CN114440759B - Flexible tensile strain sensor based on packaging material structure - Google Patents
Flexible tensile strain sensor based on packaging material structure Download PDFInfo
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- CN114440759B CN114440759B CN202210093766.3A CN202210093766A CN114440759B CN 114440759 B CN114440759 B CN 114440759B CN 202210093766 A CN202210093766 A CN 202210093766A CN 114440759 B CN114440759 B CN 114440759B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
- G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge
- G01B7/18—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge using change in resistance
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
- G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
- G01L1/2287—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges constructional details of the strain gauges
Abstract
The invention discloses a flexible tensile strain sensor based on a packaging material structure. The flexible tensile strain sensor is symmetrically arranged and comprises a strain sensor main body and a strain signal reading interface; the two ends of the strain sensor main body are respectively provided with a strain signal reading interface, and the strain signal reading interfaces at the two ends of the strain sensor main body are respectively and electrically connected with the two ends of the strain sensor main body; the strain sensor main body is composed of a strain sensor flexible substrate, a conducting layer and a main body packaging layer, the conducting layer is attached to the outer surface of the strain sensor flexible substrate, the strain sensor flexible substrate attached with the conducting layer is embedded in the main body packaging layer, and strain signal reading interfaces at two ends of the strain sensor flexible substrate penetrate through the corresponding ends of the main body packaging layer respectively and then are arranged at two ends of the strain sensor flexible substrate. The strain sensor has the characteristics of good tensile property, large measurement range and the like, and can adjust the structure of the sensor according to requirements, adjust the stress distribution of the flexible tensile strain sensor in the working process, control the mechanical property of the sensor and improve the adaptability of the sensor.
Description
Technical Field
The invention relates to a tensile strain sensor in the technical field of sensors, in particular to a flexible tensile strain sensor based on a packaging material structure.
Background
With the rapid development of artificial intelligence and robot technology and the popularization of intelligent terminals, the development of modern science and technology and the improvement of automation level, the fields of robots, wearable devices and the like present huge market prospects, and will have great influence on the future development of the human society. The flexible tensile strain sensor has wide application value in the fields of robots, wearable equipment and the like. The method is generally used in the fields of wearable human-computer interaction interfaces, wearable health monitoring equipment, soft robot flexible skin and the like. The flexible tensile strain sensor can reflect the deformation amount of the flexible tensile strain sensor through the collected electric signal data, and can be used as a wearable human-computer interaction interface, such as a data glove, for collecting the human body motion data of a user and controlling the operation of a robot or other equipment; the wearable health monitoring device can be used as a wearable health monitoring device, such as a wearable health monitoring device arranged at a knee joint and an elbow joint, and is used for collecting and monitoring joint angle data of a wearer body and estimating the limb movement health state of the wearer; the embedded soft robot body or the embedded soft robot body covers the surface of the robot body, the driving state of the soft robot is monitored in real time, for example, the bending state of the crawling soft robot body driven by positive pressure is fed back, and the crawling of the soft robot is controlled.
The traditional flexible substrate material of the flexible strain sensor is an elastic film with a smooth surface, the conductive sensitive layer embedded or covered on the surface of the flexible strain sensor is also of a planar structure, and a crack mechanism is introduced into the conductive sensitive layer, namely, the conductive sensitive layer cracks along with the tensile deformation of the strain sensor, so that the number of conductive paths is suddenly changed, and the detection sensitivity of the flexible strain sensor to micro stress can be improved. The flexible strain sensor is limited by a plane structure, the deformation capability of the flexible strain sensor in a micro-stress state is limited, and the flexible porous substrate material with a three-dimensional skeleton structure is adopted, so that the deformation capability of the flexible strain sensor in the micro-stress state can be remarkably improved, and the detection sensitivity of the flexible strain sensor to the micro-stress is enhanced. But the physical property of the flexible porous substrate material is limited, the stretching deformation quantity is determined by the material property, and the flexible stretching strain sensor based on the flexible porous substrate material generally has the defect of poor stretching mechanical property; and the pore distribution of the flexible porous material as the substrate material is not completely uniform, and the flexible porous material is stressed in the stretching process, so that the problems that the crack forming position is difficult to control, the reproducibility is low, the crack propagation degree is low, and the larger stretching deformation detection range is difficult to cover exist.
Disclosure of Invention
In order to solve the problems that a conductive layer of an existing flexible strain sensor is easy to fall off, low in stability, small in measurement range, uncontrollable in sensing performance, poor in repeatability and the like, the invention provides a flexible tensile strain sensor based on a packaging material structure, which can be applied to a human-computer interaction interface, wearable health monitoring equipment and flexible electronic skin of a soft robot, and the strain range, performance, sensitivity and stability of the strain sensor are improved.
