Design and Control of 6-Axis Articulated Robot with Fiber Coupled Laser Diode 980 nm for Laser Welding.

อาจารย์ที่ปรึกษา ผู้ช่วยศาสตราจารย์ ดร.กิตติพงษ์ เยาวาจา (Kittipong Yaovaja)
หัวหน้ากลุ่มวิจัยวิทยาการหุ่นยนต์และระบบอัตโนมัติขั้นสูง และผู้รับผิดชอบหลักสูตรหุ่นยนต์และระบบอัตโนมัติ (นานาชาติ)​ ม.เกษตรศาสตร์ วิทยาเขตศรีราชา คณะวิศวกรรมศาสตร์ศรีราชา

Design and Control of 6-Axis Articulated Robot with Fiber Coupled Laser Diode 980 nm for Laser Welding
Kaamutsup Sungkaksem1, Withit Chatrattanakulchai1 and Kittipong Yaovaja2*
1Department of Mechanical Engineering, Faculty of Engineering, Kasetsart University, 50 Ngamwongwan Rd., Chatuchak, Bangkok, 10900, Thailand.
2Robotics and Advanced Autonomous Systems Research Group, Faculty of Engineering at Sriracha, Kasetsart University Sriracha Campus, 199 Sukumvit Rd., Sriracha, Chon Buri, 20230, Thailand.

• Corresponding Author: kittipong@eng.src.ku.ac.th
  Abstract. This research aimed to design and build a 6-axis industrial articulated robot with the application of lasers for material processing. Semiconductor laser, transmits laser light through fiber optics (Fiber Coupled laser Diode), was applied for the robotics welding solution. This research focused on industrial applications in robotic welding system; integrated acquaintance in many areas of science, comprises of mechanical design, control system, electronics and computer programming. The research topics of laser engineering and optical systems in robotics are relatively novel and will be the challenges of the future. The first part of this research described the design, construction, and in-depth analysis of the 6-axis robots. There are inverse kinematics, finite element analysis of structure and force loading of parts. The mathematical model generated position, speed, acceleration setpoints, was implemented on an industrial controller; Z Motion controller. The second part; the design, components, and in-depth analysis of lasers and optical systems of semiconductor diodes fiber optic system were described. There were electrical circuits to control the laser and generating pulses for the laser, including testing of important values in the laser photonics such as a beam parameter product, beam shape values, laser power density. The final part showed the experimental results of the robot integrated with a fiber coupled laser optical head for welding sheet metals. From analysis of the weld results, based on the tensile strength such as shear test and peel test as well as the groove weld inspection measurements, showed satisfied results of the robotic laser welding application.

Keywords: Laser welding, 6-Axis robot, Diode fiber coupled laser.

