Derek Lahr

Whole Skin Locomotion Inspired by Amoeboid Motility Mechanisms

In this paper, a locomotion mechanism for mobile robots inspired by how single celled organisms use cytoplasmic streaming to generate pseudopods for locomotion is presented. Called the whole skin locomotion, it works by way of an elongated toroid, which turns itself inside out in a single continuous motion, effectively generating the overall motion of the cytoplasmic streaming ectoplasmic tube in amoebae. With an elastic membrane or a mesh of links acting as its outer skin, the robot can easily squeeze between obstacles or under a collapsed ceiling and move forward using all of its contact surfaces for traction, even squeezing itself through holes of a diameter smaller than its nominal width. Therefore this motion is well suited for search and rescue robots that need to traverse over or under rubble, or for applications where a robot needs to enter into and maneuver around tight spaces such as for robotic endoscopes. This paper summarizes the many existing theories of amoeboid motility mechanisms and examines how these can be applied on a macroscale as a mobile robot locomotion concept, illustrating how biological principles can be used for developing novel robotic mechanisms. Five specific mechanisms are introduced, which could be implemented to such a robotic system. Descriptions of an early prototype and the preliminary experimental and finite element analysis results demonstrating the feasibility of the whole skin locomotion strategy are also presented, followed by a discussion of future work.

THE OPERATION AND KINEMATIC ANALYSIS OF A NOVEL CAM-BASED INFINITELY VARIABLE TRANSMISSION

In this paper, the operation and analysis of a novel, highly configurable, infinitely variable transmission of the ratcheting drive type is presented. This particular drive uses a cam and a number of cam followers rotatably mounted to a carrier plate to generate an oscillatory motion in an equal number of planet gears. A number of indexing clutches are then used to rectify this motion into a rotational output. A full description of the mechanism, including its components, operation, and kinematic equations are presented. There are a number of inversions of this device, and their characteristics and limitations are discussed. In addition, a method is presented to select the most suitable inversion, gearing, and follower velocity for a given application.

Novel Tripedal Mobile Robot and Considerations for Gait Planning Strategies Based on Kinematics

This paper presents a novel tripedal mobile robot STriDER (Self-excited Tripedal Dynamic Experimental Robot) and considerations for gait planning strategies based on kinematics. To initiate a step, two of the robot’s legs are oriented to push the center of gravity outside the support triangle formed by the three foot contact points, utilizing a unique abductor joint mechanism. As the robot begins to fall forward, the middle leg or swing leg, swings in between the two stance legs and catches the fall. Simultaneously, the body rotates 180 degrees around a body pivot line preventing the legs from tangling up. In the first version of STriDER the concept of passive dynamic locomotion was emphasized; however for the new version, STriDER 2.0, all joints are actively controlled for robustness. Several kinematic constraints are discussed as the robot takes a step including; stability, dynamics, body height, body twisting motion, and the swing leg’s path. These guidelines will lay the foundation for future gait generation developments utilizing both the kinematics and dynamics of the system.

Embedded joint-space control of a series elastic humanoid

This paper provides an overview of the embedded joint-space control approach developed for THOR, a new series elastic humanoid. The 60 kg robot features electromechanical linear series elastic actuators (SEAs), enabling low-impedance control of each joint in the lower body via linear to rotary and parallel mechanisms. We present a distributed joint impedance control framework that leverages a custom dual-axis motor controller to track position, velocity, and torque setpoints for each pair of joints. The required actuator forces are tracked using an inner force control loop combining feedforward and PID control with a model-based disturbance observer (DOB). Unlike previous approaches, we utilize an inverse plant model based on the open-loop actuator dynamics to simplify tuning of the cascaded controller by decoupling DOB estimates from the inner loop gains. The effectiveness of the proposed approach is verified through trajectory tracking and dynamic walking experiments conducted on the THOR humanoid utilizing a complementary optimization-based whole-body controller.

Early Developments of a Parallelly Actuated Humanoid, SAFFiR

This paper presents the design of our new 33 degree of freedom full size humanoid robot, SAFFiR (Shipboard Autonomous Fire Fighting Robot). The goal of this research project is to realize a high performance mixed force and position controlled robot with parallel actuation. The robot has two 6 DOF legs and arms, a waist, neck, and 3 DOF hands/fingers. The design is characterized by a central lightweight skeleton actuated with modular ballscrew driven force controllable linear actuators arranged in a parallel fashion around the joints. Sensory feedback on board the robot includes an inertial measurement unit, force and position output of each actuator, as well as 6 axis force/torque measurements from the feet. The lower body of the robot has been fabricated and a rudimentary walking algorithm implemented while the upper body fabrication is completed. Preliminary walking experiments show that parallel actuation successfully minimizes the loads through individual actuators.Copyright

Design and Measurement Error Analysis of a Low-Friction, Lightweight Linear Series Elastic Actuator

