closed loop welding controller for manufacturing process 2

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Closed Loop Welding Controller for ManufacturingProcessTo cite this article F Bonaccorso et al 2011 IOP Conf Ser Mater Sci Eng 26 012003

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Closed Loop Welding Controller for Manufacturing Process

F Bonaccorso1 C Bruno1 L Cantelli1 D Longo1 G Muscato1 S Rapisarda1

1Universitagrave degli Studi di Catania DIEES Viale A Doria 6 95125 Catania ITALY E-mail gmuscato dieesunictit Abstract The aim of this paper is to investigate on the closed loop welding controller of a rapid manufacturing Shaped Metal Deposition (SMD) process SMD was developed and patented by Rolls-Royce in order to produce mechanical parts directly from a CAD model A simplified SMD plant has been set up in order to investigate the welding dynamics and parameters and to develop a SMD automatic controller On the basis of the experience acquired some basic control laws have been developed and a closed loop controller has been implemented This controller permits to find and to maintain the process stability condition so that the final process results totally automatic The control is performed adjusting the welding conditions on the basis of arc voltage information obtained from the welding machine during the deposition The experimental results reported confirm the validity of the proposed strategy

1 Introduction The Shaped Metal Deposition (SMD) process is a novel manufacturing technique developed and patented by Rolls-Royce a few years ago in order to produce mechanical parts directly from a CAD model The work presented in this paper has been carried out in the frame of the RAPOLAC European project [1] [2] and aims to investigate upon an automatic control system in order to free the operator from constant monitoring and manually acting on the welding process parameters

The innovative aspect of the SMD process is a reversed production philosophy Up to now the traditional manufacturing methods such as machining are based on the material removal from the work piece in order to obtain a desired final shape There are several evident drawbacks in this process as the large waste of material in scraps and the consequent increase of the costs depending of the material used

The SMD process reverses this destructive production philosophy and works by adding progressively the material to the final work piece in order to obtain the desired component final shape This means that effectively no process scraps are produced thus minimizing the material used to the strictly amount required [3] [4]

2 SMD experimental set up In order to investigate on the SMD process dynamics and control strategies a simplified SMD plant has been set up in our laboratory with a six axis industrial welding robot and a 500A TIG inverter welding machine as it is shown in Figure 1 SMD process prescribes using of a sealed chamber with 99999 purity argon However since the chamber was not available in our laboratory we concentrated our effort mainly on the control system without taking too much attention to the quality of the final pieces

Trends in Aerospace Manufacturing 2009 International Conference IOP PublishingIOP Conf Series Materials Science and Engineering 26 (2011) 012003 doi1010881757-899X261012003

Published under licence by IOP Publishing Ltd 1

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As an alternative of video feedback above mentioned there are even different aspects of the process that can be used to measure the gap which are all directly connected to the arc length such as the arc emitted light [101112] the arc emitted sound [131415] and the arc voltage as shown in Figure 7

Though also the video feedback the arc emitted light and sound were deserved in our investigation at moment the arc voltage is the parameter that permits in the simplest manner to evaluate the gap The arc voltage is the most used parameter to measure the arc length as matter of fact that all welding machines usually have an output referred to the arc voltage It is typically used in the so called automatic voltage control (AVC) systems Although the current is controlled by the power source controller the arc voltage is primarily determined by the arc length and to a secondary degree by the arc current Ordinarily the welder specifies the desired arc voltage and a classical feedback loop (the AVC) adjusts the vertical position of the torch and consequently the arc length through a servo mechanism to achieve the required voltage

On the basis of these considerations a simple but effective control strategy has been studied once defined an arc voltage reference corresponding to a certain gap an error on the arc voltage value can be interpreted not as a torch positioning error but as an erroneous position of the bead below and consequently an erroneous value of wire feeder rate

Therefore the control strategy is implemented by a linear proportional action around a stable working point as described by the equation (1)

)( refVVkWFWF minussdot+= (1)

The wire feed rate WF represents the value corresponding to the stable process working point in

which all the material coming from the wire feeder is deposited in each layer ensuring the stability of the process This value is strictly connected to the value of the other parameters amperage travel speed and desired step height The gain k represents voltage to wire feeder rate conversion factor

The control strategy adopted is briefly represented in Figure 8 as a linear action on the wire feed rate on the basis of the error between the measured arc voltage value V and the arc voltage reference Vref the wire feeder rate is corrected in order to compensate for a wrong amount of material injected For a null error the wire feeder rate WF is imposed meaning that the desired step height is respected

A saturation is performed in order to assure deposition of uniform layers

Figure 8 The control strategy characteristic adopted by the controller

5 Experimental results The reported test regards the deposition of the 50 layers cylinder shown in Figure 9 It has been performed using the following working point TS =25 mmsec SH= 1 mm Amperage=100 A WF=1300 mmmin The resulting form presents shape irregularities propagated along the structure caused by the too high mechanical time response of wire feeder

WF

V-Vref

WF

WF

V-Vref

WF


5

Figure 9 A 50 layers cylinder deposited automatically using the linear control law

Figure 10 Arc voltage (up) and wire feeder rate (down) distribution along the external surface of cylinder angle (x axis) and layer number (y axis) cylinder deposited automatically using the linear control law

Figure 10 shows a post-elaboration of the arc voltage and the wire feeder rate data stored during the

test this technique permits to evaluate the influence of every layer deposition to the successive one and to analyze the possible causes of the defects


6

Figure 10 Cylinder deposition arc voltage and wire feed rate time series As it can be observed from the time series shown in Figure 11 even though the control system

assures that the structure goes up in a regular manner it is possible to observe high wire feeder variations around the working point (1000 mmmin) responsible of the locally structure irregularities This is due to the very large value of k

