensitivity analysis2014In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 25, no 2, p. 025011-Article in journal (Refereed)
    Abstract [en]

    In order to solve the problem of non-contact temperature measurements on an object with varying emissivity, a new method is herein described and evaluated. The method uses spectral radiance measurements and converts them to temperature readings. It proves to be resilient towards changes in spectral emissivity and tolerates noisy spectral measurements. It is based on an assumption of continuous changes in emissivity and uses historical values of spectral emissivity and temperature for estimating current spectral emissivity.

    The algorithm, its constituent steps and accompanying parameters are described and discussed. A thorough sensitivity analysis of the method is carried out through simulations. No rigorous instrument calibration is needed for the presented method and is therefore industrially tractable.

  • 22.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Charles Murgau, Corinne
    University West, Department of Engineering Science, Division of Production Engineering.
    Dzevad, Imamovic
    Volvo Aero Coorporation.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    Dep of signal and systems, Chalmers.
    Towards stable high-speed metal-wire deposition, Part I: Parameter studyIn: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387Article in journal (Refereed)
  • 23.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Automatic in-process control of laser metal-wire deposition based on sensor feedback2011In: 30th Interantional Congress on Applications of Lasers & Electro-Optics, ICALEO 2011: Orlando October 23-27, 2011. Code 87581|, 2011, p. 211-220Conference paper (Refereed)
    Abstract [en]

    This paper describes alasermetaldepositionsystem that isbasedon robotizedlaserwelding andwirefiller material. The system has been found suitable for the manufacture of simple but large shapes with high metallurgical requirements such as bosses or flanges found on aero engine components. Several benefits have been identified with the usage ofwirefiller compared to powderized feedstock, such as betterprocessefficiency, higherdepositionrates, and cleaner working environment. Thewirebaseddepositionprocessis however sensitive to disturbances and thus requires continuous monitoring and adjustments.Inthis work a 3D scanning system is described forautomaticin-processcontrolof thedeposition. The goal is to obtain a flat surface for each deposited layerinorder to ensure stabledeposition. The deviationsinthe layer height are compensated by controlling thewirefeed rate. The system is tested throughdepositionof small cylindrical bosses and the results show that the proposedcontrolapproach is suitable forautomaticdepositionof such structures. The material consideredinthis paper is Ti-6Al-4V deposited on plates of same material. The paper presents the equipment and thecontrolstrategy and discusses practical issues regarding thesensorused.

  • 24.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Production Engineering.
    Ottosson, Mattias
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    Chalmers.
    Control Design for Automation of Robotized Laser Metal-Wire Deposition2008In: Proceedings of the 17th IFAC World Congress, International Federation of Automatic Control , 2008, p. 14785-14791Conference paper (Other academic)
    Abstract [en]

    In this paper a novel approach towards automation of robotized laser metal-wire deposition (RLMwD) is described. The RLMwD technique is being developed at University West in cooperation with Swedish industry for solid freeform fabrication of fully dense metal structures. The process utilizes robotized fibre laser welding and metal wire filler material, together with a layered manufacturing method, to create metal structures directly from a CAD drawing. The RLMwD process can also be used for repair or modification of existing components. This paper faces the challenge of designing a control system for maintaining stable process variables, such as a constant layer height and a stable component temperature, during the entire manufacturing process. Several problems are identified and discussed in the paper, e.g. the difficulty of obtaining the bead height in the weld pool environment. The case study is a repair application for stamping tools, where worn out trim edges are to be repaired. Issues regarding the control design, system identification, and the practical implementation of this application are discussed.

