Robot welding

1.  Robot welding

When should robots be used for welding?

             A welding process that contains repetitive tasks on similar pieces might be suitable for automation. The number of items of any type to be welded determines whether automating a process or not. If parts normally need adjustment to fit together correctly, or if joints to be welded are too wide or in different positions from piece to piece, automating the procedure will be difficult or impossible. Robots work well for repetitive tasks or similar pieces that involve welds in more than one axis or where access to the pieces is difficult. 

Why robot welding? 

             The most prominent advantages of automated welding are precision and productivity. Robot welding improves weld repeatability. Once programmed correctly, robots will give precisely the same welds every time on workpieces of the same dimensions and specifications.

            Automating the torch motions decreases the error potential which means decreased scrap and rework. With robot welding you can also get an increased output. Not only does a robot work faster, the fact that a fully equipped and optimized robot cell can run for 24 hours a day, 365 days a year without breaks makes it more efficient than a manual weld cell. 

           Another benefit of automated welding is the reduced labor costs. Robotic welding also reduces risk by moving the human welder/operator away from hazardous fumes and molten metal close to the welding arc.

What welding processes are suitable for robot welding?

           Most production welding processes can be used in automated applications. The most popular, used in perhaps 80 percent of applications, is the solid wire GMAW process. This process is best for most high production situations because no postweld cleanup is required.

2. WELDING PROCESSES

         Welding is the most economical and efficient way to join metals permanently. Welding is used to join all of the commercial metals and to join metals of different types and strengths.

        A weld is produced either by heating the materials to the welding temperature with or without the application of pressure alone with or without the use of filler metal. There are different kinds of welding processes who all use different sources of heat, for instance arc welding which uses an electric arc as a heat source. Another commonly used welding process is spot welding (resistance welding).  

      Welding is considered to be the most complex of all manufacturing technologies. In order to transform welding from a manual operation to an automated production process, it is necessary to understand the scientific principles involved.

2.1. ROBOT ARC WELDING

       Robot welding means welding that is performed and controlled by robotic equipment. In general equipment for automatic arc welding is designed differently from that used for manual arc welding. Automatic arc welding normally involves high duty cycles, and the welding equipment must be able to operate under those conditions. In addition, the equipment components must

have the necessary features and controls to interface with the main control system. 

      A special kind of electrical power is required to make an arc weld. A welding machine, also known as a power source, provides the special power. All arc-welding processes use an arc welding gun or torch to transmit welding current from a welding cable to the electrode. They also provide for shielding the weld area from the atmosphere. 

     The nozzle of the torch is close to the arc and will gradually pick up spatter. A torch cleaner (normally automatic) is often used in robot arc welding systems to remove the spatter. All of the continuous electrode wire arc processes require an electrode feeder to feed the consumable electrode wire into the arc.

    Welding fixtures and workpiece manipulators hold and position parts to ensure precise welding by the robot. The productivity of the robot-welding cell is speeded up by having an automatically rotating or switching fixture, so that the operator can be fixing one set of parts while the robot is welding another.

To be able to guarantee that the electrode tip and the tool frame are accurately known with respect to each other, the calibration process of  the TCP (Tool Center Point) is important. An automatic TCP calibration device facilitates this time consuming task.

TYPICAL COMPONENTS OF AN INTEGRATED ROBOTIC ARC-WELDING CELL:

1. Arc welding robot
2. Power source
3. Welding torch
4. Wire feeder
5. Welding fixtures and work piece positioners

6. Torch cleaner

7. TCP calibration unit

2.1.1 ARC WELDING ROBOT

      During the short time that industrial welding robots have been in use, the jointed arm or revolute type has become by far the most popular. For welding it

has almost entirely replaced the other types except for the Cartesian, see (robot kinematics), which is used for very large and very small robots. The reason for the popularity of the jointed arm type is that it allows the welding torch to be manipulated in almost the same fashion as a human being would manipulate it. The torch angle and travel angle can be changed to make good quality welds in all positions. Jointed arm robots also allow the arc to weld in areas that are difficult to reach. Even so, a robot cannot provide the same manipulative motion as a human being, although it can come extremely close. In addition, jointed arm robots are the most compact and provide the largest work envelope relative to their size. Usually arc-welding robots have five or six free programmable arms or axes.                                                           

      Off-the-shelf programmable robot arms are today available from different suppliers such as ABB, FANUC, PANASONIC, KUKA, MOTOMAN.

