Spin doctor

UK engineers pioneer the use of eddy current probe technology to monitor the health of individual turbine blades as they turn. Siobhan Wagner reports


Two UK companies have developed a turbine condition monitoring system that they claim can pinpoint a damaged blade — even when it is spinning.

The T3, from condition monitoring specialist Monitran Technology and defence group Qinetiq, combines an eddy current probe with an advanced driver to monitor the time it takes the tips of the blades to spin round in a turbine and the vibration frequency of each individual blade.

It could have applications for both airborne and ground-based industrial turbine systems.

‘If you damage a blade or if it becomes misaligned, it will change its time of arrival and its vibration frequency,’ said Donald Lyon, managing director of Monitran. ‘The eddy current probe is able to measure this feedback and determine the health of a blade.’

The probe begins to work when a current is sent around an electro-magnetic coil, which produces a magnetic field. If a conducting material, such as a turbine blade, passes through this field a current will start flowing. ‘That is the eddy current,’ said Lyon.

‘The current will then produce an opposite magnetic field and this can be detected back through a response coil — the second in the probe.’

To detect differences in a blade’s current, operators would have to know each blade’s signature current when they are new as the signature will change as the blade wears or becomes damaged.

Lyon said his company and Qinetiq have perfected the technology so that it could pinpoint a damaged blade spinning round thousands of times a second.

‘It’s very dramatic really,’ he said. ‘People who saw this being tested were amazed you could detect which blade was damaged when all of them are going round so fast.’

The T3 was recently trialled on the F-35B, the vertical take-off and landing (VTOL) variant of the Joint Strike Fighter, which is set to replace the Harrier. The tests were carried out on the aircraft’s uplift fan, which is an important part because it provides the vertical take off. ‘It spins around a couple of inches behind the pilot’s neck so you need to monitor that fan very carefully,’ said Lyon.

For this type of military application, Monitran is working to develop a digital signal processing system (DSP) that will automatically eject a pilot if something is critically wrong with the uplift fan.

While the system has undergone testing with F-35B, Lyon said this does not mean T3 is limited to airborne applications.

‘We’re looking to put this on ground-based gas turbines,’ he said. ‘It has all been developed on aerospace engines, which are very rigorous. In a sense, if you’ve developed it on an aerospace engine you can bet it will work on a standard gas turbine.’

The technology works with turbines that run at temperatures of up to 250ºC, but with a few material changes could work with turbines that run at temperatures as high as 1,000ºC. For example, the body of the eddy current probe could be made of machineable ceramic, and ceramic could be used for the winding and the coil.

Lyon said the main selling point for the T3 is that is performs more reliably than optical sensors, which depend on lasers bouncing off the blades. The ray comes back into the sensor and monitors how the blade behaves from the reflection of the light.

‘Obviously in that system contamination can happen very easily,’ said Lyon, adding that just dust, grease, oil or even rainwater can distort the signal from the optical sensor. He claimed the future is in eddy current technology.

‘The eddy current probes have much more promise [than other systems] because you put them on an engine and don’t need to maintain them, shut them down or clean as often,’ he said. ‘It’s probably a good idea to keep things clean generally, but it is not going to be critical to the function of the sensor.’