## 4 Results and Discussion  
### Multimodal Sensing Capabilities of the Tactile Sensor  
The repeatability and stability characteristics of a sensor are as important as multimodal sensing capabilities when discriminating the textures of objects based on various tactile modalities. Even if a sensor is able to measure numerous physical quantities, the reliability of its measurements is compromised if the sensor has low stability and repeatability characteristics. For example, a sensor with such characteristics would produce fluctuating readings even when measuring a single sample, greatly hindering the effectiveness of learning. As such, it is vital to conduct repeatability and stability evaluations in addition to evaluations of the physical quantities that can be detected by the robot fingertip module with the tactile sensor. To evaluate the vertical (Z-directional) force measurement capability, the robot fingertip module was placed on a precision balance (BSA6202S-CW, Sartorius, Gottingen, Germany) with a resolution of 0.01 g and a metal tip with a diameter of 3 mm was attached to the indenter connected to the 3-axis stage [16]. The metal tip was placed on the center of the bump of the tactile sensor built into the robot fingertip module, and the vertical force measurement capability of the sensor was evaluated. The vertical direction (Z-axis) force measurement capability (\(R_{\mathrm{z}}\)) serves as a measure of the two aforementioned characteristics as well as the range of force the sensor is capable of detecting, and is calculated as the average rate of change in resistance of the four strain gauges [33]. This can be expressed by the following equation:  
\[R_{\mathrm{z}}=\left(\frac{1}{4}\times\left(\frac{\Delta R_{\mathrm{N}}}{(R_ {\mathrm{N}})_{0}}+\frac{\Delta R_{\mathrm{E}}}{(R_{\mathrm{E}})_{0}}+\frac{ \Delta R_{\mathrm{S}}}{(R_{\mathrm{S}})_{0}}+\frac{\Delta R_{\mathrm{W}}}{(R _{\mathrm{W}})_{0}}\right)\right)\times 100(\%) \tag{1}\]  
\[\Delta R_{\mathrm{i}}=R_{\mathrm{i}}-\left(R_{\mathrm{i}}\right)_{0} \tag{2}\]  
where \(\Delta R_{\mathrm{i}}\) is the change in resistance of strain gauge \(\mathrm{i}\); \(\left(R_{\mathrm{i}}\right)_{0}\) is the initial resistance value of strain gauge \(\mathrm{i}\); and \(\mathrm{i}\) refers to the strain gauge located in the N, E, S, or W directions. As a result of evaluating the vertical direction (Z-axis) force measurement capability using this equation, it was found that the hysteresis error was low at 2.66% within the force range of 0-100 gf, and the sensor possessed the sensitivity to sufficiently discriminate forces as small as 0.5 gf. In addition, the maximum detection force (100 gf) was applied to the module 1000 times to evaluate the repeatability characteristics. As a result, the repeatability error  
Figure 5: The picture of 16 tactile samples used in the tactile identification experiment. was also low at 1.96%, highlighting the excellent repeatability and stability characteristics of the sensor. Data on the sensor characteristics are shown in Figure 6a,b. The horizontal direction (X- and Y-axes) force measurement capabilities of the sensor are important as these physical quantities are used to measure the friction coefficient of samples, which is not possible with the vertical direction (Z-axis) force measurement capability. The difference in resistance change rates between the two gauges located along the X-axis was used to calculate the X-axis force (\(R_{\mathrm{x}}\)), and the Y-axis force (\(R_{\mathrm{y}}\)) was calculated in the same manner using the difference in resistance change rates of the two gauges located along in the Y-axis [33].