Three-dimensional force fingertip sensor based on screw and strain mapping and its preparation method

Abstract

The present invention discloses a three-dimensional force fingertip sensor based on screw and strain mapping and a preparation method thereof. The sensor comprises: a flexible detection layer, an elastic layer, a support seat, a ball joint mechanism, an angle sensor, a printed circuit board, a flexible flat cable and a fingertip base. The flexible detection layer covers the surface of the elastic layer; the elastic layer is fixed on the support seat; the ball joint mechanism is connected to the angle sensor welded on the printed circuit board and is encapsulated inside the fingertip base; the support seat is used to connect the fingertip base and the flexible detection layer; the flexible flat cable welded to the angle sensor and the lower flexible circuit board in the flexible detection layer serve as signal output terminals. The sensor of the present invention has a small size, simple decoupling of the output multi-dimensional force signal, and low manufacturing cost, and can be used as a dexterous fingertip.

Description

Technical Field

[0001] The present invention relates to the field of robot sensing technology, and in particular to a three-dimensional force fingertip sensor based on screw and strain mapping and a preparation method thereof. Background Art

[0002] Three-dimensional force sensors have the ability to measure tangential and normal forces, and they have indispensable application value in the field of robotics. For example, a manipulator equipped with a three-dimensional force sensor can perform tasks such as stable object grasping, dexterous manipulation, and safe and smooth handover based on the force feedback provided by the sensor. At present, researchers have developed a variety of three-dimensional force sensors based on different physical mechanisms.

[0003] However, most of the developed three-dimensional force sensors are difficult to be well applied in robot tasks. After analysis, the main reasons can be summarized as follows: (1) The response performance of three-dimensional force sensors is largely limited by the characteristics of the sensing material. Taking capacitive sensors as an example, their output is easily affected by parasitic capacitance, and factors such as material aging of the dielectric layer and external electromagnetic interference will cause the instability of their output information. (2) There is signal coupling between multi-dimensional forces. When the sensor is subjected to tangential force and normal force at the same time, its output cannot accurately invert these two physical quantities. Only when the force is applied to a single axis can the sensor correctly reflect the force information. At present, the mainstream methods to solve the coupling problem of multi-dimensional force sensors include mechanical modeling and neural network training. However, for sensors with complex structures, the accuracy of mechanical modeling is often low. In addition, the neural network-based method requires a large amount of labeled data. Using this method on sensors with poor response performance and stability often requires recalibration for different scenarios. (3) Large size. For dexterous manipulation tasks, large fingertip sensors will limit the flexibility of the robot, making it difficult to handle small or geometrically complex objects.

[0004] Therefore, it is of great significance for robotic arms to develop a three-dimensional force fingertip sensor with stable output performance, good decoupling of multi-dimensional force signals and moderate size. Summary of the invention

[0005] In view of the technical problems of unstable response performance, multi-dimensional force output signal coupling, and large external dimensions limiting the flexibility of the manipulator in the existing robot sensing technology, the purpose of the present invention is to provide a three-dimensional force fingertip sensor based on screw and strain mapping and a preparation method thereof. In view of the problem of unstable sensor output signal caused by the characteristics of the sensing material, stable tangential force detection is achieved by combining a commercial angle sensor with a designed mechanical ball joint mechanism. In view of the problem of multi-dimensional force output signal coupling, the present invention adopts a layered design scheme to separate the detection sources of tangential force and normal force from each other, and designs the size and shape of the sensor based on the three-dimensional size data of real human fingertips.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A three-dimensional force fingertip sensor based on screw and strain mapping, the sensor comprising:

[0008] Ball joint mechanism, elastic layer, flexible detection layer, support seat, 4 angle sensors, 4 PCBs and fingertip base;