  • 25.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    Dep of signal and systems, Chalmers.
    Height control of laser metal-wire deposition based on iterative learning control and 3D scanning2012In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 50, no 9, p. 1230-1241Article in journal (Refereed)
    Abstract [en]

    Laser Metal-wire Deposition is an additive manufacturing technique for solid freeform fabrication of fully dense metal structures. The technique is based on robotized laser welding and wire filler material, and the structures are built up layer by layer. The deposition process is, however, sensitive to disturbances and thus requires continuous monitoring and adjustments. In this work a 3D scanning system is developed and integrated with the robot control system for automatic in-process control of the deposition. The goal is to ensure stable deposition, by means of choosing a correct offset of the robot in the vertical direction, and obtaining a flat surface, for each deposited layer. The deviations in the layer height are compensated by controlling the wire feed rate on next deposition layer, based on the 3D scanned data, by means of iterative learning control. The system is tested through deposition of bosses, which is expected to be a typical application for this technique in the manufacture of jet engine components. The results show that iterative learning control including 3D scanning is a suitable method for automatic deposition of such structures. This paper presents the equipment, the control strategy and demonstrates the proposed approach with practical experiments.

  • 26.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Lennartson, Bengt
    Dep of signal and systems, Chalmers.
    Towards stable high-speed metal-wire deposition, Part II: Automatic deposition using feedback controlArticle in journal (Refereed)
  • 27.
    Heralic, Almir
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Ottosson, Mattias
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Hurtig, Kjell
    University West, Department of Technology, Mathematics and Computer Science, Division for Mechanical Engineering.
    Lennartson, Bengt
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Automation of Robotized Laser Metal-Wire Deposition2007In: Proceedings of the ninth IASTED International Conference on Control and Applications: Montreal, Canada, ACTA Press , 2007, p. ID 658-075-Conference paper (Refereed)
  • 28.
    Heralic, Almir
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Ottosson, Mattias
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Hurtig, Kjell
    University West, Department of Technology, Mathematics and Computer Science, Division for Mechanical Engineering.
    Lennartson, Bengt
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Freeform Fabrication using Laser Metal-wire Deposition2007In: Proceedings from the 1st Swedish Production Symposium: 28-30 August, Gothenburg, Sweden, 2007, p. session 1.2-Conference paper (Other academic)
  • 29.
    Heralic, Almir
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Ottosson, Mattias
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Hurtig, Kjell
    University West, Department of Engineering Science, Division of Production Engineering.
    Kristiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Visual feed-back for operator interaction in robotized laser metal deposition2008In: Proceedings of the 22nd International Conference on Surface Modification Technologies SMT22: Held at University West, Trollhättan, Sweden September 22-24, 2008 / [ed] T.S. Sudarshan & Per Nylen, 2008, p. 297-304Conference paper (Other academic)
  • 30.
    Lennartson, Bengt
    et al.
    Chalmers University of Technology, Control Engineering Lab, Göteborg, Sweden.
    Middleton, R.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    Unified, periodic & sampled data H control using the delta operator2004In: Decision and Control, 2004. CDC. 43rd IEEE Conference on, 2004, Vol. 3, p. 2364-2369Conference paper (Refereed)
    Abstract [en]

    A unified and general framework is presented for H<sub>&infin;</sub> control of mixed continuous-time and discrete-time time-varying (periodic) systems. Using the delta operator, a close relationship is shown between the continuous- and discrete-time solutions. No assumptions are made on certain system matrices being zero or normalized, which makes the approach general and easy to apply. A combined continuous/discrete-time lifting procedure is shown to be useful, especially for ill-conditioned systems. This procedure together with the delta formalism results in a numerically robust design method concerning both short and long sampling periods for systems with W-conditioned dynamics, including widely spread eigenvalues.

  • 31.
    Lennartson, Bengt
    et al.
    Chalmers University of Technology, Control Engineering Lab, Göteborg, Sweden.
    Middleton, R.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation and Computer Engineering.
    McKelvey, T.
    Low order sampled data H control using the delta operator and LMIs2004In: Decision and Control, 2004. CDC. 43rd IEEE Conference on, 2004, Vol. 4, p. 4479-4484Conference paper (Refereed)
    Abstract [en]

    A procedure for H optimization of low order controllers for discrete-time and sampled-data systems is presented in this paper. Generally, low order H controllers may be achieved by solving bilinear matrix inequalities (BMIs). In this paper an iterative alternation between two LMIs gives a suboptimal solution. To avoid local minima in this search the initial controller is obtained by a frequency weighted controller reduction scheme, where the closed loop properties of a full order controller is taken into account. A minimal number of parameters in the state space realization of the controller also reduces the complexity and improves numerical robustness. The complete presentation is based on delta operator models, which shows a close relationship between the continuous- and discrete-time solutions. The sensitivity of the ordinary discrete-time shift operator LMI formulation to small sampling periods is also analyzed.

