Driving in Milli-G: The Flight Model of the MMX Rover Locomotion Subsystem and its Integration & Testing in the Rover

IDEFIX is a 25 kg four-wheeled rover that will explore the surface of the Martian Moon Phobos in 2027. The rover is jointly developed by the German Aerospace Center (DLR) and the Centre National d'Etudes Spatiales (CNES) and will be brought to Phobos within the Japan Aerospace Exploration Agency's (JAXA) Martian Moon eXploration (MMX) mission. Being the world's first wheeled system to drive in milli-gravity, IDEFIX's locomotion deserves special attention. This paper gives an overview of the locomotion subsystem (LSS) of the rover, which is entirely developed and built by the Robotics and Mechatronics Center of DLR (DLR-RMC). A representative LSS, mounted on an IDEFIX prototype, is shown in Figure 1. The LSS is tailored to the needs for the IDEFIX rover and the most important, sizing challenges and functional requirements are summarized. It is then shown how the final flight model (FM) design answers to these requirements. The assembly, integration and testing (AIT) with respect to the LSS consists of several steps of integration and testing at different facilities as well as a comprehensive test sequence once the rover is mostly integrated. Since the LSS is an important, interconnected and the functionally most complex subsystem of the rover, some functionalities could only be tested once the LSS was integrated into IDEFIX. These AIT aspects are therefore summarized in this paper as well

Fault Detection, Isolation and Recovery in the MMX Rover Locomotion Subsystem

In any mechatronic system, faults can occur. Likewise also in the MMX rover, which is a wheeled rover mutually developed by CNES (Centre national d'études spatiales) and DLR (German Aerospace Center), intended to land on Phobos. An essential part of the MMX rover is the locomotion subsystem which includes several sensors and eight motors actuating the four legs and the four wheels. In each of these components and their interfaces, there is a possibility that faults arise and lead to subsystem failures, which would mean that the rover cannot move anymore. To reduce this risk, the possible faults of the MMX locomotion subsystem were identified in a FMECA study and their criticality was classified, which is presented in here. During this examination, the criticality was graded depending on different mission phases. With the help of this study, the hardware, firmware and software design were enhanced. Fur- ther, certain fault detection, isolation and recovery strategies were implemented in the locomotion firmware and software as well as in the full rover software

Mobility on the Surface of Phobos for the MMX Rover - Simulation-aided Movement planning

The MMX Rover, recently named IDEFIX, will be the first wheeled robotic system to be operated in a milli-g environment. The mobility in this environment, particularly in combination with the interrupted communication schedule and the activation of on-board autonomous functions such as attitude control requires efficient planning. The Mobility Group within the MMX Rovers Team is tasked with proposing optimal solutions to move the rover safely and efficiently to its destination so that it may achieve its scientific goals. These movements combine various commands to the locomotion system and to the navigation systems developed by both institutions. In the mission's early phase, these actions will rely heavily on manual driving commands to the locomotion system until the rover behavior and environment assumptions are confirmed. Planning safe and efficient rover movements is a multi-step process. This paper focuses on the challenges and limitations in sequencing movements for a Rover on Phobos in the context of the MMX Mission. The context in which this process takes place is described in terms of available data and operational constraints

MMX - development of a rover locomotion system for Phobos

The MMX mission (Martian Moons eXploration) is a robotic sample return mission of the JAXA (Japan Aerospace Exploration Agency), CNES (Centre National d'Etudes Spatiales ) and DLR (German Aerospace Center) for launch in 2024. The mission aims to answer the question on the origin of Phobos and Deimos which will also help to understand the material transport in the earliest period of our solar system and the most important question how was the water brought on Earth. Besides the MMX mothership (JAXA) which is responsible for sampling and sample return to Earth a small rover which is built by CNES and DLR shall land on Phobos for in-situ measurements similar to MASCOT (Mobile Asteroid Surface Scout) on Ryugu. The MMX rover is a four wheel driven autonomous system with a size of 41 cm x 37 cm x 30 cm and a weight of approx. 25 kg. Multiple science instruments and cameras are integrated in the rover body. The rover body is basically a rectangular box, attached at the sides are four legs with one wheel per leg. When the rover is detached from the mothership, the legs are folded together at the side of the rover body. When the rover has landed passively (no parachute, braking rockets) on Phobos, the legs are autonomously controlled to bring the rover in an upright orientation. One Phobos day lasts 7 earth hours, which gives for the total mission time of 3 earth months, the number of about 300 extreme temperature cycles. These cycles and the wide span of surface temperature between day and night are main design drivers for the rover. This paper gives a short overview on the MMX mission, the MMX rover and a detailed view on the development of the MMX rover locomotion subsystem

