Forbidden region virtual fixture for nonlinear teleoperation systems with variable time delays

Teleoperation system performance suffers from the delays in communication lines. These delays are usually variable time delays. The customary wave variable technique used for constant time delays cannot provide satisfactory results to stabilize the system under variable delays. Another problem is observed in the position tracking performance of the limited-workspace teleoperation systems. Although wave variable technique is proven to track position demands successfully under constant time delays given that there is no communication failure, position tracking performance also degrades under variable delays. In this paper, a new controller is proposed to stabilize the system and enhance position tracking performance under variable time delays. This proposed controller is then applied to an identical master-slave teleoperation configuration which qualifies as a limited-workspace teleoperation system.

A forbidden-region virtual fixture (FRVF) is a computer-generated constraint that displays position or force limitations to a robot manipulator or operator, in order to prevent motion into forbidden regions of the workspace. We compare nine FRVFs on each of four common telemanipulator control architectures: position forward, position exchange, position forward/force feedback, and position exchange/force feedback. A one-degree-of-freedom telemanipulation system was used in an experiment designed to simulate users working near a known forbidden region. The metrics of tracking, safety, and submittance were used to analyze the performance of the system with six different users. The results indicate that different FRVF architectures perform best for each of the three metrics. No single FRVF scheme is the best over all metrics, so selection of an FRVF architecture should be an application-dependent weighting of the three metrics. Across all control architectures, the results indicate that a very strong FRVF at the slave device in combination with no FRVF at the master device leads to poor telepresence.

Improved stabilization criteria for Takagi–Sugeno fuzzy systems with variable delays

Signal correlator with programmable variable time delay based on optical coherent transients

In this paper, two novel adaptive finite-time control schemes are proposed for position tracking of nonlinear teleoperation system, which dynamic uncertainties, actuator saturation, and time-varying communication delays are considered. First, a novel auxiliary variable is designed to provide more stable performance. The radial basis function (RBF) neural network is introduced to estimate dynamic uncertainties. Second, two adaptive finite-time control schemes are investigated. In control scheme I, the RBF neural network and the gain switching strategy are applied to compensate the actuator saturation. In control scheme II, an auxiliary compensation filter and the compensation adaptive update laws, which contain the finite-time structure, are developed for dealing with saturation. Third, the finite-time adaptive controller is designed in each of these two control schemes. Based on the multiple Lyapunov function method, the closed-loop teleoperation system with these two control methods is proved to be bounded and finite-time stability. Finally, the simulation experiments are performed and the comparisons with other control methods are shown. The effectiveness of the proposed control schemes is demonstrated.

Bilateral teleoperation systems have been widely applied for human operators to manipulate slave robots in accomplishing various tasks within remote environments. However, most teleoperation systems developed previously have heavily relied on continuous communication or a network with higher sampling rates. In this study, the authors propose novel control schemes to guarantee stability and tracking performance of bilateral teleoperation by considering an event-based transmission. Based on local output signals and the proposed triggering mechanisms, the master and slave robots transmit signals over a communication network only when the triggering conditions are satisfied. An event-triggered coordinating controller is presented to guarantee stability and position tracking for non-linear teleoperators. Input-to-state stability of the proposed teleoperation system with bounded external forces is studied with guaranteed static force reflection. Subsequently, a transformation-based framework is addressed with the event-triggered coordinating control to cope with bilateral teleoperation systems under time delays. The Zeno behaviour is demonstrated to be excluded in the proposed event-triggered teleoperation systems. Numerical examples and experiments are presented to discuss the efficacy of the proposed event-triggered teleoperation systems.

In this paper, a new robust adaptive nonlinear teleoperation system using an improved extended active observer (IEAOB), adaptive Smith predictor (ASP) and sliding mode control is developed to address the time delay in the communication channels and the nonlinear robot model uncertainties. Firstly, an ASP based on Padé approximation and active observer is designed to compensate for the time delay effect. Specifically, the total network time delay is modelled by Padé approximation, and then an active observer is deployed to estimate the time delay. To ensure the time-varying delay effect is completely suppressed, a sliding mode control algorithm is further developed. The main added value of this teleoperation approach is that it requires neither specific mathematic delay-time model, nor strict assumptions on time delay from a practical point of view. Finally, the stability of the designed teleoperation system is theoretically studied and the system effectiveness is demonstrated by applying it to a pair of Phantom Omni haptic devices connected via a communication channel with time-varying delays.

This paper presents the design of a state convergence (SC)-based bilateral controller for a nonlinear teleoperation system, which has been approximated by a Takagi–Sugeno (TS) fuzzy model. The selection of SC is made due to the advantages offered by this scheme both in the modeling and control design stages. The modeling stage considers master/slave systems, which can be represented by the $n$ th order differential equations, while the control design stage offers an easy way to determine the control gains required for assigning desired closed loop dynamics to teleoperation system. After the master/slave systems are represented by TS fuzzy models, a stabilizing fuzzy law is adopted which allows deploying the SC scheme with all its benefits to design the fuzzy bilateral controller. In this way, not only the simplicity of the design scheme is ensured but also the existing SC scheme is able to control a nonlinear teleoperation system based on its TS fuzzy model description. As an additional advantage, the SC-based existing linear bilateral controller can be easily derived from the SC-based proposed fuzzy bilateral controller. Various cases of master/slave systems originally reported in terms of their linear model representation and communication in the absence/presence of time delay are all discussed in the corresponding fuzzy framework. The MATLAB simulations, considering a one-degree-of-freedom teleoperation system, are performed to validate the proposed methodology for controlling a nonlinear teleoperation system.

