Bio-inspired friction switches: adaptive pulley systems

Frictional influences in tendon-driven robotic systems are generally unwanted, with efforts towards minimizing them where possible. In the human hand however, the tendon-pulley system is found to be frictional with a difference between high-loaded static post-eccentric and post-concentric force production of 9-12% of the total output force. This difference can be directly attributed to tendon-pulley friction. Exploiting this phenomenon for robotic and prosthetic applications we can achieve a reduction of actuator size, weight and consequently energy consumption. In this study, we present the design of a bio-inspired friction switch. The adaptive pulley is designed to minimize the influence of frictional forces under low and medium-loading conditions and maximize it under high-loading conditions. This is achieved with a dual-material system that consists of a high-friction silicone substrate and low-friction polished steel pins. The system, designed to switch its frictional properties between the low-loaded and high-loaded conditions, is described and its behavior experimentally validated with respect to the number and spacing of pins. The results validate its intended behavior, making it a viable choice for robotic tendon-driven systems.Comment: Conference. First submission, before review

Electromechanical Performance of HASEL Actuators: Fundamentals and Applications

Soft robotic systems are well-suited to unstructured, dynamic tasks and environments, but are currently limited by the soft actuators that power them. Most current soft actuators are based on pneumatics or shape-memory alloys, which have issues with efficiency, response speed, and portability. More recently, dielectric elastomer actuators (DEAs) have shown promise, but they have limited material selection, fabrication methods, and modes of actuation. As such, there remains a need for new types of high performance and well-rounded soft actuators. This dissertation focuses on the development and exploration of a new class of soft electrohydraulic actuators called hydraulically amplified self-healing electrostatic (HASEL) actuators. The first part of this dissertation (Chapter 2) presents of a subclass of HASEL actuators called Peano-HASELs that simultaneously introduce a breakthrough materials system based on thermoplastic films as well as a novel contractile mode of actuation. This approach enables industrially-amenable fabrication techniques, vastly expands the usable materials for actuator construction, and results in high performance actuators with fast linear contraction on activation. The second part of this dissertation (Chapter 3) elucidates the fundamentals of the electromechanical coupling that drives HASEL actuators. An analytical model is developed that accurately describes the quasi-static actuation behavior of Peano-HASEL actuators without relying on fitting parameters. Using this model, we identify a theory-driven approach to actuator design, including a roadmap for actuators with drastically improved specific energies. The final section of this dissertation (Chapter 4 and 5) looks towards more integrated and applied designs of HASEL actuators. First, a new type of articulating actuator is presented that integrates both compliant and rigid components. These spider-inspired electrohydraulic soft-actuated (SES) joints demonstrate high torque and high-speed actuation in an independently-addressable multi-joint limb, a bidirectional actuator, and a versatile gripper. Second, a biodegradable materials system is presented for high performance Peano-HASEL actuators that reduce environmental impact. Finally, a new method of capacitive self-sensing is presented that enables inexpensive and compact circuits that use only off-the-shelf and low voltage components. The results presented in this dissertation provide a framework for the development of high-performance actuators that may one day power the next generation of capable soft robots.</p

Control of Bio-Inspired Sprawling Posture Quadruped Robots with an Actuated Spine

Sprawling posture robots are characterized by upper limb segments protruding horizontally from the body, resulting in lower body height and wider support on the ground. Combined with an actuated segmented spine and tail, such morphology resembles that of salamanders or crocodiles. Although bio-inspired salamander-like robots with simple rotational limbs have been created, not much research has been done on kinematically redundant bio-mimetic robots that can closely replicate kinematics of sprawling animal gaits. Being bio-mimetic could allow a robot to have some of the locomotion skills observed in those animals, expanding its potential applications in challenging scenarios. At the same time, the robot could be used to answer questions about the animal's locomotion. This thesis is focused on developing locomotion controllers for such robots. Due to their high number of degrees of freedom (DoF), the control is based on solving the limb and spine inverse kinematics to properly coordinate different body parts. It is demonstrated how active use of a spine improves the robot's walking and turning performance. Further performance improvement across a variety of gaits is achieved by using model predictive control (MPC) methods to dictate the motion of the robot's center of mass (CoM). The locomotion controller is reused on an another robot (OroBOT) with similar morphology, designed to mimic the kinematics of a fossil belonging to Orobates, an extinct early tetrapod. Being capable of generating different gaits and quantitatively measuring their characteristics, OroBOT was used to find the most probable way the animal moved. This is useful because understanding locomotion of extinct vertebrates helps to conceptualize major transitions in their evolution. To tackle field applications, e.g. in disaster response missions, a new generation of field-oriented sprawling posture robots was built. The robustness of their initial crocodile-inspired design was tested in the animal's natural habitat (Uganda, Africa) and subsequently enhanced with additional sensors, cameras and computer. The improvements to the software framework involved a smartphone user interface visualizing the robot's state and camera feed to improve the ease of use for the operator. Using force sensors, the locomotion controller is expanded with a set of reflex control modules. It is demonstrated how these modules improve the robot's performance on rough and unstructured terrain. The robot's design and its low profile allow it to traverse low passages. To also tackle narrow passages like pipes, an unconventional crawling gait is explored. While using it, the robot lies on the ground and pushes against the pipe walls to move the body. To achieve such a task, several new control and estimation modules were developed. By exploring these problems, this thesis illustrates fruitful interactions that can take place between robotics, biology and paleontology

Scaling Laws in Robotics

AbstractScaling laws are pervasive in biological systems, found in a large number of life processes, and across 27 orders of magnitude. Recent findings show both biological and engineered motors adhering to two fundamental regimes for the mass scaling of maximum force output. This scaling law is of particular interest for the robotics field as it can affect the design stage of a robot. In this study we present data of motors commonly used in robotic applications and find an adherence to a similar power law of mass scaling of maximum torque output in two groups, group a, (Ga ∝ m1.00) and group b (Gb ∝ m1.27). Findings imply that there could exist an upper motor limit of maximum specific torque/force that should be taken under consideration in robot design. Additionally, we show how a robot's minimum mass can be calculated with motor mass being the only necessary parameter