Please cite this article in press as: T. Li, H. Ren, A hybrid FBG displacement and force sensor with a suspended and bent optical fiber configuration, Sens. Actuators A: Phys. (2017), https://doi.org/10.1016/j.sna.2017.11.032 ARTICLE IN PRESS G Model SNA-10462; No. of Pages 9 Sensors and Actuators A xxx (2017) xxx–xxx Contents lists available at ScienceDirect Sensors and Actuators A: Physical j ourna l h o mepage: www.elsevier.com/locate/sna A hybrid FBG displacement and force sensor with a suspended and bent optical fiber configuration Tianliang Li, Hongliang Ren ∗ Department of Biomedical Engineering, National University of Singapore, 117575, Singapore, Singapore a r t i c l e i n f o Article history: Received 2 June 2017 Received in revised form 17 November 2017 Accepted 17 November 2017 Available online xxx Keywords: Displacement sensor Force sensor Fiber bragg grating Bent optical fiber a b s t r a c t This paper has proposed a novel fiber Bragg grating (FBG)-based hybrid displacement and force sensor with a compact structure and excellent resolution by using the transverse property of a tightly suspended and slightly bent optical fiber. The optical fiber, embedded with an FBG element, has been suspended with its ends fixed on the sensor frame and implemented with a pre-tension force by the displacement loading along its vertical direction to form a bent shape. A conversion mechanism has been designed to convert the displacement and force inputs into the transverse movement of different points along the suspended fiber. Experimental results show that the displacement sensitivity and force sensitivity are −219.69 pm/mm within the range of 0–2.5 mm and −345.2 pm/N with a high calculated resolution of 2.9 mN, respectively. Results from both the displacement and force experiments have illustrated a close agreement with values from commercial sensors. © 2017 Published by Elsevier B.V. 1. Introduction Displacement measurement is a significant monitoring index for evaluating structural health conditions in many fields such as, civil, industrial and aerospace engineering [1,2]. Various electri- cal sensing techniques such as capacitance and magnetoresistive displacement sensors, have been widely utilized to measure dis- placement for structural health monitoring [3–5]. However, these techniques are susceptible to high-intensity electromagnetic (EM) interference, and make multi-point distributed detection difficult to achieve. Compared with the above-mentioned traditional displacement sensors, FBG-based displacement sensors offer remarkable advan- tages such as miniature size, strong corrosion resistance capacity, immunity from EM interference, and multiplexing capabilities [6–9]. Therefore, they have attracted considerable attention and have been increasingly introduced for displacement detection. Many researchers have proposed FBG-enabled displacement sen- sors based on cantilever beams, improved beam forms, or other mechanical elastomers in Fig. 1a) [10–17]. Most applied the bandwidth demodulation of the reflected spectrum from optical spectrum analyzer (OSA) due to easy implementation [10–15]. For instance, Yu et al. have designed a composite isosceles cantilever ∗ Corresponding author. E-mail address: ren@nus.edu.sg (H. Ren). beam with two different sections. An FBG element was glued across the joint between two sections of the composite beam. The FBG reflective spectrum bandwidth was measured to determine the dis- placement exerted at the free end of the designed beam. However, the length of its elastomer structure is up to 112.5 mm, and the sensing properties are limited by the low resolution of OSA [10]. The implementations in [10–14] have only verified the working principle of the proposed FBG-based displacement measurement methods, but without fabricating sensor prototypes. Moreover, it is difficult for these proposed sensors to support multi-point distributed detection. To overcome these limitations, the wave- length demodulation has been applied to improve the displacement measurement resolution by pasting FBG elements on the elas- tomer surface [16–18]. This approach can overcome the common drawbacks associated with the use of an OSA. However, their imple- mentations are limited by the FBG-gluing process and suffer from uniform strain distribution along the pasted FBG element. To solve these aforementioned issues, some scholars have directly used the axial property of a suspended optical fiber inscribed with FBG elements to detect displacement. The embed- ded FBG elements are compressed and stretched to sense the produced strain along the axial direction, as shown in Fig. 1b) [19–21]. However, they suffered from some drawbacks, such as a low sensitivity (∼30 pm/mm [19]), a very small working range (400 m [20], and 140 m [21]), and a poor resistance capacity for compressive impact loading. Moreover, it is difficult for this configuration to achieve distributed displacement detection due https://doi.org/10.1016/j.sna.2017.11.032 0924-4247/© 2017 Published by Elsevier B.V.