Foot stress sensor based on conductive polymer, method for manufacturing a sensing element, and sensor calibration test device and method

Abstract

The application discloses a sole stress sensor based on a conductive polymer, a preparation method of a sensing element, and a sensor calibration testing device and method, and aims to solve the problems of poor practicability, complex structure and high cost of the sensor when the sensor is used to detect normal force and shear force on the feet of a diabetic patient, wherein a sensor assembly and multiple supporting columns are installed between an upper clamping plate and a base plate, the bottom end of the sensor assembly is fixedly connected with the end face of the base plate, and the pressure borne by the upper clamping plate and the base plate is monitored through the sensor assembly. A normal force loading mechanism and a tangential force loading mechanism are fixed on an anti-vibration workbench through a fixing frame, a stress sensor is arranged on the anti-vibration workbench, a linear brake is fixed above the stress sensor, the loading end of the normal force loading mechanism is arranged on the upper end face of the linear brake, and the four tangential force loading mechanisms are arranged on the side faces of the linear brake. The application belongs to the field of sensors and preparation thereof.

Description

TECHNICAL FIELD

[0001] The present application relates to a stress sensor, a preparation method of a sensing element, and a sensor calibration testing device and method, in particular to a conductive polymer-based plantar stress sensor, a preparation method of a sensing element, and a sensor calibration testing device and method, and belongs to the technical field of sensors and preparation thereof. BACKGROUND

[0002] Diabetes is a chronic disease marked by high blood sugar caused by absolute or relative insulin secretion deficiency and utilization disorders. Most diabetes patients will have nerve damage or neuropathy in their limbs (such as feet) due to disease complications, resulting in the loss of tactile perception. This loss of tactile perception can cause the formation of ulcers or open wounds on the skin of the patient's feet. Recent studies have shown that such foot ulcers are related to the normal force and shear force on the plantar part or the bottom of the foot. Therefore, various measurement systems have been developed to attempt to use load sensors and pressure sensors to detect the force applied to the feet of diabetes patients in real time. For example, Systems and Systems use a pressure sensor array to measure the pressure distribution of the foot. However, the disadvantage of these force measurement systems is that although they are sensitive to normal force, they have poor tactile perception of shear force.

[0003] In addition, some sensor array devices have been developed to detect normal force and shear force simultaneously. However, such sensor array devices are large in size and difficult to directly attach to the patient's foot, and have poor practicability. Some compact sensors have also been developed to overcome the above-mentioned disadvantages, integrating two shear sensors and one pressure sensor into an insole. However, due to the complex design, such devices are extremely expensive, and many diabetes patients who can benefit from wearable force monitoring devices cannot afford them. Therefore, there is an urgent need for a small, high-precision, and large-deformation-adaptive foot stress sensor that can dynamically and synchronously measure the normal force and shear force applied to the feet of diabetes patients, and is low in cost. SUMMARY

[0004] The purpose of the present application is to solve the problems of poor practicability, complex structure, and high cost of sensors when detecting normal force and shear force on the feet of diabetes patients using sensors, and to provide a conductive polymer-based plantar stress sensor, a preparation method of a sensing element, and a sensor calibration testing device and method.

[0005] The technical scheme of the present application is as follows:

[0006] The conductive polymer-based plantar stress sensor comprises an upper clamping plate, a substrate, a sensing assembly, and a plurality of support columns.

[0007] The upper clamping plate and the base plate are oppositely arranged, the sensor assembly and the plurality of support columns are installed between the upper clamping plate and the base plate, the bottom end of the sensor assembly is fixedly connected with the end face of the base plate, and the top of the sensor assembly is in contact with the upper clamping plate, and the pressure borne by the upper clamping plate and the base plate is monitored through the sensor assembly.

[0008] Further, the plurality of support columns are evenly arranged around the sensor assembly.

[0009] Further, the sensor assembly comprises a column, a resistance tester and five sensing elements, the column is a cuboid structure, the five sensing elements are arranged on the bottom end face and four side faces of the cuboid respectively, the five sensing elements are connected with one resistance tester, and the five sensing elements are arranged in parallel.

[0010] Further, the sensing element comprises a PDMS elastic layer, a PEDOT:PSS conductive layer, a base layer and two copper electrodes.

[0011] The PDMS elastic layer, the PEDOT:PSS conductive layer, the two copper electrodes and the base layer are sequentially fixedly pasted together, the PDMS elastic layer of each sensing element is in contact with the column, the two parallel copper electrodes are oppositely arranged between the PEDOT:PSS conductive layer and the base layer, and a gap is arranged between the two copper electrodes.

[0012] Further, the support column is a support column made of rubber material, and the column is a cubic column made of polymethyl methacrylate material.

