Technical Details
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Motion Model
Figure 1: The predicted acceleration of the welder in the X, Y, and Z axes
An ideal welder would have periodic acceleration along the X and Y axes and essentially no acceleration on the Z. This is shown by figure 1.
For a weld to be as strong as possible the welder needs to be consistent in the size and speed of their circles while keeping the mig gun at a constant height. This would lead to periodic acceleration along 2 axis and no movement along the third, assuming the sensor was mounted aligned with the axis of motion
Due to limitations in how the phone could be held, the data was not collected in such a way that one of the axes would not have acceleration. Given that, we are expecting to see acceleration of a similar frequency in all three axis. Based off initial tests and discussions with welders, a frequency of between .5 and 1 hz is expected.
Figure 2: A MIG welder welding a T joint coupon. The periodic motion modeled above is visible.
Beginner Welder
Beginner welders often move at inconsistent speed, struggle to move the MIG gun in consistent shape, and don't keep the MIG gun at a consistent height. Moving at inconsistent speed and shape would appear on the sensor as variations in the frequency and amplitude of X and Y acceleration. Moving in the Z axis would appear on the sensor as Z acceleration.
Figure 3: This weld has inconsistent bead size indicating inconsistent travel speed. The penetration and thus weld strength would be low
Experienced Welder
An experienced welder would have some/all of the issues that a beginner would, but generally on a small scale. The frequency would be more consistent than a beginner, and the Z access acceleration would be lower.
Figure 4: The bead is consistent in size, but the tow of the weld isn't whetted as well as it ideally should be. This 'cold' weld indicates either slightly high torch height or incorrect welder settings.
Expert Welder
An expert welder would have consistent frequency in the X and Y axes and very little motion along the Z axes
Figure 5: The weld is clearly consistent in terms of motion, and there is good penetration yielding a strong weld joint.
Fourier Transform
The data is converted from the time domain into the frequency domain to analyze frequency consistnency