# Human-Touch-Inspired Material Recognition for Robotic Tactile Sensing
## 3 Methodology
### Experimental Setup  
**Tactile Sensor**: A tactile sensor (Pressure Mapping Sensor 5101, Tekscan, South Boston, MA, USA) is employed to acquire the tactile data, as shown in Figure 2a. The sensor consists of two thin, flexible polyester sheets that have electrically conductive electrodes deposited in varying patterns. The inner surface of one sheet forms a row pattern, while the inner surface of the other employs a column pattern. The intersection of these rows and columns creates a sensing element, and a force sensitive material is placed between these two mating sheets. The spacing between the rows and columns is 2.5 mm, and the total thickness is 0.1 mm. Since the sensor is flexible, large-scale, and ultra-thin, it can be overlaid on the robot, making material recognition more achievable. The sensor consists of \(44\times 44\) sensing elements, it is high-density, and capable of monitoring the pressure distribution of the contact surface and generating tactile images that are relevant to the compliance and texture of the material, which enriches the tactile data and allows robots to collect enough data in a short-duration exploration. Pressures are converted into intensities of tactile images, which range from 0 to 255. Similar to human skin, a full range of information, including sustained downward pressure, slippage, object position, object shape, and vibration, can be detected by processing the tactile data. Figure 1: The processes of the material recognition method. **Slide Action**: Mimicking the dynamic tactile sensing that humans employ to explore an unknown material [30], a short-duration slide action was designed on the test materials to collect tactile data in 1 s. For robots, implementing the slide action is easier than grasp and other actions; for safety, the slide action is less harmful than the press action. Additionally, vibration caused by slide action is useful for material recognition. Hence, the slide action is adopted as the exploratory procedure in this paper. Moreover, 1 s sliding time improves the rapidness efficiency of the method. In the slide action, the sensor was dragged across the surface of the material at a constant force. In our experiments, a linear motion platform (M-403, Physik Instruments, Karlsruhe, Germany) was used to drag the sensor at a speed of 2.5 mm/s, while a standard 200 g weight is placed on the sensor to provide the constant interactive force. The dragging speed is slower than the sliding speed used by humans, as the slow slide can prevent robots from harming humans or objects. The 200 g load makes the interactive force fluctuate within a range that will not hurt objects or humans. Moreover, the exploration area on the material surface of each slide action is different, which increases the robustness of the tactile data. Figure 2b illustrates the slide action. **Test materials**: To make the test materials representative, 12 materials with different textures and compliances were selected. The 12 materials are foamed silicone rubber (FSR), sponge, cotton, polyvinylchloride (PVC), silicone rubber (SR), rubber, sandpaper (80 Grit), wood, iron (unpolished), ceramics, copper, and acrylic (shown in Figure 2c). These materials were divided into four categories: soft-rough, soft-smooth, rigid-rough, and rigid-smooth, which covers properties of most objects. The individual materials and the categories to which they belong are shown in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Soft-Rough** & **Soft-Smooth** & **Rigid-Rough** & **Rigid-Smooth** \\ \hline FSR & PVC & Sandpaper & Ceramics \\ Sponge & SR & Wood & Copper \\ Cotton & Rubber & Iron & Acrylic \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The test materials and the categories to which they belong. Figure 2: The tactile data acquisition of test materials (**a**) The tactile sensor; (**b**) the sensor was slid across the material surface by the motion platform.