[0013] The preparation method of the sensing element is realized according to the following steps:

[0014] Step one: the silicon substrate is cleaned and treated, and a mixture of PDMS prepolymer and curing agent is spin-coated on the silicon substrate, and a PDMS elastic layer with a thickness of about 2mm is formed by curing;

[0015] Step two: the PEDOT:PSS dispersion liquid is mixed with about 5% volume of dimethyl sulfoxide, and the PEDOT:PSS dispersion liquid and dimethyl sulfoxide mixture is coated on the PDMS elastic layer formed in step one, and a PEDOT:PSS conductive layer with a thickness of about 10μm is formed after drying;

[0016] Step three: the copper electrode is pasted on the same end face of the PEDOT:PSS conductive layer dried in step two through conductive epoxy adhesive, the copper electrode and the PDMS elastic layer are respectively arranged at both ends of the conductive layer, and the PDMS elastic layer dried in step one is cut and separated on the silicon substrate;

[0017] Step four: the two copper electrodes are pasted on the PMMA through ultraviolet curing adhesive, and the sensing element is completed.

[0018] Further, in step one, the elastic layer is prepared: a silicon wafer substrate with a diameter of 100 mm is placed in a cleaning container and ultrasonically cleaned with acetone and isopropyl alcohol for 10 minutes to remove surface impurities, then dried with high-purity nitrogen, PDMS prepolymer and curing agent are taken and mixed according to a mass ratio of 10:1, mechanically stirred for 5 minutes until mixed evenly, the mixed solution is degassed under vacuum for 30 minutes to eliminate bubbles, 25 ml of the mixed solution of the mixed PDMS prepolymer and curing agent is poured onto the surface of the silicon wafer substrate,

[0019] A two-step spin coating process is used to form a uniform thin film: the first step is to spin coat at 500 rpm for 10 seconds to evenly spread the PDMS prepolymer and curing agent mixture on the substrate; the second step is to spin coat at 1000 rpm for 30 seconds, and finally a thin film of the elastic layer with a thickness of about 2 mm is obtained, then the PDMS-coated substrate is placed in a vacuum oven and cured at 80°C for 5 hours to ensure that the PDMS prepolymer and curing agent mixture is fully crosslinked and cured;

[0020] In step two, the conductive layer is prepared: prepare a PEDOT:PSS aqueous dispersion and 5% by volume of dimethyl sulfoxide, add dimethyl sulfoxide to the PEDOT:PSS aqueous dispersion and stir magnetically for 2 hours, place the cured PDMS elastic layer of step one on a spin coating device and spin coat the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution mixture at a speed of 1000 rpm for 30 seconds to form a conductive layer thin film with a thickness of about 10 μm, then place the conductive layer thin film in a drying oven and dry it in an inert nitrogen environment at about 50°C for 1 hour to fully film the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution mixture and improve the uniformity of the conductive layer;

[0021] In step three, the copper electrodes are pasted: a diamond scribe is used to precisely cut the PDMS elastic layer of step one and the PEDOT:PSS conductive layer of step two according to a size of 1.5 mm x 1.5 mm, and two copper pieces with a size of 1.5 mm x 0.5 mm x 1.5 mm are pasted to the same end face of the PEDOT:PSS conductive layer through conductive epoxy, after pasting, the pasted PDMS elastic layer, PEDOT:PSS conductive layer and copper electrodes are heated and cured at 120°C for 30 minutes, after curing, the silicon wafer substrate of step one is separated on the elastic layer, then a PMMA base layer with a size of 1.5 mm x 1.5 mm x 1 mm is pasted and fixed to the two copper electrodes using ultraviolet curing glue, and the ultraviolet curing glue is cured by irradiation with 365 nm ultraviolet light for 60 seconds, completing the fabrication of a single sensing element.

[0022] The foot stress sensor calibration test device based on conductive polymer includes a shockproof workbench, a fixing frame, a linear brake, a normal force loading mechanism and four tangential force loading mechanisms;

[0023] The normal force loading mechanism and the tangential force loading mechanism are fixed to the anti-vibration workbench through a fixing frame. A foot stress sensor based on a conductive polymer is set on the anti-vibration workbench. The linear brake is fixed above the foot stress sensor based on a conductive polymer. The loading end of the normal force loading mechanism is pressed against the upper end surface of the linear brake, and the four tangential force loading mechanisms are respectively pressed against the side surfaces of the linear brake.

[0024] Furthermore, the normal force loading mechanism includes a normal loading cylinder, a normal air supply tank and a normal solenoid valve; the tangential force loading mechanism includes a tangential loading cylinder, a tangential air supply tank and a tangential solenoid valve;

[0025] The shell of the normal loading cylinder is fixedly installed on the fixed frame, the normal air supply tank is connected to the normal loading cylinder and supplies air, the normal solenoid valve is installed on the normal air supply tank and controls the operation of the normal air supply tank, the piston end of the normal loading cylinder is at the center position of the upper end surface of the linear brake, the shell of the tangential loading cylinder is fixedly installed on the fixed frame, the tangential air supply tank is connected to the tangential loading cylinder and supplies air, the tangential solenoid valve is installed on the tangential air supply tank and controls the operation of the tangential air supply tank, and the piston end of the tangential loading cylinder is at the side end surface of the linear brake.