The technical scheme adopted by the invention for solving the problems is as follows:
the flexible tensile strain sensors are symmetrically arranged and comprise strain sensor main bodies and strain signal reading interfaces;
the two ends of the strain sensor main body are respectively provided with a strain signal reading interface, and the strain signal reading interfaces at the two ends of the strain sensor main body are respectively and electrically connected with the two ends of the strain sensor main body;
the strain sensor main body is composed of a strain sensor flexible substrate, a conducting layer and a main body packaging layer, the conducting layer is attached to the outer surface of the strain sensor flexible substrate, the strain sensor flexible substrate attached with the conducting layer is embedded in the main body packaging layer, and strain signal reading interfaces at two ends of the strain sensor main body penetrate through the corresponding ends of the main body packaging layer respectively and then are arranged at two ends of the strain sensor flexible substrate.
At least one side of four sides of a main body packaging layer of the strain sensor main body is provided with a protruding structure, the protruding structure is composed of a plurality of protruding units which are arranged in parallel and at intervals along the stretching direction of the strain sensor, and each protruding unit is arranged on the current side.
The protruding units have the same or different sizes, and the intervals between the adjacent protruding units are the same or different.
At least one side of four sides of a main body packaging layer of the strain sensor main body is provided with a concave structure, the concave structure is composed of a plurality of concave units which are arranged in parallel along the stretching direction of the strain sensor at intervals, and each concave unit is arranged on the current side.
The sizes of the concave units are the same or different, and the intervals between the adjacent concave units are the same or different.
The strain sensor body is prepared by the following method:
firstly, carrying out laser cutting on a flexible porous material to obtain a flexible substrate of the strain sensor, dissolving a nano conductive material in a dispersing agent and uniformly stirring to obtain a nano conductive material mixed solution, then placing the flexible substrate of the strain sensor in the nano conductive material mixed solution to be fully soaked and dried until the dispersing agent is completely volatilized, so that a conductive layer is formed on the outer surface of the flexible substrate of the strain sensor, and the flexible substrate of the conductive strain sensor is obtained; and finally, placing the flexible substrate of the conductive strain sensor in a mold, pouring an external packaging material into the mold, heating, curing and molding to form a main body packaging layer, and obtaining the strain sensor main body.
The strain signal reading interface mainly comprises a copper needle and a signal transmission lead connected with the copper needle, the copper needle is arranged in the strain sensor main body, and the copper needle is electrically connected with the strain sensor main body.
The axial direction of the copper needle is parallel to or perpendicular to the stretching direction of the stretching strain sensor.
The working principle of the invention is as follows:
the flexible strain sensor is internally deformed when being subjected to external tensile force, so that the number of microscopic filamentous conductive paths inside the strain sensor is changed, the resistance value is changed, and the magnitude of the electrical parameter resistance value of the strain sensor can be used for detecting external force and deformation.
The flexible substrate of the strain sensor has a skeleton structure, and a plurality of micro-cracks can be generated during stretching to cause irregular breakage of the substrate, so that the main reasons for uneven breakage are as follows: errors exist in the forming process of the flexible substrate of the strain sensor, so that the geometric shape of the outer part of the flexible substrate of the strain sensor is not completely symmetrical; errors exist in the porous structure of the flexible substrate of the strain sensor in the forming process, so that the size and the distribution of gaps of the porous structure in the flexible substrate of the strain sensor are not uniform; the inner pores of the silica gel filled sponge are not uniform; the position precision of the flexible substrate of the strain sensor is low and the flexible substrate is not positioned in the positive center of the packaging material; the outer packaging material has defects such as bubbles, which cause tensile stress concentration.
In order to solve the problem of irregular fracture of the flexible substrate of the strain sensor, the invention designs a plurality of fracture-controllable external packaging material structures, so that fracture controllability of the flexible substrate of the strain sensor in the sensor is realized, and the linearity and the fitting accuracy of signals are improved.
The technical scheme for realizing controllable fracture is as follows: constructing regular convex structures or concave structures on the left side surface, the right side surface or the front side surface and the rear side surface of the strain sensor main body, wherein the shapes of the convex structures or the concave structures are semi-ellipses and are connected with the surface of the strain sensor main body through fillets; the stress concentration phenomenon of the strain sensor main body corresponding to the structure in the stretching process can be reduced through the convex structure, the breakage of the flexible substrate of the strain sensor and the conductive sensitive material attached to the surface of the substrate is inhibited, and the stability of an electric signal is ensured; the stress concentration position of the strain sensor main body in the stretching process can be controlled by designing the position and the size of the concave structure, so that the fracture crack position of the flexible substrate of the conductive strain sensor is controllable, and the regulation and control of the sensing performance are realized according to actual requirements.
