CN110987029A - A kind of multifunctional flexible sensor and its preparation method and application - Google Patents
A kind of multifunctional flexible sensor and its preparation method and application Download PDFInfo
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Abstract
本发明属于电子皮肤领域，并公开了一种多功能柔性传感器及其制备方法和应用。该多功能柔性传感器包括电容层，该电容层从下自上包括底层柔性衬底、底电极、中间介电层、导电电极、顶电极和顶层柔性衬底，其中，底电极为平板电极，中间介电层上设置有多个高低起伏的柔性凸起，顶电极上每个平行电极上设置有一根引线，两个平板电极上的引线末端相对设置形成缺口，导电电极设置在缺口正下方，当导电电极未嵌入缺口时，两个平板电极形成电容，当顶层柔性衬底受到外力时，顶电极向下运动，导电电极嵌入缺口，两个平板电极导通形成电阻，同时顶电极与底电极形成电容。通过本发明，实现温度、距离和应力的测量，降低对引线的数量和引线接口的需求。
The invention belongs to the field of electronic skin and discloses a multifunctional flexible sensor and a preparation method and application thereof. The multifunctional flexible sensor includes a capacitive layer, which from bottom to top includes a bottom flexible substrate, a bottom electrode, an intermediate dielectric layer, a conductive electrode, a top electrode and a top flexible substrate, wherein the bottom electrode is a flat electrode, and the middle A plurality of undulating flexible protrusions are arranged on the dielectric layer, a lead is arranged on each parallel electrode on the top electrode, the ends of the leads on the two flat electrodes are oppositely arranged to form a gap, and the conductive electrodes are arranged directly under the gap. When the conductive electrode is not embedded in the gap, the two flat electrodes form a capacitor. When the top flexible substrate is subjected to an external force, the top electrode moves downward, the conductive electrode is embedded in the gap, and the two flat electrodes are connected to form a resistance. At the same time, the top electrode and the bottom electrode form a capacitance. Through the present invention, the measurement of temperature, distance and stress is realized, and the requirement for the number of lead wires and the lead wire interface is reduced.
Description
Technical Field
The invention belongs to the field of electronic skin, and particularly relates to a multifunctional flexible sensor and a preparation method and application thereof.
Background
With the discovery of new materials and the progress of processes, the field of flexible electronics has been developed greatly, and in order to obtain a flexible pressure sensing device, organic or inorganic electronic materials are integrated on a flexible substrate, and a certain microstructure is adopted, so that the performance of the flexible pressure sensing device is not lower than or even better than that of a traditional rigid pressure sensing device. The skin is the largest organ of the human body, and a large and dense sensing network is distributed in the skin tissue, for example, various sensors are distributed in the hand of the human body, including pain sensors, temperature sensors, mechanical stimulators and the like, and can be used for static force, stretching of the skin, vibration, temperature, sliding of an object, identification of texture and hardness and the like. The various receptors have high distribution density, such as the density of the temperature receptors between human fingers is 4/cm2The number of the baroreceptors is 70/cm2The number of static force sensors is 48/cm2The number of dynamic force sensors is 163/cm2. Therefore, in order to make the mechanical hand have the sensing capability like a human hand and even exceed the sensing capability of the human hand, it is very important to develop an electronic skin with flexible large-area, multifunctional and high-integration sensing.
But is about 1cm2Hundreds of sensing units with different functions are integrated in the device, each unit is an independent sensing module, the occupied space and the number of interface leads are not interfered with each other, and therefore the number of port leads needed by the device is extremely large. The invention patent CN106595940A proposes a flexible multifunctional sensor, which uses the resistance change of the sensitive material to realize the measurement of pressure and bending, but the invention integrates less physical quantity, has small area, and the two quantities can be coupled with each other. Patent CN201210243282.9 proposes a flexible composite array sensor for robot skin, in which a pressure sensor, a temperature sensor and a humidity sensor are respectively disposed on the same surface of a substrate, and this integration method occupies a large space and is not suitable for large-area high-density integration. Hua et alThe measuring units of temperature, strain, heat, magnetic field, humidity, approach and light intensity are integrated on the flexible electronic skin by means of longitudinally adding functional layers (documents' Hua Q, Sun J, Liu H, ethyl. skin-induced high strain and compatible matrix networks for multi-functional sensing [ -J ]]Nature communications,2018,9(1):244. "). Although the multi-function sensor in the above patent and document can measure a plurality of physical quantities, it is necessary to add one function, that is, to add one functional layer, that is, to add a function layer number and a function number in a one-to-one relationship. With the adoption of the integration mode, with the increase of the number of functions and the increase of the sensing density, the process is complicated, the lead interfaces are increased, and the consumption of hardware resources is increased. In addition, a mode that one functional layer corresponds to two functions is adopted, but the physical quantities measured by the two functions are mutually coupled, and the method is not advisable.
