Covering shock waves on mars induced by insight’s HP3-mole - efficient co-simulation using dem and multi-domain dynamics

NASAs discovery mission InSight (Interior Seismic Investigations, Geodesy and Heat Transport) scheduled for launch in 2018 will investigate the interior of Mars. For the subsurface locomotion, DLR’s self-impelling nail nicknamed the ”Mole” needs to hammer itself down to 5 m into the martian soil with less than 5 W of input power. A major focus during the Mole’s development has been on simulation and analysis using virtual prototypes. As certain aspects of the environmental conditions on Mars cannot be recreated and tested on earth, high-fidelity coupled simulations are required to achieve the accuracy needed. These co-simulations are composed by a multi-body based cross domain model for the hammering device and a discrete element model for the soil. The article focuses on the coupling strategy of both models as well solutions in terms of communication. Furthermore efficiency improvements of the computationally expensive DEM models will be presented. Using these approaches the detailed stroke cycle and shock wave propagation are analyzed. Allowing to evaluate the performance under martian or terrestrial conditions

Hammering beneath the Surface of Mars . Modellbildung und Optimierung des HP3-Mole

Um Antworten auf die Frage nach der Entstehung erdähnlicher Planeten zu geben, werden im Rahmen der NASA-Marsmission InSight Untersuchungen des Wärmestroms im Planeteninneren durchgeführt. Damit der Wärmestrom im Inneren bestimmt werden kann, schlägt sich der HP3 Mole tiefer in des- sen Boden als alle Messinstrumente jemals zuvor. Um das gewünschte Ziel von fünf Metern Tiefe zu erreichen, wird ein spezieller Mechanismus als „selbsteinschlagender Nagel“ verwendet. Mehrkörpersimulationen des Systems unter Berücksichtigung der Kontaktdynamik ermöglichen einen Einblick in das mechanisch-dynamische Verhalten und die Wechselwirkungen beim Betrieb des Moles. Diese Modelle bilden die Ausgangsbasis für die Einbindung in eine Optimierungsumgebung, und damit die Optimierung des Mechanismus selbst. To answer the question for the evolution of earth-like planets, NASA’s In-sight mission addresses heat flow experiments of Mars’ interior. For measuring the red planet’s internal heat flow, the HP³ Mole hammers deeper below Mars’ surface than any instrument before. To achieve the desired penetration depth of five meters, a special hammering mechanism is used. Multibody simulations, combined with contact dynamics are used to gain further knowledge about the dynamic behaviour and interactions of the system. Based on these models optimizations using an optimization framework are carried out

Coaching Mascot for broad-jumping: Multi-criterial optimization of the arm trajectories for Mascot’s hopping locomotion

After a cruise phase of four years, Mascot will land on the asteroid 1999JU3. Due to the complex interaction of the lander with the terrain in low-gravity environments, a certain orientation of Mascot after descent cannot be achieved directly. Thus the mobility unit developed in the DLR Robotics and Mechatronics Center enables Mascot to upright into the nominal position and to relocate by hopping motion. As the dependence of the desired jumping trajectory on the trajectory of the mobility eccentric arm is complex, a suitable trajectory cannot be determined beforehand. As even parabolic flight campaigns do not allow for sufficiently long low-gravity phases, it is also not possible to define the trajectories based on measurements. Additionally the zero or low gravity phase during parabolic flights needs to be quite precise. Thus Mascot’s multibody dynamics model is used to check a priori created trajectories. Therefore the model has been verified using both parabolic flight campaigns as well as high precision reaction force measurements in order to determine the applicable frequency range the model is able to reproduce. These comparisons have shown, that the range important for jumping is covered by the model and only higher frequency structural vibrations are not covered yet. As contact dynamics between the lander and the asteroid are crucial to cover the post-impact behaviour, the ground contact has been modeled as an elasto-plastic surface based on the currently available but yet limited knowledge on 1999JU3. Applying the aforementioned model to multi-criterial optimization enables to systematically search for suitable trajectories in an automated process. Using the optimization framework MOPS (Multi-Objective Parameter Synthesis) developed by DLR Institute of System Dynamics and Control it is possible to find global optima for the trajectories using evolutionary strategies. The objectives for this optimization are specifically defined for the hopping scenario. Due to the micro-gravity environment it is also crucial to keep the upwards velocity safely below the escape speed. By using the multi-criterial approach it can be always maintained that Mascot’s escape velocity is never reached and minimized, while also improving performance and reliability of the hopping locomotion. As the evolutionary algorithms usually need a certain number of individuals to find optima, these numerous simulations can be used in order to further investigate the complex low-gravity interaction of the lander and the asteroid. Using these preliminary sets of training data, Mascot’s short mission phase can be supported and enhanced by the deeper insight into the dynamic interaction between lander and Asteroid

