Krzysztof Skonieczny

Design and field experimentation of a prototype Lunar prospector

Scarab is a prototype rover for Lunar missions to survey resources in polar craters. It is designed as a prospector that would use a deep coring drill and apply soil analysis instruments to measure the abundance of elements of hydrogen and oxygen and other volatiles including water. Scarab’s chassis can adjust the wheelbase and height to stabilize its drill in contact with the ground and can also adjust posture to better ascend and descend steep slopes. This enables unique control of posture when moving and introduces new planning issues. Scarab has undergone field testing at Lunar-analog sites in Washington and Hawaii in an effort to quantify and validate its mobility and navigation capabilities. We report on results of the experiments in slope ascent and descent and in autonomous kilometer-distance navigation in darkness.

Soil behavior of wheels with grousers for planetary rovers

The performance of wheels operating in loose granular material for the application of planetary vehicles is well researched but little effort has been made to study the soil shearing which governs traction. Net traction measurements and application of energy metrics have been solely relied upon to investigate performance but lack the ability to evaluate or describe soil-wheel interaction leading to thrust and resistances. The complexity of rim and grouser interaction with the ground has also prevented adequate models from being formulated. This work relies on empirical data gathered in attempt to study the effects of rim surface on soil shearing and ultimately how this governs traction. A novel experimentation and analysis technique was developed to enable investigation of terramechanics fundamentals in great detail. This technique, the Shear Interface Imaging Analysis Tool, is utilized to provide visualization and analysis capability of soil motion at and below the wheel-soil interface. Analysis of the resulting displacement field identifies clusters of soil motion and shear interfaces. Complexities in soil flow patterns greatly affect soil structure below the wheel and the resulting tractive capability. Grouser parameter variations, spacing and height, are studied for a rigid wheel. The results of soil shear interface analysis for wheels with grousers are presented. The processes of thrust and resistances are investigated and behavior characterized for grousered wheels.

Inching locomotion for planetary rover mobility

New articulated planetary rovers offer alternative locomotion modalities beyond conventional rolling wheel mobility.12 These new modalities should be explored to overcome the limitations of traditional rolling mobility, and expand the areas of planetary surfaces amenable to exploration. The topic of this study is a hybrid push-roll locomotion mode called inching. Static (non-rolling) wheels are used in conjunction with the rolling wheels of a vehicle in order to increase net traction potential. Preliminary experiments have shown an approximate doubling in drawbar pull for the inching locomotion mode relative to pure rolling. This improvement is not accounted for by reductions to wheel motion resistance alone, and furthermore evidence is provided that static wheels are capable of reacting more ground thrust than rolling wheels. Further investigations using a transparent soil tank, and novel image processing techniques, reveal key differences in the soil shear failure interface under rolling and static wheels. For the cases studied, static wheels generated much deeper and more unified soil failure masses than rolling wheels. Further investigation is recommended to clarify the physics of these thrust development processes, and ultimately to populate the vehicle design space for inching locomotion.

A grouser spacing equation for determining appropriate geometry of planetary rover wheels

Grousers, sometimes called lugs, are recognized as a way to improve wheel performance and traction, but there have been, to date, no comprehensive guidelines for choosing grouser patterns. This work presents a quantitative expression for determining appropriate grouser spacing for rigid wheels. Past empirical studies have shown that increasing grouser height and number can improve performance, to a point. The newly proposed grouser spacing equation is based on observations that wheels with an inadequate number of grousers induce forward soil flow ahead of the wheel, and thus rolling resistance. The equation relates geometric wheel parameters (wheel radius, grouser height and spacing) and operating parameters (slip and sinkage), and predicts a maximum allowable grouser spacing (or, equivalently, a minimum number of grousers). Experiments with various grouser heights and numbers demonstrate good correspondence to the proposed equation, as increases in number of grousers beyond the predicted minimum number stop improving performance. A grouser spacing equation is particularly useful for designing efficient wheels. The proposed relation includes slip and sinkage, parameters that cannot be assumed constant or known a priori, but this work shows that wheels designed using the proposed equation are robust to changing operating scenarios even if they degrade beyond estimated nominal conditions.

Productive Lightweight Robotic Excavation for the Moon and Mars

AbstractExploration of the Moon and Mars calls for robots that are increasingly capable in regolith, or granular soil. Beyond traversing and avoiding entrapment, future robots will excavate and process regolith as a resource. This work distinguishes concerns governing the performance of regolith operations, based on load-haul-dump tasks motivated by in situ resource utilization and lunar outpost site work. Payload ratio (mass of regolith payload capacity normalized by robot mass) and driving speed are identified as key parameters governing the productivity of small site work robots. Other parameters, such as number of wheels, are not as important. Experiments with a small robotic excavator and task-level simulations (for which a modeling framework is described) determine the relative sensitivity of productivity to changes in these variables. These findings provide direction for the development of future lightweight robotic excavators.