Therefore, with the increasing number of the sensing and exciting modules, the ratio of the number of the functions of the flexible sensor to the number of the ports (or leads) is low, which results in high system resource burden, complex manufacturing process of the device, low space utilization rate, and higher cost of the circuit hardware part, the electronic skin with bending, vibration, three-way pressure, object approach and temperature measurement is developed, the problems of single function, more lead interfaces and low space utilization rate existing in the electronic skin are solved, and the reduction of the tactile sensation of the manipulator is the research target of the invention.
Disclosure of Invention
In view of the above drawbacks or needs for improvement in the prior art, the present invention provides a multifunctional flexible sensor, a method for manufacturing the same, and an application of the same, which implement free switching between resistive and capacitive electrodes when stressed through design of a bottom electrode and a top electrode layer in a capacitive layer, especially design of a top electrode, and implement measurement of bending, vibration, temperature, distance, and three-dimensional stress (normal stress, shear stress, and combinations thereof) by using an array combination of capacitive sensors n × n in combination with switching of measurement modes of piezoelectric layer electrodes, thereby reducing the number of functional layers and the requirements for lead interfaces.
To achieve the above object, according to one aspect of the present invention, there is provided a multifunctional flexible sensor comprising a capacitance layer including, in order from bottom to top, a bottom flexible substrate, a bottom electrode, an intermediate dielectric layer, a conductive electrode, a top electrode, and a top flexible substrate, wherein,
the bottom electrode is a flat plate electrode and is arranged on the bottom layer flexible substrate, a plurality of flexible protrusions with different heights are arranged on the middle dielectric layer, the top electrode comprises two flat plate electrodes, each parallel electrode is provided with a lead, the tail ends of the leads on the two flat plate electrodes are oppositely arranged to form a notch, the conductive electrode is arranged on the flexible protrusions and is arranged right below the notch,
when the conductive electrode is not embedded into the notch, the two plate electrodes on the top electrode form a capacitor, and when the capacitor changes due to the external environment, the change of the external environment is reflected by measuring the change of the capacitor between the two plate electrodes;
when the top layer flexible substrate is subjected to downward external force, the top electrode moves downwards, the conductive electrode is embedded into the notch, the two flat electrodes are conducted to form a resistor, when the resistor changes due to the external environment, the change of the external environment is reflected by measuring the change of the resistor, meanwhile, when the two flat electrodes are conducted to form the resistor, the whole top electrode serves as one flat electrode and forms a top electrode-bottom electrode capacitor with the bottom electrode, and when the capacitor between the top electrode and the bottom electrode changes due to the change of the external environment, the change of the external environment is reflected by measuring the change of the capacitor between the top electrode and the bottom electrode.
Further preferably, the material of the bottom flexible substrate is preferably PDMS or Ecoflex, the material of the bottom electrode is preferably Cu or Au, the material of the intermediate dielectric layer is preferably PDMS or Ecoflex, the material of the conductive electrode is preferably conductive silver paste, the material of the top electrode is preferably Cu or Au, and the material of the top flexible substrate is preferably PDMS or Ecoflex;
the thickness of the bottom layer flexible substrate is preferably 10-20 μm, the thickness of the bottom electrode is preferably 200-400 nm, the thickness of the middle dielectric layer is preferably 60-80 μm, the thickness of the conductive electrode is preferably 200-400 nm, the thickness of the top electrode is preferably 200-400 nm, and the thickness of the top layer flexible substrate is preferably 10-20 μm.
Further preferably, the leads on the parallel electrodes are in a spiral shape, the two spiral leads on the two parallel electrodes are arranged in a staggered mode, and the two spiral leads are arranged in a staggered mode in a spiral mode, so that the length of the leads is increased, and the sensitivity of temperature measurement is improved.
Further preferably, the flexible sensor further comprises a substrate and a piezoelectric layer, the substrate is arranged below the piezoelectric layer, namely the carrier of the piezoelectric layer, the piezoelectric layer is arranged below the capacitance layer and comprises a piezoelectric material layer and an electrode layer, the piezoelectric material layer is used for sensing pressure and converting the sensed pressure into electric charges, the electrode layer is used for leading out the electric charges, two sets of orthogonal interdigital transducers are arranged on the electrode layer, and each set of orthogonal interdigital transducers comprises a transverse pair of interdigital transducers and a longitudinal pair of interdigital transducers.
Further preferably, the flexible sensor is a module, and a multi-module sensor array is formed by combining n × n modules, and the multi-module sensor array can be used for measuring the shear force.
Further preferably, the flexible sensor further comprises a force-bearing boss disposed at the center of the upper surface of the capacitor layer.
Further preferably, the multifunctional flexible sensor is an island, a sensor network, i.e. a flexible electronic skin, is formed by the combination of a plurality of islands, wherein the islands are connected to each other by a meandering distribution of wires, which makes the sensor network stretchable, thereby making the coverage area of the sensor network adjustable.
