Preparation and installation method of film strain sensor and harmonic reducer

Abstract

The invention relates to the technical field of film strain, in particular to a preparation and installation method of a film strain sensor and a harmonic reducer. The preparation and installation method comprises the steps that the surface of a workpiece to be installed is coated with a polyamide acid solution, then heating is conducted, and polyamide acid is subjected to dehydration cyclization to form a polyimide film attached to the surface of the workpiece to be installed; and then, forming a laser-induced graphene electrode pattern on the polyimide film. The harmonic reducer comprises a wave generator, a rigid gear, a flexible gear and a thin film strain sensor, the flexible gear is connected into the rigid gear in a toothed mode, and the wave generator is arranged in the flexible gear; the film strain sensor comprises a strain film and a graphene electrode, the graphene electrode is formed on the strain film, and the strain film is in direct contact connection with the surface of the flexible gear. Through the film strain sensor adhered to the surface of the workpiece to be installed, in-situ torque detection can be achieved, measurement errors, hysteresis, instability and the like caused by a glue joint layer are avoided, and measurement is more accurate.

Description

Technical Field

[0001] This invention relates to the field of thin film strain technology, and in particular to a method for fabricating and installing a thin film strain sensor and a harmonic reducer. Background Technology

[0002] Harmonic reducers are widely used in applications requiring precise control, such as robotics, CNC machine tools, medical equipment, aerospace, and semiconductor manufacturing, due to their lightweight, small size, large transmission ratio, high precision, and smooth operation. A harmonic reducer generally consists of three main components: a flexible gear, a rigid gear, and a wave generator. The input shaft drives the wave generator to rotate, causing the flexible gear to undergo periodic deformation. This achieves a small tooth difference internal meshing between the flexible and rigid gears, thereby transmitting motion and torque.

[0003] In the aforementioned applications, some situations require harmonic reducers to have torque sensing capabilities to ensure their safety or to achieve force feedback control. One related technology discloses a method for detecting the torque of a harmonic reducer by attaching strain gauges to the outer surface of the flexspline. This method of manually attaching strain gauges directly cannot guarantee the positional accuracy of the strain gauges. Furthermore, the adhesive used to attach the strain gauges can introduce measurement errors, making in-situ detection impossible. Another related technology discloses positioning the strain gauges using a mounting flange that contacts the harmonic reducer, attaching the non-adhesive side of the strain gauge to the pressure bar with double-sided adhesive, applying adhesive to the adhesive side, and then applying pressure to bond it. While this method improves the positional accuracy of the strain gauge attachment to some extent, the force resulting from the separation of the double-sided adhesive when removing the mounting device after installation can affect the tightness of the strain gauge attachment. Summary of the Invention

[0004] In view of this, the present invention provides a method for fabricating and installing a thin-film strain sensor and a harmonic reducer to solve the problems of poor position accuracy, poor stability and detection hysteresis in the strain detection of the prior art.

[0005] To achieve the above objectives, the present invention mainly provides the following technical solutions: On one hand, embodiments of the present invention provide a method for preparing and installing a thin-film strain sensor, wherein a polyamic acid solution is coated on the surface of the workpiece to be installed, and then heated to dehydrate and cyclize the polyamic acid to form a polyimide film adhering to the surface of the workpiece; subsequently, a laser-induced graphene electrode pattern is formed on the polyimide film.

[0006] In some embodiments of the preparation and installation method, the thickness of the polyamic acid solution coated on the surface of the workpiece to be installed is no more than 1 mm.

[0007] In some embodiments of the preparation and installation method, the heating is a step-by-step temperature increase to 240-260°C for baking.

[0008] In some embodiments of the preparation and installation method, an electrode protective layer is also formed on the electrode pattern.

[0009] In some embodiments of the fabrication and mounting method, forming a laser-induced graphene electrode pattern on the polyimide film includes: Finding the optimal parameters for laser processing; The polyimide film is subjected to laser ablation treatment in a laser engraving machine using the aforementioned optimal parameters.

[0010] In some embodiments of the preparation and installation method, finding the optimal parameters for laser processing includes: Select a smooth flat plate made of the same material as the workpiece to be installed, form a polyimide film on the smooth flat plate, and divide it into multiple detection blocks; Three of the four parameters—laser power, laser scanning speed, dots per inch, and pixels per inch—are fixed. A range of values ​​is set for the unfixed parameters. Multiple values ​​are taken within the range and the detection block is processed. The sheet resistance of the processed detection block is measured, and the value with the smallest sheet resistance is selected as the optimal value of the unfixed parameters.

