4:30 PM - 4:45 PM
[PCG17-03] Development of the Autofocusing Subsystem for the Raman Spectrometer on the MMX Rover
Keywords:Martian Moons Exploration (MMX), Phobos, Raman Spectroscopy
Raman spectroscopy is a useful approach for the identification of minerals. Raman spectrometers have been developed or are under development for planetary missions. For example, NASA’s Perseverance rover has two Raman spectrometers, SuperCam [1] and Sherloc [2]. The Rosalind Franklin rover of ESA’s ExoMars mission incorporates the Raman Laser Spectrometer (RLS) [3] on board as well, where elements have been inherited for RAX. The rover of JAXA’s Martian Moons Exploration (MMX) has a Raman Spectrometer (RAX) an instrument to determine the surface composition and the formation conditions of Phobos. It also plays a critical role in the selection of the samples to be returned to the Earth [4]. RAX is much more compact as compared to those aboard the Mars rovers because of the requirements to fit in the volume of the much smaller MMX rover; the volume of the RAX main box is 81×98×125 mm3 [5, 6]. Its weight is only 1.4 kg.
In-situ Raman measurements require the high transmission of the optical path and precise focusing in order to acquire high-quality Raman spectra, since the intensity of Raman scattering is very low (10-4 of Rayleigh scattering). Thus, a small, lightweight, robust, and power-saving focusing system (the Autofocusing Subsystem, AFS) is designed as a part of the RAX instrument. The AFS is primarily made of two parts, Light Shuttle Objective (LSO) and actuator system [7]. The LSO moves up and down for focusing the laser spot onto the surface of Phobos and to collect the Raman scattered light from the sample. It is attached to the lead screw and driven by a stepping motor. Because of the small volume, no encoder was implemented into the system; the position of the LSO is estimated by counting the pulses transmitted to the motor.
To verify the capability of this design, the development model (DM) of the AFS was manufactured for the first integrated performance test. The DM has the same design as the engineering/qualification model (EQM), but is not subject to the qualification tests. For the DM, we conducted three test campaigns: functional test to verify the design and function of the AFS, performance test using a breadboard laser/spectrometer to acquire Raman spectra of actual samples, and vibration test to detect any workmanship errors during its assembly. These test campaigns were all successful [8].
Then the EQM was manufactured and a thermal vacuum test was conducted. The EQM was subject to a bakeout for more than 72 hours at a temperature of 70℃. Then it was cooled down to 15℃ at a rate of 10℃ per hour. When the temperature was stabilized at 15℃, the pressure was 2.0×10-4 Pa. Under this condition, the LSO was moved for 200 up-and-down cycles. During the movement, no stuttering or sudden temperature rise was observed. We also observed that the LSO moved smoothly at the temperature of -20℃ and the pressure of 1.6×10-4 Pa. These results proof the capability of the AFS to function under a cold representative thermal environment consistent with the environment aboard the rover on Phobos.
[1] Wiens, R. et al. (2021) Space Sci. Rev., 217, 4.
[2] Beegle, L. et al., (2015) SHERLOC: Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals, IEEE Aerosp. Conf. Proc.
[3] Rull, F. et al. (2017) Astrobiology 17, 627-654.
[4] Cho, Y. et al. (2021) “In-situ science on Phobos with the Raman spectrometer for MMX (RAX): preliminary design and feasibility of Raman measurements” Earth, Planets and Space (under review).
[5] Schröder, S. et al. (2020) LPSC-51. Abstract #2019.
[6] Hagelschuer, T. et al. (2019) “The Raman spectrometer onboard the MMX rover for Phobos”. IAC 2019.
[7] Rodd-Routley, S. et al. (2021) LPSC-52. Abstract #1923.
[8] Mori, S. et al. (2021) LPSC-52. Abstract #1757.
In-situ Raman measurements require the high transmission of the optical path and precise focusing in order to acquire high-quality Raman spectra, since the intensity of Raman scattering is very low (10-4 of Rayleigh scattering). Thus, a small, lightweight, robust, and power-saving focusing system (the Autofocusing Subsystem, AFS) is designed as a part of the RAX instrument. The AFS is primarily made of two parts, Light Shuttle Objective (LSO) and actuator system [7]. The LSO moves up and down for focusing the laser spot onto the surface of Phobos and to collect the Raman scattered light from the sample. It is attached to the lead screw and driven by a stepping motor. Because of the small volume, no encoder was implemented into the system; the position of the LSO is estimated by counting the pulses transmitted to the motor.
To verify the capability of this design, the development model (DM) of the AFS was manufactured for the first integrated performance test. The DM has the same design as the engineering/qualification model (EQM), but is not subject to the qualification tests. For the DM, we conducted three test campaigns: functional test to verify the design and function of the AFS, performance test using a breadboard laser/spectrometer to acquire Raman spectra of actual samples, and vibration test to detect any workmanship errors during its assembly. These test campaigns were all successful [8].
Then the EQM was manufactured and a thermal vacuum test was conducted. The EQM was subject to a bakeout for more than 72 hours at a temperature of 70℃. Then it was cooled down to 15℃ at a rate of 10℃ per hour. When the temperature was stabilized at 15℃, the pressure was 2.0×10-4 Pa. Under this condition, the LSO was moved for 200 up-and-down cycles. During the movement, no stuttering or sudden temperature rise was observed. We also observed that the LSO moved smoothly at the temperature of -20℃ and the pressure of 1.6×10-4 Pa. These results proof the capability of the AFS to function under a cold representative thermal environment consistent with the environment aboard the rover on Phobos.
[1] Wiens, R. et al. (2021) Space Sci. Rev., 217, 4.
[2] Beegle, L. et al., (2015) SHERLOC: Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals, IEEE Aerosp. Conf. Proc.
[3] Rull, F. et al. (2017) Astrobiology 17, 627-654.
[4] Cho, Y. et al. (2021) “In-situ science on Phobos with the Raman spectrometer for MMX (RAX): preliminary design and feasibility of Raman measurements” Earth, Planets and Space (under review).
[5] Schröder, S. et al. (2020) LPSC-51. Abstract #2019.
[6] Hagelschuer, T. et al. (2019) “The Raman spectrometer onboard the MMX rover for Phobos”. IAC 2019.
[7] Rodd-Routley, S. et al. (2021) LPSC-52. Abstract #1923.
[8] Mori, S. et al. (2021) LPSC-52. Abstract #1757.