LOCO - Software for the MMX Rover Locomotion System

In situ exploration of the surface of celestial bodies requires hardware purposely developed for the target environment and sophisticated software architectures for efficient operation. So does the Martian Moons Exploration (MMX) rover IDEFIX, which is developed by the French Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR) to explore the surface of the Martian moon Phobos. It is equipped with a locomotion system responsible for the movements.The software for the rover locomotion system (LOCO-SW) translates system-level commands into actions executed by the locomotion system while monitoring the status of its mechanical and electrical components. To do so, it takes care of the following tasks: Reception of telecommands, sending of housekeeping telemetry, timing, inter-subsystem communication and signaling, processing of telecommands, SpaceWire communication with the LOCO electronic box for actuator control and monitoring of the sensor data. Few functionalities can rest upon existing frameworks, though many features have to be designed particularly for IDEFIX. The paper will present how they are defined and realized while various quality goals are fulfilled. Comprehensive testing is conducted to ensure the reliability and effectiveness of the software architecture. The test suite comprises unit tests and regression testing on hardware similar to flight hardware, as well as tests in simulation. These tests validate the functionality and performance of the software architecture and show its suitability for the MMX locomotion system. This paper focuses on the architecture, design and testing of the LOCO-SW. The design of the software will be presented, and the decisions will be linked back to the requirements and use cases that motivate the design.

1. Introduction

In 2024, the Japanese Aerospace Exploration Agency (JAXA) will launch the MMX mission to explore the origins of the Martian moons. The mission will involve a main spacecraft landing on the larger moon, Phobos, and carrying a small rover named Idefix, developed jointly by the Centre National d’Études Spatiales (CNES) and the German Aerospace Center (DLR). The rover, shown in Figure 1, weighs around 25 kg and will be dropped from a height of approximately 40 m onto the surface of Phobos. Once it reaches the surface, it will perform autonomous uprighting and begin its primary mission of exploring the surrounding area. The Locomotion Subsystem (LSS) realizes rover movement on the surface. It comprises three main components. The first component is a set of four shoulder modules. Each shoulder has one leg and one wheel attached, respectively, where their motors lie within the shoulder inside the chassis to be able to regulate their environmental temperature. Further, each of the four modules are equipped with an accelerometer and a torque sensor measuring the total torque applied to the leg. The second component is the electronic box (E-Box), which holds the required components to operate the motors and sensors in the locomotion modules, internal sensors like a gyroscope and a radiation monitor, and an FPGA for low-level control and monitoring. The locomotion module’s third component is the software (LOCO-SW) which runs the rover’s main onboard computer (OBC), transforms telecommands (TCs) received from Earth or other subsystems into trajectories commands for the E-Box and performs higher-level control and monitoring.