The use of a tether in mobile robotics provides a method to safely explore steep terrain and harsh environments considered too dangerous for humans and beyond the capability of standard ground rovers. However, there are significant challenges yet to be addressed concerning mobility while under tension, autonomous tether management, and the methods by which an environment is assessed. As an incremental step towards solving these problems, this paper outlines the design and testing of a center-pivoting tether management payload enabling a four-wheeled rover to access and map steep terrain. The chosen design permits a tether to attach and rotate passively near the rover's center-of-mass in the direction of applied tension. Prior design approaches in tethered climbing robotics are presented for comparison. Tests of our integrated payload and rover, Tethered Robotic Explorer (TReX), show full rotational freedom while under tension on steep terrain, and basic autonomy during flat-ground tether management. Extensions for steep-terrain tether management are also discussed. Lastly, a planar lidar fixed to a tether spool is used to demonstrate a 3D mapping capability during a tethered traverse. Using visual odometry to construct local point-cloud maps over short distances, a globally-aligned 3D map is reconstructed using a variant of the Iterative Closest Point (ICP) algorithm.
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... One particular thrust in this area of research is the development of ground robots capable of navigating vertically challenging terrain (e.g., steep slopes, rocky outcroppings, and uneven surfaces) [8]- [10]. Achieving reliable and robust mobility in these environments is challenging due to the intricate nature of the terrain, the complex vehicle-terrain interactions, the adverse impact caused by gravity, and the potential deformation of the vehicle chassis. ...
... Achieving reliable and robust mobility in these environments is challenging due to the intricate nature of the terrain, the complex vehicle-terrain interactions, the adverse impact caused by gravity, and the potential deformation of the vehicle chassis. Despite these difficulties, such mobility has been made possible mainly through advancement in hardware, including the development of specialized robot platforms [3], [4], [10]- [12] and the use of new materials [13]. These robots are capable of climbing walls [13], scaling cliffs [10], and traversing rough terrain with ease [11], [12], making them suitable for a wide range of applications. ...
... Despite these difficulties, such mobility has been made possible mainly through advancement in hardware, including the development of specialized robot platforms [3], [4], [10]- [12] and the use of new materials [13]. These robots are capable of climbing walls [13], scaling cliffs [10], and traversing rough terrain with ease [11], [12], making them suitable for a wide range of applications. ...
Most conventional wheeled robots can only move in flat environments and simply divide their planar workspaces into free spaces and obstacles. Deeming obstacles as non-traversable significantly limits wheeled robots' mobility in real-world, non-flat, off-road environments, where part of the terrain (e.g., steep slopes, rugged boulders) will be treated as non-traversable obstacles. To improve wheeled mobility in those non-flat environments with vertically challenging terrain, we present two wheeled platforms with little hardware modification compared to conventional wheeled robots; we collect datasets of our wheeled robots crawling over previously non-traversable, vertically challenging terrain to facilitate data-driven mobility; we also present algorithms and their experimental results to show that conventional wheeled robots have previously unrealized potential of moving through vertically challenging terrain. We make our platforms, datasets, and algorithms publicly available to facilitate future research on wheeled mobility.
... However, recent advances in wheeled mobility have shown that even conventional wheeled robots (i.e., without extensive hardware modification such as active suspensions [7,8,9] or adhesive materials [10]) have previously unrealized potential to move over vertically challenging terrain (e.g., in mountain passes with large boulders or dense forests with fallen trees) [11,12,13], where vehicle motion is no longer constrained to a 2D plane [14] (Figure 1). In those environments, neither assumptions of rigid vehicle chassis and clear delineation between obstacles and free spaces in a simple 2D plane are valid [15,16,17,18]. ...
... Most research aiming at allowing robots to move in vertically challenging environments are from the hardware side. Still treating vehicles as rigid bodies, tracked vehicles are expected to crawl over more rugged terrain than wheeled platforms due to the increased surface contact and therefore propulsion [31], while adhesive materials [10] and tethers [13] allow robots to overcome gravity while climbing vertical slopes. Relaxing the assumption of rigid body, vehicles with active suspen-sions have been developed [7,8,9] to proactively maintain a stable pose of the chassis on vertically challenging terrain. ...
