Muhammad E. Abdallah

Robonaut 2 - The first humanoid robot in space

NASA and General Motors have developed the second generation Robonaut, Robonaut 2 or R2, and it is scheduled to arrive on the International Space Station in early 2011 and undergo initial testing by mid-year. This state of the art, dexterous, anthropomorphic robotic torso has significant technical improvements over its predecessor making it a far more valuable tool for astronauts. Upgrades include: increased force sensing, greater range of motion, higher bandwidth, and improved dexterity. R2s integrated mechatronic design results in a more compact and robust distributed control system with a fraction of the wiring of the original Robonaut. Modularity is prevalent throughout the hardware and software along with innovative and layered approaches for sensing and control. The most important aspects of the Robonaut philosophy are clearly present in this latest models ability to allow comfortable human interaction and in its design to perform significant work using the same hardware and interfaces used by people. The following describes the mechanisms, integrated electronics, control strategies, and user interface that make R2 a promising addition to the Space Station and other environments where humanoid robots can assist people.

A Biomechanically Motivated Two-Phase Strategy for Biped Upright Balance Control

Balance maintenance and upright posture recovery under unexpected environmental forces are key requirements for safe and successful co-existence of humanoid robots in normal human environments. In this paper we present a two-phase control strategy for robust balance maintenance under a force disturbance. The first phase, called the reflex phase, is designed to withstand the immediate effect of the force. The second phase is the recovery phase where the system is steered back to a statically stable “home” posture. The reflex control law employs angular momentum and is characterized by its counter-intuitive quality of “yielding” to the disturbance. The recovery control employs a general scheme of seeking to maximize the potential energy and is robust to local ground surface feature. Biomechanics literature indicates a similar strategy in play during human balance maintenance.

The Robonaut 2 hand - designed to do work with tools

The second generation Robonaut hand has many advantages over its predecessor. This mechatronic device is more dexterous and has improved force control and sensing giving it the capability to grasp and actuate a wider range of tools. It can achieve higher peak forces at higher speeds than the original. Developed as part of a partnership between General Motors and NASA, the hand is designed to more closely approximate a human hand. Having a more anthropomorphic design allows the hand to attain a larger set of useful grasps for working with human interfaces. Key to the hands improved performance is the use of lower friction drive elements and a redistribution of components from the hand to the forearm, permitting more sensing in the fingers and palm where it is most important. The following describes the design, mechanical/electrical integration, and control features of the hand. Lessons learned during the development and initial operations along with planned refinements to make it more effective are presented.

Multiple-priority impedance control

Impedance control is well-suited to robot manipulation applications because it gives the designer a measure of control over how the manipulator to conforms to the environment. However, in the context of end-effector impedance control when the robot manipulator is redundant with respect to end-effector configuration, the question arises regarding how to control the impedance of the redundant joints. This paper considers multi-priority impedance control where a second-priority joint space impedance operates in the null space of a first-priority Cartesian impedance at the end-effector. A control law is proposed that realizes both impedances while observing the priority constraint such that a weighted quadratic error function is optimized. This control law is shown to be a generalization of several motion and impedance control laws found in the literature. The paper makes explicit two forms of the control law. In the first, parametrization by passive inertia values allows the control law to be implemented without requiring end-effector force measurements. In the second, a class of parametrizations is introduced that makes the null space impedance independent of end-effector forces. The theoretical results are illustrated in simulation.

Applied joint-space torque and stiffness control of tendon-driven fingers

Existing tendon-driven fingers have applied force control through independent tension controllers on each tendon, i.e. in the tendon-space. The coupled kinematics of the tendons, however, cause such controllers to exhibit a transient coupling in their response. This problem can be resolved by alternatively framing the controllers in the joint-space of the manipulator. This work presents a joint-space torque control law that demonstrates both a decoupled and significantly faster response than an equivalent tendon-space formulation. The law also demonstrates greater speed and robustness than comparable PI controllers. In addition, a tension distribution algorithm is presented here to allocate forces from the joints to the tendons. It allocates the tensions so that they satisfy both an upper and lower bound, and it does so without requiring linear programming or open-ended iterations. The control law and tension distribution algorithm are implemented on the robotic hand of Robonaut-2.

