Details of a Researcher - ISHIGAMI Genya

Papers - ISHIGAMI Genya

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Evaluating and advancing scaling methods for reliable wheel mobility prediction in low-gravity environments

Omura T., Ishigami G.

Journal of Terramechanics 122 2026.04

ISSN 00224898

Predicting wheel mobility in low-gravity environments through Earth-based gravity tests offers a practical alternative to expensive parabolic flights and computationally intensive numerical simulations. However, an optimal scaling method for varying wheel speeds remains unidentified. This study systematically evaluated three scaling methods — Granular Scaling Laws (GSL), reduced-weight tests, and equal-mass tests — using Discrete Element Method simulations at three wheel angular velocities (π/10, π, and 2π rad/s). The methods were assessed based on their accuracy in predicting horizontal velocity, slip ratio, sinkage, and power consumption under free-driving conditions. GSL maintained errors below 5% across all conditions, while the equal-mass test showed velocity-dependent degradation with errors reaching 234% at high speeds. The reduced-weight test underestimated sinkage by over 100%, risking vehicle immobilization. An analytical framework employing an inertial number was developed to quantify soil flow characteristics, facilitating a comprehensive comparative analysis of the scaling methods. This analysis revealed that the equal-mass test inadequately captured dynamic flow phenomena, accounting for its velocity-dependent degradation. Conversely, GSL accurately reproduced soil flow characteristics under all conditions, enabling precise mobility predictions over a broad velocity range. These findings establish GSL as the most accurate and practical scaling approach for extraterrestrial rover mobility design and analysis.

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Deep probabilistic traversability with test-time adaptation for uncertainty-aware planetary rover navigation

Endo, M., Taniai, T., Ishigami, G.

SCIENTIFIC REPORTS 16 ( 1 ) 2026.02

ISSN 2045-2322

Traversability assessment of deformable terrain is vital for safe rover navigation on planetary surfaces. Machine learning (ML) is a powerful tool for traversability prediction but faces predictive uncertainty. This uncertainty leads to prediction errors, increasing the risk of wheel slips and immobilization for planetary rovers. To address this issue, we integrate principal approaches to uncertainty handling -- quantification, exploitation, and adaptation -- into a single learning and planning framework for rover navigation. The key concept is \emph{deep probabilistic traversability}, forming the basis of an end-to-end probabilistic ML model that predicts slip distributions directly from rover traverse observations. This probabilistic model quantifies uncertainties in slip prediction and exploits them as traversability costs in path planning. Its end-to-end nature also allows adaptation of pre-trained models with in-situ traverse experience to reduce uncertainties. We perform extensive simulations in synthetic environments that pose representative uncertainties in planetary analog terrains. Experimental results show that our method achieves more robust path planning under novel environmental conditions than existing approaches.

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Exhaustive evaluation of microgravity landing dynamics for small body exploration

Otsuki M., Baba M., Imada T., Yamato M., Ohashi T., Takahashi M., Himeno T., Maeda T., Ozaki S., Ishigami G., Kobayashi T., Kitazono K.

Acta Astronautica (Elsevier BV) 238 1 - 12 2026.01

ISSN 00945765

Landing dynamics play a crucial role in the success of planetary missions, particularly in low-gravity environments, and determining optimal landing sites continues to be challenging. This paper reports a method for landing safety evaluation based on spacecraft dynamics for landings on the surface layer of small bodies in the Martian moons exploration project. Unlike conventional short-duration contact explorations, spacecraft designed for the exploration of Martian moons will remain on the surface of Phobos for several hours post landing. Owing to low gravitational forces, small bodies exhibit an inadequate restoring force, which can result in overturning and rebounding of the spacecraft; moreover, the gravitational forces on Martian moons are several tens of times greater than that on other small bodies, such as Ryugu and Itokawa, necessitating a consideration of the impact acceleration during landing. In this study, a distinctive model was constructed based on low-gravity experimental results, with the capacity to replicate the flexibility of the spacecraft structure, tank sloshing, shock absorption, and interaction with the surface layer. Subsequently, we confirmed the unique behaviors of the spacecraft during its landing simulation on authentic terrain, and candidate landing areas were evaluated based on nine safety criteria. This evaluation process entailed the identification of safe landing areas, yielding a 20% reduction in the number of safe landing sites compared with that based exclusively on terrain.

