基于微纳光纤的柔性仿生微结构触觉传感器研究封面文章
The rapid evolution of bionic flexible tactile sensors is driven by the overarching goal of emulating human tactile perception to augment robots' perceptual acuity. Conventional electric sensing paradigms grapple with a myriad of challenges, including elevated manufacturing costs and susceptibility to signal interference. Meanwhile, due to the small size, strong flexibility, and high sensitivity, optical sensing modalities are pushing micro/nano fibers (MNFs) into the spotlight. Domestically, the Zhejiang Lab is at the forefront of developing various MNF-based sensors, enabling single/dual-modal detection for applications in human-machine interaction and physiological parameter monitoring. Nevertheless, the challenge of balancing sensitivity and operational range remains unresolved in current methods, compounded by susceptibility to wear-related issues. Thus, we introduce a micro/nano fiber-based flexible tactile sensor unit inspired by fingertip skin microstructures (FIMF). By simulating the biological microstructures and tactile conduction mechanisms of fingertip skin, FIMF achieves the detection of mechanical stimuli and object feature recognition. The advanced sensor structure and functional attributes are significant for applications in flexible bionic devices and advanced robotics technology.
Firstly, the proposed flexible tactile sensing unit FIMF is inspired by the microstructure of fingertip skin and is achieved by embedding an MNF between two layers of polydimethylsiloxane (PDMS) films. The structure is further enhanced by introducing two layers of elastic resin annular ridges on the surface, each with varying stiffness. This design aims to replicate the intricate microstructure of biological fingertip skin and its underlying tactile conduction mechanism. Subsequently, we delve into the influence of PDMS film thickness and the dimensions of the annular ridges on the tactile pressure response of the FIMF sensor. Based on meticulous simulation results, the optimal sensor parameters are identified with a PDMS film thickness of 50 µm, an upper annular ridge thickness of 0.2 mm, and a lower annular ridge thickness of 0.4 mm. Furthermore, we extensively examine the FIMF sensor's response to diverse tactile stimuli, including static and dynamic pressure, and vibrations. Finally, the FIMF's ability to discern object hardness and surface textures is investigated by employing a synergistic approach integrating the mechanical finger's travel distance and the FIMF force feedback to discern object hardness characteristics. Meanwhile, we conduct waveform analysis of transmitted intensity changes over time to perceive and compute object texture. The pursuit of further insight into different textures is accomplished by the application of short-time Fourier transform (STFT) to extract frequency domain features.
The experimental findings underscore that the devised FIMF inspired by the microstructures of fingertip skin presents an amalgamation of wide-ranging dynamic detection capabilities and high sensitivity. Remarkably, it boasts response and recovery times of less than 100 ms, providing the sensor with the capacity to swiftly discern mechanical stimuli (Fig. 7). Furthermore, the sensor exhibits exceptional robustness and elevated static/dynamic stability, which is a testament to the robust encapsulation of its diverse structural layers (Fig. 8). Expanding its sensing range is proven instrumental in significantly enhancing the sensitivity for minute pressure ranges (0-2 N), thereby achieving an enhancement of approximately fourfold compared to recently reported MNF tactile sensors. A pivotal facet arises from the microstructure integration to amplify tactile mechanical stimuli and translate them into MNF deformations. This innovative approach does not need to employ tapering processes that would require reducing the MNF diameter to below 2 µm, which not only streamlines manufacturing but also augments the overall structural resilience (Table 1). In object hardness/texture perception, the FIMF divulges a pertinent trait that the transmitted intensity diminishes with the escalating hardness. This phenomenon arises because stiffer objects induce greater forces and stresses during contact, thus culminating in a more conspicuous attenuation of optical intensity (Fig. 9). The FIMF employs a spatial frequency-based characterization for discerning object texture, and the texture wavelength is derived by dividing the sliding speed by the dominant frequency. Additionally, the STFT of the transmitted light intensity signal provides a richer depiction of intensity fluctuations over time. During scans across regular surface patterns, the light intensity signal engenders periodic motifs at frequencies below 10 Hz. Notably, the number and positioning of these motifs amplify in tandem with increased scanning speeds in the temporal domain (Fig. 11).
