E. Alberti

MIMA, a miniaturized infrared spectrometer for Mars ground exploration: Part III. Thermomechanical design

The Mars Infrared MApper (MIMA) is a FT-IR miniaturized spectrometer which is being developed for ESA ExoMars Pasteur mission. MIMA will be mounted on the rover mast and so it must be compact and light-weight. The scientific goals and its optical design are presented in two companion papers [1] [2]; the focus of this work is on the thermomechanical design and testing. The instrument design faces challenging constraints both from the expected environment and the allocated resources. The temperatures during operation are expected to be from -120 °C to +30 °C with the presence of a low density but thermally effective atmosphere. Severe dynamic loads are foreseen during launch and moreover at landing on Mars. The overall size is limited to an envelope of 140 mm x 140 mm x 120 mm and the mass to less than 1 kg. The expected performances of this instrument should be comparable with those of much heavier ones built in the past. An instrument compliant with these constraints has been conceived, introducing many innovative solution with respect to the past experiences and making use of intensive modeling and testing to prove the survival to the harsh environment. Among the most challenging problems the mounting of the brittle KBr optics and the matching of its thermal expansion coefficient with that of the supporting aluminium structure, in a temperature interval of more than 200 °C. Most of the components have undergone thermovacuum tests in the low temperature range because none of them was expected to be used in the -100 °C range.

MIMA, a miniaturized Fourier infrared spectrometer for Mars ground exploration: Part I. Concept and expected performance

The Mars Infrared MApper (MIMA) is a FT-IR miniaturised spectrometer which is being developed for ESA ExoMars Pasteur mission. The Martian Infrared MApper Fourier Spectrometer is designed to provide remote measurements of mineralogy and atmosphere of the scene surrounding a Martian rover and guide it to key targets for detailed in situ measurements by other rover experiments. Among the main scientific objectives of the MIMA instrument are to assist the rover in rock/soils selection for further in-situ investigation and to identify rocks and soils on the Martian surface which provide evidence of past/present biological activity. The instrument is also designed to measure the water vapour abundance and vertical distribution and its diurnal and seasonal variation, dust opacity, optical properties, composition, diurnal and seasonal variation. The instrument is a double pendulum interferometer providing spectra in the 2 - 25 μm wavelength domain with a resolving power of 1000 at 2 μm and 80 at 25 μm. The radiometric performances are SNR > 40 in the near infrared and a NEDe = 0.002 in the thermal region. The instrument design is very compact, with a total mass of 1kg and an average power consumption of 5 W.

Mechanical Filters for Accelerometers: Design and Metrological Characterization

Accelerometers saturation may occur when measuring mechanical events characterized by a wide frequency bandwidth. The problem usually arises when the mechanical vibration frequency band exceeds the accelerometer bandwidth. Mechanical filtering is then a smart technique to reject undesired contribution above a certain cut-off frequency and avoid transducer saturation. Commercially available mechanical filters exhibit limits, in particular regarding cut-off frequency selection and insertion error. This paper describes a method to realize a mechanical filter that takes advantage of the mechanical link between the vibrating surface and the accelerometer to obtain the user-demanded characteristics for both mono-axial and tri-axial accelerometers. The designed filter frequency response function is first estimated by means of finite elements method simulations, and then experimentally verified. The effects of temperature and aging on filter performances are also presented

Experimental characterisation and modelling of a pyroelectric sensor

This paper presents the activities performed for the modelling and experimental characterisation of a pyroelectric infrared detector. The work focuses on a LiTaO3 sensor which has been used as detector in the Long Wavelength Channel of a double channel IR spectrometer devoted to the study of Mars atmosphere, the MarsExpress Planetary Fourier Spectrometer, PFS. The need for an experimental characterization arise from the need of modelling the complete spectrometer for a correct interpretation of the scientific data collected while orbiting around Mars. The sensor of interest has been characterised along with its amplifying and conditioning proximity electronics. Because of the final use of the detector, i.e. FTIR spectrometry, the experimental characterization focuses on the frequency response and non-linear behaviour which respectively affects spectral responsivity and the presence of spectral features ghosts. Mathematical models available in literature describing the pyroelectric phenomena usually neglect the dependence of thermal characteristics on temperature and are intrinsically linear, therefore unfit for our needs. Because of the lack of information about the detector building characteristics, an accurate a priori model could not be straightforward implemented. An a posteriori model, derived from an identification process based on the detector testing has been developed and validated in order to have a simulation tool for the full spectrometer. The sensor exhibit nonlinearities, depending on all factors influencing the sensing element average temperature: incident infrared power, housing temperature. These nonlinearities can be traced back to the dependence on temperature of thermal characteristics of the sensing element, pyroelectric coefficient and the thermal capacity of LiTaO3 and on the nonlinearity of the radiative heat exchanges.