Although deformable linear objects (DLOs), such as cables, are widely used in the majority of life fields and activities, the robotic manipulation of these objects is considerably more complex compared to the rigid-body manipulation and still an open challenge. In this paper, we introduce a new framework using two robotic arms cooperatively manipulating a DLO from an initial shape to a desired one. Based on visual servoing and computer vision techniques, a perception approach is proposed to detect and sample the DLO as a set of virtual feature points. Then a manipulation planning approach is introduced to map between the motion of the manipulators end effectors and the DLO points by a Jacobian matrix. To avoid excessive stretching of the DLO, the planning approach generates a path for each DLO point forming profiles between the initial and desired shapes. It is guaranteed that all these intershape profiles are reachable and maintain the cable length constraint. The framework and the aforementioned approaches are validated in real-life experiments.

This paper is devoted to the design of gravity compensators for prismatic joints. The proposed compensator depends on the suspension of linear springs together with mechanical transmission mechanisms to achieve the constant application of force along the sliding span of the joint. The use of self-locking worm gears ensures the isolation of spring forces. A constantforce mechanism is proposed to generate counterbalance force along the motion span of the prismatic joint. The constant-force mechanism is coupled with a pin-slot mechanism to transform to adjust the spring tension to counterbalance the effect of rotation of the revolute joint. Two springs were used to counterbalance the gravity torque of the revolute joint. One of the springs has a moving pin-point that is passively adjusted in proportion with the moving mass of the prismatic joint. To derive the model of the compensator, a 2-DoF system which consists of a revolute and a prismatic joint is investigated. In contrast to previous work, the proposed compensator considers the combined motion of rotation and translation. The obtained results were tested in simulation based on the dynamic model of the derived system. The simulation shows the effectiveness of the proposed compensator as it significantly reduces the effort required by the actuators to support the manipulator against gravity. The derived compensator model provides the necessary constraints on the design parameters.

This paper deals with the stiffness modeling of the double pantograph transmission system. The main focus is on the comparison analysis of different stiffness modeling approaches: virtual joint modeling (VJM) and matrix structural analysis (MSA). The aim of this work is to investigate the limitations of the considered approaches. To address this issue, corresponding MSA-based and VJM-based stiffness models were derived. To evaluate the deflections of the end effector, the external loads were applied in different directions at multiple points in the robot workspace. The computational cost and the difference in end-effector deflections were studied and compared. MSA was found to be 2 times faster than VJM for this structure. The results obtained showed that the MSA approach is more appropriate for the double pantograph mechanism.

Any obstacle-free path planning algorithm, in general, gives a sequence of waypoints that connect start and goal positions by a sequence of straight lines, which does not ensure the smoothness and the dynamic feasibility to maneuver the MAV. Kinodynamic-based motion planning is one of the ways to impose dynamic feasibility in planning. However, kinodynamic motion planning is not an optimal solution due to high computational demands for real-time applications. Thus, we explore path planning followed by kinodynamic smoothing while ensuring the dynamic feasibility of MAV. The main difference in the proposed technique is not to use kinodynamic planning when finding a feasible path, but rather to apply kinodynamic smoothing along the obtained feasible path. We have chosen a geometric-based path planning algorithm “RRT*” as the path finding algorithm. In the proposed technique, we modified the original RRT* introducing an adaptive search space and a steering function that helps to increase the consistency of the planner. Moreover, we propose a multiple RRT* that generates a set of desired paths. The optimal path from the generated paths is selected based on a cost function. Afterwards, we apply kinodynamic smoothing that will result in a dynamically feasible as well as obstacle-free path. Thereafter, a b-spline-based trajectory is generated to maneuver the vehicle autonomously in unknown environments. Finally, we have tested the proposed technique in various simulated environments. According to the experiment results, we were able to speed up the path planning task by 1.3 times when using the proposed multiple RRT* over the original RRT*.

The accuracy of the robot positioning during material processing can be improved if the deformation under the load is taken into account. A manipulator stiffness model can be obtained using various approaches which differ in the degree of detail and computational complexity. Regardless of the model, its practical application requires knowledge of the stiffness parameters of the robot components, which implies solving the identification problem.
In this work, we consider a reduced stiffness model, which assumes that the manipulator links are rigid, while the joints are compliant and include both elasticities in the joints themselves and the elastic properties of the links. This simplification reduces the accuracy of the model, but allows us to identify the stiffness parameters, which makes it suitable for practical application. In combination with a double encoders measurement system, this model allows for real-time compensation of compliance errors, that is, the deviation of the real end-effector position from the calculated one due to the deformation of the robot under load.
The paper proposes an algebraic approach to determining the parameters of the reduced model in a general form. It also demonstrates several steps that can be done to simplify computations. First, it introduces the backward semianalytical Jacobian computation technique, which allows reducing the number of operations for the manipulator with virtual joints. Second, it provides an algorithm for the calculation of the required intermediate matrices without explicit Jacobian calculation and using more compact expressions.
To compare the proposed techniques with the experimental approach, the robot deformation under load is simulated and the tool displacement is estimated. It is shown that both approaches are equivalent in terms of accuracy. While the experimental method is easier to implement, the algebraic approach allows analyzing the contribution of each link in a reduced model of elasticity. Besides, the obtained estimate of the parameters does not depend on the accuracy of measurements and configurations used in the identification process.