Aleksandar Timcenko

Real-time visual servoing

A real-time tracking algorithm in conjunction with a predictive filter to allow real-time visual servoing of a robotic arm that is tracking a moving object is described. The system consists of two calibrated (but unregistered) cameras that provide images to a real-time, pipeline-parallel optic-flow algorithm that can robustly compute optic-flow and calculate the 3-D position of a moving object at approximately 5-Hz rates. These 3-D positions of the moving object serve as input to a predictive kinematic control algorithm that uses an alpha - beta - gamma filter to update the position of a robotic arm tracking the moving object. Experimental results are presented for the tracking of a moving model train in a variety of different trajectories.<<ETX>>

Trajectory filtering and prediction for automated tracking and grasping of a moving object

The authors explore the requirements for grasping a moving object. This task requires proper coordination between at least three separate subsystems: real-time vision sensing, trajectory-planning/arm-control, and grasp planning. As with humans, the system first visually tracks the objects 3D position. Because the object is in motion, this must be done in real-time to coordinate the motion of the robotic arm as it tracks the object. The vision system is used to feed an arm control algorithm that plans a trajectory. The arm control algorithm is implemented into two steps: filtering and prediction and kinematic transformation computation. Once the trajectory of the object is tracked, the hand must intercept the object to actually grasp it. Experimental results are presented in which which a moving model train was tracked, stably grasped, and picked up by the system.<<ETX>>

Position control of robot manipulators with elastic joints using force feedback

Transmission flexibility, as a predominant source of mechanical resonance, often results in inaccurate tracking of prescribed robot trajectories. Thus, the control algorithm must compensate not only for nonlinear dynamics of the arm and payload variations, but also for resonant effects in joints. The compensation for all of these effects results in a very complicated control algorithms which are difficult to apply in practice. In this article we suggest the measurement of torque/force on the output shaft of elastic reducer in order to compensate for elastic effects. The torque measurement can be replaced by the simultaneous measurement of the joint and the motor-shaft angle. Then, we introduce a dynamic compensator with two zeros and two poles to filtrate the torque signal. It appears that the obtained control law is very simple and compensates for nonlinear system dynamics, variable payload parameters and resonant effects in joints. The proposed control scheme is illustrated by computer simulation of a six-degree-of-freedom robot.

Probability-driven motion planning for mobile robots

This paper proposes a path-planning method for mobile robots in the presence of uncertainty. We analyze environment and control uncertainty and propose methods for incorporating each of them into the planning algorithm. We model the environment using the pyramid structure that encodes the information on occupancy probabilities for each pixel as well as the partial information on conditional probabilities among different pixels. This structure allows for efficient and accurate computation of collision probabilities in the presence of environment uncertainty. The control uncertainty is mainly characterized by its expansion in space and time and is accordingly modeled by a stochastic differential equation that mathematically captures this phenomenon. Models that we develop are inevitably approximate but experiments confirm that they can be used as a reasonable model for motion planning. We have conducted a series of experiments on the mobile platform and some of these results are presented.<<ETX>>

SYM-program environment for manipulator modeling, control and simulation

The structure of SYM, a program package for manipulator modeling, control law synthesis, and simulation, is described. SYMs research and educational aims are emphasized. The control law synthesis in symbolic form and the system simulation are discussed in detail. Several examples of SYM outputs which depict the main steps in the manipulator control system creation process, namely, manipulator structure as a 3-D scheme, control law definition form, and system simulation results as 2-D plots are presented.<<ETX>>

Computation of customized symbolic robot models on peripheral array processors

The authors address the problem of the optimal evaluation of robot inverse dynamics on array processors. The inverse dynamics models used are the symbolic customized models with near-minimum numerical complexity, which are computer-generated given the robot arm parameters. Such models represent the input for a proposed scheduling algorithm that distributes the computation of the model over several multipliers and adders. The scheduling algorithm is automatic and minimizes the number of microcycles. The algorithm was tested on several standard robots, and processor efficiency up to 84% was achieved. Experimental results on a 30 MFLOPS array processor showed that the inverse dynamics of a three-link PUMA-like robot requires 25.5 mu s. For a six-link robot, the computation of the inverse dynamics and the control law takes about 100 mu s on this processor. Thus, the control of high-speed robots could be achieved by attaching low-cost array processors (10 MFLOPS) to the control processor of the robot controller.<<ETX>>

Distributed general-purpose robot controller

A general-purpose robot controller is described. The controller is based on a distributed multiprocessor architecture that provides real-time response. The high sampling rate of up to 6 kHz for each controlled joint allows successful applications of tasks, such as operation in very rigid environments. The software supports real-time operation, network communication, interrupt handling and code migration among CPUs that are part of the controller. The interface between four main hierarchical levels in the controller is defined, and its implementation on the distributed multitasking system is described. The controller is used in a testbed that consists of a 7-degree-of-freedom manipulator equipped with joint position and torque sensors as well as a force/torque sensor mounted on the manipulators tip.<<ETX>>

Planning velocity profiles from task-level constraints and environment uncertainties

A method for parameterizing robot trajectories in the presence of uncertainties is presented. The planning process is defined as a problem of constrained optimization and the concept of a tasks difficulty is used as an optimization criterion. The task difficulty, as defined by the authors, comprises the combined effects of velocity and uncertainty, mimicking human perception of difficulty in positioning tasks. The success probability is used as a constraint necessary for planning tasks with contradicting requirements. This planning paradigm is demonstrated with an experiment that contains opposing requirements: reaching the obstacle in a given time, but without exceeding certain maximal impact force. The planner is implemented on a real system.<<ETX>>

Symbolic modeling in robotics: Genesis, applicaton, and future prospects

This survey article gives an overview of software packages for generating numerically efficient manipulator models in symbolic form, i.e. as computer programs written in a high-level language such as C or FORTRAN. We chronicle the history of computational robot dynamics and, to some extent, multibody systems dynamics. We survey several mechanical computer-aided engineering software packages because they have charted the course for symbolic robot modeling software. The attractive features of various programs regarding areas of application (vehicles, robots, satellites, etc.) and design possibilities (kinematic and dynamic analysis, modal analysis, optimization of mechanical design, numerical efficiency of generated symbolic models, etc.) are emphasized. Finally, as an example, we present the SYM program package in more detail and point out the new strategic area of robotics which has emerged during the last two years: computer-aided generation of control laws.

Program Package for Generation of Control Laws for Robot Manipulators in Symbolic Form

Abstract This paper presents a new program package for the generation of control laws for robot manipulators in symbolic form. Since the computational efficiency of the generated control laws is extremely high, the real-time implementation become possible even on low-cost microcomputers. This program package represents an extension of the previously developed program package SYM which is designed to generate efficient manipulator models (Timcenko, Kircanski, Vukobratovic 1991). The basic algorithm belongs to the class of customized algorithms that reduce the computational burden by taking into account the specific characteristics of the manipulator to be controlled. It generates the high-level program code for computing various control laws based on inverse robot dynamics. The program code is highly optimized from the standpoint of numerical complexity. It represents a series of assignment statements with simple float variables and constants. Matrix and vector computations, loops, and conditional branches are avoided. The software modules for generation of various kinds of control laws as well as for simulation of robot moving along a defined path are incorporated into SYM software package and demonstrated in this paper. Generated models and control laws are implemented in several robot controllers designed to drive a 6 degrees of freedom robots.