[0011] In some embodiments of the preparation and installation method, the workpiece to be installed is the flex wheel of a harmonic reducer.

[0012] On the other hand, embodiments of the present invention also disclose a harmonic reducer, which includes a wave generator, a rigid wheel, a flexible wheel, and a thin-film strain sensor, wherein the thin-film strain sensor is used to detect torque, wherein... The flexible gear is toothed inside the rigid gear, and the wave generator is disposed inside the flexible gear; The thin-film strain sensor includes a strain film and a graphene electrode, wherein the graphene electrode is formed on the strain film, and the strain film is in direct contact with the surface of the flexible wheel. The thin-film strain sensor is prepared and installed using the method described above.

[0013] In some embodiments, the thickness of the polyimide film is less than 1 mm.

[0014] In some embodiments, the flexible wheel includes a cylindrical body with external teeth disposed on the outer surface of one end of the cylindrical body; the other end of the cylindrical body forms an annular disk from the cylindrical wall toward a direction away from the center, the annular disk including an annular deformable thin-walled diaphragm; the strain film is disposed in direct contact on the diaphragm.

[0015] In some embodiments, the pattern formed by the graphene electrodes includes a main trunk and several branches, the branches extending from the main trunk in a direction away from the main trunk; an angle is formed between the center line of the main trunk and the center line of the branches; The position of the thin-film strain sensor is configured such that the centerline of the main trunk coincides with the ray emitted from the center of the cylindrical body.

[0016] In some embodiments, the thin-film strain sensor includes multiple sensors, which are equally spaced along the circumference of the diaphragm, and two sensors arranged in the same diameter direction are grouped together; the included angle of the graphene electrode patterns of adjacent groups of thin-film strain sensors is different.

[0017] In some embodiments, the thin-film strain sensors comprise four sensors arranged at 90° intervals along the circumference of the diaphragm; wherein one set has an included angle of 45° and another set has an included angle of 135°.

[0018] In some embodiments, the harmonic reducer further includes wires and a circuit board; the wires extend from the end of the graphene electrode and are connected to the circuit board.

[0019] Compared with the prior art, the preparation and installation method of the thin-film strain sensor and the harmonic reducer of the present invention have at least the following beneficial effects: The method for fabricating and installing a thin-film strain sensor provided in this invention involves dehydrating and cyclizing polyamic acid to form a polyimide film that adheres to the surface of the workpiece to be installed. This allows direct contact between the polyimide film and the workpiece without an adhesive layer. Subsequently, a laser-induced graphene electrode pattern is formed on the polyimide film to further form a thin-film strain sensor. This method enables in-situ detection of the torque of the workpiece, such as a flexible wheel, and can directly monitor the output torque of a harmonic reducer during operation. This avoids measurement errors, hysteresis, and instability caused by adhesive layers, resulting in more accurate measurements.

[0020] Furthermore, the method for fabricating and installing the thin-film strain sensor provided in this embodiment of the invention adopts a method of first attaching the patch and then processing the sensor. The formation position of the sensor on the flexure can be precisely controlled by computer software, which solves the problems of difficulty in ensuring installation accuracy due to the small size of the strain gauge, the compact structure of the workpiece to be installed, such as the flexure, and the small reserved installation position, as well as the contamination of the reducer assembly surface.

[0021] This invention uses laser engraving to generate a strain sensor on a flexible wheel for detecting the output torque of the flexible wheel, which can improve the installation position accuracy of the strain gauge and reduce measurement errors.

[0022] On the other hand, the thin-film strain sensor of the harmonic reducer provided in the embodiments of the present invention can be prepared and installed using the preparation and installation method of the thin-film strain sensor in the aforementioned embodiments. Its beneficial effects are the same as those of the preparation and installation method of the thin-film strain sensor described above, and will not be repeated here.