[0026] A conductive polymer-based foot stress sensor calibration and testing method is implemented as follows: normal pressure is applied to align the piston end of the normal loading cylinder with the center of the linear brake, the output pressure is adjusted by the normal loading cylinder, tangential pressure is applied to align the piston end of the tangential loading cylinder with the side of the linear brake, a resistance tester is connected in parallel to the copper electrodes of five sensing elements, a 1kHz measurement signal is introduced, and the resistance change of each sensing element is recorded in real time.

[0027] First, the initial resistance of each sensor element was measured in a stress-free state to confirm that its baseline was approximately 50 kΩ. Normal stress calibration was then performed: the normal load cylinder pressure was gradually increased to 530 kPa and held for 10 seconds. The resistance of the sensor element located at the bottom end face of sensor assembly 3 was recorded as a function of stress. The loading and unloading cycles were repeated multiple times to evaluate the hysteresis error of the sensor output.

[0028] Then, perform shear pressure calibration: calibrate in four directions separately, gradually increasing the tangential loading cylinder to 530kPa for 10 seconds in each direction, and record the curve of the resistance of the sensor element on the side of the sensor assembly as a function of stress. Repeat the loading and unloading cycles in this direction multiple times to evaluate the hysteresis error of the sensor output.

[0029] When the sensor element is subjected to external force and its resistance changes, the voltage across it will change accordingly. By utilizing the corresponding relationship between resistance and stress obtained from the aforementioned calibration test, the measured voltage signal can be converted into the corresponding stress value, realizing real-time measurement of plantar stress.

[0030] Compared with the prior art, the present application has the following effects:

[0031] The present application uses basalt fiber three-dimensional grid as the reinforcing material of the concrete matrix. The warp and weft of the basalt fiber three-dimensional grid intersect longitudinally and latitudinally to form upper and lower surface layers, and the core column connects the two surface layers to form a whole.

[0032] 1. The present application stretches the PEDOT:PSS conductive layer through the deformation of the PDMS layer, causing the resistance to change from about 50kΩ to about 250kΩ, realizing stress-resistance signal conversion. The sensor uses PEDOT:PSS conductive polymer instead of traditional conductive elastomers or metal strain gauges, and has high precision (error <5%), multi-axis synchronous detection and large deformation adaptability, solving the limitations of large error, high brittleness and single-axis detection in the prior art. The total size of the sensor is about 14mmx14mmx8.5mm, which is suitable for foot pressure distribution analysis, medical rehabilitation and sports biomechanics monitoring fields. The preparation method includes PDMS spin coating film, PEDOT:PSS film forming treatment, precision cutting and assembly assembly, and the process has strong controllability and is suitable for mass production.

[0033] 2. The present application uses PEDOT:PSS conductive polymer as the sensing layer, which has higher flexibility and sensitivity than traditional metal or carbon-based materials; by adjusting the elastic modulus of the PDMS elastic layer (about 400kPa) and the chloroprene rubber support layer (about 50kPa), the detection sensitivity of normal stress and shear stress is optimized respectively; a double-layer composite structure (PDMS+PEDOT:PSS) is designed to improve the durability and signal repeatability of the sensor; through geometric structure design, the sensor can obtain a larger resistance change amplitude under lower stress, thereby improving the sensitivity; copper spacers are used as electrode supports to form fixed clamping ends, enhancing the stability of the sensor structure; the thickness of the conductive polymer PEDOT:PSS layer (about 10μm) is optimized to ensure that the sensor still has good recovery performance after multiple cycle loads.

[0034] 3. The present application adopts a five-axis decoupling force structure, which can realize independent measurement of normal and shear stress with small mutual interference. Through structural design, the normal pressure is concentrated from the center PMMA cube to the center sensing element, while the shear force causes horizontal displacement of the cube, which is sensed by the edge sensing elements around. With the elastic isolation design of the rubber support layer, each sensing element responds independently to stress changes in different axes, effectively reducing the coupling error between axes, and compared with the traditional multi-strain gauge rosette solving method, the present application is more simple and has higher precision.

[0035] 4. The sensor of the present application has excellent repeatability and fatigue resistance.

[0036] By optimizing the thickness of the PDMS elastic layer and the PEDOT:PSS conductive film, the sensor can still maintain stable resistance response in multiple loading-unloading cycles, with hysteresis error less than 3% and resistance drift less than 5%. Compared with traditional conductive rubber or carbon-based composite materials, the PEDOT:PSS thin film conductive path is more stable and less likely to drift due to particle migration, making it suitable for long-term repeated use.

[0037] 5、The sensor material of the application has good biocompatibility and safety.

[0038] The materials used, such as PDMS, PMMA, PEDOT:PSS, and chlorobutyl rubber, are polymer materials widely used in the biomedical field, with low toxicity, no skin irritation, softness, and other characteristics, making them suitable for use in diabetic foot patients, the elderly or children with sensitive skin. The device has no metal peeling and no risk of hard structure damage, making it safer than lead-containing ceramic piezoelectric materials or optical fiber structures.

[0039] 6、The preparation process of the application is simple and suitable for low-cost mass production.

