Veljko Potkonjak

ZMP: A REVIEW OF SOME BASIC MISUNDERSTANDINGS

One of basic characteristics of the regular bipedal walk of humanoid robots is the maintenance of their dynamic balance during the walk, whereby a decisive role is played by the unpowered degrees of freedom arising at the foot–ground contact. Hence, the role of the Zero-Moment Point (ZMP) as an indicator of dynamic balance is indispensable. This paper gives a detailed discussion of some basic theoretical assumptions related to the ZMP in the light of imprecise, and even incorrect, interpretations that have recently appeared, and which have led to some erroneous conclusions. Examples are given to show some erroneous basic attitudes and the genesis of some of them is indicated. It is also pointed out that in the domain of bipedal walk there are still notions that are not clearly defined and their meanings differentiated in some related branches of science and engineering. One of the examples is dynamic balance and stability, which are often used interchangeably.

Virtual laboratories for education in science, technology, and engineering

Within education, concepts such as distance learning, and open universities, are now becoming more widely used for teaching and learning. However, due to the nature of the subject domain, the teaching of Science, Technology, and Engineering are still relatively behind when using new technological approaches (particularly for online distance learning). The reason for this discrepancy lies in the fact that these fields often require laboratory exercises to provide effective skill acquisition and hands-on experience. Often it is difficult to make these laboratories accessible for online access. Either the real lab needs to be enabled for remote access or it needs to be replicated as a fully software-based virtual lab. We argue for the latter concept since it offers some advantages over remotely controlled real labs, which will be elaborated further in this paper.We are now seeing new emerging technologies that can overcome some of the potential difficulties in this area. These include: computer graphics, augmented reality, computational dynamics, and virtual worlds. This paper summarizes the state of the art in virtual laboratories and virtual worlds in the fields of science, technology, and engineering. The main research activity in these fields is discussed but special emphasis is put on the field of robotics due to the maturity of this area within the virtual-education community. This is not a coincidence; starting from its widely multidisciplinary character, robotics is a perfect example where all the other fields of engineering and physics can contribute. Thus, the use of virtual labs for other scientific and non-robotic engineering uses can be seen to share many of the same learning processes. This can include supporting the introduction of new concepts as part of learning about science and technology, and introducing more general engineering knowledge, through to supporting more constructive (and collaborative) education and training activities in a more complex engineering topic such as robotics. The objective of this paper is to outline this problem space in more detail and to create a valuable source of information that can help to define the starting position for future research. State of the art in dynamics-based virtual laboratories.Defining the criteria for critical evaluation of existing technologies.State of the art in virtual worlds.Future advances in the field of virtual-world based laboratories.

Redundancy problem in writing: from human to anthropomorphic robot arm

This paper presents the analysis of motion of a redundant anthropomorphic arm during the writing. The modeling is based on the separation of the prescribed movement into two motions: smooth global, and fast local motion, called distributed positioning (DP). The distribution of these motions to arm joints is discussed. It is based on the inertial properties and actuation capabilities of joints. The approach suggested allows unique solution of the inverse kinematics of redundant mechanisms such as human arm and anthropomorphic robot arm. Distributed positioning is an inherent property of biological systems. Humans, when writing, as shown in literature and in our earlier work control their proximal joints, while the movement of distal joints follow them (synergy). To enhance capabilities of robots, new control schema are necessary. We show that robot control can be improved if it is biological analog. The major aim of this study is to promote such a hypothesis by using anthropomorphic robot arm in writing as an example.

Dynamics of contact tasks in robotics. Part I : General model of robot interacting with environment

Abstract Modelling the dynamics of contact tasks is carried out in a completely general way. Two dynamic systems are brought into contact and the relevant dynamic effects analysed. The effects are: rigid-body-contact force, elastodynamics in contact zone, friction in contact points, elastic deformation in torque transmission, impact, etc. The general model is then applied to some more concrete problems in order to discuss some effects in detail. The control strategy is considered for each concrete problem and the simulation results are presented. The work is organized in two parts dedicated to first the general model, and second model application and simulation.

Toward anthropomimetic robotics: Development, simulation, and control of a musculoskeletal torso

Anthropomimetic robotics differs from conventional approaches by capitalizing on the replication of the inner structures of the human body, such as muscles, tendons, bones, and joints. Here we present our results of more than three years of research in constructing, simulating, and, most importantly, controlling anthropomimetic robots. We manufactured four physical torsos, each more complex than its predecessor, and developed the tools required to simulate their behavior. Furthermore, six different control approaches, inspired by classical control theory, machine learning, and neuroscience, were developed and evaluated via these simulations or in small-scale setups. While the obtained results are encouraging, we are aware that we have barely exploited the potential of the anthropomimetic design so far. But, with the tools developed, we are confident that this novel approach will contribute to our understanding of morphological computation and human motor control in the future.

