Robot Welding

The essential guide

Welding robots
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Robots for welding

The term "robot" is derived from the Slavic word "robota", which means forced labour (see introduction to robotics). The robot itself is nothing more than a programmable mechanical arm that is controlled from a computer system, the robot controller. A MIG welding robot holds a welding torch and manipulates this around a work piece in a similar way as a human welder.
Definition of a robot according to European standard EN 10218-1 (previously EN 775)
A robot/industrial robot is an automatically controlled, freely programmable, multi-purpose manipulator that can be programmed in three of more axes, is used in automation systems, and can be either fixed in place or mobile.

The robot consists of:
The robot arm (including actuators)
The control device including teach pendant and all communications interfaces (hardware and software)

This includes all external axes controlled by the robot controller.

The industry standard

The industry standard is a machine with about 1.4.m reach from the centre of the base to the knuckle joint near the wrist. It is however very useful to have the option of a robot that has a longer reach, which is achieved by having a longer upper arm option, typically 1.6m, 1.8m and 2.0m The choice of robot depends on the assembly that needs to be welded. A disadvantage is that a longer arm makes the robot less agile to reach into tight areas, so it really depends on the application which robot should be selected. The naming convention of KUKA robot is the payload in kg after KR and the reach in mm after R. The KR 10 R1420 is therefore a 10kg robot with a reach of 1420mm.

The KUKA KR30 L16-2 is an example of a robot with an extremely long reach of 3102mm. It would be unusual to use such a machine, but depending on the application it could be more economical to use this robot, rather than a standard robot on a track or an inverted robot that is suspended from a gantry system. An inverted robot will use the top area of the robot's working envelope and this can be useful if the robot needs to cover an assembly that is too wide or too deep for a floor mounted robot.

Robots specifically designed for arc welding

Traditional robots are still extensively used for arc welding. However dedicated arc welding robots have some benefits. The upper arm accommodates the welding hose bundle or is routed very close to it and then passes through the centre of the wrist. This arrangement offers higher protection to wear and tear compared to the traditional dressing method where the hose bundle runs over the top of the upper arm. In addition it facilitates robot programming since the programmer does not need to consider the hose bundle which could otherwise get snagged up in the assembly or fixture. A fringe benefit is also that the Tool Centre Point (the reference point for programming, in this case the end of the welding wire) is more consistent due the improved routing and shorter hose bundle. This in turn can give a quality improvement when welding conditions are critical.

As for regular industrial robots various arm lenghts are availble up to a reach of 2.0m.

KR 8 R1420 arc hollow wrist robot
KR 10 R1420 robot

Working envelope

The working envelope of a robot is shaped like a fist and when it is floor mounted the maximum reach is usually in line with the motor for the lower arm joint. For this reason the robot may be placed on a base frame. The envelope of the robot will extent in a circular motion and the work piece will be placed inside the working envelope. The area at the top is wider and this may a  reason for inverting the robot if it has to weld an assembly, that is too wide to fit into the envelope in front of the robot and if a long reach robot is not suitable for reaching into the assembly.

The example below shows the working envelope of the KR 10 R1420 arc robot, which has a hollow wrist.

Robot controller

Inside the cabinet is a power supply, a PC based computer, servo amplifiers and communication boards. These can be simple I/O boards or field bus boards. The welding interface can be via the traditional analogue interface card or the more modern fieldbus or Ethernet interface. The robot motion control software of most industrial robots is based on VXWorks wiith some sort of robot programming language interface or alternative software to make robot programming user friendly. The system also comprises a teach pendant which is used to program the robot.

Standard size robot controller with pendant
Space Mouse with 6 degrees of freedom for guiding the robot
The KUKA pendant is called a smartPAD
Alternatively conventional toggle keys can be used to guide the robot, or if preferred a combination of both

Robot axes

The articulated arm of the robot has 6 degrees of freedom or axes, which are driven by electric, brushless AC motors from the robot controller. These are referred to as axes 1 to 3 for the major axes and as axes 4 to 6 for the minor axes. On the teach pendant are six jog keys, which are clearly marked so the user will find it easy to identify the correct one. The KUKA controller is unique in as much that it features a teach pendant (Smart Pad) which in addition to the jog keys, has a Space Mouse that allows the user to select any of the robot axes from a single control knob.

Robot axes
A1: base rotation
A2: lower arm back and forth
A3: upper arm up and down
A4: upper arm rotate
A5: wrist bend
A6: wrist rotate

Robot speed

Robot manufacturers will specify the relevant speeds for each of their robot axes. In an arc welding robot program there are often very small moves between welds and these do not allow the robot to reach its maximum speed. The most important factor is for the robot to accelerate as fast as possible, attempting to reach its maximum speed and then decelerate to the next welding position as fast as possible. This kind of control is taken care of within the dynamic model, that is part of the motion control software. The software also considers the mass of the tool at the end of the robot arm with its centre of gravite, and additional loading such as the wire feeder, friction in the joints and any other external forces that act upon the robot such as gravity. This dynamic model ensures, that all the motors work in harmony with each other resulting in good path following accuracy and positional accuracy.

Generally the speed of the robot is restricted to its slowest axis and for that reason it is a good idea to select "point to point moves" for air moves between welds. In this mode the robot will move to its programmed position in the fastest possible way.

Robots feature AC servo motors with either encloder or resolvers, that provide high resultion positional accuracy and monitoring functionalilty.The position of the robot can be recovered after a power cut. These motors are capable of moving the robot at very high speeds without undesirable vibration or backlash. The servo motors are controlled from servo drive units inside the robot controller. The programmer does not get involved in the detail of how the robot plans its path and merely uses a teach pendant to instruct the robot which postions to visit and when to start and stop welding.

Welding software

In order for the robot to weld properly, it has to be given some intelligence, which is provided through a software interface that takes care of the communications between the robot controller and the power sources. The typical control over the welding process includes these functions:

Gas pre-flow and post flow

Start & end voltage with time

Main seam voltage or trim if in synergic mode

Main seam wire feed speed

Start & end wire feed speed

Weaving patterns

Re-start function

Pulse patterns trim

Crater fill

Burn back control

Automatic re strike

In the early days of robot welding most of these parameters had to be programmed by the robot programmer, who therefore needed to understand the affect on the welding process, how to achieve the functionality by programming various parameters in the robot controller and above all understand the welding process. The settings were made from the teach pendant and the process became quite complicated, which meant that in some challenging applications, a robotic welding engineer was responsible for programming the robot.

Since then there has been a shift change in technology and the power source is now able to hold and control the majority of parameters, which are largely automatically optimised using synergic lines through built in welding software. Of course the programmer still needs a fairly good understanding of the welding process, but it is a lot easier and faster to get good results. The welding parameters tend to be stored inside the power source as a series of schedules containing various parameters, that are needed to achieve good results for any given joint. The schedules are simply selected by the programmer and then selected from the robot to the controller via digital interface.

One of the advantages of this kind of interface is that it is easier to identify any faults since the tasks are clearly identified with the robot being responsible for moving the torch and the power source being responsible for the process. This principle of interface works as follows:

At the start of the joint the robot controller sends a digital output to the power source to select a particular weld schedule (start, main and end conditions).

The robot controller then initiates a signal for the power source to switch on the welding process.

The robot power source returns a signal to the robot controller that an arc has been established.

The robot controller then initiates the movement of the robot.

At the end of the joint the robot sends a signal to the power source to stop the process and extinguish the arc.

The robot moves to the next position in its program, etc.

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