Software-in-the-Loop Simulation of a Planetary Rover

The development of autonomous navigation algorithms for planetary rovers often hinges on access to rover hardware. Yet this access is usually very limited. In order to facilitate the continued development of these algorithms even when the hardware is temporarily unavailable, simulations are used. To minimize any additional work, these simulations must tightly integrate with the rover’s software infrastructure. They are then called Software-in-the-Loop simulators. In preparation for the 2015 DLR SpaceBot Camp, a simulation of the DLR LRU rover became necessary to ensure a timely progress of the navigation algorithms development. This paper presents the Software-in-the-loop simulator of the LRU, including details on the implementation and application

Optimizing the Shape of Planetary Rover Wheels using the Discrete Element Method and Bayesian Optimization

The MMX Rover is a contribution by CNES and DLR to JAXA's Martian Moons eXploration (MMX) mission and will explore the surface of the Mars moon Phobos. As there haven’t been any successful landers on Phobos, little is known about the conditions on the moon’s surface. Additionally, the gravity on Phobos is only about 1/2000 of Earth’s gravity and the behavior of regolith in such low gravity is still a topic of active research. In order to design a suitable wheel for the MMX Rover, we made worst-case assumptions about the soil conditions and implemented them in a simulation model using the Discrete Element Method. This simulation is then used within an optimization loop that automatically tests different wheel shapes on their suitability for reliable locomotion on Phobos. The result is an optimized wheel shape as well as a dataset showing how different wheel shapes affect wheel performance. These results are now being used to design the wheels for the MMX Rover

Hammering beneath the surface of Mars - Modellbildung und Optimierung des HP3-Mole

Um Antworten auf die Frage nach der Entstehung erdähnlicher Planeten zu geben, werden im Rahmen der NASA-Marsmission InSight Untersuchungen des Wärmestroms im Planeteninneren durchgeführt. Damit der Wärmestrom im Inneren bestimmt werden kann, schlägt sich der HP3 Mole tiefer in dessen Boden als alle Messinstrumente jemals zuvor. Um das gewünschte Ziel von fünf Metern Tiefe zu erreichen, wird ein spezieller Mechanismus als „selbsteinschlagender Nagel“ verwendet. Mehrkörpersimulationen des Systems unter Berücksichtigung der Kontaktdynamik ermöglichen einen Einblick in das mechanisch-dynamische Verhalten und die Wechselwirkungen beim Betrieb des Moles. Diese Modelle bilden die Ausgangsbasis für die Einbindung in eine Optimierungsumgebung, und damit die Optimierung des Mechanismus selbst.To answer the question for the evolution of earth-like planets, NASA’s In-Sight mission addresses heat flow experiments of Mars’ interior. For measuring the red planet’s internal heat flow, the HP³ Mole hammers deeper below Mars’ surface than any instrument before. To achieve the desired penetration depth of five meters, a special hammering mechanism is used. Multibody simulations, combined with contact dynamics are used to gain further knowledge about the dynamic behaviour and interactions of the system. Based on these models optimizations using an optimization framework are carried out

