On the accuracy of N − 1 wire-driven parallel robots

A N−1 wire-driven parallel robot is a robot for which all the N ≥ 3 wires are connected at the same point of the platform, allowing to control the location of this point. We are interested in the positioning accuracy of such a robot. If the wires are not elastic we show that the influence on the accuracy of the co-location errors of the wire anchor points on the platform is moderate, although a full analysis is a very difficult task. If the wires are elastic we study the influence of the the wire lengths measurement errors and inaccurate estimation of the stiffness of the wires. Again we show a moderate influence but very large changes in the tensions in the wires that probably prohibit the use of the redundancy to optimize the tension in the wires. In all cases the complexity of the forward kinematics of such a robot makes accuracy analysis a very demanding task that requires an in-depth investigation. 1 The N − 1 wire-driven parallel robot In a wire-driven parallel robot (WDPR) wires are attached at specific anchor points on the robot platform and can be coiled and uncoiled through an actuation system with a fixed output point for the wires. WDPR have been introduced in the 80’s (Landsberger and Sheridan, 1985),(Miura and Furuya, 1984) as an alternate to parallel robot with rigid links. They share with them the ability to manipulate large load and to be energy efficient (Li and Bone, 2001) while they allow for larger workspace (as the amount of leg lengths variation is much larger) and present a simpler mechanical design. However their major drawback is that wires can be pulled but not pushed, which increases the complexity of their kinematics as statics has to be taken into account (for example the forward kinematic problem is an open issue (Carricato and Merlet, 2011)). ∗ The author acknowledges the partial support of the EU through the grant 285404