Felix Huber

Robots Driven by Compliant Actuators: Optimal Control Under Actuation Constraints

Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and impedance (stiffness and/or damping) simultaneously, continuously, and independently. These actuators are, however, nonlinear and assert numerous constraints, e.g., range, rate, and effort limits on the dynamics. Finding a control strategy that makes use of the intrinsic dynamics and capacity of compliant actuators for such redundant, nonlinear, and constrained systems is nontrivial. In this study, we propose a framework for optimization of torque and impedance profiles in order to maximize task performance, which is tuned to the complex hardware and incorporating real-world actuation constraints. Simulation study and hardware experiments 1) demonstrate the effects of actuation constraints during impedance control, 2) show applicability of the present framework to simultaneous torque and temporal stiffness optimization under constraints that are imposed by real-world actuators, and 3) validate the benefits of the proposed approach under experimental conditions.

On-Orbit Servicing Missions: Challenges and Solutions for Spacecraft Operations

The DLR German Space Operations Center (GSOC) is currently involved in the preparation of two On-Orbit Servicing missions, DEOS and OLEV. Due to the many new challenges within those missions the ground segment design requires new concepts. Accordingly, the paper presents the challenges and solutions regarding the communication architecture including teleoperation and extended contact time. Additionally, we discuss a method of vision based navigation which bridges the gap between absolute and purely geometric navigation. Finally, an integrated system test including GSOC’s new EPOS facility is described.

Optimal control for exploiting the natural dynamics of Variable Stiffness robots

In contrast to common rigid or actively compliant systems, Variable Stiffness Arms are capable of storing potential energy in their joint and convert it into kinetic energy, respectively speed. This capability is well known from humans and is a good example for the outstanding performance of biological systems. However, only since some years intrinsic compliance is considered as a key feature and not a drawback in robot design. Therefore, only very little work has been carried out on exploiting the natural dynamics of elastic arms for such explosive motion sequences. In this paper, we treat the problem of how to optimally achieve maximum link velocity at a given final time for Variable Stiffness Arms. We show that solutions to this problem lead to excitation motions, which enable the robot to move on the link side at much higher speed than on the motor side. In particular, the robot uses the dynamic transfer of elastic joint energy into link side kinetic energy for further acceleration. In our work we consider the practically relevant input and state constraints, and give experimental verification of the developed methods on the new DLR Hand-Arm system.

First analysis and experiments in aerial manipulation using fully actuated redundant robot arm

In this paper we describe a system for aerial manipulation composed of a helicopter platform and a fully actuated seven Degree of Freedom (DoF) redundant industrial robotic arm. We present the first analysis of such kind of systems and show that the dynamic coupling between helicopter and arm can generate diverging oscillations with very slow frequency which we called phase circles. Based on the presented analysis, we propose a control approach for the whole system. The partial decoupling between helicopter and arm - which eliminates the phase circles - is achieved by means of special movement of robotic arm utilizing its redundant DoF. For the underlying arm control a specially designed impedance controller was proposed. In different flight experiments we showcase that the proposed kind of system type might be used in the future for practically relevant tasks. In an integrated experiment we demonstrate a basic manipulation task - impedance based grasping of an object from the environment underlaying a visual object tracking control loop.

Aerial manipulation robot composed of an autonomous helicopter and a 7 degrees of freedom industrial manipulator

This paper is devoted to a system for aerial manipulation, composed of a helicopter and an industrial manipulator. The usage of an industrial manipulator is motivated by practical applications which were identified in different cooperation projects with the industry. We address the coupling between manipulator and helicopter and show that even in case when we have an ideal controller for manipulator and a highperformance controller for helicopter, an unbounded energy flow can be generated by internal forces between helicopter and manipulator if both controllers are used independently. To solve this problem we propose a new kinematical coupling for control by introducing an additional manipulation DoF realized by helicopter rotation around its yaw axis. The new experimental setup and required modifications in the manipulator controller for this purpose are described. Further, we propose dynamical coupling which is implemented by modification of the helicopter controller feeding the interaction force/torque, measured between manipulator base and fuselage, directly to the actuators of the rotor blades. At the end, we present experimental results for aerial manipulation and their analysis.

Optimal torque and stiffness control in compliantly actuated robots

Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and passive impedance (stiffness and/or damping) simultaneously and independently. Here, we propose a framework for simultaneous optimisation of torque and impedance (stiffness) profiles in order to optimise task performance, tuned to the complex hardware and incorporating real-world constraints. Simulation and hardware experiments validate the viability of this approach to complex, state dependent constraints and demonstrate task performance benefits of optimal temporal impedance modulation.