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Slope-aware Adaptive Gaussian Process Sampling for Robotic Information Gathering on Rough Terrain

Tazaki M., Ishigami G.

2026 IEEE SICE International Symposium on System Integration Sii 2026 1619 - 1625 2026

Wheeled robots are increasingly utilized for environmental exploration and data collection in environments inaccessible to humans, such as the lunar surface. While adaptive sampling methods like the Upper Confidence Bound (UCB) can balance the trade-off between exploration and measurement data exploitation to model Regions of Interest (ROI), they do not explicitly incorporate robot traversability into the trade-off. Consequently, when faced with multiple scientifically promising locations, the UCB may select paths for the robot that are costly in terms of travel time, leading to time-inefficient surveys, particularly in sloped terrain. To address this limitation, we propose the Slope-aware Upper Confidence Bound (SaUCB), a novel acquisition function that integrates a traversability score based on terrain slope directly into the decision-making process. This allows the robot to explicitly balance exploration, exploitation, and traversal time. Through an extensive simulation study in realistic geological features of lunar terrain models, we demonstrate that the proposed approach generates significantly more time-efficient survey paths. Our method also demonstrates an improved trade-off between investigation time and modeling accuracy within the ROI compared to conventional approaches.

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Maneuverability Ellipsoid for Analyzing Sampling Space of Model Predictive Path-Integral Control for 4WIDS Robot Navigation

Ueda R., Ishigami G.

2026 IEEE SICE International Symposium on System Integration Sii 2026 545 - 552 2026

Four-wheel independent drive and steering (4WIDS) robot possesses high maneuverability, with exploiting its stemming 8-DoF (Degrees of Freedom) control inputs, for navigation itself in complex, obstacle-rich environments. In such scenarios, Model Predictive Path Integral (MPPI) control, a sampling-based approach to model predictive control, has emerged as a powerful technique. While MPPI can effectively handle nonlinear dynamics and non-differentiable cost functions, it confronts the fundamental challenge of curse of dimensionality, where the number of required samples grows exponentially with the dimension of the control space. Although dimensionality reduction of the control space is a known countermeasure for suppressing the computational burden of MPPI, a systematic analysis for rationally designing the reduced space remains as an open issue. This research therefore addresses this issue by first proposing a novel metric, Maneuverability Ellipsoid of the 4WIDS, to quantify the robot's maneuvering capability with regard to the multiple DoFs of the robot control inputs. Based on this ellipsoid, we numerically analyze a sampling method for selecting variables that contribute higher maneuverability in the MPPI framework. The robot maneuverability index is also proposed that is quantified by the size and shape of the manipulability ellipsoid. Through simulations, we demonstrate that this index significantly correlates with the success rate of robot navigation.

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Energy-Efficient Objects Collection with Tethered Mobile Robots Twins with Dynamic Grouping of Scattered Objects

Nakamura H., Nagai H., Ishigami G.

2026 IEEE SICE International Symposium on System Integration Sii 2026 1579 - 1586 2026

This paper addresses an energy-efficient object collection problem using two mobile robots twins, equipped with a flexible tool such as a net or tether. This problem is subject to a routing problem for a collection sequence that minimizes total energy consumption of both robots for collecting scattered, multiple objects. To this end, we propose a dynamic programming-based route planning method, where the robot state is defined by energy consumption and its position. As the flexible tool allows the robots to collect a number of objects all at once, we implement a grouping circle of the tool, the radius of which depends on the number of the collected objects. The method incorporates a load-dependent velocity model as well as the load-dependent radius of object grouping circles to realistically reflect robot dynamics during the cooperative collection and transportation of the objects. Simulations were conducted under various scenarios with different object distributions, start positions of the robots, and the maximum velocities of the robots. The simulation results demonstrate that the proposed method reduces both energy consumption and travel time compared to baseline methods.