[0023] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a harmonic reducer with torque detection function provided by an embodiment of the present invention; Figure 2a and Figure 2b This is a schematic diagram of the structure of a thin-film strain sensor provided in an embodiment of the present invention. In its graphene electrode pattern, the center lines of the branches and the main branches are at 45° and 135° angles, respectively. Figure 3 This is a schematic diagram showing the distribution of the thin-film strain sensor in the flexure according to an embodiment of the present invention; Figure 4 This is a schematic diagram of a full-bridge circuit connection for flexspline torque detection in a harmonic reducer provided by an embodiment of the present invention; Figure 5 This is a schematic diagram of a ring circuit board for a harmonic reducer provided in an embodiment of the present invention; Figure 6 yes Figure 1 Enlarged view of point A.

[0025] Among them, 1. Wave generator; 2. Rigid wheel; 3. Rigid bearing; 31. Rigid bearing inner ring; 32. Rigid bearing outer ring; 4. Flexible wheel; 41. External toothed part; 42. Diaphragm; 43. Flange; 5. Thin film strain sensor; 51-54. Four thin film strain gauges (thin film strain sensors); 511. Graphene electrode; 512. Strain film substrate; 513. Electrode terminal; 6. Ring circuit board. Detailed Implementation

[0026] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the specific embodiments, structures, features, and effects according to the present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of the present invention.

[0027] In the description of this invention, it should be understood that the orientation or positional relationship indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, horizontal" and "top, bottom" is generally based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing this invention and simplifying the description. Unless otherwise stated, these directional terms do not indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the scope of protection of this invention; the directional terms "inner" and "outer" refer to the inner and outer contours relative to the outline of each component itself.

[0028] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used here to describe the spatial positional relationship of a device or feature as shown in the figure with other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation in addition to the orientation of the device as described in the figure. For example, if a device in the figure is inverted, a device described as "above" or "on top of" other devices or structures will subsequently be positioned as "below" or "under" other devices or structures.

[0029] Existing strain detection technologies suffer from poor positional accuracy, instability, and detection lag. Thin-film strain gauge fabrication methods have been considered, involving alternating processes such as ion pre-sputtering cleaning, DC magnetron sputtering, plasma-enhanced chemical vapor deposition, and laser engraving. Ultimately, a thin-film strain gauge was fabricated on the surface of the gear being measured. While this method solved the problems of large detection lag, instability, and low sensitivity caused by the presence of the adhesive layer and the limitations of the adhesive properties when the strain gauge is bonded to the surface of the measured component, its process is complex and costly. Furthermore, the positional accuracy during fabrication depends on the precision of the mask placement.

[0030] This invention discloses a method for fabricating and installing a thin-film strain sensor. When used for torque detection of the flexspline in a harmonic reducer, it can directly monitor the output torque of the reducer during operation, resulting in more accurate measurements. Furthermore, the fabrication process is relatively simple. The specific fabrication and installation method includes: coating a polyamic acid solution onto the surface of the workpiece to be installed, then heating it to dehydrate and cyclize the polyamic acid, forming a polyimide film adhered to the surface of the workpiece. Subsequently, a laser-induced graphene electrode pattern is formed on the polyimide film. Heating can be performed using a stepped temperature increase to 240-260°C, preferably 250°C. The purpose of the stepped temperature increase is to make the surface of the formed polyimide film more uniform and smooth, avoiding uneven internal distribution and inconsistent cyclization processes caused by direct heating.

[0031] For example, the coated workpiece to be installed can be placed in an oven for baking, causing the polyamic acid solution to dehydrate and cyclize to form a polyimide film (PI film), which then adheres firmly to the workpiece. When the workpiece to be installed is a flexible wheel of a harmonic reducer, the surface of the flexible wheel can be used as the placement area for the strain film of a thin-film strain sensor, and a flexible sensor for detecting the torque of the flexible wheel can be processed on it.

[0032] This embodiment forms a polyimide film that adheres to the surface of the workpiece by dehydrating and cyclizing the polyamic acid. Then, a graphene electrode is formed by laser ablation of the film, thereby forming a strain sensor. This replaces the glue required for adhesion in normal strain gauge use, allowing direct contact between the polyimide film and the workpiece without an adhesive layer. This enables in-situ detection of the torque on the surface of the workpiece, such as a flexible wheel, and direct monitoring of the output torque of a harmonic reducer during operation. It avoids measurement errors, hysteresis, and instability caused by adhesive layers, resulting in more accurate measurements.

