Anti-magnetic field interference magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing

Abstract

The invention discloses an anti-magnetic-field-interference magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing. The anti-magnetic-field-interference magnetic tactile sensor comprises a bottom-layer flexible circuit board, a top-layer flexible circuit board, a magnetic film, a bottom tunnel magnetoresistance element, an elastic body and a top tunnel magnetoresistance element. A hollow area is arranged in the center of the magnetic thin film, the magnetic thin film is pasted on the bottom layer flexible circuit board, the magnetizing direction of the magnetic thin film is upward along the height of the sensor, and the bottom tunnel magnetoresistive element is embedded in the hollow area of the magnetic thin film and is connected with the bottom layer flexible circuit board; the top tunnel magnetic resistance element is located over the bottom tunnel magnetic resistance element and connected with the top flexible circuit board, and the two tunnel magnetic resistance elements are connected through an elastic body. The external magnetic field interference is converted into a common-mode signal which can be directly counteracted by acquiring the difference of the output voltages of the two tunnel magnetoresistive elements, and the local magnetic field change caused by the real contact force is used as a differential-mode signal to be sensed with high sensitivity, so that the physical suppression of the external magnetic field interference is realized.

Description

Technical Field

[0001] This invention relates to the field of tactile sensor technology, and in particular to an anti-magnetic field interference magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing. It effectively suppresses the influence of external magnetic field interference on the sensor output voltage, improves the accuracy and reliability of tactile detection, and is suitable for applications with complex electromagnetic environments and high requirements for sensor accuracy, such as industrial humanoid robots and the power industry. Background Technology

[0002] With the rapid development of robotics technology, tactile sensors have demonstrated significant application value in tasks such as precision grasping, surface recognition, and force feedback. Magnetic tactile sensors have attracted widespread attention due to their high sensitivity, fast response, and good durability. However, these sensors are extremely sensitive to changes in external magnetic fields, especially in industrial environments with electromagnetic interference sources such as motors, frequency converters, and magnetic materials. In these environments, their measurement signals are prone to drift, resulting in a significant decrease in stability. When grasping metal objects, traditional magnetic tactile sensors can also suffer from output signal distortion due to interference from the metal object's own magnetic permeability, severely limiting their applicability in real-world work scenarios.

[0003] Traditional magnetic tactile sensors typically use a magnetic thin film to generate a magnetic field, usually located at the top of the sensor, while the magnetic sensing structure (such as a tunnel magnetoresistive element) lies beneath it. When external pressure is applied to the magnetic thin film, the magnetic sensing structure detects the change in the magnetic field and outputs a voltage. However, electromagnetic interference or application to metallic objects can affect the magnetic field of the thin film, leading to inaccurate detection and consequently, inaccurate sensor output voltage. Furthermore, traditional magnetic tactile sensors often use a high-permeability shielding layer on the sensor surface. When external pressure is applied, this shielding layer deforms, causing changes in the magnetic field and introducing new interference, further contributing to inaccurate sensor detection.

[0004] While some existing magnetic tactile sensors possess basic tactile sensing capabilities, they struggle to cope with complex magnetic field environments. For example, patent document CN118533331A discloses an arc-shaped soft tactile sensor for texture sensing. Although it has certain advantages in texture and pressure sensing, its single tunnel magnetoresistive element and solid magnetic thin film structure are extremely sensitive to changes in the spatial magnetic field. Furthermore, it lacks an effective anti-magnetic interference mechanism, resulting in severe signal drift when operating near electromagnetic devices, leading to measurement inaccuracies. Moreover, it cannot stably grasp ferromagnetic materials, as the permeability of ferromagnetic materials significantly disturbs the sensor's original magnetic field distribution, introducing huge detection errors and severely limiting its application in complex magnetic field environments. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing that is resistant to magnetic field interference. Through a unique spatial structure design, it achieves physical suppression of external magnetic field interference, which not only significantly improves the measurement accuracy and reliability of the magnetic tactile sensor in industrial strong magnetic field environments, but also expands its application in key fields such as automated assembly, metal sorting, and the power industry.

