D Dragan Kostic

Modeling and identification for high-performance robot control: an RRR-robotic arm case study

This paper explains a procedure for getting models of robot kinematics and dynamics that are appropriate for robot control design. The procedure consists of the following steps: 1) derivation of robot kinematic and dynamic models and establishing correctness of their structures; 2) experimental estimation of the model parameters; 3) model validation; and 4) identification of the remaining robot dynamics, not covered with the derived model. We give particular attention to the design of identification experiments and to online reconstruction of state coordinates, as these strongly influence the quality of the estimation process. The importance of correct friction modeling and the estimation of friction parameters are illuminated. The models of robot kinematics and dynamics can be used in model-based nonlinear control. The remaining dynamics cannot be ignored if high-performance robot operation with adequate robustness is required. The complete procedure is demonstrated for a direct-drive robotic arm with three rotational joints.

A virtual structure approach to formation control of unicycle mobile robots using mutual coupling

In this article, the formation control problem for unicycle mobile robots is studied. A distributed virtual structure control strategy with mutual coupling between the robots is proposed. The rationale behind the introduction of the coupling terms is the fact that these introduce additional robustness of the formation with respect to perturbations as compared to typical leader–follower approaches. The applicability of the proposed approach is shown in simulations and experiments with a group of wirelessly controlled mobile robots.

Learning-based identification and iterative learning control of direct-drive robots

A combination of model-based and iterative learning control (ILC) is proposed as a method to achieve high-quality motion control of direct-drive robots in repetitive motion tasks. We include both model-based and learning components in the total control law, as their individual properties influence the performance of motion control. The model-based part of the controller compensates much of the nonlinear and coupled robot dynamics. A new procedure for estimating the parameters of the rigid body model, implemented in this part of the controller, is used. This procedure is based on a batch-adaptive control algorithm that estimates the model parameters online. Information about the dynamics not covered by the rigid body model, due to flexibilities, is acquired experimentally, by identification. The models of the flexibilities are used in the design of the iterative learning controllers for the individual joints. Use of the models facilitates quantitative prediction of performance improvement via ILC. The effectiveness of the combination of the model-based and the iterative learning controllers is demonstrated in experiments on a spatial serial direct-drive robot with revolute joints.

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.

Human-Like Behavior of Robot Arms: General Considerations and the Handwriting Task-Part I: Mathematical Description of Human-Like Motion: Distributed Positioning and Virtual Fatigue

Abstract This two-part paper is concerned with the analysis and achievement of human-like behavior by robot arms (manipulators). The analysis involves three issues: (i) the resolution of the inverse kinematics problem of redundant robots, (ii) the separation of the end-effectors motion into two components, i.e. the smooth (low accelerated) component and the fast (accelerated) component, and (iii) the fatigue of the motors (actuators) of the robot joints. In the absence of the fatigue, the human-like performance is achieved by using the partitioning of the robot joints into “smooth” and “accelerated” ones (called distributed positioning—DP). The actuator fatigue is represented by the so-called “virtual fatigue” (VF) concept. When fatigue starts, the human-like performance is achieved by engaging more the joints (motors) that are less fatigued, as does the human arm. Part I of the paper provides the theoretical issues of the above approach, while Part II applies it to the handwriting task and provides extensive simulation results that support the theoretical expectations.

Saturated control of time-varying formations and trajectory tracking for unicycle multi-agent systems

We propose a synchronization approach to solve a problem where multiple unicycle agents are required to follow individual reference trajectories while maintaining a time-varying formation. Motions of the agents are synchronized thanks to coupling terms in their feedback control laws. Under saturation constraints on the control signals, our control laws guarantee global asymptotic zeroing of the synchronization errors and global asymptotic stability of the tracking error dynamics. For stronger controller couplings, the robustness of formation keeping to perturbations is increased. The proposed approach is successfully validated in experiments.

Foot placement for planar bipeds with point feet

When humanoid robots are going to be used in society, they should be capable to maintain the balance. Knowing where to step appears to be crucially important to remain balanced. This paper contributes the foot placement indicator (FPI), an extension to the foot placement estimator (FPE) for planar bipeds with point feet and an arbitrary number of non-massless links. The method uses conservation of energy to determine where the planar biped needs to step to remain in balance. Simulations of the FPI show improved foot placement for balance with respect to the FPE.

Human-like behavior of robot arms: general considerations and the handwriting task*Part II: the robot arm in handwriting

Abstract This paper (Part II) investigates the motion of a redundant anthropomorphic arm during the writing task. Two approaches are applied. The first is based on the concept of distributed positioning which is suitable to model the “writing” task before the occurrence of fatigue symptoms. The second approach uses the concept of “virtual fatigue” (VF) which is a variable that dynamically behaves in a way analogous to the biological fatigue. VF enables the arm to reconfigure itself and take postures appropriate for the current level of fatigue. The study includes the analysis of legibility and inclination of handwriting, and a set of simulation results that show most practical aspects of robot human-like performance.

Representation of robot motion control skill

Development of skilled robotics draws clues from model based theories of human motor control. Thus, a comprehensive anthropomorphic background is given. Skills in robotics are viewed as a tool for fast and efficient real time control that can handle complexity and nonlinearity of robots, generally aiming at robot autonomy. In particular, a skill of redundancy resolution is addressed through a skill representation problem based on Function Approximator. The task of the robot is approximated by a set of parameterized motion primitives. Adopted parameters are also parameters of the function approximator, i.e., skill used. Redundancy is resolved during skill learning based on available expert knowledge, yielding parameterized joint motions. The approximation procedure (Successive Approximations), a major contribution of the paper, is used for batch compilation of parameterized examples, resulting in a parameterized skill model. Such skill enables a user, inexpert in redundancy resolution, to gain benefits from redundant robots. All properties of the Successive Approximations procedure such as accuracy in interpolation and extrapolation, acceleration in redundancy resolution and upgrading to new skill regarding the task variation, are discussed in the example of a five degrees-of-freedom planar redundant robot, performing parameterized ellipse as motion primitive.

On the stability of bipedal walking

Stability of bipedal locomotion is analyzed using a model of a planar biped written in the framework of systems with unilateral constraints. Based on this model, two different stable walking gaits are derived: one which fulfills the widely used criterion of the Zero Moment Point (ZMP) and another one violating this criterion. Both gaits are determined using systematic model-based designs. The model and the two gaits are used in simulations to illustrate conservatisms of two commonly used methods for stability analysis of bipedal walking: the ZMP criterion and Poincare return map method. We show that none of these two methods can give us a general qualification of bipedal walking stability.