An Optimal Traction Control Scheme for Off-Road Operation of Robotic Vehicles

Active degrees of freedom provide a robotic vehicle the ability to enhance its performance in all terrain conditions. While active suspension systems are now commonplace in on-road vehicles, their application to off-road terrains has been little investigated. A fundamental component of such an application is a need to translate desired body motion commands into actuator values through the use of proprioceptive algorithms. The diverse nature of the terrains that might be encountered places variable demands upon the operation of the vehicle. This entails the potential use of a diverse set of algorithms designed to optimize mobility and performance. This paper presents a cohesive control scheme designed for the operation of an autonomous vehicle under all conditions. The ideas presented have been tested in simulation, and some have been used extensively in the field

Decoupled torque control of tendon-driven fingers with tension management

To facilitate human assembly tasks, Robonaut 2 is equipped with a dexterous, compact hand featuring fingers driven remotely by tendons. This work outlines the force-control strategy for the fingers, which are actuated by an “n + 1” tendon arrangement. Existing tendon-driven fingers have applied force control through independent tension controllers on each tendon, in other words, in the tendon space. The coupled kinematics of the tendons, however, cause such controllers to exhibit a transient coupling in their response. This problem can be resolved by alternatively framing the controllers in the joint space of the finger. A joint-space torque control law is proposed here that demonstrates a decoupled response with a faster settling time than an equivalent tendon-space formulation. In addition, a tension distribution algorithm is presented here to translate joint torque commands into tendon tensions. It guarantees that each tendon tension respects both an upper and a lower bound, using an efficient, finitely-convergent algorithm. These two contributions provide for a compliant, well-controlled hand, aptly suited for unstructured interaction.

Multi-Priority Cartesian Impedance Control.

Manipulator compliance is well known to be important to robot manipulation and assembly. Recently, this has been highlighted by the development of new higly-compliant robot manipulators such as the Barrett arm or the DLR lightweight manipulator [1], [2]. It is also clear that dexterous manipulation involves touching the environment at different locations simultaneously (perhaps at different points on the robot hand or fingers). In these situations, it is particularly attractive to control the system using a multi-priority strategy where several contact points are commanded in parallel. Multi-priority Cartesian impedance control is the natural combination of these two ideas. The system realizes several impedances with different reference positions at different points on the robot with a specified order of priority. We find a controller that minimizes an arbitrary quadratic norm on the second-priority impedance error subject to constraints deriving from the first priority impedance task. We also show that the locally optimal controller does not require force feedback in its implementation for passive desired inertias. The results are illustrated in simulation.

A miniature load cell suitable for mounting on the phalanges of human-sized robot fingers

It is frequently accepted that tactile sensing must play a key role in robust manipulation and assembly. The potential exists to complement the gross shape information that vision or range sensors can provide with fine-scale information about the texture, stiffness, and shape of the object grasped. Nevertheless, no widely accepted tactile sensing technology currently exists for robot hands. Furthermore, while several proposals exist in the robotics literature regarding how to use tactile sensors to improve manipulation, there is little consensus. This paper describes the electro-mechanical design of the Robonaut 2 phalange load cell. This is a miniature load cell suitable for mounting on the phalanges of humanoid robot fingers. The important design characteristics of these load cells are the shape of the load cell spring element and the routing of small-gauge wires from the sensor onto a circuit board. The paper reports results from a stress analysis of the spring element and establishes the theoretical sensitivity of the device to loads in different directions. The paper also compares calibrated load cell data to ground truth load measurements for four different manufactured sensors. Finally, the paper analyzes the response of the load cells in the context of a flexible materials localization task.

Object impedance control using a closed-chain task definition

Humanoid robots are intended to interact with unstructured environments and to perform diverse applications. Often, such work involves manipulating an object cooperatively with multiple hands or fingers. This work presents an impedance based control framework for such cases with multi-priority tasking. The primary task governs the impedance response of the object and a secondary task governs the impedance response of the joints. Using a novel transformation, the primary task may specify a subset of the object degrees of freedom (DOFs), allocating the remaining DOFs to the secondary task. This results in an integrated null space that includes not only the redundant DOFs of each manipulator independantly, but also the free DOFs of the object shared across the manipulators.