Optimizing the Shape of Planetary Rover Wheels using the Discrete Element Method and Bayesian Optimization

The Martian Moons eXploration mission will include a rover to conduct in situ science and exploration on the Martian moon Phobos. The rover's locomotion system and especially its wheels require special consideration, as wheeled locomotion has never been employed in a comparably low gravity environment. This paper shows an automated approach to design the shape of rover wheels using high-fidelity simulation models based on the Discrete Element Method and a Bayesian Optimization algorithm. Four main parameters were selected to describe the wheel shape: grouser height, grouser number, grouser chevron angle and rim curvature. On Phobos, three main scenarios are important to ensure the rover's successful operation: Driving forward, backward, and up slopes. The optimization loop thus generates wheels within given limits for the four wheel shape parameters, runs a simulation of a single wheel experiment with free slip conditions for each scenario, and assesses the wheel's performance based on the distance traveled. The found wheel shape performs 64 % better than the previous prototype and, in combination with the found parameter relations, will guide the design of the wheels for the MMX mission

Stress Analysis of Soil Beneath Wheel for Planetary Rover by Using Discrete Element Method

Modeling the interaction between rover's wheel and soft terrain is of great importance in predicting or evaluating wheel performance for lunar and planetary rovers. The current wheel-soil interaction models predict or evaluate wheel performance under certain conditions. However, most of them do not consider the soil flow and deformation, and thus, they cannot capture the physical phenomena of wheel-soil interaction. Developing a new model that includes such physical phenomena contributes to the improvement of prediction accuracy. To develop such a model, it is necessary to analyze soil flow and deformation beneath the wheel. This study analyzes the stress distributions in the soil and soil flow fields beneath the grouser wheel by performing experiments using the discrete element method (DEM) with the particle simulation tool "Sir partsival". In addition to the single wheel simulation, two simple test simulations - an angle of repose test and a shear test - are performed to confirm the soil flow fields and stress distributions in the soil. In the field of fluid dynamics, (shear) stress generally exists along high gradients of flow velocity. These two tests confirm if the soil stress shows the same trend. The wheel simulations are performed under several slip conditions to investigate their influences on soil flow characteristics. The shape of the soil flow region - the shape of the slip line - can be divided into two patterns depending on the slip conditions. The stress increases along the slip line in all simulations. The findings of this study contribute to understanding the relationship between soil velocity field and stress distribution in the soil

Soil Flow Analysis for Planetary Rovers Based on Particle Image Velocimetry and Discrete Element Method

Planetary rovers commonly have grouser wheels to improve locomotion performance on deformable terrains such as the surfaces of the Moon or Mars. The biggest difference between the wheel with grousers and without grousers is soil behavior underneath the wheel since the grousers shovel the sand. Hence, analyzing soil flow gives us beneficial information on wheel-soil interaction. The detailed investigation for micro-scale soil behavior and gravity effect, which are difficult to see in the laboratory test, contributes to further understanding of wheel-soil interaction mechanics. This paper presents a two-dimensional discrete element method (DEM) simulation to analyze soil flow beneath the grouser wheel. The soil flow in the simulation is validated by comparing it with that of the measurements, which is visualized by particle image velocimetry (PIV). The comparison results are discussed from four perspectives: 1) wheel slip ratio, 2) traces formed behind the wheel travels, 3) entrance and leaving angles of the grousers, 4) soil velocity field. The results indicate that DEM could describe the soil deformation. This work would contribute to further investigations of the state inside the soil by using developed DEM simulation

Numerics of Discrete Element Simulations in Milli-g Environments: Challenges and Solutions

This work gives an overview of numerical issues arising when performing DEM soil simulations in milli-g environments. The cause for those issues as well as their effect on simulation results and numerical stability are discussed. Lastly, strategies and solutions to avoid numerical issues with floating-point arithmetics in milli-g environments are proposed, implemented and their effectiveness is reviewed