# Soft Gripper with EGaIn Soft Sensor for Detecting Grasp Status
###### Abstract
Thus, tendon-driven soft grippers are more prevalent, as the system's stiffness can be varied. If the stiffness of the system increases, the gripping force increases, so the weight of the object that the soft gripper can lift increases. However, if these soft grippers gripped objects of more than 2 kg, they would slide due to a weight greater than the maximal static friction force. Therefore, to improve the performance of the soft gripper, it is necessary to monitor the grip of the soft gripper to make sure that it is gripping within the gripper system state. The rigid condition of typical sensors has limited the ability to monitor grip forces. Because these sensors have a fixed shape, they cannot always produce accurate outputs for every action completed, as their shape can differ from that of the gripper. Soft sensors that conform to the shape of a gripper provide a solution for this discrepancy of shape. A range of soft sensors for monitoring gripping forces has been studied. Reference [11] describes a soft sensor consisting of hydrophilic PTFE as the sensitive dielectric element of the relative humidity-based grasp sensor. In contrast, [12] describes a capacitive strain sensor for force monitoring created by combining silicone and fabric, providing a larger capacity and stiffness than fiber-based sensors and a larger elastic range than silicone-based sensors. Sensors that use EGaIn, a liquid eutectic mixture of gallium and indium as a sensing element, have also been studied. To respond to microstrain, [PERSON] et al., developed an EGaIn sensor with a wavy pattern to reduce hysteresis [13]. [PERSON] et al., improved the sensing response of their EGaIn sensor by applying pressure to it through a rigid structural block [14]. [PERSON] et al., designed their EGaIn sensor path to be thin and fabricated the sensor as a multilayer structure to increase its responsiveness [15]. Finally, [PERSON] et al., produced a hybrid soft elastomer strain sensor in which signal routing was isolated from detection, such that the sensor reacted less sensitively to unnecessary stresses on wiring [16]. In this paper, a silicone integument was added to the gripper to increase the gripping friction force. Various soft sensors have been considered for application to the integument of this gripper fingertip. Therefore, there is a need for a manufacturing method that can be easily applied to the curved shape and integument of the soft gripper fingertip. The manufacturingmethod of the EGaIn soft sensor is made easy by injecting EGaIn into a fabricated path. In addition, EGaIn is suitable because of its high conductivity and the small number of processes required for fabricating the soft sensor. The soft gripper is designed based on previous studies [4; 5] that performed the optimal design of variable-stiffness soft grippers. The previous study established the basis of the characteristics of each parameter of the variable-stiffness structure and the shape of the gripper. In this study, a soft gripper with a variable-stiffness structure driven by a tendon was characterized based on the deformation according to the gripping target. This gripper was applied with an EGaIn soft sensor at the end of the grip to increase the gripping freedom and detect the state of the object grip. This EGaIn soft sensor measures the gripping force applied to an object, and the performance of the sensor was verified through an experiment to monitor the sensing response. The soft gripper is capable of autonomous gripping that enables it to firmly fix various objects without damage by checking the sensing value, grasping the grip state, and adjusting the appropriate gripping force. The weight of the gripping target could be inferred from the sensor's value, and the verification confirmed that an autonomous grip that can firmly grip various objects without damage was possible.