I.

INTRODUCTION

The Fiber Sensor Demonstrator (FSD) provides temperature and pressure monitoring for various Proba-2 subsystems, including a hybrid propulsion system. It includes a redundant architecture developed for critical fiber-optic components, and a lightweight, ruggedized fiber-optic signal harness for use in space.

Spacecraft propulsion subsystems [1] require an array of different sensors to monitor the propellant tank pressure and integrity, the gas-line valve temperatures and status, the propellant mass flow rate and the thruster temperatures. This is currently performed using various electronic sensors at a substantial mass and performance penalty due to the shielding requirements of these sensors. The current electrical sensors have a number of drawbacks, including sensitivity to EMI, susceptibility to sparking, low sensor capacity per wire, sensor response times, and the proximity requirements for the associated electronics. The optical fiber sensors, provide an alternative that can solve many of these issues [2].

II.

FIBER SENSOR DEMONSTRATOR SYSTEM DESCRIPTION

FSD has been launched on ESA’s Proba-2 satellite in November 2009. It contains sixteen temperature sensors to measure the temperature at different locations in the satellite, and one High-Temperature (HT) sensor to measure the transient high temperature in the thruster, as well as one Pressure/Temperature (P/T) sensor to measure the xenon tank pressure. First set of on-orbit test data were obtained in January 2010.

The advantages of the FBG approach include a central interrogation module that controls the variety of different fiber-optic sensors including temperature, pressure, actuator status, and propellant leakage. The combination of both parallel signal distribution and serial wavelength division sensor multiplexing along single strands of fiber enables high sensor capacities.

Fiber-optic sensors employ a signal link via an optical fiber, allowing the subsequent electronic processing to be located remotely from critical areas of the spacecraft. This facilitates minimization of electromagnetic interferences and avoids the safety issues associated with electronics near the propulsion subsystem. Signals on a fiber-optic line are bidirectional, allowing a single fiber to carry both the source signal to the optical sensor and the reflected return signal back to the interrogation system. The temperature of FSD Interrogation module is also acquired through two internal thermistors AD590.

The FSD employs a redundant temperature-compensated narrow-band reference FBG in series with the external FBG sensors (see Fig. 1) for each of the six sensor lines to provide an absolute wavelength reference. The FBG reference is kept in a small athermal packaging, keeping its wavelength practically independent from temperature (about 0.001 nm/C). The two thermistors ADS590, permit to confirm the accuracy of the FBG against long term potential shift of the FBG wavelength. In addition there are the readings of the laser diodes internal temperature provided by the diodes circuit. The thermistors, the FBG references and the temperature of the laser diodes give a good picture of the temperature evolution within the Interrogation module.

Fig. 1

Schematic of FSD fiber-optic sensor signal harness.

The Pressure/Temperature (P/T) sensor employs a heat-treated, orbitally-welded stainless-steel housing that is suitable for direct contact with propellants such as hydrazine. It can be employed for either single-ended or differential pressure measurements. The P/T sensor was proof-tested in N2 at 1200 psi, relative to the maximum operating pressure of 600 psi. The FSD innovative P/T sensor uses multiple FBG gratings with special mounting to provide simultaneous pressure (P-sense) and temperature (T-sense) measurements. The pressure readings are independent of the gas composition (see Fig. 5). The integral temperature measurements are employed to correct for the effects of temperature on the pressure readings.

The High Temperature (HT) Sensor measures the transient response of the SSTL thruster during firing, it is protected in a microtubing tip that is only 0.34 mm O.D. for minimal thermal mass.

The Four lines of Distributed FBG Sensors have a special proprietary packaging that was developed to nearly triple the effective sensor sensitivity to temperature (Δλ/ΔT ≅0.03 nm/°C), permitting a resolution of 0.05 °C.

Table 1:

Specifications of the FSD Sensors.

A tuneable Fabry Perot, controlled by a fast piezo-electric element, permits the fiber laser to sweep the spectral range and read the intensity at each wavelength between 1520 and 1560 nm. A fixed Fabry-Perot interferometer of 50 GHz follows the tuneable one and is the basic component to measure the wavelength of the FBGs. Its accuracy is a good sign of the sensor functionality in space.

