Plane for all to see

A next-generation test probe is designed to speed up the inspection of aircraft structures.

Inspecting the structural integrity of aircraft is regularly carried out by technicians using eddy-current test systems that provide an indication of any subsurface corrosion and cracks that might be present. In existing eddy-current test systems, a pen-shaped probe is placed in close proximity to an electrically conductive fuselage or aircraft wing. Alternating current flowing through a coil in the probe then creates a changing magnetic field that induces eddy currents in the structure under test.

If there are variations in the electrical conductivity of the test object due to flaws or cracks, these disturb the path of the eddy currents. This can be detected by measuring the change in the impedance of the coil or using a secondary detection coil.

Engineers at St Albans-based GE Sensing and Inspection Technologies, working with designers at Cambridge Industrial Design, have developed a next-generation probe that they claim boasts significant advantages over existing approaches.

Rather than create a pen probe that would only allow a user to examine one specific point on the aircraft structure at a time, the engineers at GE realised that if they were to build one using a large linear sensor array, it would enable a user to inspect whole swathes of a structure at once, speeding up the inspection process enormously.

As if that wasn’t enough of a challenge, the engineers also wanted the new system to probe far deeper into the surfaces under test than existing instrumentation. To do just that, they had to develop both a drive coil for the probe, as well as deploy new sensor technology.

‘Conventional eddy-current inspection techniques use a sinusoidal alternating current to excite a coil in a probe,’ said Ian Mayes, manager of EC development at GE Inspection Technologies. ‘But we decided to build a new drive coil that could be excited by a pulsed eddy-current step function waveform that contains a spectrum of frequencies, most importantly low frequencies that can penetrate deeper into the structure under test.’

To pick up the low-frequency signals generated by the coil, the GE designers chose to use an array of giant magneto resistive (GMR) sensors in the pick-up sensor head itself, rather than use a traditional coil. The design was based on a concept developed and patented by a team at GE’s Global Research Centre in Schenectady, US.

‘We wanted to be able to look for corrosion rather than just surface cracks and, by generating a pulsed low-frequency signal and then using a GMR sensor array with a frequency response from DC to a few megahertz to detect the signal, we are able to probe up to 10mm into the material,’ added Mayes.

One advantage of the new approach was that the electromagnetic response of the material to several different frequencies could be measured in a single step. As a wide range of frequencies is both generated and received by the system, the operator does not have to select a specific inspection frequency in which to operate the probe, as in previous systems. The system also makes it easier for the user to select an optimum single inspection frequency for follow-up testing or analysis.

Since an aircraft wing or fuselage is an uneven, jointed and riveted surface, the designers also needed to design a mechanism that would ensure that the drive coil and GMR sensor could be kept at close proximity to the surface at all times – close enough, at least, that any movement that did occur could be compensated for by software when the data from the sensor was analysed.

‘To overcome that issue, we enclosed the GMR sensor and coil assembly in a cradle, with two chamfered edges at its base, and then mounted the cradle onto a four-spring suspension system so that the assembly could float as it moves, always ensuring that it was close enough to the surface even as it moves over joins and rivets in the structure,’ said Alex Jones, managing director of Cambridge Industrial Design.

Another design challenge was to develop a means that would help the user to sweep the surface of the aircraft structure under test at a uniform rate during the inspection procedure. If it was moved too quickly, the probe would not produce accurate or meaningful results and the exercise would have to be performed again.

To provide feedback to the operator on the speed of the probe, Jones and the GE team incorporated a small rotary encoder at the back of the unit, which was driven by the movement of the two back wheels via a small belt drive. This allowed the speed of the rear wheels and hence the speed of the unit to be tracked and used to provide a visual feedback of its speed through a flashing light-emitting diode on the front of the probe.
‘It’s a useful feedback mechanism that teaches the user to perform the scanning operation at the right speed,’ said Jones.

‘More importantly,’ added Mayes, ‘the signals from the encoder are used to construct an image that locates the defects in the structure.’
One other issue the designers faced was to ensure that the user would move the probe in a linear path across the surface of the airframe. Despite the fact that the four wheels on the unit provide a smooth crab-like motion, the team realised that an operator might still have difficulty keeping the unit on a fixed linear path.

As a solution, the designers developed a set of transparent polycarbonate guide rails that could be affixed to the surface under test by suction cups. In use, the operator simply needs to position protruding guide sites on the front of the probe at a fixed position on the guide beam and then move it slowly along the beam to make a measurement.

Once the signals have been acquired by the handheld probe, the data is sent over a cable at the back of the unit to a standalone companion system that performs real-time analysis on the data, as well as data recording and storage.

‘Importantly, to ensure that the data is interpreted correctly before it is presented to the user, the standalone processing unit features a lift-off algorithm – a software routine that compensates for any variations in paint thickness and surface roughness that may cause the sensor head to move away from its optimum measuring position in the probe during the scanning procedure,’ said Mayes. Once analysed and processed, the data is then displayed on the screen on the unit as a C-scan that presents the user with a colour image of the layers of the material scanned, highlighting in a variety of colours any areas where flaws may exist.

Design essentials

The key facts to take away from this article

  • Large linear sensor array inspects whole swathes of aircraft structures at once
  • Test probe goes deeper into surfaces under test than existing systems
  • Electromagnetic responses can be measured in one step
  • GMR sensor and coil are enclosed in a cradle with chamfered edges at base