Robotic Wires Manipulation for Switchgear Cabling and Wiring Harness Manufacturing

—This paper describes the development of a system for a wiring operation, composed by a robotic manipulator and a gripper with tactile sensors. The manipulation of electrical wires, and more in general deformable linear objects, is a challenging task of large interest in many industrial applications. The solution studied, is designed to shape a cable along a given path with ﬁxing points and obstacles exploited to shape the cable itself. The wire position is determined starting from a list of point and then converted to a joint reference pose for the manipulator. Moreover, tension of the wire is controlled on the basis of both the estimation of the robot external wrench and the tactile data by acting on the gripper ﬁnger opening. An experimental setup in which a cable must be routed along two linear paths connected by a turn and with four ﬁxing points has been used to validate the proposed solution.


I. INTRODUCTION
For the relevance in several industrial applications, the manipulation of wires and cables (DLOs) is been widely discussed their behaviour has been studied with different approaches [1], [2]. Is also been examinated their application in a DLO-in-hole problem [3], [4] or for their manipulation [5]. In the field of industrial manufacturing, some studies can be found related to the assembling of electrical harnesses [6], [7] in which the idea of using environmental contacts to shape the wire along a desired path or to place it in a certain position is exploited. Another relevant problem is the DLOs detection is the perception where tactile sensors are usually the only way due to the occlusion provided by the gripping device itself [8] to detect the manipulated object. This work focuses on the DLOs routing problem addressing the trajectory planning and two online controllers for the tension and the position of the wire inside the gripper using a pair of tactile sensors placed on the finger's tips. The planner use as input the list of points over which the cable must pass and their semantic description, from which it elaborates a feasible trajectory for the robot that fulfil the expected DLO desired configuration and assembly sequence. The robot approach the clamp laterally to perform a circular movement to insert it inside the clamp. On the right, the cornerfix trajectory where the wire is elongated using the peg placed in the corner and is used to ris used for the rotation semantic description denominated as "labels", which is an identifier of the sequence of operations that must be executed in the specific point. The labels' list is elaborated in order to have a feasible list of action for the wiring and from that it generates a vector of "Secondary Trajectories" assosciated to each label. The planning is completed with the interpolation of the points and the conversion in the joint space. The secondary trajectories can be described as follows. 1) Fix Trajectory: The Fix Trajectory is characterized by the presence of a clamp along the expected path of the wires, used to hold and collect the wires in position after they are successfully placed. The clamp's insertion area is facing up, with a chamfer to helps the wire to slide in case of small misalignment error. The insertion starts with a lateral alignment at P start , then it moves laterally with respect to the clamp position P fix preserving the cable tension. Then, with a combination of a semi-circular movement and a forward movement the robot to bring the cable in front of the clamp ready to be inserted while keeping the wire tensioned to avoid the formation of knots.
2) CornerFix Trajectory: In the presence of a clamp right after a corner, a second movement denominated "Corner-fix is been designed. In this case, a tensioner is necessary to keep the wire tensioned in the corner. From the starting point of this traj. P start , the robot moves forward avoiding the peg positioned in P peg to elongate the wire and then proceeds to an intermediate point P int. spaced at a distance equal to the sum of the length P start P peg and P peg P end . The gripper rotates around the corner 2021 I-RIM Conference October 8-10, Rome, Italy ISBN: 9788894580525 DOI:10.5281/zenodo.5900503 point P peg maintaining the cable tensioned with the ending point P end ahead of the actual clamp position P fix . The gripper rotate to keep the fingers always aligned to centre of the corner.
3) Corner rounding: The corner rounding is inserted whenever a change in the cable direction is needed. The detection of the corners rounding is accomplished by checking if three points are not aligned, by looking at the sum of the points distance. The algorithm proceed with the insertion of a rounded trajectory obtained using a quadratic Bezier curve. The orientation is changed using the Spherical Linear Interpolation [SLERP] that linearly interpolate the orientation of the gripper.

III. ACTIVECABLE CONTROLLERS
To achieve a better manipulation, an active tensioning controller is been introduced in order to keep the tension of the wire constant during the execution to permit the cable to be inserted in the clamps without damaging the cable or other connectors. The controller Fig. 2a is based on PID controller that control the distance between the two gripper's fingers, using as input the estimation of the external forces from the reading of the torque at each joint, starting from: The relationship can be inverted to isolate the external wrench: Where J # (q) is the pseudoinverse of the Jacobian transpose compute using the pseudoinverse of J # (q) with the Singular Value Decomposition [SVD] method. The data are filtered using a Moving Average filter. An active heigh controller is used to ensure that the cable remain aligned with the centre of the gripper without the necessity of any holding device to counter act the gravity effect on the cable. A pair of tactile sensors Fig. 2 detect the position of the wire and modify the elevation of the EE.

IV. EXPERIMENTAL EVALUATION AND CONCLUSION
The test bench for the evaluation reproduces a path in a switchgear box composed by two linear traits connect by a 90°corner Fig.3, where the trajectories can be studied to evaluate their effectiveness in the insertion of the wire and during the placement obtaining a success rate close to 100%. These preliminary results open the way to industrial applications like switchgear manufacturing or wiring harnesses assembling. Additionally, the tactile sensor can be used to extract more information about the cable position and can be used for specific trajectory controller such as the highcontroller to keep the wire inside the fingers. Future works will be devoted to evaluate the performances on more complex and realistic scenarios.