2.1.2 ARC WELDING POWER SOURCES

      A welding power source must deliver controllable current at a voltage according to the requirements of the welding process. Normally, the power required is from 10 to 35 V and from 5 to 500 A. The various welding processes and procedures have specific arc characteristics that demand specific outputs of the welding machine.

      Automatic arc welding machines may require power sources more complex than those used for semi-automated welding. An automatic welding machine

usually electronically communicates with the power source to control the welding power program for optimum performance. A power source for arc welding is designed to provide electric power of the proper values and characteristics to maintain a stable arc suitable for welding. 

      There are three types of arc welding power sources, distinguished according to their static characteristics output curve. The constant-power (CP) is the conventional type of power source that has been used for many years for shielded metal arc welding using stick electrodes. It can be used for submerged arc welding and gas tungsten arc welding.  The constant-voltage (CV) power source is the type normally used for gas metal arc and flux cored arc welding using small-diameter electrode wire. The constant-current (CC) power source is normally used for gas tungsten arc and plasma arc welding. 

      The selection of a welding power source is based on

1.      The process or processes to be used

2.      The amount of current required

3.      The power available at the job site

4.      Economic factors and convenience

                                  

 

  2.1.3 WELDING TORCH

      A welding torch is used in an automatic welding system to direct the welding electrode into the arc, to conduct welding power to the electrode, and to provide shielding of the arc area. There are many types of welding torches, and the choice depends on the welding process, the welding process variation, welding current, electrode size and shielding medium

      Welding torches can be categorized according to the way in which they are cooled. They may be water-cooled with circulating cooling water or air-cooled with ambient air. A torch can be used for a consumable electrode welding process such as gas metal arc or flux cored arc welding, and shielding gas may or may not be employed. 

         

         A torch can be described according to whether it is a straight torch or has a bend in its barrel. A torch with a bend is often used for robotic arc welding applications to provide access for the weld.

     

      The major function of the torch is to deliver the welding current to the electrode. For consumable electrode process this means transferring the current to the electrode as the electrode moves through the torch. 

      A second major task of the torch is to deliver the shielding gas, if one is used, to the arc area. Gas metal arc welding uses a shielding gas that may be an active gas usually carbon dioxide or a mixture of an inert gas, normally argon, with CO₂ or oxygen.

      The welding torch is mounted to the robot flange with a matching mounting arm. Preferably an anti collision clutch is used to prevent damages on expensive weld equipment in case of sticking electrode and crashes during installation and start-up.

2.1.4 WIRE FEEDER

      Wire feeders are used to add filler metal during robotic welding. This allows flexibility in establishing various welding wire feed rates to suit specific requirements for an assembly. Normally, the wire feeder for robotic welding is mounted on the robot arm, separate from the power supply. For robotic welding, a control interfaces between the robot controllers, the power supply and wire

feeder is needed. The wire feeding system must be matched to the welding process and the type of power source being used. 

      There are two basic types of wire feeders. The first type is used for the consumable electrode wire process and is known as an electrode wire feeder. The electrode is part of the welding circuit, and the melted metal from the electrode crosses the arc to become the weld deposit. There are two different types of electrode wire feeders. The constant-power power source requires a voltage-sensing wire feed system in which the feed rate may be changing continously. The constant-voltage system requires a constant feed rate during the welding operation.

      The second type of wire feeder is known as a cold wire feeder and is especially used for gas tungsten arc welding. The electrode is not part of the circuit, and the filler wire fed into the arc area melts from the heat of the arc and becomes the weld metal.