[0040] The sensor preparation includes standard processes such as PDMS spin coating, PEDOT:PSS film formation, laser / blade cutting, copper electrode bonding, and component assembly. No high-temperature sintering, vacuum deposition, or photolithography process is required, and all steps can be completed at room temperature, facilitating pipeline batch manufacturing. Compared with solutions requiring MEMS processing or expensive optical fiber accessories, the cost is significantly reduced, making it suitable for widespread promotion.

[0041] 7、The sensor of the application has strong compatibility with conventional embedded electronic systems and simple signal reading.

[0042] The sensor element is a resistance output, and the voltage division reading scheme is simple and reliable, suitable for low-power and low-cost wireless modules. The sensor resistance range is several tens of kilo-ohms, which can be directly sampled by a microcontroller ADC without the need for external complex conditioning circuits or shielding modules, making it particularly suitable for wearable device system integration. Compared with capacitive, piezoelectric, and optical fiber sensors that rely on specialized circuits, the present application is easier to promote. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 is a schematic diagram of a conductive polymer-based foot stress sensor;

[0044] Figure 2 is a schematic diagram of the installation of the upper splint 1, the base plate 2, and the sensor assembly 3;

[0045] Figure 3 is a schematic diagram of the connection of the five sensor elements with the resistance tester;

[0046] Figure 4is a schematic diagram of a conductive polymer-based insole stress sensor subjected to normal loading;

[0047] Figure 5 is a schematic diagram of a conductive polymer-based insole stress sensor subjected to tangential loading;

[0048] Figure 6 is a schematic diagram of a sensing element;

[0049] Figure 7 is a schematic diagram of a conductive polymer-based insole stress sensor mounted on a calibration test device;

[0050] Figure 8 is a diagram of a conductive polymer-based insole stress sensor subjected to normal loading;

[0051] Figure 9 is a diagram of a conductive polymer-based insole stress sensor subjected to tangential loading;

[0052] Figure 10 is a diagram of a conductive polymer-based insole stress sensor mounted on a calibration test device. DETAILED DESCRIPTION

[0053] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations.

[0054] DETAILED DESCRIPTION Figures 1-3 In this embodiment, the conductive polymer-based insole stress sensor includes an upper clamping plate 1, a base plate 2, a sensing assembly 3 and a plurality of support columns 4.

[0055] The upper clamping plate 1 and the base plate 2 are oppositely arranged, the sensing assembly 3 and the plurality of support columns 4 are installed between the upper clamping plate 1 and the base plate 2, the bottom end of the sensing assembly 3 is fixedly connected with the end face of the base plate 2, the top of the sensing assembly 3 is in contact with the upper clamping plate 1, and the pressure borne by the upper clamping plate 1 and the base plate 2 is monitored through the sensing assembly 3.

[0056] In this embodiment, the base plate 2 is subjected to plasma surface treatment, for example, 50W power treatment for 2 minutes in an oxygen plasma cleaning machine, so as to improve the surface adhesion performance thereof. High-strength epoxy adhesives, for example, 3M DP460, are coated on the four corners of the base plate 2 respectively, four support columns 4 are fixed, the size of the support columns 4 is about 2mm×2mm×3mm, about 1N pressure is applied and maintained for 5 minutes, and it is ensured that the support columns 4 are firmly adhered to the base plate 2.

[0057] The sensing assembly 3 is fixed on the substrate 2 in this embodiment.

[0058] Specific embodiment two: combination Figures 1-3 In this embodiment, the conductive polymer-based plantar stress sensor includes a plurality of support columns 4 arranged around the sensing assembly 3. The other components and connection relationships are the same as those in specific embodiment one.

[0059] Specific embodiment three: combination Figures 1-3 In this embodiment, the conductive polymer-based plantar stress sensor includes a sensing assembly 3 including a column, a resistance tester, and five sensing elements. The column is a cuboid structure, and the five sensing elements are arranged on the bottom end surface and four side surfaces of the cuboid, respectively. The five sensing elements are connected to one resistance tester and are arranged in parallel. The other components and connection relationships are the same as those in specific embodiment two.

[0060] In this embodiment, the substrate 2 is made of polymethyl methacrylate (PMMA), and a PMMA upper clamping plate 1 is arranged in parallel with the substrate 2. A PMMA column is fixed at the center of the top plate. Four support columns 4 are fixed to the four corners of the substrate 2 to provide support and deformation space. Five sensing elements are arranged, including a first sensing element arranged vertically at the center of the substrate 2 and directly below the PMMA cuboid, for detecting the normal stress in the z direction of the foot bottom. The second to fifth sensing elements are arranged around the column of the PMMA cuboid and are used to detect the shear stress in the x and y directions of the foot bottom, respectively. The resistance tester is an LCR tester, such as the E4980A model from Keysight Company.

[0061] Specific embodiment four: combination Figures 1-3 In this embodiment, the conductive polymer-based plantar stress sensor includes a sensing element including a PDMS elastic layer, a PEDOT:PSS conductive layer, a base layer, and two copper electrodes.

