The SEIS seismometer is based on a six-axis hybrid instrument composed of:
- a sphere including three Very Broad Band (VBB) seismic probes and their temperature sensors,
- three Short Period (SP) seismic probes and their temperature sensors,
- an acquisition electronics box (e-box: SEIS AC, SEIS DC/DC, ASICS) and the feedback boards for the VBB, SP probes and the MDE deployment system,
- a deployment system (DPL),
- a software (S/W).
Its mass is about 3 kg.
Its consumption varies around 1W depending on the modes.
SEIS seismometer main performances are:
VBB -9 m.s-² Hz-½ from 10-³ up to 10 Hz
SP < 5 10-8 m.s-² Hz-½ from 10-² up to 100 Hzx
The sphere harbours the VBB probes (long period seismometers). It is the "noble part" of the instrument. It has House Keeping for the best functioning of the VBB probes.
- It has a thermal screen and torlon plots to reduce the temperature variations of the seismometers as much as possible
- It keeps the probes in vacuum
- It contains temperature sensors (House Keeping - HK) and inclinometers for the exploitation of the data measured by the VBB.
The broad-band probes (VBB) are oblique pendulums (see the picture below).
How does it work?
The spring and the pendulum mass are perfectly balanced. When the ground moves, the pendulum begins to move. This movement is registered by the DCS sensor. The balance mechanism can adjust the pendulum balance in real use conditions (poorly known gravity, levelling flaw, influence of the temperature on the pendulum balance). The pivot should enable the rotation of the mobile part around its axis without any friction.
The displacement sensor is constituted of electrodes placed on the fixed and mobile parts. The electrical characteristics thus constituted form an image of the position of the sensor's mobile part.
The proximity electronics transforms these characteristics in easily measured tension. It is transmitted to the acquisition electronics. The feedback coil allows the servitude of the pendulum to improve the performances (increase of the bandwidth). The intensity that runs in the coil is delivered by the feedback board "SEIS-FB" located in the e-box. This intensity is generated depending on the measurement of the pendulum displacement.
The short-period probes
The three short-period probes record the ground acceleration along their sensibility axes. The movement of a mobile mass is detected by a synchronous detection sensor. To extend the bandwidth, the mobile mass is locked on the zero thanks to the feedback loop, which acts on a magnetic actuator of the coil-magnet kind. The feedback loop controls the current injected in the coil, which is plunged in the constant field.
The deployment system
The deployment structure is composed of a frame with three deployable feet and the sphere. The sphere support must ensure an inclination of ±18° in the lander. Two approaches are envisioned for the deployment system: a cardanic system (composed of one cardan ring and two motors) and a three-axis system (screw-nuts). Lastly, a system enables to lower the seismometer on the ground.
The flight software
One of the software programmes will run in the on-board computer of the lander (CDMS). It interacts with the seismometer and the lander in order to store the data in the mass memory and to transmit them to the Earth.
Its main functions are:
- to ensure the operational status of the instrument
- to record seismic data
- to perform the processing (compression)
- to generate the Telecommands (TC) received
- to send the pertinent data back by Telemetry (TM).
The software programme receives and interprets the commands sent from the Earth (TC). It switches on the instrument deployment and enables the installation sequence (sphere levelling, VBB seismometers re-centring) of the seismometer on Mars surface.
Once SEIS is installed and ready for the seismology experiment, the software programme switches to the operational mode. Its role is then to manage the instrument and collect data.
The instrument management consists in switching from one mode to the other to adjust seismometer operation depending on the available power (ex: "winter" mode when the power available is low, or "campaign" mode to maximize the scientific feedback when possible).
To collect data, the software programme performs several tasks:
- Recording and compressing measurements to minimize the space occupied in the memory.
- Generating "quick look data". These highly compressed data are systematically sent to the Earth. They record when interesting seismic events happen.
- Downloading (in telemetry) raw data (without information loss) corresponding to the periods selected by the scientists.
- Erasing useless data.
The flight complexity of the flight software programme is due to the automation of all the operations on the seismometer, to the low electric power available, and to the volume of data to be stored. In fact, the seismology requires continuous measurement recording, owing to the impossibility to forecast the seism. The volume of data thus recorded is much higher than the transmission capabilities between Mars and the Earth.