[0009] The ball joint mechanism is composed of a force transmission rod, a first rotating base, a second rotating base, a first rotating guide rail, a second rotating guide rail and a bearing; the first rotating base and the second rotating base are respectively provided with slots on their inner sides, and the force transmission rod is embedded in the middle of the first rotating base and the second rotating base; the first rotating base and the second rotating base are respectively provided with slots on both sides, and the slots are respectively embedded with bearings; the first rotating guide rail and the second rotating guide rail are arranged in a cross shape, and the first rotating guide rail and the second rotating guide rail are interference fit with the inner hole of the bearing through the inner rotating shaft, and the wall surface of the U-shaped slide groove of the first rotating guide rail and the second rotating guide rail is tangent to the surface of the end of the force transmission rod, and the end of the force transmission rod can move freely in its slide groove;

[0010] The ball joint mechanism is provided with two bosses respectively on the first rotating guide rail and the second rotating guide rail, and is connected to four angle sensors respectively through the bosses, and the angle sensors are respectively welded to the convex printed circuit board and the concave printed circuit board; when the sensor is subjected to a tangential force, the force transmission rod can rotate in the spherical groove of the base;

[0011] The flexible flat cable is welded to the bottom pad of the angle sensor;

[0012] The ball joint mechanism, angle sensor, convex printed circuit board and concave printed circuit board are packaged together inside the fingertip base, and the four angle sensors are placed in the card slot at the bottom of the fingertip base; the fingertip base is not only used to connect various components but also to protect the sensor element, and the upper end of the fingertip base is connected to the support base; the upper end of the support base is connected to the elastic layer;

[0013] The bottom layer of the elastic layer is provided with a slot hole, and the slot hole wraps the top of the force transmission rod; when the tangential force acts on the fingertip, the elastic layer drives the force transmission rod to rotate in the spherical groove of the base, and when the force is removed, the elastic layer resets the force transmission rod to the initial position;

[0014] The outer surface of the elastic layer is covered with a flexible detection layer, which is composed of an upper flexible circuit board, a middle conductive film and a lower flexible circuit board; the conductive film is placed between the upper and lower flexible circuit boards and fits tightly with them respectively; the lower flexible circuit board is welded to the upper flexible circuit board.

[0015] Furthermore, the material of the packaging layer of the flexible circuit board is a polyimide film.

[0016] Furthermore, the conductive film is made of Velostat piezoresistive material, which responds significantly to normal force but is sensitive to tangential force.

[0017] Furthermore, the bearing is a deep groove ball bearing with an inner diameter of 1 mm, an outer diameter of 3 mm and a thickness of 1 mm.

[0018] Furthermore, the elastic layer material is Ecoflex0030.

[0019] Furthermore, the angle sensor model is SV01A103AEA01R00.

[0020] Furthermore, the support seat, force transmission rod, first rotating base, second rotating base, first rotating guide rail, second rotating guide rail and fingertip base are all manufactured by 3D printing, and the manufacturing material is epoxy resin.

[0021] Furthermore, the upper flexible circuit board has 5 pins, and the lower flexible circuit board has 10 pins.

[0022] A method for preparing a three-dimensional force fingertip sensor based on screw and strain mapping comprises the following steps:

[0023] S1. Copper electrodes are coated on the PI film by electroplating technology; then, it is covered with an insulating layer, and the electrodes are exposed at the pads and contact positions to obtain an upper and lower flexible circuit board;

[0024] S2, using the steel mesh printing process, apply 3M liquid glue on the surface of the conductive film, combine it with the flexible circuit board prepared in step S1, and sandwich it between the upper and lower layers of flexible circuit boards, fully contact with the exposed electrodes, align the pins of the upper flexible circuit board with the pads of the lower flexible circuit board for welding, and then put it in an oven at 60-80°C for drying for 20-30 minutes, vacuumize it after drying, and finally apply a circle of glue on the edge to ensure that it is completely sealed to obtain a flexible detection layer, wherein the opening shape of the steel mesh is the insulating area except for the exposed electrode position in step S1;

[0025] S3, fully mix the A component and the B component of Ecoflex0030 in a mass ratio of 1:1, then evacuate the mixture in a vacuum barrel to remove bubbles, pour the mixture into a molding mold, dry the mold in an oven at 60-100°C for 1-2 hours to accelerate the curing of the silicone, and finally mold the mixture to obtain an elastic layer;