Configuring innovative regolith moving techniques for lunar outposts

The NASA exploration vision calls for extended human presence at lunar outposts within the coming decades. Any permanent outpost requires a significant amount of infrastructure and a cost-effective way of preparing this infrastructure is to utilize native materials such as regolith and rocks inherently present. This work investigates techniques for excavating, transporting, and building up regolith in the context of berm building, surface stabilization, and other critical tasks using small (100 kg to 300 kg) robots. Terrestrial excavation techniques and machines are reviewed. REMOTE (the Regolith Excavation MObility & Tooling Environment), a simulated task model that accounts for the special requirements of excavating in the harsh lunar environment, is presented. The model is used to quantitatively compare excavation systems according to key metrics including production ratio. It is shown that the teleoperated lunar berm construction robots achieve a production ratio less than 1/10th that of commercial equipment employed in terrestrial construction. A preliminary sensitivity analysis shows that these results are affected by the operating velocity as well as excavation blade design. A prototype of a rock rake for soil stabilization is also demonstrated. The goal of this work is to arrive at innovative robotic approaches that are best suited for excavation and infrastructure preparation tasks on the moon.

Advantages of continuous excavation in lightweight planetary robotic operations

Planetary excavator robots face unique and extreme engineering constraints relative to terrestrial counterparts. In space missions mass is always at a premium because it is the main driver behind launch costs. Lightweight operation, due to low mass and reduced gravity, hinders excavation and mobility by reducing the forces a robot can effect on its environment. This work shows that there is a quantifiable, non-dimensional threshold that distinguishes the regimes of lightweight and nominal excavation. This threshold is crossed at lower weights for continuous excavators (e.g. bucket-wheels) than discrete scrapers. This research introduces novel experimentation that for the first time subjects excavators to gravity offload (a cable pulls up on the robot with five-sixths its weight, to simulate lunar gravity) while they dig. A 300 kg excavator robot offloaded to 1/6 g successfully collects 0.5 kg/s using a bucket-wheel, with no discernible effect on mobility. For a discrete scraper of the same weight, production rapidly declines as rising excavation resistance stalls the robot. These experiments suggest caution in interpreting low-gravity performance predictions based solely on testing in Earth gravity. Experiments were conducted in GRC-1, a washed industrial silica-sand devoid of agglutinates and of the sub-75-micron basaltic fines that make up 40% of lunar regolith. The important dangers related to dust are thus not directly addressed. The achieved densities for experimentation are 1640 kg/m3 (very loose/loose) and 1720 kg/m3 (medium dense). This work develops a novel robotic bucket-wheel excavator, featuring unique direct transfer from bucket-wheel to dump bed as a solution to material transfer difficulties identified in past literature.

Motion Analysis System for Robot Traction Device Evaluation and Design

Though much research has been conducted regarding traction of tires in soft granular terrain, little empirical data exist on the motion of soil particles beneath a tire. A novel experimentation and analysis technique has been developed to enable detailed investigation of robot interactions with granular soil. This technique, the Shear Interface Imaging Analysis method, provides visualization and analysis capability of soil shearing and flow as it is influenced by a wheel or excavation tool. The method places a half-width implement (wheel, excavation bucket, etc.) of symmetrical design in granular soil up against a transparent glass sidewall. During controlled motion of the implement, high-speed images are taken of the sub-surface soil, and are processed via optical flow software. The resulting soil displacement field is of very high fidelity and can be used for various analysis types. Identification of clusters of soil motion, shear interfaces and shearing direction/magnitude allow for analysis of the soil mechanics governing traction. The Shear Interface Imaging Analysis Tool enables analysis of robot-soil interactions in richer detail than possible before. Prior state-of-art technique relied on long-exposure images that provided only qualitative insight, while the new processing technique identifies sub-millimeter gradations in motion and can do so even for high frequency changes in motion. Results are presented for various wheel types and locomotion modes: small/large diameter, rigid/compliant rim, grouser implementation, and push-roll locomotion.

Parameters Governing Regolith Site Work by Small Robots

Continued exploration of the Moon and Mars will call for mobile robots that are increasingly capable in regolith; not only traversing it without getting stuck, but also manipulating and shaping it to suit mission needs. This work distinguishes issues governing performance of regolith operations, based on tasks motivated by preparatory site work for lunar outposts. Payload ratio (pound-forpound regolith-moving capacity) of small site work robots is identified as the key parameter governing metrics such as completion time for tasks ranging from berm building to trench digging. In addition to payload ratio, driving speed also governs berm building with small robots, while soil interaction parameters including regolith cohesion and soil-tool friction dictate trench digging performance. Furthermore, 3-dimensional bucket modeling highlights cohesion and friction as risks to the viability of using the smallest proposed class of excavation robots (those with mass of 250 kg or less). For trench digging especially, machines with higher mass (500 kg or more) may be required. Results are based on parameter sensitivity analyses using REMOTE, a task-level site work simulator.

Novel Experimental Technique for Visualizing and Analyzing Robot- Soil Interactions

A novel experimentation and analysis technique has been developed to enable detailed investigation of robot interactions with granular regolith. This technique, the Shear Interface Imaging Analysis Tool, provides visualization and analysis capability of soil shearing and flow as it is influenced by a wheel or excavation tool. The method places an implement (wheel, excavation bucket, etc.) in granular soil up against a transparent sidewall. During controlled motion of the implement, images are taken of the sub-surface soil, and are processed with optical flow software. Analysis of the resulting displacement field identifies clusters of soil motion and shear interfaces. The Shear Interface Imaging Analysis Tool enables analysis of robot-soil interactions in richer detail than possible before. Prior art relied on long-exposure images that provided only qualitative insight, while the new processing technique identifies sub-millimeter gradations in motion and can do so even for high frequency changes in motion (several Hz). Results are presented for various wheel types and locomotion modes: small/large diameter, rigid/compliant, with/without grousers, and rolling/inching. Results are also presented for an excavation bucket horizontally cutting granular soil.