Triaxial 3-D-Channeled Soft Optical Sensor for Tactile Robots
Robotics is an expanding field with various applications in industries like healthcare, manufacturing, and logistics. Tactile sensing enables robots to interact and perform challenging tasks. The existing optical sensors often either lack the flexibility of soft materials or do not provide the precision needed for accurate measurement in three dimensions. Hence, soft optical sensors have emerged as an enabling technology with several advantages, including durability and immunity to electromagnetic interference.
The paper introduces a novel triaxial 3D-channeled soft optical sensor designed to measure forces in three axes (x, y, and z) using light signals, making it robust, versatile, and suitable for applications in robotics and tactile sensing. Fabricated with a 3D-printed mold, the sensor uses polydimethylsiloxane (PDMS) infused with black ink for enhanced light absorption and titanium dioxide for shielding against external interference. The materials are cured at 60°C, and although the outer surface shows minor imperfections, these do not impact performance.
The sensor’s dome-shaped, flexible structure includes a central light emitter (PE) at the top, which directs light into optical channels. Three light detectors (PRs) spaced 120° apart at the base capture the transmitted light. Optical channels guide the light efficiently, while a dark absorbing spot, which shifts position under applied forces, alters the light path to measure changes in force. Reflective materials surrounding the structure ensure light remains confined within the channels and improve precision.
When no force is applied, light flows evenly through all channels. A normal force causes equal reductions in light across all channels due to the absorbing spot’s movement. A tangential force shifts the absorbing spot unevenly, altering light signals depending on the force’s direction. This dynamic enables accurate detection of both force magnitude and direction.
Simulations using COMSOL Multiphysics validated the sensor’s design, and tests were conducted on three prototypes: one with an 8.5 mm base radius and 4.4 mm channels, a scaled-down version with a 3.9 mm base radius and 2 mm channels, and a bulk model without channels. The channeled design proved superior, capturing 15–20 times more light than the bulk model. The larger prototypes consistently produced higher voltage outputs and improved force detection capabilities.
The sensor demonstrated high sensitivity, with an output of 11.02 mV/N for the z-axis, detecting forces as small as 3 mN and handling forces up to 8.5 N (z-axis) and 4.05 N (x-y plane) without saturation. It accurately converted forces into Cartesian coordinates with an angular error of 16.33°, ensuring practical usability. In a robotic setup, the sensor measured indentations up to 2 mm at 0.1 mm/s across 24 directions in the x-y plane, showing precision in detecting force magnitude and direction.
The sensors integrated into a robotic hand enabled secure object handling, weight change detection, and slip prevention, demonstrating the potential for applications in robotics, tactile feedback systems, and soft grippers. Challenges in fabricating smaller prototypes highlight opportunities for further refinement and development. This sensor has potential applications in robotics, where accurate force sensing is essential for dynamic interactions.