Fig.2 presents the photographs of the location of the different sensors installed on Proba-2, and Fig.3 illustrates the Interrogation module box.

Fig. 2:

Photograph of flight P/T sensor integrated with the SSTL propulsion system for Proba-2

• (b) FBG Sensors integrated with SSTL Propulsion Subsystem

• (c) Photograph of a packaged flight FBG T-sensor line with protective cabling and four sensors

• (d) FBG Sensors integrated with Propulsion thruster

• (e) Pressure/Temperature (P/T) and High Temperature (HT) sensors mounting on Proba-2

Fig. 3:

Photograph of the Interrogation module

The interrogation module (15cmx17cmx12cm) is under 1.3 kg, and requires less than 2.5 W peak power.

The DAQ PCB is employing spacegrade and MIL-883 milgrade components for the critical functions, including a 16 bit A/D converter and serial interface D/A drivers from Maxwell. To minimize the power consumption, micropower op amps are employed for the analog signal processing. The DAQ PCB includes the control/monitoring electronics for the tuneable fiber-laser diode pumps and filters, as well as eight fiber-coupled InGaAs detector channels with individual preamplifiers for the six external FBG sensor lines and two internal reference lines.

III.

ON ORBIT MEASUREMENTS

MPBC is monitoring periodically the performance of fiber laser and the FBG sensors. The FSD unit measures Proba-2 temperature variations in increments over an orbit, when requested by ESA. The following figures provide examples of the measurements.

Fig. 4:

Temperature measurements with FBG sensor at the thruster top, and a thermocouple at the thruster bottom, during a preheat experiment

Fig. 5:

Scan of the wavelengths of the Line during the orbit cycle

Fig. 6:

Temperature measurements provided by the four sensors of Fiber Line_sens6(Bus)

Fig. 7:

Intensity of the fixed Fabry Perot Scans (50 GHz/scan) in a Zoom over the pixels 2700-3000 of the tuneable Fabry-Perot (total 45 nm/4000 pixel width)

Fig. 8:

Main FSD parameter evolution since the last on ground measurement. Only the temperature follows the primary vertical axis (left side), all the other parameters follows the secondary axis (right)

Fig. 9:

Calibration of the RADFET vs the time spent in space

Fig. 10:

Main FSD parameter evolution since the last on ground measurement

IV.

FBG TEST WITH HIGH GAMMA RADIATION DOSE

A test was made by a third party (Kayser-threde, Germany) addressing the stability of the FBGs in equivalent harsh radiation -space environment. MPBC prepared three kinds of packaging:

The FBGs were submitted to high radiation level (25Mega-rad at 420 krad/h) - 250 times larger than the 100 krad commonly requested. The effect on the FBGs was a small shift (200 pm, equivalent to the shift in the Central Wavelength (CWL). A second radiation test (250 krad, at 4.1 krad/h) confirmed the small CWL shift. For 100 krad the CWL shift is less than 5 pm at the 4.1 krad/h, and less than 10 pm at 420 krad/h.

Fig. 11:

Central-Wave-Length shift with 25 Mrad at a dose rate of 420rad/h

Fig. 12:

CWL shift at 300 krad total dose, comparison of two dose rates (420 vs 4.1 rad/h)

V.

DISCUSSIONS AND CONCLUSION

The Fixed Fabry Perot signal is reduced by 15-20% after the 6.5 years. All the other FSD parameters seem more constant, with a trend of decreasing signal of the Laser diode pumps by about 5-10% over the 6.5 years.

The FBG sensors are at the end of the circuit and shall see the effect of all the effects received by the other components in the opto-electronic circuits. The evolution of the FBGs peaks from three different lines showed about 3-10% reduction in their intensity (Fig. 10), depending on the sensor itself.

The FSD confirmed the reliability to use Optical Fiber Sensors in space missions and demonstrate its capability to monitor different parameters such as pressure and temperature at different level. The FSD on Proba-2 opens the door to a variety of other fiber sensor applications such as launch vibrations, liquid level in tanks and high temperature atmospheric re-entry.