Homayoun Seraji

Configuration control of redundant manipulators: theory and implementation

A simple approach for controlling the manipulator configuration over the entire motion, which is based on augmentation of the manipulator forward kinematics, is presented. A set of kinematic functions is defined in Cartesian or joint space to reflect the desirable configuration. The user-defined kinematic functions and the end-effector Cartesian coordinates are combined to form a set of task-related configuration variables as generalized coordinates for the manipulator. A task-based adaptive scheme is then utilized to control directly the configuration variables so as to achieve tracking of some desired reference trajectories throughout the robot motion. This accomplishes the basic task of desired end-effector motion, while utilizing the redundancy to achieve any additional task through the desired time variation of the kinematic functions. Simulation results for a direct-drive two-link arm are given to illustrate the proposed control scheme. The scheme has also been implemented for real-time control of three links of a PUMA 560 industrial robot, and experimental results are presented and discussed. The simulation and experimental results validate the configuration control scheme, and demonstrate its capabilities for performing various realistic tasks. >

Behavior-based robot navigation on challenging terrain: A fuzzy logic approach

This paper presents a new strategy for behavior-based navigation of field mobile robots on challenging terrain, using a fuzzy logic approach and a novel measure of terrain traversability. A key feature of the proposed approach is real-time assessment of terrain characteristics and incorporation of this information in the robot navigation strategy. Three terrain characteristics that strongly affect its traversability, namely, roughness, slope, and discontinuity, are extracted from video images obtained by on-board cameras. This traversability data is used to infer, in real time, the terrain Fuzzy Rule-Based Traversability Index, which succinctly quantifies the ease of traversal of the regional terrain by the mobile robot. A new traverse-terrain behavior is introduced that uses the regional traversability index to guide the robot to the safest and the most traversable terrain region. The regional traverse-terrain behavior is complemented by two other behaviors, local avoid-obstacle and global seek-goal. The recommendations of these three behaviors are integrated through adjustable weighting factors to generate the final motion command for the robot. The weighting factors are adjusted automatically, based on the situational context of the robot. The terrain assessment and robot navigation algorithms Are implemented on a Pioneer commercial robot and field-test studies are conducted. These studies demonstrate that the robot possesses intelligent decision-making capabilities that are brought to bear in negotiating hazardous terrain conditions during the robot motion.

Decentralized adaptive control of manipulators: theory, simulation, and experimentation

The author presents a simple decentralized adaptive control scheme for multijoint robot manipulators based on the independent joint control concept. The control objective is to achieve accurate tracking of desired joint trajectories. The proposed control scheme does not use the complex manipulator dynamic model, and each joint is controlled simply by a PID (proportional-integral-derivative) feedback controller and a position-velocity-acceleration feedforward controller, both with adjustable gains. Simulation results are given for a two-link direct-drive manipulator under adaptive independent joint control. The results illustrate trajectory tracking under coupled dynamics and varying payload. The proposed scheme is implemented on a MicroVAX II computer for motion control of the three major joints of a PUMA 560 arm. Experimental results are presented to demonstrate that trajectory tracking is achieved despite coupled nonlinear joint dynamics. >

Force tracking in impedance control

This article presents two simple on-line schemes for force tracking within the impedance-control framework. The force- tracking capability of impedance control is particularly important for providing robustness in the presence of large uncertainties or variations in environmental parameters. The two proposed schemes generate the reference position trajec tory required to produce a desired contact force despite lack of knowledge of the environmental stiffness and location. The first scheme uses direct adaptive control to generate the refer ence position on-line as a function of the force-tracking error. Alternatively, the second scheme utilizes an indirect adaptive strategy in which the environmental parameters are estimated on-line, and the required reference position is computed based on these estimates. In both schemes, adaptation allows au tomatic gain adjustment to provide a uniform performance despite variations in the environmental parameters. Simula tion studies are presented for a 7-DOF Robotics Research arm using full arm dynamics, demonstrating that the adaptive schemes are able to compensate for uncertainties in both the environmental stiffness and location. The simulation studies also highlight the limitations of pure impedance control without the force-tracking capability for robust execution of realistic contact tasks. Experimental results are also presented for the Robotics Research arm to demonstrate that the end effector applies the desired contact force while exhibiting the specified impedance dynamics.

