We present a low-cost, three-degree-of-freedom (3-DOF) prismatic-spherical-revolute (PSR) parallel mechanism used as a testing platform for an unmanned aerial vehicle (UAV) tethered to an unmanned surface vehicle (USV). The mechanism has three actuated linear rails kinematically linked to a platform which replicates boat motion up to 2.5 m vertical heave (sea state 4, Douglas Sea Scale). A lookup table relating relative slider heights to platform roll and pitch was developed numerically leveraging geometric constraints. A design parameter study optimized the arm length, platform size, and ball joint mounting angle relative to the overall radius to maximize the workspace. For this design, a maximum roll and pitch range from -32° to 32° and -25° to 35°, respectively, is achievable. A prototype was manufactured to carry the tethered UAV winch payload. Experimental testing confirmed the workspace and demonstrated boat motion replication, validated using an inertial measurement unit (IMU).

Our Tensegrity-Inspired Compliant Three degree-of-freedom (DOF) robotic joint adds omnidirectional compliance to robotic limbs while reducing sprung mass through base mounted actuation. This enables a robotic limb which is safer to operate alongside humans and fragile equipment while still capable of generating quick movements and large forces if required. Unlike many other soft robotic systems which leverage continuously soft materials, our joint is simpler to model with low order dynamic systems and has a host of embedded sensing which provide ample information of its position and velocity. We first discuss geometry selection and optimization to maximize the theoretical configuration space of the joint. We then show several of our mechatronic design solutions, which are easily generalized to a multitude of cable-driven mechanisms, and demonstrate the performance of these mechanisms within the context of our hardware prototype. We then present results on the controllable stiffness of our physical prototype. Finally, we demonstrate the strength of our prototype which is capable of lifting a 7 kg mass at a distance of 0.95 meters from the joint.

The quasi-static catenary curve of a semi-slack tether between an essentially stationary unmanned air vehicle (UAV) and a small unmanned surface vehicle (USV) is investigated and characterized. An empirical analysis, performed over a discretized space of vertical and horizontal separations of the two vehicles, determines an optimum cable length & tension for maximizing system robustness during the vertical heave of the USV due to high seas. Operating at this optimum condition allows for equal displacements of the USV in the up and down directions, minimizing the possibility of both fouling (with the tether touching the water) and excessive downforce on the UAV (with the tether pulled taut) during dynamic heave events. Scaling the horizontal offset, tether length, and tension by the flying height collapses all empirical results into convenient curves depending only on a nondimensional relative position parameter (Δx/Δy), accurately fit by low order polynomials. This eliminates the need for a lookup table, and decreases computation time during implementation. The heave robustness analysis results in a recommended operating relative position of Δx/Δy ≈ .46. Experimental results are presented and confirm the catenary analysis for the proposed tether.

This paper presents a (patent-pending) small, quasi-static, minimal-complexity Stair Climbing Robot (SCR). The vehicle design is given simply by adding a third motor to a (Segway-like) Mobile Inverted Pendulum (MIP), enabling it to maneuver up stairs, leveraging feedback control, by planting it's “foot” onto the ground in front of the next step, lifting the chassis/wheel assembly up it's own “leg”, leaning over onto the top of the next step, self uprighting, and repeating for the following step(s). Fore/aft stabilization during leg balancing is given by using the MIP drive wheels as reaction wheels, while left/right stability is given by the width of the foot itself. The design is small and simple enough to potentially be ruggedized as a stair-climbing throwbot, akin to the Recon Scout (but able to climb up stairs) for reconnaissance in military and homeland security applications.

Duct exploration and maintenance is a task well suited for small agile robots, which must be capable of navigating complex and irregular systems of ducts. Previously, we presented a tensegrity robot, DuCTT (Duct Climbing Tetrahedral Tensegrity), which demonstrated the plausibility of such a robot for duct exploration but was never able to successfully demonstrate climbing. Here we present DuCTTv2, redesigned from the ground up to address issues with actuator power, cable routing, compliance and synchronized control present in our first prototype. These improvements allow the prototype to be the first tensegrity robot to demonstrate duct climbing, and does so with an average climb speed of 1.4 cm/s. We also demonstrate initial tests of the prototypes ability to bend and translate its two segments relative to one another, which will allow it to navigate T-junctions and sharp corners commonly found in duct systems. Testing of the prototype is conducted to demonstrate the new faster and more robust control of motion, and analysis of dynamic simulations is presented.

Ball-balancing robots (BBRs) are endowed with rich dynamics. When properly designed and stabilized via feedback to eliminate jitter, and intuitively coordinated with a well-designed smartphone interface, BBRs exhibit a uniquely fluid and organic motion. Unlike mobile inverted pendulums (MIPs, akin to unmanned Segways), BBRs stabilize both fore/aft and left/right motions with feedback, and bank when turning. Previous research on BBRs focused on vehicles from 50cm to 2m in height; the present work is the first to build significantly smaller BBRs, with heights under 25cm. We consider the unique issues arising when miniaturizing a BBR to such a scale, which are characterized by faster time scales and reduced weight (and, thus, reduced normal force and stiction between the omniwheels and the ball). Two key patent-pending aspects of our design are (a) moving the omniwheels to contact the ball down to around 20 to 30 deg N latitude, which increases the normal force between the omniwheels and the ball, and (b) orienting the omniwheels into mutually-orthogonal planes, which improves efficiency. Design iterations were facilitated by rapid prototyping and leveraged low-cost manufacturing principles and inexpensive components. Classical successive loop closure control strategies are implemented, which prove to be remarkably effective when the BBR isn't spinning quickly, and thus the left/right and fore/aft stabilization problems decompose into two decoupled MIP problems.