1. Introduction.
   The increasing demands of laser applications for metal cutting, welding, and 3D metal printing, has driven the need for higher laser power and positional accuracy. This demand has led to the development of new laser technologies; fiber lasers powered by semiconductor diodes were replacing traditional solid-state Nd:YAG lasers that use xenon lamps for excitation. Semiconductor laser, a type of diode that transmits laser light through fiber optics (Fiber Coupled laser Diode), is a technology that plays an important role in both research and development sectors such as the application of lasers to maintain the state of quantum bits in quantum computers, telecommunications in space, military work, laser weapons. Those also applied for laser material processing in industrial sectors such as welding metal, plastic, 3D printing the molds, and soldering printed circuit boards. There are medical uses of the lasers to remove hairs, acnes, blemishes, and freckles [1]-[4].
   The 6-axis robotic systems with fiber-coupled diode laser systems have been developed for accurate position control; had gaining increased attention due to their versatility and the advantages they offer over human labor particularly in terms of skill replacement and fatigue reduction in hazardous environments. The rapid advancement in robotic systems for laser applications is evident, but the performance of these systems is still influenced by the precision of their mechanical components and controllers. The 6-axis robot integrated with a laser system provides precise laser path trajectories; are especially significance in processes such as laser material processing, plastic welding in automotive parts, laser soldering on printed circuit boards, and 3D metal laser printing. [5]-[9].
   This research focused on the design, implementation, and development of a 6-degree-of-freedom (DOF) robotic arm equipped with a 300-watt, 980-nanometer fiber-coupled diode laser system, and applied for laser welding applications. The properties of the 6-DOF robotic arm and the characteristics of the 980-nanometer fiber diode laser system were investigated; focused on the mechanical and electrical design, control programming, inverse kinematics, path programming, laser source and controllers, electronics circuit, and laser welding application. The robot operation was the movement of an end-effector or tool center point (TCP) on a two-dimensional plane to guide the optical laser equipment along a predefined straight-line geometric path for welding thin sheet metal. It was observed that the fiber-coupled laser diode system successfully performed laser welding on two stacked stainless steel sheets with a thickness of 0.3 mm each. The laser was optically aligned using a collimating lens with a focal length of 50 mm to produce a parallel beam, which then passed through a focusing lens with a focal length of 100 mm. The laser wavelength used for this operation was 980 nanometers. The experimental results of the welds of metal sheets with specific parameters were performed. The precision of the robotic laser welding was observed using high-precision measurements. Assessment the quality of the weld beam through tensile strength testing on welded sheet metal aimed to determine the tensile force required to break the weld.
2. Design The Prototype 6-Axis Articulated Robot
   The prototype 6-axis articulated robot, equipped with a fiber-coupled laser diode is shown in Figure 1 (left). The prototype robot with a reach of 550 mm, and a payload capacity of 0.5 kg was produced. The has a base with a diameter of 324 mm, a height of 750 mm with a total robot weight of 12.5 kg (excluding the base slide rail). The design concept was inspired by the AR3 articulated robot developed by an American robotic engineer; Chris Annin. It was designed by using a combination of 6061 aluminum and high-strength 3D-printed components. Most of the 3D-printed structural parts were produced with an infill of 50%, except for the joint-2, joint-3 rotational axes and the tension adjustment rings, which were printed with 90% infill. The components were printed with a layer height of 2 mm and a shell thickness of 5 layers. The robot was constructed using ABS and special printing material igumid P150-PF, a 3D printing filament from IGUS, known for its exceptional strength and rigidity due to fiber reinforcement. This material is ideal for lightweight structural components, offering high bending strength of 87 MPa and a flexural modulus of 5GPa, thereby maintaining high strength and stiffness. The repeatability of ±0.5 mm was satisfied for the design.

Figure 1. (left)The prototype 6-axis articulated robot, equipped with a fiber-coupled laser diode, (right) The robot with the base slide rail and controller.
There are six stepper motors with encoders installed on the prototype robot. The first joint has a 39Ncm motor with a 10:1 gear box. The second joint has a 102Ncm motor with a 50:1 gear box. The third joint has a 39Ncm motor with a 50:1 gear box. The four joint has a 14Ncm motor with a 16:1 gear box. The fifth joint has a 44 Ncm motor with a lead screw with pitch 8 mm. The second joint has a 17Ncm motor with a 20:1 gear box. There are 6 stepper motor drivers.
The 6-DOF robotic arm was designed to capable of handling load; up to 0.5 kg. The CAD model of the robot is shown in Figure 2 (left). Under the condition, finite-element analysis simulation was conducted, as shown in Figure 2 (right), and showed the maximum deviation was less than 0.06 mm.

Figure 2. (left) CAD model of the robot, (right) finite-element analysis of the robot.

According to the three-dimensional model with a CNC machining center; the design of the mechanical arm’s structure is combination of 3D printing with special grade fiberglass reinforced plastic parts and 6061 aluminum parts using traditional mechanical machining methods. Components of the first and second joints of the robot were constructed using high-strength 3D-printed plastic. However, these components showed cracks due to residual stresses from movement. To resolve these issues, the parts were replaced with aluminum components, as the cost difference between the two materials was minimal.