Improved configuration control for redundant robots

This article presents a singularity-robust task-prioritized reformulation of the configuration control scheme for redundant robot manipulators. This reformulation suppresses large joint velocities near singularities, at the expense of small task trajectory errors. This is achieved by optimally reducing the joint velocities to induce minimal errors in the task performance by modifying the task trajectories. Furthermore, the same framework provides a means for assignment of priorities between the basic task of end-effector motion and the user-defined additional task for utilizing redundancy. This allows automatic relaxation of the additional task constraints in favor of the desired end-effector motion, when both cannot be achieved exactly. The improved configuration control scheme is illustrated for a variety of additional tasks, and extensive simulation results are presented.

A unified approach to motion control of mobile manipulators

This paper presents a simple on-line approach for motion control of mobile manipulators comprising a manipulator arm mounted on a mobile base. The proposed approach is equally applicable to nonholonomic mobile robots such as rover-mounted manipulators and holonomic mobile robots such as tracked robots and compound manipulators. For wheeled mobile robots, the nonholonomic base constraints are incorporated directly into the task formulation as kinematic constraints. The configuration control approach is ex tended to exploit the redundancy introduced by the base mobility to perform a set of user-specified additional tasks during the end- effector motion while satisfying the nonholonomic base constraints (if applicable). This approach treats the base nonholonomy and the kinematic redundancy in a unified manner to formulate new task constraints. The computational efficiency of the proposed control scheme makes it particularly suitable for real-time implementation. Two simulation studies are presented to demonstrate the applica tions of the motion control scheme to a rover-mounted arm (non holonomic system) and to a tracked robot (holonomic system).

An on-line approach to coordinated mobility and manipulation

The author presents simple and computationally efficient approach for online coordinated control of mobile robots consisting of a manipulator arm mounted on a mobile base. The effect of base mobility on the end-effector manipulability index is discussed. The base mobility and arm manipulation degrees of freedom are treated equally as the joints of a kinematically redundant composite robot. The redundancy introduced by the mobile base is exploited to satisfy a set of user-defined additional tasks during the end-effector motion. A simple online control scheme that allows the user to assign weighting factors to individual degrees-of-mobility and degrees-of-manipulation, as well as to each task specification, is proposed. The computational efficiency of the control algorithm makes it particularly suitable for real-time implementations. A computer simulation study for a spatial three-jointed manipulator mounted on a one degree-of-freedom mobile platform, which demonstrates the online coordination of mobility and manipulation, is presented.<<ETX>>

Obstacle avoidance for redundant robots using configuration control

A redundant robot control scheme is provided for avoiding obstacles in a workspace during motion of an end effector along a preselected trajectory by stopping motion of the critical point on the robot closest to the obstacle when the distance therebetween is reduced to a predetermined sphere of influence surrounding the obstacle. Algorithms are provided for conveniently determining the critical point and critical distance.

Vision‐based terrain characterization and traversability assessment

© 2001 John Wiley & Sons, Inc. Published online in Wiley InterScience (www.interscience.wiley.com)

Kinematic analysis of 7-DOF manipulators

This article presents a kinematic analysis of seven-degree-of- freedom serial link spatial manipulators with revolute joints. To uniquely determine the joint angles for a given end-effector position and orientation, the redundancy is parameterized by a scalar variable that defines the angle between the arm plane and a reference plane. The forward kinematic mappings from joint space to end-effector coordinates and arm angle and the augmented Jacobian matrix that gives end-effector and arm angle rates as functions of joint rates are presented. Conditions under which the augmented Jacobian becomes singular are also given and are shown to correspond to the arm being either at a kinematically singular configuration or at a nonsingular configuration for which the arm angle ceases to parameterize the redundancy.