A robot with the ability to traverse complex duct systems requires a large range of controllable motions as well as the ability to grip the duct walls in vertical shafts. We present a tensegrity robot with two linked tetrahedral frames, each containing a linear actuator, connected by a system of eight actuated cables. The robot climbs by alternately wedging each tetrahedron within the duct and moving one tetrahedron relative to the other. We first introduce our physical prototype, called DuCTT (Duct Climbing Tetrahedral Tensegrity). We next discuss the inverse kinematic control strategy used to actuate the robot and analyze the controller's capabilities within a physics simulation. Finally, we discuss the hardware prototype and compare its performance with simulation.

SkySweeper is a mobile robot designed to operate in an environment of cables, wires, power lines, ropes, et cetera. The robot is comprised of two links pivotally connected at one end; a series elastic actuator at this “elbow” joint can actuate relative rotation between the two links. At the opposite end of each link is an actuated three-position clamp. The clamp can either be open, partially closed, such that the clamp can roll (translate) along the cable, or fully closed, such that the clamp can only pivot on the cable. By actuating the elbow joint and cleverly choosing the positions of the clamps, the robot can locomote on the cable in a number of different ways. The particular method of locomotion can be chosen to minimize energy consumption, maximize speed, or traverse an obstacle (e.g. a support from which the cable is suspended). SkySweeper has the potential to locomote in a more energy efficient manner than existing cable-locomoting robots. It also operates with a minimal number of actuators, which reduces cost significantly. Potential applications include power and communication line inspection, suspension bridge inspection and construction, as well as entertainment. Data from a prototype, consisting largely of 3D-printed and off-the-shelf parts, are compared to dynamic simulation results.

The geometry, kinematics, and mechanical development of a symmetric and adjustable arm suspension mechanism for an actively stabilized single-legged hopping robot are presented. This mechanism is a key enabling design feature of a recently developed two-armed reaction wheel stabilized monopod, that is capable of conventional wheeled roving, continuous hopping, and self-uprighting. The mechanism is shown to behave essentially as a digressive-rate torsional spring placed between the two arms, in series with a weaker progressive rate torsional spring tied to the leg/central body. This makes it well-suited to recovering substantial energy from hopping motion, whilst presenting negligible resistance to antisymmetric motions used to maintain side-to-side stability. Furthermore, by storing energy purely in tension, the problem of buckling encountered with compression springs is avoided, and, the effective torsional spring constant may be varied by adjusting the spring pretension. Finally, the mechanism is self-latching near maximum deflection, allowing the vehicle to fold into a compact roving configuration, and enabling "running jumps" via the release of this gradually accumulated spring energy.

A versatile unmanned ground vehicle (UGV) should be able to traverse rough terrain while retaining a small form factor for navigating confined spaces. Such a (patent pending) vehicle, dubbed Switchblade, is developed in the present work via an effective combination of a novel transforming mechanical design, capable onboard electronics, and advanced feedback control algorithms. A single chassis holds the actuators, sensors, electronics, and battery. Shafts protruding from either side of this chassis connect to tread assemblies. Rotation of this shaft causes the treads to advance for translational movement; rotation about this shaft causes the entire tread assembly to rotate with respect to the chassis. Vehicle orientation is estimated via onboard filtering of optical encoders and MEMS accelerometers and gyros. In its horizontal configuration, Switchblade operates as a differential-drive treaded platform. In its various upright configurations, Switchblade operates as a mobile inverted pendulum, capable of surmounting obstacles, including stairs, that would otherwise be impassable by a vehicle of its size. Design-for-manufacturing (DFM) and design-for-assembly (DFA) techniques are employed to reduce cost, part count, complexity, and assembly time without sacrificing system capabilities. Results from a working prototype are discussed. The resulting platform is well suited for a variety of socially-relevant applications, including reconnaissance, mine exploration, and search & rescue.

A method for estimating the rotation center position (RCP) of a rigid body in the x-y plane using two offset accelerometers is presented. RCP estimation via inertial measurement is motivated by the related problems of detecting foot slippage of legged robots and detecting stair edges for treaded robots, for applications in which alternative methods such as discontinuity recognition, visual tracking, and/or tactile feedback are impractical. The RCP may be directly solved for as a function of the two offset tangential acceleration measurements, when the RCP is colinear with the two accelerometers, and when the common-mode tangential accelerations, due to linear acceleration and/or gravity, can be independently measured or estimated. Angular velocity estimates may be enhanced by combining calculated angular acceleration with gyroscope measurements, even when both the RCP and common-mode tangential accelerations cannot be independently measured. An input variance modulated variable cutoff low-pass filter is also proposed for RCP estimation in the absence of independent measurements, which is validated on a balance-beam inverted-pendulum apparatus.