A Capacitive-Proximity-Based Gripper for Service Robots to Measure the Liquid Level in Opaque Cups

Service robots are often required to determine the liquid level inside a container. However, traditional approaches using camera-based or ultrasonic wave sensors are unreliable and can introduce challenges in movement trajectory planning. In this study, we developed a novel gripper system that is equipped with a capacitive proximity sensor, enabling the robot to sense the liquid through the container. In addition, we developed a control module that ensures easy integration with a robot operating system (ROS)-based control systems commonly used in service robots. Our gripper represents a significant improvement over traditional camera-based or ultrasonic wave sensor solutions. Laboratory experiments revealed that our gripper can perform data-lossless detection in just 0.085 s (11.8 Hz) and dynamic liquid-level measurement in 0.31 s. Furthermore, the liquid-level measurement resolution of the gripper is sufficient for functional applications of service robots in a domestic environment. The present work demonstrates that a gripper with capacitive proximity sensors is a practical and cost-effective solution for service robots to manipulate opaque cups filled with liquid, which are frequently encountered in domestic environments.