Series elastic actuators (SEAs) have many benefits for force controlled robotic applications. Placing an elastic member in series with a rigid actuator output enables more-stable force control and the potential for energy storage while sacrificing position control bandwidth. This paper presents the design and measurement error analysis of a low-friction, lightweight linear SEA used in the Shipboard Autonomous Fire Fighting Robot (SAFFiR). The SAFFiR SEA pairs a stand-alone linear actuator with a configurable compliant member. Unlike most electric linear actuators, this actuator does not use a linear guide, which reduces friction and weight. Unlike other SEAs which measure the force by measuring the spring deflection, a tension and compression load cell is integrated into the design for accurate force measurements. The configurable compliant member is a titanium cantilever with manually adjustable length. The final SEA weighs 0.82[kg] with a maximum force of 1,000[N]. The configurable compliant mechanism has in a spring constant range of 145–512[kN/m]. Having no linear guide and incorporating the load cell into the universal joint both introduce measurement errors. The length error across a parallel ankle joint is less than 0.015[mm] and the force measurement error is less than 0.25% of the actual force. Finally, several changes are suggested for the next iteration of the SEA to improve its usability on future robots.© 2013 ASME

Configurable Compliance for Series Elastic Actuators

Variable compliance has been a growing topic of interest in legged robotics due to recent studies showing that animals adjust their leg and joint stiffness to adjust their natural dynamics and to accommodate changes in their environment. However, existing designs add significant weight, size, and complexity. Series Elastic Actuators, on the other hand, are designed with a set stiffness usually tuned for actuator performance. We propose a new concept for implementing a physical spring in series with a linear SEA using a cantilevered spring. A movable pivot is used to adjust the stiffness by changing the effective length of the cantilever. While the proposed design does not allow for variable compliance, it does retain many of the benefits of passive spring elements such as absorbing impacts, storing energy, and enabling force control. The primary advantage of the design is the ability to adjust the stiffness of each joint individually without the increased weight and complexity of variable stiffness designs. This paper introduces the motivation for configurable compliance, describes the proposed design concept, explains the design methods, and presents experimental data from a completed prototype.Copyright

Operation and Kinematic Analysis of a Cam-Based Infinitely Variable Transmission

In this paper, the operation and analysis of a novel, highly configurable, infinitely variable transmission of the ratcheting drive type is presented. This particular drive uses a cam and a number of cam followers rotatably mounted to a carrier plate to generate an oscillatory motion in an equal number of planet gears. A number of indexing clutches are then used to rectify this motion into a rotational output. A full description of the mechanism, including its components, operation, and kinematic equations, are presented. There are a number of inversions of this device, and their characteristics and limitations are discussed. In addition, a method is presented to select the most suitable inversion, gearing, and maximum follower velocity for a given set of requirements.

Synthesis of the Body Swing Rotator Joint Aligning Mechanism for the Abductor Joint of a Novel Tripedal Locomotion Robot

The unique three-legged walking robot STriDER (Self-excited Tripedal Dynamic Experimental Robot) utilizes a novel tripedal gait which incorporates aspects of passive dynamic walking into a stable tripedal platform to walk efficiently, and is also capable of changing directions. This unique tripedal gait, however, requires three abductor joints to align two of the three body swing rotator joints in the body, depending on the direction of the step the robot takes. In an earlier prototype of STriDER, the three abductor joints were independently actuated using three DC motors to align the rotator joints which made the robot heavy and inefficient. In this paper, we present the synthesis, analysis, and mechanical design of a novel mechanism for actuating the three abductor joints of this unique three-legged walking robot to generate the required motion using only a single actuator. The mechanism utilizes an internal gear set to generate a Hypotrochoid path curve and uses pin-in-slot joints to coordinate the motion of the three abductor joints to guide them through the four sets of positions required to enable the robot to walk efficiently. A brief description and background of the tripedal locomotion robot STriDER is presented first, followed by the design constraints and requirements of the abductor joint mechanism. Synthesis and kinematic analysis of the mechanism is presented with a study of the force transmission characteristics for a quasi-static case. A description of the detailed mechanical design, results from the experiments, and a conclusion with a discussion for future work is presented next.Copyright

The Development of an Incrementally Loaded Finite Element Model of the Whole Skin Locomotion Mechanism: Discovering the Relationship Between Its Shape and Motion

The Whole Skin Locomotion (WSL) robotic platform is a novel biologically inspired robot that uses a fundamentally different locomotion strategy than other robots. Its motion is similar to the cytoplasmic streaming action seen in single celled organisms such as the amoeba. The robot is composed of a closed volume, fluid filled skin which generally takes the shape of an elongated torus. When in motion the outer skin is used as the traction surface. It is actuated by embedded smart material rings which undergo cyclical contractions and relaxations, generating an everting motion in the torroidially shaped skin. To better understand, design, and optimize this mechanism, it is necessary to have a model of the skin, fluid, and actuators and their interactions with the environment. This paper details the first steps in the development of a non-linear finite element (FE) model which will allow us to study these interactions and predict the shape and motion of the robot under various actuation strategies. A simple membrane element model is introduced from literature and is modified such that an incremental loading strategy can be employed. Finally, an underlying physical mechanism is introduced which could possibly describe the relationship between the shape of and pressure within the membrane skin and motion of the whole skin locomotion robot.Copyright