Time Domain Passivity Approach Research Articles

Improvement in the conservatism of the time domain passivity approach in delayed position-position teleoperation system

Improvement in the conservatism of the time domain passivity approach in delayed position-position teleoperation system

Relaxing Conservatism for Enhanced Impedance Range and Transparency in Haptic Interaction.

The Time Domain Passivity Approach (TDPA) has been accepted as one of least conservative tools for designing stabilizing controllers in haptics and teleoperation, but it still suffers from conservatism because it is based on passivity. Additionally, high-frequency, immediate control actions lead to a degradation of transparency. In this paper, we propose a method to relax the conservatism of haptic interaction and enhance stable impedance range while maintaining high transparency. Based on the observation of energy exchange behavior in pressing and releasing paths in haptic interaction, we introduce an energy cycle as a completion of a pressing and releasing path. With this new concept, we compare the energies at the end of each energy cycle to estimate the energy generation and inject adaptive damping to regulate it over upcoming cycles. Because we wait a pressing-releasing cycle is completed, we allow energy to be generated, but we regulate the amount of generated energy over upcoming cycles by injecting adaptive damping. In this way, we perform low-frequency control actions on system dynamics. These in turn enable us to achieve high transparency. We show the validity of the proposed approach through several simulations and experiments, and show that it enhances the stable impedance range and transparency compared to the TDPA.

Chattering-Free Time Domain Passivity Approach.

The time-domain passivity approach (TDPA) has gained considerable attention (even beyond the fields of haptics and teleoperation) because it can ensure passivity while maintaining high system performance. Despite its simplicity and effectiveness, the practical applications of this method are limited by its high-frequency force modification, so-called chattering. Virtual mass-spring (VMS) filters have been proposed to overcome this limitation. However, such approaches also suffer from certain limitations attributable to the distorted forces. Herein, we propose a method to mitigate the chattering of the TDPA, while also retaining its merits. We introduce a non-zero velocity threshold, within which the adaptive damping of the TDPA is scaled down, to attenuate the chattering while maintaining a stable response significantly. The feasibility and effectiveness of the proposed method are evaluated using a time-delayed bilateral teleoperation system. We confirm that the proposed method can significantly outperform the conventional and VMS filter-based TDPAs.

A Novel Energy Compensation Scheme for Quality Enhancement in Time-Delayed Teleoperation With Multi-DoF Haptic Data Reduction and Communication.

Efficient and low-delay exchange of hapticinformation plays a key role for the design of high-fidelity teleoperation systems that operate over real-world communication networks. In the presence of communication unreliabilities such as delay and packet loss, a combination of stability-ensuring control schemes and haptic data reduction approaches is essential to address the challenges from both the control and communication perspective. In this paper, we extend our previous one degree-of-freedom (DoF) solution to 3DoF that combines the time-domain passivity approach (TDPA) and the perceptual deadband-based (DB) haptic data reduction approach. We also extend the 1DoF energy compensation (EC) scheme to 3DoF for mitigating the control artifacts introduced by the inter-play of the TDPA and DB approaches. The 3DoF solution is analyzed in detail and verified through a real-world teleoperation setup. The performance of the proposed method and the existing solutions is compared though objective and subjective assessments. Experimental results show that by jointly considering the evaluated objective/subjective quality and the required packet rate, the proposed 3DoF solution with the EC scheme improves system performance significantly compared to the state-of-the-art solutions.

Position Synchronization Through the Energy-Reflection Based Time Domain Passivity Approach in Position-Position Architectures

Position synchronization is a primary control challenge in the delayed coupling of robotic systems. While in tele-operation, the interaction force of the robot is often the preferred feedback for high degrees of transparency, positions are exchanged in the coupling of autonomous systems for improved position matching. This letter proposes the extension of the energy-reflection based Time Domain Passivity Approach to position-position control for delayed coupled network systems. By considering the physical behavior of the two coupling springs as a reference for the desired energy exchange of the agents, a real passive coupling 2-port is achieved at low conservatism. Furthermore, position drift is completely avoided in contrast to state-of-the-art approaches. The high modularity of the approach allows a straightforward application in multilateral setups. Other benefits are its robustness in case of active environments and an undisturbed force transmission to the input device in tele-operation setups. The approach is validated in bilateral and multilateral experiments with delays up to 800 ms and compared to a state-of-the-art approach. The results confirm the high position tracking and force-feedback quality of the proposed approach as well as its robustness against active environments.