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Granular Scaling Laws for accurate prediction of wheel mobility on slopes in low-gravity environments

Takuya Omura, Genya Ishigami

Journal of Terramechanics (Elsevier BV) 120 101085 - 101085 2025.10

ISSN 00224898

This study proposes an enhanced Granular Scaling Law (GSL) for accurately predicting wheel mobility on sloped terrain in low-gravity environments. First, wheel driving simulations were conducted using the Discrete Element Method (DEM) with a lunar regolith simulant under Earth's gravity. The GSL was then applied to convert simulation data obtained under various slope angles and wheel angular velocities into corresponding predictions for lunar gravity. These GSL-based predictions were validated against additional DEM simulations conducted directly under lunar gravity. The comparison revealed that the enhanced GSL accurately predicts four key mobility parameters: wheel horizontal velocity, slip ratio, sinkage, and power consumption, with a mean square percentage error of less than 5%. Notably, the study confirmed that GSL remains effective even in transient states and high-speed driving conditions, where wheel-soil interactions become particularly complex. This comprehensive verification, covering slopes up to ±10∘ and slip ratios ranging from negative to approximately 0.9, demonstrates that the enhanced GSL serves as a highly effective predictive method. It provides a practical framework for exploration rover design and mission planning in low-gravity environments, where precise consideration of wheel-soil interactions is critical.

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DEM-based analysis and optimization of an excavation bucket drum for in-situ resource utilization

Nakano T., Omura T., Galati R., Ishigami G.

Journal of Terramechanics 120 2025.10

ISSN 00224898

The bucket drum, a cylindrical rotating mechanism that efficiently excavates and collects lunar regolith, is essential for in-situ resource utilization (ISRU). While bucket drums offer advantages in energy efficiency and adaptability to low-gravity conditions, optimizing their shape and motion remains an open issue. Therefore, this paper aims to find an optimal design of the bucket drum using the discrete element method (DEM). We focus on five key design parameters were analyzed: two related to bucket shape (scoop throat length and scoop inlet number) and three related to motion (bucket vertical force, horizontal velocity, and angular velocity). These parameters were evaluated based on two performance indices: sand fill ratio and excavation power consumption. By solving the multi-objective problem of maximizing fill ratio while minimizing power consumption, an optimal design of the bucket drum was identified. Subsequently, a bucket drum with the optimal shape was 3D-printed, and the optimal motion of the bucket drum was applied using the soil excavation test apparatus. The test results qualitatively matched the results derived from the DEM analysis. This highlights the validity of the relationship between the design parameters and two performance indices, contributing to the advancement of ISRU excavation technologies.

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Mechanical properties of loose soil during dynamic penetration of landing pad under microgravity

Otsuki M., Maeda T., Ozaki S., Ishigami G., Kobayashi T.

Journal of Terramechanics 120 2025.10

ISSN 00224898

The safety of lunar and planetary exploration is contingent upon a preliminary assessment of the interaction between loose soils composed of regolith that covers a celestial surface and the landing gear of spacecraft, which should be designed in accordance with the findings of the assessment. However, given the issues associated with replicating the surface layer of celestial bodies on Earth, the verification of the gear design can be challenging. Conducting a real-size low-gravity test in the design verification process of a large spacecraft remains cumbersome and resource intensive. As such, drawing valuable knowledge from scale model tests is the only available solution. This paper presents the results and findings of a dynamic penetration test conducted on a scaled landing pad under microgravity using a drop tower. The experimental findings demonstrate that the results of the dynamic penetration of the pad into the regolith simulant are not necessarily indicative of the worst-case scenario of low-gravity conditions and that they are equivalent to the results obtained in the gravitational environment of the earth. This finding suggests that design verification may be feasible through tests using a full-scale spacecraft model.