  • 32.
    Lennartson, Bengt
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Sampled-data Hcontrol applied to the choice of controller order and sampling rate2010Manuscript (preprint) (Other academic)
  • 33.
    Nilsen, Morgan
    et al.
    University West, Department of Engineering Science, Division of Production System.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Production System.
    Christiansson, Anna Karin
    University West, Department of Engineering Science, Division of Production System.
    Joint tracking in zero gap laser beam welding using a vision sensor2016In: The 7th International Swedish Production Symposium, SPS16, Conference Proceedings: 25th – 27th of October 2016, Lund: Swedish Production Academy , 2016, p. 1-7Conference paper (Refereed)
    Abstract [en]

    The paper describes a robust vision sensor system that can find very narrow gaps between metal sheets to be butt welded together. The use of automated laser beam welding is seen as a key enabler for efficient manufacturing by enabling narrow and deep welds with a limited heat affected zone and low thermal distortion of the welded components. It is sensitive to positioning the laser beam with respect to the joint position. Even a small off-set from the actual joint could result in detrimental lack of fusion. The system comprises a CMOS camera with optic filters integrated in the welding optics and appropriate LED illumination of the work piece. By analysing the spectral emissions during welding, illumination and matching optic filters have been chosen in a spectral range where the process disturbances are relatively low. In this way it has been shown possible to detect the joint position even during harsh welding conditions. Preliminary results from the first experiments show promising results, however more tests will be performed using different weld geometries etc. to verify the robustness of the algorithm.

  • 34.
    Nilsen, Morgan
    et al.
    University West, Department of Engineering Science, Division of Production System.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Production System.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Production System.
    Ancona, Antonio
    University West, Department of Engineering Science, Division of Production System.
    In-process Monitoring and Control of Robotized Laser Beam Welding of Closed Square Butt Joints2018In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 25, p. 511-516Article in journal (Other academic)
    Abstract [en]

    In robotized laser welding of technical zero gap closed square butt joints it is critical to position the laser beam correct with regardsto the joint. Welding with an offset from the joint may cause lack of sidewall fusion, a serious defect that is hard to detect and gives a weak weld . When using machined parts with gap and misalignment between the parts that is close to zero, existing joint tracking systems will probably fail to track the joint. A camera based system using LED illumination and matching optical filters is proposed in this paper to address this issue. A high dynamic range CMOS camera and the LED illumination is integrated into the laser tool. The camera captures images of the area in front of the melt pool where the joint is visible and an algorithm based on the Hough transform and a Kalman filter estimates the offset between the laser spot and the joint position. Welding experiments, using a 6 kW fiber laser, have been conducted to evaluate the performance of the system. Promising results are obtained that can be used in the further development of a closed loop controlled joint tracking system.

  • 35.
    Nilsen, Morgan
    et al.
    University West, Department of Engineering Science, Division of Production System.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Production System.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Production System.
    Ancona, Antonio
    University West, Department of Engineering Science, Division of Production System. IFN-CNR Institute for Photonics and Nanotechnologies, Physics Department, via Amendola 173, 70126 BARI, Italy.
    Monitoring of Varying Joint Gap Width During Laser Beam Welding by a Dual Vision and Spectroscopic Sensing System2017In: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 89, p. 100-107Article in journal (Refereed)
    Abstract [en]