Whole-Body Teleoperation and Shared Control of Redundant Robots with Applications to Aerial Manipulation

This paper introduces a passivity-based control framework for multi-task time-delayed bilateral teleoperation and shared control of kinematically-redundant robots. The proposed method can be seen as extension of state-of-the art hierarchical whole-body control as it allows for some of the tasks to be commanded by a remotely-located human operator through a haptic device while the others are autonomously performed. The operator is able to switch among tasks at any time without compromising the stability of the system. To enforce the passivity of the communication channel as well as to dissipate the energy generated by the null-space projectors used to enforce the hierarchy among the tasks, the Time-Domain Passivity Approach (TDPA) is applied. The efficacy of the approach is demonstrated through its application to the DLR Suspended Aerial Manipulator (SAM) in a real telemanipulation scenario with variable time delay, jitter, and package loss.

The 6-DoF Implementation of the Energy-Reflection Based Time Domain Passivity Approach With Preservation of Physical Coupling Behavior

Instability due to delayed communication is one of the main challenges in the coupling of autonomous robots but also in teleoperation with applications reaching from space to tele-healthcare scenarios. The Time Domain Passivity Approach assures stability despite delay and has already been validated in teleoperation scenarios from the International Space Station. It has been improved by a method considering energy reflection of the coupling controller recently. This extension has been shown to provide better performance in terms of position tracking and transmitted impedances which promises increased transparency for a human operator. This letter presents the 6-DoF implementation of the energy-reflection based approach and of an extended gradient method which promises to maintain the physical coupling behavior despite delay. An intense experimental validation confirms the performance increase due to both methods at delays up to 600 ms in the 6-DoF case.

Multilateral Teleoperation Over Communication Time Delay Using the Time-Domain Passivity Approach

A general framework to stabilize multilateral teleoperation system over a communication time delay based on the well-known time-domain passivity approach (TDPA) is proposed. The uniqueness of this framework is that it is independent of the amount of communication time delay, the multilateral control architecture, and the number of masters and slaves. The multilateral system was first decoupled into subsystems with respect to each terminal by identifying the input signals’ contributions to the output terminals, which was not straightforward due to the coupled nature of the multilateral teleoperation system. The decoupled subsystems were then converted into an electrical circuit using a mechanical–electrical analogy. Time-delay power network (TDPN) was introduced to clarify active energy sources from the time delay and passivity observer/passivity controller (PO/PC) was utilized to dissipate those active energies. A less-conservative method compared with prior work was proposed to guarantee the stability. Experiments with a trilateral teleoperation system and with a multilateral teleoperation system with a dual master and dual slave were conducted to validate the proposed framework.

Multi-DoFs Exoskeleton-Based Bilateral Teleoperation with the Time-Domain Passivity Approach

SummaryIt is well known that the sense of presence in a tele-robot system for both home-based tele-rehabilitation and rescue operations is enhanced by haptic feedback. Beyond several advantages, in the presence of communication delay haptic feedback can lead to an unstable teleoperation system. During the last decades, several control techniques have been proposed to ensure a good trade-off between transparency and stability in bilateral teleoperation systems under time delays. These proposed control approaches have been extensively tested with teleoperation systems based on identical master and slave robots having few degrees of freedom (DoF). However, a small number of DoFs cannot ensure both an effective restoration of the multi-joint coordination in tele-rehabilitation and an adequate dexterity during manipulation tasks in rescue scenario. Thus, a deep understanding of the applicability of such control techniques on a real bilateral teleoperation setup is needed. In this work, we investigated the behavior of the time-domain passivity approach (TDPA) applied on an asymmetrical teleoperator system composed by a 5-DoFs impedance designed upper-limb exoskeleton and a 4-DoFs admittance designed anthropomorphic robot. The conceived teleoperation architecture is based on a velocity–force (measured) architecture with position drift compensation and has been tested with a representative set of tasks under communication delay (80 ms round-trip). The results have shown that the TDPA is suitable for a multi-DoFs asymmetrical setup composed by two isomorphic haptic interfaces characterized by different mechanical features. The stability of the teleoperator has been proved during several (1) high-force contacts against stiff wall that involve more Cartesian axes simultaneously, (2) continuous contacts with a stiff edge tests, (3) heavy-load handling tests while following a predefined path and (4) high-force contacts against stiff wall while handling a load. The found results demonstrated that the TDPA could be used in several teleoperation scenarios like home-based tele-rehabilitation and rescue operations.