3. Robot kinematics model
   The kinematics of the 6-axis articulated robot was conducted using a mathematical model of the robot; defined by the Denavit-Hartenberg parameters in Table 1. The actual DH parameters of the prototype robot were set to a robot controller; the inverse kinematics performed the transformation of the robot end-effector trajectory to the robot’s joints trajectories.

Table 1. DH parameters
joint Θ α d a
1 0.000 -1.571 169.770 64.200
2 -1.571 0.000 0.000 305.000
3 3.142 1.571 0.000 0.000
4 0.000 -1.571 222.630 0.000
5 1.571 1.571 0.000 0.000
6 0.000 0.000 41.000 0.000

The robot targets, positions and orientations, were generally predefined by robot instructions. The geometric path expresses the path of the end-effector from the start point to the end point. The planning of trajectory, can be carried out in the operational space, is a motion law that defines time according to a given geometric path. The trajectory of end-effector in position, velocity, and acceleration were converted to joint trajectory in order to control the motion of each joint. In Figure 3(A) and 3(B) show displacement trajectory and velocity trajectory of each joint when set the tool center point (TCP) of the robot to change from one target to another. It showed the acceleration time and deceleration time were both set to 0.4 seconds.

Figure 3. (A) Displacement trajectory of each joint. (B) Velocity trajectory of each joint.

4. Control Unit and Embedded System
   The robot’s control unit, Z Motion Controller, is capable to control up to 8 axes as well as 6 degrees of freedom (DOF) movement of the prototype robot. The controller, which can be developed by multiple programming languages such as LabVIEW, Python, C++, and PLC Ladder diagram, is flexible to interface with an HMI touch screen; communicates to devices via Ethernet TCP/IP. The controller also provides digital input/output, analog input/output.
   The control unit utilizes the Z Motion Controller HMI, the ZMC406 model, which is an EtherCAT 6-axis motion controller. The ZMC406 is a high-performance EtherCAT motion controller with a base configuration of 6 axes, expandable up to 32 axes, and supports various types of robotic configuration. It offers faster response times, with 6-axis encoders embedded, enabling the control of 6 to 32 axes via EtherCAT digital servo axes, as illustrated in Figure 4 (left). The HMI interface in Figure 4 (right); control system was developed by the researcher and interfaced with a custom developed laser control system unit.

Figure 4. (left) ZMC406 Motion Controller (right) HMI interface

The touch panel display or graphical user interface (GUI) is required for setting procedures with the ZMotion motion controller; supports the XPLC programming software. The Z Development operating system software facilitates this integration. The screen interface of the operating system allows an operator jogging the robot or adding instructions to the robot in positioning and movement in Figure 5 9 (left) and (right), respectively.

Figure 5. GUI of Z Motion Controller (left) jogging (right) robot programming

5. Design and Development of a Fiber-Coupled Diode Laser System
   The laser; ‘Light Amplification by Stimulated Emission of Radiation’, describes the emission of electromagnetic radiation that is amplified by light signals when electrons are excited and shifted down an energy level. Light is an electromagnetic wave, which propagates in the direction given by the equation: E (x,t) = E0  sin (t – kx) (1)

5.1. Fundamentals and Configuration of High-Power Diode Laser Technology
Theory of atomic stimulation, Albert Einstein presented in 1905, radiation occurs in the electromagnetic wave region that light radiation hits. The surface of the atom or molecule causes the absorption of the beam at the microscopic level as in equation (2). In the design of compact lasers using diode laser stacks, minimal component count and active cooling technology are essential for the most compact design and cost-effective production of modular and reliable laser.

dNabsorption = N1  u (v)  B12  dt    (2)

5.2. Design of Diode Bars and Stacks with Integration into Optical Systems and Fiber Optics
The design involved integrating diode bars and stacks, followed by combining the light into an optical system and lenses compatible with optical fiber. Figure 6 (B) illustrates the coupling of light from diode lasers into optical fibers. The light is reflected off the walls of the fiber core (total internal reflection) due to the differing refractive indices between the core and the cladding material. The maximum angle for total internal reflection (fiber acceptance angle) defines the numerical aperture (NA) of the fiber. For a NA of 0.22, calculated from a sine value of 12.71°, it is essential to compute the NA when focusing light into the optical fiber. Exceeding this angle can damage the fiber, making it crucial to ensure the angle remains within the acceptable range to avoid fiber damage. Infrared high power fiber-coupled laser diode module as shown in Figure 6 (C), courtesy of Dr. Vsevolod Mazo by Frankfurt & EIT Lasertechnik, has high power output up to 300 W, 200 um core multimode optical fiber, NA of 0.22, with fiber laser feedback protection.