[0033] In the above embodiments, the thickness of the polyamic acid solution coated on the surface of the workpiece to be installed is generally no more than 1 mm. This is to ensure that the formed polyimide film (i.e., strain film) adheres more stably. If it is too thick, it may lead to a decrease in measurement accuracy. When used in harmonic reducers, a suitable thickness allows the strain film to adhere more firmly during the cyclic deformation of the flexure during the operation of the harmonic reducer. If the thickness is too small, it may also be affected by the surface of the workpiece to be installed, resulting in an uneven and smooth surface of the generated strain film, thus affecting the measurement results.

[0034] During coating, the polyamic acid solution can generally be applied to the surface of the workpiece (flexible wheel) to be installed by using a syringe, dripping, or a coating machine, or by 3D printing. Preferably, the height of the polyamic acid solution on the surface of the flexible wheel is controlled to be 1 mm by using a coating machine, so that the thickness of the polyimide film formed after dehydration and cyclization is less than 1 mm.

[0035] To achieve torsional strain detection, in the above embodiments, after forming the polyimide film, a laser-induced graphene electrode pattern (such as...) needs to be formed on the polyimide film. Figure 2a (As shown in 2b), wires are led out from the electrode pattern for signal transmission and connected to the ring circuit board (as shown in 2b). Figure 5 As shown in the diagram, the wires can be passed through the holes in the protruding part. Correspondingly, the harmonic reducer also has holes and slots for placing the annular circuit board 6. For the torque detection of the flexspline in the harmonic reducer, the circuit can use... Figure 4 The Wheatstone full-bridge circuit connection shown is implemented. An electrode protective layer can also be formed on the laser-induced graphene electrode pattern to protect the electrode pattern. The electrode protective layer can be a commonly used protective layer in existing technologies; for example, a layer of polydimethylsiloxane (PDMS) for electrode protection can be coated onto its surface, and the coating method can be the same as that used for coating polyamic acid solutions.

[0036] To form laser-induced graphene electrode patterns on a polyimide film, a high-energy laser (typically a pulsed laser) can be used to irradiate the surface of the polyimide film, causing it to pyrolyze and form a dense graphene structure. Specifically, before processing the workpiece, optimal laser processing parameters can be identified, and then these optimal parameters can be used to perform laser ablation on the polyimide film in a laser engraving machine. Generally, identifying optimal laser processing parameters may include: Select a smooth flat plate made of the same material as the workpiece to be installed, form a polyimide film on the smooth flat plate, and divide it into multiple detection blocks; Three of the four parameters—laser power, laser scanning speed, dots per inch, and pixels per inch—are fixed. A range of values ​​is set for the unfixed parameters. Multiple values ​​are processed for the detection block within the range of values. The sheet resistance of the detection block is measured, and the value with the smallest sheet resistance is selected as the optimal value of the unfixed parameters.

[0037] In some specific implementation schemes, to find the optimal parameters for laser processing, a smooth flat plate made of the same material as the flexible wheel can be selected. A polyimide film (PI film) is then fabricated on the PI film using the method described above. Small square blocks with sides of 5mm are then processed on the PI film using a laser engraving machine, arranged in a 4x4 grid in one continuous operation. The sheet resistance of the four middle squares is measured and recorded. The following parameters are mainly controlled during laser processing: laser power, laser scanning speed, dots per inch (DPI), and pixels per inch (PPI). The specific implementation process for optimizing these four processing parameters sequentially is as follows: First, within a certain laser power range, the graphene blocks are divided into multiple different laser powers. Then, the remaining three processing parameters are fixed, and square laser-induced graphene blocks for measuring sheet resistance are processed in the manner described above. The sheet resistance of the four graphene blocks at the center of the array is measured, and the sheet resistance values ​​are recorded in ascending order of laser power. When the recorded sheet resistance has a minimum value within the selected laser power range, that laser power is selected as the optimal processing parameter. When the sheet resistance increases or decreases constantly within the selected laser power range, a new set of laser power values ​​is added to the side with the smallest sheet resistance value, and sheet resistance measurements are performed until the minimum sheet resistance is found within the selected laser power range.

[0038] The process of finding the optimal parameters for laser scanning speed is the same as that of finding laser power. Laser power, DPI, and PPI need to be fixed, with the laser power using the previously found optimal parameters. The subsequent optimal parameters for DPI and PPI can also be obtained by repeating the above steps.

