WELDING ROBOTS - Full report

Welding technology has obtained access virtually to every branch of manufacturing; to name a few bridges, ships, rail road equipments, building constructions, boilers, pressure vessels, pipe lines, automobiles, aircrafts, launch vehicles, and nuclear power plants. Especially in India, welding technology needs constant upgrading, particularly in field of industrial and power generation boilers, high voltage generation equipment and transformers and in nuclear aero-space industry.

            Computers have already entered the field of welding and the situation today is that the welding engineer who has little or no computer skills will soon be hard-pressed to meet the welding challenges of our technological times. In order for the computer solution to be implemented, educational institutions cannot escape their share of responsibilities.     

INTRODUCTION TO AUTOMATION AND ROBOTICS

            Automation and robotics are two closely related technologies. In an industrial context, we can define automation as a technology that is concerned with the use of mechanical, electronics and computer-based systems in the operation and control of production. Examples of this technology include transfer lines, mechanized assembly machines, feed back control systems, numerically controlled machine tools, and robots. Accordingly, robotics is a form of industrial automation.

            There are three broad classes of industrial automation: fixed automaton, programmable automation, and flexible automation. Fixed automation is used when the volume of production is very high and it is therefore appropriate to design specialized equipment to process the product very efficiently and at high production rates. A good example of fixed automation can be found in the automobile industry, where highly integrated transfer lines consisting of several dozen work stations are used to perform machining operations on engine and transmission components. The economics of fixed automation are such that the cost of the special equipment can be divided over a large number of units, and resulting unit cost are low relative to alternative methods of production. The risk encountered with fixed automation is this; since the initial investment cost is high, if the volume of production turns out to be lower than anticipated, then the unit costs become greater than anticipated. Another problem in fixed automation is that the equipment is specially designed to produce the one product, and after that products life cycle is finished, the equipment is likely to become obsolete. For products with short life cycle, the use of fixed automation represents a big gamble.

            Programmable automation is used when the volume of production is relatively low and there are a variety of products to be made. In this case, the production equipment is designed to be adaptable to variations in product configuration. This adaptability feature is accomplished by operating the equipment under the control of “program” of instructions which has been prepared especially for the given product. The program is read into the production equipment, and the equipment performs the particular sequence of processing operations to make that product. In terms of economics, the cost of programmable equipment can be spread over a large number of products even though the products are different. Because of the programming feature, and the resulting adaptability of the equipment, many different and unique products can be made economically in small batches.

            There is a third category between fixed automation and programmable automation, which is called “flexible automation”. This is more suitable for the mid volume production range. It must be programmed for different product configurations, but the variety of configurations is usually non-limited than for a programmable configuration.

Of the three types of automation, robotics coincides most closely with programmable automation. An industrial robot is a general-purpose, programmable machine which possesses certain human like characteristics of present-day robots is their movable arms. The robots can be programmed to move its arm through a sequence of in order to perform some useful task. It will repeat that motion pattern over and over until reprogrammed to perform some other task. Hence the programming feature allows robots to be used for a variety of different industrial operations. Like machine loading and unloading, spot welding, continuous arc welding, spray painting etc.

            The official definition of an industrial robot provided by the Robotics Industrial Association (RIA) is as follows:” An industrial robot is a reprogrammable multifunctional manipulation designed to move materials, parts, tools or special devices through programmed motions for the performance of a variety of tasks.” 

INTRODUCTION TO WELDING

            Welding is a process of joining different materials. The large bulk of materials that are welded are metals and their alloys although welding is also applied to the joining of other materials such as thermoplastics. Welding joins different metals or alloys with help of a number of processes in which heat is supplied either electrically or by means of a gas torch.

SPOT WELDING

            As he term suggests, spot welding is a process in which two sheet metal parts are fused together at localized points by passing a large electric current using two copper electrodes, hence producing the weld. For relatively small parts a spot welding machine is used in which the parts are inserted between the pair of electrodes that are maintained in a fixed position. Where as for larger works such as in automobile bodies a portable welding gun is used which consists of a pair of electrodes and a frame to open and close the electrodes.

