Impedance control with on-line neural-network compensator for robot contact tasks

In this paper, a force-tracking impedance controller with an on-line neural-network compensator is shown to be able to track a reference force in the presence of unknown environmental dynamics. The controller can be partitioned into three parts. The computed torque method is used to linearize and decouple the dynamics of a manipulator. An impedance controller is then added to regulate the mechanical impedance between the manipulator and its environment. In order to track a reference force signal, an on-line neural network is used to compensate the effect of unknown parameters of the manipulator and environment.     

Adaptive control of robot contact tasks with on-line learning of planar surfaces

In robot constrained motion problems on planar surfaces with frictional contacts, uncertainties on the contacted surface not only affect the control system performance but also distort control targets. The surface normal direction cosines are in this case uncertain parameters that are involved in both the control law and the control targets. This work proposes an adaptive learning controller that uses force and joint position/velocity measurements to simultaneously learn the surface orientation and achieve the desired goal. Simulation examples for a 6 dof robot are used to illustrate the theoretical results and the performance of the proposed controller in practical cases.     

Dynamic modeling of an anthropomimetic robot in contact tasks

Considerations of energy efficiency and safe human-robot interaction have led to an increase in the exploitation of compliance in robotics, and much of this work has been inspired by biological systems. As a consequence, new analytical tools are now required in order to enable the dynamic analysis of these novel compliant robots, especially in their interactions with the environment. This paper extends the ‘Flier’ approach to show how it could be applied to the dynamic analysis of contact tasks involving a highly compliant biologically inspired robot – in this case an anthropomimetic robot, a humanoid with a human-like skeleton and artificial muscles, in which the joints are actuated by DC motors acting via compliant tendon transmissions. First, a computer-based model of the robot’s dynamics is developed. Various constraints are then introduced to describe the contacts (including impacts) with the ground, and with objects in the environment. Simulation results are presented for two types of interactions with the external world: a grasping task and the case of the robot moving on a mobile base.     

Decision-making problems in industrial control tasks

This paper considers the design of industrial control tasks with regard to the human operator's decision-making skills and limitations. Four control tasks are described to provide illustrative material for a number of typical problems. The problems discussed are the need for memory and planning aids, the choice between on-line and off-line control, workload peaking, conflicting objectives, and the introduction of computer aids. Practical approaches towards minimising the difficulties and improving the performance of the human operator are discussed.     

Reasoning about nature of contact for planning manipulators tasks

Extracting information about contact between two convex bodies from the measured force vector is a prerequisite for any fine compliant motion control strategy. Contact information contains the direction and orientation of the contact surface normal and its relative location and orientation with respect to the compliant reference frame system.A method for interpreting the contact force feedback during compliant robot motion control, using kinematic screws, is presented. Domain specific rules combined with partial a priori knowledge of mating parts geometry and interpreted force signals are used to reason and make inferences about the initial contact configuration. The likely contact surfaces are predicted and point(s) or line(s) of contact are fully defined. These surfaces are idealized and represented by quadratic equations or polyhedral surfaces. The geometric properties of surfaces at the contact location are used to select the contact configuration when multiple solutions exist.An algorithm for predicting the Expected Contact Configuration (ECC) has been developed and is illustrated here with examples. Experimental validation of the developed expert system prototype, using a 6R manipulator, a six-axis force sensor, and a host computer is described.     

Estimation of the risk for an unstable behaviour of feedback systems in the presence of nonlinear distortions

A criterion to verify experimentally the stability of a nonlinear system, captured in a feedback loop, is proposed. The basic idea is to split the output power in coherent (linearly related to the input) and noncoherent power (the remaining power). A nonlinear power gain, measuring the sensitivity of the noncoherent output power to input variations, is introduced. Using the small gain theorem, it is possible to check the local stability of the feedback for the actual class of excitation signals: the risk is estimated that the feedback system would become unstable for another realization of the experiment using extreme value statistics.     

Experiments in contact control

This article describes the implementation, experimentation, and application of contact control schemes for a 7-DOF Robotics Research arm. The contact forces and torques are measured in the sensor frame by the 6-axis force/torque sensor mounted at the wrist, are compensated for gravity, and then are transformed to the tool frame in which the contact task is defined and executed. The contact control schemes are implemented on the existing robot Cartesian position control system at 400Hz, do not require force rate information, and are extremely simple and computationally fast. Three types of contact control schemes are presented: compliance control, force control, and dual-mode control. In the compliance control scheme, the contact force is fed back through a lag-plus-feedforward compliance controller so that the end-effector behaves like a spring with adjustable stiffness; thus the contact force can be controlled by the reference position command. In the force control scheme, a force setpoint is used as the command input and a proportional-plus-integral force controller is employed to ensure that the contact force tracks the force setpoint accurately. In the dual-mode control scheme, the end-effector approaches and impacts the reaction surface in compliance mode, and the control scheme is then switched automatically to force mode after the initial contact has been established. Experimental results are presented to demonstrate contact with hard and soft surfaces under the three proposed control schemes. The article is concluded with the application of the proposed schemes to perform a contact-based eddy-current inspection task. In this task, the robot first approaches the inspection surface in compliance control until it feels that it has touched the surface, and then automatically levels the end-effector on the surface. The robot control system then transitions to force control and applies the desired force on the surface while executing a scanning motion. At the completion of the inspection task, the robot first relaxes the applied force and then retracts from the surface. © 1996 John Wiley & Sons, Inc.     

