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A Robot is a reprogrammable, multifunctional manipulator designed to move parts, materials, tools, or special devices through variable programmed motions for the performance of a variety of different tasks. The industrial robot is a tool that is used in the manufacturing environment to increase productivity. It can perform jobs that might be hazardous to the human workers. Many of the applications for the robot are in the industrial area. In the past, robots were used primarily in applications where human risk factors were very high. Today, robots find many other applications, such as Material handling, Machine loading and unloading, Die casting, Welding, Inspection, Assembly, Spray painting, etc.
1.1 Robotic Welding
There are two popular types of industrial welding robots. The two are articulating robots and rectilinear robots. Robotics controls the movement of a rotating wrist in space. Descriptions of some of these welding robots are described below:
Rectilinear robots move in line in any of three axes (X, Y, Z). In addition to linear movement of the robot along axes there is a wrist attached to the robot to allow rotational movement. This creates a robotic working zone that is box shaped.
Articulating robots employ arms and rotating joints. These robots move like a human arm with a rotating wrist at the end. This creates an irregularly shaped robotic working zone.
1.2 Considerations of a robotic welding:
There are many factors that need to be considered when setting up a robotic welding facility. Robotic welding needs to be engineered differently than manual welding. Some of the considerations for a robotic welding facility are listed below:
Number of axes
Seam tracking systems
Arc welding equipment
1.3 Planning for Robotic Welding:
A robotic welding system may perform more repeat ably than a manual welder because of the monotony of the task. However, robots may necessitate regular recalibration or reprogramming.
Robots should have the number of axes necessary to permit the proper range of motion. The robot arm should be able to approach the work from multiple angles.
Robotic welding systems are able to operate continuously, provided appropriate maintenance procedures are adhered to. Continuous production line interruptions can be minimized with proper robotic system design. Planning for the following contingencies needs to be completed:
Rapid substitution of the inoperable robots.
Installing backup robots in the production line
Redistributing the welding of broken robots to functioning robots.
1.4 The challenges of robotic MIG welding:
When compared to steel, aluminum and its alloys are more difficult to weld. The most significant obstacle for producing aluminum TWBs is the under developed technology for welding aluminum, and the lack of knowledge about the formability of different aluminum alloys. Commonly faced welding difficulties include these:
Excessive porosity and hot cracking of the fusion zone
Poor coupling during welding because of the high reflectivity of the metal
Degradation of strength, and loss of alloying elements in the fusion zone
Recent studies suggest that robotic MIG welding is most likely to be successful.
Robotic MIG welding of aluminum alloys has been an industry challenge for many years. Early attempts made to automate this process typically failed. The failure of early automated cells was associated with a lack of process experience or improper equipment selection. This failure rate put a "high risk" label on robotic aluminum applications, so many have avoided the practice of automating MIGW.
2.0 The basic components of a robotic MIGW system:
Fig.1.Basic configuration of a typical robotic MIGW system
The figure1 shows the basic configuration of a typical robotic MIGW system 1-Quick-change Torch, 2-Servo Torch Four-roll rive, 3-Utility Cable, 4-Utility Junction Box, 5-Insulated Wire Conduit, 6-Welding Wire,7-Digital Weld Power Supply, 8-Gas Supply, 9-Robot Connection Cables, 10-Robot Arm, 11-Work Lead.
2.1 Key issues and controls of Robotic MIGW:
2.1.1 Weld Power Source Interfacing:
The weld power source should be coupled with the robot directly through a digital network. The robot becomes the interface to the weld equipment, offering a single point of control to adjust parameters and eliminating the need for additional programming devices.
With a digital network, commanded outputs are absolute. The time it takes for the robot to talk to the power source is minimized, making on-the-fly process changes almost instantaneous.
For the operator, these can mean a decrease in setup time, an increase in integration efficiency, and accurate process control. Accurate process control is critical when welding aluminum. The objective should be to have maximum process control through a single source, the robot.
2.1.2 Wire Drive Design:
The wire drive design should be robust and able to feed the wire at the commanded rate without wire slippage. A high-torque, four-roll, servo-driven wire feeder can meet the requirements to feed aluminum wire successfully. The drive should have a high pulling torque (10-kilogram force) to feed wire from conventional wire spools or boxed wires. The four-roll design increases the surface area driving the wire, providing positive placement. Wire slippage doesn't occur, and the welding process is stable.
Processes such as touch-retract or hot starting can be used to draw the arc and preheat the start of the weld, preventing cold starts and wire buckling, otherwise known as bird nesting. Process switching combined with ramping can be used to end the weld and fill the crater, minimizing crater cracks commonly associated with aluminum MIGW.
From a processing perspective, the drive becomes an extension of the robot, and wire placement is coupled to the robot's tool-center point (TCP). A properly designed drive that is integrated into the control architecture of the robot provides stable wire feeding with processing control to overcome application problems associated with MIGW of aluminum.