6 Conclusion A closed loop welding controller for manufacturing purposes has been presented

An SMD plant has been set up in order to observe and study various aspects of this process In this the process parameters and their ties have been identified

The developed control strategy permits to maintain the process stability condition so that the process results totally automatic

The control is performed locally during the single layer deposition in order to eventually correct the basement or previous layer imperfections

All tests have been performed using stainless steel 316 It should be observed that the irregularities on the surface of the manufactured objects are due to

the fact that our plant is a laboratory reproduction of the SMD and is not shielded from the external atmosphere Consequently the surface is subject to rapid oxidation Moreover a rotating table is not present in our plant

However all the points described are valid only if the stable process working point is known a priori Future developments will regard the utilization of last layers errors taking advantage from the iterativity of the process in order to automatically tune the process

All the work that has been presented is also part of a patent deposited by DIEES-UNICT

7 References [1] RAPOLAC Project home page [Online] httpwwwRAPOLACeu [2] AMR home page [Online] httpwwwamrcouk

refV

WF

1000

WF

1000


7

[3] Hensinger D M Ames A L and Kuhlmann J L 2000 Motion Planning for a Direct Metal Deposition Rapid Prototyping System Proc of the 2000 IEEE International Conference on Robotics amp Automation San Francisco CA

[4] Zhang Y et al 2003 Weld deposition based rapid prototyping a preliminary study J of Materials Processing Technology 135 347ndash357

[5] Moore D S Naidu S and Ozcelik K L 2003 Modeling Sensing and Control of Gas Metal Arc Welding (Elsevier)

[6] Cheni S B Zhang Y Qiu T Lin T 2003 Robotic Welding Systems with Vision-Sensing and Self-learning Neuron Control of Arc Welding Dynamic Process J of Intelligent and Robotic Systems 36 191ndash208

[7] Wu C S Gao J Q Liu X F and Zhao Y H 2003 Vision ndash based measurement of weld pool geometry in constant ndash current gas tungsten arc welding Proc of the Institution of Mechanical Engineers Part B Journal of Engineering Manufacture 217 879-882

[8] Messer R W 1999 Principles of Welding Processes Physics Chemistry and Metallurgy (Wiley-Interscience)

[9] Tarn Tzyh-Jong Chen Shan-Ben Zhou Changjiu 2007 Robotic Welding Intelligence and Automation Lecture Notes in Control and Information Sciences 362

[10] Zhang P J and Li Y M 2001 Precision Sensing of Arc Length in GTAW Based on Arc Light Spectrum Journal of Manufacturing Science and Engineering 123 62-65

[11] Sciaky C A and Johnson A M 1966 System For Controlling Length Of Welding 3236997 US [12] Ralchenko Yu Kramida A E Reader J and NIST ASD Team 2008 NIST Atomic Spectra

Database (version 315) [Online] National Institute of Standard and Technology [Cited 08 06 2008] httpphysicsnistgovasd3

[13] Tam J 2005 Methods of Characterizing Gas-Metal Arc Welding Acoustics for Process Automation University of Waterloo

[14] Jolly W D 1969 Acoustic emission exposes cracks during welding Welding Journal 48 21-27

[15] Drouet M G and Nadeau F 1979 Pressure waves due to arcing faults in a substation IEEE Transactions on Power Apparatus and Systems PAS-98 1632-1635


8

Closed Loop Welding Controller for Manufacturing Process

F Bonaccorso1 C Bruno1 L Cantelli1 D Longo1 G Muscato1 S Rapisarda1

1Universitagrave degli Studi di Catania DIEES Viale A Doria 6 95125 Catania ITALY E-mail gmuscato dieesunictit Abstract The aim of this paper is to investigate on the closed loop welding controller of a rapid manufacturing Shaped Metal Deposition (SMD) process SMD was developed and patented by Rolls-Royce in order to produce mechanical parts directly from a CAD model A simplified SMD plant has been set up in order to investigate the welding dynamics and parameters and to develop a SMD automatic controller On the basis of the experience acquired some basic control laws have been developed and a closed loop controller has been implemented This controller permits to find and to maintain the process stability condition so that the final process results totally automatic The control is performed adjusting the welding conditions on the basis of arc voltage information obtained from the welding machine during the deposition The experimental results reported confirm the validity of the proposed strategy

1 Introduction The Shaped Metal Deposition (SMD) process is a novel manufacturing technique developed and patented by Rolls-Royce a few years ago in order to produce mechanical parts directly from a CAD model The work presented in this paper has been carried out in the frame of the RAPOLAC European project [1] [2] and aims to investigate upon an automatic control system in order to free the operator from constant monitoring and manually acting on the welding process parameters

The innovative aspect of the SMD process is a reversed production philosophy Up to now the traditional manufacturing methods such as machining are based on the material removal from the work piece in order to obtain a desired final shape There are several evident drawbacks in this process as the large waste of material in scraps and the consequent increase of the costs depending of the material used

The SMD process reverses this destructive production philosophy and works by adding progressively the material to the final work piece in order to obtain the desired component final shape This means that effectively no process scraps are produced thus minimizing the material used to the strictly amount required [3] [4]

2 SMD experimental set up In order to investigate on the SMD process dynamics and control strategies a simplified SMD plant has been set up in our laboratory with a six axis industrial welding robot and a 500A TIG inverter welding machine as it is shown in Figure 1 SMD process prescribes using of a sealed chamber with 99999 purity argon However since the chamber was not available in our laboratory we concentrated our effort mainly on the control system without taking too much attention to the quality of the final pieces


Published under licence by IOP Publishing Ltd 1

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closed loop welding controller for manufacturing process 2

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