Tom Graver

Process for Mounting and Packaging of Fiber Bragg Grating Strain Sensors for use in Harsh Environment Applications

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating strain and temperature sensors for use in harsh environments. The compound is based on a combination of ceramic fillers with an epoxy binder that is applied with a brush technique. Samples of optical fiber Bragg gratings were successfully encapsulated and mounted on metal shims. The packaged sensors were tested for strain (+/- 1000µε) and temperature (-20 to +120 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of 24.37pm/°C, and a strain gauge factor of 1.25pm/με.

On-line structural health and fire monitoring of a composite personal aircraft using an FBG sensing system

We report in this paper on the design and development of a novel on-line structural health monitoring and fire detection system based on an array of optical fiber Bragg grating (FBG) sensors and interrogation system installed on a new, precommercial compact aircraft. A combined total of 17 FBG sensors - strain, temperature and high-temperature - were installed at critical locations in an around the wings, fuselage and engine compartment of a prototype, Comp Air CA 12 all-composite, ten-passenger personal airplane powered by a 1,650 hp turbine engine. The sensors are interrogated online and in real time by a swept laser FBG interrogator (Micron Optics sm125-700) mounted on board the plane. Sensors readings are then combined with the planes avionics system and displayed on the pilots aviation control panel. This system represents the first of its kind in commercial, small frame, airplanes and a first for optical fiber sensors.

Structural Monitoring of Wind Turbine Blades Using Fiber Optic Bragg Grating Strain Sensors

Over the last few years, fiber optic sensors (FOS) have seen increased acceptance and widespread use in civil engineering, aerospace, marine, oil & gas, composites and smart structure applications. More and more, different research groups and blade manufacturers worldwide have started adopting fiber sensors and fiber Bragg gratings (FBGs) in particular, as practical sensing technology for wind blades. FOS are an attractive technology and reliable sensing solution due to the fact that are completely immune to electromagnetic interference, lightning and electric noise, unlike more conventional electronic sensors that are prone to failure given the harsh and exposed environmental conditions under which wind turbines normally operate. Typically, FBG sensor arrays–either surface-mounted or embedded–have been used to monitor the mechanical behavior of composite rotor blades during the design and qualification stages, as well as in service, to help monitor, on–line, the blades’ condition under rotating, stationary and different wind load conditions. In this paper, will present test field results on the mechanical measurements from an experimental composite blade developed under Sandia Lab’s S–Blade experimental wind turbine program, instrumented with FBG temperature and strain sensors. A discussion of the methodology, on-line monitoring electronic system, and results obtained will be presented.

Packaging of surface relief fiber Bragg gratings for use as strain sensors at high temperature

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments - where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using surface relief fiber Bragg gratings (SR-FBG). These gratings, instead of being written in the core of a photosensitive or hydrogen-loaded fiber, are formed by introducing a periodic surface relief - through photolithographic and etching processes - in the cladding above the core. Samples of SR-FBGs were successfully encapsulated and mounted onto metal shims. The packaged sensors displayed a linear response with temperature and a sensitivity factor of 11pm/°C.

FlexPatch: a rugged miniature FBG strain sensor

The design and development of a novel opto-mechanical strain sensor-called FlexPatch-based on the use of an optical fiber Bragg grating (FBG) mounted into a custom-made miniature metallic flexure is reported. The FBG sensing element is attached to a carrier flexure using proprietary bonding process which ensures a linear, drift-free and repeatable strain response even under severe moisture and temperature conditions. The sensor is uncompensated for temperature effects, but has undergone extensive mechanical and environmental testing and is qualified for use in a strain range of +/- 2,500&mgr;&egr; with a gage factor of 1.2pm/&mgr;&egr; over a temperature range from -40° to 120°C, and a fatigue life >100x106 cycles. The FlexPatch is intended for use in diverse sensing and monitoring applications and can be installed onto surfaces by epoxy bonding or spot welding.

