With the significant advancement of sensor and communication technology and the reliable application of obstacle detection techniques and algorithms, automated driving is becoming a pivotal technology that can revolutionize the future of transportation and mobility. Sensors are fundamental to the perception of vehicle surroundings in an automated driving system, and the use and performance of multiple integrated sensors can directly determine the safety and feasibility of automated driving vehicles. Sensor calibration is the foundation block of any autonomous system and its constituent sensors and must be performed correctly before sensor fusion and obstacle detection processes may be implemented. This paper evaluates the capabilities and the technical performance of sensors which are commonly employed in autonomous vehicles, primarily focusing on a large selection of vision cameras, LiDAR sensors, and radar sensors and the various conditions in which such sensors may operate in practice. We present an overview of the three primary categories of sensor calibration and review existing open-source calibration packages for multi-sensor calibration and their compatibility with numerous commercial sensors. We also summarize the three main approaches to sensor fusion and review current state-of-the-art multi-sensor fusion techniques and algorithms for object detection in autonomous driving applications. The current paper, therefore, provides an end-to-end review of the hardware and software methods required for sensor fusion object detection. We conclude by highlighting some of the challenges in the sensor fusion field and propose possible future research directions for automated driving systems.

Sensor and Sensor Fusion Technology in Autonomous Vehicles: A Review

Individuals with poststroke hemiparesis often exhibit mobility deficits, particularly during tasks requiring high lower limb torques. The sit-to-stand transition is consistently marked by asymmetrical weight-bearing between the paretic and unaffected legs. One way to improve characteristics of stroke sit-to-stand may be to provide assistive knee extension torque with a powered exoskeleton. To perform research on the biomechanical effects of assisting sit-to-stand, a unilateral powered knee exoskeleton is required, which can accurately control torque. This paper introduces a novel series elastic actuator capable of producing the full torques and speeds required for sit-to-stand (80 Nm, 3 rad/s). It utilizes a unique transmission configuration with a series fiberglass beam spring that improves torque-control and reduces output impedance. The actuator is incorporated into a unilateral orthosis and a high-level sit-to-stand controller is implemented. A small validation study with three able-bodied subjects performing sit-to-stand is presented, demonstrating the ability to appropriately provide assistance.

Design and Validation of a Torque-Controllable Knee Exoskeleton for Sit-to-Stand Assistance

This review paper provides a synthetic yet critical overview of the key biomechanical principles of human bipedal walking and their current implementation in robotic platforms. We describe the functional role of human joints, addressing in particular the relevance of the compliant properties of the different degrees of freedom throughout the gait cycle. We focused on three basic functional units involved in locomotion, i.e. the ankle-foot complex, the knee, and the hip-pelvis complex, and their relevance to whole-body performance. We present an extensive review of the current implementations of these mechanisms into robotic platforms, discussing their potentialities and limitations from the functional and energetic perspectives. We specifically targeted humanoid robots, but also revised evidence from the field of lower-limb prosthetics, which presents innovative solutions still unexploited in the current humanoids. Finally, we identified the main critical aspects of the process of translating human principles into actual machines, providing a number of relevant challenges that should be addressed in future research.

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Human-like compliant locomotion: state of the art of robotic implementations.

In order for the lower limb exoskeletons to realize their considerable potential, a greater understanding of optimal assistive performance is required. While others have shown positive results, the fundamental question of how the exoskeleton interacts with the human remains unknown. Understanding the optimal assistance magnitude is not simply relevant for control, it is a critical knowledge for exoskeleton designers. An accurate understanding of assistance levels will enable the designers to minimize exoskeleton mass and improve the performance by avoiding excessive actuators and drivetrains. We explored the relationship between the assistance magnitude and the energetic cost benefits by using a series elastic actuator driven powered hip exoskeleton. The exoskeleton controller mimics a human biological hip moment to provide the assistance during the gait cycle. Ten able-bodied subjects walked using the exoskeleton with different magnitudes of assistance in both hip flexion and extension. Generally, the resulting metabolic cost across different assistance conditions showed a U-shape trend which was consistent across all subjects (p < 0.01). The interpreted optimal assistance point through the quadratic fit resulted in a 6% metabolic cost reduction with respect to the no-assistance condition. The study validated that simply increasing the assistance level did not yield higher energetic return.

The Effect of Hip Assistance Levels on Human Energetic Cost Using Robotic Hip Exoskeletons

Soft robots are one of the most significant recent evolutions in robotics. They rely on compliant physical structures purposefully designed to embody desired characteristics. Since their introduction, they have shown remarkable applicability in overcoming their rigid counterparts in such areas as interaction with humans, adaptability, energy efficiency, and maximization of peak performance. Nonetheless, we believe that research on novel soft robot applications is still slowed by the difficulty in obtaining or developing a working soft robot structure to explore novel applications.

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The Quest for Natural Machine Motion: An Open Platform to Fast-Prototyping Articulated Soft Robots

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 ESCHER's 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.

Design of a series elastic humanoid for the DARPA Robotics Challenge

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.

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

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 © 2014 by ASME

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

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 © 2014 by ASME

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

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 © 2014 by ASME

Coleman Knabe

Papers

Design of a series elastic humanoid for the DARPA Robotics Challenge

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

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

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

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