According to another aspect of the present invention, there is provided a method for preparing the multifunctional flexible sensor, comprising the following steps:
s1, selecting a hard substrate, coating a corrosion inhibitor on the substrate to be used as a sacrificial layer, then depositing the bottom electrode, obtaining the bottom electrode layer after curing, coating a layer of material of the flexible substrate on the bottom electrode layer to form the bottom flexible substrate on the bottom electrode layer, and removing the sacrificial layer to separate the bottom electrode layer from the hard substrate to obtain the bottom flexible substrate and the bottom electrode layer;
s2, selecting a mold, and pouring silicon rubber liquid on the mold to obtain the required intermediate dielectric layer, wherein the silicon rubber is preferably PDMS;
s3, forming the conductive electrode on a preset flexible bulge on the intermediate dielectric layer by adopting an electric spray printing method, so as to obtain the intermediate dielectric layer provided with the conductive electrode on the preset flexible bulge;
s4, selecting a hard base, coating a sacrificial layer on the hard base, then coating a material of a flexible substrate on the sacrificial layer, curing to obtain the flexible substrate, removing the sacrificial layer, and separating the flexible substrate from the hard base to obtain the required top-layer flexible substrate;
and S5, sequentially stacking the bottom layer flexible substrate, the bottom electrode layer, the middle medium layer with the preset flexible protrusions provided with the conductive electrodes and the flexible substrate layer by layer from bottom to top, and thus obtaining the required capacitor layer.
According to another aspect of the present invention, there is provided a method for manufacturing the flexible sensor, wherein the method for manufacturing the piezoelectric layer includes the steps of:
s6, selecting a hard substrate, uniformly coating a sacrificial layer on the hard substrate, and depositing a layer of interdigital transducer on the sacrificial layer to obtain the electrode layer;
s7, coating a layer of piezoelectric material on the electrode layer, heating and curing, and then annealing to obtain the piezoelectric material layer;
s8, coating a layer of substrate material on the piezoelectric material layer, and curing to obtain the substrate, thereby obtaining the required piezoelectric layer on the hard substrate;
s9 removing the sacrificial layer by laser lift-off technique, and peeling the piezoelectric layer from the rigid substrate to obtain the desired piezoelectric layer.
According to a further aspect of the present invention there is provided a use of the flexible electronic skin described above for measuring distance, temperature, pressure, bending, vibration and shear stress measurements.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
1. the flexible sensor provided by the application is used as a capacitance sensor when not subjected to external force, and is used as a resistance sensor and a capacitance sensor when being subjected to external force, so that high integration of a resistor and the capacitance sensor is realized;
2. the flexible sensor provided by the invention realizes the integrated design of two capacitance sensors and one resistance sensor, can realize the measurement of the distance, the temperature and the pressure of an object to be measured, reduces the complex work of respectively adopting three different sensors for measurement, reduces the workload and shortens the time cost;
3. the flexible electronic skin provided by the invention realizes the sensing of multiple functions through the switching of the acquisition circuit or the reconstruction and combination of the sensor structure, and can be particularly used for measuring bending, vibration, three-way stress (including positive pressure, shear stress and combination thereof), the proximity degree and temperature of an object, and all physical quantities are not coupled with each other, so that the requirements on the number of functional layers and lead interfaces are reduced, and the tactile feeling of a robot is realized.
Drawings
FIG. 1 is a schematic diagram of a multi-module flexible sensor formed by combining 2X2 modules constructed in accordance with a preferred embodiment of the present invention;
FIG. 2 is a top view of the electrode layer 3 of the multi-module flexible sensor formed of 2X2 modules of FIG. 1 constructed in accordance with a preferred embodiment of the present invention, wherein (a) is a signal input and output diagram for bending measurement, and (b) is a signal output diagram for vibration measurement;
FIG. 3(a) is a top view of the top electrode layer of the capacitive layer of the multi-module flexible sensor formed of 2 by 2 modules of FIG. 1 constructed in accordance with a preferred embodiment of the present invention;
FIG. 3(b) is a schematic illustration of the change in capacitance of the top electrode as an object constructed in accordance with a preferred embodiment of the present invention approaches the sensor;
FIG. 4 is a schematic diagram of a multi-module sensor constructed in accordance with a preferred embodiment of the present invention for static pressure measurement;
FIG. 5(a) is a schematic diagram of a multi-module sensor constructed in accordance with a preferred embodiment of the present invention for temperature measurement of an object;
FIG. 5(b) is a schematic diagram of the switched measurement principle of the top electrode as a resistance temperature sensor and a capacitance sensor constructed in accordance with the preferred embodiment of the present invention;
FIG. 6(a) is a schematic diagram of a 2 × 2 small array of four modular sensors constructed in accordance with a preferred embodiment of the present invention to achieve measurement of forward force;
FIG. 6(b) is a schematic diagram of a 2 × 2 small array of four modular sensors configured in accordance with a preferred embodiment of the present invention to achieve shear force measurement;
FIG. 7 is a schematic diagram of a stretchable serpentine substrate with a plurality of sensor assemblies connected to form a network constructed in accordance with a preferred embodiment of the present invention;
FIG. 8 is a schematic diagram of a plurality of sensors configured in accordance with a preferred embodiment of the present invention connected in combination to form a flexible electronic skin;
FIG. 9 is a schematic illustration of a plurality of sensor assemblies constructed in accordance with a preferred embodiment of the present invention connected to form a flexible e-skin application;
FIG. 10 is a logic block diagram of a measurement procedure for flexible electronic skin constructed in accordance with a preferred embodiment of the present invention;
fig. 11 is an exploded view of a manufacturing process for a multi-module flexible sensor constructed in accordance with a preferred embodiment of the present invention.
