Temperature compensated diaphragm based Fiber Bragg Grating (FBG) sensor for high pressure measurement for space applications

Highlights

• Diaphragm based FBG high pressure sensor is developed to measure pressures upto 700 bar with a sensitivity of 3.64 pm/bar.

• The pressure sensor is compensated for a wide temperature range from −40 °C to 90 °C to suit space applications.

• A complete analysis of stress transfer from APX-4 steel diaphragm to polymer adhesive to FBG has been carried out.

• The experimental results are validated with thermal and strain transfer analysis using COMSOL Multiphysics and are in good agreement.

• Sensitivity and stability of diaphragm based FBG high pressure sensor is tested in shock test facility for its suitability in space environment.

Abstract

A temperature compensated diaphragm-type Fiber Bragg Grating (FBG) sensor for high pressure measurement, with a maximum pressure up to 700 bar, has been developed and packaged for different applications including space. In this device, a pressure sensitive FBG is bonded at the centre of a Martensitic Stainless Steel (APX-4) diaphragm and a temperature compensation FBG is bonded in the strain-free region of the same diaphragm. The experimental results obtained indicate a pressure sensitivity of 3.64 pm/bar and a non-linearity + Hysteresis error of 0.75% of full-scale pressure in the range of 0 to 700 bar, with a correlative coefficient of 99.99%. Structural and thermal stress analyses of the diaphragm and strain transfer analysis from the diaphragm to FBG have been carried out using Comsol Multiphysics software, to validate the experimental results. Shock tube and vibration tests have also been carried out to study the dynamic characteristics and stability of the sensor. The pressure sensor developed, can be used to measure both static/dynamic pressures of cryogenic propellants as well as pneumatic pressure in rockets, missiles and launch vehicles.

Introduction

Accurate, stable and precise pressure measurement is of prime importance in several engineering applications, especially in aerospace domain. The space environment is characterized by harsh conditions such as radiation (high energy protons, heavy ions, γ rays), microgravity, vacuum, large range of thermal fluctuations, shock and mechanical vibrations [1]. Further, electrical pressure sensors are vulnerable to electrical sparking which may lead to explosions and they are prone to electromagnetic interference (EMI). Fiber optic sensing systems offer several advantages, especially for space applications, such as immunity to EMI, freedom from electrical sparking, lightweight and flexible harness that reduces mass significantly, distributed/quasi distributed sensing at remote locations, high sensor capacity due to efficient multiplexability, low power requirement per sensor, multi-parameter sensing, etc. [2].

In the category of fiber optic sensors, Fiber Bragg Grating (FBG) sensors are being employed in a variety of applications for measuring strain, temperature, pressure, displacement, etc. FBG sensors are also being explored for measuring different parameters in space applications [2]. Further, considering the harsh environment and typical conditions, there is a need to develop a stable, high-pressure optical sensor network for space applications. In this work, we have developed a FBG based sensor for high pressure measurements (up to 700 bar) that can be used in launch vehicles and satellites.

In a FBG sensor, pressure can be applied on the FBG as a uniform pressure, transverse compressive load or longitudinal tensile/compressive load [3]. Bare FBG sensor cannot be used directly for pressure measurement due to its comparatively low sensitivity to pressure as well as its fragility. Xu et al. first reported that a bare FBG sensor has a pressure sensitivity of 3.04 pm/MPa, which is too low for practical applications. Bare FBGs also have a temperature sensitivity of 10.45 pm/⁰C [4]. Several techniques have been reported in literature, to enhance the pressure sensitivity like polymer coated FBGs [[5], [6], [7], [8], [9], [10]] and mechanical amplification techniques [[11], [12], [13], [14], [15]]. However, some of these methods have relatively complicated construction and packaging methods. Among the polymer coated FBG pressure sensing techniques, the maximum pressure measured is below 100 bar. Long period gratings and photonic crystal fibres have also been used for pressure measurements; however, their measurement range is limited, from 0 to 10 bar.

Mechanical amplification techniques include the use of an elastic diaphragm or a combination of a diaphragm and a cantilever beam/fixed beam, where the diaphragm acts as a force collector. The sensitivity of these sensors depends on the material and geometric properties of the diaphragm and beam structure. Nellen et al. (2003) have demonstrated a pressure sensor with FBG mounted longitudinally between the ends of two concentric steel tubes, with a pressure sensitivity of 25 pm/MPa in the range of 0–1000 bar [16]. In this design, the temperature compensation is complicated as it required the outer tube to be constructed of 2 sections with different thermal expansion coefficients. Qi Jiang et al. (2011) have reported a lateral hydraulic pressure sensor with FBG bonded on the centre of an elastic metal film [17]. This sensor exhibited a pressure sensitivity of 23.8 pm/MPa in the range of 0–600 bar. Minfu Liang et al. (2017) have developed a FBG pressure sensor which utilized a diaphragm cantilever structure to measure pressure in the range of 0–100 bar with a sensitivity of 33.99 pm/bar; in this case the temperature compensation has been achieved in the range 5–70 °C [18]. Yong Zhao et al. (2018) have reported an FBG pressure sensor based on a diaphragm cantilever with a sensitivity of 25.82 pm/bar in the range of 0–20 bar [19]. The diaphragm-beam structure is relatively complex in construction and difficult to package. Liang et al. (2018) have reported a diaphragm based FBG pressure sensor capable of measuring pressures up to 500 bar with a pressure sensitivity of 3.57 pm/bar; here, the deformation of the diaphragm under pressure is transferred to a vertically bonded FBG [20]. It is interesting to note here that there are not many FBG based pressure sensors reported in the literature to measure high pressures (>500 bar). Also, FBG sensors are sensitive to strain and temperature. Typical value of strain sensitivity is 1.2 pm/με and temperature sensitivity is 13.7 pm/⁰C. Therefore, temperature compensation is necessary for accurate measurement of pressure and eliminate cross-sensitivity of FBG to temperature variations. Many temperature compensation techniques have been reported in literature, which include the reference grating method and Dual grating difference output method [21].

In this paper, a compact, diaphragm based, temperature compensated FBG high pressure sensor (up to 700 bar) is designed and developed for space applications. A complete design and analysis of diaphragm based FBG sensor performance, specially designed for high pressure measurement in launch vehicles/satellites is reported. With reference grating temperature compensation technique, the pressure sensor can accurately measure pressure in the temperature range −40 °C to 90 °C. Structural and thermal stress analyses of the diaphragm and strain transfer analysis from diaphragm to FBG have been carried out using COMSOL Multiphysics software to validate the experimental results. Shock and vibration studies have also been carried out to study the sensitivity, stability and dynamic characteristics of the sensor in view of space application.