Most autonomous navigation systems assume wheeled robots are rigid bodies and their 2D planar workspaces can be divided into free spaces and obstacles. However, recent wheeled mobility research, showing that wheeled platforms have the potential of moving over vertically challenging terrain (e.g., rocky outcroppings, rugged boulders, and fallen tree trunks), invalidate both assumptions. Navigating off-road vehicle chassis with long suspension travel and low tire pressure in places where the boundary between obstacles and free spaces is blurry requires precise 3D modeling of the interaction between the chassis and the terrain, which is complicated by suspension and tire deformation, varying tire-terrain friction, vehicle weight distribution and momentum, etc. In this paper, we present a learning approach to model wheeled mobility, i.e., in terms of vehicle-terrain forward dynamics, and plan feasible, stable, and efficient motion to drive over vertically challenging terrain without rolling over or getting stuck. We present physical experiments on two wheeled robots and show that planning using our learned model can achieve up to 60% improvement in navigation success rate and 46% reduction in unstable chassis roll and pitch angles.
... By taking advantage of fixed anchors, tetherworld friction supports exponential amplification of ground traction forces and the effective load holding capacity of simple lightweight mobile robots. agents tethered to a "mother-ship" [3], [4]. Systems overcome obstacles in irregular environments using robot-robot cooperation through pushing [5], [6] and exploiting tethers [7]- [9]. ...
... These works assume robust attachment or traction with the world, achieved through specialized gripping or anchoring mechanisms [1], [2], [17], [18], making an agent massive enough to be assumed unmovable [3], or manually predeploying secure anchor points by researchers [9]. Applications outside of these specific scenarios warrant new robust and more generalized lightweight methods for the mobile creation of secure anchor points on the fly in the field. ...
Reduced traction limits the ability of mobile robotic systems to resist or apply large external loads, such as tugging a massive payload. One simple and versatile solution is to wrap a tether around naturally occurring objects to leverage the capstan effect and create exponentially-amplified holding forces. Experiments show that an idealized capstan model explains force amplification experienced on common irregular outdoor objects - trees, rocks, posts. Robust to variable environmental conditions, this exponential amplification method can harness single or multiple capstan objects, either in series or in parallel with a team of robots. This adaptability allows for a range of potential configurations especially useful for when objects cannot be fully encircled or gripped. These principles are demonstrated with mobile platforms to (1) control the lowering and arrest of a payload, (2) to achieve planar control of a payload, and (3) to act as an anchor point for a more massive platform to winch towards. We show the simple addition of a tether, wrapped around shallow stones in sand, amplifies holding force of a low-traction platform by up to 774x.
... The use of tethers has also been applied to UGVs for exploration. In [7] a tether is used to anchor the UGV to objects in the environment in order to explore a steep terrain. In [8] a UGV is tethered to a UAV, used as environment sensing assistance and also as an anchor to structures for climbing steep terrain. ...
This letter addresses the problem of trajectory planning in a marsupial robotic system consisting of an unmanned aerial vehicle (UAV) linked to an unmanned ground vehicle (UGV) through a non-taut tether withcontrollable length. To the best of our knowledge, this is the first method that addresses the trajectory planning of a marsupial UGV-UAV with a non-taut tether. The objective is to determine a synchronized collision-free trajectory for the three marsupial system agents: UAV, UGV, and tether. First, we present a path planning solution based on optimal Rapidly-exploring Random Trees (RRT*) with novel sampling and steering techniques to speed-up the computation. This algorithm is able to obtain collision-free paths for the UAV and the UGV, taking into account the 3D environment and the tether. Then, the paper presents a trajectory planner based on non-linear least squares. The optimizer takes into account aspects not considered in the path planning, like temporal constraints of the motion imposed by limits on the velocities and accelerations of the robots , or raising the tether's clearance. Simulated and field test results demonstrate that the approach generates obstacle-free, smooth, and feasible trajectories for the marsupial system.
... These tasks require sufficiently sized and powered robots that can access these extreme terrains. Tethered rappelling rovers [18], [19] are capable of carrying large payloads and accessing vertical surfaces below the initial landing point. This makes them ideal platforms for missions that are targeting environments such as cliffs, crevasses, and pits. ...
... In an opposite direction, several preliminary works use a tether as a mean of navigation, especially when big efforts are required in climbing. In [4] a navi-guider system for a robot driven by a tether pulled by a person is proposed, whereas in [5] an innovative approach for center-pivoting tether management payload is presented. Such a strategy enables a four-wheeled rover to access and map steep terrain, where a winch is mounted on-board the rover, while the free end of the cable is anchored to a fixed point in the terrain and the tether is used to help the rover climbing high slopes. ...