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Orbital magnetic moments in FeCr2S4 studied by x-ray magnetic circular dichroism

V. K. Verma, J. Patra, V. R. Singh, Y. Nonaka, G. Shibata, K. Ishigami, A. Tanaka, K. Ohgushi, Y. Tokura, T. Koide, A. Fujimori

2025.08

We have investigated the element specific magnetic characteristics of single-crystal FeCr2S4 using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). We have found that the Fe L2,3-edge XAS spectra do not exhibit clear multiplet structures, indicating strong hybridization between the Fe 3d and S 3p orbitals, leading to delocalized rather than localized electronic states. The Fe 3d and Cr 3d spin moments are antiferromagnetically coupled, consistent with the Goodenough-Kanamori rule. The orbital magnetic moments of Fe and Cr are determined to be -0.23 and -0.017 μB/ion, respectively. The large orbital magnetic moment of Fe is due to the d6 configuration under the relatively weak tetrahedra crystal field at the Fe site, and the delocalized Fe electrons maintain the orbital degree of freedom in spite of their itinerant nature. To understand phenomena such as the gigantic Kerr rotation, it is essential to consider not only the orbital degrees of freedom but also the role of spin-orbit coupling, which induces a finite orbital magnetic moment through t2 and e level hybridization under the tetrahedral crystal field. This finite orbital moment serves as a direct indicator of spin-orbit interaction strength and links element-specific orbital magnetism to the large Kerr rotation. On the other hand, the octahedral crystal-field splitting of the Cr 3d level is large enough to result in the quenching of the orbital moment of the Cr ion in FeCr2S4.

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Design and Development of Jamming-based Stiffness-Adjustable Wheel on Soft Terrain

Suthisomboon, T., Bonardi, S., Ishigami. G.,

Journal of Terramechanics (PERGAMON-ELSEVIER SCIENCE LTD) 117 101014 (11pages) 2025

Research paper (scientific journal), Joint Work, Lead author, Corresponding author, Accepted, ISSN 00224898

Traversing soft terrain poses a major challenge for planetary wheeled rovers, and various studies have demonstrated ways to enhance rover mobility by transforming the wheel structure or adjusting the wheel's stiffness, which results in a change in wheel contact area on the terrain. This paper presents a novel idea using the jamming mechanism for modulating the wheel's stiffness. The developed wheel consists of the core body, wheel outer rim, inner flexure, and cable tension mechanism. The jamming mechanism is realized by adjusting the cable tension inserted between the outer rim of the wheel. The wheel stiffness measuement test confirms that the wheel with low stiffness can reduce its stiffness for 75% of the high stiffness configuration. The wheel's traversability on soft terrain are also evaluated based on slip ratio and current consumption. The results demonstrate that the lower-stiffness configuration outperforms the higher-stiffness wheel under various conditions. These findings, being consistent with previous works on flexible wheels, highlight the potential benefits of the jamming-based stiffness-adjustable wheel for rough terrain traverse with various payload conditions.

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Simultaneous Planning of Path-and-Rendezvous PointsUsing Bidirectional Informed RRT* for UAV-UGV Coordination

Tazaki Minori, Ishigami Genya

Journal of the Robotics Society of Japan (The Robotics Society of Japan) 43 ( 8 ) 811 - 814 2025

ISSN 02891824

Cooperative exploration by unmanned aerial vehicles (UAVs) and unmanned ground vehicles (UGVs) is efficient for long-duration surveys and wider exploration areas. We propose a rendezvous point determination and path planning method that reduces the travel distance between survey points in a UAV-UGV cooperative system, in which the UAV can recharge itself by landing on the UGV. In this study, Bidirectional Informed Rapidly-exploring Random Tree* that selectively extends paths for UAV and UGV in a specific direction is developed. Simulation study in three different terrains confirms that this method can reduce the travel distance by 5–7% compared to existing methods.

DRPA-MPPI: Dynamic Repulsive Potential Augmented MPPI for Reactive Navigation in Unstructured Environments

Fuke T., Endo M., Honda K., Ishigami G.

IEEE International Conference on Automation Science and Engineering 961 - 968 2025

ISSN 21618070

Reactive mobile robot navigation in unstructured environments is challenging when robots encounter unexpected obstacles that invalidate previously planned trajectories. Model predictive path integral control (MPPI) enables reactive planning, but still suffers from limited prediction horizons that lead to local minima traps near obstacles. Current solutions rely on heuristic cost design or scenario-specific pre-training, which often limits their adaptability to new environments. We introduce dynamic repulsive potential augmented MPPI (DRPA-MPPI), which dynamically detects potential entrapments on the predicted trajectories. Upon detecting local minima, DRPA-MPPI automatically switches between standard goal-oriented optimization and a modified cost function that generates repulsive forces away from local minima. Comprehensive testing in simulated obstacle-rich environments confirms DRPA-MPPI's superior navigation performance and safety compared to conventional methods with less computational burden.