Reducing the conservatism of the time domain passivity approach through consideration of energy reflection in delayed coupled network systems

The Time Domain Passivity Approach (TDPA), a method to inject adaptive damping to satisfy the passivity condition in real-time, has emerged as a powerful tool to stabilize coupled network systems with/without communication delay, such as haptic and teleoperation systems, due to its simplicity and effectiveness. However, we found that the conventional TDPA has unnecessary conservatism especially in delayed coupled network systems due to direction dependent energy calculation and dissipation. In this paper, we propose a new time domain passivity approach with reduced conservatism by considering the energy reflection from an energy storage element in the network. The method is generally formulated with a delayed 2-port network system including an energy storage element and implemented to a teleoperation system. The proposed method is experimentally tested and a comparison with the conventional TDPA reveals improved kinesthetic coupling and transparency in terms of position tracking and force reflection.

Preserving the Physical Coupling in Teleoperation despite Time Delay through Observer-Based Gradient Control

In recent past, a variety of control approaches were proposed that serve the stabilization of coupled network systems despite time delay. Especially in teleoperation setups, where the input device displays the interaction forces and torques of the remote coupled robot with its environment to a human operator. This force feedback should represent the physical behavior of a coupling at zero time delay. However, most control approaches aim to limit the force feedback for the sake of stability which does not necessarily lead to a reasonable presentation of the remote side impedances. Recently, Singh et al. (2019) proposed an observer-based gradient controller to eliminate the force jittering on the master side. It rectified the delayed feedback force by removing the undesired increase in force which is generated by the delay in communication channel. The stability of the system was assured via the Time Domain Passivity Approach which leads to an undesired position drift. In this work the observer-based gradient approach is modified to increase it’s force transparency and implemented to another recently developed position-drift free Time Domain Passivity Approach, Panzirsch et al. (2019). The new amalgamated approach is validated and compared to the state-of-the-art in experiments.

Type-2 Fuzzy logic based time-delayed shared control in online-switching tele-operated and autonomous systems

This paper develops a novel shared control scheme for online-switching tele-operated and autonomous system with time-varying delays. Type-2 Takagi–Sugeno (T–S) fuzzy model is used to describe the dynamics of master and slave robots in this system. A novel non-singular fast terminal siding mode (NFTSM)-based algorithm combined with an extended wave-based time domain passivity approach (TDPA) is presented to enhance the master–slave motion synchronization in the tele-operated mode and reference-slave motion synchronization in the autonomous mode, while simultaneously ensuring the stability of the overall system in the presence of arbitrary time delays. In addition, based on the Type-2 Fuzzy model, a new torque observer is designed to estimate the external torques and then the torque tracking method is employed in the control laws to let the slave apply the designated force to further improve the operator’s force perception for the environment. The stability of the closed-loop system is proven using the Lyapunov–Krasovskii functions. Finally, experiments using two haptic devices prove the superiority of the proposed strategy.

Non-Linear Local Force Feedback Control for Haptic Interfaces

Abstract In this paper, a time-continuous, non-linear local force feedback control is proposed which produces low inertia of the haptic interface during free slave motion as well as higher transparency during slave-environment interactions, by modifying the local force gain as a function of the measured forces of the slave-environment interaction. Stability of the system is proven using damping injection based on a model-based passivation approach and a model-free time domain passivity approach. Teleoperation experiments with time-delay on a KUKA light weight robot-based master device show improvements in performance while using the proposed method.

The Input-to-State Stable (ISS) Approach for Stabilizing Haptic Interaction With Virtual Environments

Passivity has been a major criterion for designing a stable haptic interface due to its numerous advantages. However, passivity-based controllers have suffered from the design conservatism of the passivity criterion, particularly when users want to increase the maximum apparent impedance. Based on the input-to-state stable (ISS) criterion and an analogy between haptic interfaces and systems with hysteresis, this paper proposes a control framework that is less conservative than passivity-based controllers. The proposed ISS approach allows a non-predetermined finite amount of output energy to be extracted from the system. Therefore, the proposed method can increase the maximum apparent impedance compared with passivity-based approaches. The focus of this paper is on how the proposed approach is designed to satisfy the input-to-state stability criterion in real time without prior knowledge of the system. This paper also extends the primary single-port ISS approach to a two-port ISS approach for multiple-degree-of-freedom generalization. The experimental and numerical results demonstrate that the proposed ISS approach is able to stabilize a higher impedance range than the time-domain passivity approach. The experimental results also confirm that the proposed approach provides higher actual apparent impedance to the operator compared with the energy-bounding and force-bounding approaches.

Neural Network-Based Passivity Control of Teleoperation System Under Time-Varying Delays.

In this paper, a novel neural network (NN)-based four-channel wave-based time domain passivity approach (TDPA) is proposed for a teleoperation system with time-varying delays. The designed wave-based TDPA aims to robustly guarantee the channels passivity and provide higher transparency than the previous power-based TDPA. The applied NN is used to estimate and eliminate the system's dynamic uncertainties. The system stability with linearity assumption on human and environment has been analyzed using Lyapunov method. The proposed algorithm is validated through experimental work based on a 3-DOF bilateral teleoperation platform in the presence of different time delays.