We propose a novel micro-nano optical fiber flexible tactile pressure sensor inspired by the fingertip skin microstructure. This sensor combines force sensing with object hardness/texture detection capabilities. The sensor's force conduction performance is enhanced by bionic design to offer a wide detection range (0-16 N), high sensitivity (20.58% N-1), short response time (86 ms), extended lifespan, and low cost. By demonstrating its functionality, we directly connect this soft sensor to a robotic manipulator, enabling it to differentiate between soft and hard objects, perceive object textures, and measure gripping forces. Consequently, this sensor is suitable for robotic gripping operations. Thus, the proposed sensor possesses structural and functional features reminiscent of human fingertip skin and has promising potential for applications in bionic artificial skin and advanced robotics technology.
1 引 言
仿生柔性触觉传感器[1]能够更好地模拟人类的触觉感知,可感知和量化触觉刺激,例如应力、硬度和表面纹理,实现精准、快速和灵活的感知,从而提升机器人的感知和应对能力,近年来得到快速的发展。基于电传感方案的柔性触觉传感器,包括电容、电阻、压电和摩擦电机制等在过去十年中已得到广泛报道,这些电子触觉传感器通常模拟人类皮肤中精细触觉的生物特征,例如表皮-真皮界面、感觉受体、指纹模式和传入神经元中的离子刺激[2-5]。在电子触觉传感器中也尝试了多模传感能力和其他特殊特性,如自愈、自供电、能量收集、刺激可视化到环境适应,但它们仍然存在一些缺陷,包括制造成本高、寄生效应、电路复杂、信号串扰等可能会限制其在机器人中的实际应用[6-10]。基于光学传感方案的柔性触觉传感器是一种很有前途的方法,其中,微纳光纤(MNF)具有尺寸小、柔韧性强、灵敏度高、抗电磁干扰、易于制作等优点[11-13],在柔性触觉传感领域拥有独特的优势。近年来,国内之江实验室研究开发了若干类型的微纳触觉传感器,实现在应力[14-15]、应变[16]、滑动[17]、物体硬度[18]的单参数检测以及应力-温度[19]、应力-湿度[20]的双模态检测,并进一步探究在人机交互[21-24]和生理参数检测方面的应用,包括手势识别、脉搏波检测等。目前,基于微纳光纤的柔性触觉传感器多采用聚二甲基硅氧烷(PDMS)单一材料封装MNF,其传感范围和触感灵敏度无法兼顾,且存在容易磨损而造成MNF的损坏问题,适当的引入微结构设计能够有效解决此类问题。
本文提出一种受指尖皮肤微结构启发的微纳光纤柔性触觉传感单元(FIMF),以解决机器人集成光学皮肤中触感灵敏度和传感范围不能兼顾的问题,并实现对抓握过程的力传感和物体的特征识别。FIMF通过模拟指尖皮肤独特的生物微结构和触觉传导机制将触觉机械刺激转换为以透射光输出强度变化的形式,能够有效地检测静压、动压、振动等机械刺激。所提出的FIMF微结构由3D打印设备制造,这是一种高效、精度高和可定制化的制备方法。总体而言,该传感器制造技术是一种低成本的方法,不需要使用光刻或真空系统,所有使用的材料都是高度柔软的,有利于集成到不同表面系数的机器人设备中。这种先进可重复传感器的结构和功能特点是柔性仿生设备和先进机器人技术中非常需要的。
2 原理与仿真
2.1 传感器结构设计
人体指尖皮肤是一种特殊的皮肤类型,具有高度的触觉灵敏度,能够感知和区分静态和动态力、摩擦、振动等各种时空触觉刺激的能力,并能识别压力/滑动,感知所抓物体的形状、硬度和纹理[25]。如
图 1. 模拟人体手指皮肤的FIMF传感器示意图。(a)指尖皮肤的结构;(b)FIMF传感器的结构
Fig. 1. Schematic diagram of a FIMF sensor simulating human finger skin. (a) Structure of fingertip skin; (b) structure of FIMF sensor
受人类指尖皮肤独特的生物微结构和触觉传导机制的启发,本文提出了一种具有指纹结构和互锁微结构的FIMF触觉传感器,如
2.2 传感器结构力学仿真
为进一步研究FIMF的传感特性并优化传感性能,使用有限元软件COMSOL对FIMF触觉传感器进行参数化建模以及力学仿真。仿真参数如下:弹性树脂为超弹性材料,仿真采用Mooney-Rivlin双参数超弹性模型,其中,C10=3.7×105 Pa,C01=1.1×105 Pa,体积模量K=1×107 Pa,密度ρ=1.1×104 kg/m3,所制备的微纳光纤直径在10 µm以下,相对于各个层级的尺寸来说小一个数量级,所以仿真的模型不包括微纳光纤本身。
