Long distance teleoperation over asynchronous transmission links makes many classical teleoperation schemes unstable. The use of this kind of media involves varying transmission delays that may become prohibitive even at short distance. However, they are standardized, cheap and widespread over the planet. The paper presents our latest works on the improvement of teleoperation loops, by providing a low-level architecture which permits the use of classical teleoperation controls over asynchronous digital links.

A mobile robotic manipulator, i.e. a manipulator mounted on a vehicle, is a very useful system for tasks in dangerous environments. Unfortunately the use of a wheeled vehicle introduces non-holonomic constraints, and the combination of a vehicle and a manipulator generally introduces kinematic redundancy. The goal of this paper is to propose a method to determine a feasible path between two fixed configurations for a mobile manipulator whose vehicle is non-holonomic. For this purpose, we write the global displacement of the system in a symbolic way, using two representation tools: homogeneous matrices and dual quaternions. Then, we compute the corresponding joint parameters to make the desired displacement coincide with the computed symbolic displacement. The method is described and some simulation results are presented.

A mobile manipulator, composed of a manipulator mounted on a vehicle, is a very useful system to achieve tasks in dangerous environments. Unfortunately it is a very complicated system to model and control. One solution is to control the manipulator and the vehicle independently with advanced control schemes. In particular motions of the vehicle on uneven terrains induce disturbances on the arm behaviour that must be either taken into account or rejected by the arm control scheme. The aim of this paper is to compare the robustness of hybrid position/force and external force control structures with respect to external disturbances, for a manipulator mounted on a vehicle. The principles of both control structures are recalled. Some theoretical analysis tends to prove the superiority of the external control structure. This is confirmed by experiments with our real testbed. Results are presented in the form of graphs.

A mobile manipulator, composed of a manipulator mounted on a vehicle, is a very useful system to achieve tasks in dangerous environments. However, it is a very complicated system to model and control. The use of a wheeled vehicle generally introduces nonholonomic constraints, and the combination of a vehicle and a manipulator generally introduces some kinematic redundancy. The aim of this paper is to investigate the use of dual quaternions for the motion generation of a mobile manipulator with a nonholonomic vehicle. In this preliminary work, only planar cases are considered. Therefore, the principles of the planar quaternion theory are recalled. The description of the system in the planar quaternion space is developed. Some simulation results are then presented.

Robustness is a very important feature of robot controllers. Obviously, dynamic control is not the good way to obtain the best robustness performance, especially when the payload is unknown. Moreover, this kind of control laws are not able to compensate for Coulomb friction. Our approach is based on the variable structure control (VSC) with sliding mode, but it is well known that VSC is unstable for physical sampled systems. Therefore, the concept of fuzzy logic is used to implement fuzzy sliding mode control on our six-degree-of-freedom fast parallel robot. Both simulated and real experiments are reported in this paper.

Concerns the control of the deformation of a robot manipulator exerting large forces and moments on its environment. This was motivated by a real application consisting in inserting rivets with a two-arm robot (to assemble a wing or a fuselage on an airplane structure). The solution consists in performing the complete task rivet after rivet. To do so, it is necessary that the two parts be compressed together to avoid any introduction of turnings between them during drilling. Therefore, the forces that the arms have to exert exceed 1000 N. With such constraints, a position controlled robot deforms and the end-effecters are not orthogonal to the parts anymore, which is not acceptable. These deformations cannot be detected by the optical encoders mounted on the motors shafts, because they are due mainly to the flexibility of the joints. We have developed a force control scheme with explicit force feedback to compensate for these deformations by servoing the forces and moments at the level of the end-effector: the desired values are zero except for the force orthogonal to the parts being assembled. To avoid using force sensors in a production line, we propose to use a force control scheme to derive a reference model. This model furnishes at every point the position increments to be applied to the robot in order to compensate for its nondesired deformations.>

The use of a two-arm robot for assembling two objects, with each being held by one arm, is presented. More precisely, the authors propose some solutions for determining how and where such an assembly task should be performed. For the approach phase (before contact), they define the motion of one end-effector with respect to the other, without reference to any fixed frame. This allows them to take advantage of the two-arm robot redundancy. For the insertion phase, they use a symmetric hybrid control scheme. Real experiments with two PUMA robots have shown that the success of the assembly is highly dependent on the location where the assembly is performed. This is due to the control of contact forces between the objects, which can be done more or less properly. This problem is treated by using the concept of manipulability, represented by ellipsoids.

The paper presents an original hybrid position/force control scheme. The novelty of the control law lies in the absence of Cartesian position servoloop. It is shown that when one tries to implement a hybrid control based upon a previously published scheme and task description, the Cartesian position servoloop automatically disappears. This is an essential result that is taken into account to construct the control diagram. Two types of experimental results are presented. The first experiments are aimed at validating the theoretical results we have obtained; the second experiments prove the efficiency of our hybrid control.

The authors present solutions for manipulating objects with two robotic arms, when force control is necessary. The applications of interest are the transport and assembly of rigid objects, the deformation, transport, and assembly of flexible objects, and the assembly in space of two objects, each being held by one arm. The authors present a theoretical solution for modeling the tasks and practical solutions implemented for controlling two six-axis arms.>

Two parallel robots are presented. The first is a three-degree-of-freedom lightweight robot called DELTA. The equations corresponding to different models of this robot (forward and inverse kinematics as well as inverse dynamics) are given. The important feature of the method used in deriving these models is the use of a 'good' set of parameters to simplify the equations. These models have been employed to control a real DELTA with a low-cost controller. An attempt was also made to extend the principle of the DELTA mechanical structure to a six-degree-of-freedom parallel robot. A design called the HEXA has been developed. The authors present this robot and show that it should have the same dynamic capabilities as the DELTA because, like the later, it can be built with light material and can be easily modeled.>

In order to perform complex coordinated tasks with a two-arm robot, some kind of robust hybrid position/force control is necessary. As a first step towards such a control scheme, the authors present mainly the results they have obtained in simulation with a single-arm robot. They have developed a simulator which allows one to test various control algorithms. In particular, they have tested a variable structure system for the hybrid control of a Puma robot. Several experimental results are presented in this case. Controlled variables for a two-arm system are also proposed.>

A symmetric scheme-not a master/slave scheme-is proposed for the coordination of two robots to manipulate a single object jointly. The definition of workspace force, velocity, and position vectors as symmetric functions of the joint space force, velocity and position vectors of the two robots is essential to implementing the symmetric scheme. These workspace vectors are derived by first analyzing the statics of the closed kinematic chain consisting of the two robots and the object, and then calculating velocities using the force-velocity duality. The workspace-position vector is defined by integrating the velocities. The derived workspace vectors are used successfully to implement the symmetric scheme.>

This paper deals with the problem of true cooperation of robots working on the same task. We propose a method to carry out various tasks where both robots move together a same rigid object. Firstly we develop the theorical aspect and then we present the exchange problem between two grippers and an assembly task using two arms.