The same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
1-substrate, 2-piezoelectric material layer, 3-electrode layer, 4-bottom flexible substrate, 5-bottom electrode, 6-middle dielectric layer, 7-conductive electrode, 8-top electrode, 9-top flexible substrate, 10-stress boss, 11-bridge, 12-electric jet printing nozzle, 13-flexible electronic skin, 14-robot hand, 15-water cup, 16-aluminum die, 17-silicon chip, 18-stripping resist, 19-sapphire substrate, 20-PZT sacrificial layer, 21-laser beam, 22- <100> crystal orientation silicon chip, and 23-photoresist.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in fig. 1, the multifunctional flexible sensor comprises a capacitor layer, the capacitor layer comprises a bottom layer flexible substrate 4, a bottom electrode 5, a middle dielectric layer 6, a conductive electrode 7, a top electrode 8 and a top layer flexible substrate 9 from bottom to top in sequence, the bottom layer flexible substrate 4 and the top layer flexible substrate 9 are used as substrates and are isolated from the outside, a flexible insulating material is adopted, the middle dielectric layer 6 is composed of a plurality of levels of flexible pyramid-shaped protruding structures with different heights, the sensitivity and the range of pressure measurement are increased, the top electrode 8 is composed of two spiral coils and a flat plate electrode, each flat plate electrode is connected with one spiral coil, the tail ends of the two spiral coils are opposite to form a gap, the conductive electrode 7 is arranged on the flexible pyramid-shaped protruding structure corresponding to the position right below the gap, when the conductive electrode is embedded into the gap, the two flat plate electrodes are conducted to form a resistor for measuring the temperature of an object applying pressure, the two plate electrodes form a plane capacitor for measuring the distance when an object approaches, the bottom electrode 5 is a plate electrode, and the bottom electrode 5 and the top electrode 8 form a parallel plate capacitor which changes under the action of pressure and is used for measuring static pressure.
The multifunctional flexible sensor also comprises a substrate 1 and a piezoelectric layer, wherein the substrate 1 is a carrier of other components in the flexible sensor, the piezoelectric layer comprises a piezoelectric material layer 2 and an electrode layer 3 positioned above the piezoelectric material layer, the piezoelectric material layer 2 is used for sensing pressure and converting the sensed pressure into electric charge, the electrode layer 3 is used for leading out the electric charge, the electrodes are arranged in an interdigital pair mode, two groups of orthogonal interdigital transducer pairs are arranged on the electrode layer 3, each group of orthogonal interdigital transducers comprises a transverse pair of interdigital transducers and a longitudinal pair of interdigital transducers, one of the two interdigital transducers in the transverse/longitudinal pair of interdigital transducers is used as the input of a signal, the other interdigital transducer is used for measuring the bending degree when the signal is output, and the other interdigital transducers are used for measuring the vibration signal when the two interdigital transducers are used as the output of the signal.
The flexible sensor also comprises a stress boss 10, and the stress boss 10 is arranged in the middle of the upper surface of the capacitor layer and is used as a medium for the flexible sensor to contact with the external environment.
The flexible sensor comprising a substrate, a piezoelectric layer and a capacitor layer is taken as a module, as shown in fig. 1, a multifunctional flexible sensing array formed by combining 2 × 2 modules is shown in the figure, and a stress boss 10 is arranged in the center of the upper surface of the capacitor layer of the array.
When an object approaches the flexible sensor, the planar capacitance formed by the two flat electrodes on the top electrode 8 changes, and the distance between the object to be grabbed and the flexible sensor is reflected through the change of the measured capacitance, so that the distance measurement is realized;
the bending of the manipulator causes the path between the interdigital transducer pair of the piezoelectric layer to change, and the measurement of the bending degree of the flexible sensor is realized by measuring the voltage amplitude and the phase of a receiving interdigital transducer in the piezoelectric layer;
the flexible sensor is under the action of pressure, so that the conductive electrode is embedded into the notch, the resistance formed by the two flat electrodes is changed due to the temperature change of the object to be grabbed, and the temperature of the object to be grabbed is obtained by measuring the resistance at the two ends of the two flat electrodes, so that the temperature measurement is realized;
the capacitance between the top electrode and the bottom electrode is changed by the pressure and the shear stress when the manipulator grabs an object, and the magnitude and the direction of the pressure and the shear stress can be decoupled by measuring the variation trend and the variation magnitude of the 4 capacitances combined by the 2 multiplied by 2 array;
meanwhile, the interdigital transducer pairs of the piezoelectric layer are all used as output electrodes so as to sense dynamic pressure signals in the grabbing process.