 

 

 

 

2.1.5 WORKPIECE FIXATION AND POSITIONING

      In order to join parts successfully in a robotic welding application, individual parts must be aligned precisely and held securely in place while the welding is proceeding. An important consideration, then, is the design of a fixture which holds the individual parts in the proper alignment. The tool must allow for quick and easy loading, it must hold the parts in place securely until they are welded together and must allow the welding gun unrestricted access to each weld point. 

One starting point for positioning the workpiece for robotic welding may be the fixture already used for manual welding even though specialized positioners are used to improve the versatility and to extend the range of robotic arc welding systems. The usable portion of a robot work envelope can be limited becuse the

welding torch mounting method does not allow the torch to reach the joint properly. Special positioners eliminate some of these limitations by making the workpiece more accessible to the robot welding torch. 

      The positioners used with robots also have to be more accurate than required for manual or semiautomatic welding. In addition the robot positioner controls must be compatible and controllable by the robot controller in order to have simultaneous coordinated motion of several axes while welding.

However, loading and unloading stationary jigs of the robot cell can be time consuming and impractical. It is often more efficient to have two or more fixtures on a revolving workpiece positioner, despite a higher initial cost. With a revolving table for instance, the operator can load and unload while the robot is welding. Obviously, this speeds up the process and keeps the robot welding as much of the time as possible.

2.1.6 TORCH CLEANER

      Periodic cleaning of arc welding guns is required for proper and reliable operation of robotic arc welding equipment. The high duty cycle of an automatic operation may require automated gun cleaning. Systems are available that spray an antispatter agent into the nozzle of the gun. Additionally, tools that ream the nozzle to remove accumulated spatter and cut the wire are available. The cleaning system is automatically activated at required intervals by the welding control system. 

2.1.7 TCP-CALIBRATION UNIT

      End-of-arm sensor and tool centre point calibration is a critical aspect of successful system implementation. End-of-arm sensing, in the context of robotic welding, is used to detect the actual position of the seam on the workpiece with respect to the robot tool frame. 

      Analysis of the profile data yields the relative position of the seam with respect to the sensor reference frame. If the sensor reference frame pose is known with respect to the end-frame of the robot, and the tool frame pose is known with respect to the end-frame, then the sensor data may be used to accurately position the tool centre point (TCP) with respect to the workpiece.

      While end-of-arm sensor based control would appear to solve both robot accuracy and workpiece position error problems, this is only so if the sensor frame, end frame, and tool frame are accurately known with respect to each other. 

      Should the sensor be accidentally knocked out of position, the robot system becomes a highly consistent scrap production facility. Indeed, this very concern has been one of the reasons why some companies that would benefit from a sensor based correction system have been reluctant to implement such a system. What is required is not only a technique that enables the frames to be automatically calibrated, but that also enables the system to quickly determine if recalibration is necessary. This second capability is perhaps the more important in practice, since it can be reasonably assumed that any calibration error will be caused by an unanticipated event that could occur during any welding cycle. 

 

 

 

2.2 ROBOT SPOT WELDING

      Automatic welding imposes specific demands on resistance welding equipment. Often, equipment must be specially designed and welding procedures developed to meet robot welding requirements. 

      The spot welding robot is the most important component of a robotized spot welding installation. Welding robots are available in various sizes, rated by payload capacity and reach. The number of axes also classifies robots.  A spot welding gun applies appropriate pressure and current to the sheets to be welded. There are different types of welding guns, used for different applications, available.  An automatic   weld-timer initiates and times the duration of current. 

  

      During the resistance welding process the welding electrodes are exposed to severe heat and pressure. In time, these factors begin to deform (mushroom) the electrodes. To restore the shape of the electrodes, an automatic tip-dresser is used. 

      One problem when welding with robots is that the cables and hoses used for current and air etc. tend to limit the capacity of movement of the robot wrist. A solution to this problem is the swivel, which permits passage of compressed air, cooling water, electric current and signals within a single rotating unit. The swivel unit also enables off-line programming as all cables and hoses can be routed along defined paths of the robot arm.