Dynamic optimality in real-time: A learning framework for near-optimal robot motions

Elastic robots have a distinct feature that makes them especially interesting to optimal control: their ability to mechanically store and release potential energy. However, solving any kind of optimal control problem for such highly nonlinear dynamics is feasible only numerically, i.e. offline. In turn, optimal solutions would only contribute a clear benefit for dynamic environments/tasks (apart from rather general insights), if they would be accessible/generalizable in real-time. In this paper, we propose a framework for executing near-optimal motions for elastic arms in real-time. We approach the problem as follows. First, we define a set of prototypical optimal control problems. These represent a reasonable set of motions that an intrinsically elastic robot arm is sought to execute. Exemplary, we solve the optimal control problem for some of these prototypes in a roughly covered task space. Then, we encode the resulting optimal trajectories in a dynamical system via Dynamic Movement Primitives (DMPs). Finally, a distance and cost function based metric forms the basis to generalize from the learned parameterizations to a new unsolved optimal control problem in real-time. In short, we intend to overcome the well known problems of optimal control and learning with associated generalization: being offline and being suboptimal, respectively.

Connected Component Labeling algorithm for very complex and high-resolution images on an FPGA platform

Connected Component Labeling (CCL) is a basic algorithm in image processing and an essential step in nearly every application dealing with object detection. It groups together pixels belonging to the same connected component (e.g. object). Special architectures such as ASICs, FPGAs and GPUs were utilised for achieving high data throughput, primarily for video processing. In this article, the FPGA implementation of a CCL method is presented, which was specially designed to process high resolution images with complex structure at high speed, generating a label mask. In general, CCL is a dynamic task and therefore not well suited for parallelisation, which is needed to achieve high processing speed with an FPGA. Facing this issue, most of the FPGA CCL implementations are restricted to low or medium resolution images (≤ 2048 ∗ 2048 pixels) with lower complexity, where the fastest implementations do not create a label mask. Instead, they extract object features like size and position directly, which can be realized with high performance and perfectly suits the need for many video applications. Since these restrictions are incompatible with the requirements to label high resolution images with highly complex structures and the need for generating a label mask, a new approach was required. The CCL method presented in this work is based on a two-pass CCL algorithm, which was modified with respect to low memory consumption and suitability for an FPGA implementation. Nevertheless, since not all parts of CCL can be parallelised, a stop-and-go high-performance pipeline processing CCL module was designed. The algorithm, the performance and the hardware requirements of a prototype implementation are presented. Furthermore, a clock-accurate runtime analysis is shown, which illustrates the dependency between processing speed and image complexity in detail. Finally, the performance of the FPGA implementation is compared with that of a software implementation on modern embedded platforms.

Technical studies for operations with real-time communications in robotic missions

Robotic telepresence operations between earth and space are of high research value for science as they enable operators on ground to perform physical tasks in space without the need of human presence. Real-Time telepresence with haptic-feedback and stereoscopic imaging, however, poses new requirements to physical parameters of the communication channel like loss, delay and jitter as well as to the protocols spoken between the participants. To meet the new requirements, past robotic missions like ROKVISS chose to use specialized and dedicated communication channels while bypassing the established ground station network infrastructure. However, performing robotic and standard TM/TC operations in parallel was impossible because the Space Link could only be locked by either of the communication chains. For future missions, we present a setup that multiplexes robotic science data and standard TM/TC into one physical channel. Real-time requirements are met because the setup makes use of several FPGAs that forward UDP packets in synchronization with a common master clock. We present test results and test measurements of this technology and compare the proposed setup to a software based solution. Furthermore we present general approaches, tools and techniques for real-time related tasks. Finally we discuss the use of Space Link and Space Link Extension protocols in the communication chain and their impact on the real-time requirements. Operational aspects of the new setup and protocols are discussed as well.

EFAL: EDRS Feeder Link from Antarctic Latitudes - System Architecture and Operations Concept

The goal of near real-time delivery of data from spacecraft in polar low Earth orbits to the customers can be achieved by utilization of Arctic and Antarctic hubs in a Ground Station Network. Additionally, the commanding of the satellites from these hubs can provide novel opportunities for spacecraft operations. The challenge of high-rate data repatriation from such remote sites can be met by means of laser communications from ground to geostationary satellites. We present system architecture and operations concept for a setup employing the European Data Relay System (EDRS) for data uplinks from an Antarctic ground station taking into account the impact of local atmospheric conditions.