The invention improves the sensitivity of the flexible sensor and is embodied in that: the thin flexible porous material is adopted as the substrate material, the substrate material has a framework structure, a plurality of microcracks can be generated during stretching to increase the resistance, the electric signal change is obvious when unit deformation occurs, and the electric conduction sensitivity of the substrate material is improved. In addition, the strong adsorbability of the porous material also provides a proper carrier for the conductive sensitive material, and the skeleton structure of the porous material enables the porous material to deform greatly, so that the stretchability is improved.
The invention improves the tensile property of the flexible tensile strain sensor by the following steps: a layer of material is packaged outside the flexible substrate of the strain sensor and covers the surface of the flexible substrate of the traditional flexible strain sensor, so that the flexible substrate of the strain sensor and the conductive material are both embedded in the external packaging material, and when external tensile stress acts on the upper end and the lower end of the strain sensor body, the external packaging material has stretchability, namely, a larger elastic deformation range. The silica gel has higher stretchability, can be used as an external packaging material, and simultaneously isolates the substrate material from directly contacting the outside with the conductive layer to prevent the substrate material from rubbing to cause the conductive sensitive material to fall off; the flexible porous material is prevented from absorbing substances such as water vapor in the air to cause property change, and the stability of the sensor is improved.
The invention improves the repeatability of the manufacture of the flexible tensile strain sensor and is characterized in that: the special structural treatment is carried out on the packaging structure, so that the stress dispersion and the fracture distribution are more uniform, the cracks are controllable, the larger deformation can be borne, the stretchability of the sensor is improved, and the consistency of the cracks is also improved. The corresponding mould is designed to ensure that the packaging structure formed each time is the same, and the mould is fixed to ensure that the flexible substrate of the strain sensor can be arranged at the central position of the packaging material and distributed in central symmetry to ensure that the sensor can be manufactured repeatedly.
The method comprises the steps of dissolving a nano conductive material (such as a carbon nano material, nano metal particles, nano metal wires, nano metal sheets and the like) in a dispersing agent (such as n-hexane and the like), solidifying the nano conductive material on the surface of a flexible substrate (such as polyurethane sponge and the like) of the strain sensor, placing the flexible substrate in a designed mould, taking silica gel as an encapsulating material, removing residual air in the external encapsulating material by using a vacuum defoaming machine, preparing a sponge dielectric layer with an excellent strain-resistance curve, and completing the preparation of the flexible strain sensor by a simple method.
The invention has the beneficial effects that:
the unique external packaging structure of the flexible sensor is easy to deform such as stretching, has the advantages of larger deformation amount than an unpackaged structure, larger measurement range, capability of recovering the original state and keeping the sensing function under the condition of large deformation amount under the condition of the same external force action, can effectively buffer the contact of an object (such as a human body) and a sensing device, reduce the interference of the external environment to the sensor, improve the stability of the sensor during working, be applied to various working scenes and prolong the fatigue life. High strength, tensile structure through optimal design strain sensor main part are showing and are improving the tensile properties who uses the flexible strain sensor of porous material as the basement, have effectively overcome traditional packaging structure's flexible strain sensor mechanical properties poor, easy fracture, and measuring range is little and the lower problem of sensitivity. Through the design of the special structure of the external packaging material, the fracture position of the flexible substrate of the strain sensor is controllable, and the stability and the linearity of electric signal transmission are improved.
On the other hand, the invention can change the characteristics of the sensor by changing the structural design, allows a user to freely adjust and obtain flexible sensors with different sensitivities and measurement ranges according to different use conditions to adapt to different requirements, and has a wider use range. The arrangement surface, arrangement mode and size of the convex structure or the concave structure can be customized, adjusted and optimized according to the actual use condition, for example: when the strain sensor is attached to the surface of a human body and used as a bending strain sensor, the surface, which is used as a bending and stretching strain sensor and is attached to the surface of the human body, needs to be flat and has no special structure, so that the comfort is ensured, and the special structure on the other surface plays a role in performance regulation. The stress on different positions is not uniform, for example, when the sensor is attached to the outer surface of an elbow joint, the stress on the salient point of the elbow joint is the largest, so that the convex units or the concave units are densely arranged in the middle and sparse at two ends, and the uniform stress on sensitive materials in the sensor can be ensured; the size of the convex unit or the concave unit in the middle of the strain sensor main body is required to be enlarged, the size of the convex unit or the concave unit at the two ends of the strain sensor main body is required to be reduced, the stress action can be enhanced in the middle to initiate fracture, or the stress is reduced to inhibit fracture, and then the sensing performance is improved in the aspects of linearity, tensile strain degree and the like.
