Werner Kraus

IPAnema: A family of Cable-Driven Parallel Robots for Industrial Applications

Nowadays there are very little robot systems in operation in the field of medium to large-scale handling and assembly mostly due to lack of repetitive processes or shortcomings in programming and configuring such robots. In this paper we introduce a family of cable-driven parallel robot called IPAnema that are designed for industrial processes. We address the system architecture, key components such as winches and controller, as well as design tools. Furthermore, some experimental data from the evaluation are presented to illustrate the performance of cable robots.

Case studies in the manipulation of flexible parts using a hybrid position/force approach

A unified method for the planning and control of tasks involving flexible parts is applied to two case studies: the bending of sheet metal and the insertion of a beam into a hole with friction. The method is formulated on a hybrid position/force framework so as to exploit the advantages brought about by load elasticity. The unifying design guideline is the maintenance of deformations on the load so that a region of reduced stiffness is presented to the force controller. The main focus of this paper is the experimental results obtained from the implementation of the tasks.

Force fields in the manipulation of flexible materials

Assembly operations that involve flexible parts differ greatly from rigid-part assembly due to the capacity of the parts to comply with motions of the manipulators by yielding to the contact forces. This natural compliance can be exploited in the design of tasks involving the manipulation of flexible loads. Namely, using knowledge about the force vector field generated by conservative restoring forces of the load, a trajectory for the manipulator end-effector can be specified. The idea is illustrated through two case studies: an analysis of the problem of bending a flexible sheet, and experiments on an insertion task where a flexible beam has to be inserted in a hole with friction in the direction of insertion.

An Elastic Cable Model for Cable-Driven Parallel Robots Including Hysteresis Effects

Experimental results indicate that time invariant linear elastic models for cable-driven parallel robots show a significant error in the force prediction during operation. This paper proposes the use of an extended model for polymer cables which allows to regard the hysteresis effects depending on the excitation amplitude, frequency, and initial tension level. The experimental design as well as the parameter identification are regarded.

Hybrid position/force coordination for dual-arm manipulation of flexible materials

The problem of coordinating two arms for the manipulation of flexible materials is studied through the bending of a flexible beam. The solution proposed in this paper is based on a hybrid position/force approach. The choice of controlled variables in each direction is determined by minimum-effort optimality criteria. The bending effort is defined as the overall bending moment and force applied by the grippers. The control of forces exploits the materials elasticity, which is an impedance for the manipulators, The force controller then behaves as an admittance, in the form of an accommodation (inverse damping) law. Experimental results show the efficacy of the approach in achieving task requirements.

Control of Flexible Load Deformations and Environment Contact Forces in Dual-Arm Manipulation

Abstract A pseudo-inverse-Jacobian scheme to control deformations of a flexible load while attaining contact force goals with an external environment is discussed in this paper. The flexible load is manipulated by two arms. A kinematic description of the flexible load is obtained from a lumped-parameter model. Coordinated motion of the arms is achieved through kinematic redundancy resolution at the differential kinematic level. The strategy proposed here uses load flexibility to accommodate for position errors that would result in the build-up of internal forces in a rigid-load case. Simulation results are presented that illustrate the success of the method.

Modelling and Control of Interaction Forces in Nonlinear-Stiffness Types of Contact

Abstract This paper analyses a force control scheme used for interactions between a manipulator and a nonlinearly stiff environment. Initially, a nonlinear model for the contact stiffness is developed, and the idea of using an accommodation law in controlling force is introduced. Accommodation is the inverse of damping, and corresponds to a force input/velocity output mapping. The stability of the scheme is analysed using the second method of Lyapunov. The analysis is new since it takes into account the variation in stiffness caused by changes in the deformation of the load (or environment). The results are applicable to many types of contact that can be described by nonlinear stiffness models. Such models are more general than the linear ones which have been the base for the results available in the literature.

Minimum-time control for the transport of flexible loads by two manipulators

Abstract The minimum-time pointwise motion of two manipulators grasping the same flexible object is examined in this paper. We model the object as a spring with an associated damping factor. The coordinated motion is accomplished through the specification of the trajectory for the mid-point between the two manipulators. The separate trajectories for each manipulator are not relevant per se, since they only need to preserve the desired motion of the mid-point. This freedom in the motion together with the presence of the flexible material causes the minimum time to be less than the minimum time required to move two manipulators with a rigid link between them. This result shows the advantage of exploiting the flexibility of the material being worked by two manipulators.