Biomimic Hairy Skin Tactile Sensor Based on Ferromagnetic Microwires

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Bio-mimic Hairy Skin Tactile Sensor based on Ferromagnetic Microwires Jian Zhang, Lifeng Hao, Fan Yang, Weicheng Jiao, Wenbo Liu, Yibin Li, Rongguo Wang, and Xiaodong He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14236 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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ACS Applied Materials & Interfaces

Bio-mimic Hairy Skin Tactile Sensor based on Ferromagnetic Microwires Jian Zhang1, Lifeng Hao1,*,Fan Yang1, Weicheng Jiao1, Wenbo Liu2, Yibin Li1 , Rongguo Wang1,*,Xiaodong He1. 1.Center for Composite Materials and Structures, Harbin Institute of Technology, 150080, China. 2.School of Materials Science and Engineering, Harbin Institute of Technology, 150001, China

KEYWORDS : multifunctional sensor, skin sensor, hair sensor, artificial hairy skin sensor, ferromagnetic microwires

ABSTRACT: We present a multifunctional tactile sensor inspired by human hairy skin structure, in which the sensitive hair sensor and the robust skin sensor are integrated into a single device via a pair of Co-based ferromagnetic microwire arrays in a very simple manner. The sensor possesses a self-tunable effective compliance with respect to the magnitude of the stimulus, allowing a wide range of loading force to be measured. The sensor also exhibits some amazing functions, such as air-flow detection, material property characterization and excellent damage resistance. The novel sensing mechanism and structure provide a new strategy for designing multifunctional tactile sensors and shows great potential applications on intelligent robot and sensing in harsh environments.

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Introduction In order to make robots to behave as smart as living creatures, multifunctional tactile sensors are highly desired. Several attempts have been carried out on multifunctional sensors.1-3 Currently, the sensing mechanisms of tactile sensors mainly include transistor sensing, capacitive sensing, piezoelectric sensing, and piezo-resistive sensing. There are plenty of hair sensors based on MEMS (Micro-electromechanical Systems) to sense air-flow and artificial skin sensor based on various mechanisms (interlocking, contact area changing, field effect, etc) to detect pressure or load force. 4-10 Lee fabricated poly(vinylidene fluoride) and ZnO nanostructures hybrid thick film to measure pressure and temperature; Zhao reported PET-based Ag serpentine-shaped electrode sensor array for static and dynamic mapping; Harada designed a strain-engineered three-axis tactile force sensor and temperature sensor array to detect the slip force, tactile force, and temperature. Park demonstrated an ultra sensitive strain gauge sensor based on nanoscale crack and studied the crack depth effect to the sensitivity. 11-15The hair sensor is highly sensitive to detect air-flow while the sensing elements lack flexibility to sense contact force, so traditional hair sensor is mainly utilized to sense air-flow. The artificial skin sensor design principle is to pursue high sensitivity.16-22So these sensors are based on various kinds of nanostructure or complicated manufacture process.23-27 Restricted by their meticulous construction, the sensors are vulnerable and could not detect wide range force which constrains their applications on practical situations. If there is a rational sensor which can combine hair sensor’s high sensitivity with skin sensor’s wide detection range, the novel structure may generate a multifunctional sensor with considerable high sensitivity and wide detection range. Sensing a diversity of properties simultaneously via direct physical contact plays a key role for living creatures to handle different kinds of external stimuli and to survive in a complex


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environment.28,29 Human skin tactile force sensory function feature can be summarized as high sensitivity, wide force detection range, long-term durability, abrasion resistance and material rigidity identification.30 There are two types of skin, with fine hair such as opisthenar and without fine hair such as fingertip. Skin with fine hair cover over 95% human body which is more sensitive to tiny stimuli like air-flow and gentle touch due to deformation of the flexible hair.31,32 So hairy skin structure is a idea sensor model while the key problem is that sensing element must be flexible and tough like human fine hair. In order to realize stable and durable sensing activity, the sensor structure must be as concise as possible. The need to develop force sensors with capabilities in high sensitivity, wide pressure rage detection, long-term durability and low cost production requires the innovative design of sensor structure and outstanding materials as sensory component. In present research, we describe a new type of tactile force sensor inspired by hair-skin structure of human skin. The key innovation of material design, Co-based amorphous glass-coated micro-wires are known to feature excellent mechanical property and magnetic performance. Some outstandingly sensitive magnetic effects such as Barkhausen effect, GMI (Giant magneto-impedance) and GSI (Giant stress-impedance) have been found in Cobased microwires. Co-based magnetic microwires are traditional sensing element widely used in engineering field such as GMI magnetic sensor. However, traditional applications based on Cobased microwires are mainly focus on its magnetic performance and neglect its flexibility, mechanical property and fiber shape. The flexibility of the Co-based microwire is as outstanding as human hair while it cannot be broken even under the knotting operation, which make is suitable to be the sensing element of hairy skin sensor. The fiber shape makes Co-based microwire applicable to be sensing component of the hair sensor. Outstanding mechanical characteristic make it is capable of withstanding large effective strain and stress. For the first


