Constant Distance and Orientation Following of an Unknown Surface with a Cable-Driven Parallel Robot

Abstract

Cable-Driven Parallel Robots (CDPRs) are welladapted to large workspaces since they replace rigid links by cables. However, they lack in positioning accuracy and new control methods are necessary to achieve profile-following tasks. This paper presents a control scheme designed for these tasks, relying on a combination of accurate boarded distance sensors and of a less accurate remote camera. The profile-following task is divided into two subtasks that are partially conflicting: maintaining a parallel orientation and a constant distance with the surface to follow, and following a trajectory between two points on the surface. The data fusion to solve the redundancy is based on the Gradient Projection Method. This control scheme is validated experimentally on a CDPR prototype and shown to provide the expected behaviour.

Stability Analysis of Profile Following by a CDPR Using Distance and Vision Sensors

“…This paper is organized as follows: Section 2 presents the two sensors and their kinematics modeling. The control law is recalled from [7] in Section 3. Section 4 focuses on the design of the stability criterion and Section 5 presents the numerical results of the stability analysis.…”

“…The control law, pictured in Fig. 3 and performing the fusion described above, is detailed in [7]. It relies on the projection operator P 1 on the kernel of the primary task given by:…”

“…However, vision-based control has some important limitations: embedded cameras suffer from a limited field of view, while external cameras provide a global overview of the workspace but at the cost of worse accuracy. A solution to these shortcomings was proposed in [7] where measurements from both distance sensors and an external camera are combined using the Gradient Projection Method (GPM) [8,9] to prioritize tasks. The robustness of this control law was not considered in [7].…”

“…A solution to these shortcomings was proposed in [7] where measurements from both distance sensors and an external camera are combined using the Gradient Projection Method (GPM) [8,9] to prioritize tasks. The robustness of this control law was not considered in [7]. While the stability analysis for N-tasks problems was investigated in [10], the proposed stability criterion is only valid in the ideal case while a specific study based on the quality of the estimations and possible sensor biases is required in our case.…”

“…Finally, a combined analysis is run in the robot operating conditions to confirm its robustness. Since experimental results have already been presented in [7], only simulation results are discussed in this paper. A view of the experimental platform used in this paper, ACROBOT, a six degrees of freedom (DoF) suspended CDPR, is shown in Fig.…”