Jordi Artigas

On-orbit servicing

Space robots were the topic of this paper. While on earth, nobody would follow such advice; in space, there are few other options than to replace a malfunctioning spacecraft. There are no repair shops and gas stations in the Earth orbit. Because of the lack of so-called on-orbit servicing (OOS) opportunities, some malfunctioning spacecraft continue operational work with reduced or hardly any performance. The only general modification, which can currently be undertaken to an arbitrary spacecraft in orbit, is a software update. In this paper, exploration and manipulation capabilities of space robots were discussed. Teleprescence through a data relay satellite and teleoperation capabilities were mentioned and discussed.

Network representation and passivity of delayed teleoperation systems

The paper proposes a general network based analysis and design guidelines for teleoperation systems. The electrical domain is appealing because it enjoys proficient analysis and design tools and allows a one step higher abstraction element, the network. Thus, in order to analyze the system by means of network elements the mechanical system must be first modeled as an electric circuit. Only then power ports become apparent and networks can be defined. This kind of analysis has been previously performed in systems with well defined causalities, specially in the communication channel. Indeed, a communication channel exchanging flow-like and effort-like signals, as for instance velocity and computed force, has a well defined causality and can thus be directly mapped as a two-port electrical network. However, this is only one of the many possible system architectures. This paper investigates how other architectures, including those with ambiguous causalities, can be modeled by means of networks, even in the lack of flow or effort being transmitted, and how they can be made passive for any communication channel characteristic (delay, package-loss and jitter). The methods are exposed in the form of design guidelines sustained with an example and validated with experimental results.

The DLR bimanual haptic device with optimized workspace

This article accompanies a video that presents a bimanual haptic device composed of two DLR/KUKA Light-Weight Robot (LWR) arms. The LWRs have similar dimensions to human arms, and can be operated in torque and position control mode at an update rate of 1 kHz. The two robots are mounted behind the user, such that the intersecting workspace of the robots and the human arms becomes maximal. In order to enhance user interaction, various hand interfaces and additional tactile feedback devices can be used together with the robots. The presented system is equipped with a thorough safety architecture that assures safe operation for human and robot. Additionally, sophisticated control strategies improve performance and guarantee stability. The introduced haptic system is well suited for versatile applications in remote and virtual environments, especially for large unscaled movements.

Bilateral energy transfer in delayed teleoperation on the time domain

The time domain passivity framework is attracting interest as a method for granting stability in both telerobotics and haptic contexts; this paper employs this approach in order to introduce a novel concept, the Bilateral Energy Transfer for haptic telepresence. Loosely speaking, the Bilateral Energy Transfer is the straightforward transfer of energy between the two opposite sides of a teleoperation network, the master and slave robots. In an ideal telepresence scenario master and slave robots behave as rigid connected masses [1], and their power exchange is lossless; conversely, realistic scenarios include sources of energy leaks, i.e. elements that modify the power flows in the network. Moreover, if energy leaks have an active nature, they become source of instability for the system. This work isolates two sources of instability normally present in a teleoperation system, i.e. the delayed communication channel and robot velocity estimation based on digital position acquisition. These energy leaks are counterbalanced by two independent controllers, whose design is based on energetic consideration, and whose employment allows to achieve the Bilateral Energy Transfer. The presented arguments are sustained by simulations and experiments.

Time Domain Passivity Control-based Telepresence with Time Delay

This paper analyses the time domain passivity control approach in the time-delayed telepresence context, and proposes a method which provides stable operation. The passivity controller for the two-port network which is created by the bilateral control and communication elements in (J. H. Ryu, et al., May 2002) is shown to be not valid if a time delay is introduced in the communication channel. Classical stability analysis for the delayed system is presented and used as argument and benchmark for the proposed solution. Simulations and experimental results are discussed and compared with classical stability analysis