Drawings
FIG. 1 is a schematic diagram of the basic structure of a high tensile flexibility strain sensor based on porous flexibility material; wherein the copper pins are arranged longitudinally in the strain sensor;
FIG. 2 is a front view and a half sectional view of a high tensile flexibility strain sensor based on a porous flexible material according to the present invention;
FIG. 3 is a schematic diagram of a structure in which copper pins are arranged in a strain sensor in a transverse direction;
FIG. 4 is a schematic view of two positions of a raised structure on a strain sensor frame;
FIG. 5 is a schematic view of two positions of a recessed feature on a strain sensor frame;
FIG. 6 is a process flow of manufacturing a sensor body, taking a basic structure as an example;
FIG. 7 is a schematic diagram of a mold in a manufacturing process of a sensor body with a bump structure;
FIG. 8 is a schematic diagram of a mold in a manufacturing process of a sensor body with a recessed structure;
in the figure: the strain sensor comprises a strain sensor body 1, a strain signal reading interface 2, a strain sensor flexible substrate 3, a conductive layer 4, a body packaging layer 5, copper pins 6, signal transmission leads 7, left and right side surfaces 8, upper and lower side surfaces 9, a convex unit 10 and a concave unit 11.
Detailed Description
The invention is further described below with reference to the following examples:
as shown in fig. 1 and 2 (a) and 2 (b), the flexible tensile strain sensor of the present invention is symmetrically arranged, and comprises a strain sensor body 1 and a strain signal reading interface 2;
the two ends of the strain sensor main body 1 are respectively provided with a strain signal reading interface, and the strain signal reading interfaces at the two ends of the strain sensor main body 1 are respectively and electrically connected with the two ends of the strain sensor main body;
the strain sensor main body 1 is composed of a strain sensor flexible substrate 3, a conducting layer 4 and a main body packaging layer 5, the conducting layer 4 is attached to the outer surface of the strain sensor flexible substrate 3, the strain sensor flexible substrate 3 attached with the conducting layer 4 is embedded in the main body packaging layer 5, the main body packaging layer 5 completely wraps the strain sensor flexible substrate 3 attached with the conducting layer 4, and strain signal reading interfaces at two ends of the strain sensor flexible substrate 3 penetrate through corresponding ends of the main body packaging layer 5 respectively and then are arranged at two ends of the strain sensor flexible substrate 3.
At least one of the four outer side surfaces of the strain sensor body 1 is provided with a protruding structure, the protruding structure is composed of a plurality of protruding units 10 which are arranged in parallel and at intervals along the stretching direction of the strain sensor, each protruding unit 10 is arranged on the current side surface, and two side surfaces of each protruding unit 10 are flush with two adjacent side surfaces of the current side surface. As shown in fig. 4, (a) and (b) of fig. 4 are schematic views in which the left and right side surfaces 8, and the upper and lower side surfaces 9 of the strain sensor frame are provided with the convex structures, respectively. It is effective in suppressing breakage at a position where breakage is likely, but tensile properties are inferior to those of the basic structure because the internal conductive sponge is not broken. In addition to the transverse rupture of the sponge caused by the width shrinkage during stretching, the sponge is also ruptured in the longitudinal direction due to the reduction of the thickness. The breaking position is controllable, so that the conductivity of the sponge substrate is uniform, but the resistance of the breaking position is suddenly changed.
The sizes of the protruding units 10 are the same or different, and the distances between the adjacent protruding units 10 are the same or different, and are specifically set according to actual situations.
Be provided with sunk structure on at least one side in four lateral surfaces of strain sensor main part 1, sunk structure comprises a plurality of sunken units 11 that are parallel and the interval arrangement along strain sensor's tensile direction, and each sunken unit 11 is all seted up on current side, and two side notches of every sunken unit 11 flush with two adjacent sides of current side. As shown in fig. 5, in which (a) and (b) of fig. 5 are schematic views of the strain sensor frame in which the left and right side surfaces 8 and the upper and lower side surfaces 9 are provided with the concave structures, respectively, the longitudinal breakage of the conductive sponge can be controlled.
The sizes of the concave units 11 are the same or different, the distances between the adjacent concave units 11 are the same or different, and the concave units are specifically arranged according to actual conditions, in the embodiment, the convex units 10 and the concave units 11 are both semi-elliptical and are excessively connected with the surface of the strain sensor body 1 through round corners; the stress concentration phenomenon of the strain sensor main body 1 corresponding to the structure in the stretching process can be reduced through the convex structure, and the fracture of the flexible substrate 3 of the strain sensor and the conducting layer 4 attached to the surface of the substrate is inhibited; the stress concentration position of the strain sensor main body 1 in the stretching process can be controlled by designing the position and the size of the concave structure, so that the fracture crack position of the flexible substrate 3 of the conductive strain sensor is controllable, and the sensing performance is further regulated and controlled.