CONTINUOUS ARC WELDING

            Arc welding is a continuous process as opposed to spot welding which might be called a discontinuous process. Continuous arc welding is used to make long welding joints in which an air tight seal is often required between the two pieces of metals being joined. The process uses an electrode in the form of a rod or a wire of metal to supply the high electric current needed for establishing the arc. Currents are typically 100 to 300A at voltages of 10 to 30GV. The arc between the welding rod and the metal parts to be joined produces temperatures that are sufficiently high to form a pool of molten metal to fuse the two pieces together. The electrode can also be used to contribute to the molten pool, depending on the type of welding process.

            For robot applications two types of arc welding processes seems to be most practical, namely: gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). Gas tungsten arc welding is also called MIG welding for metal inert gas welding. 

WHY CONTINUOUS ROBOTIC ARC WELDING?

            Arc welding is performed by skilled workers who are assisted by a person called fitter. The purpose of the fitter is to organize the work and fixture the parts of the welder. The working condition of the welder is typically unpleasant and hazardous. The arc from the welding process emits ultra-violet radiations which is injurious to human vision. As a result welders are required to wear eye protection in the form of a welding helmet with a dark window. The dark window filters out the dangerous, but it so dark that the welder is virtually blind while wearing the helmet except when the arc is struck. Other aspects of the process are also hazardous. The high temperature created in arc welding and the resulting molten metals are inherently dangerous. The high electric current used to create the arc is also unsafe. Sparks and smoke are generated during the process are a potential threat to operators. Because of the hazards for human workers in continuous arc welding, it is logical to consider industrial robots for the purpose.

BENEFITS OF ROBOT ARC WELDING

1. HIGHER PRODUCTIVITY

   

            Factors that contribute to the increased rate when robots used in batch production is the elimination of fatigue factor. Robots do not experience fatigue in the sense that human workers do. A robot can continue to operate in the entire shift with need of periodic rest breaks.

2. IMPROVED SAFTEY AND QUALITY-OF-WORK LIFE

            Improved safety and quality-of-work environment result from removing the human operator from an uncomfortable, fatiguing and potentially dangerous work situation.

3. GREATER QUALITY OF PRODUCT

            Greater product quality in robot arc welding results from the capability of the robot to perform the welding cycle with accuracy and repeatability than its human counterpart. This translates into a more consistent welding seam; one that is free of the start-and-stop builds up of filler metal in the seam that is the characteristic of many welds accomplished by human welders.

FEATURES OF ARC WELDING ROBOTS

An industrial robot that performs welding must possess certain features and capabilities. Some of the technical considerations in arc welding applications are discussed in the following.

1.      WORK VOLUME AND DEGREES OF FREEDOM

            The robot’s work volume must be large enough for the size of the parts to be welded. A sufficient allowance must be made for the manipulation of the welding torch. Five or six degrees of freedom are generally required for arc welding robots. The number is influenced by the characteristics of the welding job and motion capabilities of the parts manipulator. If the parts manipulator has two degrees of freedom, this tends to reduce the requirement on the number of degrees of freedom possessed by the robot.     

2. MOTION CONTROL SYSTEM

            Continuous path control is required for arc welding. The robot must be capable of smooth continuous motion in order to maintain uniformity of welding seam.

3. PRECISION OF MOTION

            The accuracy and repeatability of the robot determines to a large extend for the quality of welding job. The precision requirements of welding job vary according to size and industry purpose, and these requirements should be defined by each individual user before selecting the most appropriate robot.

4. INTERFACE WITH OTHER SYSTEM

            The robot must be provided with sufficient input/output and control capabilities to work with other equipments in the cell. These other pieces of equipments are automobile fixturing units, conveyors, and parts of positioners. The cell controller unit must co-ordinate the path and path of robot with operation of parts manipulator and the welding parameters such as wire feed rate and power level.

5. PROGRAMMING

            Programming the robot for continuous arc welding must be considered carefully. To facilitate the input of the program for welding paths with irregular shapes; it is convenient to use the walk through method in which the robot wrist is physically moved through its motion path. For straight welding paths, the robot should possess the capability for linear interpolation between two points in the space. This permits the programmer to define the beginning and points of the path the robot is capable of computing the straight trajectory between the points.