    A vision and spectroscopic system for estimation of the joint gap width in autogenous laser beam butt welding is presented. Variations in joint gap width can introduce imperfections in the butt joint seam, which in turn influence fatigue life and structural integrity. The aim of the monitoring approach explored here is to acquire sufficiently robust process data to be used to guide post inspection activities and/or to enable feedback control for a decreased process variability. The dual-sensing approach includes a calibrated CMOS camera and a miniature spectrometer integrated with a laser beam tool. The camera system includes LED illumination and matching optical filters and captures images of the area in front of the melt pool in order to estimate the joint gap width from the information in the image. The intensity of different spectral lines acquired by the spectrometer has been investigated and the correlation between the intensity of representative lines and the joint gap width has been studied. Welding experiments have been conducted using a 6 kW fiber laser. Results from both systems are promising, the camera system is able to give good estimations of the joint gap width, and good correlations between the signal from the spectrometer and the joint gap width have been found. However, developments of the camera setup and vision algorithm can further improve the joint gap estimations and more experimental work is needed in order to evaluate the robustness of the systems.

  • 36.
    Nilsen, Morgan
    et al.
    University West, Department of Engineering Science, Division of Production System.
    Sikström, Fredrik
    University West, Department of Engineering Science, Division of Production System.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Production System.
    Ancona, Antonio
    University West, Department of Engineering Science, Division of Production System. IFN-CNR Institute for Photonics and Nanotechnologies, Physics Department, via Amendola 173, 70126 BARI, Italy.
    Vision and spectroscopic sensing for joint tracing in narrow gap laser butt welding2017In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 96, p. 107-116Article in journal (Refereed)
    Abstract [en]

    The automated laser beam butt welding process is sensitive to positioning the laser beam with respect to the joint because a small offset may result in detrimental lack of sidewall fusion. This problem is even more pronounced incase of narrow gap butt welding, where most of the commercial automatic joint tracing system fail to detect the exact position and size of the gap. In this work, adual vision and spectroscopic sensing approach is proposed to trace narrow gap butt joints during laser welding. The system consists of a camera with suitable illumination and matched optical filters and a fast miniature spectrometer. An image processing algorithm of the camera recordings has been developed in order to estimate the laser spot position relative the joint position. The spectral emissions from the laser induced plasma plume has been acquired by the spectrometer, and based on the measurements of the intensities of selected lines of the spectrum, the electron temperature signal has been calculated and correlated to variations of process conditions. The individual performances of these two systems have been experimentally investigated and evaluated offline by data from several welding experiments where artificial abrupt as well as gradual excursions of the laser beam out of the joint were produced. Results indicate thata combination of the information provided by the vision and spectroscopic systems is beneficial for development of a hybrid sensing system for joint tracing.

  • 37.
    Ryberg, Anders
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Eriksson, Kenneth
    University West, Department of Engineering Science, Divison of Natural Sciences, Surveying and Mechanical Engineering.
    Accuracy Investigation of a Vision Based System for Pose Measurements2006In: Control, Automation, Robotics and Vision, 2006. ICARCV '06. 9th International Conference on, 2006, p. 1-6Conference paper (Other academic)
    Abstract [en]

    The accuracy of a pose measurement system, called PosEye, is investigated. PosEye is a system for real time measurement of the position and orientation, the pose, of a camera (sensor) using the information in its image. This sensor is aimed to be mounted on an industrial robot for welding. The investigation was done by comparing the PosEye system position output to that of a coordinate measuring machine. Sources of errors are identified, and suggestions for improvements are made

  • 38.
    Ryberg, Anders
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Eriksson, Kenneth
    University West, Department of Technology, Mathematics and Computer Science, Division for Mathematics and Sciences.
    Lennartson, Bengt
    Chalmers University of Technology.
    A new Camera Model and Algorithms for higher Accuracy and better Convergence in Vision-based Pose Calculations2006In: Mechatronics and Automation, Proceedings of the 2006 IEEE International Conference on, 2006, p. 194-199Conference paper (Refereed)
    Abstract [en]

    This paper presents novel strategies for better calibration and pose calculations of a system for determining the pose, i.e. position and orientation, of a camera. The system in question has a camera aimed to be placed on the hand of an industrial robot for welding, but is useful for any application with a need for measuring position and/or orientation. To calculate the pose of the camera circular reference points that can be recognized in the images are distributed in the working area. From their 2D image coordinates the 6D pose of the camera can be calculated. First the system is calibrated, i.e. the positions of the reference points and the camera parameters are determined. This is done by first taking images of the reference points from different locations, and then do a "total calibration" procedure to calculate the unknown parameters. For a specific system, called PosEye, it was concluded that the accuracy needs to be improved for welding applications. Also a method for making the calculations converge more easily, was needed. To meet these demands a new camera model is proposed, and three preprocessing calculation steps are presented. The new camera model increases accuracy by considering more distortion effects. The preprocessing steps give better initial values for more robust convergence of the algorithms and increased accuracy

