Keyvan Hashtrudi-Zaad

Transparency in time-delayed systems and the effect of local force feedback for transparent teleoperation

This paper first investigates the issue of transparency in time-delayed teleoperation. It then studies the advantages of employing local force feedback for enhanced stability and performance. In addition, two classes of three-channel control architectures, that are perfectly transparent under ideal conditions are introduced. The stability robustness of the proposed architectures to delays is rigorously analyzed, leading to certain bounds on force feedforward control parameters. Experimental results are included in support of the theoretical work.

Analysis of Control Architectures for Teleoperation Systems with Impedance/Admittance Master and Slave Manipulators

A large number of bilateral teleoperation control architectures in the literature have been designed based on assumed impedance models of the master and slave manipulators. However, hydraulic or heavily geared and many other manipulators cannot be properly described by impedance models. In this paper, a common four-channel bilateral control architecture designed for the above impedance models is extended to teleoperation systems with master and slave manipulators of either the admittance or impedance type. Furthermore, control parameters that provide perfect transparency under ideal conditions are found for each type of teleoperation system. Because in practice such parameters may not lead to systems that are robust to time delays and model uncertainties, an analysis of the stability and performance robustness of this very general architecture and two-channel architectures is also presented. The analysis uses the passivity-based Llewellyn two-port network absolute stability criterion, as well as bounds on the minimum and range of values of the impedance transmitted to the operator. The results of these evaluations provide design guidelines on choosing a particular control architecture and its parameters given different master and slave manipulator structures.

Bilateral parallel force/position teleoperation control

The extension of parallel force/position control to teleoperation systems is considered in this article. In the proposed four-channel bilateral controller, higher priority is granted to position control at the master side and to force control at the slave side. The primary goal of this control architecture is the enhancement of force and position tracking performance in the presence of uncertainties in the system and environment. The stability and performance of the proposed controller is investigated by analyzing the three decoupled single-degree-of-freedom systems obtained from decoupling and projecting the closed-loop system dynamics onto the slave task-space orthogonal directions. Experimental results demonstrate significant improvement in transparency.

Transparent Bilateral Teleoperation under Position and Rate Control

A four-channel control architecture has been suggested in the literature to achieve transparency for master-slave teleoperator systems under position control. In this paper, the result is generalized to include teleoperator systems that are under rate control or more general master-slave kinematic correspondence laws, such as a mixedposition/rate mode. A one-degree-of-freedom example is given to outline the design and analysis of such a system for transparency and stability. The validity of the theory has been verified through simulations and experiments.

Impedance control of a teleoperated excavator

Earth-moving machines such as hydraulic excavators are usually used for carrying out contact tasks. Impedance control can be employed as an approach for achieving compliant motion in such tasks. This paper describes a position-based impedance controller that has been developed in our laboratory for excavator-type manipulators, and presents the supporting experimental results. First, the problem of impedance control for a single hydraulic cylinder is addressed and a method is presented to analyze the system stability. The steady-state position and force tracking accuracy of the closed-loop system is also studied. Next, the problem of impedance control for a multi-link hydraulic excavator is addressed and the arm Jacobian and accurate estimates of the arm inertial terms are employed to map the desired impedance of the end-effector (bucket of the excavator) onto the hydraulic cylinders. Various contact experiments carried out using an instrumented mini-excavator demonstrate that the proposed impedance controller has very good performance for both single-link and multilink cases.

Bounded-Impedance Absolute Stability of Bilateral Teleoperation Control Systems

Available passivity-based robust stability methods for bilateral teleoperation control systems are generally conservative, as they consider an unbounded range of dynamics for the class of passive operators and environments in the complex plane. In this paper, we introduce a powerful 3D geometrical robust stability analysis method based on the notions of wave variables and scattering parameters. The methodology, which was originally a 2D graphical method used in microwave systems for single-frequency analysis [1], is further developed in this paper for teleoperation and haptic systems. The proposed method provides both mathematical and visual aids to determine bounds or regions on the complex frequency response of the passive environment impedance parameters for which a potentially unstable system connected to any passive operator is stable, and vice-versa. Furthermore, the method allows for the design of bilateral controllers when such bounds are known, or can even be utilized when the environment dynamics are active. The geometrical test can also be replaced by an equivalent mathematical condition, which can easily be checked via a new stability parameter. The proposed method results in less conservative guaranteed stability conditions compared to the Llewellyns criterion; thus, promising a better compromise between stability and performance. The new method is numerically evaluated for two bilateral control architectures.

Analysis and evaluation of stability and performance robustness for teleoperation control architectures

Teleoperation systems are subject to operator and environment dynamic uncertainties as well as communication-channel delays. For the first time in the context of teleoperation, the passivity-based Llewellyns two-port network absolute stability criterion as well as the minima and the dynamic ranges (Z-widths) of the operator and environment transmitted impedances are employed to analyze stability and performance robustness of two and four channels bilateral control architectures. The results of these evaluations and the above analysis tools provide a framework for robust bilateral controller design.

Smith Predictor Type Control Architectures for Time Delayed Teleoperation

An early control methodology for time delayed plants is the Smith predictor, in which the plant model is utilized to predict the non-delayed output of the plant and move the delay out of the control loop. Recent Smith predictor based teleoperation control architectures have used linear or fixed-parameter dynamic approximations of the slave/environment at the master for environment contact prediction. This paper discusses and analyzes the performance of the previous work and proposes new architectures to overcome their shortcomings. The proposed architectures consist of a novel pseudo two-channel nonlinear predictive controller and its variations that use neural networks for online estimation of the slave and environment dynamics to replicate the environment contact force at the master using a similar local network. Intermittent contact experiments are conducted on a teleoperation test-bed consisting of two Planar Twin-Pantograph haptic devices. The experimental results with half a second delay demonstrate significant improvement in stability and performance by the proposed neural network based predictive control architectures over traditional force-position and linear Smith predictor based control architectures.

Dual-User Teleoperation Systems: New Multilateral Shared Control Architecture and Kinesthetic Performance Measures

This paper proposes a novel six-channel multilateral shared control architecture for dual-user teleoperation systems. The proposed controller allows interaction between two users as well as the slave and environment through a dominance factor. The dominance factor adjusts the authority of the users over the slave robot and environment. The proposed controller is implemented on a haptic simulation test bed consisting of two planar twin pantograph haptic devices and a simulated pantograph as the slave robot. To analyze the kinesthetic performance of the proposed multilateral shared controller, a number of performance measures are extended or proposed. These measures are evaluated analytically and experimentally for various types of environments, users’ grasps, and levels of dominance of the users over the task.

Estimation of Elbow-Induced Wrist Force With EMG Signals Using Fast Orthogonal Search

In many studies and applications that include direct human involvement-such as human-robot interaction, control of prosthetic arms, and human factor studies-hand force is needed for monitoring or control purposes. The use of inexpensive and easily portable active electromyogram (EMG) electrodes and position sensors would be advantageous in these applications compared to the use of force sensors, which are often very expensive and require bulky frames. Multilayer perceptron artificial neural networks (MLPANN) have been used commonly in the literature to model the relationship between surface EMG signals and muscle or limb forces for different anatomies. This paper investigates the use of fast orthogonal search (FOS), a time-domain method for rapid nonlinear system identification, for elbow-induced wrist force estimation. It further compares the forces estimated using FOS with the forces estimated by MLPANN for the same human anatomy under an ensemble of operational conditions. In this paper, the EMG signal readings from upper arm muscles involved in elbow joint movement and sensed elbow angular position and velocity are utilized as inputs. A single degree-of-freedom robotic experimental testbed has been constructed and used for data collection, training and validation