Dead-zone is one of the most common hard nonlinearities ubiquitous in master–slave teleoperation systems, particularly in the slave robot joints. However, adaptive control techniques applied in teleoperation systems usually deal with dynamic uncertainty but ignore the presence of dead-zone. Dead-zone has the potential to remarkably deteriorate the transparency of a teleoperation system in the sense of position and force tracking performance or even destabilizing the system if not compensated for in the control scheme. In this paper, an adaptive bilateral control scheme is proposed for nonlinear teleoperation systems in the presence of both uncertain dynamics and dead-zone. An adaptive controller is designed for the master robot with dynamic uncertainties and the other is developed for the slave robot with both dynamic uncertainties and unknown dead-zone. The two controllers are incorporated into the four-channel bilateral teleoperation control framework to achieve transparency. The transparency and stability of the closed-loop teleoperation system is studied via a Lyapunov function analysis. Comparisons with the conventional adaptive control which merely deal with dynamic uncertainties in the simulations demonstrate the validity of the proposed approach.

In this paper, a class of single‐master‐multiple‐slaves (SMMS) bilateral teleoperation systems with the limitation of communication bandwidth is investigated. The signals are exchanged between the master and the slaves through delayed communication. Besides, to avoid data collisions, the Try‐Once‐Discard (TOD) scheduling protocol is utilized to decide which slave gets the network access at each triggered instant. A control algorithm with the event‐triggered mechanism is proposed to ensure the master–slaves synchronization. Furthermore, the closed‐loop system's stability criterion associated with the controller gains, the varying time delays' bounds, and the maximum transmission interval is established on the basis of the Lyapunov–Krasovskii analysis. Finally, the presented algorithm's performance is demonstrated by the simulation and experiment results.

In this paper, a new adaptive control design method for nonlinear telerobotic systems with varying asymmetric time delays is presented. Using the proposed controller, it is possible to synchronize the state behavior of the local and the remote robots. While prior art on adaptive teleoperation has addressed stability in such systems only for constant delays, we guarantee asymptotic stability in the presence of delays that may be time-varying and/or unequal in the forward and backward directions. Using the proposed controller, asymptotic stability of the bilateral teleoperation system subject to any bounded varying delay with a bounded rate of variation can be guaranteed. The proposed controller also has the ability to cope with parameter variations in the dynamics of the local and the remote robots. To study the transparency of the closed-loop teleoperation system, we prove that the position and velocity errors between the local and the remote manipulators converge to zero asymptotically. To show the efficiency of the proposed controller, simulation results on a pair of two-degree-of-freedom manipulators with varying time delays in the communication channel are presented.

A trigonometric approximation scheme for optimal periodic control problems with state and control dependent delays is considered. The formulation of approximate problems is here adapted to a specific form of state equation with variable time delays. Sufficient conditions for the sequence of nearly optimal solutions of approximate problems to be a generalized minimizing sequence for the basic problem are given. The application of the method proposed to a periodic control of chemical reactors with recycles and controlled piping is pointed out.

In this paper, adaptive control is proposed for hybrid position/force synchronization of master-slave teleoperation systems with time-varying delays in communication channels. Different from previous works on bilateral teleoperation systems, we investigate position/force control of bilateral teleoperation systems subject to time-varying delays and dynamical uncertainties. Using partial feedback linearization, the dynamics of both master and slave are transformed into two subsystems: local master/slave position/force control with unmodeled dynamics and delayed position/force synchronization. An adaptive control is proposed to deal with the dynamical uncertainties and robust against time delays, which guarantees the position/force synchronization trajectories converge to the desired manifolds with prescribed performance. The stability of the closed-loop system and the boundedness of synchronization tracking errors are proved using linear matrix inequalities based on Lyapunov stability synthesis. The position/force tracking error up to an ultimately bounded error is achieved. The proposed adaptive controls are robust against relative motion disturbances, parametric uncertainties, and time delays, and are validated by experimental studies.

Many robotics researchers have focused on stability in free and constrained motions, but only a few of them have considered system stability in the transition between these phases. For a teleoperation system, due to delay effects, studying this concept is even more crucial. Recently, passivity-based controller stability has been studied in free motion and constrained mode of a teleoperation system in the presence of constant time delay. However, the controller does not guarantee arbitrary switching between these modes. In this paper, a hybrid system theorem was employed to treat delayed teleoperative tasks that involve phase transition or switching. Applying this theorem, the stability of passivity-based architecture in transition was studied. It was proved that this type of controller is stable even with arbitrary switching. Experiments on a two-degrees-of-freedom nonlinear teleoperation system were conducted to verify the approach. Experimental results have shown that passivity-based architecture, as expected in controller design, is well suited for delayed teleoperation in transition. Moreover, even without force sensing, the system’s force tracking in dealing with a hard environment is admissible.

In this paper, a novel control scheme is proposed to guarantee global asymptotic stability of bilateral teleoperation systems that are subject to time-varying time delays in their communication channel and “sandwich linearity” in their actuators. This extends prior art concerning control of nonlinear bilateral teleoperation systems under time-varying time delays to the case where the local and the remote robots' control signals pass through saturation or similar nonlinearities that belong to a class of systems we name sandwich linear systems. We call the proposed method nonlinear proportional plus damping (nP+ D). The asymptotic stability of the closed-loop system is established using a Lyapunov-Krasovskii functional under conditions on the controller parameters, the actuator saturation characteristics, and the maximum values of the time-varying time delays. The controller is experimentally validated on a pair of 3-DOF PHANToM Premium 1.5A robots, which have limited actuation capacity, that form a teleoperation system with a varying-delay communication channel.