[0006] The present invention solves the aforementioned technical problem by adopting the following technical solution: An anti-magnetic field interference magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing includes a bottom flexible circuit board, a top flexible circuit board, a magnetic film, a bottom tunnel magnetoresistive element, an elastomer, and a top tunnel magnetoresistive element. The magnetic film has a hollow area at its center and is attached to the bottom flexible circuit board. The magnetization direction of the magnetic film is upward along the height of the sensor. The bottom tunnel magnetoresistive element is embedded in the hollow area of ​​the magnetic film and connected to the bottom flexible circuit board. The top tunnel magnetoresistive element is located directly above the bottom tunnel magnetoresistive element and is connected to the top flexible circuit board. The two tunnel magnetoresistive elements are connected by the elastomer.

[0007] Furthermore, the magnetic film is formed by mixing and curing neodymium iron boron particles and organosilicon elastomer, with a mass ratio of neodymium iron boron particles to organosilicon elastomer of 3:1 to 5:1, and the diameter of the neodymium iron boron particles is 200 to 300 μm.

[0008] Furthermore, the magnetic film has a length and width of 6-9 mm, a thickness of 1-2.5 mm, and a cutout area with a length and width of 3-5 mm.

[0009] Furthermore, the elastomer is a thermoplastic elastomer, and the length, width, and height of the elastomer are 3~4 mm.

[0010] Compared with the prior art, the beneficial effects of the present invention are: 1. Compared to traditional magnetic tactile sensors that place the magnetic film on top, the structure of this invention is drastically different. The magnetic film is placed at the bottom of the sensor as a fixed magnetic field source. A pair of tunnel magnetoresistive elements with identical characteristics are arranged at specific positions above and below the magnetic film. One tunnel magnetoresistive element is soldered to the top flexible circuit board and inverted above the cutout area of ​​the magnetic film, directly contacting external pressure. The other tunnel magnetoresistive element is rigidly fixed to the bottom flexible circuit board. When the sensor contacts an object, the pressure applied to the tactile contact surface forces the top tunnel magnetoresistive element to undergo a slight displacement relative to the fixed bottom magnetic film, thus sensitively sensing the magnetic field change caused by the distance change. Meanwhile, the bottom tunnel magnetoresistive element, serving as a reference, maintains a stable reference magnetic field signal from the magnetic film due to its fixed spatial position.

[0011] Any magnetic field interference in the external environment will have almost the same effect on the two tunnel magnetoresistive elements, while the local magnetic field changes caused by actual tactile pressure will only be captured by the slightly movable top tunnel magnetoresistive element. By obtaining the differential value of the output voltage of the two tunnel magnetoresistive elements, the environmental magnetic field interference is converted into a common-mode signal that can be directly canceled and effectively suppressed, while the local magnetic field changes caused by actual contact force are perceived with high sensitivity as the differential-mode signal of the two tunnel magnetoresistive elements, thus achieving stable and reliable tactile perception of metal workpieces or under magnetic field interference conditions. This "common-mode suppression, differential-mode detection" working mechanism achieves essential suppression of external magnetic field interference from the source of the detection principle. It can obtain a stable output with a high signal-to-noise ratio in complex electromagnetic noise environments without relying on an additional magnetic shielding layer, realizing stable and reliable grasping and recognition of metal and non-metal objects in complex environments, and solving the signal distortion problem of traditional magnetic tactile sensors when grasping metal objects and dealing with magnetic field interference.

[0012] 2. The anti-interference capability of this invention mainly stems from its ingenious structural design, namely the difference in spatial layout between the top tunnel magnetoresistive element and the bottom tunnel magnetoresistive element. This design eliminates the need for complex active signal processing algorithms or additional shielding layers, resulting in a simpler overall structure. This not only reduces material costs but also simplifies assembly and calibration processes, improving the reliability and consistency of the sensor. It is very suitable for large-scale production and application deployment.

