## 3 Experimental Setup & Procedure for Texture Discrimination  
### Experimental Setup  
For the robot fingertip module with multimodal sensing capabilities to discriminate the textures of objects, the module should be capable of vertically pressing (static contact) and rubbing (dynamic contact) objects. Hence, we used a motorized stage to develop a texture discrimination platform capable of both types of actions, as shown in Figure 2. The platform included a Z-axis motion stage, which enables the robot fingertip module with the mounted multimodal tactile sensor to touch the surface of samples in the vertical (Z-axis) direction, and an X-axis motion stage, which enables the module to scan the surface of an object it is in contact with at a constant velocity. A laser displacement sensor (ZX-LD100L, OMRON, Kyoto, Japan) was installed on the Z-axis motion stage to measure the distance between the module and the object to enable control of the speed at which the module approaches the object. Notably, the displacement data from the sensor becomes the standard for measuring the hardness of the object. If an object is pressed (indentation) with the same displacement, harder objects will produce larger contact forces whereas soft objects like a sponge will produce smaller contact forces. In addition, a tension block that provides compliance was installed to allow the robot finger mounted on the Z-axis stage to move according to irregularities on the object surface and maintain a preloaded force in a series of experiments so that variations of the contact pressure at a start of the sliding motion among samples could be minimized. Furthermore, the robot fingertip module was mounted on a rotary stage that was installed on the Z-axis stage; this enabled control over the area of the module that was in contact with the sample by changing the contact angle. A holder to place the texture sample corresponding to the object was installed on the X-axis motion stage. The holder, which had dimensions of W: 60 mm \(\times\) H: 150 mm, was designed in the form of a flat and angular u-shape with a depth of 2 mm so that a 140 mm-long texture sample could be fixed. In addition, the holder was made of steel to enable a pad-shaped rubber magnet located under each sample to attach to the holder, which aimed to minimize the movement of the sample during the measurement process. To embed the circuit board that processes the measurement signals from the tactile sensor onto the module, the circuit board was developed with a two layer-structure (dimensions: W: 18 mm \(\times\) H: 14.5 mm \(\times\) t: 5.2 mm). One layer consists of an analog strain gauge signal amplifier circuit and the other layer consists of an analog to digital converter and a digital circuit that performs computation and communication functions. Additionally, the length of wiring to connect the sensor and the circuit board was minimized, which also reduced the effects of external noise. Therefore, the noise levels of the gauges processed using the developed circuit board were maintained below 0.02%, which enabled sufficient discrimination of 0.5 gf Z-axis force signals. Furthermore, by setting the internal timer interrupt generated every 2 ms in the MCU (STM8L151G6) and measuring the voltage every interrupt cycle, the sensor signals could be processed at a sampling rate of 500 Hz. Figure 3 shows the communication method used to control the platform and the circuit board design.  
Figure 2: Photo showing the experiment of measuring a hard sponge (# 12) sample with a motorized texture discrimination platform equipped with a robot fingertip module.