The setting surface of the convex structure or the concave structure can be customized, adjusted and optimized according to the actual use condition, for example: when the strain sensor is required to be attached to the surface of a human body, the surface, which is used as a bending and stretching strain sensor and is attached to the surface of the human body, is required to be flat and has no special structure, so that the comfort is ensured, and the special structure on the other surface plays a role in performance regulation. Therefore, the arrangement surface of the convex structure or the concave structure can be adjusted according to actual use requirements, and the front surface 9 and the rear surface 10 are not necessarily both provided with structures;
the arrangement mode of the convex structures or the concave structures can be customized, adjusted and optimized according to the actual use condition, for example: when the strain sensor is attached to the surface of a human body and used as a bending strain sensor, the stress borne by different positions is not uniform, for example, when the strain sensor is attached to the outer surface of an elbow joint, the stress at the salient point of the elbow joint is the largest, so that the structure arrangement of the convex units or the concave units is dense in the middle and sparse at two ends, the stress uniformity of sensitive materials in the sensor can be ensured, and the sensing performance is improved from the aspects of linear degree, tensile strain degree and the like; therefore, the arrangement mode of the convex structures or the concave structures can be adjusted according to the actual use requirement and is not necessarily evenly arranged at equal intervals.
The size of the convex structure or the concave structure can be customized, adjusted and optimized according to the actual use condition, for example: when the strain sensor is attached to the surface of a human body and used as a bending strain sensor, the stress applied to different positions is not uniform, for example, when the strain sensor is attached to the outer surface of an elbow joint, the stress at the salient point of the elbow joint is the largest, so that the size of the convex unit 10 or the concave unit 11 in the middle of the strain sensor main body 1 is expanded, the size of the convex unit 10 or the concave unit 11 at two ends of the strain sensor main body 1 is reduced, the stress action can be enhanced at the middle to cause fracture, or the stress is reduced to inhibit fracture, and the sensing performance is further improved in the aspects of linearity, tensile strain degree and the like; in addition, the shape of the convex unit 10 or the concave unit 11 can be changed, such as a triangle, which can improve the stress concentration effect; therefore, the size and shape of the convex structure or the concave structure can be adjusted according to the actual use requirement, and are not necessarily fixed.
The structure of the strain sensor main body 1 is a thin structure with a large length-width ratio, and the left side surface 8, the right side surface 8, the upper side surface 9 and the lower side surface 9 of the strain sensor main body can be provided with regular convex structures or concave structures; the stress concentration phenomenon of the strain sensor main body 1 corresponding to the structure in the stretching process can be reduced through the convex structure, and the fracture of the flexible substrate 3 of the strain sensor and the conductive sensitive material of the conductive layer 4 attached to the surface of the substrate is inhibited; the stress concentration position of the strain sensor main body 1 in the stretching process can be controlled by designing the position and the size of the concave structure, so that the fracture crack position of the strain sensor flexible substrate 3 attached with the conductive layer 4 is controllable, and the sensing performance is further regulated and controlled; because of the stability and the sealing property of the silica gel, the silica gel can be used as an external packaging material 5 to ensure the stable work of the sensor, and can be used for various working conditions of humidity and underwater.
The strain sensor body 1 is prepared by the following method:
firstly, a flexible substrate 3 of the strain sensor is obtained after flexible porous materials (such as polyurethane sponge and melamine sponge) are subjected to laser cutting, the flexible substrate 3 of the strain sensor is in a long and narrow thin film structure and has a high length-width ratio, and the flexible substrate is ensured to be easily packaged and easily applied to the fields of wearable health monitoring equipment, human-computer interaction interfaces and soft robot skin; the internal microscopic porous structure of the conductive sensitive material can be used as a carrier of the conductive sensitive material, so that the continuous transmission of electric signals is ensured, and the electrical property of the conductive sensitive material is changed by changing the deformation degree of the internal microscopic structure; dissolving a nano conductive material (such as a carbon nano material, a nano metal particle, a nano metal wire and a nano metal sheet) in a dispersing agent (deionized water, ethanol, isopropanol and n-hexane) according to the mass concentration of 30g/L, uniformly stirring to obtain a nano conductive material mixed solution, then placing the flexible substrate 3 of the strain sensor in the nano conductive material mixed solution, fully soaking, and drying until the dispersing agent is completely volatilized, so that a conductive layer 4 is attached to the flexible substrate 3 of the strain sensor, namely the nano conductive material is fully covered on the flexible substrate of the porous strain sensor to obtain the flexible substrate of the conductive strain sensor, and the nano conductive material forms a conductive path on the surface of the flexible porous substrate to obtain the flexible substrate of the conductive strain sensor; and finally, placing the flexible substrate of the conductive strain sensor in a mold, pouring an external packaging material (namely a silicone-like soft material with a liquid precursor, such as polydimethylsiloxane) into the mold, and heating, curing and molding by a mold method to form a main body packaging layer 5, thereby obtaining the strain sensor main body 1.