MMX Rover Locomotion Subsystem - Development and Testing towards the Flight Model

Wheeled rovers have been successfully used as mobile landers on Mars and Moon and more such missions are in the planning. For the Martian Moon eXploration (MMX) mission of the Japan Aerospace Exploration Agency (JAXA), such a wheeled rover will be used on the Marsian Moon Phobos. This is the first rover that will be used under such low gravity, called milli-g, which imposes many challenges to the design of the locomotion subsystem (LSS). The LSS is used for unfolding, standing up, driving, aligning and lowering the rover on Phobos. It is a entirely new developed highly-integrated mechatronic system that is specifically designed for Phobos. Since the Phase A concept of the LSS, which was presented two years ago [1], a lot of testing, optimization and design improvements have been done. Following the tight mission schedule, the LSS qualification and flight models (QM and FM) assembly has started in Summer 2021. In this work, the final FM design is presented together with selected test and optimization results that led to the final state. More specifically, advances in the mechanics, electronics, thermal, sensor, firmware and software design are presented. The LSS QM and FM will undergo a comprehensive qualification and acceptance testing campaign, respectively, in the first half of 2022 before the FM will be integrated into the rove

Analysis of Phase Shifts for a Rimless Wheel Rover

Space missions to explore Mars and Moon in situ are of high interest, but hazard terrain with steep slopes, soft soil, loose stones or high obstacles are a barrier for exploration rovers with common wheels. It was shown that rimless wheel rovers yield an enhanced performance in those environments while letting the complexity of the rover unchanged. They can cross higher obstacles, have a significant lower likelihood to get stuck and can move faster compared to common wheel rovers. At the Institute of System Dynamics and Control at the German Aerospace Center (DLR), a modular rimless wheel rover with flexible elements was designed for the purpose of planetary exploration in the above mentioned terrain. Due to the rimless wheels, the wheel circumference is not uniform. Hence, for an improved controllability of the full rover locomotion, it is reasonable to have control over the angular position of each wheel and especially over their angular position relative to each other. In this paper, several of these phase shifts are presented and compared. A setup in which the rover is driving straight forward is chosen to compare the behavior and performance of the different wheel configurations. The roll angle as well as the vertical movement of each rover segment are measured in simulation for different rover speeds and different phase shifts. A comparison between the different phase shift schemes in the sense of their waviness is made. The phase offsets have to be adjusted by means of a controller such that disturbances do not interfere significantly. For that reason, a PI controller was implemented. The study presented in this paper shows how the locomotion performance of a rimless wheel rover with compliant spokes can be improved by a well-tuned wheel position controller and the right choice of the phase shift configuration

Mathematical modelling of the elastic deformation of rimless wheels for a scout rover

In the present thesis, the mathematical modelling of the elastic deformation of a rimless wheel for a scout rover is performed. Thereby, the focus is on the elastic deformation of a single spoke. This spoke is supposed to be curved and compliant, moreover an external force is expected to impact on the spoke. To satisfy all of this, an Euler-Bernoulli cantilever beam is considered. In chapter 2 some assumptions for the mathematical modelling are stated to frame the problem. After that, the static elastic deformation of an initially straight Euler-Bernoulli cantilever beam is studied for small and large deformations in chapter 3. Thereafter the governing equation for the dynamic deformation of the intitially straight beam is stated and solved: First for the case that no external force is impacting on the beam by the method of separation of variables and after that the entire equation by Laplace transformations. Finally, in chapter 4 the elastic deformation of a beam that is curved at rest is studied. Therefore, the governing equation is deduced again, this time considering the curvature of the beam. This leads to a partial differerential equation of order six. Another approach is finally made by conformal mappings

