Custom-built loggers help weld improvements

A self-contained data-logging system based on National Instruments hardware and software has helped GKN Wheels improve its butt welding processes.

Sigmapi Systems was approached by GKN Wheels, one of the world's leading manufacturers of wheels for any kind of light or heavy agricultural, construction and off-highway machinery, with the following challenge.

The company explained that when it used a range of materials in the construction of wheels, it experienced a high level of bad welds.

What it wanted was the ability to monitor several properties of the butt weld process in order to improve the success rate.

Specifically, GKN Wheels wished to monitor the current usage (up to 3kA to an accuracy of 1%), the voltage (up to 14V to an accuracy of just a few millivolts), the pressure (up to 150bar to an accuracy of 1%) and displacement (0-50mm with an accuracy of 1%).

The data had to be collected at 1000 samples per second per channel during the weld process and stored for later analysis and comparison.

The data logging equipment would have to withstand the incredibly electrically noisy environment of industrial welding and, for health and safety reasons, the operator could get no closer than 4m from the welding station to run the tests.

Finally, the equipment had to be portable so that it could easily be moved from site to site anywhere in the world, which also meant that it had to be capable of monitoring different types of butt welds (for example, some are AC powered, others use DC).

Sigmapi Systems decided the best way to solve this challenging problem was to enclose the data logging system in a portable, purpose-built carrying case, which also served as the data capture and analysis unit with compartments to hold all the sensors and cables when the unit was being transported to different sites.

Thus, the whole unit became self-contained.

The company decided to use a National Instruments solution for the signal conditioning and data acquisition.

The SCC unit provides portable, modular signal conditioning, comprising a very low profile carrier unit and a wide range of SCC modules that plug into the carrier.

By selecting this system, extra sensors can be added if the customer wished to monitor other properties of the weld process, by adding extra SCC modules.

Calibration of the system was achieved using National Instruments' Measurements and Automation Explorer (MAX).

For data acquisition, a NI PCMCIA DAQ card in a laptop was used.

This was ideal for applications with high-voltage signal or sensor measurements, allowing 1Ksample/s sampling rate at 16bit resolution for up to 16 analogue channels It was decided that the trigger for starting the test should be a digital input from a remote triggering device (which could be either wired or wireless to give the customer maximum flexibility), thus enabling the operator to stay well clear of the welding station.

A LED on the case would inform the operator when a test was running to provide feedback.

The software for test setup and data logging was written in LabView as this provides all the necessary drivers for the hardware we were using, making the system much quicker to develop.

The software system was designed and written to allow the operator to set up tests, remotely trigger a test, log the data to spreadsheet files and launch the files in Excel for analysis.

The unit was delivered to GKN and, following tests at the company's UK site, it was then taken to its Denmark, Italy and USA sites.

The unit was used on a variety of butt-welding equipment (including AC flash butt and DC butt welding) and was capable of measuring the key parameters on both CAM operated and hydraulic servo machines.

Sigmapi Systems was told that the unit had paid for itself three times over in the first month of use: "due to the high capture rates, it has been possible to diagnose equipment faults prior to complete failure as well as detect weld procedure faults not visible to the naked eye".

It was originally conceived that the unit would be used by the four GKN Wheels sites.

However, the unit proved so successful that each plant decided to purchase its own unit.

And so the company ordered three more units for its UK, Denmark and Italy sites.

From lessons learned with the first unit, several changes were made to produce a Mark II unit for the three plants.

This upgraded unit could measure each weld automatically, allowing direct comparison of each weld in Excel.

This would give the additional benefit of being able to define working parameters for each machine range from which good welds can be expected.

The original Mark I unit had been to four different countries on numerous visits; it travelled in the holds of aeroplanes and had been in almost constant use.

It has now been adopted by the USA plant, which returned it to Sigmapi Systems (travel worn and covered in travel stickers) for a complete refurbishment and upgrade to Mark II.

The beauty of the unit is that the design was so modular that it can be set up to log signals from any analogue sensor (up to 16 channels).

It can stand up to harsh environments (allowing remote triggering) and it travels well.

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