Sensor developments enable measurement in harsh environments

Written by: Justin Cunningham | Published:
Sensor developments enable measurement in harsh environments

Sensing in harsh and hostile environments doesn't come much more intense than measuring the combustion process inside a jet engine. Yet Oxford based Oxsensis has managed to produce sensors that can be relied upon to withstand the heat.

The sensors are actually tiny microphones and measure the noise produced by the combustion process. They are in fact so small that they can be encompassed within the combustion chamber itself, termed a combustor on a gas turbine.

Sensors are currently used to measure instabilities that occur during the combustion process. However, due to extreme nature of the combustor, sensors cannot normally be placed there, having to be placed further back within the engine structure. However, this has meant that high frequencies are highly attenuated, making accurate measurements and instabilities more difficult to detect. However, early detection of combustion instabilities is essential to preventing damage. Additionally, accurate measurement of the combustor could allow aircraft engine manufacturers to reduce emissions.

"The best way to do that is by having a very lean fuel-to-air mix," says Alex Winterburn, business development engineer at Oxsensis. "You are running it like a candle that is about to run out of wax, the flame kind of splutters. If that splutter gets too big, though, it starts to rattle the whole structure, which can cause damage to the engine and even the engine blades.

"By monitoring the behaviour of the flame inside the combustor it has promoted a new level of understanding and management of what exactly is going on inside."

This splutter is extremely loud – some 140dB plus – and so the best way to measure it is acoustically. The team has developed what essentially can be described as a very high-temperature microphone. The microphone needs a lot of noise – and consequently a lot of pressure – to actually register an output. In fact the microphone will not register anything under 138dB.

Single-crystal sapphire is used to provide the super heat resistance needed and this essentially acts as a diaphragm. With a melting point of over 2000°C it is combined with innovative fibre optic interrogation techniques to give the sensitivity and immunity from electro-magnetic interference (EMI) effects which are common in turbomachinery such as gas turbines.

"The idea came from a lecture that a Rolls-Royce engineer gave at the IMechE [Institute of Mechanical Engineers) in London," says Winterburn. "They were saying they could not get a sensor to work in the heart of the gas turbine." The founder of the company had a background in telecomms and knew of materials that could be used to cope with such extreme temperatures. After coming up with a few ideas, he got some initial funding from the TSB and started working on a small Rolls-Royce Viper engine.

The sensor uses a light that shines on the sapphire membrane and bounces back inside. The pressure waves inside the combustor push against it, changing the size of the vacuum cavity. Additionally, the piece of sapphire thermally expands. So, by measuring the change in these two thicknesses, both pressure and temperature can be calculated.

The company has also developed the sensor for a small helicopter engine, and has since installed and trialled the sensor in a giant 200MW gas turbine, which has been running at Didcot power station since 2009.

"We started off in the R&D world, but once manufacturers get more comfortable with the technology, then hopefully we will branch out to supplying sensors that will actually go into engines in the field," adds Winterburn.

The company has won further funding from the TSB to develop an aircraft fuel quantity indication system with Parker Hannifin and the Science and Technology Facilities Council (STFC). The project known as the Silicon-based Optical High Accuracy Pressure Sensor (SOHAPS) will deliver high-accuracy, multi-parameter optical sensors for the measurement of pressure and temperature within the next generation of aircraft fuel systems. Oxsensis will design the sensor and draw on the expertise of the STFC to model and fabricate novel sensor head elements.

There is a trend in the aviation industry towards the use of composite materials, particularly in wing construction and therefore the EMI immunity of optical sensors is a major advantage. This improves the intrinsic safety of the fuel system. The sensor technology developed in SOHAPS, has potential to be applied to engine driven lubrication pumps, hydraulic systems, main engine bearings, landing gear and other major systems.

Another fuel sensor innovation comes from UK based Gill Sensors. It recently helped play an important part helping a UAV Factory team smash a world record for the longest recorded flight for a small unmanned aircraft. In July 2012, the Penguin B aircraft stayed in the air for 54.5 hours, breaking the previous record by over 16 hours.

Gill Sensors developed an innovative fuel level sensor to enable accurate and reliable monitoring of remaining fuel in the 7.5 litre tank. The main challenge was that space was extremely limited. The irregular shape of the fuel tank meant there was no space to mount the sensor through the top of the tank, as would normally be the case.

Engineers at Gill Sensors therefore designed and produced a micro-liquid level sensor that could be mounted through the side wall of the tank that used a specially angled probe to allow accurate monitoring of the depth of fuel.

Mike Rees, head of marketing at Gill Sensors says: "We are able to utilise the proven microelectronic level sensor technology that is currently supplied by Gill into other specialist applications."

Optical sensing has also recently found application in the offshore renewable industry. RWE Innogy and the Carbon Trust's Offshore Wind Accelerator programme have set about testing innovative wind measuring buoys.

Two Light Detection and Ranging units (LIDAR) are being mounted on buoys ten miles off the North Wales coast. Both units will collect wind data which can then be compared with information from the met mast. This will be used to build confidence for future wind farm developments.

Chief operational officer at RWE Innogy, Paul Coffey, says: "The need for research in the field of offshore wind power continues to be immense. The construction of measuring stations is an important step towards recording and analysing local wind conditions. The data is of fundamental significance for the development, construction and operation of offshore wind power plants."

If the trials are successful, LIDAR devices are expected to be simpler, quicker, more effective and cheaper alternatives to met masts during offshore wind project development. The two models being trialled, one manufactured by the Belgian company FLiDAR, the other by the British producer Babcock International Group, differ particularly in terms of design.

The prototype developed by FLiDAR floats on the waves and is undergoing a trial for wave motion compensation. The prototype has already been successfully used in the Belgian North Sea for accurate wind data collection. The measuring buoy from Babcock is currently under construction and is characterised by its low motion buoy design.

Both prototypes will be towed by ship to the chosen measuring sites where they will be anchored to the seabed. Electricity will be supplied by photovoltaic panels and micro wind turbines installed on the buoy. Like a conventional met mast, the buoys will supply weather data on wind velocities and wind direction. These trial laser-based measuring systems will be used to record wind velocity and wind direction both horizontally and vertically up to a height of 200m and data could be critical to the development of future offshore wind projects.

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