[0039] like Figures 1-5 As shown, some embodiments of the present invention also disclose a harmonic reducer, including a wave generator 1, a rigid wheel 2, a flexible wheel 4, and a thin-film strain sensor 5. The thin-film strain sensor 5 is used to detect torque. The flexible wheel 4 is toothed inside the rigid wheel 2, and the wave generator 1 is disposed inside the flexible wheel 4. The thin-film strain sensor 5 includes a strain film and a graphene electrode 511. The graphene electrode 511 is formed on the strain film 512, and the strain film 512 is in direct contact with the surface of the flexible wheel 4. The thin-film strain sensor 5 is prepared and installed by the aforementioned method.

[0040] The harmonic reducer configured in this way relies on the rotation of wave generator 1 to cause the flexible gear to produce controllable elastic deformation, and then mesh with the rigid gear to transmit motion and power.

[0041] This invention provides a harmonic reducer, in which a thin-film strain sensor is used to detect torque. The strain sensor includes a strain film (substrate) and a graphene electrode formed thereon. The strain film is in direct contact with the surface of the flexible wheel 4, and the graphene electrode is formed on the strain film. Preferably, the strain film is a polyimide film, and the graphene electrode is formed by laser ablation of the film. When an external load is applied to the harmonic reducer, the flexible wheel 2 undergoes elastic deformation, and the thin-film strain sensor on its surface also undergoes corresponding deformation. The deformation is then superimposed through a Wheatstone bridge circuit and output to detect the torque of the harmonic reducer. By detecting the deformation of the flexible wheel under load, the output torque of the harmonic reducer is obtained. This introduces torque measurement functionality while transmitting power. Furthermore, the rigid wheel of the harmonic reducer is fixed during implementation, facilitating torque signal acquisition without requiring additional mechanical structures, making installation convenient.

[0042] Preferably, the flexible wheel 4 includes a cylindrical body, with external teeth disposed on the outer surface of one end of the cylindrical body (see...). Figure 1 The outer toothed portion 41); the other end of the cylindrical body forms an annular disk from the cylindrical wall toward the direction away from the center, the annular disk including an annular deformable thin-walled diaphragm 42; the strain film 512 is directly disposed on the diaphragm.

[0043] Preferably, the annular disc further includes a flange 43, which is located outside the annular diaphragm 42 and has a thickness greater than the thin wall of the annular diaphragm 42. Further, the flange 43 has a groove protruding from the inner wall of the annular diaphragm 42, and the annular circuit board 6 forms a convex-concave fit with this groove.

[0044] Specifically, the cylindrical body is in the shape of a (thin-walled) cylinder, and the flexible wheel 4 is generally shaped like a cap. The wave generator 1 is installed inside the cylinder at the end of the flexible wheel 4 with external teeth; the rigid wheel 2 is provided with internal teeth, which are installed at the external tooth end of the flexible wheel 4 containing the wave generator 1 and mesh with the external teeth of the flexible wheel 4, that is, the flexible wheel 4 is toothed into the rigid wheel 2; the rigid bearing 3 is installed between the flexible wheel 4 and the rigid wheel 2.

[0045] Preferably, in some embodiments, such as Figure 1 , 6 As shown, the rigid bearing 3 includes a rigid bearing inner ring 31 and a rigid bearing outer ring 32. The rigid bearing inner ring 31 can be connected to the rigid wheel 2, and the rigid bearing outer ring 32 can be connected to the flange 43.

[0046] Because the thin-film strain sensor 5 and the diaphragm 42 of the flexible wheel 4 are directly connected, there is no longer an adhesive layer between them. During the torque detection process, the measurement error, hysteresis and instability caused by the adhesive layer can be effectively avoided. The output torque of the harmonic reducer can be directly monitored during operation, and the measurement is more accurate.

[0047] This invention also discloses a harmonic reducer. Based on the above embodiments, the strain film 512 is made of polyimide film. The polyimide film can be prepared and installed using the method for preparing and installing thin-film strain sensors disclosed in the foregoing embodiments.

[0048] Specifically, the diaphragm surface of the flexible wheel 4 is used as the placement area for the thin-film strain sensor, and a flexible sensor for detecting the torque of the flexible wheel 4 is fabricated on it. First, the surface of the parts is cleaned with a cleaning agent to ensure that the surface of the flexible wheel 4 is free of burrs, debris and contaminants, and then it is placed in an oven to dry.

