Passivity-based approaches to bilateral teleoperation sacrifice performance to achieve robust stability against timevarying delays. Typically, force and velocity signals are exchanged in passivity-based bilateral teleoperation resulting in good velocity tracking, but may accrue a position drift. Recently, a power-based time domain passivity approach (TDPA) was proposed to passivate the communication channel in bilateral teleoperation with time-varying delays, which has the potential to be less conservative than other time-invariant passivity-based approaches. Several approaches have been proposed to address the problem of position drift in time-invariant passivity-based approaches to bilateral teleoperation, but the problem of position drift with powerbased TDPA remains unsolved. We propose a feedback passivitycontrol-based scheme to achieve position synchronization in bilateral teleoperation with power-based TDPA. Our proposed method encodes position information with velocity to construct a composite signal, which is transmitted across the communication channel to attain position tracking. The proposed method utilizes time delay power network formulation, enabling extension to positionmeasured force bilateral teleoperation scheme. Simulations and experiments conducted on a custom one degree of freedom teleoperation setup demonstrate robust position tracking performance with our approach under time-varying communication delays and remote environment conditions.
Passivity based approaches to bilateral teleoperation control ensure robust stability against disruptive effects of communication delays. These approaches, while achieving velocity tracking, cannot guarantee position tracking in general. Recently, the Time Domain Passivity Approach (TDPA) has been gaining interest in field of bilateral teleoperation due to its simplicity, ease of implementation, robustness to communication delays, and adaptive control design which promises less conservative performance than frequency domain passivity approaches. Several techniques have been proposed to counter the position drift with conventional passivity based approaches, but not much work has been done to address the problem of position drift with TDPA based control of teleoperation. We propose a novel position drift compensation architecture employing a virtual dependent energy source which leverages the passivity margins allowed by the communication channel to inject energy and recover position tracking without compromising system passivity. A drift compensation scheme is developed within this architecture that ensures synchronization of master and slave robot trajectories. The proposed method is generalizable to all bilateral teleoperation control architectures, and is robust against different communication delay and remote environment conditions. Experiments are conducted to validate the efficacy of the approach, and demonstrate position tracking with up to 1000 ms round-trip delays in free space motion and hard wall contact scenarios.
Abstract-This paper comparatively evaluates the effect of real-time velocity estimation methods on passivity and fidelity of virtual walls implemented using haptic interfaces. Impedance width, or Z-width is a fundamental measure of performance in haptic devices. Limited accuracy of velocity estimates from position encoder data is an impediment in improving the Zwidth in haptic interfaces. We study the efficacy of Levant's differentiator as a velocity estimator, to allow passive implementation of higher stiffness virtual walls as compared to some of the commonly used velocity estimators in the field of haptics. We first experimentally demonstrate feasibility of Levant's differentiator as a velocity estimator for haptics applications by comparing Z-width performance achieved with Levant's differentiator and commonly used Finite Difference Method (FDM) cascaded with a lowpass filter. A novel Z-width plotting technique combining passivity and fidelity of haptic rendering is proposed, and used to compare the haptic device performance obtained with Levant's differentiator, FDM+lowpass filter, First Order Adaptive Windowing and Kalman filter based velocity estimation methods. Simulations and experiments conducted on a custom single degree of freedom haptic device demonstrate that the stiffest virtual walls are rendered with velocity estimated using Levant's differentiator, and highest wall rendering fidelity is achieved by First Order Adaptive Windowing based velocity estimation scheme.
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