Dual-layer orthogonal fiber Bragg grating mesh ... - OSA Publishing

Vol. 25, No. 20 | 2 Oct 2017 | OPTICS EXPRESS 24727

Dual-layer orthogonal fiber Bragg grating mesh based soft sensor for 3-dimensional shape sensing LI XU, JIA GE, JAY H. PATEL, AND MABLE P. FOK* Lightwave and Microwave Photonics Laboratory, College of Engineering, The University of Georgia, Athens, GA 30606, USA * [email protected]

Abstract: A soft shape sensor for 3-dimensional object shape measurement is demonstrated. The proposed sensor is based on dual-layer fiber Bragg grating arrays with an orthogonal mesh structure, which enables multi-point bi-directional shape sensing. The 3D shape reconstruction is based on a bi-directional curvature measurement at each sensing point, which is achieved through measuring the direction and amount of wavelength shift of each off-center embedded FBG. The conversion coefficient between the wavelength shift and bending curvatures is acquired and is used to convert the change in FBG to the corresponding bending curvatures and bending direction. The measurement error on the bending radius of each sensing point is about 2.7%. A 3D shape of the object surface is reconstructed with the help of a curve fitting method based on the curvature information across the whole FBG mesh. This design successfully achieved visualized 3D shape sensing, which has great practical value in soft robotics and biomedical applications. © 2017 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.3735) Fiber Bragg gratings; (150.6910) Three-dimensional sensing.

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#303289 Journal © 2017

https://doi.org/10.1364/OE.25.024727 Received 26 Jul 2017; revised 31 Aug 2017; accepted 31 Aug 2017; published 28 Sep 2017


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1. Introduction 3-Dimensional (3D) shape sensor is an essential tool in a variety of applications such as soft robotics, minimally invasive surgery and human body monitoring. In soft robotics, 3D shape sensing is typically used for the real time positional and shape information estimation [1], while in minimally invasive surgery shape sensing focuses on orientation measurement along a 3D curve [2]. Human body monitoring requires a shape sensor to monitor shape and volumetric changes of the human body [3]. In recent years, electronics based 3D shape sensors have been proposed to accomplish 3D geometry reconstruction [1,4], body shape change measurement [3], and surface shape measurement [5,6]. However, electronics based approaches are easily interfered by electromagnetic signals, they usually have limited performance in extreme environment. Due to the unique features of fiber optics – small size, high sensitivity, and immunity to electromagnetic interference [7], fiber optics technique is considered as a promising candidate and high-efficient technique for shape sensing. Over the last decade, researchers have been exploring different fiber optics schemes to demonstrate shape sensing for a wide range of applications. Most existing fiber optics approaches [8–10] show outstanding performances and capabilities comparing with electronics approaches, however, they are focusing mainly on 3D curve sensing along an optical fiber or a line instead of shape sensing of an object surface. Among the fiber optics approaches, fiber Bragg grating (FBG) with various structures [11– 19] show promising sensing capability. Unlike other techniques that require complicated sensor fabrication techniques, standard FBG is a simple device and has been a mature technology for sensing. However, most of the standard FBG based shape sensing approaches have limitations on 3D shape sensing – uni-directional curve sensing [13,14] or having limited resolution [19]. Thus, a new simple 3D shape sensor with higher resolution is highly desired in biomedical and soft robotics applications. In this paper, a compact soft 3D shape sensor utilizing a dual-layer orthogonal FBG mesh that can measure the complete 3D shape of an object surface is proposed and demonstrated. The sensor consists of 18 standard FBGs that are embedded inside a thin and flexible silicone rubber sheet. The 18 FBGs are aligned orthogonally in a dual-layer mesh structure with 9 FBGs in the x-direction at the top layer and 9 FBGs in the y-direction at the bottom layer. Embedding the FBGs in two layers enables the sensing of both convex and concave object surfaces, while the two orthogonal layers decompose a curved surface into two orthogonal axes. The FBGs are embedded in and protected by the soft silicone rubber, allowing the FBGs to be bend along the bending curvature of the measured object due to the flexible property of silicone. The proposed scheme can measure the complete 3D shape of an object surface simply by placing the sensor on top of the object. The measured results are plotted out in a 3D visual figure, which provides good intuition of the 3D shape of the measured object. The compact and soft silicone rubber is highly compatible with human body, making the proposed sensor a promising design for wearable monitoring devices and medical robotics.


2. Principle and fabrication The 3D shape sensor is composited of 18 FBGs and a silicone sheet of 84 mm × 84 mm in size and 5 mm in thickness. 18 FBGs with different center wavelengths are embedded in the silicone sheet, resulting in 18 sensing points in two orthogonal directions. The FBGs are embedded in two off-center layers (red and green) that enables the measurement of shape change in both concave (-z) and convex ( + z) directions. The top layer FBGs (green) are embedded 1 mm below the top surface, while the bottom layer FBGs (red) are embedded 4 mm below the top surface (i.e. 1 mm above the bottom surface). According to our previous study [16], 1-mm silicone sheet outside the fiber can provide enough protection to the FBGs while enabling good sensitivity at the same time. The top and bottom layers each has 9 FBGs embedded orthogonally in x- and y- directions, such that the curvature information of an object surface can be decomposed into two orthogonal axes. The x-direction FBGs (green) are arranged into three rows, while the y-direction FBGs (red) are arranged into three columns. Each FBG is 15 mm long and is placed with a 10-mm gap between adjacent FBG. The distance between adjacent row/column is 26 mm and is 16 mm from the edge of the silicone sheet. All the FBGs are written on the same fiber, enabling one single measurement for capturing information from all the 18 FBGs. The design of the proposed soft 3D shape sensor is shown in Fig. 1.

Fig. 1. (a) Schematic diagram of the proposed 3D shape sensor based on dual-layer orthogonal FBG mesh. Green gratings: FBGs at the top layer; Red gratings: FBGs at the bottom layer. (b) Prototype of the fabricated soft 3D shape sensor. (c) Illustration of different layers of the silicone sheet and the dual-layer orthogonal FBG mesh structure.

To fabricate the sensor, Ecoflex® 00-10 from Smooth-On Inc. is used as the silicone material, which is flexible, ductile, and can be recovered to the original shape after many times of measurement, such that a good repeatability is obtained for the shape measurement. The silicone material has tensile strength of 120 psi, maximum elongation of 800%, shrinkage of