Rapidly Deployable Cable-Driven Parallel Robots

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Cable--driven parallel robots (CDPRs) consist of an end effector suspended by a number of cables. Each cable is wound onto a servo-actuated spool, which is rigidly attached to a fixed frame. The end-effector displacements are controlled by simultaneously winding or unwinding the cables on their respective spools. Our interest in CDPRs stems from four advantages they hold over classical rigid-link robots: large workspaces; low cost; high speeds and accelerations; portability and reconfigurability. In our opinion, this latter advantage, the portability and reconfigurability of CDPRs, has been relatively neglected in previous research. We believe that this could be significantly improved by solving three problems: (1-2) making the CDPR calibration fully autonomous (3) designing new spools that can be mounted on towers rather than buttressed structures and (4-5) quickly tracing and certifying their workspace. The hardware will not have to be rebuilt from scratch for each project, because a dynamically reconfigurable CDPR is available at the Laboratoire de robotique. In projects (1-2), we will devise a mathematical method that allows to identify the CDPR mechanical model without the prior estimate of its parameters required by current calibration methods. Measuring the dimensions of a newly deployed cable robot is time consuming and constitutes an important limitation to its portability. The proposed method will be demonstrated through the dynamically reconfigurable CDPR already available and by designing a new highly portable lightweight CDPR. Project (3) was established from the observation that current CDPRs resemble more gantry cranes than tower cranes. To minimise the CDPR structure, we want to attach its spools on towers free of large buttresses, stabilisers or tethers. We will achieve this objective by cancelling the moment induced at the base of the tower by the cable tension applied at its top. A moving counterweight driven by the winch and cable direction will be used to cancel the moment. We will demonstrate this concept on a two--degree--of-freedom cable--suspended parallel robot in the vertical plane. In projects (4-5), we wish to develop faster methods for tracing and certifying the wrench-feasible workspace of CDPRs. The fastest available methods for tracing the workspace are based on line search, following rays emanating from a point until they intersect the workspace surface. We will propose a method where a path is followed along the workspace surface by using a predictor-corrector scheme. This method should be faster, as every point predicted along the path will be close to the true workspace surface, ensuring a fast correction step. As this type of workspace tracing method is not guaranteed to find the whole workspace, we will also develop a certification method. This method will consist in a branch-and-prune algorithm in which the lower bounds are computed through convex relaxations.