[0026] S4. Apply lubricant to the hemispherical grooves of the first rotating base and the second rotating base to make the force transmission rod rotate smoothly, place the force transmission rod in the hemispherical groove and apply adhesive to the surfaces to be combined with the first rotating base and the second rotating base, then bond the two fixed bases together so that the force transmission rod is embedded in the two fixed bases, embed the four bearings into the slots on the outer surfaces of the first rotating base and the second rotating base respectively, then pass the U-shaped groove of the first rotating guide through the end of the force transmission rod while the rotating shaft thereof is interference fit with the inner hole of the bearing, the second rotating guide is installed in the same manner, and the two rotating guides are arranged in a cross shape to form a ball joint mechanism;

[0027] S5, align the pins of the angle sensor with the pads of the convex printed circuit board and the concave printed circuit board for welding, then the boss of the rotating guide rail is interference-fitted with the inner hole of the angle sensor, the convex printed circuit board and the concave printed circuit board are mutually engaged and welded and fixed, and finally the flexible flat cable is aligned with the pads of the convex printed circuit board for welding and leading out, and together with the ball joint mechanism assembled in step S4, a tangential force detection mechanism is formed;

[0028] S6. Place the tangential force detection mechanism assembled in step S5 into the fingertip base, which is designed with a slot for fixing the position of the mechanism. Then use resin glue to connect the support base to the fingertip base, wherein a notch is provided at the bottom of the support base to facilitate the lead-out of the soft flat cable. Then use Sil-Poxy adhesive to fix the elastic layer on the upper surface of the support base, wherein the slot of the elastic layer wraps the top of the force transmission rod. Finally, use Sil-Poxy adhesive to cover the flexible detection layer prepared in step S1 on the outer surface of the elastic layer to obtain a sensor.

[0029] The beneficial effects of the present invention are:

[0030] (1) The sensor can detect the contact position and normal force based on the flexible detection layer, and can detect the tangential force based on the ball joint mechanism;

[0031] (2) Through a hierarchical design scheme, the normal force and tangential force detection sources are independent, thereby effectively decoupling the multi-dimensional force output signal;

[0032] (3) By combining a commercial angle sensor with a designed mechanical ball joint mechanism, stable tangential force detection can be achieved without being limited by the properties of the sensing material;

[0033] (4) The dimensions of the sensor refer to the three-dimensional data of real human fingertips, making it closer to human fingertips in appearance, laying a certain hardware foundation for the robot hand to achieve human-like dexterous operation;

[0034] (5) The cost of manufacturing the sensor is low and the process is simple, making it suitable for large-scale commercial applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 3 is a structural diagram of a three-dimensional force sensor of the present invention, wherein (a) is a dimensional diagram of the three-dimensional force sensor of the present invention, (b) is a cross-sectional view of a right view of the three-dimensional force sensor of the present invention, and (c) is a cross-sectional view of a front view of the three-dimensional force sensor of the present invention;

[0036] Figure 2 is an exploded schematic diagram of the three-dimensional force sensor of the present invention;

[0037] Figure 3 is an exploded schematic diagram of the ball joint mechanism of the present invention;

[0038] Figure 4 1 is a schematic diagram of force signal detection principle of the three-dimensional force sensor of the present invention, wherein (a) is a schematic diagram of tangential force detection principle of the three-dimensional force sensor of the present invention, (b) is a schematic diagram of the motion of the ball joint mechanism of the present invention under the action of tangential force, and (c) is a schematic diagram of normal force and contact position detection principle of the three-dimensional force sensor of the present invention;

[0039] Figure 5 Schematic diagram of the flexible detection layer of the present invention, wherein (a) is a structural diagram of the flexible detection layer of the present invention, (b) is a schematic diagram of the upper electrode of the flexible detection layer of the present invention, and (c) is a schematic diagram of the lower electrode of the flexible detection layer of the present invention;

[0040] Figure 6 It is a schematic diagram of signal collection principle of the present invention.