Fig. 2 is a top view of the electrode layer 3 of the sensor array formed of 2 × 2 modules in fig. 1, and (a) in fig. 2 is a view showing two interdigital transducers of a transverse/longitudinal pair of interdigital transducers, one as an input and one as an output of a signal, one of which is referred to as an input transducer, converting an input electrical signal into an acoustic signal by inverse piezoelectric effect, the acoustic signal propagating along the surface of the substrate, and finally converting the acoustic signal into an electrical signal output by the other transducer, which is referred to as an output transducer, when used for bending measurement.
In fig. 2 (b), when the sensor is used as a vibration sensor, the two interdigital transducers are used as outputs, and under the action of pressure, electric charges are generated on the two polar plates due to the piezoelectric effect, and the generated electric charges are in proportion to a vibration signal.
For isotropic piezoelectric materials, the output voltage of an interdigital transducer pair is related to its geometrical parameters, which are as follows:
where h is a constant relating to the piezoelectric material and its physical properties, t is time, and VRIs the propagation velocity of Rayleigh wave, L is the distance between the interdigital connected with input voltage by the input transducer and the interdigital connected with output voltage by the output transducer, a is the distance between the interdigital electrodes, m, n are the number of the interdigital of the input transducer and the interdigital of the output transducer respectively, vinIs the excitation voltage, v, of the input transduceroutIs the output voltage of the output transducer. As can be seen from the above equation, the output voltage varies as the dimensional parameters a and L of the transducer pair vary. Bending deformation along the transducer direction can cause anisotropy of the piezoelectric material, affecting the meterThe amplitude and the phase of the output voltage are changed by the propagation of the surface acoustic wave, and the amplitude and the phase of the voltage received by the output transducer are proportional to the bending radius, namely, a single pair of transducers can represent one-dimensional bending morphology along the direction of the transducers. According to the superposition principle, any non-overlapping three-dimensional surface can be decomposed into two-dimensional surfaces with unidirectional bending, so that three-dimensional deformation can be analyzed by adopting two pairs of orthogonal transducers.
Fig. 3(a) is a top view of the top electrode 8 of the sensor array formed by 2 × 2 modules in fig. 1, fig. 3(b) is a schematic diagram of capacitance variation of the top electrode when an object approaches the sensor, the top electrode 8 of the capacitance layer is composed of a pair of spiral electrodes, and constitutes a planar capacitance sensor for measuring an approach distance of the object, the size of the planar capacitance C is related to size parameters of the electrode plate and electric field distribution near the electrode plate, when the object approaches, the electric field distribution near the electrode plate is changed, thus resulting in a change of capacitance value, and the amount of change of the planar capacitance C is different according to the approach distance of the object, and in addition, physical properties of different objects are different, the amount of change of the planar capacitance is different when the object approaches, thus being also used for identification of object types.
As shown in fig. 4, the capacitance layer of the sensor is used for measuring static pressure, the capacitance layer comprises a bottom electrode, a dielectric layer and a top electrode, the bottom electrode and the top electrode form a parallel plate capacitor for measuring static pressure, under the action of pressure, the middle dielectric layer is pressed and deformed, the distance between the two plates is reduced, and the parallel plate capacitance is increased. The method comprises the following specific steps:
according to the parallel plate capacitance formula:
wherein epsilon0Is a vacuum dielectric constant of ∈rThe relative dielectric constant of the middle dielectric layer, A is the facing area of the upper and lower electrode plates, and d is the distance between the upper and lower electrode plates, i.e. the thickness of the middle dielectric layer, which varies with the pressure. In order to increase the sensitivity and the measuring range of the pressure sensor, the intermediate dielectric layer adopts a multi-stage pyramid micro-boss structure, namely, a multi-stage pyramid micro-boss structurePyramid micro-bosses are uniformly distributed at different heights. Under the action of small pressure, the highest pyramid is firstly contacted with the top electrode layer, and the top electrode layer continuously moves downwards along with the increase of the pressure to contact with the next highest pyramid and then sequentially moves downwards. Compared with the traditional equal-height pyramid microstructure, the tensile rigidity of the middle dielectric layer is reduced, the pressure measuring sensitivity is improved, and meanwhile, due to the existence of multiple stages, the sensor cannot enter a saturation region too fast, so that the measuring range is small. In addition, the contact area of the pyramid microstructure and the top electrode after the pyramid microstructure is deformed under pressure is reduced, so that the hysteresis phenomenon of the sensor is also reduced.
As shown in fig. 5(a), under the action of positive pressure, the middle pyramid electrode part of the middle dielectric layer of the sensor contacts the top electrode 8 of the capacitance layer, so that the spiral electrode pair of the top electrode is connected to serve as a temperature sensor, and the resistance of the temperature sensor changes along with the difference of temperature, thereby realizing the measurement of the temperature. Therefore, the top electrode is made of a temperature sensitive material, preferably, platinum metal with good conductivity can be selected, and meanwhile, the double-helix structural design increases the sensitivity of temperature measurement. The top electrode of the capacitive layer has two modes of operation for object proximity and temperature measurement, respectively, while pressure acts as a switch for the two modes. Fig. 5(b) is a schematic diagram of circuit switching to realize time-sharing and co-location measurement when the resistance temperature sensor and the parallel plate capacitance sensor of the capacitance layer share the top electrode, that is, the resistance of the top electrode is measured at different time intervals by switching the switch, the capacitance between the top electrode and the bottom electrode is measured at different time intervals, and the measurement of three-way force and temperature at the same position is realized.
