Welding robots are industrial robots engaged in welding( including cutting and spraying).  According to the International Organization for Standardization (ISO)Industrial Machines Man is defined as a standard welding robot, an industrial robot is a versatile, programmable, automatic control operator(Manipulator)with three or more programmable axes  for industrial  automation. To accommodate different uses, the robot’s last shaft has a mechanical interface, usually a connecting flange, which can be fitted with different tools or end actuators.  Welding robots are industrial robots whose last-axis flanges are fitted with welding pliers or welding (cutting) guns so that they can be welded, cut or hot-sprayed.

With the development of electronic technology, computer technology, numerical control and robotics technology, automatic welding robots, since the 1960s began to be used in production, its technology has become increasingly mature, mainly has the following advantages:

1) Stabilize and improve the welding quality, can reflect the welding quality in numerical form;

2) Improve labor productivity;

3) Improve the labor intensity of workers, can work in harmful environments;

4) Reduce the requirements for workers’ operating skills;

5) Shorten the preparation cycle of product modification and change, reduce the corresponding equipment investment.

Therefore, in all walks of life has been widely used.

Welding robot mainly includes two parts: robot and welding equipment. The robot consists of a robot body and control cabinet (hardware and software). Welding equipment, taking arc welding and spot welding as an example, is composed of welding power supply( including its control system), wire feeder (arc welding), welding gun (clamp) and so on. For intelligent robots, there should also be sensing systems, such as laser or camera sensors and their controls.

Welding robot diagram

Welding robots produced around the world are basically joint robots, the vast majority of which have six axes. Among them, 1, 2, 3 axes can send the end tool to different spatial positions, while 4, 5, 6 axis to solve the different requirements of tool posture. There are two main forms of mechanical structure of welding robot body: one is a parallelogram structure and the other is a side-mounted (swing) structure. The main advantage of the side-mounted (swing) structure is the large range of activities of the upper and lower arms, which enables the robot’s working space to reach almost a sphere. As a result, the robot can work upside down on racks to save floor space and facilitate the flow of objects on the ground. However, this side-mounted robot, 2 and 3 axes for the cantilever structure, reduce the stiffness of the robot, generally suitable for smaller load robots, for arc welding, cutting or spraying. The parallelogram robot’s upper arm is driven by a lever. The lever forms two sides of a parallelogram with the lower arm. So it’s named. The early development of parallelogram robot workspace is relatively small (limited to the front of the robot), it is difficult to hang upside down work. However, the new parallelogram robot (parallel robot) developed since the late 1980s  has been able to extend the workspace to the top, back and bottom of the robot, without the stiffness of the measuring robot, so it has been widely paid attention to. This structure is suitable not only for light but also for heavy-duty robots. In recent years, spot welding robots (load 100 to 150 kg) mostly choose parallelogram structure form robots.

Each of the shafts of the above two robots is used for swing motion, so the servo motor is driven by a swing needle wheel (RV) reducer (1 to 3 axes) and a harmonic reducer (1 to 6 axes). Before the mid-1980s, electric-driven robots were powered by DC servo motors, and since the late 1980s, countries have switched to AC servo motors.  Because AC motors do not have carbon brushes, good dynamic characteristics, so that the new robot not only low accident rate, but also maintenance-free time greatly increased, plus (minus) speed is also fast. Some new lightweight robots with loads of less than 16 kg have a maximum motion speed of more than 3m/s at their tool center point (TCP), accurate positioning and low vibration. At the same time, the robot’s control cabinet also used a 32-bit microcomputer and a new algorithm, so that it has the function of optimizing the path itself, running the trajectory closer to the trajectory of teaching.

peculiarity

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Spot welding is not very demanding on welding robots. Because spot welding only needs point control, as for the welding pliers between the point and the point of movement trajectory is not strict requirements, which is the robot can only be used for spot welding at the earliest reason. Spot welding robot not only has sufficient load capacity, but also in the point-to-point shift speed is fast, the action should be smooth, the positioning should be accurate, in order to reduce the shift time, lift

High productivity. How much load capacity a spot welding robot requires depends on the form of the welding clamp used. For welding pliers separated from transformers, a 30 to 45 kg load of robots is sufficient. However, on the one hand, this kind of welding clamp is due to the long secondary cable line, the power loss is large, it is not conducive to the robot to weld the welding pliers into the inside of the workpiece, on the other hand, the cable line swings with the robot movement, the cable damage is faster. Therefore, the use of integrated welding pliers is gradually increasing. This welding clamp, along with the transformer, has a mass of about 70 kg. Considering that the robot should have sufficient load capacity, welded pliers to a space position for welding at a large acceleration, heavy-duty robots with a load of 100 to 150 kg are generally selected. In order to meet the requirements of short-distance rapid displacement of weld clamps during continuous spot welding. The new heavy-duty robot adds the ability to complete 50mm displacement in 0.3s. This puts forward higher requirements for the performance of the motor, the computing speed and the algorithm of the microcomputer.

Structural design

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Because the design of welding robot is in the quasi-plane, narrow space environment, in order to ensure that the robot can track the welding of the weld according to the deviation information of the arc sensor, the robot should be designed compact, flexible movement and stable work. In view of the characteristics of narrow space, a small mobile welding robot is developed, according to the motion characteristics of each structure of the robot, using modular design method, the robot mechanism is divided into three parts: wheeled mobile platform, torch adjuster and arc sensor. Among them, wheeled mobile platform because of its inertia, slow response, mainly on the weld rough tracking, the torch adjustment mechanism is responsible for the precise tracking of the weld, arc sensor to complete the weld deviation real-time identification. In addition, the robot controller and motor driver are integrated on the robot mobile platform, making it smaller. At the same time, in order to reduce the influence of dust on moving parts in harsh welding environment, a fully enclosed structure is used to improve the reliability ^of its system.

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The welding equipment of the spot welding robot, because of the use of integrated welding pliers, welding transformers installed behind the welding pliers, so the transformer must be as small as possible. For smaller transformers can use 50Hz frequency AC, and for larger transformers, inverter technology has been used to change 50Hz frequency AC into 600 to 700Hz AC, so that the size of the transformer is reduced and reduced. After the variable pressure can be directly with 600 to 700Hz AC welding, can also be re-rectified, with DIRECT welding. Welding parameters are adjusted by the timer.  The new timer has been microcomputed, so the robot control cabinet can directly control the timer without the need for an additional interface. Spot welding robot welding pliers, usually with pneumatic welding pliers, pneumatic welding pliers between the two electrodes of the opening degree is generally only two strokes. And once the electrode pressure is adjusted, it cannot be changed at will. In recent years, a new type of electric servo spot welding clamps has appeared. The opening and closing of the welding pliers is driven by a servo motor, and the code plate feedback allows the opening of the pliers to be arbitrarily selected and preset according to actual needs. And the pressure force between the electrodes can also be adjusted without stage. This new electric servo spot welder has the following advantages:

1) The welding cycle of each welding point can be greatly reduced, because the degree of opening of the welding pliers is precisely controlled by the robot, the robot between the point and the point of movement process, welding pliers can start to close;

2) The opening degree of the welding clamp can be adjusted according to the condition of the workpiece, as long as there is no collision or interference to minimize the degree of opening, in order to save the opening degree of the welding clamp, in order to save the time occupied by the opening and closing of the welding clamp.

3) When the welding clamps are closed and pressurized, not only the pressure size can be adjusted, but also when closed, the electrodes are gently closed, reducing impact deformation and noise.

Spot welding robot FANUC R-2000iB

Welding applications

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