Wave-based passive control for transparent micro-teleoperation system

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Abstract

A wave-based bilateral controller for a micro-teleoperation system is presented for stability and transparency via a passivity approach. We showed that the application of wave variable formalism allows the passivity of the system in spite of external perturbations and environment uncertainties. While most of the existing wave-based approach assumes LTI and fixed time-delay, this paper extends the research domain to a scaled teleoperation system with variable time-delay, asymmetric gains in force and velocity variables or model uncertainties in the microenvironment. Based on the wave-based analysis of a time-varying system, a variable gain controller is proposed in order to guarantee the passivity of the system. Furthermore, in scaled teleoperation there exist positive gain matrices (scaling position and force factors) that preserve its passivity only if gain matrices are equal, which is hard to find in a real system. In case the equality condition is not satisfied, we introduced filter functions with a set of conditions for two operating cases: a structurally dominated system and a surface-dominated system. Finally, as the nonlinear nature of small scale environments with unknown dynamic passivity condition can not be satisfied, we analyzed the constraints inevitably occurring in a micro-teleoperation system and derived conditions between the parameters. The stability is analyzed based on the passivity of the resultant operator–telemanipulator–environment system. The experimental results show clearly that the proposed passivity conditions ensure stability and transparency performances against variable time-delays and scaling factors.

Introduction

Force reflecting micro-teleoperation systems allow a human operator (master) to handle micro-objects (slave) in a dangerous or inaccessible environment from a distant site. The current applications concern the biological remote control handling or the assembly of microsystems. In most of these applications, master and slave sites are separated by a long distance and communicate through an Internet support. Variable delays are then inevitable, inducing oscillations and strong instabilities. Furthermore, considering the variety of micro-objects to be manipulated (soft, fragile), a strong variability of the scaling factors in position and in force exists. Colgate [1] has shown that a synthesis of a controller that takes into account the variability of scaled factors could improve drastically the overall performances of the closed loop system. In recent years, the passivity based approach has been proved to be effective for the stability analysis of teleoperation systems. The two major issues in scaled teleoperation are stability robustness and transparency performances. However, the transparency and robust stability (passivity) are conflicting design goals in micro-teleoperation systems via the passivity approach. That is to say, it is impossible to make a micro-teleoperation system passive due to varying time-delays, scaling effects and uncertainties on working microenvironment. This paper was in part motivated by the fact that prior publications on the topic seem to uniformly incorporate network-based concepts to address transparency, and passivity-based concepts to address stability, the former necessitating the latter. Many researchers have employed velocity, force or impedance information to propose a variety of transparency-optimized bilateral controllers. For the delay problem, the majority of the research adopts concepts of passivity to ensure stability in the presence of time-delay. Anderson and Spong [2] derived a control law based on passivity and scattering theory to ensure teleoperative stability subject to any time-delay, but performance was shown to degrade as the time-delay was increased. Niemeyer and Slotine [3] also proposed an approach based on passivity and scattering theory to address time-delay in teleoperation. The authors additionally present prediction techniques that further improve the system’s performance under time-delay. Lawrence [4] addressed time-delay in four-channel bilateral telemanipulation. Using passivity theory, filters were derived that ensured the stability. Yoshikawa and Ueda [5] used scattering theory to assess the stability of four conventional teleoperation architectures subject to time delay. Munir and Book [6] incorporated a Smith predictor and Kalman filter to improve the performance of a wave-based teleoperator in the presence of a varying-time delay.

Although such approaches compensate constant and/or varying time-delays, few works take into account variation of force scaling factors and dynamics of environment in a scaled bilateral manipulator controller structure. Colgate [1] was the first to rigorously utilize passivity concepts for macro–micro bilateral manipulation by proposing impedance shaping bilateral control, and deriving a general condition that guarantees that the coupled manipulator, impedance shaping filter and passive environment together exhibit passive behavior. Based on this concept, different works have been derived [7], [8] but the choice of the scaling gain to properly reshape the master–slave–environment is still a remaining problem. The transparency is altered since the reshaped microenvironment loses its dynamic character. To solve this problem, Park [9] has introduced a velocity–force scaling property which happens at micro-teleoperation and used modified four-channel architecture (4C) control, originated by Lawrence [4] and Hashtrudi-Zaad et al. [10], which has local force feedback. By this method we can satisfy the passivity condition for the micro-teleoperation system handling a small inertial object. However, the transparency and the stability is not guaranteed in the presence of significant time-delays (variable or constant).