As shown in fig. 6(a), four sensors form a 2 × 2 small array to measure three-dimensional force, i.e., normal stress in the vertical direction (z direction) and shear stress in the horizontal front-back direction (y direction) and horizontal left-right direction (x direction). In order to further increase the force measuring sensitivity, a miniature circular boss is added at the top, so that the stress concentration is facilitated. Wherein two top electrodes and bottom electrodes arranged in front horizontal left-right direction form a capacitor C1、C2Two top electrodes and bottom electrodes arranged horizontally and left and right at the rear part form a capacitor C3、C4. The three-dimensional stress in any direction can be decomposed into stress tau along the directions of x, y and zx、τy、PzThen the parallel plate capacitance is expressed as:
in the formula of0Is a vacuum dielectric constant of ∈rIs the relative dielectric constant of the intermediate dielectric layer, d is the initial thickness of the intermediate dielectric layer, A is the initial facing area between the two plates of the parallel plate capacitor, Δ d1、Δd2、Δd3、Δd4Are respectively C1、C2、C3、C4The distance between two polar plates is reduced under the action of pressure. As shown in FIG. 6(a), under positive pressure in the z-direction, the capacitance C1、C2、C3、C4The distance between the polar plates is reduced, and the capacitance is increased. As shown in fig. 6(b), C is under shear stress in the x-direction1、C3Decrease in inter-plate distance, increase in capacitance, and C2、C4The distance between the polar plates is increased, the capacitance is reduced, the direction of shear stress can be judged, the shear stress in the front-back direction can cause similar changes, and the magnitude and the direction of the three-way force can be decoupled by synthesizing the change trend and the change magnitude of the four capacitances under the action of the three-way force.
As shown in fig. 7, a schematic diagram of a stretchable winding substrate of a large-area flexible electronic skin formed by connecting a plurality of sensors in combination according to the present invention is shown, and in order to implement a large-area sensor network, a flexible hollow-out type "island-bridge" structure design method is adopted. The islands are multifunctional flexible sensing units, which in the present invention consist of 2x2 sensors, which can conform to the surface of the robot, and the bridge 11 connects each sensing unit, which consists of stretchable meandering wires, so that the sensor network has great stretching ability, and the gap between each unit can be adjusted as required to change the measurement density. As shown in fig. 8, in order to further increase the area of the electronic skin, after the stretched sensor networks are attached to the robot hand, the circuit pins of each sensor network are interconnected by means of electro-jet printing to form an electronic skin with a larger area, and the method can be expanded arbitrarily according to different application objects.
As shown in fig. 9, which is a schematic diagram of the application of the large-area flexible electronic skin integrated with multiple sensors according to the present invention, the process of grasping and sensing a cup of hot water 15 by the robot hand 14 attached with the flexible electronic skin 13 is mainly divided into three stages, I is approaching stage, II is contacting stage, and III is grasping stage. In the stage I, the manipulator needs to adjust the posture of the hand according to the shape of the cup, and the piezoelectric layer is mainly used for measuring the bending of the manipulator, namely one of the pair of interdigital transducers is used as an input transducer, and the other one is used as an output transducer, so that the real-time monitoring of the posture of the manipulator is realized. At the moment, the electronic skin is not acted by external positive pressure, the top electrode of the capacitor layer serves as a planar capacitor sensor, and when the manipulator is gradually close to the water cup, the electric field distribution of the planar capacitor is changed, so that the capacitance value of the manipulator is changed, and the distance between the manipulator and the water cup is monitored. In the stage II, the manipulator is just contacted with the water cup, and at the moment, under the action of positive pressure, the electrode on the middle pyramid of the middle node layer of the capacitor layer is contacted with the top electrode of the capacitor layer, so that the planar capacitor sensor is converted into a resistance-type temperature sensor, and the temperature of the water cup is measured. In stage III, the bottom electrode, the top electrode and the middle multi-level pyramid structure of the capacitance layer form a parallel plate capacitance sensor. The manipulator gradually grips the water cup, and the middle dielectric region of the capacitor layer is extruded under the action of pressure, so that the capacitor is changed, and the static pressure is measured. At this time, the top electrode also serves as a resistance temperature sensor, and the temperature and the pressure are measured in a coordinated and time-sharing manner through switching on a circuit. The measuring unit of the three-way force is composed of four multifunctional sensing units, under the action of the horizontal force, the change of the parallel plate capacitance in the four sensing units is different, and the magnitude and the direction of the horizontal force can be judged through decoupling of an algorithm. At this time, all the interdigital electrode pairs of the piezoelectric layer are used as outputs for sensing vibration signals in the gripping process.