TYPICAL COMPONENTS OF AN INTEGRATED ROBOTIC SPOT WELDING CELL:

      1) Spot welding robot
      2) Spot welding gun
      3) Weld timer
      4) Electrode tip dresser
      5) Spot welding swivel

 

2.2.1 SPOT WELDING ROBOT

      A robot can repeatedly move the welding gun to each weld location and position it perpendicular to the weld seam. It can also replay programmed welding schedules. A manual welding operator is less likely to perform as well because of the weight of the gun and monotony of the task. 

   

  Spot welding robots should have six ore more axes of motion and be capable of approaching points in the work envelope from any angle. This permits the robot to be flexible in positioning a welding gun to weld an assembly. Some movements that are awkward for an operator, such as positioning the welding gun upside down, are easily performed by a robot. 

2.2.2 SPOT WELDING GUNS

Spot welding guns are normally designed to fit the assembly. Many basic types of guns are available, the two most commonly used being the direct acting type, generally known as a “C”-type gun, where the operating cylinder is connected directly to the moving electrode, and the “X”-type (also known as "Scissors" or "Pinch") where the operating cylinder is remote from the moving electrode, the force being applied to it by means of a lever arm. C guns are generally the cheapest and the most commonly used. There are many variations available in each basic type with regard to the shape and style of the frame and arms, and also the duty for which the gun is designed with reference to welding pressure and current. 

 Pneumatic guns are usually preferred because they are faster, and they apply a uniform electrode force. Hydraulic spot welding guns are normally used where space is limited or where high electrode forces are required

2.2.3 WELD TIMER

      An automated spot welding cell needs control equipment to initiate and time the duration of current. A spot weld timer (weld control unit) automatically controls welding time when spot-welding. It also may control the current magnitude as well as sequence and time of other parts of the welding cycle.

2.2.4 ELECTRODE TIP DRESSER

      The function of the electrodes is to conduct the current and to withstand the high pressures in order to maintain a uniform contact area and to ensure the continued proper relationship between selected current and pressure. Uniform contacting areas should therefore be maintained. 

      Good weld quality is essential and depends, to a considerable degree, upon uniformity of the electrode contact surface. This surface tends to be deformed (mushroomed) with each weld. Primary causes for mushrooming are too soft electrode material, too high welding pressure, too small electrode contact surface, and most importantly, too high welding current. These conditions cause excessive heat build-up and softening of electrode tips. Welding of today’s coated materials also tends to contaminate the face of the electrodes. 

      As the electrode deforms, the weld control is called upon to "step" up the welding current in order to compensate for "mushroomed" weld tips. Eventually, the production line will have to be shut down in order to replace the electrodes or to manually go in and hand dress the electrodes. This process will improve the weld cycle but in either case, the line is stopped and time is lost. Furthermore the deformed electrodes have caused unnecessary high consumption of energy and electrodes.

      In automatic tip dressing, a tip dresser is mounted on the line where it can be accessed by the welding robot. The robot is programmed to dress the electrodes at regular time intervals. The dressing can be done after each working cycle, after every second cycle, and so on. It depends upon how many spot-welds are done in each cycle. For welding in galvanized sheet, dressing after about 25 spot-welds is recommended. The dressing takes approximately 1 to 2 seconds, and is performed when the work pieces are loaded, unloaded and transported.  Maintaining proper electrode geometry minimizes production downtime and utility costs and increases weld efficiency.

2.2.5 SPOT WELDING SWIVEL

      A major advancement in resistance spot welding is the swivel. This unit permits passage of compressed air, cooling water, electric current and signals through different channels within a single rotating unit. 

   This invention greatly improves total efficiency of robotic spot-weld installations. Electrical connection between swivel and transformer is minimal thus permitting maximum utilization of access to spot-weld areas.