[0013] 3. The top tunnel magnetoresistive element directly contacts external pressure through the top flexible circuit board, directly converting minute pressure changes into its own displacement relative to the magnetic thin film (as a fixed magnetic source), thereby causing sensitive changes in magnetic flux. This direct mechanical conduction path with minimal energy loss, combined with the inherent high sensitivity of the tunnel magnetoresistive element, enables the sensor to have extremely high responsiveness to weak tactile signals, accurately detecting micron-level surface texture changes or millinewton-level minute pressure. The static performance test results of Example 1 show that the sensor's sensitivity is 266.7 mV / N, the force measurement range is 0~3 N, the force measurement resolution is 0.05 N, and the sensitivity drift during loading and unloading processes does not exceed 5%.

[0014] 4. Due to the absence of a thick magnetic shielding layer, the magnetic film is fixed at the bottom layer, and a low-inertia elastomer is used, reducing the overall mechanical damping of the sensor and significantly improving the dynamic response speed. The dynamic performance test results from Example 2 show that the sensor maintains good output performance under a 10Hz dynamic force, with a response time of 25ms and a recovery time of 30ms. Under 10,000 cycles of dynamic force application, the sensor's output signal error does not exceed 10%.

[0015] 5. The top and bottom tunnel magnetoresistive elements are close together, and can generate nearly the same voltage signal when subjected to external magnetic field interference, which facilitates subsequent signal decoupling and processing.

[0016] 6. The anti-interference method proposed in this invention does not rely on specific signal post-processing algorithms. Its output differential signal is stable, intuitive, and easily read and processed by various standard data acquisition systems. Therefore, this sensor can be easily integrated into existing industrial robots, collaborative robots, or precision operating equipment, greatly expanding the application boundaries of magnetic tactile sensors in environments with strong electromagnetic interference, such as automated assembly, product quality inspection, and logistics sorting.

[0017] 7. Compared with existing magnetic tactile sensors, this invention places the magnetic film at the bottom layer and has a hollow area in the center, which avoids the inability to decouple interference signals from useful signals due to the change of the whole magnetic field caused by pressure on the magnetic film. This achieves anti-interference performance without reducing sensitivity. Attached Figure Description

[0018] Figure 1 This is an overall structural diagram of the present invention; Figure 2 This is an exploded view of the present invention; Figure 3 This is a top view of the present invention; Figure 4 This is a top view of the flexible circuit board of the present invention; Figure 5 This is a diagram showing the output voltage of the sensor of the present invention under different static pressures; Figure 6 This is a diagram showing the output voltage of the sensor of the present invention under dynamic pressure; Figure 7 These are the response time and recovery time curves of the sensor of this invention; Figure 8 This is a graph showing the pressure and output voltage of the sensor of the present invention under different magnetic field interferences; Figure 9 This is a diagram showing the output voltage of the sensor in this invention when it grasps iron foam; Among them, 1-bottom flexible circuit board; 2-bottom tunnel magnetoresistive element; 3-magnetic film; 4-elastic body; 5-top tunnel magnetoresistive element; 6-top flexible circuit board. Detailed Implementation

[0019] Specific embodiments are given below with reference to the accompanying drawings. These specific embodiments are only used to describe the technical solution of the present invention in detail, and are not intended to limit the scope of protection of this application.

[0020] like Figures 1-4As shown, this invention provides an anti-magnetic field interference magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing, including a bottom flexible circuit board 1, a bottom tunnel magnetoresistive element 2, a magnetic film 3, an elastomer 4, a top tunnel magnetoresistive element 5, and a top flexible circuit board 6; the magnetic film 3 has a hollow area at its center, and the magnetic film 3 is pasted on the bottom flexible circuit board 1; the bottom tunnel magnetoresistive element 2 is embedded in the hollow area of ​​the magnetic film 3 and welded to the bottom flexible circuit board 1; the top tunnel magnetoresistive element 5 is located directly above the bottom tunnel magnetoresistive element 2 and is welded together with the top flexible circuit board 6; the two tunnel magnetoresistive elements are connected together by the elastomer 4.

