UNIVERSITY

Cranfield

1 Literature Review.1.1 Introduction

Robotic automation systems are quickly taking the place of the human’s work force. One of thebenefits is that this change provides the human work force with the time to spend on morecreative tasks. The highest amount of robots work in spot welding, spray painting, materialhandling, and arc welding.Development and research of automatic and highly intelligent control systems is future tendency of robotic arc welding systems. Such development has important role cause of provision high weld quality, increasing productivity and improving work conditions for labor. To achieve such high results is necessary to use sensors. Sensors are collecting information from the various sources such as robot and peripheral devices. Information transfer to controller, process and creates indispensable corrections. In MIG/MAG welding, it is critical to consider distortion from high heat input and, consequently, a closed loop control with a sensor is necessary. As well, in an robot welding system the errors caused by manufacturing tolerances of robot system itself and tolerances of non welded parts have to be estimated.1.2 Types of seam tracking sensors.

Considerable progress has been made after several decades of development, study and practical work. Number of types seam-tracking sensors have been developed in last few decades of last century.Sensors can be subdivided in categories by principles of sensing. Usually sensors installed close to welding torch and detecting position of robot arm relative to seam location. Most of seam tracking sensors can be subdivided on mechanical, electromagnetic, optical devices and arc sensors. SeeFigure 1.

1.2.1 Contact seam tracking sensor.The typical structure of contact sensor shown on Figure 2 Reference (1).

The probe slips in grove and detects deflection of torch from proper position in grove. Micro switch detects direction of deflection. Other types of sensing devices can be utilized such as potentiometer, electromagnetic or electro optical. Advantages of mechanical contact sensors are simplicity and low cost, that’s why they are so widely use in industry at present moment. Disadvantages are that sensors are only applicable to welds which have grooves provided (strait first root pass welds or T joints). Also mechanical contact sensors require different probes for different grove shapes, probes as well deform, wear. Tack welds in joint makes impossible to use mechanical contact sensors. Mechanical sensors are not applicable for high speed welding.

Sensors for seam tracking

Attached sensor

Arc Sensor

Bent wire

Two wires in parallel

Oscillating tungsten

Oscillating Arc

Rotating Arc

Contact

Eddy current

Electro-magnetic

Optical

Others

Figure 1

Figure 1 Reference (1)

Figure 2 Reference (1)

UNIVERSITY

Cranfield

1.2.2 Electromagnetic seam tracking sensor.

Electromagnetic sensors based on principles of high frequency transformer Figure 3 Reference (1).High frequency current on primary coil U1 will induce a potential on secondary coil U21 and U22. Misalignment of work piece creates asymmetry of magnetic path and creates different potentials on coils U21 and U22. Electromagnetic sensors useful for butt welds, lap welds and T joints, but this type of sensors large, not flexible, sensitive to magnetic interference with arc and assembly accuracy.

1.2.3 Optical seam tracking sensor.Optical sensors have been intensive developed in last decade. Many different mode of operation can be

obtained by using optical sensors. For example relative position of torch, shape of groove, arc molten pool penetration etc. Optical sensors can be divided to several categories:

Single spot light sensor. Photo electronic sensor used for groove detection, laser spot source can be used as well.

Linear light sensor. See Figure 4 Reference (3). Light line projected to work piece, CCD camera receive reflected light and processed by controller to locate center and width of groove.

Scanning light sensor. Laser beam scans over groove, CCD camera receive reflected light and processed by controller to locate center and width of the groove.

CCD image sensor. See Figure 5 Reference (4). CCD image sensor observes tip to work distance, center of groove, and molten pool. Image processed by modern sophisticated software to locate center, width

of groove and other parameters. Optical sensors have high precision and repeatability. Disadvantages are high capital cost, sophisticated and expensive software, distance between torch and sensor creates extra errors, limited accessibility of torch sensor system. Due to disadvantages they are not highly utilized in industry.

1.2.4 Arc seam tracking sensors. The first time arc sensors were utilized more than 30 years ago. They have some advantages compare with mentioned attached sensors: Detection point at the end welding wire (no distance between sensor and torch), good accessibility, low cost, durable system. Principals of operation based on variation of welding current during arc oscillation across the groove. Figure 6 Reference (1) show current variation due to variation tip to work distance on CV drooping characteristic.