The flexible porous material has the characteristics of easy deformation, low density and strong adsorbability, and a conductive path can be constructed on the framework of the flexible porous material by adsorbing the conductive sensitive material to transmit an electric signal; by utilizing the characteristic of the flexible porous material, the sensing principle of the flexible tensile strain sensor is as follows: after the conductive sensitive material is attached to the surface of the flexible porous material, external tensile stress acts on the upper end and the lower end of the strain sensor main body 1, so that the strain sensor main body 1 generates tensile strain, the number of microscopic filiform conductive paths in the conductive layer 4 attached to the flexible substrate 3 of the strain sensor is changed, the resistance value of the sensor main body is triggered to change, and the tensile stress and the tensile strain applied to the strain sensor main body 1 from the outside are reflected by using the resistance value change;
when no external packaging material is added, external tensile stress acts on the upper end and the lower end of the conductive layer 4 attached to the flexible substrate 3 of the strain sensor, so that the conductive layer 4 is cracked, the microscopic filiform conductive paths are cracked, the number of the conductive paths is reduced, and when the flexible substrate 3 of the strain sensor is completely cracked, the electric conduction cannot be realized, the open circuit is realized, and the recovery cannot be realized;
after an external packaging material is added to form a main body packaging layer 5, a flexible substrate 3 of the strain sensor and a conducting layer 4 containing a nano conducting material are both embedded in the main body packaging layer 5, when external tensile stress acts on the upper end and the lower end of a strain sensor main body 1, because the external packaging material has stretchability, namely, a large elastic deformation range, when the strain sensor is subjected to external tensile stress, the flexible substrate 3 of the strain sensor is stretched, microscopic filamentous conducting paths of the nano conducting material are reduced, but because the flexible substrate 3 of the strain sensor and the conducting layer 4 containing the nano conducting material are both embedded in the main body packaging layer 5, the microscopic filamentous conducting paths cannot be completely broken, and the microscopic filamentous conducting paths recover to the original state when the external tensile stress is cancelled, and the shapes and the electrical properties of the flexible substrate 3 of the strain sensor, the conducting layer 4 and the main body packaging layer 5 are recovered to the unstretched state;
therefore, the use of the external packaging material plays a role in improving the sensing performance of the sensor based on the flexible porous material, the measuring range (stretchability) can be expanded, the mechanical property of the sensor can be improved, the fatigue life can be prolonged, and the signal stability can be improved.
The strain signal reading interface 2 mainly comprises a copper needle 6 and a signal transmission lead 7 connected with the copper needle 6, wherein the copper needle 6 is arranged in the strain sensor body 1, and the copper needle 6 is electrically connected with the strain sensor body 1. The copper needle is used as an electrode, is a common copper needle electrode, has the size of more than 500 micrometers, even millimeter level, is slightly larger than the aperture of the strain sensor main body 1, when the electrode is inserted into the strain sensor main body 1, the electrode can be stably contacted with the surface of the strain sensor main body to form a stable circuit for transmitting an electric signal, and is suitable for being used as an electric signal reading interface of a porous material, one end of the exposed copper needle is welded with a signal transmission lead 7, and the electric signal is transmitted to external sensing signal detection equipment.
As shown in fig. 3, the axial direction of the copper needle 6 is parallel or perpendicular to the tensile direction of the tensile strain sensor. Namely, the copper needle 6 has two insertion modes: longitudinal insertion and transverse insertion. The copper needle 6 is longitudinally inserted to be longer in exposed length and easy to connect with a signal transmission lead 7, but the copper needle 6 is made of a metal material and does not have ductility, and the flexible substrate 3 of the strain sensor is made of a flexible porous material and has ductility, so that the copper needle 6 and the flexible substrate 3 of the strain sensor can generate relative displacement in the sensor stretching process; the mode of transversely inserting the copper pin 6 is beneficial to reducing relative displacement, the stability is higher, but the length of the copper pin extending out of the flexible substrate 3 of the strain sensor is limited, the signal transmission lead 7 is inconvenient to connect, and the symmetrical copper pin 6 needs to symmetrically penetrate through the whole flexible substrate 3 of the strain sensor in order to meet the requirement;
after the copper needle 6 is inserted, the main body packaging layer 5 plays a role in fixing and packaging the copper needle 6, so that the copper needle 6 can be tightly attached to the flexible substrate 3 and the conductive layer 4 of the strain sensor, and the stability of the copper needle 6 and the stability of signal reading are ensured.