Time domain passivity control for position-position teleoperation architectures

This article presents a method for passivating the communication channel of a symmetric position-position teleoperation architecture on the time domain. The time domain passivity control approach has recently gained appeal in the context of timedelayed teleoperation because passivity is not established as a design constraint, which often forces conservative rules, but rather as a property which the system must preserve during operation. Since passivity is a network property, the first design rule within this framework is to represent consistent and comprehensible circuit (i.e., network) representations of the mechanical teleoperation system. In particular, the energetic behavior of these networks is interesting because it allows straightforward conclusions about system stability. By means of so-called passivity observers (PO) and passivity controllers (PC) (Hannaford & Ryu, 2001), the energetic response of a delayed communication channel is captured and modulated over time so that the network in question never becomes nonpassive. The case analyzed in this paper tackles a communication channel that conveys position data back and forth. This type of channel does not offer intuitive network representation since only flows are actually being transmitted. Although energy clearly travels from one side to the other, port power identification, as defined by the correlated pair flow and effort, is not evident. This work first investigates how this kind of channel can be represented by means of circuit networks even with the lack of physical effort being transmitted through the channel, and identifies which networks are susceptible to become nonpassive due to the channel characteristics (i.e., time delay, discretization or package loss). Once achieved, a distributed control structure is presented based on a PC series that keeps the system at the verge of passivity (and therefore stability) independent from the channel properties. The results obtained by the simulation and by experiment sustain the presented approach.

The OOS-SIM: An on-ground simulation facility for on-orbit servicing robotic operations

On-orbit servicing involves a new class of space missions in which a servicer spacecraft is launched into the orbit of a target spacecraft, the client. The servicer navigates to the client with the intention of manipulating it, using a robotic arm. Within this framework, this work presents a new robotic experimental facility which was recently built at the DLR to support the development and experimental validation of such orbital servicing robots. The facility allows reproducing a close-proximity scenario under realistic three-dimensional orbital dynamics conditions. Its salient features are described here, to include a fully actuated macro-micro system with multiple sensing capabilities, and analyses on its performance including the amount of space environment volume that can be simulated.

Time domain passivity for delayed haptic telepresence with energy reference

This paper presents a new control strategy based on the time domain passivity control approach which copes with the active nature of delayed communication channels. Describing the system by means of network elements, the energy of the communication channel can be computed in real time and subsequently dissipated, thus providing stable operation. This is done bilaterally, since the system energy may flow from master to slave and from slave to master. The approach is accompanied with some experiments which validate the method.

Position drift compensation in time domain passivity based teleoperation

The Time Domain Passivity Control Approach is gathering interest in the robotics field. Simplicity and flexibility and the fact that system design emerges from ideal cases make it a powerful stability tool for teleoperation systems. Communication time delay is an inherent attribute of nearly every realistic teleoperation system. Unless the communication channel guarantees transmission delays of less than the system sampling time, the delay must be considered in the design in order to guarantee stability and satisfy a desired degree of performance. In previous work it has been shown how passivity can be considered in the time domain and how control rules are derived from it in order to dissipate the energy produced by the delayed communication. However, a weakness of these approaches is the impossibility of observing the exact amount of energy stored in the communication channel due to its delayed nature. A passive estimation is therefore needed which outcomes in an over-dissipation and in turn impacts on transparency. In constrained communications over-dissipation may become apparent in the form of a non-neglectful position drift between master and slave. This paper tackles the over-dissipative behavior of the Passivity Controller by resembling the energetic behavior of an ideal communication, i.e. where no delay is present and the transmission is lossless. Thus, the communication channel is not just controlled to be passive, as has been the case up to now, but also lossless. Energy can be dissipated to prevent activity, but activity can be also produced to prevent dissipative behaviors. The approach is sustained with experimental results.

Passivity of virtual free-floating dynamics rendered on robotic facilities

This paper describes a control strategy to achieve high fidelity dynamics simulation rendered on admittance controlled robotic facilities. It explores the reasons for an increasing energy found in the virtual dynamics of a free-floating satellite rendered on a six degree of freedom robot, which can lead the system to become unstable and proposes a method to cope with it. The proposed method identifies the sources of intrinsic instability provoked by time delays that are found in the computational loop of the rendered dynamics and counteracts their destabilizing effects using the passivity criteria. The performance of the system and the benefits of the method are shown in simulations and are verified experimentally.