[0021] The bottom tunnel magnetoresistive element 2 and the top tunnel magnetoresistive element 5 serve as magnetic field detection elements. When the sensor is subjected to a vertically downward external pressure, the elastic body 4 is compressed and contracts, causing the top tunnel magnetoresistive element 5 to move slightly downward. This reduces the distance between the top tunnel magnetoresistive element 5 and the magnetic film 3, resulting in a change in the detected magnetic field and an output voltage signal. However, the relative position of the bottom tunnel magnetoresistive element 2 and the magnetic film 3 remains unchanged, so the detected magnetic field of the bottom tunnel magnetoresistive element 2 does not change, and it outputs no voltage signal. Therefore, the output voltage of the top tunnel magnetoresistive element 5 reflects the pressure magnitude. When the sensor is interfered with by an external magnetic field, both tunnel magnetoresistive elements generate output voltages due to the interference. When a vertically downward pressure is applied to the sensor, the top tunnel magnetoresistive element 5, being closer to the interfering magnetic field, will have a higher output voltage than the bottom tunnel magnetoresistive element 2. The difference in output voltage between the two tunnel magnetoresistive elements can be used to determine the true pressure magnitude.

[0022] The magnetic film 3 is formed by curing a mixture of neodymium iron boron particles and silicone elastomer, with a mass ratio of neodymium iron boron particles to silicone elastomer of 3:1 to 5:1. The diameter of the neodymium iron boron particles is 200 to 300 μm. The silicone elastomer is made by mixing Smooth-On's Ecoflex 00-30 series A agent and B agent in a 1:1 ratio. The magnetic film 3 has a length and width of 6 to 9 mm, a thickness of 1 to 2.5 mm, and a perforated area with a length and width of 3 to 5 mm. The magnetic film 3 is magnetized upwards along the height of the sensor, with a surface magnetic field strength of 25 to 50 mT, ensuring that both the bottom tunnel magnetoresistive element 2 and the top tunnel magnetoresistive element 5 operate within the linear range.

[0023] Both the bottom tunnel magnetoresistive element 2 and the top tunnel magnetoresistive element 5 are Y-axis sensitive, model TMR2185G, with a linear range of ±1000GS and dimensions of 2×1.5×0.73mm, and are soldered to the center of their respective flexible circuit boards.

[0024] The elastic body 4 serves as a pressure buffer. In order to ensure that the sensor operates in the linear range and that the bottom tunnel magnetoresistive element 2 and the top tunnel magnetoresistive element 5 are affected by external magnetic field interference in a consistent manner, the length, width and height of the elastic body 4 are set to 3~4mm. The elastic body 4 is made of TPE, which can be compressed under external force and can rebound quickly after the external force is removed. It is necessary to ensure that no plastic deformation occurs within the force measurement range.

[0025] The bottom flexible circuit board 1 and the top flexible circuit board 6 are 0.05 mm thick. The substrate material of the flexible circuit boards is polyimide, and four through-holes are provided at the tail for soldering wires. A top view of the flexible circuit board is shown below. Figure 4 As shown, VCC represents the 5V DC power supply of the sensor, GND is the power ground terminal, V+ represents the positive voltage output of the tunnel magnetoresistive element, and V- represents the negative voltage output of the tunnel magnetoresistive element. The V+ and V- terminals of the two flexible circuit boards are connected to the data acquisition card, so that the two tunnel magnetoresistive elements are connected to channel one and channel two of the data acquisition card for acquiring the output voltage of the two tunnel magnetoresistive elements.

[0026] Example 1 The sensor parameter settings in this embodiment include: the magnetic film 3 has a size of 6mm×6mm×1mm, the length and width of the hollow area are 3mm, the mass ratio of neodymium iron boron particles to silicone elastomer is 3:1, the magnetization direction is upward along the sensor height, and the surface magnetic field strength is 27.2mT; the elastomer 4 has a size of 3mm×3mm×2.5mm.

[0027] The main purpose of this embodiment is to test the force sensitivity and force range of the magnetic tactile sensor. After the sensor is assembled, it is installed on a static force testing platform, which consists of a digital push-pull force gauge (0-50N), a 5V adjustable DC power supply, a multi-channel analog voltage acquisition card, and a PC computer. The digital push-pull force gauge outputs adjustable pressure by rotating a knob. The DC power supply provides a stable 5V voltage to the bottom tunnel magnetoresistive element 2 and the top tunnel magnetoresistive element 5 in the sensor. The data acquisition card acquires the output voltage of the two tunnel magnetoresistive elements and transmits it to the PC computer for processing and display.