The preparation process of the flexible tensile strain sensor is as follows:
completely immersing the cut and formed strain sensor flexible substrate 3 (such as polyurethane sponge and melamine sponge) into a solution prepared by a conductive sensitive material (such as carbon nano material, nano metal particles, nano metal wires and nano metal sheets) which is obtained by dissolving a nano conductive material (such as deionized water, ethanol, isopropanol and n-hexane) in a dispersing agent (such as deionized water, ethanol, isopropanol and n-hexane) according to a certain proportion and uniformly stirring, putting the strain sensor flexible substrate 3 which is repeatedly extruded and soaked into an oven and drying until the dispersing agent is completely volatilized, taking out the dried strain sensor flexible substrate 3, washing the strain sensor flexible substrate 3 with the dispersing agent and drying again, and fully covering the nano conductive material on the surface of porous material fibers to form a plurality of conductive paths to obtain the strain sensor flexible substrate 3 attached with the conductive layer 4.
Placing the dried strain sensor flexible substrate 3 attached with the conductive layer 4 in a mold with a corresponding convex structure or concave structure, injecting an external packaging material 5 such as polydimethylsiloxane into the mold, uniformly covering, extracting residual air in the external packaging material 5 by using a vacuum defoaming machine, and then placing in an oven to quickly cure the external packaging material 5; the external packaging material 5 has good fluidity before curing, and can fully fill the pores of the porous material; the cured porous material has good stretchability, the irregular stretch fracture of the porous material is effectively inhibited, and the measuring range of the sensor is improved.
Taking out the external packaging material 5 and the flexible substrate 3 of the strain sensor from the die, inserting copper pins 6 into two sides of the flexible substrate of the conductive strain sensor, and welding signal transmission leads 7; and then placing the flexible tensile strain sensor in a mould without a groove, slowly injecting an external packaging material 5 into the mould until the mould is filled, placing the mould in a vacuum defoaming machine to extract residual air, and then placing the mould in an oven until the external packaging material 5 is cured to obtain the flexible tensile strain sensor.
For the above steps, as the residual air in the external packaging material 5 is extracted by using the vacuum defoaming machine so that the external packaging material 5 is completely immersed into the porous structure of the flexible substrate 3 of the strain sensor, the initial resistance of the sensor is large, the relative change percentage of the resistance is small when the deformation degree is constant, and the sensitivity is low; however, the external packaging material 5 is completely filled in the porous structure, so that the tensile strain strength of the prepared flexible tensile strain sensor can be improved.
If the vacuumizing link is omitted and other manufacturing links are unchanged, the interior of the flexible substrate 3 of the strain sensor is still in a porous structure, the initial resistance is small, the relative change percentage of the resistance is large when the deformation degree is fixed, and the sensitivity is high; and because the external packaging material 5 is not filled in the porous structure, the tensile strain strength of the prepared flexible tensile strain sensor is lower.
The molds are mainly classified into grooved molds and non-grooved molds, and are manufactured by 3D printing, the grooved molds being provided as shown in fig. 7 (a) and (c), fig. 8 (a) and (c), and the non-grooved molds being shown as fig. 7 (b) and (D), fig. 8 (b) and (D). The groove die is mainly used for fixing the flexible substrate of the strain sensor, and the symmetry of the structure and the position accuracy of the substrate are guaranteed. Placing the flexible substrate of the strain sensor in the groove, wherein the lower half part of the flexible substrate of the strain sensor is shielded by the mold, and injecting silica gel into the mold to realize the encapsulation of the upper half part of the silica gel; a groove die is not arranged for supplementing silica gel, the part of the flexible substrate of the strain sensor, which is packaged and connected with the strain signal reading interface, is arranged in the die downwards, the upper half part of the flexible substrate of the strain sensor is shielded by the groove die and is not covered, and the silica gel is injected into the die to realize the packaging of the upper half part of the flexible substrate of the strain sensor, so that the packaging of the whole flexible substrate of the strain sensor is realized. For a particular configuration, the use of a mold configuration as shown in fig. 7 and 8 can be used to make four configurations of sensor bodies.
In order to improve the tensile property, a high length-width ratio is required, and meanwhile, in consideration of the aspects of strength, use scene and the like, a basic structure shown in fig. 1 is designed, wherein the inner part is rectangular thin sponge, and the outer part is rectangular colloidal silica.
The preparation conditions of the high-tensile flexible strain sensor based on the porous flexible material are shown in table 1.
TABLE 1
| Nano conductive material (carbon nano tube) | 3.000g |
| Dispersant (n-hexane) | 100ml |
Taking the basic structure as an example, as shown in fig. 6, the flexible sensor is obtained by the following steps:
i. adding a certain mass of carbon nano tubes into a clean and dry beaker, and adding n-hexane according to the proportion shown in the table 1 to obtain a mixed solution required for soaking. Adding a magnetic stirrer into the mixed solution, and stirring the mixture to be uniform by using a magnetic stirrer. And completely soaking the cut sponge into the mixed solution for about 5 minutes, and continuously extruding the sponge by using a glass rod in the process. And putting the sponge which is repeatedly extruded and soaked into an oven for drying until the dispersing agent is completely volatilized, taking out the dried sponge, cleaning the sponge with n-hexane, drying again, and fully covering the surface of the porous material fiber with the nano conductive material to form a plurality of conductive paths to obtain the porous sponge attached with the nano conductive material.