  • 39.
    Ryberg, Anders
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Eriksson, Kenneth
    University West, Department of Technology, Mathematics and Computer Science, Division for Mathematics and Sciences.
    Lennartson, Bengt
    Chalmers University of Technology.
    A new Camera Model for Higher Accuracy Pose Calculations2006In: Industrial Electronics, 2006 IEEE International Symposium on, 2006, p. 2798-2802Conference paper (Refereed)
    Abstract [en]

    A position and orientation (pose) measurement system is being developed. The system, called PosEye, is based on a camera and by using the information in the image, the pose of the camera taking the image can be calculated. The system is aimed to be placed on an industrial robot for welding, but it is flexible and can also be used in many other applications. The accuracy has been measured, and it is concluded that the accuracy needs to be improved for welding applications. To make the pose measurement, reference points, that can be recognized in the image, are distributed in the working area. The positions of the reference points and the parameters in a camera model are initially computed automatically from sample images from a number of directions to the reference points. After this calibration, the pose can be calculated at each sample image. For high accuracy there is a need to have a camera model that takes into account a number of distortion effects, which are further developed in this paper. The new model is used to express an optimization cost function that can be used for both the pose calculation, and the extensive calibration, that determines camera parameters in the camera model and the positions of the reference points

  • 40.
    Ryberg, Anders
    et al.
    University West, Department of Engineering Science.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Production System.
    Eriksson, Kenneth
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Camera Modelling and Calibration - with Applications2008In: Computer Vision / [ed] Zhihui, X., Vienna: I-Tech Education and Publishing , 2008, p. 303-332Chapter in book (Other academic)
  • 41.
    Ryberg, Anders
    et al.
    University West, Department of Engineering Science, Division of Production Engineering.
    Ericsson, Mikael
    University West, Department of Engineering Science, Division of Production Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Production Engineering.
    Eriksson, Kenneth
    University West, Department of Engineering Science, Division of Land Surveying and Mathematics.
    Nilsson, Jim
    University West, Department of Engineering Science.
    Larsson, Matthias
    University West, Department of Engineering Science.
    Stereo vision for path correction in off-line programmed robot welding2010In: Proceedings of the IEEE International Conference on Industrial Technology, 2010, p. 1700-1705Conference paper (Refereed)
    Abstract [en]

    The paper describes a versatile machine vision system for correcting off-line programmed nominal robot trajectories for advanced welding. Weld trajectory corrections are needed due to slight variations in weld joints. Such variations occur naturally because of varying tolerances in parts and to heat induced deformations during earlier weld sequences. The developed system uses one camera and a weld tool mounted on the robot hand. As a first step, the whole system, including the camera, is calibrated. Then the system takes images of the weld joint from different positions and orientations, and determines the weld joint geometry in 3D using a stereo vision algorithm and a novel camera model. The weld trajectory is then updated in the robot control system, and weld operation is performed. These steps are repeated for all weld sequences of the work piece. The strategy has successfully been demonstrated for a standard industrial welding robot and a standard FireWire CMOS camera. The maximum deviation of the trajectory found by the system compared to a reference (coordinate measuring machine) is 0.7 mm and the mean deviation is 0.23 mm. Thus, the system shows high potential for industrial implementation. ©2010 IEEE.