[0041] In the figure:

[0042] Force transmitting rod 1, first rotating base 2, second rotating base 3, bearing 4, first rotating guide rail 5, second rotating guide rail 6, ball joint mechanism 7, angle sensor 8, convex printed circuit board 9, concave printed circuit board 10, flexible flat cable 11, fingertip base 12, support base 13, elastic layer 14, lower flexible circuit board 15, conductive film 16, upper flexible circuit board 17. DETAILED DESCRIPTION

[0043] The above scheme is further described below in conjunction with specific embodiments and drawings. The basic situation of the embodiments of the present invention is summarized as follows: a three-dimensional force fingertip sensor based on screw and strain mapping.

[0044] like Figure 2 As shown, the three-dimensional force fingertip sensor in the embodiment of the present invention includes: a ball joint mechanism 7, an elastic layer 14, a flexible detection layer, a support seat 13, an angle sensor 8, a convex printed circuit board 9, a concave printed circuit board 10, a flexible flat cable 11 and a fingertip base 12.

[0045] like Figure 3 As shown, it is an exploded view of the ball joint mechanism 7, which consists of a force transmission rod 1, a first rotating base 2, a second rotating base 3, a first rotating guide rail 5, a second rotating guide rail 6 and a bearing 4; the bearing 4 is a deep groove ball bearing with an inner diameter of 1 mm, an outer diameter of 3 mm and a thickness of 1 mm; the first rotating base 2 and the second rotating base 3 are respectively provided with slots on their inner sides, and the force transmission rod 1 is embedded in the middle of the first rotating base 2 and the second rotating base 3; the first rotating base 2 and the second rotating base 3 are respectively provided with slots on both sides, and the bearings 4 are respectively embedded in the slots; the first rotating guide rail 5 and the second rotating guide rail 6 are arranged in a cross shape, and the first rotating guide rail 5 and the second rotating guide rail 6 are interference fit with the inner hole of the bearing 4 through the inner rotating axis, and the wall surface of the U-shaped slide groove of the first rotating guide rail 5 and the second rotating guide rail 6 is tangent to the surface of the end of the force transmission rod 1.

[0046] The ball joint mechanism 7 is respectively provided with two bosses on the first rotating guide rail 5 and the second rotating guide rail 6, and is connected to four angle sensors 8 through the bosses. The angle sensors 8 are respectively welded to the pads of the convex printed circuit board 9 and the concave printed circuit board 10; the model of the angle sensor 8 is SV01A103AEA01R00; the flexible flat cable 11 is welded to the bottom pad of the angle sensor 8.

[0047] The ball joint mechanism 7, angle sensor 8, convex printed circuit board 9 and concave printed circuit board 10 are encapsulated inside the fingertip base 12, and the four angle sensors 8 are placed in the card slots at the bottom of the fingertip base 12; the upper end of the fingertip base 12 is connected to the support base 13; the upper end of the support base 13 is connected to the elastic layer 14; the material of the elastic layer 14 is Ecoflex0030.

[0048] The bottom layer of the elastic layer 14 is provided with slots, and the slots wrap the top end of the force transmission rod 1 .

[0049] The outer surface of the elastic layer 14 is covered with a flexible detection layer, such as Figure 5 As shown in (a), (b), and (c), the flexible detection layer is composed of an upper flexible circuit board 17 with 5 pins, a middle conductive film 16, and a lower flexible circuit board 15 with 10 pins; the material of the flexible circuit board encapsulation layer is a polyimide film; the material of the conductive film 16 is a Velostat piezoresistive material; the conductive film 16 is placed between the upper and lower flexible circuit boards and fits tightly therewith respectively; the lower flexible circuit board 15 is welded to the upper flexible circuit board 17.

[0050] The support base 13, the force transmission rod 1, the first rotating base 2, the second rotating base 3, the first rotating guide rail 5, the second rotating guide rail 6 and the fingertip base 12 are all manufactured by 3D printing, and the manufacturing material is epoxy resin.