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Drawbar pull experiment and analysis of grousered-track unit on sandy terrain

Azuma R., Maeda T., Horie K., Miyai S., Sugimura S., Ishigami G.

Proceedings of the 55th Conference of the Istvs Istvs 2025 52 - 57 2025

Tracked vehicles are commonly used on construction sites because of their high vehicle stability and traversability with large ground contact area. Grousers attached to the track shoe enhance vehicle stability and traversability, mitigating the risk of slipping on rough terrain. While many past studies investigated the mechanical effect of a single grouser shoe, experimental test of an entire grousered-track unit remains limited due to non-uniform stress distribution of the track unit on the ground. In this study, we developed an experimental apparatus with a grousered-track unit and conducted drawbar pull experiment on sandy terrain. We measured the slip ratio and sinkage of the track unit with various traction load corresponding to the drawbar pull of the track unit. The relationship between the drawbar pull coefficient, which is the ratio of the drawbar pull to the vertical load of the track, and the slip ratio was examined. The experiments confirmed that the slip ratio remains constant when the drawbar pull coefficient is approximately 0.4 or lower. Within this range, the track unit maintains small sinkage and a stable driving state. However, when the drawbar pull coefficient exceeds approximately 0.45, the slip ratio sharply increases as larger traction load cases the track unit to an unstable driving state by repeating the move-and-stop motion. This trend was observed regardless of the varied vertical load. These findings suggest that the relationship of the slip-drawbar pull coefficient and the threshold for stable/unstable driving state are useful for evaluating the traversability of the grousered-track unit.

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Physics-Based Torque Prediction Model for Excavating Drums on Granular Soil

Galati R., Nakano T., Ishigami G., Reina G.

Journal of Field Robotics 43 ( 3 ) 2237 - 2253 2025

ISSN 15564959

Accurate torque prediction for excavating drums is critical for the design and autonomous control of planetary rovers operating in loose, unstructured regolith. Existing terramechanics models may fail to capture the full dynamics of rotating excavation drums. In this paper, we present a unified, physics-based torque prediction model with two novel dynamic effects: a longitudinal, velocity-sensitive bulldozing resistance term that grows logarithmically with forward speed, and a regolith-accumulation torque that evolves as the drum buckets fill. We validate our approach on a custom four-wheel holonomic rover excavating Silica Sand No. 5 across multiple test sequences spanning different angular and translational velocities. The model achieves a normalized RMSE generally below 12%, an MAE under 0.15 Nm, and (Formula presented.), in nearly all cases, accurately reproducing the temporal evolution toward steady-state torque plateaus. Its modular structure and reliance on minimal empirical tuning make it suitable for real-time torque estimation in future autonomous planetary missions.

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Development of Bucket Drum Excavation System with Reaction Force Control for In-Situ Resource Utilization

Nakano T., Omura T., Ishigami G.

2025 International Conference on Space Robotics Isparo 2025 490 - 497 2025

This paper presents the development and experimental evaluation of a bucket drum excavation system for InSitu Resource Utilization (ISRU). A custom-designed bucket drum, equipped with a vertical lift mechanism for reaction force control during excavation, has been developed and mounted on a mobile robotic platform. Experiments under varying target reaction force conditions demonstrated the controller's suitability and effectiveness. Additionally, excavation performance was compared under two scenarios: when the robot was stationary and while it was in motion. Results showed that the excavation by bucket drum in robot motion was more energy efficient since the bucket drum can excavate fresh and uncompacted soil. These findings highlight the potential of reaction force control for the bucket drum mounted on mobile robotic platforms.