This letter addresses the problem of trajectory planning in a marsupial robotic system consisting of an unmanned aerial vehicle (UAV) linked to an unmanned ground vehicle (UGV) through a non-taut tether with controllable length. To the best of our knowledge, this is the first method that addresses the trajectory planning of a marsupial UGV-UAV with a non-taut tether. The objective is to determine a synchronized collision-free trajectory for the three marsupial system agents: UAV, UGV, and tether. First, we present a path planning solution based on optimal Rapidly-exploring Random Trees (RRT*) with novel sampling and steering techniques to speed-up the computation. This algorithm is able to obtain collision-free paths for the UAV and the UGV, taking into account the 3D environment and the tether. Then, the letter presents a trajectory planner based on non-linear least squares. The optimizer takes into account aspects not considered in the path planning, like temporal constraints of the motion imposed by limits on the velocities and accelerations of the robots, or raising the tether's clearance. Simulated and field test results demonstrate that the approach generates obstacle-free, smooth, and feasible trajectories for the marsupial system.
Remote Operated Vehicles are widely used in underwater operation mainly because the tether that links the robot to its floating base provides inexhaustible energy and gives live feedback which alleviates two major issues in autonomous underwater vehicles : autonomous decision making and power consumption. Yet deploying and handling a tether is not without drawbacks. Tether dragging or entanglement can hamper the ROV motion and it could make it difficult to navigate in narrow and confined spaces such as wreck or cave. In this paper we introduce the concept of line of ROVs : adding intermediate robot along the tether can be a simple and practical solution to properly handle tether shape. In this paper we propose to investigate the use of a local visual servoing.
The domain and technology of mobile robotic space exploration are fast moving from brief visits to benign Mars surface regions to more challenging terrain and sustained exploration. Further, the overall venue and concept of space robotic exploration are expanding---"from flatland to 3D"---from the surface, to sub-surface and aerial theatres on disparate large and small planetary bodies, including Mars, Venus, Titan, Europa, and small asteroids. These new space robotic system developments are being facilitated by concurrent, synergistic advances in software and hardware technologies for robotic mobility, particularly as regard on-board system autonomy and novel thermo-mechanical design. We outline these directions of emerging mobile science mission interest and technology enablement, including illustrative work at JPL on terrain-adaptive and multi-robot cooperative rover systems, aerobotic mobility, and subsurface ice explorers.
Many modern sensors used for mapping produce 3D point clouds, which are typically registered together using the iterative closest point (ICP) algorithm. Because ICP has many variants whose performances depend on the environment and the sensor, hundreds of variations have been published. However, no comparison frameworks are available, leading to an arduous selection of an appropriate variant for particular experimental conditions. The first contribution of this paper consists of a protocol that allows for a comparison between ICP variants, taking into account a broad range of inputs. The second contribution is an open-source ICP library, which is fast enough to be usable in multiple real-world applications, while being modular enough to ease comparison of multiple solutions. This paper presents two examples of these field applications. The last contribution is the comparison of two baseline ICP variants using data sets that cover a rich variety of environments. Besides demonstrating the need for improved ICP methods for natural, unstructured and information-deprived environments, these baseline variants also provide a solid basis to which novel solutions could be compared. The combination of our protocol, software, and baseline results demonstrate convincingly how open-source software can push forward the research in mapping and navigation.
RGB-D cameras provide both a color image and per-pixel depth esti-mates. The richness of their data and the recent development of low-cost sensors have combined to present an attractive opportunity for mobile robotics research. In this paper, we describe a system for visual odometry and mapping using an RGB-D camera, and its application to autonomous flight. By leveraging results from recent state-of-the-art algorithms and hardware, our system enables 3D flight in cluttered environments using only onboard sensor data. All computation and sensing required for local position control are performed onboard the vehicle, eliminating its depen-dence on unreliable wireless links. We evaluate the effectiveness of our system for stabilizing and controlling a quadrotor micro-air vehicle, demonstrate its use for constructing detailed 3D maps of an indoor environment, and discuss its limitations.
A micro rover, code-named Moonraker, was developed to demonstrate the feasibility of 10kg-class lunar rover missions. Requirements were established based on the Google Lunar X-Prize mission guidelines in order to effectively evaluate the prototype. A 4-wheel skid steer configuration was determined to be effective to reduce mass, maximize regolith traversability, and fit within realistic restrictions on the rover’s envelope by utilizing the top corners of the volume.
