The essential guide

ROBOT WELDING

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Off-line programming

Off-line programming and simulation software runs on a PC and may also run the actual robot control software in the back ground so performance from the software closely matches reality. Depending on the software this can also be simulated.

Advantages and disadvantages of various software suppliers

The advantage of  robot supplier software is that it represents a lower cost compared to third party software, but it is restricted to one brand of robot and may not have the same enhanced functionality. There are some independent suppliers of off-line programming software, such as IGRIP, Delfoi and RobCad, that in general have high level of acceptance in the market for high profile off-line programming and simulation. This kind of software can use the kinetic model of the particular robot based on the control software code that can be purchased from the robot supplier or it can simulate the machine code. When programming off-line the user can build a virtual cell from a library of 3D robot models and tools. It is also possible to import externally created 3D models in a variety of file formats such as stepp and igis.

Functionality

The simulation makes it possible to calculate cycle times; detect collision risks and visualise the robot’s path. The robot program can then be uploaded to the actual robot controller using Ethernet, a serial link or a USB stick. Although the robot has a very high repeatability it has a somewhat poorer absolute accuracy and not any two robots will ever be 100% identical. Due to its polar construction very small inaccuracies in the mechanical arm can result in deviations at the TCP. For this reason it will be necessary to calibrate the actual cell, construct a frame work of a number of points and link the actual robot program to this. The robot robot program will then be fairly precise but it will still be necessary to touch up some positional information.

The advantage of off-line programming is that costly mistakes can be eliminated by doing the work up front. Design changes can easily be accommodated without having cut any metal and installation and commissioning times can greatly be reduced for very large production lines. Off-line programming can in some cases save time if a number of similar assemblies need to be addressed. The biggest advantage is that the robot can be kept in production. In most cases it is a balance as to what is more economical. For the average arc welding application off-line programming does not appear to be cost effective.

Complete body in white systems in the car industry tend to be programmed off-line, but bear in mind that the level of expertise to do so will be quite high and the time taken is substantial. There are hundreds of spot welds on a car body and the average time to program a single one by the time the system is simulated etc is about 3 hours!

Absolute accuracy and repeatability

These are two different concepts that can be confusing. A robot can have a very good repeatability but a poor absolute accuracy. For offline programming it is important that the real robot matches the theoretical model as closely as possible to reduce the amount of touching up after off-line programming. Some suppliers can offer high accuracy versions of their robot, which have specially selected gearboxes and castings that fall within higher tolerances. With KUKA robots it is also possible to specify a high accuracy calibrated robot. Prior to delivery a test is conducted using a laser vision system that measures specific points in the robot test programme. The position of  these points are referenced against the theoretical positions and stored in a data file which is uploaded to the robot controller. When this type of robot is in production, the absolute accuracy and repeatability of  the robot closely matches the theoretical robot.

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KukaSim simulation