Fiber Optic Multi-Sensing Platforms
Aerospace and Defense Technology, October 2016, by Michael Heflin, former CEO, Sensuron
Eventually, technology advances to the point where solutions that have been “good enough” for decades are no longer “good enough” for the innovations of today. The philosophy of “good enough” is widely applied when developing a new product or solution. Businesses have to make decisions about what new technology features will receive the most attention to keep projects within scope and completed on time.
This philosophy has a downside when applied to testing and monitoring. For decades, point sensing solutions like strain gauges and thermocouples have been “good enough” for testing aircraft durability, however, the mindset of “good enough” often blocks innovation. It is not uncommon for researchers to find that they have innovated beyond their ability to test with legacy technologies.
Sometimes it takes a significant, ex-pensive failure to admit that this is a human issue and not a technology issue. New, robust sensing technologies that can monitor beyond the scope of point sensors are necessary to enable the next generation of aircraft designs. Fiber optic multi-sensing platforms available today are capable of obtaining spatially continuous data and varying degrees of multi-sensing capabilities that will accelerate advances in aircraft design and lifecycle management.
Multi-Sensing Platforms Defined
Back in 2003, NASA experienced the downside to “good enough” sensing solutions firsthand when the Helios aircraft broke up over the Pacific Ocean. Thankfully, it was an unmanned vehicle so no one was injured. Unfortunately, millions of dollars of investment literally sank before their eyes. This spurred the team at NASA Armstrong (formerly NASA Dryden) to develop a sensing technology capable of keeping pace with their innovations. They realized that breakthroughs they made in developing a fiber optic sensing platform had vast applications and benefits across multiple industries due to its ability to obtain real-time, spatially continuous information of multiple parameters. The foundation they laid enabled the technology to be developed into a commercially available platform to help organizations across industries drive their innovations forward.
Multi-sensing platforms, simply put, are sensor technologies that can monitor multiple parameters (strain, temperature, deflection, etc.) simultaneously and are robust enough that they can be deployed in multiple applications across an organization and utilized throughout the product life cycle. It’s not just about being able to monitor different parameters using the same data acquisition hardware. More than that, a multi-sensing platform can consolidate sensing technology so the same hardware, with minor changes in application techniques and sensor packaging, can adapt to cover multiple testing and monitoring needs of an organization.
To do this, the sensing system must obtain data in real time and obtain spatially continuous information. Point sensors can miss events that occur between critical points, leaving blank spaces in the picture painted by the data. These features allow multi-sensing platforms to be deployed in lifecycle monitoring applications from design validation to providing feedback for flight control systems.
Sensing Technologies Today
Data acquisition hardware can collect data from strain gauges, thermocouples, and other point sensors simultaneously, but these systems fall short of the definition of a multi-sensing platform in a number of ways. First, they are well suited for periodic tests but are not designed for long-term monitoring. Additionally, they only obtain points of information.
While it is possible to get thousands of data points on a test article from strain gauges, the wiring and acquisition hardware necessary makes the task cumbersome and expensive due to extensive time and expertise requirements for installation. This limits the extent to which engineers can test and monitor their innovations.
Other technologies are capable of obtaining distributed measurements such as digital image correlation, which has numerous benefits in laboratory settings due to its ability to comprehensively measure such things as full-field material deformation. However, its spatial coverage is minimal, and it cannot be readily applied outside of a laboratory environment.
This technology will continue to gain traction in some settings. However, it is not well suited, nor designed, for environments outside the laboratory and can be cumbersome to implement.
Fiber optic sensing (FOS) technologies offer multiple advantages that other sensing technologies cannot. FOS technologies are also able to operate in temperatures ranging from -270°C to 900°C, depending on the technology, making them well suited for harsh environments. Historically fiber optic sensors have had mixed results when it comes to accuracy; however, systems today are equally or more accurate than legacy sensing technologies such as strain gauges and thermocouples.