PROBLEMS FOR ROBOTS IN ARC WELDING

1.      A related problem is that arc welding is often performed in confined areas that are difficult to access, such as insides of tanks, pressure vessels, and ship hulls. Humans can position in to these areas more readily than robots.

2.      One of the most difficult technical problems is the variation in the dimensions of the parts in a batch production job. This type of dimensional variations means that the arc-welding path to be followed will change slightly from part to part.

3.      Another technical difficulty is the variations in the edges and surfaces to be welded together. Instead of being straight and regular, the edges are typically irregular. This causes variations in the gap between the parts and other problems in the way the pieces mate together prior to the welding process.

      

            Human welders are able to compensate for both these variations by certain parameters in the welding process. Industrial robots provided with sensors to monitor the variations in the welding process and the control logic to compensate for part and weld gap irregularities.

WHY ROBOT SPOT WELDING?

          For larger works on spot welding the welding guns with cables attached is quite heavy and can easily exceed 100lb in weight. To assist the operator in manipulating the gun, the apparatus is suspended from an overhead hoist system. Even with this assistance, the spot-welding gun represents a heavy mass and is difficult to manipulate by a human worker at high rates of production desired on a car body assembly line. There are often problems with the consistency of the welded products made on such a manual line as a consequence of this difficulty.

            As a result of these difficulties robots have been employed with great success on this type of production line to perform some or all of the welding operations. A welding gun is attached as the end effector to each robot’s wrist, and the robot is programmed to perform a sequence of welds on the product as it arrives at the workstation. Some robot spot-welding lines operate with several dozens of robots all programmed to perform different welding cycles on the product. Today, the automobile manufacturers make extensive use of robots for spot-welding.

BENEFITS OF ROBOT SPOT WELDING

1. IMPROVED PRODUCT QUALITY

               

            Improved quality is in the form of more consistent welds and better repeatability in the location of welds. Even robots with relatively unimpressive repeatability specifications are able to locate the spot welds more accurately than human operators.

2. OPERATOR SAFETY

            Improved safety results simply because the human is removed from the work environment where there are hazards from electrical shocks and burns.

3. BETTER CONTROL OVER PRODUCTION OPERATION

            The use of robots to automate the spot welding process should also result in improvements in area such as production scheduling and in process inventory control.

            The maintenance of the robots and welding equipment becomes an important factor in the successful operation of an automated spot welding production line.

FEATURES OF SPOT-WELDING ROBOTS

1.      Robots must be relatively large. It must have sufficient payload capacity to readily manipulate the welding gun for the application.

2.      The work volume must be adequate for the size of the product.

3.      The robot must be able to position and orient the welding gun in places on the product that might be difficult to access. This might result in need for an increased number of freedoms.

4.      The controller memory must have enough capacity to accomplish the many positioning steps required for the spot-welding cycle. In some applications, the welding line is designed to produce several different models of the product. Accordingly, the robot must be able to switch from one programmed welding sequence to another as the models change.

CONCLUSION

            A substantial opportunity exists in the technology of robotics to relieve people from boring, repetitive, hazardous and unpleasant work in all forms of a human labour. There is a social value as well as a commercial value in pursuing this opportunity. The commercial value of robotics is obvious. Properly applied, robots can accomplish routine, undesirable work better than humans at a lower cost. As the technology advances, and more people learn how to use robots, the robotics market will grow at a rate that will approach the growth of the computer market over the past thirty years. One might even consider robotics to be a mechanical extension of computer technology.

            The social value of robotics is that these wonderfully subservient machines will permit humans more time to do work that is more challenging, creative, conceptual, constructive and co-operative than at present. There is every reason to believe that the automation of work through robotics will lead to substantial increases in productivity, and that these productivity increases year by year will permit humans to engage in activities that are cultural and recreational.

Not only will robotics improve our standard of living, it will also improve our standard of life.   

REFERENCES

1. Robotics by Fu, Gonzalez, LEE
2. Robot Technology  by James G. Keramas
3. Industrial Robotics by Mikell P. Groover, Nicholas G. ODREY, MITCHEL Weiss
4. www.faculty.washington.edu/robot_weld_engl.html
5. www.thetech.mit.edu/professional_robot.html