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time we utilized Co-based magnetic microwires as sensing element of hairy skin tactile sensor which expands its application in artificial skin sensor. The sensor imparts ultra-high sensitivity and reproducible sensing characteristics benefited by the innovative structure. Novel sensing mechanism based on Barkhausen effect of Co-based magnetic microwires and magnetic induction was introduced which enable the multifunction such as material property detection compared to the traditional artificial skin sensor. Experiment section In present research, we describe a cost efficient tactile sensor inspired by human hairy skin structure, as shown in Figure 1b, which possesses multi-functions including wide-range force measurement, air flow detection and material properties identification. The structure of the hairy skin sensor is very simple and similar to the natural one as shown in Figure 1a. The sensor consists of an array of Co-rich amorphous ferromagnetic microwires (Co72.5Si12.5B15 from MFTI company) as hair follicle shaft and a copper coil as transducer to mimic the mechanoreceptors. The fabrication technology of the glass-coated ferromagnetic microwires is mainly based on Taylor-Ulitovsky procedure (quenching and drawing procedure). While the Co72.5Si12.5B15 ferromagnetic microwires exhibit excellent mechanical and magnetic property as listed in Table 1. Figure 1c shows the fabrication process of the sensor. Ten pieces of glass coated microwires with core radius of 24µm and a glass insulation layer of 5µm thick are wound into an array with a separation of 0.5mm on a custom-made single filament winding machine (Figure S1 in Supporting Information). The array is cut into a rectangular piece 30mm in length. A coil of 200cycle enameled copper wire is manually wound onto one end of the array with a length of about 5mm. Two identical coil-wrapped microwire arrays are face-to-face embedded into a silicon rubber matrix and healed inside a mold to form a bridge-shaped structure, with a small portion


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(about 10mm in length) protruding out of the center of the matrix. The thickness of the matrix is 0.5mm in center and 2mm at ends. Both ends of the bridge structure are glued onto a piece of PCB board which is 30mm long and 10mm wide. A piece of rubber triangular prism is put right underneath the center of the matrix which is used to make the protruded wires pointed upward a little bit and also to have the embedded wires suspended. The simple structure mentioned above forms a multifunctional tactile sensor with the protruded microwires working as hair sensor, and the embedded ones together with the rubber matrix as the skin sensor. Table 1. Magnetic and mechanical properties of Co72.5Si12.5B15 microwires Composition

Co72.5Si12.5B15

Saturation magnetization

Saturation magnetostriction

Ms(T)

λ×10-6

0.64

-3.0

Tensile

Tensile

Strength

Modulus

σ(GPa)

E(GPa)

2

150

The sensing mechanism is based on Barkhausen effect and magnetic induction. The Barkhausen effect will occur in Co-based glass-coated amorphous microwires because of the high tensile stressed layer introduced during rapid quenching and quenching under tension after cold-drawing. The Co-based micro-wire may generate stable and sharp voltage impulse under low amplitude (about 30A/m) but wide bandwidth altering magnetic field, which simplify the structure of sensor, enable the miniaturization, integration and stability. From a circuit point view, the sensor resembles two mutual inductance coils with the pair of the microwire arrays as the inner core. During operation, an alternating current I1 is fed into the left coil, providing excitation, creating a magnetic flux inside the left coil. Due to the Faraday–Lenz law, a potential difference V2 is developed in the right coil, which can be expressed as


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dI1 dt

(1)

M 21 = k L1L2

(2)

L1 = µ1n12 Sh

(3)

L2 = µ2 n22 Sh

(4)

V2 = − M 21

where M21 is the mutual inductance coefficient between the two coils, k is the coupling coefficient, L1, L2, n1 and n2 are the self-inductance coefficient and the turn of the left and right coil respectively, µ1 and µ2 are the permeability of left and right microwires array, S is the sectional area of the coil, and h is the length of the coil along the magnetic field direction. Substituting equations. (2-4) into equation. (1), we get

V2 = -n1n2 Shk µ1 µ 2

dI1 dt

(5)

In which n1, n2, S and h are constant for a sensor, dI/dt is determined by the magnitude of the excitation, and k µ1µ2 is related to the property variations of microwires under loading. µ1 and µ2 are determined by the force applied to the embedded microwires in the matrix, hence showing the response of the skin sensor. This is due to the fact that microwires are sensitive to tensile stress and only the embedded wire is loaded. k depends on the exact postures and relative positions of the microwire arrays, therefore is mainly determined by the microwires protruded outside the matrix. Since only the protruded microwires are capable to make large deformation, so it behaved as hair sensor. As a result, the output of the sensor consists of the signals from both skin and hair sensors. In order to discriminate the two signals, the frequency dependence of k is considered.


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The frequency dependence of k is fundamentally determined by the domain wall propagation in the magnetic induction process. If the excitation frequency is low enough, domain walls are able to travel all the length of the microwire and result in a single domain structure as shown in Figure 1e. The microwire behaves as single magnet with the magnetic field distributed in a large region surrounding the wire. Thus, even tiny microwire posture change can cause obvious variation in k. On the contrary, at high frequency, the domain propagation cannot catch the excitation frequency and multiple small domain structure will be developed as shown in Figure 1f. Here the magnetic field can only be distributed in the adjacent region of the microwires, hence the coupling between the mcirowire arrays only takes place at the joint position between the arrays. As a result, the exact posture of the hair sensor does not matter, and k is only determined by the overall torque applied. For the microwires used here, the domain length is about 2mm and the domain wall propagation speed is about 500m/s, therefore, a critical frequency can be calculated and is about 25kHz.33,34 Hence two frequencies are chosen as excitations for the sensor: 20kHz as the low frequency channel and 1MHz as the high frequency channel. So by using different excitation frequency, the force stimulus and the gentle touch stimulus can be separated effectively. It is noteworthy that multiple-frequencies can be applied to the sensor simultaneously to detect different stimuli at the same time. Results and discussion Sensing property of the sensor. By combining the soft hair sensor and the robust skin sensor together, the hairy skin sensor is capable to detect forces in a wide range from

Biomimic Hairy Skin Tactile Sensor Based on Ferromagnetic Microwires

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