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触觉作为人体重要的感官之一,是感知外部环境信息、与外界信息进行交互的主要媒介[1-3]。近年来,电子皮肤(e-skin)基于力学敏感材料模拟人体皮肤在触觉感知方面的优异功能得到了广泛关注,通过重塑触觉感知功能在可穿戴电子、软体机器人、医疗健康、虚拟现实和人工智能等领域展现出重要的应用价值[4-8]。传统的硅基、金属应变片式触觉传感器用作电子皮肤时在柔性、延展性及穿戴舒适性等方面存在弊端,为模仿人体皮肤的触觉感知特性,具备柔性、可拉伸、高灵敏度等特点的触觉传感器成为国内外电子皮肤的研究热点。
随着智能材料与制备工艺的不断发展,旋涂成膜、微流体成型、层层组装、浸渍包覆、3D打印等技术被广泛应用于柔性电子学领域,如柔性天线[9-12]、柔性电子皮肤[13-15]、柔性电极[16]等。柔性触觉传感器按敏感机理可分为电阻式、压电式、光电式和电容式等几类[17-18],其中,电容式柔性触觉传感器因具备优良的动态响应特性和检测灵敏度在电子皮肤研究中得到了广泛应用[19-20]。
设计具有微结构特点的复合介质层是提升电容式柔性触觉传感器灵敏度的常用方法,文献[21]通过在纸基表面依次制备柔性电极和表面粗糙状弹性介质层,并通过层层组装工艺提出了一种高灵敏度电容式柔性触觉传感器。文献[22]基于荷叶疏水特性,以荷叶表面固有的微结构为模板制备复合介质层电容式柔性触觉传感器,可实现高灵敏(0.815 k·Pa-1)和快速(~38 ms)触觉感知。制备微结构复合介质层通常需要繁琐的工艺且多应用于类平行板结构的电容式柔性触觉传感器研究中,可实现高灵敏度法向力感知,然而不具备切向力检测能力或灵敏度较低。为此,研发具有高灵敏度法向力和切向力触觉感知功能的电容式柔性触觉传感器仍是电子皮肤研究所面临的问题之一。
本文通过在半球型柔性腔体内壁等分设置4个呈空间立体排布的球曲面感应极板,与底部柔性公共极板组成电容柔性触觉传感器,并构成差分式结构,相比于传统类平行板结构的电容柔性触觉传感器,呈空间立体排布的球曲面感应极板和差分式结构特点更有利于提升触觉感知灵敏度。结合理论计算与ANSYS有限元仿真阐述了本文电容式柔性触觉传感器的感知机理,同时,基于AD7147-1和STM32构建容性触觉信息采集与分析系统,并对本文提出的触觉传感器进行性能表征,实验结果论证了基于球曲面极板的电容式柔性触觉传感器用作电子皮肤的可行性。
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基于球曲面极板的电容式柔性触觉传感器的工作原理可等效为变极板间距式类平行板电容器,首先,通过理论计算分析本文电容式柔性触觉传感器输出电容与结构参数之间的函数关系,考虑其极板结构具有非平面、面积不相等的特点,基于微积分原理,将球曲面感应极板电容器等效为无限多个微电容器(类平行板电容器)的级联[24],并根据电容器级联规律进行积分。图 3为单个球曲面极板电容器参数结构示意图,如图 3a所示,假定半球型触头内腔半径为r,感应极板(厚度忽略不计)两端距离半球型触头内腔垂直中心边和水平边的弧长分别为m和n(0 < m,n < πr/2)。如图 3b所示,球曲面极板上各点与水平底端夹角θ和底端投影长度l满足式(1),当夹角增加dθ时,其投影宽度为rdθ,在dθ极小时,该微电容可视为类平行板电容器,微电容满足式(2),将球曲面极板电容器看作各微电容并联而成,则总电容C如式(3)所示。
$$l = r\cos \theta \left( {\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r} - \frac{n}{r}} \right)$$ (1) $${\rm{d}}C = \frac{{{\varepsilon _r}{\varepsilon _0}r\cos \theta \left( {\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r} - \frac{n}{r}} \right)r{\rm{d}}\theta }}{{r\sin \theta }}$$ (2) $$C = \int_{\frac{n}{r}}^{\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r}} {\frac{{{\varepsilon _r}{\varepsilon _0}r\cos \theta \left( {\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r} - \frac{n}{r}} \right)r{\rm{d}}\theta }}{{r\sin \theta }}} =\\{\varepsilon _r}{\varepsilon _0}r\left( {\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r} - \frac{n}{r}} \right)\ln \frac{{\sin \left( {\frac{{\rm{ \mathsf{ π} }}}{2} - \frac{m}{r}} \right)}}{{\sin \frac{n}{r}}}$$ (3) 式中,ε0为真空介电常数(ε0=8.85×10-12 F/m);εr为相对介电常数。