[0062] The PDMS elastic layer, the PEDOT:PSS conductive layer, the two copper electrodes, and the base layer are sequentially fixed and pasted together. The PDMS elastic layer of each sensing element is in contact with the column. The two parallel copper electrodes are arranged opposite to each other between the PEDOT:PSS conductive layer and the base layer, and a gap is provided between the two copper electrodes. The other components and connection relationships are the same as those in specific embodiment three.

[0063] When the normal pressure is applied, the PDMS elastic layer is compressed, the length of the PEDOT:PSS conductive layer is increased, and the thickness of the PEDOT:PSS conductive layer is reduced, so as to cause the resistance to be increased; when the tangential force is applied, the column of the PMMA cube is offset, the PDMS elastic layer of the edge sensing element is extruded and deformed, the thickness of the PEDOT:PSS conductive layer is changed, so as to cause the resistance to be changed, and the stress-resistance conversion is realized.

[0064] The conductivity of the PEDOT:PSS conductive layer is controlled by adjustment, and the resistance change under deformation follows the formula:

[0065] R = p L / A, wherein L is the length of the conductive layer, A is the cross-sectional area, and p represents the resistivity.

[0066] Each sensing element has an upper layer of polydimethylsiloxane (PDMS) elastic layer with a size of about 1.5 mm x 1.5 mm x 2 mm, which is used to transmit stress and produce deformation; a middle layer of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) conductive layer with a thickness of about 10 μm, whose resistance changes with the deformation; and a base layer made of PMMA material, two copper electrodes are used to lead out the resistance signal of the conductive layer.

[0067] The base layer of the first sensing element is fixedly attached to the upper surface of the substrate 2, and the base layers of the second to fifth sensing elements are attached to the substrate 2 at the positions where the side surfaces thereof are in contact with the upper surface of the substrate 2. The first sensing element is vertically arranged at the center of the substrate and located directly below the PMMA cube, and is used for detecting the normal stress (σz); the second to fifth sensing elements are vertically distributed around the PMMA cube, and are respectively used for detecting the shear stresses (σx, σy) in the x direction and the y direction.

[0068] Specific embodiment five: combination Figures 1-3 In this embodiment, the insole stress sensor based on the conductive polymer has a support column 4 made of rubber material and a column made of polymethyl methacrylate material. The other components and connection relationships are the same as those in the fourth embodiment.

[0069] Specific embodiment six: combination Figures 1-3 In this embodiment, the preparation method of the sensing element is realized according to the following steps:

[0070] Step one: the silicon substrate is cleaned and treated, and a mixture of PDMS prepolymer and curing agent is spin-coated on the silicon substrate to form a PDMS elastic layer with a thickness of about 2 mm;

[0071] Step two: add PEDOT:PSS dispersion liquid to about 5% volume of dimethyl sulfoxide mixture, and coat the PEDOT:PSS dispersion liquid and dimethyl sulfoxide mixture on the PDMS elastic layer formed in step one, and dry to form a PEDOT:PSS conductive layer with a thickness of about 10 μm;

[0072] Step three: paste a copper electrode on the same end face of the PEDOT:PSS conductive layer dried in step two through conductive epoxy, and the copper electrode and the PDMS elastic layer are respectively arranged at both ends of the conductive layer, and cut and separate the PDMS elastic layer dried in step one on the silicon substrate;

[0073] Step four: paste the two copper electrodes on PMMA through ultraviolet curing glue, and complete the preparation of the sensing element.

[0074] Specific implementation seven: combined Figures 1-3 The preparation method of the sensing element of the embodiment is described,

[0075] Step one: place a silicon wafer substrate with a diameter of 100 mm in a cleaning container, and sequentially clean the surface impurities with acetone and isopropyl alcohol for 10 minutes, then blow dry with high-purity nitrogen, take PDMS prepolymer and curing agent, mix according to the mass ratio of 10:1, the curing agent is Dow Corning Sylgard 184, mechanically stir for 5 minutes until the mixture is uniform, and then degas the mixture under vacuum for 30 minutes to eliminate bubbles, pour 25 ml of the mixed PDMS prepolymer and curing agent mixture evenly on the surface of the silicon wafer substrate,

[0076] A two-step spin coating process is used to form a uniform thin film: in the first step, spin at 500 rpm for 10 seconds to evenly spread the PDMS prepolymer and curing agent mixture on the substrate; in the second step, spin at 1000 rpm for 30 seconds, and finally obtain an elastic layer thin film with a thickness of about 2 mm, then place the PDMS-coated substrate in a vacuum oven and cure at 80°C for 5 hours to ensure that the PDMS prepolymer and curing agent mixture is fully cross-linked and cured;