首先,考虑封装结构中PDMS上下两层厚度对FIMF性能的影响。分别设置两层PDMS的厚度范围为0~1 mm,其中PDMS接触层为MNF所在的平面,以其平均受力的大小表征MNF的应力情况。仿真结果如
图 2. 传感器各层厚度对性能的影响。(a)PDMS厚度对应力的影响;(b)环形脊厚度对应力的影响
Fig. 2. Effect of sensor layer thickness on performance. (a) Effect of PDMS thickness on stress; (b) effect of annular ridge thickness on stress
其次,在仿真中对FIMF施加1 N的法向力,结果如
图 3. 传感器力学响应。(a)光纤所在平面形变分布;(b)不同情况下应力分布;(c)手指按压滑动下的传感器输出响应
Fig. 3. Sensor mechanical response. (a) Planar deformation distribution where the optical fiber is located; (b) stress distribution under different conditions; (c) sensor output response under finger press sliding
3 传感器制备
利用单模光纤熔融拉锥可以得到非绝热锥形结构的MNF,以往的研究表明,当MNF直径小于12 μm时,模式干涉主要发生在HE11和HE12模式之间[27]。
式中:
式中:
图 4. 双模干涉式MNF结构示意图
Fig. 4. Schematic diagram of the structure of dual-mode interference MNF
模式有效折射率通过微纳光纤的色散方程[27]求得
式中:J1为一阶第一类贝塞尔函数;K1为一阶第二类修正贝塞尔函数;
图 5. 微纳光纤。(a)当波长为1550 nm时,HE11、HE12模式的有效折射率与MNF直径的变化;(b)直径为10 μm、5 μm和2.3 μm的MNF模拟光谱;(c)不同直径MNF的FSR在1550 nm附近的变化
Fig. 5. Micro-nano fiber. (a) Change of effective refractive index and MNF diameter of HE11 and HE12 modes when the wavelength is 1550 nm; (b) MNF analog spectra with diameters of 10 μm, 5 μm, and 2.3 μm; (c) changes in FSR around 1550 nm for different diameters of MNF
FIMF制作流程:将单模光纤剥去涂覆层用乙醇擦拭干净,将其放入光纤熔接机,设置放电量为105 bit,放电时间为2000 ms,放电次数为2次,进行放电操作,得到直径为70 μm的锥形结构,再采用火焰熔融拉锥的方法进一步制作得到非绝热突变锥,光纤拉锥机的氢气流速设置为225 mL/min,预绘制速度为120 µm/s,预拉制长度为10000 µm。所制作的MNF直径约为5 μm,腰椎长约为2200 μm的MNF显微镜图像如
图 6. 传感器实物图。(a)直径为5 μm的MNF显微镜图;(b)FIMF实物图;(c)封装前后透射光谱
Fig. 6. Physical diagram of the sensor. (a) Microscope diagram of MNF with a diameter of 5 μm; (b) physical drawing of FIMF;(c) transmission spectra before and after packaging
4 实验与讨论
4.1 力学性能实验及分析
如
图 7. 力学性能对比和标定。(a)力学性能测试实验系统图;(b)不同结构封装的传感器对比;(c)FIMF压力光谱响应;(d)FIMF灵敏度;(e)重复性;(f)响应/恢复时间
Fig. 7. Comparison and calibration of mechanical properties. (a) Diagram of the experimental system for mechanical property testing; (b) comparison of sensors with different structural packages; (c) FIMF pressure spectral response; (d) FIMF sensitivity; (e) repeatability; (f) response/recovery time
通过制备三种不同结构封装的MNF传感器(包括仅PDMS薄膜封装、上层环形脊加PDMS封装、互锁微结构加PDMS封装),实验对比研究FIMF结构各部分对于传感器性能的影响,如
表 1. 与其他微纳光纤触压觉传感器的性能比较
Table 1. Performance comparison with other micro-nano fiber tactile pressure sensors
|
依次对FIMF递增压力值,光透射强度逐渐降低,表现出稳定和连续的波动[
图 8. 力学性能测试。(a)递增压力响应曲线;(b)重复性测试;(c)1 N压力下不同频率传感器响应;(d)0.5 Hz频率下不同压力传感器响应