基于球曲面极板的电容式柔性触觉传感器工作原理如下:触觉传感器在未受力时,设初始电容分别为$C_1^0$、$C_2^0$、$C_3^0$和$C_4^0$,受力F作用时4个电容的输出变化量分别为$\Delta {C_1}$、$\Delta {C_2}$、$\Delta {C_3}$和$\Delta {C_4}$,基于ANSYS有限元仿真分析外力作用下对球曲面感应极板的影响规律,简化后的球曲面极板电容式柔性触觉传感器应力应变结果如图 4所示。由仿真结果可以看出,在法向力FZ作用下(图 4a),球曲面感应极板与柔性公共极板间距减少,引起4个电容增加量相等;在切向力FX/FY作用(图 4b),X轴(Y轴)方向差分电容输出增加,Y轴(X轴)方向差分电容输出无变化。此外,本文提出的基于球曲面的电容式柔性触觉传感器可用于三维力检测,在三维力F=(FX FY FZ)T作用下,各个分量力对应的电容传感器输出电容值(${\tilde C_X}$、${\tilde C_Y}$和${\tilde C_Z}$)满足式(4)~式(6)中关系[25],通过标定${\tilde C_X}$、${\tilde C_Y}$和${\tilde C_Z}$与三维力分量FX、FY和FZ之间的关系,即可反演出三维力信息,从而实现三维力触觉感知功能。
$${\tilde C_X} = \frac{1}{4}\left[ {\frac{{\Delta {C_1}}}{{C_1^0}} + \frac{{\Delta {C_2}}}{{C_2^0}} - \frac{{\Delta {C_3}}}{{C_3^0}} - \frac{{\Delta {C_4}}}{{C_4^0}}} \right]\left[ {\frac{1}{{1 + {{\tilde C}_Z}}}} \right]$$ (4) $${\tilde C_Y} = \frac{1}{4}\left[ { - \frac{{\Delta {C_1}}}{{C_1^0}} + \frac{{\Delta {C_2}}}{{C_2^0}} + \frac{{\Delta {C_3}}}{{C_3^0}} - \frac{{\Delta {C_4}}}{{C_4^0}}} \right]\left[ {\frac{1}{{1 + {{\tilde C}_Z}}}} \right]$$ (5) $${\tilde C_Z} = \frac{1}{4}\left[ {\frac{{\Delta {C_1}}}{{C_1^0}} + \frac{{\Delta {C_2}}}{{C_2^0}} + \frac{{\Delta {C_3}}}{{C_3^0}} + \frac{{\Delta {C_4}}}{{C_4^0}}} \right]$$ (6) -
传统的电容测量方法主要有谐振法、中和电流法、脉宽调制法、数字相敏检波器法和交流电桥法等,通过分立元件将电容信号转化为电压、电流、频率或脉宽等信号,上述方法存在电路设计复杂、测量精度低等弊端。目前,集成电容检测芯片可分为CAV424/444系列,Pcap01/02/03系列和AD(ANALOG DEVICES)公司的AD774X与AD714X系列的电容数字转换器[26]。依据式(3),取r=5 mm,m=n=1 mm,计算单个球曲面电容式触觉传感单元输出电容值约为0.255 5 pF(忽略柔性公共极板表面硅橡胶隔离层)。
考虑到球曲面电容式触觉传感单元输出电容变化范围、测量精度及实时性等因素,本文选择高性能电容数字转换器AD7147-1和STM32微处理器搭建容性触觉信号采集与处理系统,如图 5所示。AD7147-1拥有I2C接口及片内环境自校准功能,其有源交流屏蔽技术有效地消除了容性输入端与触觉传感单元间的寄生电容,高达16位CDC精度,13路容性输入,结合32位微处理器可方便的实现多路容性触觉信息的采集[27],其容性触觉传感信息采集流程图如图 6所示。下位机对容性触觉信息进行实时采集并通过串口传至上位机,上位机接收到数据验证无误后保存以便进一步分析。
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导电银颗粒在基体中的分布状态是影响柔性极板电学性能的重要因素,图 7a和图 7b分别为柔性极板的DMM-200C型金相电子显微镜(上海蔡康光学仪器有限公司)和SU8020型场发射扫描电子显微镜(日本日立公司)的微观表征结果。可以看出,有机硅导电银胶固化后表面均匀平滑,且导电银颗粒均匀分布且相互连接,有利于形成较为稳定的导电网络,从物理结构层面为柔性极板实现良好的电学特性提供了保障。
使用LS-WD-100型万能拉压力机(深圳力森科技有限公司)对本文球曲面极板电容式柔性触觉传感器进行力学特性表征,并通过容性触觉信息采集与处理系统实时采集触觉传感器输出。图 8为0~5 N范围内法向力作用下,球曲面极板电容式柔性触觉传感单元输出特性曲线。可以看出,法向力作用下,球曲面感应极板与柔性公共极板间距减小,输出电容值均呈增加趋势,与上述分析结果保持一致,且在0~2 N和2~5 N范围内具有良好的分段线性,其检测灵敏度分别为0.807 N-1和0.278 N-1。在0~5 N范围内切向力作用下电容式柔性触觉传感器的输出特性曲线如图 9所示,在切向力作用下,球曲面感应极板与柔性公共极板间距减小,同时,沿切向力方向的球曲面感应极板与柔性公共极板之间的等效极板面积发生变化,最终表现为:差分电容C1-C3呈现增加趋势,C2-C4基本保持不变,由此实现切向力检测。
