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Sigmapi Systems has reported how it was able to assist GKN Wheels in its drive to monitor several properties of the butt-weld process in order to improve success rates.

GKN 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.

Because of the large variation in equipment, age and type, a versatile machine was required.

Specifically, it wished to monitor the current usage (up to 3000A to an accuracy of one per cent), the voltage (up to 14V to an accuracy of a few mV), the pressure (up to 2200lb/in2 to an accuracy of one per cent) and displacement (0-50 mm with an accuracy of one per cent).

The data had to be collected at 1,000 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 to 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 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.

With this solution, the whole unit became self-contained.

Sigmapi decided to use a National Instruments (NI) solution for the signal conditioning and data acquisition.

Its SCC unit provides portable, modular signal conditioning, comprising a low-profile carrier unit and a 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 NI’s 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 1kS/s sampling rate at 16-bit 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 set-up and data logging was written in Labview, as this provides all the necessary drivers for the hardware Sigmapi was 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 its UK site, it was then taken to its Denmark, Italy and US 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 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 utilised 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.

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