[0077] The conductive layer in step two is prepared: prepare a PEDOT:PSS aqueous dispersion and 5% by volume of dimethyl sulfoxide, the PEDOT:PSS aqueous dispersion is Clevios PH1000 type, add dimethyl sulfoxide to the PEDOT:PSS aqueous dispersion, and magnetically stir for 2 hours, place the PDMS elastic layer solidified in step one on a spin coating device, spin coat the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution mixture at a speed of 1000 rpm for 30 seconds to form a conductive layer film with a thickness of about 10 μm, then place the conductive layer film in a drying oven and dry it in an inert nitrogen environment at about 50°C for 1 hour to fully film the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution mixture and improve the uniformity of the conductive layer;

[0078] The copper electrodes are pasted in step three: a diamond scribe is used to precisely cut the PDMS elastic layer in step one and the PEDOT:PSS conductive layer in step two according to a size of 1.5 mm x 1.5 mm, the blade speed is about 30000 rpm and the feed speed is 1 mm / s during cutting, the size tolerance is controlled within ±0.05 mm, and two copper pieces with a size of 1.5 mm x 0.5 mm x 1.5 mm are pasted on the same end face of the PEDOT:PSS conductive layer through conductive epoxy, after pasting, the pasted PDMS elastic layer, PEDOT:PSS conductive layer and copper electrodes are heated and cured at 120°C for 30 minutes, then the silicon wafer substrate in step one is separated on the elastic layer, a PMMA base layer with a size of 1.5 mm x 1.5 mm x 1 mm is pasted and fixed with the two copper electrodes using ultraviolet curing glue, and the glue is cured by irradiating 365 nm ultraviolet light for 60 seconds, completing the production of a single sensing element.

[0079] Specific implementation method eight: combination Figures 1-4 This embodiment is a foot stress sensor calibration test device based on a conductive polymer, which includes a shockproof workbench 10, a fixing frame 20, a linear brake 50, a normal force loading mechanism 30 and four tangential force loading mechanisms 40.

[0080] The normal force loading mechanism 30 and the tangential force loading mechanism 40 are fixed on the shockproof workbench 10 through the fixing frame 20, the shockproof workbench 10 is a TMC 63-500 type damping vibration isolation table, a foot stress sensor based on a conductive polymer is arranged on the shockproof workbench 10, the linear brake 50 is fixed above the foot stress sensor based on a conductive polymer, the loading end of the normal force loading mechanism 30 is on the upper end face of the linear brake 50, and the four tangential force loading mechanisms 40 are respectively on the side faces of the linear brake 50.

[0081] The linear brake 50 in this embodiment is of Thorlabs MLS203 model, which can exert a tangential stress of up to about 38 kPa in the x and y directions, with a displacement control accuracy of ±1 μm.

[0082] Specific embodiment nine: combination Figures 1-4 In this embodiment, the normal force loading mechanism 30 includes a normal loading cylinder, a normal air supply tank, and a normal electromagnetic valve; the tangential force loading mechanism 40 includes a tangential loading cylinder, a tangential air supply tank, and a tangential electromagnetic valve.

[0083] The housing of the normal loading cylinder is fixedly installed on the fixed frame 20, the normal air supply tank is connected with the normal loading cylinder and supplies air, the normal electromagnetic valve is installed on the normal air supply tank and controls the operation of the normal air supply tank, the piston end of the normal loading cylinder is positioned on the center of the end face of the linear brake 50, the housing of the tangential loading cylinder is fixedly installed on the fixed frame 20, the tangential air supply tank is connected with the tangential loading cylinder and supplies air, the tangential electromagnetic valve is installed on the tangential air supply tank and controls the operation of the tangential air supply tank, and the piston end of the tangential loading cylinder is positioned on the side face of the linear brake 50. The other components and connection relationships are the same as those in specific embodiment one.

[0084] In this embodiment, the pressure range of the normal loading cylinder and the tangential loading cylinder is 0-600 kPa, and the resolution is 0.1 kPa.

[0085] Specific embodiment ten: combination Figures 1-3 In this embodiment, the method for calibrating and testing the conductive polymer-based foot stress sensor is implemented in the following manner: a normal pressure is applied to align the piston end of the normal loading cylinder with the center of the linear brake 50, the output pressure is adjusted by the normal loading cylinder, a tangential pressure is applied to align the piston end of the tangential loading cylinder with the side face of the linear brake 50, a resistance tester is connected in parallel with the copper electrodes of the five sensing elements, the size of the sensing elements is 1.5 mm x 1.5 mm x 1 mm, the size of the copper electrodes is 1.5 mm x 0.5 mm x 1.5 mm, a 1 kHz measurement signal is introduced, and the resistance changes of the sensing elements are recorded in real time,

[0086] First, the initial resistance of each sensing element is measured in a stress-free state, and it is confirmed that the baseline is about 50 kΩ. The normal stress calibration is performed: the normal loading cylinder is gradually increased to 530 kPa, and kept for 10 seconds. The curve of the resistance of the sensing element located at the bottom end face of the sensing assembly 3 with the stress change is recorded. The loading and unloading cycles are repeated multiple times to evaluate the hysteresis error of the sensor output.