(A)
(B) (C)
Figure 6. (A) high power diode laser technology, (B) coupling of light from diode lasers into optical fibers, (C) Infrared high power fiber-coupled laser diode module.
5.3. Diode Driver Circuit Design for Semiconductor Diode Lasers
The driver circuit utilized is the LDP-CW 20-50 model, as shown in Figure 7, by Picolas GmbH. The LDP-CW 20-50 laser diode driver provides high voltage and current, offering an efficient and compact solution for diode lasers. It operates in continuous wave (CW) mode with a power output of 10 kW. The output can be adjusted up to 50 kHz, allowing rapid current modulation with rise times of less than 20 µs.

Figure 7. LDP-CW 20-50 Diode Driver Circuit

In Figure 8 (lef), the LDP-CW 20-50 diode driver operates using four parallel buck converters (S1, S2, D1, D2, L1; S3, S4, D3, D4, L2; S5, S6, D5, D6, L3; S7, S8, D7, D8, L4). Each converter has an independent control loop with current sensors (Imeas1, Imeas2, Imeas3, and Imeas4). The designated current is evenly distributed among the four converters. The diode laser and driver are protected from damage by D8, which guards the laser diode against reverse current, while D7 protects the driver in case of load faults. In the event of an error, the control unit will deactivate the driver. The Soft-start mechanism gradually increases the current after the driver is activated. The current driving characteristics of the diode are illustrated in Figure 8 (right).

Figure 8. (left) Electrical Schematic Diagram of the LDP-CW 20-50,
(right) Soft-start current monitor output
5.4. Control and User Interface for the Laser System
PicoLAS; HMI Laser User Interface PLB-21 is shown in Figure 9 (left). The current control is controlled by the PI current control loop. Parameters Kp = 2400 and Ki = 2500 provided almost zero overcurrent operation. External set-point can be applied to the LDP-CW 20-50.

Figure 9. (left) PicoLAS; HMI Laser User Interface PLB-21 (right) Fiber-Coupled Diode Laser System with 300-Watt Power Output and Control Interface

5.5. Optical System Design for Laser beam focusing.

The optical system for light collection is designed using a collimating lens and a focusing lens, as depicted in Figure 10. Optical system for light collection was using a 50 mm collimating lens and a 100 mm focusing lens. The system features a beam spot diameter of 0.1 mm, with a red laser light at 635 nm and a 300-watt fiber-coupled diode laser.

Figure 10. Optical System for Light Collection
(Courtesy of Edmund Optics Inc.)
5.6. Experiments and Results for Diode Fiber-Coupled Laser

The analysis of laser beam quality was conducted using THORLAB equipment and software, as shown in Figure 10. The results of the beam shape analysis were displayed via power density and beam parameter product, in Gaussian mode. It describes the optically control the laser beam and measure these parameters by image analysis. The intensity is at the climax at the center and decreases exponentially to reach zero while moving away from the center.

Figure 11. (left) THORLAB equipment and software (right) power density or intensity of the beam shape

6. Experiments and Results of the Robotic Laser System
   6.1. Laser Welding Test
   The laser welding tests were performed with the prototype robot move the laser tool along with predefine the welding path of 30 mm displacement as shown in Figure 12. The welding tests were performed with 0.3 mm Thickness Stainless Steel Sheets with Overlapping Joints welding with 300W CW continuous mode with 80% duty cycle of power robot feed rate 25mm/s.

