Aaron M. Dollar

Lower Extremity Exoskeletons and Active Orthoses: Challenges and State-of-the-Art

In the nearly six decades since researchers began to explore methods of creating them, exoskeletons have progressed from the stuff of science fiction to nearly commercialized products. While there are still many challenges associated with exoskeleton development that have yet to be perfected, the advances in the field have been enormous. In this paper, we review the history and discuss the state-of-the-art of lower limb exoskeletons and active orthoses. We provide a design overview of hardware, actuation, sensory, and control systems for most of the devices that have been described in the literature, and end with a discussion of the major advances that have been made and hurdles yet to be overcome.

The Highly Adaptive SDM Hand: Design and Performance Evaluation

The inherent uncertainty associated with unstructured environments makes establishing a successful grasp difficult. Traditional approaches to this problem involve hands that are complex, fragile, require elaborate sensor suites, and are difficult to control. Alternatively, by carefully designing the mechanical structure of the hand to incorporate features such as compliance and adaptability, the uncertainty inherent in unstructured grasping tasks can be more easily accommodated. In this paper, we demonstrate a novel adaptive and compliant grasper that can grasp objects spanning a wide range of size, shape, mass, and position/orientation using only a single actuator. The hand is constructed using polymer-based Shape Deposition Manufacturing (SDM) and has superior robustness properties, making it able to withstand large impacts without damage. We also present the results of two experiments to demonstrate that the SDM Hand can reliably grasp objects in the presence of large positioning errors, while keeping acquisition contact forces low. In the first, we evaluate the amount of allowable manipulator positioning error that results in a successful grasp. In the second experiment, the hand autonomously grasps a wide range of spherical objects positioned randomly across the workspace, guided by only a single image from an overhead camera, using feed-forward control of the hand.

A compliant, underactuated hand for robust manipulation

This paper introduces the iRobot-Harvard-Yale (iHY) Hand, an underactuated hand driven by five actuators that is capable of performing a wide range of grasping and in-hand repositioning tasks. This hand was designed to address the need for a durable, inexpensive, moderately dexterous hand suitable for use on mobile robots. The primary focus of this paper will be on the novel simplified design of the iHY Hand, which was developed by choosing a set of target tasks around which the hand was optimized. Particular emphasis is placed on the development of underactuated fingers that are capable of both firm power grasps and low-stiffness fingertip grasps using only the compliant mechanics of the fingers. Experimental results demonstrate successful grasping of a wide range of target objects, the stability of fingertip grasping, and the ability to adjust the force exerted on grasped objects using high-impedance actuators and underactuated fingers.

Mechanical design and performance specifications of anthropomorphic prosthetic hands: A review

In this article, we set forth a detailed analysis of the mechanical characteristics of anthropomorphic prosthetic hands. We report on an empirical study concerning the performance of several commercially available myoelectric prosthetic hands, including the Vincent, iLimb, iLimb Pulse, Bebionic, Bebionic v2, and Michelangelo hands. We investigated the finger design and kinematics, mechanical joint coupling, and actuation methods of these commercial prosthetic hands. The empirical findings are supplemented with a compilation of published data on both commercial and prototype research prosthetic hands. We discuss numerous mechanical design parameters by referencing examples in the literature. Crucial design trade-offs are highlighted, including number of actuators and hand complexity, hand weight, and grasp force. Finally, we offer a set of rules of thumb regarding the mechanical design of anthropomorphic prosthetic hands.

Grasping from the air: Hovering capture and load stability

This paper reports recent research efforts to advance the functionality of Unmanned Aerial Vehicles (UAVs) beyond passive observation to active interaction with and manipulation of objects. The archetypical aerial manipulation task — grasping objects during flight — is difficult due to the unstable dynamics of rotorcraft and coupled object-aircraft motion. In this paper, we analyze key challenges encountered when lifting a grasped object and transitioning into laden free-flight. We demonstrate that dynamic load disturbances introduced by the load mass will be rejected by a helicopter with PID flight control. We determine stability bounds in which the changing mass-inertia parameters of the system due to the grasped object will not destabilize this flight controller. The conditions under which transient partial contact mechanics of objects resting on a surface will not induce instability are identified. We demonstrate grasping and retrieval of a variety of objects while hovering, without touching the ground, using the Yale Aerial Manipulator testbed.

A robust compliant grasper via shape deposition manufacturing

Joint compliance can enable successful robot grasping despite uncertainties in target object location. Compliance also enhances manipulator robustness by minimizing contact forces in the event of unintended contacts or impacts. In this paper, we describe the design, fabrication, and evaluation of a novel compliant robotic grasper constructed using polymer-based shape deposition manufacturing. Joints are formed by elastomeric flexures, and actuator and sensor components are embedded in tough rigid polymers. The result is a robot gripper with the functionality of conventional metal prototypes for grasping in unstructured environments but with robustness properties that allow for large forces due to inadvertent contact.

Stability of small-scale UAV helicopters and quadrotors with added payload mass under PID control

The application of rotorcraft to autonomous load carrying and transport is a new frontier for Unmanned Aerial Vehicles (UAVs). This task requires that hovering vehicles remain stable and balanced in flight as payload mass is added to the vehicle. If payload is not loaded centered or the vehicle properly trimmed for offset loads, the robot will experience bias forces that must be rejected. In this paper, we explore the effect of dynamic load disturbances introduced by instantaneously increased payload mass and how those affect helicopters and quadrotors under Proportional-Integral-Derivative flight control. We determine stability bounds within which the changing mass-inertia parameters of the system due to the acquired object will not destabilize these aircraft with this standard flight controller. Additionally, we demonstrate experimentally the stability behavior of a helicopter undergoing a range of instantaneous step payload changes.

Towards grasping in unstructured environments: grasper compliance and configuration optimization

This paper examines the role of grasper compliance and kinematic configuration in environments where object size and location may not be well known. A grasper consisting of a pair of two-link planar fingers with compliant revolute joints was simulated as it passively deflected during contact with a target object. The kinematic configuration and joint stiffness values of the grasper were varied in order to maximize successful grasp range and minimize contact forces for a wide range of target object size. Joint rest angles around 25–45 degrees produced near-optimal results if the stiffness of the base joint was much smaller than the intermediate joint, as confirmed experimentally.

An investigation of grasp type and frequency in daily household and machine shop tasks

This paper presents a study on the usage frequency of different grasp types throughout the daily functions of a professional house maid and a machinist. Subjects wore a head-mounted camera that recorded their hand usage during their daily work activities. This video was then analyzed, recording grasp type and associated time stamps, as well as information related to the task and object. The results show that nearly 80% of the time the house maid used just six grasps and the machinist used nine. This data, in conjunction with established grasp taxonomies, will enable a better understanding of how people utilize different grasps to accomplish tasks throughout the day, as well as inform the design of robotic and prosthetic hands.

Performance characteristics of anthropomorphic prosthetic hands

In this paper we set forth a review of performance characteristics for both common commercial prosthetics as well as anthropomorphic research devices. Based on these specifications as well as surveyed results from prosthetic users, ranges of hand attributes are evaluated and discussed. End user information is used to describe the performance requirements for prosthetic hands for clinical use.