Fig. 8. Mechanical property tests. (a) Response curve for incremental pressure; (b) repeatability testing; (c) sensor response of different frequencies at 1 N pressure; (d) different pressure sensors response at 0.5 Hz
4.2 硬度感知实验及分析
在一定条件下,硬度与弹性模量之间成近似的线性关系,胡克定律定义发生弹性形变时,弹性体的形变量
图 9. 硬度感知。(a)实验系统图;(b)3个循环的硬度变化与透射强度的关系;(c)单次循环的强度变化与硬度变化的关系;(d)30次循环的硬度与透射强度的关系;(e)FIMF集成到机械手;(f)使用集成有FIMF的机械手抓握不同物体时的强度变化波形
Fig. 9. Hardness perception. (a) Diagram of the experimental system; (b) relationship between the change in the transmission strength and hardness of the three cycles ; (c) relationship between the change in strength and the change in hardness of a single cycle; (d) relationship between hardness and transmission strength of 30 cycles; (e) FIMF integration into the manipulator; (f) waveforms of intensity changes when gripping different objects using a robotic hand integrated with FIMF
接下来,将FIMF集成到双指机械手中[
4.3 纹理感知实验及分析
在使用FIMF感知物体纹理时,感知对象是物体表面微小的几何形状的变化,在这项工作中,纹理是基于空间频率来表征的。实验系统如
图 10. 纹理感知。(a)实验系统示意图;(b)纹理间距为4 mm的测试物;(c)接触扫描的响应波形;(d)不同压力下扫描的响应波形;(e)50 mm/s、100 mm/s扫描速度下的响应波形
Fig. 10. Texture perception. (a) Schematic diagram of the experimental system; (b) test objects with a spacing of 4 mm; (c) response waveform of the contact scan; (d) response waveforms scanned at different pressures; (e) response waveform at 50 mm/s and 100 mm/s scanning speed
图 11. 不同扫描速度。(a)响应波形;(b)FFT变换的频谱图;(c)FFT变换的时频图
Fig. 11. Different scanning speeds. (a) Response waveform; (b) spectrograms of FFT transforms; (c) time-frequency diagram of FFT transform
最后,为了验证该传感器的普遍适用性。利用3D打印技术制备了更精细的纹理测试物,并用1 mm/s的扫描速度扫描测试物。
图 12. 不同纹理测试物。(a)纹理间距为1 mm、0.5 mm测试物实物图;(b)响应波形;(c)FFT变换的时频图
Fig. 12. Test objects with different textures. (a) Physical drawings of test objects with texture spacing of 1 mm and 0.5 mm; (b) response waveform; (c) time-frequency diagram of FFT transform
5 结 论
本文提出了一种新型的基于手指皮肤微结构的微纳光纤柔性触压觉传感器,它同时具有力传感和物体硬度/纹理检测功能。该传感器通过仿生设计增强了传感器的力传导性能,具有宽检测范围(0~16 N)、高灵敏度(20.58 %N-1)、响应时间短(86 ms)、寿命长、成本低等优点。通过将该传感器直接连接到机器人机械手上进行演示,传感器能够区分软硬物体、感知物体纹理、测量夹持力等,该传感器适用于机器人夹持操作。这种新型柔性触觉传感器具有与手指皮肤相似的结构和功能特征,在仿生人工皮肤和先进机器人技术中具有潜在的应用前景。
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Article Outline
范成磊, 罗彬彬, 吴德操, 邹雪, 饶洪承, 周富民, 黄玲, 石胜辉, 胡新宇. 基于微纳光纤的柔性仿生微结构触觉传感器研究[J]. 光学学报, 2023, 43(21): 2106004. Chenglei Fan, Binbin Luo, Decao Wu, Xue Zou, Hongcheng Rao, Fumin Zhou, Ling Huang, Shenghui Shi, Xinyu Hu. Flexible Bionic Microstructure Tactile Sensor Based on Micro-Nano Optical Fiber[J]. Acta Optica Sinica, 2023, 43(21): 2106004.