[0049] The flexible wheel is machined from a single metal casting. The roughness of the diaphragm, in particular, can be improved by shot peening, which enhances the adhesion of the polyimide film. During transmission, as the rotational speed increases, the diaphragm of the flexible wheel undergoes corresponding high-frequency flexible deformation. The strain sensor based on the polyimide film exhibits excellent flexibility, which can improve the service life of the strain gauge (strain sensor). Traditional metal-based strain gauges have poor flexibility, and metal sensing grids are more brittle than graphene sensing grids. Furthermore, using the polyimide film as the sensor substrate directly as the bonding layer effectively reduces sensor hysteresis and enhances adhesion.

[0050] Then, the polyamic acid solution is applied to the laser-treated surface. After all the polyamic acid has been applied, the flexible wheel 4 is placed in an oven and heated until the polyamic acid dehydrates and cyclizes to form polyimide. The polyamic acid can be applied to the surface of the diaphragm 42 of the flexible wheel 4 using a syringe, directly dripped onto the surface of the diaphragm 42 and evenly spread using a coater, or applied using 3D printing. The thickness of the polyamic acid layer must be controlled to be less than or equal to 1 mm, and then excess polyamic acid solution is cleaned off. Alternatively, the polyamic acid coating can be applied to the surface of the flexible wheel 4 using a centrifuge or other device that provides centrifugal force, controlling the coating thickness at a specific rotation speed.

[0051] During dehydration and cyclization, a stepped heating method can be used to raise the temperature to 240-260℃, preferably 250℃, for baking. The stepped heating can be done in increments of 50-70℃, with each increment lasting 30 minutes after reaching the specified temperature before proceeding to the next stage. The starting temperature can be set to 60℃ or 80℃. This results in a more uniform and smooth surface of the formed polyimide film, avoiding uneven internal distribution and inconsistent cyclization processes caused by direct high-temperature baking. Baking dehydrates and cyclizes the polyamic acid solution to form a polyimide film (PI film), ensuring it adheres firmly to the surface of the flexible wheel. After drying and forming the polyimide film, the surface is wiped clean with a lint-free cloth soaked in anhydrous ethanol to ensure no contaminants remain. The flexible wheel 4 is then placed in a laser engraving machine, where a laser ablation process creates a pre-designed laser-induced graphene electrode pattern on the polyimide film. Wires are then led out from the electrode ends for signal transmission. Specifically, a laser engraving machine is used for positioning, and the laser processing parameters are set. Generally, when the laser energy exceeds 1 J / cm², the laser is positioned correctly. 2 Then, graphene electrodes are formed on the surface of the PI film to further form a graphene electrode pattern, and then signal transmission lines are connected to the electrode ends.

[0052] Then, a layer of polydimethylsiloxane (PDMS) is coated onto its surface to protect the electrode. The PDMS solution is applied to the electrode surface, and then it is baked in an oven at 80°C–120°C for approximately 1 hour to form a protective layer. The PDMS coating method is the same as that used for the polyamic acid coating described above. Using laser ablation to produce sensing electrodes on the polyimide film can improve the installation accuracy of the strain sensor on the flexible wheel, especially on small-sized flexible wheels. Furthermore, the size of the polyimide film and the size of the graphene electrode pattern can be adjusted according to the different models of the flexible wheels.

[0053] Furthermore, the sensor structure is specifically designed accordingly. Preferably, the formed strain gauge (thin film substrate + graphene electrode pattern) structure is shown in Figure 2. The sensing electrode formed by graphene has higher sensitivity compared to traditional strain gauges. Figure 2a , Figure 2b This is a schematic diagram of the electrode pattern of the harmonic reducer provided in the embodiment of the present invention. In the figure, the outer black frame represents the polyimide film substrate, the inner black conductive grid is the electrode 511, which is also the main body of the sensor, and the connection point of the wire is the two black dots at both ends of the grid line, namely the electrode terminal 513.

[0054] Preferably, the pattern formed by the graphene electrodes is dendritic in shape, comprising a main trunk (one end of which is not closed) and several branches (white areas inside the black conductive grid), with the branches extending from the main trunk away from the main trunk; an angle is formed between the center line of the main trunk and the center line of the branches. The angle refers to the angle formed by rotating the center line of the branches clockwise toward the center line of the main trunk (the center line from the unclosed end of the main trunk toward the other end of the main trunk).

