Sensing technology has come a long way since the invention of the humble thermocouple and strain gauge. Many of these ‘classic’ sensors work by monitoring and measuring changes in wire components to calculate environmental conditions and forces such as temperatures and stress. But while this type of sensor continues to be the workhorse of many industries, including the highly demanding aerospace test environment, engineers are looking for more advanced sensors that provide single-point readings and surface-wide, high-speed data across a range of scenarios.
In a test environment, advanced sensors are required to ensure materials and components are fit for purpose, while in operational environments, they are necessary to monitor the performance of systems, including those using novel fuels and autonomous technologies.
Requirements continue to change. For example, the need to burn hydrogen and sustainable aviation fuel (SAF) changes the heat transfer properties of the combustion gases in aeroengines, necessitating redesigns and the validation of components. In addition, engineers and operators want more data. For instance, in terms of temperature sensing, they are interested in stable operating conditions and temperatures across an entire component rather than at one or two points, a limitation of thermocouples.
In response, innovative companies are developing new sensing solutions using the latest technology and materials to meet these monitoring needs.
PAINTING A PICTURE
A leading example of new sensor technology is Sensor Coating Systems’ (SCS) thermal history coatings (THC) and paint (THP), which remembers the highest temperature. “After operation, we shine a light on it and the luminescence afterglow, after calibration, tells us what the temperature was at every single point across the component. This provides entire temperature maps of components – something thermocouples can’t do,” explains Dr Jörg Feist, managing director of SCS and co-inventor of the technology.
“This is a small project with the potential to make a big impact” DAVID DEBNEY, HEAD OF TECHNOLOGY FOR WHOLE AIRCRAFT, ATI
The solution can be used on anything from combustion chambers to turbine blades and vanes, because it is manufactured similarly to traditional thermal barrier coatings. THC can endure long running intervals of tens or hundreds of hours at very high temperatures, while the paint variant, THP has already been proven to withstand more than 7,000 hours at temperatures below 550°C (1000°F).
The thermal mapping technology can also measure temperatures of up to and beyond 1,600°C (2,900°F) with an accuracy of ± 25°C (77°F). This was one of the main reasons it was awarded the UK Aerospace Technology Institute’s (ATI’s) “Making the Difference Award”.
“The project demonstrated that the thermal mapping technology can be used under multi-cycle test conditions where engine load conditions are changing,” says David Debney, head of technology for whole aircraft at the ATI, and chair of the award’s judging panel.
“Existing thermal paints and coatings today usually contain toxic materials. It’s therefore important to note that the solution developed in this project is also fully REACH compliant. This is a small project with the potential to make a big impact.”
UNDER PRESSURE
When it comes to pressure measurement, current strain gauge, piezoresistive and capacitive sensors can be limited by issues including thermal drift and degradation at high temperatures, vibration sensitivity causing inaccurate readings and limited dynamic range missing fast pressure changes.
Test and measurement technology provider HBK has developed new piezoelectric sensors which aim to overcome these challenges. “Our GaPO₄-based sensors solve these issues by operating beyond 700°C (1,300°F), where quartz and ceramics fail and by offering high-frequency response for precise combustion instability and shock wave measurement,” says Dr Christian Roethel, head of sales and product management at Piezocryst by HBK.
These sensors are suitable for environments such as jet engine and gas turbine combustion testing, where they can detect pressure pulsations and instability, and rocket engine development. They can measure dynamic pressure in extreme environments such as combustion chambers and fuel lines.
They can also support hypersonic and supersonic flow research, capturing transient pressure changes in wind tunnels, and structural health monitoring by identifying pressure-induced stress variations.
“As propulsion systems evolve toward cleaner combustion and higher efficiency, our sensors ensure safer, more precise and sustainable aerospace developments by optimizing engine performance, safety and emissions reduction through precise combustion analysis,” says Roethel.
The company continues to look for ways to advance its sensors. This includes by increasing the operating temperature limits. Hydrogen-based combustion happens at higher temperatures, increasing operating demands. Meanwhile research into miniaturization and advanced packaging is enabling the sensor’s use in compact, high-power-density turbines.
LASER VISION FOR AIR FLOW
Aerospace engineers at the University of Cincinnati (UC) in the USA have developed a laser-based system to visualize and measure airflow in unprecedented detail. Building on particle image velocimetry (PIV) techniques, the system tracks microscopic particles seeded in the air using high-speed cameras and laser pulses, enabling researchers to calculate velocity by observing particle movement between frames, explains Daniel Cuppoletti, assistant professor of aerospace engineering at UC.
Unlike traditional methods such as pitot probes or hot-wire anemometry, which only provide single-point data, the laser-based technique offers high spatial and temporal resolution, producing full-field, image-based measurements at tens of thousands of frames per second. This enables researchers to capture complex, fast-changing aerodynamic phenomena like shock waves, vortices and noise-generating structures in supersonic and hypersonic flows.
