# Design of a Planar Cable-Driven Parallel Robot for Non-Contact Tasks
###### Abstract  
Cable-driven parallel robots offer significant advantages in terms of workspace dimensions and payload capability. Their mechanical structure and transmission system consist of light and extendable cables that can withstand high tensile loads. Cables are wound and unwound by a set of motorized winches, so that the robot workspace dimensions mainly depend on the amount of cable that each drum can store. For this reason, these manipulators are attractive for many industrial tasks to be performed on a large scale, such as handling, pick-and-place, and manufacturing, without a substantial increase in costs and mechanical complexity with respect to a small-scale application. This paper presents the design of a planar overconstrained cable-driven parallel robot for quasi-static non-contact operations on planar vertical surfaces, such as laser engraving, inspection and thermal treatment. The overall mechanical structure of the robot is shown, by focusing on the actuation and guidance systems. A novel concept of the cable guidance system is outlined, which allows for a simple kinematic model to control the manipulator. As an application example, a laser diode is mounted onto the end-effector of a prototype to perform laser engraving on a paper sheet. Observations on the experiments are reported and discussed. cable-driven parallel robots; overconstrained robots; design; non-contact operations 20211949111949119491194911949119491194911949191949191949191  
suspended architectures with less cables than \(EE\)_DoFs_ can be used, though they are more difficult to control [4; 5]. Thanks to their simple architecture, high reconfigurability, easy deployability, and large workspace, _CDPRs_ have been proposed in many fields of application: entertainment [6; 7], logistics [8], construction [9; 10; 11], maintenance [12; 13], to name a few. In general, it is possible to design a general-purpose machine that can be dedicated to different tasks by changing the \(EE\) tool [14; 15] or, in alternative, a _CDPR_ for a specific purpose can be developed. In the second case, the robot geometry, the number and configuration of cables, and, consequently, additional design constraints are established by the final task. This is the case of the deployable _CDPR_ in [16]: the suspended architecture and the \(EE\) self-deployability ensure the high orientational capability required for the laser scanning of low-accessibility environments. As an additional example, the storage-retrieval _CDPR_ in [8] is equipped with eight cables, despite the planar nature of the robot task, in order to provide a suitable machine stiffness and prevent platform tilt. Usually, if the task requires a specific motion pattern of the \(EE\), it is possible to act in two different ways to limit the \(EE\) range of motion. First, an external means, which physically constrains the \(EE\), can be employed. This happens with the _CDPR_ Pickable [17], whose \(EE\) is constrained to slide on a planar surface thanks to an air-bearing system. Likewise, in [13], the \(EE\) of the robot moves in direct contact with the building facade to perform window cleaning operations. The contact of the \(EE\) with the environment requires specific contact models [18], and suitable force-control strategies [19]. For the Pickable robot, undesired phenomena, such as friction, are significantly reduced by using air bearings, whereas, for the cleaning robot, friction is part of the process itself. A different approach for constraining the \(EE\) motion consists in adopting special cable configurations, such as the parallelogram arrangements inspired by delta robots [20; 21; 22]. This expedient allows obtaining a translational motion of the \(EE\) without any external constraint, provided that all cables are taut. This is useful in the case of large-scale industrial tasks, such as pick-and-place and warehousing [23], or operations that need deployable systems, such as rescue and contour crafting [24; 25].