Influence of social presence on eye movements in visual search tasks

Abstract

This study employed an eye-tracking technique to investigate the influence of social presence on eye movements in visual search tasks. A total of 20 male subjects performed visual search tasks in a 2 (target presence: present vs. absent) × 2 (task complexity: complex vs. simple) × 2 (social presence: alone vs. a human audience) within-subject experiment. Results indicated that the presence of an audience could evoke a social facilitation effect on response time in visual search tasks. Compared with working alone, the participants made fewer and shorter fixations, larger saccades and shorter scan path in simple search tasks and more and longer fixations, smaller saccades and longer scan path in complex search tasks when working with an audience. The saccade velocity and pupil diameter in the audience-present condition were larger than those in the working-alone condition. No significant change in target fixation number was observed between two social presence conditions.

Practitioner Summary: This study employed an eye-tracking technique to examine the influence of social presence on eye movements in visual search tasks. Results clarified the variation mechanism and characteristics of oculomotor scanning induced by social presence in visual search.     

Feedback fuzzy-DVS scheduling of control tasks

To consider the energy-aware scheduling problem in computer-controlled systems is necessary to improve the control performance, to use the limited computing resource sufficiently, and to reduce the energy consumption to extend the lifetime of the whole system. In this paper, the scheduling problem of multiple control tasks is discussed based on an adjustable voltage processor. A feedback fuzzy-DVS (dynamic voltage scaling) scheduling architecture is presented by applying technologies of the feedback control and the fuzzy DVS. The simulation results show that, by using the actual utilization as the feedback information to adjust the supply voltage of processor dynamically, the high CPU utilization can be implemented under the precondition of guaranteeing the control performance, whilst the low energy consumption can be achieved as well. The proposed method can be applied to the design in computer-controlled systems based on an adjustable voltage processor.     

On iterative learning from different tracking tasks in the presence of time-varying uncertainties.

In this paper, we introduce a new iterative learning control (ILC) method, which enables learning from different tracking control tasks. The proposed method overcomes the imitation of traditional ILC in that, the target trajectories of any two consecutive iterations can be completely different. For non-linear systems with time-varying and time-invariant parametric uncertainties, the new learning method works effectively to nullify the tracking error. To facilitate the learning control system design and analysis, in the paper we use a composite energy function (CEF) index, which consists of a positive scalar function and L2 norm of the function approximation error.     

Robot motion control in zero-gravity conditions

We have considered the motion control of a space robot composed of a body and a telescopic manipulator arm. The robot is in the state of free passive flight. The vector of the number of movements and the kinetic moment of the robot relative to the center of mass are zero. The manipulator arm motion causes a corresponding motion of the robot body (change in the position of the center of mass of the body and its rotation). Unlike earlier results, we have revealed that the robot grip can be shifted from an arbitrary initial position to an arbitrary final position inside the operating area and, in addition, the required (most convenient for operations) angle between the manipulator arm and the robot body in the final position can be obtained.     

Supporting the sense of presence in control environments

This paper proposes a notion of presence which overcomes its strict interpretation in terms of physical projection and perception, and interprets the sense of being there as the understanding of the meaning of what is going on there. The domain of control environments is considered to illustrate this point and to propose technological solutions combining principles and techniques taken from Artificial Intelligence and Computer Supported Cooperative Work to enhance the interpretation and cooperation capabilities of the involved actors.     

Adaptive control in the presence of input constraints

This paper deals with the problem of adaptively controlling a linear time-invariant plant in the presence of constraints on the input amplitude. We introduce a new adaptive algorithm which ensures that the plant can be locally stabilized. In addition to the standard assumptions which are required for adaptive control in the ideal case, an upper bound on the plant control parameter is required to be known. The results are evaluated by simulation studies     