Towards a unified understanding of basic notions and terms in humanoid robotics

The intention of this paper is to contribute towards a unified understanding of the basic notions and terms in the domain of humanoid robotics, having in mind that the same notions are sometimes interpreted in different ways (some interpretations are contradictory, and some even erroneous). Hence, the first part of the paper is devoted to defining some basic notions, walk and gait being among the first. Then, the paper deals with the notion of dynamic balance and stability, particularly the difference between them, since these essentially different notions are often confused and, rarely, regarded as identical. As dynamic balance is directly related to the notion of zero-moment point (ZMP), it was necessary to touch upon some misunderstandings concerning the ZMP. Gait stability is an especially delicate category, as humanoid locomotion systems have certain specific features that are not possessed by other systems. Namely, because of external disturbances, there may appear unpowered (passive) degrees of freedom that cause loss of dynamic balance. Hence, these unpowered degrees of freedom cannot be overlooked in the stability analysis. As the stability of motion of humanoid robots is inseparably linked with control, it was also necessary to pay due attention to this notion. Finally, the paper ends with a discussion of posture and postural stability with all their specificities. The authors hope that this paper will contribute to a clearer understanding of the basic notions of humanoid robotics, especially concerning robots with high dynamic and control performances.

The Puller-Follower Control of Compliant and Noncompliant Antagonistic Tendon Drives in Robotic Systems

This paper proposes a new control strategy for noncompliant and compliant antagonistic tendon drives. It is applied to a succession of increasingly complex single-joint systems, starting with a linear and noncompliant system and ending with a revolute, nonlinearly tendon coupled and compliant system. The last configuration mimics the typical human joint structure, used as a model for certain joints of the anthropomimetic robot ECCEROBOT. The control strategy is based on a biologically inspired puller-follower concept, which distinguishes the roles of the agonist and antagonist motors. One actuator, the puller, is considered as being primarily responsible for the motion, while the follower prevents its tendon from becoming slack by maintaining its tendon force at some non-zero level. Certain movements require switching actuator roles; adaptive co-contraction is used to prevent tendons slackening, while maintaining energetic efficiency. The single-joint control strategy is then evaluated in a multi-joint system. Dealing with the gravitational and dynamic effects arising from the coupling in a multi-joint system, a robust control design has to be applied with on-line gravity compensation. Finally, an experiment corresponding to object grasping is presented to show the controllers robustness to external disturbances.

Virtual Mechatronic/Robotic laboratory - A step further in distance learning

The implementation of the distance learning and e-learning in technical disciplines (like Mechanical and Electrical Engineering) is still far behind the grown practice in narrative disciplines (like Economy, management, etc.). This comes out from the fact that education in technical disciplines inevitably involves laboratory exercises and this fact drastically increases the complexity of a potential e-learning system. New approach and new specific knowledge are needed to develop such a system. We expect to meet the requirements of distance learning by developing the software-based laboratory exercises, i.e., a virtual laboratory. To fully substitute a physical system like laboratory equipment, one must emulate its full dynamics. The mathematical model in the form of differential equations will be applied to calculate dynamics and provide the data that would otherwise be measured on a physical system - this means simulation. To prove the feasibility of the concept and make a step towards full e-learning in technical disciplines, we consider a complex technical field, Mechatronics and more precisely, Robotics being a perfect symbiosis of Mechanical and Electrical Engineering. We present the Virtual Laboratory for Robotics (VLR). It possesses all the necessary features of a virtual laboratory: user interface, simulator, and visualization.

Dynamics of anthropomorphic painting robot: Quality analysis and cost reduction

Application of robots in spray-painting tasks results in low-cost production, persistent quality and protects humans from a hostile working environment. Automated planning of applicator’s trajectory requires a model of paint deposition onto the treated surface and formulation of an appropriate criterion for the painting quality. In previous research several painting models and quality measures were derived. The research efforts were concentrated on the determination of the trajectory that provided the best quality of painting. In contrast with previous approaches, painting quality here is not considered as a criterion function, which is to be maximized, but as a constraint — we limit its lower level. This gives an opportunity for proper minimization of some additional cost function. Particular objective is ergonomic-based optimization of the painting task that results in reduced motor load, energy consumption, and control jerks, preserving the required quality. Practical fulfilment of such objectives requires a painting model applicable for an arbitrary spray-gun’s position, orientation and velocity. Since existing models only partially satisfy these requirements, our research also includes derivation of the appropriate painting model. Thus, this paper treats two topics: modeling and simulation of the spray-painting process, and dynamic optimization of an anthropomorphic painting robot. An ergonomic shape of the spray-gun’s trajectory is proposed. A set of diagrams for the optimal choice of characteristic trajectory parameters, in the sense of minimum energy consumption and preserved painting quality, is given. Benefits of the applied approach on reduced paint wastage and increased efficiency of the job, are pointed out.

Contribution to the dynamics and control of robots having elastic transmissions

This paper discusses one problem of robot dynamics rarely mentioned in papers relevent to this field. It is the problem of torsional effects in torque transmissions (reducers, shafts, transmission chains, etc.). The problem is significant since oscillations can appear to be due to these effects. The complete dynamic model, which includes these effects, is derived and the possible simplifications considered. The position of feedback transducers is discussed since it appears as an important problem when it is intended to minimize the influence of these elastic vibrations. The discussion is based on eigenvalues and simulation results.