[0051] like Figure 1 (a) shows the design dimensions of the sensor, which are based on the three-dimensional data of a real fingertip. The length, width, and height are 23 mm, 21.85 mm, and 19.5 mm, respectively. The spatial layout of each component is shown in Figure 1. Figure 1 (b) and Figure 1 As shown in (c), the flexible detection layer for measuring normal force and the ball joint mechanism 7 for measuring tangential force adopt a layered design scheme. Then, 10 pins are led out from the lower flexible circuit board 15 as the input and output ends of the flexible detection layer, and then 4 pins are led out by the soft flat cable as the input and output ends of the ball joint mechanism 7. Among them, 5 pins of the upper flexible circuit board are welded to 5 pads of the lower flexible circuit board. Therefore, the 10 pins of the lower flexible circuit board correspond to the electrodes distributed in 10 rows and columns of the flexible detection layer, among which 5 pins are used as the input end of the row scanning power selection, and the remaining 5 pins are connected to the external acquisition circuit. Among them, the 4 pins of the soft flat cable correspond to 2 power inputs and 2 analog signal outputs respectively.

[0052] The force acting on the fingertip contact surface can be decomposed into normal force and tangential force. In the ZXY coordinate system, the force along the Z axis is defined as the normal force, and the resultant force along the X and Y axes is defined as the tangential force. The fingertip sensor based on the above structure can be used to decouple and sense three-dimensional forces and detect contact positions.

[0053] Normal force detection principle: Based on the structure of the flexible detection layer and the distribution of the upper and lower layers of flexible circuit board electrodes, the flexible detection layer can be regarded as a structure composed of a 5×5 unit resistor array. Figure 4 As shown in (c), the normal force acting on the fingertip surface will reduce the resistance value of the sensor unit. As the normal force increases, the resistance value gradually decreases. The change in resistance value can reflect the action of the normal force. In addition, based on the array arrangement of the sensor units, the contact position can be reflected according to the change in resistance value of each sensor unit.

[0054] Tangential force detection principle: When the tangential force acts on the center of the fingertip sensor surface, the elastic layer 14 deforms under the external force, driving the top of the force transmission rod 1 wrapped in the slot to rotate around the center of the ball in the inner groove of the rotating base. Then, the top of the force transmission rod 1 moves in the U-shaped slide grooves of the first rotating guide rail 5 and the second rotating guide rail 6, and pushes the first rotating guide rail 5 and the second rotating guide rail 6 to rotate around the X-axis and Y-axis respectively. Figure 4 As shown in (b), the effect of the tangential force causes the top end of the force transmission rod 1 to move in the U-shaped grooves of the two rotating guide rails. X and L Y displacement. Since the inner holes of the four angle sensors 8 are interference fit with the boss ends of the two rotating guide rails, when the first rotating guide rail 5 and the second rotating guide rail 6 rotate around the X-axis and the Y-axis respectively, the rotors of the connected and matched angle sensors 8 also rotate around the X-axis and the Y-axis. After the tangential force is removed, the top of the force transmitting rod 1 can be restored to its initial position due to the wrapping effect of the slots of the elastic layer 14 on the top of the force transmitting rod 1. The first rotating guide rail 5 and the second rotating guide rail 6 are designed to be staggered cross-shaped, and the arcs of the slide grooves of the two are in an equidistant geometric relationship. In this way, the two rotating guide rails do not interfere with each other during rotation and will not get stuck, thereby eliminating the coupling relationship between the forces in the X and Y directions. Figure 4 As shown in (a), the ball joint mechanism 7 converts the tangential forces along the X-axis and along the Y-axis into moments around the Y-axis and around the X-axis, respectively, and converts the displacement into a rotation. By using a high-precision commercial angle sensor 8 to measure the above rotation, the tangential force can be characterized. In addition, the mechanism is easy to model, and the mapping relationship of the above physical quantities can be obtained through physical modeling. Therefore, the ball joint mechanism 7 can be optimized by changing factors such as structural size, surface friction coefficient and material, thereby widening the detection range and improving sensitivity.