Flexible control in expert systems for construction tasks

In most expert systems for constructional tasks, the knowledge base consists of a set of facts or object definitions and a set of rules. These rules contain knowledge about correct or ideal solutions as well as knowledge on how to control the construction process. In this paper, we present an approach that avoids this type of rules and thus the disadvantages caused by them.We propose a static knowledge base consisting of a set of object definitions interconnected by is-a and part-of links. This conceptual hierarchy declaratively defines a taxonomy of domain objects and the aggregation of components to composite objects. Thus, the conceptual hierarchy describes the set of all admissible solutions to a constructional problem. Interdependencies between objects are represented by constraints. A solution is a syntactically complete and correct instantiation of the conceptual hierarchy.No control knowledge is included in the conceptual hierarchy. Instead, the control mechanism will use the conceptual hierarchy as a guideline. Thus it is possible to determine in which respects a current partial solution is incomplete simply by syntactical comparison with the conceptual hierarchy. The control architecture proposed here has the following characteristics: separation of control and object knowledge, declarative representation of control knowledge, and explicit control decisions in the problem solving process. Thus, a flexible control mechanism can be realized that supports interactive construction, integration of case-based approaches and simulation methods.This control method is part of an expert system kernel for planning and configuration tasks in technical domains. This kernel has been developed at the University of Hamburg and is currently applied to several domains.     

Basic tasks for knowledge-based supervision in process control

A new tasks taxonomy for knowledge-based global supervision (GS) of continuous industrial processes is introduced in this work. Possible required tasks are specified together with the analysis of their dimensions, which should be useful in the selection of the final capabilities of supervision. Moreover, these dimensions would help end-users and designers when comparing different systems. Several methodologies based on concepts such as generic task, generic operation or heuristic classification have been proposed to transform knowledge-based system (KBS) development in a systematic knowledge engineering activity. These approaches have been quite successful in domains such as medicine or mineral prospecting, identifying a large number of tasks that experts in the domain articulate to solve the problem. However, this was not the case in the process control area. The selection of tasks and their capabilities is the first step to be taken, even before choosing a KBS analysis and design methodology. Authors found a lack of facilities to do this selection in the aforementioned approaches when they tried to develop a global supervision tool in a beet sugar factory in Spain. Hence, this article describes an attempt to fill this gap. Moreover, it shows how this taxonomy supported the analysis and design stages of a supervision tool in the mentioned industrial application.     

实时控制任务的抖动研究

在实时控制系统中,由于并发和资源共享等诸多不确定的因素,导致周期任务每次被调度运行的时间有不确定性的变化,系统会变得不稳定。为了保证实时控制系统有较高的性能和可预测性,减小抖动是至关重要的。通过引入截止期缩减因子,对实时系统中抖动敏感的任务确定一个新的相对截止期,能够更好地挖掘出可用的空闲时间,从而使得系统的稳定性提高。实验结果证明提出的方法比文献中用到的其他方法更加有效和可用。     

The affect of contact force sensations on user performance in virtual assembly tasks

The development of a realistic virtual assembly environment is challenging because of the complexity of the physical processes and the limitation of available VR technology. Many research activities in this domain primarily focused on particular aspects of the assembly task such as the feasibility of assembly operations in terms of interference between the manipulated parts. The virtual assembly environment reported in this research is focused on mechanical part assembly. The approach presented addresses the problem of part-to-part contacts during the mating phase of assembly tasks. The system described calculates contact force sensations by making their intensity dependent on the depth of penetration. However the penetration is not visible to the user who sees a separate model, which does not intersect the mating part model. The two 3D models of the part, the off-screen rendered model and the on-screen rendered model are connected by a spring-dumper arrangement. The force calculated is felt by the operator through the haptic interface when parts come in contact during the mating phase of the assembly task. An evaluation study investigating the effect of contact force sensation on user performance during part-to-part interface was conducted. The results showed statistically significant effect of contact force sensation on user performance in terms of task completion time. The subjective evaluation based on feedback from users confirmed that contact force sensation is a useful cue for the operator to find the relative positions of components in the final assembly state.     

On the precision of third person perspective augmented reality for target designation tasks

The availability of powerful consumer-level smart devices and off-the-shelf software frameworks has tremendously popularized augmented reality (AR) applications. However, since the built-in cameras typically have rather limited field of view, it is usually preferable to position AR tools built upon these devices at a distance when large objects need to be tracked for augmentation. This arrangement makes it difficult or even impossible to physically interact with the augmented object. One solution is to adopt third person perspective (TPP) with which the smart device shows in real time the object to be interacted with, the AR information and the user herself, all captured by a remote camera. Through mental transformation between the user-centric coordinate space and the coordinate system of the remote camera, the user can directly interact with objects in the real world. To evaluate user performance under this cognitively demanding situation, we developed such an experimental TPP AR system and conducted experiments which required subjects to make markings on a whiteboard according to virtual marks displayed by the AR system. The same markings were also made manually with a ruler. We measured the precision of the markings as well as the time to accomplish the task. Our results show that although the AR approach was on average around half a centimeter less precise than the manual measurement, it was approximately three times as fast as the manual counterpart. Additionally, we also found that subjects could quickly adapt to the mental transformation between the two coordinate systems.