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Design, Development, and Motion Analysis of Robotic Arm for Rover Rescue from Sand Traps

Hanaoka Nahoko, Ishigami Genya

Journal of the Robotics Society of Japan (The Robotics Society of Japan) 43 ( 7 ) 699 - 702 2025

ISSN 02891824

The construction of lunar infrastructure is crucial for future human activities. Unmanned vehicles, such as rovers, serve as key mobility systems for construction tasks but face the risk of becoming trapped by sand and immobilized on uneven terrains. This paper focuses on a scenario where a rover being stuck is rescued by another rover via a robotic manipulator. We design and prototype a two-link robotic manipulator and conduct dynamic simulation along with experimental demonstrations. The experimental result are generally consistent with simulation results, particularly in terms of identifying the energy-efficient configurations of the manipulator for effective rescue operations.

Non-contact seesaw-type force sensor for evaluation of force characteristics of soft actuators

SATO Soya, HOKIGUCHI Shigetora, ASAI Makoto, ISHIGAMI Genya, TAKAHASHI Hidetoshi

The Proceedings of JSME annual Conference on Robotics and Mechatronics (Robomec) (The Japan Society of Mechanical Engineers) 2025 ( 0 ) 1A2-R05 2025

Soft actuators are constrained in their deformation when subjected to loads, so their performance needs to be evaluated similarly to that of conventional motors. In this study, we propose a seesaw-type force sensor consisting of a beam with multiple load points having different spring constants and a laser displacement meter. The relationship between force and displacement is evaluated by applying force from a soft actuator and measuring the displacement. The load points of the fabricated seesaw-type beams had spring constants ranging from 1.2 N/m to 15.9 N/m. The soft actuators were ionic polymer-metal composite (IPMC) actuators with a size of 5 mm × 25 mm and a thickness of 190 μm. In the experiment with the IPMC actuator, the force was high and the displacement was small at 5.3 mN and 0.3 mm where the spring constant was large, and the opposite trend was observed at 2.3 mN and 1.9 mm where the spring constant was small.

Towards Local Minima-free Robotic Navigation: Model Predictive Path Integral Control via Repulsive Potential Augmentation

Fuke T., Endo M., Honda K., Ishigami G.

2025 IEEE SICE International Symposium on System Integration Sii 2025 1112 - 1117 2025

Model-based control is a crucial component of robotic navigation. However, it often struggles with entrapment in local minima due to its inherent nature as a finite, myopic optimization procedure. Previous studies have addressed this issue but sacrificed either solution quality due to their reactive nature or computational efficiency in generating explicit paths for proactive guidance. To this end, we propose a motion planning method that proactively avoids local minima without any guidance from global paths. The key idea is repulsive potential augmentation, integrating high-level directional information into the Model Predictive Path Integral control as a single repulsive term through an artificial potential field. We evaluate our method through theoretical analysis and simulations in environments with obstacles that induce local minima. Results show that our method guarantees the avoidance of local minima and outperforms existing methods in terms of global optimality without decreasing computational efficiency.

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Slope-aware Maximum Uncertainty Sampling for Time-efficient Robotic Mapping on Rough Terrain

Tazaki M., Ishigami G.

2025 International Conference on Space Robotics Isparo 2025 64 - 71 2025

Robotic information gathering methods combining Gaussian Process (GP) regression and Bayesian Optimization (BO) are effective for mapping of soil mechanical properties in uncertain or unknown extraterrestrial terrain. However, standard applications of such methods often model robot traversability as a simple go/no-go constraint, without fully incorporating it as a variable cost that impacts overall efficiency. This limitation is critical on rough terrains like the lunar surface, where traversability heavily influences the trade-off between survey time and modeling accuracy. To address this gap, we propose a comprehensive approach for slope-aware robotic information gathering. The core of this approach is a novel adaptive sampling method, Slope-aware Maximum Uncertainty Sampling (SaMUS), which features a new BO acquisition function that explicitly incorporates terrain slope to balance uncertainty reduction with path traversability. We demonstrate the effectiveness of our method through an extensive simulation study in two environment types: controlled basic terrains for qualitative analysis and realistic lunar terrains for evaluating the performance of the proposed method. The results show that the proposed approach generates significantly more time-efficient survey paths by strategically avoiding highslope regions. Quantitative analysis on the realistic terrains further demonstrates that our method achieves a better tradeoff between survey time and modeling accuracy, particularly on flat and steep terrains. Finally, we provide practical guidelines for selecting optimal parameters based on terrain class and operational time constraints.

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