And ii, putting the dried porous sponge attached with the nano conductive material into a groove of a specified groove die.
iii, according to 1:1, and slowly injecting the liquid silica gel into the mould until the mould is filled.
And iv, placing the mould filled with the sponge and the silica gel into a vacuum defoaming machine to extract residual air.
v. putting the silicon gel into a constant temperature box, and heating the silicon gel for about 20 minutes at about 60 ℃ until the silicon gel is solidified.
And vi, taking out the silica gel and the sponge, inserting the copper needles 6 into the two sides of the porous sponge attached with the nano conductive material, and welding a lead.
And vii, placing the sponge welded with the lead in a non-groove mold, and slowly injecting liquid silica gel into the mold until the sponge is filled.
Repeating steps iv and v to produce a flexible sensor.
Step iv can cause the silica gel to be completely immersed into the porous structure of the sponge, so that the initial resistance of the sensor is large, the relative change percentage of the resistance is small when the deformation degree is constant, and the sensitivity is low. If step iv is omitted, other manufacturing links are unchanged, the interior of the flexible substrate of the strain sensor is still in a porous structure, the initial resistance is small, the deformation degree is fixed, the relative change percentage of the resistance is large, and the sensitivity is high; the tensile strain strength of the prepared flexible tensile strain sensor is low due to the fact that silica gel is not filled in the porous structure.
Claims (6)
1. A flexible tensile strain sensor based on a packaging material structure is characterized in that the flexible tensile strain sensor is symmetrically arranged and comprises a strain sensor main body (1) and a strain signal reading interface (2);
strain signal reading interfaces are respectively arranged at two ends of the strain sensor main body (1), and the strain signal reading interfaces at the two ends of the strain sensor main body (1) are respectively and electrically connected with the two ends of the strain sensor main body;
the strain sensor comprises a strain sensor main body (1) and a strain sensor packaging layer, wherein the strain sensor main body is composed of a strain sensor flexible substrate (3), a conducting layer (4) and a main body packaging layer (5), the conducting layer (4) is attached to the surface of the strain sensor flexible substrate (3), the strain sensor flexible substrate (3) attached with the conducting layer (4) is embedded in the main body packaging layer (5), and strain signal reading interfaces at two ends respectively penetrate through the corresponding ends of the main body packaging layer (5) and then are arranged at two ends of the strain sensor flexible substrate (3);
at least one of four side surfaces of a main body packaging layer (5) of the strain sensor main body (1) is provided with a protruding structure, the protruding structure is composed of a plurality of protruding units (10) which are arranged in parallel and at intervals along the stretching direction of the strain sensor, and each protruding unit (10) is arranged on the current side surface;
or a concave structure is arranged on at least one side surface of four side surfaces of a main body packaging layer (5) of the strain sensor main body (1), the concave structure is composed of a plurality of concave units (11) which are arranged in parallel and at intervals along the stretching direction of the strain sensor, and each concave unit (11) is arranged on the current side surface.
2. A flexible tension strain sensor based on a packaging material structure according to claim 1, characterized in that the protruding units (10) have the same or different sizes and the distance between adjacent protruding units (10) is the same or different.
3. A flexible tension strain sensor based on a packaging material structure according to claim 1, characterized in that the size of the concave units (11) is the same or different, and the distance between adjacent concave units (11) is the same or different.
4. A flexible tensile strain sensor based on an encapsulating material structure according to claim 1, characterized in that the strain sensor body (1) is prepared by the following method:
firstly, carrying out laser cutting on a flexible porous material to obtain a flexible substrate (3) of the strain sensor, dissolving a nano conductive material in a dispersing agent and uniformly stirring to obtain a nano conductive material mixed solution, then placing the flexible substrate (3) of the strain sensor in the nano conductive material mixed solution for fully soaking and drying until the dispersing agent is completely volatilized, so that a conductive layer (4) is formed on the outer surface of the flexible substrate (3) of the strain sensor to obtain the flexible substrate of the conductive strain sensor; and finally, placing the flexible substrate of the conductive strain sensor in a mold, pouring an external packaging material into the mold, heating, curing and molding to form a main body packaging layer (5), and obtaining the strain sensor main body (1).
5. A flexible tension strain sensor based on packaging material structure according to claim 1, characterized in that the strain signal reading interface (2) mainly consists of a copper pin (6) and a signal transmission lead (7) connected with the copper pin (6), the copper pin (6) is arranged in the strain sensor body (1), and the copper pin (6) is electrically connected with the strain sensor body (1).
6. A flexible tensile strain sensor based on a package material structure according to claim 5, wherein the axial direction of the copper needle (6) is parallel or perpendicular to the tensile direction of the tensile strain sensor.
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