电容式柔性触觉传感器具有良好的动态响应特性,对本文提出的球曲面电容式柔性触觉传感器施加频率为0.5 Hz的动态加载,观察其动态响应曲线如图 10所示。可以看出,在0.5 Hz的循环动态加载下,球曲面极板电容式柔性触觉传感器仍可做出快速响应并展现出良好的机械重复性和电学稳定性。
为进一步测量其动态响应时间,对本文球曲面电容式柔性触觉传感器施加一阶跃激励,测得其响应特性曲线如图 11所示,响应时间约为70 ms。
为阐述本文球曲面极板电容式柔性触觉传感器用作电子皮肤实现触觉感知的可行性,将电容式柔性触觉传感器固定于手指,并连续单击鼠标和敲击键盘输入字母‘O’‘K’等动作,同时,容性触觉信号采集与处理系统实时记录球曲面电容式柔性触觉传感器的输出,其测试结果分别如图 12和图 13所示。可以看出,敲击按键时,电容式柔性触觉传感器球曲面感应极板受力被压缩,极板等效间距减小,传感器输出达到峰值,由于每次单击按键的力度和时间间隔不同,其输出峰值也略有差异。将本文传感器布置于机械手抓取重物,并在重物下方逐渐挂上砝码以模拟三维力加载,依据式(4)~(6),解析其三维力信息如图 14所示。
通过佩戴本文提出的球曲面电容式柔性触觉传感器可实现指尖触觉感知并区分不同手指的动作。此外,球曲面电容式柔性触觉传感器可连续检测同一指尖动作,说明该触觉传感器作为电子皮肤在触觉感知中具有良好的机械鲁棒性和稳定性,进一步论证了其用作电子皮肤实现触觉感知的可行性。
Research on Capacitive Tactile Sensor Based on Spherical Surface Plate
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摘要: 为提升电子皮肤的触觉感知灵敏度,提出一种基于球曲面极板的电容式柔性触觉传感器。介绍了电容式柔性触觉传感器的结构特点与制备流程,并结合理论计算与ANSYS有限元仿真阐述其触觉感知机理,空间立体排布的球曲面感应极板和差分式结构特点可协同提升电容式柔性触觉传感器感知灵敏度。基于高性能电容数字转换器AD7147-1和STM32微处理器搭建容性触觉信号采集与处理系统,完成了电容式触觉传感器特性测试及应用研究。实验结果表明,该传感器可实现法向力与切向力的高灵敏检测和稳定性,动态响应时间约为70 ms,进一步论证了其用作电子皮肤的可行性。Abstract: To improve the sensing sensitivity of electronic skin (e-skin), a capacitive flexible tactile sensor based on spherical surface plate is proposed in this paper. The structural characteristics and fabrication process of the sensor was illustrated, meanwhile, the tactile sensing mechanism is also presented based on the combine of theoretical calculation and ANSYS finite element simulation. The sensitivity of the capacitive flexible tactile can be cooperatively improved based on the spatial arrangement of spherical surface plate and differential structure. Furthermore, the signal acquisition and processing system of the capacitive tactile sensor is constructed based on the high performance capacitive digital converter AD7147-1 and STM32 microprocessor, and the characteristic test and the application of the proposed device are also conducted. The experiment results show that the high sensitivity tactile sensing of normal force and tangential force and high stability can be obtained by using the capacitive tactile sensor with the spherical surface plate with a fast response time of 70 ms, and the feasibility of the capacitive tactile sensor used as electric skin is further demonstrated.
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Key words:
- capacitive /
- e-skin /
- flexible /
- tactile sensor /
- spherical surface plate
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