[0028] When a normal force of 0-3N is applied above the tunnel magnetoresistive element 5 at the top of the sensor in the absence of magnetic field interference, the output voltage of the sensor is as follows: Figure 5 As shown in the figure. It can be seen from the figure that the output voltage of the top tunnel magnetoresistive element 5 ( Within the range of 0~3N, the voltage increases linearly with increasing pressure. During loading and unloading, the output voltage error of the top tunnel magnetoresistive element 5 does not exceed 5% under the same pressure, exhibiting good repeatability. Within the range of 0~3N, the relative position of the bottom tunnel magnetoresistive element 2 and the magnetic film 3 remains unchanged, and the background magnetic field generated by the magnetic film 3 also remains unchanged; therefore, the bottom tunnel magnetoresistive element 2 has no voltage output. When the sensor is subjected to a pressure of 3.5N, the output voltage of the top tunnel magnetoresistive element 5 increases nonlinearly, and the output voltage of the bottom tunnel magnetoresistive element 2 increases. This is because when the pressure exceeds 3N, the compression of the elastic body 4 increases, touching the edge of the hollow area of ​​the magnetic film 3 and changing the magnetic field distribution of the magnetic film 3. Therefore, the force measurement range of the sensor in this embodiment can be determined to be 0~3N. The sensitivity of the sensor can be defined as the output voltage under 1N pressure, and the calculated sensitivity is 266.7mV / N. Simultaneously, the test results show that the minimum static force resolution is 0.05N.

[0029] Example 2 The sensor composition in this embodiment is consistent with that in Embodiment 1. The purpose of this embodiment is to test the dynamic response performance of the sensor. The sensor is mounted on a dynamic performance test platform, which consists of a vibrator, a power amplifier, a signal generator, a 5V adjustable DC power supply, a multi-channel analog voltage acquisition card, and a PC. The signal generator provides a dynamic signal and connects to the power amplifier and vibrator. The dynamic signal is transmitted to the vibrator through the power amplifier, and the vibrator outputs dynamic pressure. The vibrator contacts are equipped with piezoelectric sensors to obtain accurate pressure values. The frequency and voltage of the signal generator are adjusted so that the vibrator outputs a dynamic force with a frequency of 10Hz and a magnitude of 1N, 2N, and 3N. The sensor's output voltage and response time are as follows. Figure 6 , 7 As shown. From Figure 6 It can be seen that under different dynamic forces at 10Hz, the sensor maintains good output characteristics, and the sensor sensitivity error does not exceed 3%. From Figure 7 It can be seen that the sensor's response time is 25ms and its recovery time is 30ms, indicating that the sensor has good dynamic response characteristics.

[0030] Example 3 The sensor composition in this embodiment is consistent with that in Embodiment 1. The purpose of this embodiment is to test the sensor's resistance to external magnetic field interference. The test platform is the same as the static force test platform. A Helmholtz coil is placed above the contact of the digital push-pull force gauge, and a DC current is applied to simulate external static magnetic field interference. Changing the input current of the Helmholtz coil changes the static magnetic field strength. In the initial state, rotating the knob of the digital push-pull force gauge presses down the contact to apply pressure to the sensor. The output voltage of the sensor under different magnetic field strengths is as follows: Figure 8 As shown. When the Helmholtz coil is energized, the top tunnel magnetoresistive element 5 and the bottom tunnel magnetoresistive element 2 of the sensor simultaneously output voltage signals with similar amplitudes. When external pressure is applied to the sensor, the output voltage of the top tunnel magnetoresistive element 5 increases with the increase of pressure, while the output voltage of the bottom tunnel magnetoresistive element 2 remains unchanged with the increase of pressure. The accurate pressure value can be obtained by the difference between the output voltage of the top tunnel magnetoresistive element 5 and the output voltage of the bottom tunnel magnetoresistive element 2.

