Classification of Vision-Based Tactile Sensors: A Review

Vision-based tactile sensors (VBTSs) have gained widespread application in robotic hands, grippers, and prosthetics due to their high spatial resolution, low manufacturing costs, and ease of customization. While VBTSs have common design features, such as a camera module, they can differ in a rich diversity of sensing principles, material compositions, multimodal approaches, and data interpretation methods. Here, we propose a novel classification of VBTSs that categorizes the technology into two primary sensing principles based on the underlying transduction of contact into a tactile image: the marker-based transduction (MBT) principle and the intensity-based transduction (IBT) principle. MBT interprets tactile information by detecting marker displacement and changes in marker density. In contrast, IBT maps external disturbances with variations in pixel values. Depending on the design of the contact module, MBT can be further divided into two subtypes: simple marker-based (SMB) and morphological marker-based (MMB) mechanisms. Similarly, the IBT principle encompasses the reflective layer-based (RLB) and transparent layer-based (TLB) mechanisms. This article provides a comparative study of the hardware characteristics of these four types of sensors, including various combination types, and discusses the commonly used methods for interpreting tactile information. This comparison reveals some current challenges faced by VBTS technology and directions for future research.