“The timescales of these flows are incredibly short. This technique, operating at 50 to 100KHz, lets us resolve those very fast, transient phenomena,” Cuppoletti explains.
“Our sensors ensure safer, more precise and sustainable aerospace developments” DR CHRISTIAN ROETHEL, HEAD OF SALES AND PRODUCT MANAGEMENT, PIEZOCRYST BY HBK
While the laser solution is being used in R&D environments, it is not yet a plug-and-play solution. Calibration and setup are complex processes, especially for high-fidelity measurement. However, the technique is playing a crucial role in validating computational models and refining aircraft components. Its ability to visualize coherent structures supports research into aeroacoustics, a vital aspect of electric aircraft and advanced air mobility solutions being developed to enter service over the next decade.
FROM GROUND TO AIR
It’s not just sensors for test environments that are being advanced – aerospace is also seeing exciting developments in sensor technology that is used in operational settings.
Late last year BAE Systems was awarded a US$12 million contract by the US Defence Advanced Research Projects Agency (DARPA) to develop a microelectronic sensor capable of high-bandwidth, high dynamic-range sensing at extreme temperatures, such as on hypersonic aircraft and missiles.
Current sensors are limited in that they can’t operate over 225°C (4,400°F) due to their materials, circuitry and/or packaging. BAE Systems is therefore designing a new pressure sensor module made of an integrated transducer and signal-conditioning microelectronics, capable of operating at 800°C (1,500°F).
“Applications such as jet engine prognostics and space exploration could benefit from the collection of real-time data using high temperature pressure sensors and circuitry through the High Operational Temperature Sensors program,” says Amrita Masurkar, technology development manager at BAE Systems’ FAST Labs R&D organization.
TAMING TURBULENCE
Elsewhere, researchers at the German Aerospace Centre’s (DLR) Institute of Aerodynamics and Flow Technology are working on the optimal load-adaptive aircraft (oLAF) project. This aims to develop an intelligent load control system able to proactively respond to wind gusts and maneuvers by quickly adjusting control surfaces and flaps. This will enable the aircraft to better absorb turbulence and reduce material stress, making the plane more efficient and extending service life.
“Our accelerometers open the door to scalable deployment in precision applications” DAVID BLUMSTEIN, GENERAL MANAGER MOTION, SILICON MICROGRAVITY
The project uses laser systems and LiDAR to measure wind patterns and detect ongoing gusts earlier, enabling the aircraft to react even more precisely and proactively to external influences, explains the project’s manager Lars Reimer. The research has already shown that this solution can reduce fuel consumption by over seven percent, significantly decreasing CO emissions.
SMARTER AIRCRAFT
Another leading example of sensor innovation comes from Saab, which is using Fibre Bragg Grating (FBG) sensor technology in its overheat detection system (OHDS) for commercial aircraft. The technology enables many sensing points just a few centimeters apart, each working as an individual temperature sensor.
The key difference with the OHDS is that it is being installed into new aircraft rather than used during the development and testing of materials and systems. Sensing fibres distributed across the aircraft provide real-time monitoring of bleed air piping to detect hot air leakage, providing precise measurements that can be analyzed for trends and setting smart alarms.
Another innovative company in this space is Silicon Microgravity, which is making progress in the world of inertial sensing. Engineers here have developed a resonant micro-electromechanical systems (MEMS) accelerometer that combines the performance of quartz with the scalability and efficiency of silicon.
While the sensor’s core concept isn’t new, Silicon Microgravity’s proprietary IP enables exceptional bias stability (<0.15 μg at 25g, 0.5 μg at 100g) and low temperature sensitivity, all while benefiting from the batch-fabrication advantages of silicon.
“The result is a silicon MEMS sensor that offers near-quartz performance at a fraction of the cost, size and power demand,” says David Blumstein, general manager, motion at Silicon Microgravity.
This breakthrough addresses a long-standing trade-off in sensor design, the choice between high accuracy or affordability.
“Our accelerometers close that gap, opening the door to scalable deployment in precision applications from GPS-denied aerospace navigation to missile guidance, advanced wearables and eventually mass-market autonomous systems,” Blumstein says.
“Our goal is to replace shoebox-sized Inertial Measurement Units that cost hundreds of thousands with something that costs a couple thousand and fits in a matchbox.”
With MEMS gyroscopes and custom ASICs also on its roadmap, Silicon Microgravity clearly aims to redefine what is possible in navigation, guidance and control.
SENSORS SHAPING THE SKY
As can be judged from these few examples, sensors are evolving from test instruments to design enablers and safety guardians, helping the aerospace industry to advance and push boundaries even further.
Sensor data is no longer just for reports. It is real-time, high-resolution and feeding live systems, which in turn is enabling aircraft to respond more quickly and efficiently, providing a myriad of benefits from passenger safety and comfort through to improving sustainability, efficiency, and even capabilities.
This is just the tip of the iceberg when it comes to sensor innovation, but it’s an exciting, fast-moving area that will continue to transform aerospace design, operations and performance in the years ahead.