[0055] The present invention separates the detection of tangential force and normal force by multi-sensor fusion, and the sensing modes are independent and do not affect each other. Therefore, in the final output, there is no need to use complex algorithms to decouple force signals, nor is there a need for a large amount of training data to map force signals, thereby achieving convenient and efficient measurement.

[0056] The principle of collecting signals of the sensor in the present invention can be as follows: Figure 6 As shown. Due to the symmetrical design of the ball joint mechanism 7, it is only necessary to detect the rotation amount of the rotor of a pair of adjacent angle sensors 8, and the analog quantity is output by the 4-pin soft cable 11. The 5×5 unit resistor array structure of the flexible detection layer is detected, and its signal is collected by row and column scanning, which can reduce the number of pins at the output end and isolate the crosstalk current. The collection principle is: open the path of one row each time, and collect the analog quantities on the 5 columns of paths in turn. Repeat the above process, open the paths of 5 rows in turn, and the collection is completed. Since the reading frequency of the microcontroller is relatively high, the above process can be regarded as synchronously and real-time collection of data from 25 units. CD4067 is a 16-channel analog switch that can be used for row scanning of array structures. By controlling the level of the GPIO pin, the microcontroller can flexibly select the output path of the CD4067 analog switch, thereby outputting an input signal to different channels. For example, when the scan opens R 10 When running, R 20 , R 30 , R 40 and R 50 The line will be closed accordingly. Since the positive terminal of the operational amplifier at the output is connected to GND, the sensing unit R 11 Formed on the pathway The current, where Vcc is the input voltage, is output through the feedback resistor at the output of the operational amplifier. 02 , R 03 , R 04 and R 05 The corresponding voltage is output in each column, and then the analog quantity of 5 columns is output in sequence through CD4067 to realize column scanning. This method can reduce the number of ports of MCU ADC.

[0057] A method for preparing a three-dimensional force fingertip sensor based on screw and strain mapping comprises the following steps:

[0058] 1. Through the electroplating technology, copper electrodes are covered on the PI film, and then covered with an insulating layer to protect the wiring and solder mask, and the electrodes are exposed at the pads and contact positions, thereby manufacturing the upper flexible circuit board 17 and the lower flexible circuit board 15.

[0059] 2. Use the steel screen printing process to apply 3M liquid glue on the surface of the conductive film 16, combine it with the flexible circuit board prepared in step 1, and sandwich it between the upper and lower layers of flexible circuit boards to fully contact the exposed electrodes. Align the pins of the upper flexible circuit board 17 with the pads of the lower flexible circuit board 15 for welding, and then put it in an oven at 60-80°C for 20-30 minutes. To ensure that the air inside the interlayer is removed, vacuum treatment is performed after drying, and finally a circle of glue is applied at the edge to ensure that it is completely sealed, thereby producing a flexible detection layer, wherein the opening shape of the steel mesh is the insulating area except for the exposed electrode position in step 1.

[0060] 3. Component A and component B of Ecoflex0030 are fully mixed in a mass ratio of 1:1, and then vacuumed in a vacuum barrel to remove bubbles, and then poured into a molding mold. The mold is placed in an oven at 60°C and dried for one hour to accelerate the curing of the silicone, and finally reversed to produce an elastic layer 14.

[0061] 4. Apply lubricant to the hemispherical grooves of the first rotating base 2 and the second rotating base 3 to allow the force transmitting rod 1 to rotate smoothly, place the force transmitting rod 1 in the hemispherical groove and apply adhesive to the surface of the base to be combined, then bond the two bases together so that the force transmitting rod 1 is embedded in the fixed base, and embed the four bearings 4 into the slots on the four outer surfaces of the base respectively. Then, pass the U-shaped groove of the first rotating guide rail 5 through the end of the force transmitting rod 1 and the rotating shaft thereof is interference fit with the inner hole of the bearing 4. The second rotating guide rail 6 is installed in the same manner, and the two rotating guide rails are arranged in a "cross shape" to manufacture a ball joint mechanism 7.

