Coleman Knabe

Design of a Compact, Lightweight, Electromechanical Linear Series Elastic Actuator

Series Elastic Actuators (SEAs) have several benefits for force controlled robotic applications. Typical SEAs place an elastic element between the motor and the load, increasing shock tolerance, allowing for more accurate and stable force control, and creating the potential for energy storage. This paper presents the design of a compact, lightweight, low-friction, electromechanical linear SEA used in the lower body of the Tactical Hazardous Operations Robot (THOR). The THOR SEA is an evolutionary improvement upon the SAFFiR SEA [1]. Design changes focused on reducing the size and fixed length of the actuator while increasing its load capacity. This SEA pairs a ball screw-driven linear actuator with a configurable elastic member. The elastic element is a titanium leaf spring with a removable pivot, setting the compliance to either 650 or 372 [kN/m]. The compliant beam is positioned parallel to the actuator, reducing overall packaging size by relocating the space required for spring deflection. Unlike typical SEAs which measure force through spring deflection, the force applied to the titanium beam is measured through a tension/compression load cell located in line with each actuator, resulting in a measurable load range of +/−2225 [N] at a tolerance of +/−1 [N]. A pair of universal joints connects the actuator to the compliant beam and to the robot frame. As the size of each universal joint is greatly dependent upon its required range of motion, each joint design is tailored to fit a particular angle range to further reduce packaging size. Potential research topics involving the actuator are proposed for future work.Copyright

An unlumped model for linear series elastic actuators with ball screw drives

Series elastic actuators are frequently modeled using a conventional lumped mass model which has remained mostly unchanged since their introduction almost two decades ago. The lumped model has served well for early development but more descriptive models are now needed for new actuator designs and control approaches. In this paper we propose a new unlumped model specifically for linear series elastic actuators which uses a rack & pinion conceptualization to intuitively depict the mechanics of a linear ball screw drive. Results from hardware experiments are presented and compared to the predicted simulation results for both the conventional model and the new unlumped model. The results demonstrate that the new unlumped model is significantly more representative of the true actuator dynamics.

Design of a series elastic humanoid for the DARPA Robotics Challenge

This paper describes the mechanical design of ESCHER, a series elastic humanoid developed to compete in the DARPA Robotics Challenge (DRC). The design methodology was informed by preliminary experimental results obtained using the THOR humanoid, a prototype platform developed for the DRC Trials, relying heavily on an accurate model of the torque-controlled robot in the Gazebo simulation environment. The redesigned lower body features a unique double actuated knee; by driving the single degree of freedom joint with two identical linear series elastic actuators (SEAs), the lower body is able to meet the necessary speed and torque requirements for locomotion on rough terrain. Experimental results demonstrating ESCHERs ability to step onto a 23 cm block, representative of the stairs task at the DRC Finals, validating the proposed approach. Joint torques measured on the hardware platform approximate those in simulation, validating the proposed design methodology.

An Inverted Straight Line Mechanism for Augmenting Joint Range of Motion in a Humanoid Robot

Many robotic joints powered by linear actuators suffer from a loss of torque towards the limits of the range of motion. This paper presents the design of a fully backdriveable, force controllable rotary actuator package employed on the Tactical Hazardous Operations Robot (THOR). The assembly pairs a ball screw-driven linear Series Elastic Actuator (SEA) with a planar straight line mechanism. The mechanism is a novel inversion of a Hoeken’s four-bar linkage, using the ball screw as a linear input to actuate the rotary joint. Link length ratios of the straight line mechanism have been chosen to optimize constant angular velocity, resulting in a nearly constant mechanical advantage and peak torque of 115 [Nm] throughout the 150° range of motion. Robust force control is accomplished through means of a lookup table, which is accurate to within ±0.62% of the nominal torque profile for any load case.Copyright

Design of a Human-Like Range of Motion Hip Joint for Humanoid Robots

For a humanoid robot to have the versatility of humans, it needs to have similar motion capabilities. This paper presents the design of the hip joint of the Tactical Hazardous Operations Robot (THOR), which was created to perform disaster response duties in human-structured environments. The lower body of THOR was designed to have a similar range of motion to the average human. To accommodate the large range of motion requirements of the hip, it was divided into a parallel-actuated universal joint and a linkage-driven pin joint. The yaw and roll degrees of freedom are driven cooperatively by a pair of parallel series elastic linear actuators to provide high joint torques and low leg inertia. In yaw, the left hip can produce a peak of 115.02 [Nm] of torque with a range of motion of −20° to 45°. In roll, it can produce a peak of 174.72 [Nm] of torque with a range of motion of −30° to 45°. The pitch degree of freedom uses a Hoeken’s linkage mechanism to produce 100 [Nm] of torque with a range of motion of −120° to 30°.Copyright

Two configurations of series elastic actuators for linearly actuated humanoid robots with large range of motion

We have developed two different configurations of series elastic actuators to be used in the lower body of THOR, a full scale humanoid developed for the DARPA Robotics Challenge. Both designs utilize a ball screw transmission but use different output mechanisms. The THOR-Linear actuator uses a simple lever output while the THOR-Hoekens actuator uses a novel inversion of a Hoekens Linkage. The simpler design of the THOR-Linear actuator makes it well suited for parallel actuation applications while the THOR-Hoekens actuator features a larger range of motion and nearly constant mechanical advantage. In this video, we show early tests of the two actuator designs used in the THOR lower body. The video shows the two actuator designs, their range of motion on THOR, and a series of performance tests.