Figure 3 Reference (1)

Figure 4 Reference (3)

Figure 5 Reference (4)

UNIVERSITY

Cranfield

Figure 6 Reference (1)

Some other variations of process utilized: two wires in parallel, rotating arc. Development of high speed rotating arc has been done in Dept. of Mechatonics Eng., College of Eng., Pukyong National University, Busan, Korea. High speed rotating sensor and controller has been developed for welding of complicate curved fillet welds. Tip of the torch attached to the torch and positioned away from centerline of torch. Structure of rotating torch shown on Figure 7 Reference (5). Tip rotates around of longitudinal axis of the torch. Rotation cycle divided to 4 regions. During rotation tip define horizontal deflection of torch position in groove. Controller calculates average value of current for each region, data processed through low pass filter with the moving average method. Effectives of the sensor have been proved by series of experiments.

Seam tracking sensors based on arc sensing principals has limited number of applications: welding in groove, narrow gap welds and cannot determine centerline of lap joints, joints without grooves, unsymmetrical grooves.

1.2.5 Ultrasonic seam tracking sensor.One of the researches has been done Ajay Mahajan and Fernando Figueroa Department of Mechanical Engineering, Lake Superior State University, Sault Ste. Marie, MI 49783 (USA), Department of Mechanical Engineering, Tulane University New Orleans , LA 70118 (USA), (Published in Robotica (1997) volume 15 , pp 275 – 281 . Printed in the United Kingdom Ö 1997 Cambridge University Press) to develop seam tracking sensor based on ultrasonic principals. Developed ultrasonic sensor can track arm of the robot in 2- dimensional plane. 2 sensors utilized –transmitter and receiver. Groove detected by scanning surface in front of the torch. Amplitude of received waves processed trough controller. If there is no groove receiver will receive all transmitted waves. Ultrasonic seam tracking system proved itself as durable and intelligent sensing system. System cost effective and applicable to even polished surfaces.

1.2.6 Seam tracking software development.High results in robotic arc seam tracking have been achieved in development of filtering and processing software. Predictive seam tracking system have been developed in GRACO/UnB – Automation and Control Group / University of Brasilia, Electrical and Mechanical Engineering Depts. - 70910-900 - Brasília / DF – BRAZIL. System collect information from sensor, manipulation dynamics and a path generator model are minimize the tracking error.

Figure 7 Reference (5)

UNIVERSITY

Cranfield

Software Image processing module have been developed in College of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, People’s Republic of China. Modern circular three dimensional 3-D lasers utilizes instead of conventional 2-D stripped or 1-D spot lasers. Main advantages of image processing software module is high sensitivity and accuracy, immunity to arc radiation, sparks welding fumes, reflection of laser light from polished metal surfaces.

1.3 Conclusion.

Based on performed literature review, I can conclude: Seam tracking systems are highly utilized in modern industry. Various choices of sensors are available in market. Selection particular type of sensor will depend on application. Simple straight fillet or butt (with groove) welds will require mechanical contact seam tracking or arc seam tracking sensors, robust and cost effective solutions to improve quality and productivity. Complex geometry joints with low tolerances will require optical seam tracking sensors. In last decade development of software for seam tracking systems plays leading role.

1.4 References.

1. Jiluan Pan, Arc Welding Control, Woodhead Publishing Ltd, Abington Hall Abington Cambridge CB1 6AH England

2. Arc Welding Robot Automation Systems , Beom-Sahng Ryuh, Gordon R. Pennock Division of Jeonju, Republic of Korea, 561-756 Mechanical Engineering, Chonbuk National University, School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, 47907-1288, USA.

3 Teleteaching approach for sensor-based arc welding telerobotic system, Hai Chao Li, Hong Ming Gao and Lin Wu, Harbin Institute of Technology, Harbin, People’s Republic of China.

4 Predictive Seam-Tracking Optimization, Adolfo Bauchspiess, Sadek C. Absi Alfaro,

GRACO/UnB – Automation and Control Group / University of Brasilia, Electrical and Mechanical Engineering Depts. - 70910-900 - Brasília / DF – BRAZIL.

5 Development of a High Speed Rotating Arc Sensor System for Tracking Complicate Curved Fillet Welding Lines, Gun-You Lee, Myung-Suck Oh and Sang-Bong Kim, Dept. of Mechatonics Eng., College of Eng., Pukyong National University, Busan, Korea.

6 Intelligent seam tracking using ultrasonic sensors for robotic welding, Ajay Mahajan and Fernando Figueroa, Department of Mechanical Engineering , Lake Superior State University , Sault Ste . Marie , MI 49783 (USA) . Department of Mechanical Engineering , Tulane University New Orleans , LA 70118 (USA). Robotica (1997) volume 15, pp 275 – 281 . Printed in the United Kingdom 1997 Cambridge University Press.

7 A visual seam tracking system for robotic arc welding, Peiquan Xu & Guoxiang Xu & Xinhua Tang & Shun Yao, College of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, People’s Republic of China.

UNIVERSITY

Cranfield