[0087] Then, the shear stress calibration is performed: calibrate along four directions respectively, and increase the tangential loading cylinder to 530kPa step by step in each direction, keep for 10 seconds, record the curve of the resistance of the sensing element on the side of the sensing assembly 3 with the change of stress, repeat the loading and unloading cycle in this direction for several times, and evaluate the hysteresis error of the sensor output;

[0088] The resistance of the PEDOT:PSS conductive layer in the stress-free state is about 50kΩ, and the resistance increases to about 250kΩ when a normal stress of about 530kPa or a shear stress of about 38kPa is applied,

[0089] When the sensing element bears external force and the resistance changes, the voltage between the two ends will change, and by using the resistance-stress relationship obtained by the foregoing calibration test, the measured voltage signal can be converted into the corresponding stress value, so that the real-time measurement of the plantar stress is realized.

[0090] Note that the above is only the preferred embodiment of the present application and the technical principle applied. Those skilled in the art will understand that the present application is not limited to the specific embodiments described herein, and those skilled in the art can make various obvious changes, readjustments and substitutions without departing from the scope of the present application. Therefore, although the present application has been described in more detail through the above embodiments, the present application is not limited to the above embodiments, and can include more other equivalent embodiments without departing from the concept of the present application, and the scope of the present application is determined by the scope of the appended claims.

Claims

1. A foot stress sensor based on conductive polymer, characterized in that: The invention comprises an upper clamping plate (1), a base plate (2), a sensing component (3) and a plurality of supporting columns (4), wherein the sensing component (3) comprises a column, a resistance tester and five sensing elements, and the sensing elements comprise a PDMS elastic layer, a PEDOT:PSS conductive layer, a base layer and two copper electrodes; The upper clamping plate (1) and the substrate (2) are arranged relative to each other, the sensor component (3) and the plurality of support columns (4) are installed between the upper clamping plate (1) and the substrate (2), the bottom end of the sensor component (3) is fixedly connected to the end face of the substrate (2), the top of the sensor component (3) is arranged in contact with the upper clamping plate (1), and the pressure exerted on the upper clamping plate (1) and the substrate (2) is monitored by the sensor component (3); the column is a rectangular parallelepiped structure, the five sensor elements are respectively arranged on the bottom end face and four side faces of the rectangular parallelepiped, the five sensor elements are all connected to a resistance tester, and the five sensor elements are arranged in parallel, the PDMS elastic layer, the PEDOT:PSS conductive layer, the two copper electrodes and the base layer are fixedly adhered together in sequence, the PDMS elastic layer of each sensor element is arranged in contact with the column, the two parallel copper electrodes are arranged relative to each other between the PEDOT:PSS conductive layer and the base layer, and a gap is provided between the two copper electrodes; The preparation method of the sensor element is achieved by following the steps: Step 1: The silicon substrate is cleaned and a mixture of PDMS prepolymer and curing agent is spin-coated on the silicon substrate and cured to form a 2 mm thick PDMS elastic layer; Step 2: Add 5% volume of dimethyl sulfoxide to the PEDOT:PSS dispersion and mix them. Then, coat the PEDOT:PSS dispersion and dimethyl sulfoxide mixture on the PDMS elastic layer formed by solidification in step 1 and dry it to form a PEDOT:PSS conductive layer with a thickness of 10 μm. Step 3: Attach a copper electrode to the same end surface of the PEDOT:PSS conductive layer dried in step 2 using conductive epoxy adhesive. The copper electrode and the PDMS elastic layer are respectively located at both ends of the conductive layer. Cut and separate the PDMS elastic layer dried in step 1 on the silicon substrate. Step 4: Glue the two copper electrodes onto PMMA using UV-curable adhesive to complete the fabrication of the sensing element.

2. The conductive polymer-based foot stress sensor according to claim 1, characterized in that: A plurality of support columns (4) are evenly distributed around the sensor component (3).

3. The conductive polymer-based foot stress sensor according to claim 1, characterized in that: The support column (4) is a support column made of rubber material, and the upright column is a cubic column made of polymethyl methacrylate material.