Team VALOR's ESCHER: A Novel Electromechanical Biped for the DARPA Robotics Challenge: Team VALOR's ESCHER

The Electric Series Compliant Humanoid for Emergency Response (ESCHER) platform represents the culmination of four years of development at Virginia Tech to produce a full sized force controlled humanoid robot capable of operating in unstructured environments. ESCHER’s locomotion capability was demonstrated at the DARPA Robotics Challenge (DRC) Finals when it successfully navigated the 61m loose dirt course. Team VALOR, a Track A team, developed ESCHER leveraging and improving upon bipedal humanoid technologies implemented in previous research efforts, specifically for traversing uneven terrain and sustained untethered operation. This paper presents the hardware platform, software, and control systems developed to field ESCHER at the DRC Finals. ESCHER’s unique features include custom linear series elastic actuators (SEAs) in both single and dual actuator configurations and a whole-body control framework supporting compliant locomotion across variable and shifting terrain. A high-level software system designed using the Robot Operating System (ROS) integrated various open-source packages and interfaced with the existing whole-body motion controller. The paper discusses a detailed analysis of challenges encountered during the competition, along with lessons learned critical for transitioning research contributions to a fielded robot. Empirical data collected before, during, and after the DRC Finals validates ESCHER’s performance in fielded environments.

Experiential Learning in the Development of a DARwIn-HP Humanoid Educational Robot

The purpose of this study is to present the experiential learning in the development of a Dynamic Anthropomorphic Robot with Intelligence(DARwIn)-High Performance(HP) at Robotics and Mechanism Laboratory(RoMeLa). DARwIn-HP, an miniature-sized humanoid platform, has been developed by self-directed undergraduate students of RoMeLa for robotics in education. They, as the target consumer of educational robots, contributed to the design and manufacturing of the DARwIn-HP by sharing their thoughts on the necessary functionality of an educational humanoid robot. This hands-on experience allows them to understand the fundamental materials of a humanoid robot in theory as well as the usage of mechanical tools in practice. All undergraduates after experiencing the development process also recognize the importance as well as the interests on learning the subjects in the engineering curriculum. As of the final process and post-activity of robot development, encouraging all students to participate in extracurricular activities provides them with a chance of evaluating their work. There are positive effects to engineering students such as enriching their competitiveness for a range of challenges facing society.

Team CHARLI: RoboCup 2012 Humanoid AdultSize League Winner

Autonomous soccer-playing humanoid robots have advanced significantly in the past few years. Skill sets elementary to humans such as omnidirectional bipedal walking, path planning, and gameplay strategy have matured enough to allow for dynamic and exciting games. In this paper team CHARLI, the two-time RoboCup Humanoid AdultSize League winner, describes the design and fabrication of essential components such as the spine and mechanical structure, then overviews the increase in performance resulting from recent mechanical upgrades. Finally, we detail the custom walking controller and gameplay module changes responsible for the outstanding performance of our self-constructed lightweight full-sized humanoid platform, CHARLI-2.

Team VALOR's ESCHER: A Novel Electromechanical Biped for the DARPA Robotics Challenge

The Electric Series Compliant Humanoid for Emergency Response (ESCHER) platform represents the culmination of four years of development at Virginia Tech to produce a full sized force controlled humanoid robot capable of operating in unstructured environments. ESCHER’s locomotion capability was demonstrated at the DARPA Robotics Challenge (DRC) Finals when it successfully navigated the 61 m loose dirt course. Team VALOR, a Track A team, developed ESCHER leveraging and improving upon bipedal humanoid technologies implemented in previous research efforts, specifically for traversing uneven terrain and sustained untethered operation. This paper presents the hardware platform, software, and control systems developed to field ESCHER at the DRC Finals. ESCHER’s unique features include custom linear series elastic actuators (SEAs) in both single and dual actuator configurations and a whole-body control framework supporting compliant locomotion across variable and shifting terrain. A high-level software system designed using the Robot Operating System (ROS) integrated various open-source packages and interfaced with the existing whole-body motion controller. The paper discusses a detailed analysis of challenges encountered during the competition, along with lessons learned critical for transitioning research contributions to a fielded robot. Empirical data collected before, during, and after the DRC Finals validates ESCHER’s performance in fielded environments.