Fig. 10 is a logic diagram of a measurement procedure of flexible electronic skin. Firstly, reading the resistance R of the top electrode of the capacitor layer, and if the resistance R is infinite, the manipulator is not acted by the positive surface pressure force, so that the manipulator is judged to be in the approaching stage. The piezoelectric layer is used as a surface acoustic wave sensor, i.e. as an output and an input, respectively, in the electrode layer 3 for measuring the attitude measurement of the robot arm. Meanwhile, double-spiral top electrodes 8 of the top electrode layer in the capacitor layer are not connected with each other and are used as a planar capacitance sensor for measuring the approaching distance between the manipulator and the object. And conversely, if the R is not infinite, the manipulator is in a contact or gripping stage, and the interdigital transducers of the piezoelectric layer are used as outputs for measuring vibration signals in the gripping process. The double-helix top electrode 8 of the top electrode layer in the capacitance layer is conducted for measuring the temperature of a contact object, a parallel plate capacitance is formed between the top electrode layer and the bottom electrode layer of the capacitance layer for measuring static pressure, and the combination of the 2 multiplied by 2 sensors is used for measuring three-way pressure.
As shown in fig. 11, it is a flow chart of a manufacturing process of a multi-module flexible sensor, comprising the following steps:
preparation of first partial capacitor layer
Preparation of S1 Flexible substrate and bottom electrode layer
Selecting a hard substrate, coating a resist on the substrate as a sacrificial layer, then depositing the bottom electrode, obtaining the bottom electrode layer after curing, coating a layer of material of the flexible substrate on the bottom electrode layer to form the bottom flexible substrate on the bottom electrode layer, removing the sacrificial layer to separate the bottom electrode layer from the hard substrate, and obtaining the bottom flexible substrate and the bottom electrode layer;
preparation of S2 intermediate dielectric layer
Selecting a mold, and pouring silicone rubber liquid on the mold to obtain a required intermediate dielectric layer, wherein the silicone rubber is preferably PDMS or platinum catalyzed silicone rubber Ecoflex;
preparation of S3 conductive electrode
Forming the conductive electrode on a preset flexible bulge on the intermediate dielectric layer by adopting an electric spray printing method so as to obtain the intermediate dielectric layer provided with the conductive electrode on the preset flexible bulge;
preparation of S4 top layer flexible substrate and top electrode layer
Selecting a hard substrate, coating a sacrificial layer on the hard substrate, then coating a material of a flexible substrate on the sacrificial layer, curing to obtain a flexible substrate, removing the sacrificial layer, and separating the flexible substrate from the hard substrate to obtain a required top-layer flexible substrate;
s5 Assembly of capacitor layers
And sequentially stacking the bottom layer flexible substrate, the bottom electrode layer, the middle medium layer with the conductive electrode arranged on the preset flexible protrusion and the flexible substrate layer by layer from bottom to top so as to obtain the required capacitor layer.
Preparation of the second part of the substrate and the piezoelectric layer
Preparation of S6 electrode layer
Selecting a hard substrate, uniformly coating a sacrificial layer on the hard substrate, and depositing a layer of interdigital transducer on the sacrificial layer to obtain the electrode layer, wherein the sacrificial layer adopts …;
s7, coating a layer of piezoelectric material on the electrode layer, heating, curing and annealing to obtain the piezoelectric material layer, wherein the annealing temperature is 280-320 ℃, organic matters in the piezoelectric material are removed in 10min, and 650-750 ℃ and 1min form a perovskite phase of PZT;
s8, coating a layer of substrate material on the piezoelectric material layer, and curing to obtain the substrate, thereby obtaining the required piezoelectric layer on the hard substrate;
s9, removing the sacrificial layer by using a laser lift-off technology, and peeling the piezoelectric layer from the hard substrate to obtain the required piezoelectric layer, wherein the laser beam width of the laser lift-off is preferably 20mm and 0.5mm, the laser energy is preferably 40mJ, and the scanning speed is preferably 2 mm/S.
The present invention will be further illustrated with reference to specific examples.