2.1.3 Wire Drive Location:
The location of the wire drive is a key factor in successful aluminum wire feeding. Small-diameter aluminum wires (0.035 and 0.047 inch) doesn't have the column strength to be pushed long distances, so a pull design a wrist-mounted wire drive is required.
When a wire is pushed through a delivery conduit, resistance builds between the wire and the liner wall, and the longer the liner, the greater the resistance. The greater the resistance, the less repeatable the wire feed will be, jeopardizing the process's stability.
2.1.4 Wire Delivery Conduit:
Special care should be taken when routing the wire delivery conduit. A different set of rules applies to aluminum applications compared to steel applications. Each robotic aluminum application should have the routing of the delivery conduit optimized.
The filler wire should be located as close as possible to the drive source, minimizing the delivery conduit length. The delivery conduit should be routed separately from the torch utilities and incorporate quick connections. Quick connections eliminate the need for tools and reduce the time associated with wire replenishment procedures.
Isolation allows you to control the bend radius of the conduit through proper routing and robot programming. Aluminum-specific liners designed to reduce the drag friction must be used within all delivery conduit. Liner splices in a delivery conduit should be minimized; a single liner without splices should be used between the wire and the drive.
Splices add additional points of contact on the weld wire, which results in increased drag friction.
The purpose of a delivery conduit is to provide an isolated link between the wire source and the feeder while maintaining minimal resistance. When a delivery conduit is routed properly, the wire should pull easily through the delivery conduit. If the wire can't be pulled easily by hand through the delivery conduit, feed issues and welding problems likely will occur.
2.1.5 Contact Tip Selection:
Contact tip selection can make or break an aluminum application. Contact tips can be broken down into two categories: standard and oversized. A standard contact tip typically is the same diameter as the wire, whereas oversized contact tips typically are one wire diameter bigger than the wire being fed. Every torch manufacturer offers a contact tip for aluminum applications; choosing the correct tip becomes the challenge. A basic understanding of the mechanical advantages and disadvantages of tip sizing can make contact tip selection more effective.
2.1.6 Oversized Contact Tips:
Oversized contact tips typically are one wire diameter bigger than the wire being fed. For example, a 0.047-in. aluminum wire will use a 0.052-in. contact tip. They allow the wire to be fed through with less resistance. This decreased resistance reduces the mechanical efficiency of the welding process and increases the chance of microarcing.
2.1.7 Standard Contact Tips:
A standard contact tip typically is the same diameter as the wire. A standard tip increases the sliding friction on the wire, making the welding process more efficient. The increase in friction reduces the chance of microarcing; however, as the wire heats from the welding process, it expands, causing premature tip failure by jamming the wire inside the tip. The degree of expansion is directly related to the welding output: Higher welding currents will accelerate the expansion process.
Fig.3.effect of using oversize and standard contact tips.
Users often choose an oversized tip instead of a standard tip because the efficiency disadvantage can be overcome more easily than predicting thermal expansion rates. Thermal expansion is directly related to the welding process; as the welding process is changed, the expansion rates change. Tuning a process to a changing variable never is recommended. Because microarcing is inherent with an oversized tip, it's critical to implement a tip maintenance schedule.
Its good practice to perform a study to determine how many welds can be made before a tip failure occurs. Once a tip replacement frequency is determined, we can implement routine replacement procedures into the production schedule. A maintenance schedule minimizes lost production often associated with fused or failed tips.
Wire expansion can cause a premature tip failure or jamming of the wire inside the contact tip. Fig3 shows effect of using oversize and standard contact tips.
2.1.8 Selection of Package:
Because of robotic MIGW of aluminum has some known limitations, one should take care when selecting robotic package. The wire drive and delivery system is critical to process success.
The drive should be robust and tightly coupled to the robot control architecture, and the delivery system requires little to no line resistance.
The key to success is to understand the process limitations and design a system to overcome them.
Comparison of Robotic MIGW with others:
Fig.4.Efficiency of cycle times for robotics arc welding
The figure.4 illustrates the efficiency in meeting cycle times for robotics arc welding in comparison with semiautomatic and manual operation. The robot is about 70% efficient, while the manual operation is only about 30% efficient. The semiautomatic operation is about 45% efficient. Here semiautomatic system means human works with the robotic arc welder whereas in fully automated there is no human intervention. From figure.4 we know that robotic arc welding is more efficient than manual and semiautomatic operations.
The initial cost and effort involved in installing a fully equipped robotic welding work cell may initially seem too great for a small company, despite the long-term benefits in efficiency and productivity. However, robots are becoming increasingly affordable and cost-effective, so what was a daunting prospect a year or two ago may now be a realistic option. Unless the welding industry can reverse the decade-long decline in the number of trained welders, more automation is an inevitable trend as the industry enters the new millennium.