[0031] Example 4 The sensor composition in this embodiment is consistent with that in Embodiment 1. The purpose of this embodiment is to test the sensor's grasping stability under the influence of the magnetic permeability of the metal object itself. The sensor is installed on the inside of the two-finger robotic arm, and the robotic arm is controlled by a PC to grasp the ferrofoam (i.e., the metal object). The sensor output voltage is as follows: Figure 9 As shown. Due to the high magnetic permeability of the iron foam, when the robotic arm approaches the iron foam, it changes the magnetic field distribution near the magnetic film 3, thereby increasing the output voltage of both the top tunnel magnetoresistive element 5 and the bottom tunnel magnetoresistive element 2. When the sensor comes into contact with the iron foam, the top tunnel magnetoresistive element 5 is closer to the iron foam and is pressed down by the force of the iron foam, reducing the distance between it and the magnetic film 3. Therefore, the top tunnel magnetoresistive element 5 is affected by the magnetic field changes of both the iron foam and the magnetic film 3, resulting in a higher output voltage. Meanwhile, the distance between the bottom tunnel magnetoresistive element 2 and the magnetic film 3 remains constant, so the bottom tunnel magnetoresistive element 2 is only affected by the magnetic field changes of the iron foam, resulting in a lower output voltage. The actual gripping force can be determined by the difference in output voltage between the top tunnel magnetoresistive element 5 and the bottom tunnel magnetoresistive element 2.

[0032] As can be seen from this embodiment, the magnetic tactile sensor of the present invention can be installed on a robotic arm, enabling reliable gripping and manipulation of metal objects even in industrial environments with electromagnetic equipment such as motors. When the sensor contacts the surface of a metal object, the top tunnel magnetoresistive element undergoes a slight displacement with the tactile surface, while the bottom tunnel magnetoresistive element remains spatially fixed, resulting in a difference in the magnetic field sensing of the two relative to the magnetic film. The external ambient magnetic field is applied synchronously to both tunnel magnetoresistive elements as a common-mode signal, while the local magnetic field change caused by the actual gripping force is specifically detected by the top tunnel magnetoresistive element as a differential-mode signal. This detection method effectively eliminates the interference components of the ambient magnetic field, allowing the output voltage to accurately reflect the gripping force and contact state with the metal object, thereby achieving stable and accurate tactile sensing of the workpiece in complex electromagnetic environments.

[0033] As can be seen from the above embodiments, the magnetic tactile sensor of the present invention retains the advantages of high sensitivity and accurate detection of static / dynamic forces, and also has the performance of resisting magnetic field interference, further expanding the application scenarios of the magnetic tactile sensor.

[0034] Any aspects not mentioned in this invention are known technologies.

Claims

1. A magnetic field interference resistant magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing, comprising a bottom layer flexible circuit board, a top layer flexible circuit board and a magnetic film; characterized in that, The magnetic thin film is pasted on the bottom flexible circuit board, the magnetizing direction of the magnetic thin film is upward along the height of the sensor, the bottom tunnel magnetoresistance element is embedded in the hollow area of the magnetic thin film and connected with the bottom flexible circuit board, the top tunnel magnetoresistance element is located directly above the bottom tunnel magnetoresistance element and connected with the top flexible circuit board, and the two tunnel magnetoresistance elements are connected through the elastic body. The magnetic thin film is formed by mixing and curing neodymium iron boron particles and organic silicon elastomer, the mass ratio of the neodymium iron boron particles to the organic silicon elastomer is 3:1-5:1, and the diameter of the neodymium iron boron particles is 200-300 μm.

2. The magnetic field disturbance rejection magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing of claim 1, wherein, The length and width of the magnetic thin film are 6-9 mm, the thickness is 1-2.5 mm, and the length and width of the hollow area are 3-5 mm.

3. The magnetic field disturbance immune magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing according to claim 1 or 2, characterized in that, The elastic body is a thermoplastic elastomer, and the length, width and height of the elastic body are 3-4 mm.

4. The magnetic field disturbance rejection magnetic tactile sensor based on magnetic film inversion and differential magnetic sensing of claim 1, wherein, ​

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