Preparation of first partial capacitor layer
And S4 preparing a bottom electrode layer of the capacitor layer. Taking a clean silicon wafer 17, spin-coating and stripping the resist 18 to serve as a sacrificial layer, then spin-coating an AZ5214 positive photoresist, and after photoetching and developing, carrying out magnetron sputtering on metal chromium/gold (Cr/Au) to serve as a bottom electrode 5; polydimethylsiloxane rubber (PDMS, Sylgard 184) was formulated according to prepolymer: curing agent 10: 1, mixing and stirring uniformly, removing bubbles in vacuum, then homogenizing PDMS4 on the bottom electrode, baking and curing at 90 ℃ on a hot plate for 10 min; finally, removing the stripping resist by using AZ400K developing solution to obtain a bottom electrode layer;
preparing an intermediate dielectric layer of the capacitor layer. Taking a piece<100>A crystal-oriented silicon wafer 22 with the surface coated with SiO 300nm thick2Cleaning and drying the film; then homogenizing the photoresist S1805 at the speed of 4000rpm/S for 60S, baking the photoresist at 115 ℃ for 60S by a hot plate, and baking at 115 ℃ for 5min after exposure and development to obtain the photoresist 23 for wet etching; placing the sample in 4% tetramethylammonium hydroxide (TMAH) solution, etching at 90 ℃ to obtain a pyramid groove structure, sequentially placing the pyramid groove structure in buffer etching solution for etching for 4min, and performing etching treatment on the pyramid groove structure in a 65% HNO 3: 40% NH4F ═ 2: etching in the solution 1, carrying out hydrophilic treatment on the silicon mold by using oxygen plasma, and then carrying out hydrophobic treatment in an OTS/n-heptane solution for 1h to obtain a pyramid silicon mold with different levels and heights; pouring the prepared PDMS solution into a mold, heating and curing, and tearing off the cured PDMS rubber from the mold to obtain an intermediate dielectric layer 6 with a multilayer pyramid microstructure; adopting an electric spray printing mode to print conductive silver paste, wherein an electric spray printing nozzle 12 in the figure prints the conductive silver paste as an electrode 7 on the middle pyramid;
and preparing a top electrode layer of the capacitor layer. The process is similar to the preparation process of the bottom electrode layer, but different photoetching masks are adopted, the structural form of the top electrode is a double-spiral structure, and the material is metal platinum (Pt) sensitive to temperature;
preparation of the second part of the base layer and the piezoelectric layer
Preparation of the S1 base layer. Taking a clean round silicon wafer, uniformly gluing polymethyl methacrylate (PMMA) as a sacrificial layer, wherein the glue-spreading speed is 3000rpm, the time is 45s, the baking temperature on a hot plate is 180 ℃, and the baking time is 5 min; then homogenizing a polyimide precursor solution (Beijing Bomi science and technology Co., Ltd., viscosity 4000cp), homogenizing at 1500rpm for 120s, baking on a hot plate for 130 ℃ and 60s, repeatedly homogenizing for one time, baking in an oven at 220 ℃ for 2h, and performing imidization treatment to obtain a Polyimide (PI) layer with the thickness of 14 um; then, spin coating the positive photoresist AZ5214 at a spin coating speed of 3000rpm for 60s, a baking temperature of a hot plate of 90 ℃ for 60s, exposing for 9s by a photoetching machine, and developing for 45 s; depositing metal chromium/gold (Cr/Au) by magnetron sputtering, and removing the photoresist by using acetone to obtain an etching mask of the stretchable meandering substrate; finally, in a reactive ion etching machine, etching the polyimide by oxygen plasma, heating the polyimide in acetone by a water bath for 30min at 65 ℃ to obtain a polyimide substrate with a stretchable meandering structure;
s2 preparation of piezoelectric layer. Firstly, preparing a lead zirconate titanate piezoelectric ceramic (PZT) precursor gel solution, wherein the molar concentration ratio of metal ions is Pb: zr: 1.15-1.2% of Ti: 0.52: 0.48, weighing solutes of lead acetate trihydrate, zirconium nitrate pentahydrate and tetrabutyl titanate, dissolving the solutes in a glycol monomethyl ether or acetylacetone solvent, and forming a precursor solution of PZT through hydrolysis reaction and polymerization reaction; adjusting the pH value and fixing the volume to 0.4-0.6 mol/L, naturally cooling, standing, sealing, and aging for 40-80 hours under natural conditions to form a PZT precursor gel solution; preparing a PZT sacrificial layer, namely taking a clean sapphire substrate 19, cleaning, uniformly coating the PZT precursor gel solution on the clean sapphire substrate, curing and hardening, further heating to remove organic matters, repeatedly coating multiple layers in such a way to reach the expected thickness of the piezoelectric layer, and annealing at high temperature to form a perovskite phase as the PZT sacrificial layer 20;
then photoresist is uniformly coated, after photoetching and developing, electrode chromium/gold (Cr/Au) of the interdigital transducer pair is subjected to magnetron sputtering, and the photoresist is removed in acetone to obtain an electrode layer 3 of the piezoelectric layer;
preparing the PZT piezoelectric material layer 2, which is similar to the preparation process of the PZT sacrificial layer and is not described again; then, glue is uniformly distributed on the polyimide precursor solution, and the polyimide precursor solution is baked and cured by a hot plate at the early stage and imidized at high temperature to form a uniform Polyimide (PI) film 1 as a substrate;
releasing the piezoelectric layer from the substrate by adopting a laser stripping mode, enabling a laser beam 21 to penetrate through the sapphire substrate 19 and strike on the PZT sacrificial layer to generate interface ablation, wherein the laser beam width for laser stripping is preferably 20mm multiplied by 0.5mm, the laser energy is 40mJ, the scanning speed is 2mm/s, and the ablation thickness is about 100nm, so that the piezoelectric layer is obtained;
preparation of third part stress boss
And S5 preparing a top layer stress boss. Preparing the stressed boss by adopting an aluminum mould, pouring a prepared PDMS solution by adopting a mechanical processing mode for the aluminum mould 16, heating and curing, and carefully removing the mould by using tweezers to obtain the stressed boss 10;
s6, aligning and sticking the piezoelectric layer, the capacitance layer and the top stress boss together to form a 2x2 array multifunctional sensor, and aligning and sticking the small array to the island of the stretchable winding substrate to form a large-area flexible electronic skin.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
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