Perhaps the most significant advantage of FOS technologies is that some can obtain real-time distributed data and sense multiple parameters at the same time. In other words, there are fiber optic multi-sensing platforms currently available in the market today.
Where Multi-Sensing Platforms Provide Value
One example of how a fiber optic multi-sensing system provides value to aerospace organizations is the development and control of an adaptive trailing edge at NASA Armstrong. In aircraft design today, an adaptive trailing edge is the next wave in aircraft innovations. Offering significant cost savings to the end user by enhancing aerodynamics and reducing fuel consumption, adaptive trailing edges are one of the most sought after innovations in the aircraft design community.
Fiber optic multi-sensing systems can provide the data necessary to validate these designs and to enhance their performance by providing insights about the optimum wing shape for aerodynamic efficiency. For example, NASA Armstrong is able to monitor the load and deflection of one such design using FOS technology and continues to use it in flight-testing and operation as the primary feedback control mechanism.
An industry challenge facing the aerospace industry is the development of advanced composite material manufacturing techniques. The benefits of composite materials in aircraft design are well understood; however, there is an industry-wide need to develop advanced manufacturing techniques that ensure quality and longevity.
As composite materials have been implemented into the design of aircraft and components, the demands for sensors have evolved. Strain gauges are limited in their ability to provide the data necessary for organizations to continue innovations in the manufacturing of composite components. For example, during the curing process, wrinkles can form within the thickness of a laminate, which can produce a hidden structural deficiency. As a point sensing solution, a strain gauge would not be able to capture residual strain from such a wrinkle unless it happened on or near the surface of the part.
On the other hand, fiber optic cables can be embedded in composites enabling a multi-sensing platform to collect spatially continuous strain data in real time. This information is necessary to understand the behavior of residual strain distributions after composite curing, and ultimately, to improve manufacturing techniques.
Furthermore, a fiber optic multi-sensing platform can obtain temperature and deflection measurements at the same time, so a full picture of what is happening to the material, both spatially and environmentally, can be obtained.
Multi-Sensing Platforms in the Near Future
Being able to understand how loads are distributed throughout a wing while monitoring the shape of the wing in real time is of enormous importance for future developments regarding aerodynamics. FOS platforms are currently being embedded in the wings of aircrafts to provide real-time data as part of a flight control feedback loop.
Armed with this data, an aircraft can intelligently adapt the shape of its wing in order to optimize aerodynamics, reduce fuel consumption, and notify operators of required maintenance or failures. Over time, this adds significant value to the end user in terms of fuel savings, lifecycle management, and in-creased safety for passengers. In the future, many commercial aircraft will have this type of technology embedded in their design.
Another application of multi-sensing platforms in the near future is the development of smart aircraft in the Internet of Things (IoT). The aerospace industry will experience significant benefits from IoT technologies that enable predictive maintenance. If software is the brains of the IoT, sensors are the nervous system collecting continuous streams of data to be processed.
Fiber optic multi-sensing platforms will be a critical component in the realization of predictive maintenance for the aerospace industry. For example, in addition to informing the flight control system, the embedded fiber optic multi-sensing system will be able to provide historical strain, shape, load distribution, and temperature data to a big data analytics platform.
The analysis from the software will enable predictive maintenance on the aircraft throughout its lifecycle. The fiber optic multi-sensing technology needed to make this application a reality is available today. By implementing a multi-sensing platform, engineers can resolve multiple monitoring challenges with a single platform.
Replacing ‘Good Enough’ With ‘Best’
While legacy sensing technologies such as strain gauges have been good enough for decades, the adoption of multi-sensing platforms will allow innovators in the aerospace industry to drive developments forward.
By adopting a sensing technology that can consolidate multiple technologies into a single plat-form, aerospace organizations will be able to obtain the data they need across the lifecycle of their products in order to revolutionize aircraft design and provide value to their end user.
This article was written by Michael Heflin, CEO, Sensuron (Austin, TX).