  • 42.
    Ryberg, Anders
    et al.
    University West, Department of Engineering Science.
    Lennartson, Bengt
    University West, Department of Engineering Science.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science.
    Asplund, Lars
    Mälardalen University.
    Ericsson, Mikael
    University West, Department of Engineering Science.
    Analysis and evaluation of a general camera model2011In: Computer Vision and Image Understanding, ISSN 1077-3142, E-ISSN 1090-235X, Vol. 115, no 11, p. 1503-1515Article in journal (Refereed)
    Abstract [en]

    A versatile General Camera Model, GCM, has been developed, and is described in detail. The model is general in the sense that it can capture both fisheye and conventional as well as catadioptric cameras in a unified framework. The camera model includes efficient handling of non-central cameras as well as compensations for decentring distortion. A novel way of analysing radial distortion functions of camera models leads to a straightforward improvement of conventional models with respect to generality, accuracy and simplicity. Different camera models are experimentally compared for two cameras with conventional and fisheye lenses, and the results show that the overall performance is favourable for the GCM.

  • 43.
    Sikström, Fredrik
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Simulated Feedback Control of Metal Fusion Welding2007In: Proceedings of the ninth IASTED International Conference on Control and Applications: Montreal, Canada 2007, ACTA Press , 2007, p. 2263-2270Conference paper (Refereed)
  • 44.
    Sikström, Fredrik
    et al.
    University West, Department of Engineering Science, Division of Automation Systems.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Automation Systems.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Automation Systems. Chalmers University of Technology, Department of Signals and Systems,Gothenburgh, Sweden.
    Model based feedback control of gas tungsten arc welding: An experimental study2015In: Automation Science and Engineering (CASE), 2015 IEEE International Conference on, IEEE conference proceedings, 2015, p. 411-416Conference paper (Refereed)
    Abstract [en]

    In order to obtain high structural integrity and joint performance in welding a transient heat conduction model has been utilized to design a model based feedback controller.Gas tungsten arc welding of work-pieces of austenitic steel have been simulated by the finite element method. The basis for controller design is a low order model obtained from parametric system identification. The identification has been performed both on the finite element simulation and on physical welding. The low order model responses show a good agreement with both the finite element simulation result and the physical process response. An experimental study has been performed to verify the approach. This study also explores what experiments are needed for a successful design. It is shown that model based control successfully mitigates perturbations that occur during welding.

  • 45.
    Sikström, Fredrik
    et al.
    University West, Department of Engineering Science, Division of Process and Product Development. University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Process and Product Development.
    Model order reduction methods applied to a welding model2012In: Proceedings of the Institution of mechanical engineers. Part I, journal of systems and control engineering, ISSN 0959-6518, E-ISSN 2041-3041, Vol. 226, no 7, p. 972-984Article in journal (Refereed)
    Abstract [en]

    A  finite element representation modelling transient heat conduction of gas tungsten arc welding of stainless steel is used to evaluate different methods for model order reduction. The focus is on establishing a linear low-order model of the dynamic  relation between the welding current and the temperature measured by a radiation pyrometer. The objective of this low-order model is to design a model-based feedback controller and to investigate the consequences of applying feedback control of the process. Three different approaches for model reduction have been evaluated, namely the Krylov subspace method for moment    matching, balanced truncation and parametric system identification. The study provides a knowledge base for the selection of model order reduction methods when dealing with large-scale systems like finite element models of transient heat conduction, and it recommends parametric system identification. It renders sufficient approximations for controller design, no linearization of the finite element model is required, and there is no limit on degrees of freedom of the finite element model.

  • 46.
    Sikström, Fredrik
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Modeling and simulation aspects of feedback control in gas tungsten arc welding: An experimental study2010Manuscript (preprint) (Other academic)
  • 47.
    Sikström, Fredrik
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Lennartson, Bengt
    Chalmers University of Technology, Department of Signals and Systems.
    Modelling and Simulation for Feedback Control of Welding2007In: Proceeding (567) Modelling and Simulation (MS 2007): Montreal, CanadaMay 30 - June 1, 2007, ACTA Press , 2007, p. 131-136Conference paper (Refereed)
  • 48.
    Sikström, Fredrik
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Lennartson, Bengt
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Role of fixture forces on distortion in gas tungsten arc welding: An experimental and modelling approach2011In: Proceedings of the Institution of mechanical engineers. Part B, journal of engineering manufacture, ISSN 0954-4054, E-ISSN 2041-2975, Vol. 225, no 1, p. 140-148Article in journal (Refereed)
    Abstract [en]

