*Article* **A Tactile-Based Wire Manipulation System for Manufacturing Applications**

**Gianluca Palli 1,† and Salvatore Pirozzi 2,\*,†**


Received: 9 April 2019; Accepted: 10 June 2019; Published: 12 June 2019

**Abstract:** This paper presents experimental results developed within the WIRES experiment, whose main objective is the robotized cabling of switchgears. This task is currently executed by human operators; the WIRES Project tackles the development of a suitably designed sensorized end effector for the wire precise manipulation. In particular, the developed gripper with tactile sensors are shown and a procedure for the implementation of the insertion task is presented and discussed. Experimental results are reported both for quality of wire shape reconstruction and success rate of insertion task implementation.

**Keywords:** tactile sensors; manipulation task; assembly robot

#### **1. Introduction**

Robotic manipulation is a complex task especially when deformable and fragile objects have to be grasped. In these cases, the knowledge of geometrical and physical characteristics of the object to manipulate are fundamental for the successful implementation of the task. To this aim, specific sensing systems are developed to be integrated into robotic systems. This paper presents results of activities developed within the WIRES experiment (http://www-lar.deis.unibo.it/people/gpalli/WIRES/), where the main objective is the robotized cabling of switchgears. Switchgears are basic components in a wide range of applications. Currently, the switchgear wiring is executed by human operators due to the complex manipulation tasks. The WIRES Project tackles the development of a suitably designed end effector equipped with a vision system and a tactile sensor for wire-precise manipulation. Preliminary results have been presented in [1–3].

Standard approaches to this kind of problem use vision and/or tactile data. Often vision is used alone due to its efficiency in data collection ([4]). However, this solution may fail in the presence of varying lighting conditions and occlusions. The use of tactile sensors helps to improve the success rate by overcoming some environment limitations. As a consequence, there have been many papers where vision and tactile data are integrated in a single approach ([5–9]. The objective of these approaches is the estimation of object characteristics, such as pose, shape, surface features and so on. Among these, some researchers propose interesting algorithms for edge detection [10] that could be considered in future as alternative approaches with respect to the one proposed here in order to improve the estimation quality. At the moment, the estimation quality reached with the approach proposed here is sufficiently high for the task implementation, with a very simple formalization. Some researchers in recent papers [11] use vision systems directly integrated into fingers to evaluate both tactile and image data at the same time and with the same sensing system. Also, this approach demonstrates how the fusion among tactile and vision data can be a good approaches for manipulation tasks. However, none of these past papers tackle the estimation problem of shape and pose of flexible objects like wires.

In previous papers [2,12], the authors presented details of the tactile sensor design and a possible use of tactile data for the reconstruction of the grasped wire shape and the use of the estimated shape for the implementation of an insertion task. In those papers, the model used for the wire was constituted by a quadratic function for the grasped area and a straight line for the part outside the tactile sensor pad. The sensor was mounted on a commercial gripper and preliminary insertion tests have been carried out on a single hole of an electric component fixed on the workbench, with the same wire grasped from a single position.

This paper presents improvements with respect to the previous system in terms of mechatronic solutions that are integrated and tested in a new scenario much closer to real cases. In particular, for this paper, the tactile sensor has been integrated into the final end-effector designed for the WIRES Project, presented and equipped with an electric screwdriver used to automatically complete the connection task. A simplified solution for the wire shape estimation is considered in order to allow its integration directly into the on-board microcontroller. This solution is a subset of that proposed in [12], but is explicitly formalized to be used with the final end-effector in the current study. The quality of the reconstruction has been re-evaluated with the new model, by redefining the quality metric according to the different model, in order to check if the considered simplification does not strongly affect the expected results. Finally, unlike previous papers, the whole system has been tested in a more complex scenario, by grasping, inserting and connecting a sequence of wires in a testing switchgear as shown in the video as supplementary.

#### **2. The Tactile Sensor and the Gripper**

The tactile sensor working principle and its design is detailed in [12]. Here, a brief recalling is reported (related to the integration in the gripper). Figure 1 reports some pictures of the developed sensor where the main components are highlighted. The 16 taxels constituted by the optoelectronic components with the deformable layer bonded above represents the transduction part for the sensor. The optical signals are converted in electric signals by using simple resistors and the obtained voltage signals are acquired with a standard Analogue-to-Digital converter. All details about the components integrated in the PCB are reported in [12]. For the integration into the gripper finger, a second PCB with a microcontroller has been developed and connected to the first one. The second PCB is completed by a voltage regulator and a standard connector, which allows to interface the tactile sensor with a standard USB-TTL serial cable. A suitably designed finger case has been realized in aluminum via a 3D printing technique and the extended PCB has been integrated inside this case. The thickness of the designed case is the smallest in order to allow the insertion of the finger among the switchgear components and wires already connected. The case allows a mechanical connection to the gripper by using two screw.

The end effector developed in the WIRES experiment for the implementation of the whole cabling process can be seen in Figure 2. The end effector integrates a 2D camera providing top view of the scene, an computer-controlled screwdriver (to tight the terminal screws) and a 4-DOFs gripper equipped with the tactile sensor. The end effector is also equipped with an integrated torque/controlled screwdriver with remote PLC control and process data recording capabilities (Kolver PLUTO3CA electric screwdriver + EDU2AE/TOP/E control unit). In the final process implementation, the robot arm is used to position the screwdriver tip on the terminal screw, and the FT sensor will be used to control the contact with the screw during the tightening. Therefore, the end effector will be held in an almost fixed position, just the screw motion during the tightening will be compensated. Consequently, the wire insertion will be performed by using the gripper DOFs only. It results that the FT sensor can be used to estimate the interaction between the screwdriver and the terminal screw, but it cannot be used during the insertion and for the wire tightening check, because the magnitude of the force generated by the wire contact is much lower than the one generated by the contact between the screwdriver and the screw, making the former indistinguishable. For this reason, the use of the tactile sensor installed

into the gripper fingertips is fundamental also during the insertion and for the wire tightening check, in order to reach a suitable success rate.

**Figure 1.** Some pictures of assembled tactile sensor: (**a**,**b**) report a front view and a rear view of the PCB components, respectively, while (**c**) reports the PCB integration into the gripper finger.

**Figure 2.** The end effector developed for the WIRES experiment. It is equipped with computercontrolled screwdriver, tactile sensor, 2D camera, Hydra servo controller boards, and a 4-DOF gripper.

Stepper motors with integrated encoder and lead screws have been adopted for the actuation of the end effector. This solution significantly simplifies the control and reduces the weight, the mechanical complexity and the cost of the end effector. Limit switches have been used for absolute position detection on both sides of all the end-effector movement axes. Each motor is driven by a Hydra servo drive control board, used as HW low-level motor controllers. These control boards are arranged on the end effector itself. The communication between the motor control boards and the high level WIRES controller is implemented through CAN bus. A ROS node has been developed to allow the control of the end effector and to ease the integration with other components of the WIRES system. At low level, the motors are controlled by means of the PLCOpen standard, allowing an easy implementation of the end effector controller. The tactile sensor has been integrated into the jaw tips (fingertips). Several versions of 3D printed fingers have been produced in order to evaluated different configurations during experiments.