In the presence of a multi-objective problem, i.e., time-varying delay, variation of force scaling and uncertainty in the microenvironment, the presented passivity concepts are too conservative, and as such, compromise system performance (i.e., transparency) more than necessary. Comparing to wave-based approaches, considered to be a standard solution for delayed force reflecting systems, the method is more conservative but less transparent. In order to solve the conflicting design goals of transparency and stability (passivity) in micro-teleoperation systems via the passivity approach, this paper proposes a new wave-based controller which can satisfy the passivity condition for optimized stability and transparency. The paper is organized in the following way. In Section 2, the wave-based controller architecture subjected to time-delays, varying force–velocity scaling factors and microenvironment variations is presented. Then, Section 3 derives passivity conditions about the transparency and stability for micro-teleoperation. Finally, Section 5 presents simulation and experimental results to prove the effectiveness of the proposed approach.

Section snippets

Background on wave variables

In this section, we shall briefly summarize how wave variables work as the basic of the passivity concept. A bilateral tele-micromanipulation system is modeled with linear approximation consisting of a series of functional blocks, as shown in Fig. 1, composed mainly of the human operator (block1), the controlled scaled bilateral telemanipulator (block 2), and the working microenvironment (block 3). The block 1 is composed of a local hand controller Gm(s) represented by the haptic interface Pm(s)

Passivity of scaled human–telemanipulator–environment loop

In this section, we will discuss the stability analysis of the proposed tele-micromanipulation system including a human operator, a bilateral controller and a microenvironment based on the passivity concept. Any teleoperation system must maintain stability under operator and environment variations.

Transparency

In any bilateral teleoperation system design, we desire to transfer signals (velocities and forces) faithfully between master and slave to couple the operator as closely as possible to the task (or environment). It can be described by the following equations [24]. Fm=FeVm=Ve where the master and slave velocities are noted as Vm=ẋm and Ve=ẋe. In other words, the measured force on the slave side must be equal to the force which the operator applies to the master and the velocity of the master

Experimental setup

Fig. 7(a) shows the force-reflecting micromanipulator with a four-degree-of-freedom microgripper used in the experiments operating under the field of view of an optical microscope. It is called MMOC (Microprehensile Microrobot On Chip) fabricated at the LAB-CNRS (Laboratoire d’Automatique de Besançon, France) [25] (Fig. 7(b)). The elementary micro-actuator is a duo-bimorph, a monolithic piezoelectric actuator offering two uncoupled degrees of freedom, i.e. in-plane motion (x,y) and out-of-plane

Conclusion

In this paper, we have proposed the design of a bilateral controller for a micro-teleoperation system. This controller ensures the passivity and the transparency of the system and take into account the communication delays, the variation of the microenvironment as well as the nonlinear scaling factors. To take into consideration the effects due to the delay and the scaling factors, we used a structure of impedance filtering in order to reflect energy excess in the communication lines. Then, we

Dr. Moussa Boukhnifer was born in 1976 in Bordj Menail Algeria. He received the Engineer degree in Electrotechnics, the Magister degree in electrical engineering and control, from the Ecole Nationale Polytechnique, Algiers, Algeria, in 1998, 2001 respectively and D.E.A degree in electrical engineering from the C.E.G.E.L.Y, Institut National des Sciences Appliquées, Lyon, France, in 2002. In October 2002, he joined the Laboratoire de Vision et de Robotique, Université d’Orléans, France. In

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      Using the proposed time delay identification algorithm, the value of K is derived. One of the limitations of wave variables is the degradation of their performance in the presence of increased time delay (Boukhnifer & Ferreira, 2006). Therefore, a classic approach called ‘wave prediction’ is proposed to enhance the performance of wave-based teleoperation systems.

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    Dr. Moussa Boukhnifer was born in 1976 in Bordj Menail Algeria. He received the Engineer degree in Electrotechnics, the Magister degree in electrical engineering and control, from the Ecole Nationale Polytechnique, Algiers, Algeria, in 1998, 2001 respectively and D.E.A degree in electrical engineering from the C.E.G.E.L.Y, Institut National des Sciences Appliquées, Lyon, France, in 2002. In October 2002, he joined the Laboratoire de Vision et de Robotique, Université d’Orléans, France. In December 2005, he received the doctorat degree in Control and Robotic Engineering from Université of Orléans. France His research includes robust control, passivity control and its applications to robotics, power electronics and electrical drives.

    Dr. Antoine Ferreira is an Associate Professor of Robotics Engineering at the Laboratoire Vision et Robotique of Bourges (France). He received the MS, Ph.D. in Electrical and Electronics Engineering from the University of Franche-Comté (France) in 1993, 1996 respectively. In 1997 he was a Visiting Researcher at the ElectroTechnical Laboratory (ETL), in Tsukuba (Japan). His main research interests are focused on the design, modeling and control of micro and nanorobotic systems using active materials and micro-nano-teleoperation, multimodal human-machine interfaces for micro/nanomanipulation. He is an author of more than 90 technical publications in this area. Dr. Ferreira is a IEEE Member of the RAS and EMBS society.

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