Packaging process of fiber bragg grating strain sensors for use in high-temperature applications

In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments--where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using chemical composition gratings (CCGs). The refractive index modulation in these gratings is caused by a chemical change, which results in a higher activation energy and stable behavior up to 1000°C. Samples of CCGs were successfully encapsulated and mounted onto metal shims. The packaged sensors were tested for strain (+/- 1000με) and temperature (to +400 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of ~ 28.34pm/°C, and a strain gauge factor of 1.7pm/με.

Analysis, compensation, and correction of temperature effects on FBG strain sensors

One of the most common fiber optic sensor (FOS) types used are fiber Bragg gratings (FBG), and the most frequently measured parameter is strain. Hence, FBG strain sensors are one of the most prevalent FOS devices in use today in structural sensing and monitoring in civil engineering, aerospace, marine, oil and gas, composites and smart structure applications. However, since FBGs are simultaneously sensitive to both temperature and strain, it becomes essential to utilize sensors that are either fully temperature insensitive or, alternatively, properly temperature compensated to avoid erroneous measurements. In this paper, we introduce the concept of measured “total strain”, which is inherent and unique to optical strain sensors. We review and analyze the temperature and strain sensitivities of FBG strain sensors and decompose the total measured strain into thermal and non-thermal components. We explore the differences between substrate CTE and System Thermal Response Coefficients, which govern the type and quality of thermal strain decomposition analysis. Finally, we present specific guidelines to achieve proper temperature-insensitive strain measurements by combining adequate installation, sensor packaging and data correction techniques.

Fiber Bragg grating multichemical sensor

Fiber optic-based chemical sensors are created by coating fiber Bragg gratings (FBG) with the glassy polymer cellulose acetate (CA). CA is a polymeric matrix capable of localizing or concentrating chemical constituents within its structure. Some typical properties of CA include good rigidity (high modulus) and high transparency. With CA acting as a sensor element, immersion of the gratings in various chemical solutions causes the polymer to expand and mechanically strain the glass fiber. This elongation of the fiber sections containing the grating causes a corresponding change in the periodicity of the grating that subsequently results in a change in the Bragg-reflected wavelengths. A high-resolution tunable fiber ring laser interrogator is used to obtain room-temperature reflectance spectrograms from two fiber gratings at two different wavelengths - 1540nm and 1550nm. The graphical representation from this device enables the display of spectral shape, and not merely shifts in FBG central wavelength, thereby allowing for more comprehensive analysis of how different physical conditions cause the reflectance profile to move and alter overall form. Wavelength shifts on the order of 1 to 80 pm in the FBG transition edges and changes in spectral shape are observed in both sensors upon immersion in a diverse selection of chemical analytes.

Demodulating interferometric and FBG sensors in the spectral domain

Long-gauge SOFO sensors have been in use for the last 10 years for the monitoring of civil, geotechnical, oil & Fiber optic sensing systems are increasingly recognized as a very attractive choice for structural health monitoring. Moving form demonstration project to industrial applications requires an integrated approach where the most appropriate technologies are combined to meet the users requirements. In this context it is often necessary and desirable to combine different sensing technologies in the same project. A bridge-monitoring project might for example require long-gauge interferometric sensors to monitor the concrete deck, interferometric inclinometers for the piles and fiber Bragg grating sensors for the monitoring of the strains in the steel beams and for measuring temperatures. Although fiber optic sensors relying on different technologies can easily be combined at the packaging and cable levels, they often require dedicated instruments to be demodulated. A unified demodulation system would therefore be very attractive. This paper describes a technique relying on the analysis of reflected spectra and allowing the demodulation of interferometric (Michelson or Faby-Perot) sensors and fiber Bragg grating sensors with a single measurement system. It also compares the obtained performance in terms of resolution and dynamic range with the available dedicated systems.

Experiences with Real-Life Fiber Optic Bridge Monitoring Installations

We present a brief overview of real-life bridge structural health monitoring (SHM) installations using fiber Bragg grating sensing technology. We review and describe the associated successes, challenges and lessons learned for SHM projects. In general, project successes are coupled to improved sensing tools: better sensor packages, simpler and less expensive instrumentation, improved installation techniques, and more efficient data analysis tools. Some shortcomings are the direct result or poor project planning and communications. Particular attention is given to the benefits and economics of instrumenting civil structures – when and how it pays.