Amir Haddadi

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.

Gestures for industry: intuitive human-robot communication from human observation

Human-robot collaborative work has the potential to advance quality, efficiency and safety in manufacturing. In this paper we present a gestural communication lexicon for human-robot collaboration in industrial assembly tasks and establish methodology for producing such a lexicon. Our user experiments are grounded in a study of industry needs, providing potential real-world applicability to our results. Actions required for industrial assembly tasks are abstracted into three classes: part acquisition, part manipulation, and part operations. We analyzed the communication between human pairs performing these subtasks and derived a set of communication terms and gestures. We found that participant-provided gestures are intuitive and well suited to robotic implementation, but that interpretation is highly dependent on task context. We then implemented these gestures on a robot arm in a human-robot interaction context, and found the gestures to be easily interpreted by observers. We found that observation of human-human interaction can be effective in determining what should be communicated in a given human-robot task, how communication gestures should be executed, and priorities for robotic system implementation based on frequency of use.

Real-Time Identification of Hunt–Crossley Dynamic Models of Contact Environments

Real-time estimates of environment dynamics play an important role in the design of controllers for stable interaction between robotic manipulators and unknown environments. The Hunt-Crossley (HC) dynamic contact model has been shown to be more consistent with the physics of contact, compared with the classical linear models, such as Kelvin-Voigt (KV). This paper experimentally evaluates the authors previously proposed single-stage identification method for real-time parameter estimation of HC nonlinear dynamic models. Experiments are performed on various dynamically distinct objects, including an elastic rubber ball, a piece of sponge, a polyvinyl chloride (PVC) phantom, and a PVC phantom with a hard inclusion. A set of mild conditions for guaranteed unbiased estimation of the proposed method is discussed and experimentally evaluated. Furthermore, this paper rigorously evaluates the performance of the proposed single-stage method and compares it with those of a double-stage method for the HC model and a recursive least squares method for the KV model and its variations in terms of convergence rate, the sensitivity to parameter initialization, and the sensitivity to the changes in environment dynamic properties.

A new method for online parameter estimation of Hunt-Crossley environment dynamic models

Online estimates of unknown environment dynamics are used for the control of robotic contact tasks. The Hunt-Crossley nonlinear dynamic model of environments has been shown to be more consistent with the physics of contact, compared to the classical linear models, such as Kelvin-Voigt. This paper proposes a new method for online parameter estimation of Hunt-Crossley model and provides a mild set of conditions for guaranteed unbiased estimation. The rate and the sensitivity of convergence to parameter initialization and system parameter changes are numerically evaluated and compared for both the proposed method and an existing 2-stage identification method.

Online contact impedance identification for robotic systems

In this paper, we study the performance of various algorithms for fast online identification of environment impedance during robotic contact tasks. In particular, we evaluate and compare algorithms with regard to their convergence rate, computational complexity and sensitivity to noise for different environments using a single degree-of-freedom experimental setup. The results provide some guidelines for choosing an appropriate identification algorithm for a specific application.

Development of a dynamic model for bevel-tip flexible needle insertion into soft tissues

In this paper, we develop a mechanics-based dynamic model for bevel-tip flexible needle insertion into soft tissues. We use Newton-Euler formulation to account for the effect of actuation, friction, tissue interactions, and bevel-tip forces on the needle. The soft tissue deformation is modeled by finite element analysis, whereas the mechanics-based model is used to predict needle deflections due to bevel-tip asymmetry. The proposed needle-tissue model is then experimentally evaluated by comparing the needle deflections for various insertion depths in a tissue phantom with those achieved from simulations.

A new robust stability analysis and design tool for bilateral teleoperation control systems

In this paper, a powerful robust stability analysis technique is introduced and developed for teleoperation systems. The methodology is based on wave parameters and discusses absolute stability and potential instability using scattering and is originally used in microwave systems [1], The proposed method provides suitable mathematical and visual aids to determine bounds or regions of passive environment impedances for which a potentially unstable system connected to any passive operator is stable, and vice-versa. Furthermore, a novel stability parameter is proposed to maximize the derivation of the above bounds or regions. This results in less conservative guaranteed stability conditions compared to the Llewellyns criterion; thus, achieving a better compromise between stability and performance. The proposed methodology allows for the design of bilateral control systems when such bounds are known or even when the operator or environment dynamics are active. The new robust stability analysis and Llewellyns criterion are numerically evaluated and compared with each other on two common teleoperation control architectures.

Identifying nonverbal cues for automated human-robot turn-taking

Nonverbal communication cues play an important role in human-human interaction and are expected to take a similar role in human-robot collaboration. In current industrial practice, human-robot turn-taking is explicitly human controlled, via a command channel such as switch or button. However, such a master-slave approach does not permit collaborative interaction, and requires the human to focus on both controlling the robots behavior and on the task, thereby affecting overall performance. In this paper, implicit, nonverbal communication cues are examined as a non-explicit communication channel during a turn-taking task context. The aim of this study is to characterize the types and frequencies of nonverbal cues important to regulating turn taking during an assembly-task-type collaboration. This analysis will guide the selection of cues that can be expressed by the robot as implicit user inputs while human and robot complete a shared task.

Analysis of task-based gestures in human-robot interaction

New developments, innovations, and advancements in robotic technology are paving the way for intelligent robots to enable, support, and enhance the capabilities of human workers in manufacturing environments. We envision future industrial robot assistants that support workers in their tasks, advancing manufacturing quality and processes and increasing productivity. However, this requires new channels of fine-grained, fast and reliable communication. In this research we examined the communication required for human-robot collaboration in a vehicle door assembly scenario. We identified potential communicative gestures applicable to this scenario, implemented these gestures on a Barrett WAMTM1 manipulator, and evaluated them in terms of human recognition rate and response time in a real-time interaction. Response time analysis reveals insights into the communicative structure of robot motions; namely, key short gesture segments include the bulk of the communicative information. These results will help us design more efficient and fluid task flow in human-robot interaction scenarios.

Robust Stability of Teleoperation Systems with Time Delay: A New Approach

In this paper, we propose an approach to the control of linear teleoperation systems under time delays. Unlike traditional delay-robust control systems that guarantee passive communication channel through the transmission of wave variables, the new approach uses the concept of absolute stability for the physically expressive Lawrences four-channel structure for transmitting the standard power variables, i.e., force and position. By incorporating kinesthetic performance requirements, we derive an absolutely stable four-channel controller that is transparent when time delay is negligible. Experimentally, the study evaluates and compares the performance of the proposed controller with that of a benchmark wave variable-based controller. The results indicate contact stability for large delays, a lack of position drift, and improved position and force tracking in both the free motion and rigid contact regimes for small delays.