Figure 12. (left) Preparing the workpiece and jig-fixture (right) laser welding operation
6.2. Laser Welding Results
The welds were evaluated using shear tests, peel tests, and groove weld inspection measurements. In Figure 13 (left), the weld results from were satisfied, demonstrating the effectiveness of the combined robotic and laser systems in practical applications. Figure 13 (right), results of laser welding tests with the robot were inspected as well as examining various parameters in accordance with standards are shown in Table 2.

Figure 13. (left) weld results (right) groove weld inspection measurements

The strength test of the joint was conducted using the shear test method. The shear test and peel test were performed with the welded stainless steel work pieces. The tension test machine was applied with the shear test and peel test as shown in Figure 14(A) and Figure 14(C). The result of the shear test and peel test were 941 N and peel test 750 N., respectively; the results of the damaged workpieces are shown in Figure 14(B) and Figure 14(D).

Table 2. Results of Groove Weld Inspection Measurement.
Groove Weld Inspection Measurement Detail Measurement Value Unit
Undercut 1 0.05 mm
Undercut 2 0.03 mm
Concavity 0.08 mm
Toe Angle 1 174 Degree
Toe Angle 2 175 Degree
Reinforcement -0.05 mm
Base Width 1.3 mm
Joint Angle 0 Degree
Size Ratio (Reinforcement Height / Bead Width) 3.8 %

Figure14. (A) shear test method, (B) peel test method,
(C) shear test result, (D) peel test result

7. Conclusion
   This research focused on the design, built, and analysis of a 6-axis industrial robotic arm and a semiconductor diode laser system with fiber coupling (Diode Fiber Coupled Laser), which represents a significant technological advancement with future applications in space exploration, telecommunications, defense, medical fields, and various industries.
   In the first phase of the research, a 6-axis robotic arm was designed, constructed, and analyzed. There was inverse kinematics, finite element analysis, and structural strength evaluation. The robotic arm’s structural design combined 3D-printed parts made from glass-fiber-reinforced plastic with aluminum components manufactured using CNC machining techniques. The inverse kinematic mathematical model was developed using MATLAB to describe the movement, including position, speed, and acceleration. The Z Motion controller, programmed with Z Basic language, effectively controlled the robot and communicated with other devices via Ethernet TCP/IP, digital inputs/outputs, and analog inputs/outputs. This allowed for precise control of the laser and its power settings.
   The second part involved the design, construction, and analysis of a fiber-coupled diode laser system. This included a 300-watt diode from Frankfurt Laser GmbH, collimating lenses, and focusing lenses from II-VI. The laser control and pulse generation circuits were sourced from Picolas GmbH, a leading industry manufacturer. Key performance metrics, such as Beam Parameter Product and Beam Shape, were tested using equipment and software from THORLAB.
   In the final phase, the prototype robot was tested with the laser system. Metal sheet welding was performed, and the welds were evaluated using shear tests, peel tests, and groove weld inspection measurements. The results were satisfactory, demonstrating the effectiveness of the combined robotic and laser systems in practical applications.
   For future research, foundational work could focus on expanding applications in industrial settings. This includes designing higher-power lasers and robotic systems, and incorporating advanced laser techniques such as ultra-short pulse lasers for high-precision micro-machining. These developments would enhance the capabilities of precision manufacturing and broaden the range of industrial applications.
   On a more advanced research level, military applications could be explored. This involves using diode lasers to boost the power of solid-state lasers, enabling the generation of high-energy, concentrated beams for long-distance light transmission. Additional research could integrate radar systems for tracking airborne threats and utilize deep learning algorithms for detecting aircraft or missiles. Such advancements could also involve the development of robotic systems equipped with laser technology to engage and neutralize identified targets, improving precision and effectiveness in defense operations.
   Acknowledgments
   We would like to express our sincere gratitude to the dedicated staff at our laboratory for their invaluable support and assistance throughout this research. Their expertise, guidance, and unwavering commitment have been essential to the successful completion of this project.
   We also extend our deepest thanks to EIT Lasertechnik GmbH for their generous funding and support. Their contribution has been instrumental in advancing our research and enabling the development of innovative laser technologies. Without their support, this research would not have been possible.
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