A static, hyperbolic mirror-based omnidirectional camera was selected in order to provide full 360° views around the rover, eliminating the need for a pan/tilt mechanism and motors. A front mounted, motorless MEMS laser scanner was selected for similar mass reduction qualities. A virtual reality interface is used to allow one operator to intuitively change focus between various narrow targets of interest within the wide set of fused data available from these sensors.
Lab tests were conducted on the mobility system, as well as field tests at three locations in Japan and Mauna Kea. Moonraker was successfully teleoperated to travel over 900m up and down a peak with slopes of up to 15° These tests demonstrate the rover’s capability to traverse across lunar regolith and gather sufficient data for effective situational awareness and near real-time tele-operation.
The solar system's most scientifically tantalizing terrain remains out of reach for traditional planetary rovers, which are typically limited to driving on slopes below 30 degrees. This paper details the design of a novel robotic explorer that would open access to these previously inaccessible locales, such as Martian crater walls where evidence of salty water was recently detected, Lunar polar craters where evidence of water ice was detected, and Lunar and Martian lava tubes for future habitability. The Axel rover is a two-wheeled robot capable of rappelling down steep (even vertical) slopes supported by a tether. The DuAxel rover is comprised of two Axel vehicles docked to a central module. Unrestricted by tether length, this four-wheeled system would be capable of driving long distances from a safe landing zone to the extreme terrain of interest. Once in the vicinity of terrain in which the tether would be required, one of the Axel rovers could undock from the central chassis and rappel downslope. The other Axel and central chassis would remain topside to act as an anchor and to provide line of site to Earth (for communications) and the Sun (for energy). As the detached Axel descends into the area of interest, it would receive power and relays data through conductors in its tether. Each Axel would carry a suite of instruments in a bay that would be tucked inside the wheels. Because of the novel configuration of Axel's major degrees of freedom, these instruments could be precisely pointed at targets at any desired downslope spatial separation. These instruments could then be deployed into close proximately to the ground by means of a simple mechanism, allowing for detailed study of the strata on the slope. Axel could accommodate a host of instruments, including a microscopic imager, infrared spectrometers, thermal probes, and sample collection devices. This paper will describe the design of both the latest generation of Axel and DuAxel systems and their instrum- nt/sampling mechanisms. Results from recent field trials at a rock quarry in California and a Martian analog site in the desert of Arizona will be described.
With the continued success of the Mars Exploration Rovers and the return of humans to the Moon within the next decade, a considerable amount of research is being done on the technologies required to provide surface mobility and the tools required to provide scientific capability. Here, we explore the utility of lidar and the mobile Scene Modeler (mSM) – which is based on a stereo camera system – as scientific tools. Both of these technologies have been, or are being considered for, technological applications such as autonomous satellite rendezvous and rover navigation. We carried out a series of field tests at the 23 km diameter, 39 Ma, Haughton impact structure located on Devon Island in the Canadian Arctic. Several sites of geological interest were investigated, including polygonal terrain, gullies and channels, slump/collapse features, impact melt breccia hills, and a site of impact-associated hydrothermal mineralization. These field tests show that lidar and mSM provide a superior visual record of the terrain, from the regional (km) to outcrop (m to cm) scale and in 3-D, as compared to standard digital photography. Thus, a key strength of these technologies is in situ reconnaissance and documentation. Lidar scans also provide a wealth of geometric and structural information about a site, accomplishing the equivalent of weeks to months of manual surveying and with much greater accuracy than traditional tools, making this extremely useful for planetary exploration missions. An unexpected result of these field tests is the potential for lidar and mSM to provide qualitative, and potentially quantitative, composition information about a site. Given the high probability of lidar and mSM being used on future lunar missions, we suggest that it would be beneficial to further investigate the potential for these technologies to be used as science tools.
Dante is a tethered walking robot capable of climbing steep slopes. In 1992 it was created at Carnegie Mellon University and deployed in Antarctica to explore an active volcano, Mount Erebus. The Dante project's robot science objectives were to demonstrate a real exploration mission, rough terrain locomotion, environmental survival, and self-sustained operation in the harsh Antarctic climate. The volcano science objective was to study the unique convecting magma lake inside Mount Erebus' inner crater. The expedition demonstrated the advancing state-of-art in mobile robotics and the future potential of robotic explorers. This paper details our objectives, describes the Dante robot, overviews what happened on the expedition and discusses what did and didn't work.