Locomotion Control Functions for the Active Chassis of the MMX Rover

JAXA's Martian Moons eXploration Mission (MMX) includes the delivery of an exploration rover to the Mars moon Phobos in 2026, engineered by the Centre National d'Études Spatiales (CNES) and the German Aerospace Center (DLR). On Phobos, the gravity is about two thousand times lower than on Earth, which is why it is also called a milli-g environment. While the actual surface of Phobos is largely unknown, it is agreed within mission team that areas covered with regolith are to be expected. The design of the rover includes four actuated legs, with one rotational degree of freedom (DOF) each, and four individually driven, non-steerable wheels. The first task of the rover after landing on the surface of Phobos will be to deploy itself from its cruise configuration and to stand up on its wheels. Due to the rover dimensions, a full rotation of the legs and a wheel slip compensation are essential for this task. During operations, the locomotion system needs to provide drive and steer, as well as point turn abilities. Furthermore, the solar panel sun pointing, as well as the adjustment of scientific instruments, require that the rover aligns its body orientation and alters its body-to-ground distance. To satisfy all these requirements to the locomotion system of the MMX rover, appropriate locomotion control functions were developed. For driving curves and performing point turns, the skid steering method is applied and an "inching" locomotion mode for especially soft and steep terrain is adapted. In this inching locomotion, the front and rear wheel pair move alternately while the rover body moves up and down, which leads to enhanced traction performance compared to conventional driving in soft sand. This implies lower wheel slippage and sinkage resulting in a higher safety of the full rover system. The body orientation function, which is based on a kinematic control as well, provides a well coordinated movement and zero longitudinal slippage of the wheels during its execution. In this paper, a detailed description of the control algorithms is given and results from lab tests are presented and discussed. In a successful mission, these locomotion control functions will be the first ones actuating a wheeled mobile robot in milli-g environment

Design Of The MMX Rover Attitude Control System For Autonomous Power Supply

The Martian Moons eXploration (MMX) mission led by the Japan Aerospace Exploration Agency (JAXA) aims at studying Phobos and Deimos. It includes a small rover that will explore Phobos, carried out jointly by the French Space Agency (CNES) and the German Aerospace Center (DLR). With its fixed solar array, the rover does not receive enough energy in nominal driving orientation to recharge its battery. Therefore, an attitude control system manages the rover orientation, using the locomotion system and a sun sensor. The specifics of the control loop such as movements limitations or tranquilization periods are due to the dynamic interactions with the regolith. This paper presents the attitude control system (SKA) architecture, through the various challenges encountered: limited design flexibility with major changes during the development, small knowledge of Phobos' expected soil, very tight schedule to meet the JAXA delivery date. The design drivers were to create the simplest and most robust system to orient the rover in order to maximize the battery recharge, while ensuring the rover stability and addressing the above issues. A particular focus is made on the preliminary simulation results. Finally, the way forward is outlined with the SKA development and validation steps

Wheeled locomotion in milli-gravity: A technology experiment for the MMX Rover.

For the Martian Moons eXploration (MMX) mission of the Japan Aerospace Exploration Agency (JAXA), the French Centre National d'Études Spatiales (CNES) and the German Aerospace Center (DLR) jointly develop a wheeled exploration rover. This paper will discuss the planned analysis of wheeled locomotion of the MMX Rover. The focus will be on the expected challenges, the methods to overcome them and the plans on how to achieve a better understanding of wheeled locomotion in a milli-g environment by performing and analyzing a set of driving activities on Phobos. With the MMX Rover being the first wheeled system landing on a small body, there is no experience on how such a system will behave. Topography, rocks and regolith at the landing site will have a large impact on the achievable performance. To ensure safe operations, the locomotion system, developed at DLR-RMC, is designed to master a large variety of environments: The wheels are designed for optimal traction on soft regolith and the legs are actuated individually. Thereby, advanced maneuvers like inching locomotion allow the rover to climb slopes beyond its regular traction limits. In parallel to the hardware and software development, a detailed simulation environment was established, which both assists in the design of the rover and will serve as a planning tool during driving operations. During the mission, specific locomotion manoeuvres are performed and the downlinked data is then analyzed in detail to obtain a comprehensive understanding of the achieved performance and its constraints. The telemetry from the rover will be used to reconstruct the situation in simulation. After correlation, the simulator is used to get a deeper understanding of the wheeled locomotion dynamics under milli-g conditions. Of particular interest are the control strategies chosen and the influence of environmental characteristics on the rover's maneuverability, traction achieved, obstacle traversal, and controllability. Furthermore, the performance of specific components, like the traction generated by the wheels and the actuator performance, will be compared against expected behavior. During the mission, the findings will be directly used to improve the day-by-day planning of the rover's driving phases. Later, the results will be further analyzed and published to enable better designs with reduced uncertainties for future wheeled locomotion in low gravity environments. The paper will give details on the planned maneuvers, strategies to analyze them as well as the expected results