Time domain passivity control of time-delayed bilateral telerobotics with prescribed performance

A novel approach applying the extended prescribed performance control (PPC) and the wavebased time domain passivity approach (wave-based TDPA) to teleoperation systems is proposed. With the extende ...

Energy Prediction for Teleoperation Systems That Combine the Time Domain Passivity Approach with Perceptual Deadband-Based Haptic Data Reduction.

We study the combination of the perceptual deadband (PD)-based haptic packet rate reduction scheme with the time domain passivity approach (TDPA) for time-delayed teleoperation and propose a novel energy prediction (EP) scheme that deals with the conservative behavior of the resulting controller. The PD approach leads to irregular packet transmission, resulting in degraded system transparency and reduced teleoperation quality when the PD approach is combined with the TDPA. The proposed method (PD+TDPA+EP) adaptively predicts the system energy during communication interruptions and allows for larger energy output. This achieves less conservative control and improves the teleoperation quality. Evaluation of the displayed impedance shows that the PD+TDPA+EP method achieves improved system transparency, both objectively and subjectively, when compared with related approaches from literature. According to a subjective user study, the PD+TDPA+EP method allows for a high packet rate reduction (up to 80 percent) without noticeably distorting the perceived interaction quality. We also show that the PD+TDPA+EP method is preferred over related approaches from literature in a direct comparison test. Thus, with the proposed PD+TDPA+EP method, a high data reduction and a high teleoperation quality are simultaneously achieved for time-delayed teleoperation.

A Time-Domain Approach to Control of Series Elastic Actuators: Adaptive Torque and Passivity-Based Impedance Control

Robots are increasingly designed to physically interact with humans in unstructured environments, and as such must operate both accurately and safely. Leveraging compliant actuation, typically in the form of series elastic actuators (SEAs), can guarantee this required level of safety. To date, a number of frequency-domain techniques have been proposed, which yield effective SEA torque and impedance control; however, these methods are accompanied by undesirable stability constraints. In this paper, we instead focus on a time-domain approach to the control of SEAs, and adapt two existing control techniques for SEA platforms. First, a model reference adaptive controller is developed, which requires no prior knowledge of system parameters and can specify desired closed-loop torque characteristics. Second, the time-domain passivity approach is modified to control desired impedances in a manner that temporarily allows the SEA to passively render impedances greater than the actuator's intrinsic stiffness. This approach also provides conditions for passivity when augmenting any stable SEA torque controller with an arbitrary impedance. The resultant techniques are experimentally validated on a custom prototype SEA.

Wave-Variable-Based Passivity Control of Four-Channel Nonlinear Bilateral Teleoperation System Under Time Delays

An innovative wave-based time-domain passivity approach applied to a four-channel nonlinear teleoperation system is proposed. The primary objective of this approach is to enhance system transparency while maintaining stability in the presence of random time delays. The system stability for different scenarios of human and environment condition is analyzed. The method is validated through experimental work based on a 3-DOF bilateral teleoperation system. The experimental results show that the proposed control algorithm can robustly guarantee the system stability, and simultaneously provide better performance than methods developed in previous work.

Memory-Based Passivation Approach for Stable Haptic Interaction

Inspired from the graphical interpretation of passivity in a position versus force xy graph, this paper proposes a new concept of passivation method to increase the dynamic range of impedance in which a haptic interface can passively interact. The position versus force graph represents the energy behavior in a one-port haptic interface while the haptic interface is traveling in (pressed) or out of (released) a virtual environment (VE). It interprets that to make the one-port system passive, it is sufficient to bound the releasing path below the pressing path in the position versus force graph. To realize the bound, the computed force output from the VE is saved into a designated memory, addressed by current position, while the haptic interface is pressed. Thereafter, the saved force can be reused to upper-bound the releasing path below the saved pressing path while the haptic interface is released. The proposed method is generalized from preliminary work by including interaction with a moving virtual object and multi-DOF extension, together with more systematic and detailed explanations about the methods. In particular, we reconfigure the one-port moving virtual object as a two-port haptic controller and a connected one-port moving inertia. This reconfiguration allows implementation of the preliminary approach including the interaction with moving virtual objects by using an error versus force passivity instead of a position versus force passivity. This two-port extension also allows generalization of the proposed method to multi-DOF interactions by introducing virtual proxy as a virtual moving object. The proposed method is tested with a single and a multi-DOF haptic interface and shows better performance than the recently proposed field-programmable gate array-based time-domain passivity approach .