Basic advantages are:

·         Less work space needed -No mass of cables and hoses hanging from the robot arm, resulting in floor space economy.

·         Improved accessability - Since no limitation on the robot wrist caused by any cables or hoses.

·         Improved safety - Greatly improved safety factors through reduction of air, electric and water lines; now limited to quick-connect piping, and hoses within robot arm.

·         Saving in capital equipment - Compact weld-gun assembly accessable to areas formly blocked by transformer, cables, and control boxes. More welds per station means big savings through fewer work stations and less capital equipment.

      Reduced try-out costs - No un-defined cables exist on the robot, which reduces programming time to minimum. True off-line programming is now a real. The swivel, which fits directly onto the weld-gun fixture plate without any hoses or cables, ensures the highest quality condition of the spot weld. No electrical degeneration on cables and no hoses that wear.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.3 LASER ROBOT WELDING

      Today, there are more and more three-dimensional welding applications. Typical of many is the welding of roofs in the automobile sector. Here, the focusing unit of the laser is mounted on a 6-axis buckling arm robot, which executes the movements in space. Most frequently used are Nd: YAG lasers, which allow flexible application of the laser light through optical fibers. But CO₂ lasers combined with flexible mirror movement  can also be used.

This is how bodies are created in car construction that are significantly stiffer in case of a crash, for example, and thus provide greater safety for passengers.

Furthermore, laser welding always requires access from one side only, so newdesigns are now possible that could not nave been implemented by means of traditional resistance spot welding.

 

 

 

3. Welding Safety

      Welding is an established manufacturing process with known potential hazards. Potential safety hazards associated with arc welding include arc radiation, air contamination, electrical shock, fire and explosion, compressed gases, and other hazards. Robots were originally designed to perform the job functions of a human. They were designed to relieve humans of the drudgery of unpleasant, fatiguing, or repetitive tasks and also to remove humans from a potentially hazardous environment. In this regard, robots can replace humans in the performance of dangerous jobs and are considered beneficial for preventing industrial accidents. On the other hand, robots have caused fatal accidents.

      The introduction of robots requires appropriate safety features in order to protect both those working directly with the robot and others in the workshop who may not be aware of its potential dangers. This can be provided in a number of ways.

      One of the best solutions for robot safety is to purchase a complete welding cell from a robotic integrator. A complete cell includes barriers, all necessary safety devices, and a method of loading and unloading the workstation.

      Each robot installation must be carefully planned from safety viewpoint to eliminate hazards. When the robot is in operation it is necessary that people remain outside the work envelope. Barriers or fences should be in place around the robot. All doors and maintenance openings must be protected by safety switches, and the weld areas must be safe guarded so that the power is immediately removed from the robot when a door is opened.. Emergency stop buttons should be placed on all operator panels, robot cabinets and robot programming panels. Barriers must be designed to completely surround the robot and eliminate the possibility of people climbing over or under to get inside the barrier. Signal lights must be arranged on the robot or in the robot area to indicate that the robot is powered. 

 

 

 

 

4. ADVANTAGES IN USING WELDING ROBOT    

      At present relatively few figures are available on the economics of robot Welding machines, but it has been found that numbers of components produced by A robot are 2.5 to 3.5 times greater than that produced manually over the same Span of the time. It can be said that for an output of more than 100 parts/month which takes two or three shift per day there is an increase in number of parts output without difference in quality, which is not necessarily so with manual shift   work.

      Use of robot welding increases the flexibility. Because it is easy to change the robot work from to another just by changing the program. When the same time of work is already done, the same programme can be fed and the time and cost of programming can be eliminated completely.

      Day by day the cost of welding consumable are increasing. Using robots by Slightly changing the edge preparations from normal gap to narrow gap welding lot of consumable can be served with improved weld quality (decrease in grain size, distortion). In addition to increase the productivity it maintains the desired quality throughout the reducing the rework scrap.

      It reduces welder fatigue and welder exposure to the more hazardous atmosphere.