[0062] 5. Align the pins of the four angle sensors 8 with the pads of the four printed circuit boards and weld them. Then, make an interference fit between the boss of the rotating guide rail and the inner hole of the angle sensor 8. In order to enhance the integrity of the installation, the printed circuit board is designed as two types of plates: "convex" and "concave". The convex printed circuit board 9 and the concave printed circuit board 10 are interlocked and welded to each other. Finally, align the flexible flat cable 11 with the pads of the printed circuit board 9, weld it and lead it out, thereby forming a tangential force detection mechanism with the ball joint mechanism 7 assembled in step 4.

[0063] 6. Place the tangential force detection mechanism assembled in step 5 into the fingertip base 12, which is designed with a slot for fixing the position of the mechanism. Then use resin to connect the support base 13 to the fingertip base 12, wherein a notch is provided at the bottom of the support base 13 to facilitate the lead-out of the soft flat cable 11. Then use Sil-Poxy adhesive to fix the elastic layer 14 on the upper surface of the support base 13, wherein the slot of the elastic layer 14 wraps the top of the force transmission rod 1. Finally, use Sil-Poxy adhesive to cover the flexible detection layer prepared in step 1 on the outer surface of the elastic layer 14, thereby manufacturing a sensor.

Claims

1. A three-dimensional force fingertip sensor based on screw and strain mapping, characterized in that: include: A ball joint mechanism (7), an elastic layer (14), a flexible detection layer, a support seat (13), an angle sensor (8), a convex printed circuit board (9), a concave printed circuit board (10), a flexible flat cable (11), and a fingertip base (12); The ball joint mechanism (7) is composed of a force transmission rod (1), a first rotating base (2), a second rotating base (3), a first rotating guide rail (5), a second rotating guide rail (6) and a bearing (4); the first rotating base (2) and the second rotating base (3) are respectively provided with hemispherical grooves on their inner sides, and the force transmission rod (1) is embedded in the middle of the first rotating base (2) and the second rotating base (3); the first rotating base (2) and the second rotating base (3) are respectively provided with slots on both sides, and the slots are respectively embedded with bearings (4); the first rotating guide rail (5) and the second rotating guide rail (6) are arranged in a cross shape, and the first rotating guide rail (5) and the second rotating guide rail (6) are interference fit with the inner hole of the bearing (4) through the inner rotating shaft, and the wall surface of the U-shaped slide groove of the first rotating guide rail (5) and the second rotating guide rail (6) is tangent to the surface of the end of the force transmission rod (1); The ball joint mechanism (7) is provided with two bosses respectively on the first rotating guide rail (5) and the second rotating guide rail (6), and is connected to four angle sensors (8) respectively through the bosses. The angle sensors (8) are respectively welded to the pads of the convex printed circuit board (9) and the concave printed circuit board (10); The flexible flat cable (11) is welded to the bottom pad of the angle sensor (8); The ball joint mechanism (7), the angle sensor (8), the convex printed circuit board (9) and the concave printed circuit board (10) are encapsulated inside a fingertip base (12), and the four angle sensors (8) are placed in a slot at the bottom of the fingertip base (12); the upper end of the fingertip base (12) is connected to a support base (13); and the upper end of the support base (13) is connected to an elastic layer (14); The bottom layer of the elastic layer (14) is provided with a slot, and the slot wraps the top end of the force transmission rod (1); The outer surface of the elastic layer (14) is covered with a flexible detection layer, the flexible detection layer comprising an upper flexible circuit board (17), a middle conductive film (16), and a lower flexible circuit board (15); the conductive film (16) is placed between the upper and lower flexible circuit boards and is closely attached to them respectively; the lower flexible circuit board (15) is welded to the upper flexible circuit board (17).

2. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The material of the flexible circuit board packaging layer is polyimide film.

3. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The conductive film (16) is made of Velostat piezoresistive material.

4. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The bearing (4) is a deep groove ball bearing with an inner diameter of 1 mm, an outer diameter of 3 mm and a thickness of 1 mm.

5. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The elastic layer (14) is made of Ecoflex0030.

6. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The angle sensor (8) has a model number of SV01A103AEA01R00.

7. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The support seat (13), the force transmission rod (1), the first rotating base (2), the second rotating base (3), the first rotating guide rail (5), the second rotating guide rail (6) and the fingertip base (12) are all manufactured by 3D printing, and the manufacturing material is epoxy resin.

8. The three-dimensional force fingertip sensor based on screw and strain mapping according to claim 1, characterized in that: The upper flexible circuit board (17) has 5 pins, and the lower flexible circuit board (15) has 10 pins.

9. A method for preparing a three-dimensional force fingertip sensor based on screw and strain mapping, characterized in that: The steps include: (S1) Copper electrodes are coated on the PI film by electrodeposition technology; then, the film is covered with an insulating layer, and the electrodes are exposed at the pads and contact positions to produce a flexible circuit board with two layers; (S2) using a steel mesh printing process, applying 3M liquid glue on the surface of the conductive film (16), combining it with the flexible circuit board prepared in step (S1), and sandwiching it between the upper and lower flexible circuit boards to fully contact the exposed electrodes, aligning the pins of the upper flexible circuit board (17) with the pads of the lower flexible circuit board (15) for welding, and then placing it in an oven at 60-80°C for drying for 20-30 minutes, vacuuming it after drying, and finally applying a circle of glue on the edge to ensure that it is completely sealed, thereby obtaining a flexible detection layer, wherein the opening shape of the steel mesh is the insulating area except for the exposed electrode position in step (S1); (S3) Component A and component B of Ecoflex0030 are fully mixed in a mass ratio of 1:1, and then vacuumed in a vacuum barrel to remove bubbles therein, poured into a molding mold, and the mold is placed in an oven at 60-100° C. and dried for 1-2 hours to accelerate the curing of the silicone, and finally molded to obtain an elastic layer (14); (S4) Lubricant is applied to the hemispherical grooves of the first rotating base (2) and the second rotating base (3) to allow the force transmission rod (1) to rotate smoothly, the force transmission rod (1) is placed in the hemispherical groove and adhesive is applied to the surfaces of the first rotating base (2) and the second rotating base (3) to be combined, and then the two rotating bases are bonded together so that the force transmission rod (1) is embedded in the two rotating bases, and four bearings (4) are respectively embedded in the groove holes on the outer surfaces of the first rotating base (2) and the second rotating base (3), and then the U-shaped groove of the first rotating guide rail (5) is passed through the end of the force transmission rod (1) and the rotating shaft thereof is interference fit with the inner hole of the bearing (4). The second rotating guide rail (6) is installed in the same manner, and the two rotating guide rails are arranged in a cross shape to form a ball joint mechanism (7); (S5) aligning the pins of the angle sensor (8) with the pads of the convex printed circuit board (9) and the concave printed circuit board (10) and welding them, then interferingly fitting the boss of the rotating guide rail with the inner hole of the angle sensor (8), locking the convex printed circuit board (9) and the concave printed circuit board (10) with each other and welding them to fix them, finally aligning the flexible flat cable (11) with the pads of the convex printed circuit board (9) and welding and leading them out, and forming a tangential force detection mechanism together with the ball joint mechanism (7) assembled in step (S4); (S6) The tangential force detection mechanism assembled in step (S5) is placed in a fingertip base (12), wherein the fingertip base (12) is provided with a slot for fixing the position of the mechanism, and then the support base (13) is connected to the fingertip base (12) using resin glue, wherein a notch is provided at the bottom end of the support base (13) to facilitate the lead-out of the soft flat cable (11), and then the elastic layer (14) is fixed to the upper surface of the support base (13) using Sil-Poxy adhesive, wherein the slot of the elastic layer (14) wraps the top of the force transmission rod (1), and finally the flexible detection layer prepared in step (S1) is covered on the outer surface of the elastic layer (14) using Sil-Poxy adhesive to obtain a sensor.

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