4. The conductive polymer-based foot stress sensor according to claim 1, characterized in that: The preparation method of the sensor element specifically includes: Preparation of the elastic layer in step 1: Place a 100 mm diameter silicon wafer substrate in a cleaning container and ultrasonically clean it with acetone and isopropyl alcohol for 10 minutes to remove surface impurities. Then blow it dry with high-purity nitrogen. Take PDMS prepolymer and curing agent, mix them in a mass ratio of 10:1, and mechanically stir for 5 minutes until the mixture is uniform. Degas the mixture under vacuum for 30 minutes to eliminate bubbles. Pour 25 ml of the evenly mixed PDMS prepolymer and curing agent mixture onto the surface of the silicon wafer substrate. A two-step spin coating process was used to form a uniform film: the first step was to spin-coat at 500 rpm for 10 seconds to evenly spread the PDMS prepolymer and curing agent mixture on the substrate. The second step was to spin-coat at 1000 rpm for 30 seconds to obtain a 2 mm thick elastic layer film. The PDMS-coated substrate was then placed in a vacuum oven and cured at 80°C for 5 hours to ensure that the PDMS prepolymer and curing agent mixture were fully cross-linked and cured. Preparation of the conductive layer in step 2: Prepare a PEDOT:PSS aqueous dispersion and 5% by volume of dimethyl sulfoxide, add dimethyl sulfoxide to the PEDOT:PSS aqueous dispersion, and magnetically stir for 2 hours. Place the PDMS elastic layer cured in step 1 on a spin coating apparatus and spin-coat the mixture of the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution at 1000 rpm for 30 seconds to form a conductive layer film with a thickness of 10 µm. Subsequently, place the conductive layer film in a drying oven and dry it in a nitrogen inert environment at 50°C for 1 hour to allow the mixture of the PEDOT:PSS aqueous dispersion and dimethyl sulfoxide solution to fully form a film and improve the uniformity of the conductive layer. Paste the copper electrodes in step three: Use a diamond scriber to precisely cut the PDMS elastic layer in step one and the PEDOT:PSS conductive layer in step two into a size of 1.5 mm × 1.5 mm, and paste two copper sheets on the same end face of the PEDOT:PSS conductive layer with conductive epoxy glue. The size of the copper sheet is 1.5 mm × 0.5 mm × 1.5 mm. After pasting, heat and cure the pasted PDMS elastic layer, PEDOT:PSS conductive layer and copper electrode at 120°C for 30 minutes. After curing, separate the silicon wafer substrate in step one from the elastic layer, and then use UV curing glue to paste and fix the PMMA base layer with a size of 1.5 mm × 1.5 mm × 1 mm to the two copper electrodes, and cure the glue by irradiating it with 365 nm wavelength ultraviolet light for 60 seconds to complete the production of a single sensor element.

5. A calibration and testing device suitable for the conductive polymer-based plantar stress sensor according to any one of claims 1 to 4, characterized in that: It includes a shockproof workbench (10), a fixing frame (20), a linear brake (50), a normal force loading mechanism (30) and four tangential force loading mechanisms (40); The normal force loading mechanism (30) and the tangential force loading mechanism (40) are fixed on the anti-vibration workbench (10) through a fixing frame (20); a foot stress sensor based on a conductive polymer is arranged on the anti-vibration workbench (10); a linear brake (50) is fixed above the foot stress sensor based on a conductive polymer; a loading end of the normal force loading mechanism (30) is pressed against an upper end surface of the linear brake (50); and four tangential force loading mechanisms (40) are respectively pressed against side surfaces of the linear brake (50).

6. The calibration test device according to claim 5, characterized in that: The normal force loading mechanism (30) includes a normal loading cylinder, a normal air supply tank, and a normal electromagnetic valve; the tangential force loading mechanism (40) includes a tangential loading cylinder, a tangential air supply tank, and a tangential electromagnetic valve; The shell of the normal loading cylinder is fixedly mounted on the fixed frame (20), the normal air supply tank is connected to the normal loading cylinder and supplies air, the normal solenoid valve is mounted on the normal air supply tank and controls the normal air supply tank to work, the piston end of the normal loading cylinder is pressed against the center position of the upper end face of the linear brake (50), the shell of the tangential loading cylinder is fixedly mounted on the fixed frame (20), the tangential air supply tank is connected to the tangential loading cylinder and supplies air, the tangential solenoid valve is mounted on the tangential air supply tank and controls the tangential air supply tank to work, and the piston end of the tangential loading cylinder is pressed against the side end face of the linear brake (50).

7. A calibration test method for a foot stress sensor based on a conductive polymer, characterized in that: The calibration test device according to claim 5 or 6 is used to perform a calibration test in the following manner: applying normal pressure to align the piston end of the normal loading cylinder with the center of the linear brake (50), adjusting the output pressure by the normal loading cylinder, applying tangential pressure to align the piston end of the tangential loading cylinder with the side of the linear brake (50), connecting a resistance tester in parallel with the copper electrodes of the five sensing elements, introducing a 1 kHz measurement signal, and recording the resistance change of each sensing element in real time. First, the initial resistance of each sensor element was measured in a stress-free state to confirm that its baseline was 50 kΩ. Normal stress calibration was performed by gradually increasing the normal loading cylinder to 530 kPa and holding it for 10 seconds. The resistance curve of the sensor element located at the bottom end face of the sensor assembly (3) was recorded as a function of stress. The loading and unloading cycles were repeated several times to evaluate the hysteresis error of the sensor output. Then, shear pressure calibration is performed: calibrate in four directions respectively. When calibrating in each direction, gradually increase the tangential loading cylinder to 530 kPa and hold it for 10 seconds. Record the curve of the resistance of the sensor element located on the side of the sensor component (3) as a function of stress. Repeat the loading and unloading cycle in this direction several times to evaluate the hysteresis error of the sensor output. When the sensor element is subjected to external force and its resistance changes, the voltage across it will change accordingly. By utilizing the corresponding relationship between resistance and stress obtained from the aforementioned calibration test, the measured voltage signal is converted into the corresponding stress value, realizing real-time measurement of plantar stress.

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