[0055] like Figure 2a , Figure 2b As shown, the graphene electrode is shaped such that it starts from one end and bends continuously until the other end, resulting in a pattern where one end of the main trunk is not closed. The two ends of the grid lines of the graphene electrode are connected to wires. Furthermore, the position of the thin-film strain sensor is set so that the center line of the main trunk coincides with the ray (i.e., the axis) emanating from the center of the flexible cylindrical body (the center of the cross-section). This configuration allows the thin-film strain sensor to withstand the maximum stress during torsion, making detection convenient and accurate. The dendritic graphene electrode pattern can accommodate more electrode grids in a smaller usable area. Thus, when the flexible wheel deforms, the overall deformation of the electrode grids increases, improving the area utilization for strain detection. The more black grid lines in the figure, the greater the cumulative deformation, the greater the overall elongation or compression of the electrode, the more signals received, and the higher the detection sensitivity.

[0056] Preferably, the thin-film strain sensor 5 includes a plurality (an even number), which are equally spaced along the circumference of the diaphragm, and two of them are arranged in the same diameter direction as a group; the included angle of the graphene electrode patterns of adjacent groups of thin-film strain sensors is different.

[0057] The inventors discovered that the flexible deformation is particularly pronounced at the major and minor axes of the wave generator. Therefore, it is preferable to form strain films at four circumferentially spaced 90° intervals along the diaphragm of the flexible wheel. Four thin-film strain sensors 5 are configured, spaced 90° apart along the circumferential axis of the diaphragm. The angles between the branches and the main branches in the graphene electrode pattern of these thin-film strain sensors are 45° and 135°, respectively. This means that when the electrode circuit forms angles of 45° and 135° with the axis (i.e., the ray emitted from the center of the flexible wheel), the deformation trend can be captured to the greatest extent, resulting in a larger detection signal value and reduced noise. In other words, the graphene electrode structure of this application can capture the deformation of the flexible wheel 4 caused by torque to a greater extent, improving the detection sensitivity. Specifically, four strain sensors are distributed on the surface of the diaphragm 42 of the flexible wheel 4, and arranged according to… Figure 4 The full-bridge circuit is connected in this manner. Meanwhile, as... Figure 1 As shown, a groove for fixing the annular circuit board 6 and a through hole for wiring are machined at the flange 43 corresponding to the flexible wheel 4.

[0058] There are two sets of thin-film strain gauges. One set has its electrode grid forming a first angle (preferably 45°) with the axis, while the other set has its electrode grid forming a second angle (preferably 135°) with the axis. The two sets of strain gauges are placed alternately. For example... Figure 3 As shown, the angles of thin film strain gauges 51 and 53 and 52 and 54 should be exactly 90°. That is, if the angle between 51 and 52 is 45°, then the angle between 52 and 54 is 135°. 51, 52, 53, and 54 also correspond exactly... Figure 4 R51, R52, R53, and R54.

[0059] Therefore, when the entire flexible wheel is subjected to a torsional torque in a certain direction, if the thin film strain gauge adopts the same included angle direction of the electrode pattern, its torsional torque will be largely canceled out. However, with the above distribution method, when 51 and 53 are subjected to torsional torque and generate tensile strain (denoted as positive), 52 and 54 are subjected to torsional torque and generate compressive strain (denoted as negative), which can increase the signal value collected by the full-bridge circuit and improve the detection accuracy.

[0060] It should be noted that the number of strain gauges on the surface of the diaphragm 42 of the flexible wheel 4 is not limited to 4, but can be more than 4, as long as they appear in pairs.

[0061] like Figure 5 As shown, the annular circuit board has eight protruding parts, four of which are used to fix it to the flexible wheel, and the other four are used to connect the strain gauge and the circuit board with wires.

[0062] In summary, the fabrication and installation method of the thin-film strain sensor and the harmonic reducer disclosed in this invention embodiment: They employ a method of directly detecting torque on the flexible wheel (the workpiece to be installed), which allows for direct monitoring of the output torque during the operation of the harmonic reducer, resulting in more accurate measurements; they utilize laser processing technology, solving the problems of large measurement errors and surface contamination associated with previous manual patch methods; they employ an integrated adhesive layer and sensing layer method, directly fixing the thin-film strain sensor substrate to the surface of the flexible wheel, and forming the strain sensor by laser ablation of the thin film, avoiding detection errors and hysteresis caused by the adhesive layer, and preventing detachment and dimensional interference, enabling in-situ detection; they use a method of patching first and then processing the sensor, allowing for precise control of the sensor's formation position on the flexible wheel via computer software, solving the shortcomings of difficulty in ensuring installation accuracy and contamination of the reducer assembly surface caused by the small size of the strain gauge, the compact structure of the flexible wheel, and the limited reserved installation space. Laser-induced graphene refers to the thermal decomposition of a polyimide film surface by high-energy laser (generally a pulsed laser) to form a dense graphene structure. Furthermore, the sensing electrodes formed from graphene have higher sensitivity compared to traditional strain gauges. This achieves higher torque measurement accuracy, stronger stability, and faster detection, without excessively increasing the mass of the reducer, thus ensuring the reducer's rigidity and demonstrating good market prospects.