    Simulation and experiments show that the fixture clamping force has a significant influence on the structural integrity of a welded workpiece. This understanding is of great importance for the manufacture of aerospace components with tight tolerances in the specifications. The focus in the present study is on the temperature history during welding and residual deformation; its main contribution is a demonstrator designed for evaluation of the influence of fixture clamping forces and validation of the simulation results. The demonstrator concerns a simplified situation considering gas tungsten arc welding of a nickel-based metal plate fixed by a specially designed fixture, where one side of the plate was clamped with different levels of force. The temperature history was measured during the weld sequence and deformation measurements were performed after cooling and release of the workpiece from the fixture. The results from simulation and experimentation showed good agreement. The proposed strategy is industrially competitive and has shown that the looser the fixture clamps, the smaller the residual deformation. Furthermore, the study provides a knowledge base for selection of active fixture concepts in that the fixture clamping force can be determined in advance and possibly also be subject to force control.

  • 49.
    Sikström, Fredrik
    et al.
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Christiansson, Anna-Karin
    University West, Department of Technology, Mathematics and Computer Science, Division for Electrical Engineering and Land Surveying.
    Lennartson, Bengt
    Chalmers University of Technology, Department of Signals and Systems.
    Simulation for Design of Automated Welding2007In: EUROCON, 2007. The International Conference on "Computer as a Tool": Warsaw, Poland, 2007, IEEE Explore , 2007, p. 2263-2270Conference paper (Refereed)
    Abstract [en]

    This paper describes a promising approach where simulations were used in the design of real-time control for automated welding. A finite element method has been used for thermal modelling of gas tungsten arc welding on a simplified test object. Measurement data for model calibration and validation was acquired through thermal imaging during weld experiments on test objects of the alloy Fe-316. An optimisation scheme for inverse modelling was employed in the calibration of the distributed weld process model. Frequency weighted model reduction and parametric system identification were applied and evaluated to get a low order model of the single-input single-output dynamics between a simulated weld heat source (actuator) and a sensor. This low order model was then successfully used for controller design where the control signal was weld current and the measured output was a moving spot temperature. Finally, the closed-loop performance was evaluated by simulation of the weld process model showing improved temperature stability relative to open loop.

  • 50.
    Sikström, Fredrik
    et al.
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Ericsson, Mikael
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Christiansson, Anna-Karin
    University West, Department of Engineering Science, Division of Electrical and Automation Engineering.
    Niklasson, Kjell
    University West, Department of Engineering Science, Division of Process and Product Development.
    Tools for simulation based fixture design to reduce deformation in advanced fusion welding2008In: Intelligent Robotics and ApplicationsLecture Notes in Computer Science Volume 5315,  2008: First International Conference, ICIRA 2008 Wuhan, China, October 15-17, 2008 Proceedings, Part II / [ed] Youlun Xioun, Springer, 2008, Vol. 5315 LNAI, no PART 2, p. 398-407Conference paper (Refereed)
    Abstract [en]

    The traditional fusion welding and fixture simulations are performed using advanced finite element simulation tools, commonly used are e.g. MSC.Marc, ANSYS, ABACUS and COMSOL Multiphysics. These simulations are made one at a time and separately due to heavy calculation load for each case. Such an approach does not give a full description of the integrated work piece and fixture behaviour. We propose a strategy to decrease the computational time and solve the problem accurately enough for industrial needs. Focus of the simulation result is on residual deformation. The work piece is a simplified component composed by metal sheets, and rigid and loose clamping was investigated. Simulation results give the size of forces and deformations in the clamped edge. Deformation measurements are performed using 3D-scanning of the work piece after cooling and released from fixture, same situation as in the FE-simulations. The proposed strategy has shown to be useful and is industrially competitive due to reduced engineering manpower, computation time, and need for practical experiments. The strategy is to use full off-line programming where computer aided robotics for weld sequencies is integrated with finite element modelling in order to obtain weld parameters and fixture design.

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