Andrew P. Sabelhaus

Design and Control of Compliant Tensegrity Robots Through Simulation and Hardware Validation

To better understand the role of tensegrity structures in biological systems and their application to robotics, the Dynamic Tensegrity Robotics Lab at NASA Ames Research Center, Moffett Field, CA, USA, has developed and validated two software environments for the analysis, simulation and design of tensegrity robots. These tools, along with new control methodologies and the modular hardware components developed to validate them, are presented as a system for the design of actuated tensegrity structures. As evidenced from their appearance in many biological systems, tensegrity (‘tensile–integrity’) structures have unique physical properties that make them ideal for interaction with uncertain environments. Yet, these characteristics make design and control of bioinspired tensegrity robots extremely challenging. This work presents the progress our tools have made in tackling the design and control challenges of spherical tensegrity structures. We focus on this shape since it lends itself to rolling locomotion. The results of our analyses include multiple novel control approaches for mobility and terrain interaction of spherical tensegrity structures that have been tested in simulation. A hardware prototype of a spherical six-bar tensegrity, the Reservoir Compliant Tensegrity Robot, is used to empirically validate the accuracy of simulation.

Design and evolution of a modular tensegrity robot platform

NASA Ames Research Center is developing a compliant modular tensegrity robotic platform for planetary exploration. In this paper we present the design and evolution of the platforms main hardware component, an untethered, robust tensegrity strut, with rich sensor feedback and cable actuation. Each strut is a complete robot, and multiple struts can be combined together to form a wide range of complex tensegrity robots. Our current goal for the tensegrity robotic platform is the development of SUPERball, a 6-strut icosahedron underactuated tensegrity robot aimed at dynamic locomotion for planetary exploration rovers and landers, but the aim is for the modular strut to enable a wide range of tensegrity morphologies. SUPERball is a second generation prototype, evolving from the tensegrity robot ReCTeR, which is also a modular, lightweight, highly compliant 6-strut tensegrity robot that was used to validate our physics based NASA Tensegrity Robot Toolkit (NTRT) simulator. Many hardware design parameters of the SUPERball were driven by locomotion results obtained in our validated simulator. These evolutionary explorations helped constrain motor torque and speed parameters, along with strut and string stress. As construction of the hardware has finalized, we have also used the same evolutionary framework to evolve controllers that respect the built hardware parameters.

System design and locomotion of SUPERball, an untethered tensegrity robot

The Spherical Underactuated Planetary Exploration Robot ball (SUPERball) is an ongoing project within NASA Ames Research Centers Intelligent Robotics Group and the Dynamic Tensegrity Robotics Lab (DTRL). The current SUPERball is the first full prototype of this tensegrity robot platform, eventually destined for space exploration missions. This work, building on prior published discussions of individual components, presents the fully-constructed robot. Various design improvements are discussed, as well as testing results of the sensors and actuators that illustrate system performance. Basic low-level motor position controls are implemented and validated against sensor data, which show SUPERball to be uniquely suited for highly dynamic state trajectory tracking. Finally, SUPERball is shown in a simple example of locomotion. This implementation of a basic motion primitive shows SUPERball in untethered control.

Mechanism Design and Simulation of the ULTRA Spine: A Tensegrity Robot

The Underactuated Lightweight Tensegrity Robotic Assistive Spine (ULTRA Spine) project is an ongoing effort to create a compliant, cable-driven, 3-degree-of-freedom, underactuated tensegrity core for quadruped robots. This work presents simulations and preliminary mechanism designs of that robot. Design goals and the iterative design process for an ULTRA Spine prototype are discussed. Inverse kinematics simulations are used to develop engineering characteristics for the robot, and forward kinematics simulations are used to verify these parameters. Then, multiple novel mechanism designs are presented that address challenges for this structure, in the context of design for prototyping and assembly. These include the spine robot’s multiple-gear-ratio actuators, spine link structure, spine link assembly locks, and the multiple-spring cable compliance system.Copyright

Model-Predictive Control of a flexible spine robot

The Underactuated Lightweight Tensegrity Robotic Assistive Spine (ULTRA Spine) project is an ongoing effort to develop a flexible, actuated backbone for quadruped robots. In this work, model-predictive control is used to track a trajectory in the robots state space, in simulation. This is the first work that tracks an arbitrary trajectory, in closed-loop, in the state space of a spine-like tensegrity robot. The state trajectory used here corresponds to a bending motion of the spine, with translations and rotations of the three moving vertebrae. The controller uses a linearized model of the system dynamics, computed at each timestep, and has both constraints and weighted penalties to reduce linearization errors. For this robot, which measures 26cm × 26cm × 45cm, the tracking errors converge to less than 0.5cm even with disturbances, indicating that the controller is stable and could be used on a physical robot in future work.

Modular Elastic Lattice Platform for Rapid Prototyping of Tensegrity Robots

This paper presents a new platform for prototyping tensegrity robots that uses an elastic lattice structure for the robots’ tension network. This approach significantly reduces the time required for design, manufacturing, and assembly, while increasing experimental repeatability and symmetry of the tensioned robot. The platform allows more scientific experiments to be performed in less time and with higher quality. This lattice platform, with associated laser-cutting design techniques developed in this work, has been applied to three types of tensegrity structures: 6-bar spheres, 12-bar spheres, and multiple-vertebra tensegrity spines. For the 12-bar tensegrity case in particular, this new lattice platform has allowed multiple different shapes to be explored as designs for future robots. Basic testing confirmed a reduction in robot assembly time from multiple hours down to a mean of one-two minutes for the 6-bar prototype, five-ten minutes for the various 12-bar prototypes, and approximately seven minutes for the spine. ∗Address all correspondence to this author. †Authors also with the BEST Lab at UC Berkeley. FIGURE 1: The TT-4mini prototype, the first tensegrity robot that uses the elastic lattice platform. This robot moves by adjusting the lengths of its cables with respect to its elastic lattice. INTRODUCTION AND PRIOR RESEARCH Challenging environments for robot locomotion, such as those in space applications, have motivated recent research into tensegrity (tension-integrity) robots [1, 2, 3, 4, 5]. These robots 1 Copyright c

Dna structured linear actuator

A DNA-structured linear actuator comprised of a ladder-like structure that twists to generate linear motion. In its base state, the DNA structured linear actuator best resembles a rope ladder. When this ladder is twisted, it takes on the appearance of a DNA double-helix structure. By application of a torsional force on one end, the ladder-like structure extends or contracts to allow linear translation of one end of the structure.