[0063] In summary, it is readily understood by those skilled in the art that, without conflict, the aforementioned advantageous technical features can be freely combined and superimposed.

[0064] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method of preparing and installing a thin film strain sensor, characterized by, A polyamide acid solution is coated on the surface of a workpiece to be mounted, and then heated to cause the polyamide acid to undergo dehydration and cyclization to form a polyimide film adhering to the surface of the workpiece to be mounted; and then a laser-induced graphene electrode pattern is formed on the polyimide film.

2. The method of claim 1, wherein, The thickness of the polyamide acid solution coated on the surface of the workpiece to be mounted is not greater than 1 mm.

3. The method of claim 1, wherein, The heating is stepwise heating to 240-260 ℃ baking.

4. The method of claim 1, wherein, The method further comprises forming an electrode protection layer on the electrode pattern.

5. The method of claim 1, wherein, The method of forming a laser-induced graphene electrode pattern on the polyimide film comprises: Finding optimal parameters for laser processing; Using the optimal parameters to perform laser ablation processing on the polyimide film in a laser engraving machine.

6. The method of claim 5, wherein, The method of finding optimal parameters for laser processing comprises: Selecting a smooth flat plate of the same material as the workpiece to be mounted, forming a polyimide film on the smooth flat plate and dividing it into multiple detection blocks; Fixing three of the four parameters of laser power, laser scanning speed, dots per inch and pixels per inch, setting a value range for the unfixed parameter, performing multiple value settings in the value range and processing the detection blocks, measuring the sheet resistance of the processed detection blocks, and selecting the value at which the sheet resistance is the smallest as the optimal value of the unfixed parameter.

7. A harmonic reducer characterized by, The harmonic reducer further comprises a wave generator, a rigid wheel, a flexible wheel and a thin film strain sensor for detecting torque, wherein, The teeth of the flexible wheel are connected to the rigid wheel, and the wave generator is arranged in the flexible wheel; The thin film strain sensor comprises a strain film and a graphene electrode, the graphene electrode is formed on the strain film, and the strain film is directly connected to the surface of the flexible wheel; The thin film strain sensor is installed by the method of any one of claims 1-6.

8. The harmonic reducer of claim 7, wherein, The flexible wheel comprises a cylindrical body, and external teeth are arranged on the outer surface of one end of the cylindrical body; the other end of the cylindrical body forms a ring-shaped disc from the cylinder wall towards the direction away from the center, and the ring-shaped disc comprises a ring-shaped deformable thin-walled diaphragm; and the strain film is directly arranged on the diaphragm.

9. The harmonic reducer of claim 7, wherein, The pattern surrounded by the graphene electrode comprises a main stem and a plurality of branch stems, the branch stems extend away from the main stem in a direction away from the main stem; and an included angle is formed between the center line of the main stem and the center line of the branch stem; The position of the thin film strain sensor is arranged such that the center line of the main stem coincides with a ray emitted from the center of the cylindrical body.

10. The harmonic reducer of claim 9, wherein, The thin film strain sensor comprises a plurality of sensors, which are arranged at equal intervals along the circumference of the diaphragm, and two sensors arranged in the same radial direction form a group; the included angles of the graphene electrode patterns of adjacent groups of thin film strain sensors are different.

11. The harmonic reducer of claim 10, wherein, The thin film strain sensor comprises four sensors, which are arranged at intervals of 90° along the circumference of the diaphragm; wherein one group has an included angle of 45°, and the other group has an included angle of 135°.

12. The harmonic reducer of claim 7, wherein, The harmonic reducer further comprises a wire and a circuit board; the wire is led out from the end of the graphene electrode and connected to the circuit board.

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