4.1 Sensing platform
In order to measure the stress and strain for tactile sensing with flexible and stretchable waveguide, a flexible tactile sensing platform was built. The whole flexible and stretchable optical waveguide experimental platform is shown in Fig. 6(a), mainly includes the following process: 1) Incident light source. A laser point with wavelength located at 632.8 nm is selected as incident light. 2) Light source position and posture adjustment device. It is a mechanical device used to fix the position of incident light source and adjust its incident angle in real time. 3) Tensile measurement device. The tensile measurement device composed of Vernier Caliper and non-standard fixed parts, which can be used to accurately measure the initial length of flexible and stretchable optical waveguide and the corresponding stretching length variation during the experiment. 4) Photodetector. The photodetector PM100D (Thorlabs, Inc.) has a light intensity detection range from 500nW to 500 mW. In this experimental platform, the photodetector is used to detect the output light intensity variation of the flexible and stretchable PDMS based optical waveguide, and the related pressure and strain can be calculated based on the change amount of output light intensity. This tactile sensing experimental platform is low cost, compactable and can be used to detect pressure and strain for tactile sensing. The strain precision can reach to 0.14%, since the original length of the flexible and stretchable optical waveguide is 14 mm and the precision of Vernier Caliper is 0.02 mm. At the same time, the photodetector is used to detect the variation of the output light intensity, and the resolution of photodiode probe is 10 PW. The flexible and stretchable optical waveguide fabricated by nano replica molding is shown in the figure. The colorful square area is the input and output part of the flexible and stretchable optical waveguide, and the transparent area in the middle area is the light transmission area. The colorful effect is generated by light diffraction on the grating surface. The flexible stretchable optical waveguide is shown in Fig. 6(b), the colored area is the input and output port of the flexible stretchable optical waveguide, and the middle transparent area is the transmission area of the optical waveguide. The color image of the grating coupling input and output port is caused by the diffraction of light on the grating surface.
4.2 Tactile Sensing Experiments
In tactile sensing, pressure and strain are two physical quantities that are often involved in robotic tactile sensors when interact with external environment. Real-time and accurate perception of pressure and strain can enable robots precisely capture the degree of mechanical deformation in interaction with external objects, so as to facilitate subsequent optimal feedback operation.
The testing method for the flexible and stretchable optical waveguide is as follows:
1) A stable light beam is used to incident into the waveguide transmission layer of the flexible and stretchable optical waveguide through the coupling grating at a fixed angle. At the other end of the optical waveguide device, a photodetector is used to gather the output light from the output grating coupler. 2) When an external force is applied on the flexible and stretchable optical waveguide, the structure if the optical waveguide will change, which leads the attenuation of the output light intensity. By analyzing the attenuation of the light intensity, the external force can be accurately measured. 3) When an external strain is applied on the flexible and stretchable optical waveguide, the strain can also be accurately measured according to the variation of output light intensity. The pressure test for flexible and stretchable optical waveguide was carried out. In this experiment, the flexible and stretchable optical waveguide is fixed by two sliding heads of Vernier Calipers, and a 632.8 nm laser source is tuned to couple into the input grating port at an optimal angle. The position of the optimal angle is related to the maximum power received by the power meter at the output end of the grating. In the middle region of the flexible and stretchable optical waveguide, a pressure meter is used to gradually apply pressure to it, and the corresponding data of the pressure value and the light intensity is recorded.
The experimental results are shown in Fig. 7(a). According to the figure, the output light intensity of the optical waveguide decreases with the increase of applied pressure, and there is a linear correlation between the pressure change and the output coupling light intensity. The pressure sensing range of the flexible and stretchable optical waveguide is 0 to 25 × 10− 3 N.
The strain sensing experiment of flexible and stretchable optical waveguide is carried out by stretching mechanism with scale. Firstly, the flexible and stretchable waveguide is prestretched to avoid bending due to gravity, so that it is in horizontal state, and its initial length L0 is recorded by the vernier caliper. Then, the flexible and stretchable optical waveguide is stretched by the clamping mechanism at both ends of the vernier caliper, and the length after stretching is recorded as L, then the corresponding strain S can be calculated as:
(4-1)
The experimental results of strain sensing based on flexible and stretchable optical waveguide are shown in the figure. According to the figure, with the increase of applied strain, the output optical intensity of the flexible and stretchable optical waveguide decreased gradually. Moreover, the optical power decreased as applied strain increasing, and there is a linear correlation between them. Meanwhile, the strain sensing range of flexible and stretchable optical waveguide is 0 to 13%, with a strain precision of 0.14%, as shown in Fig. 7(b).