A pressure sensor based on giant magneto-impedance effect and manufacturing method thereof

Abstract

The application discloses a pressure sensor based on giant magneto-impedance effect and a manufacturing method thereof, which comprises an upper shell and a lower shell, the upper and lower shells are fixed by screwing a threaded structure, the bottom of the lower shell is fixed with a pasting plate, and the pasting plate is pasted with a sensitive element wound by an exciting coil. The upper shell is fixed with a first elastic film and a second elastic film at the upper and lower ends respectively, a non-compressible cavity in the shape of an inverted cone is fixed between the first elastic film and the second elastic film, a semi-rigid film is fixed above the first elastic film and is used for receiving the pressure applied from outside, a small magnet is fixed below the second elastic film, the two ends of the sensitive element are connected with a signal processing circuit, when the semi-rigid film applies pressure, the displacement of the small magnet and the sensitive element changes, the sensitive element outputs corresponding voltage signals to the signal processing circuit, the signal processing circuit calculates the relationship between the pressure and the output voltage, and then the corresponding pressure value is obtained according to the voltage value.

Description

Technical Field

[0001] This invention belongs to the field of pressure detection, specifically relating to the structure and manufacturing method of a pressure sensor prepared using the magnetic properties of cobalt-based materials. Background Technology

[0002] In 1992, Mohri et al. from Nagoya University in Japan discovered that in cobalt-based soft magnetic amorphous wires with zero or negative magnetostriction coefficients, the impedance changes with the applied magnetic field when directly magnetized by high-frequency AC current. This phenomenon is highly sensitive and is therefore termed the giant magnetoresistance effect. After heat treatment, the internal stress of cobalt-based amorphous alloy strips is fully released, resulting in a more pronounced giant magnetoresistance effect. With its significant advantages such as high measurement sensitivity, fast response speed, small size, and low power consumption, the development of high-precision magnetic sensors based on the giant magnetoresistance effect has become a hot topic.

[0003] Pressure sensors are among the most commonly used sensors in industrial practice, widely applied in various industrial automation environments, including water conservancy and hydropower, railway transportation, intelligent buildings, production automation, aerospace, and many other industries, and currently showing promising development prospects in the medical field. Recent research abroad has demonstrated the high performance of magnetometers fabricated using the giant magnetoresistance effect, and the subsequent design of pressure sensors based on this technology, deposited on flexible substrates. These sensors can be used for pressure measurement, microfluidic systems, and biomedical applications. Therefore, pressure sensors based on the giant magnetoresistance effect represent a novel development direction in the field of pressure sensors.

[0004] Therefore, due to the large size, heavy weight, and high price of traditional pressure sensors, and the slow development of sensors based on the giant magnetoresistance effect, this invention proposes a pressure sensor based on the giant magnetoresistance effect. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the aforementioned background technology. Specifically, the following technical solution is adopted:

[0006] A pressure sensor based on the giant magnetoresistance effect, the fabrication of its sensing element includes:

[0007] Step 1: Cobalt-based amorphous ribbon was selected as the sensing material. This material was produced by Shanghai Baosteel Iron & Steel Research Institute. The material was cut to a length of 25 mm, a width of 1.5 mm, and a thickness of 34 μm.

[0008] Step 2: Connect the material to an annealing circuit consisting of a relay, a DC power supply, and a function signal generator for heat treatment, such as... Figure 3As shown, the relay acts as a switch. When the relay is in the open state, the current generated by the DC power supply flows through the sensitive material. When the relay is in the closed state, the current generated by the DC power supply does not flow through the sensitive material, thus achieving a pulsed current flowing through both ends of the material. The function signal generator controls the switching speed of the relay, thereby controlling the pulse width. The DC power supply controls the current magnitude and pulse current density. When the pulse reaches the appropriate parameters, annealing begins. Annealing involves heating the sensitive material with the pulsed current. After heating for a suitable time, the annealing circuit is disconnected, and annealing ends. The annealing parameters are: pulse frequency of 1Hz, annealing time of 50s, pulse density of 127A / mm², duty cycle of 50%, and pulse width of 0.5s.

[0009] Step 3: Make electrodes with thin copper sheets at both ends of the cut sensitive material, and weld wires onto the thin copper sheets.

[0010] Step 4: Attach the sensitive material with electrodes and wires to a plastic substrate with the same length and width as the material, and insert the whole assembly into the cylindrical skeleton.

[0011] Step 5: Wind 120 turns of copper wire around the cylindrical frame to form an excitation coil. One end of the coil is welded to the electrode of the sensitive material, completing the fabrication of the sensitive element. The other end of the electrode of the sensitive element, along with the other end of the coil, serves as the signal output port of the sensitive element.

[0012] A pressure sensor based on the giant magnetoresistance effect comprises: a bottom plate fixed to the bottom of the lower housing, on which a sensitive element wound with an excitation coil is attached, and one end of the excitation coil is connected to one end of the sensitive element to improve the sensitivity of the sensitive element (10) to the magnetic field. A first elastic diaphragm and a second elastic diaphragm are fixed to the upper and lower ends of the upper housing, respectively. An incompressible cavity in the shape of an inverted cone is fixed between the first and second elastic diaphragms. A semi-rigid membrane is fixed above the first elastic diaphragm to receive external pressure, and a small magnet is fixed below the second elastic diaphragm. Signal processing circuits are connected to both ends of the sensitive element. The upper and lower housings are tightened together by a threaded structure.

[0013] like Figure 4As shown, the signal processing circuit includes: (1) a signal generation circuit, which serves as the excitation signal to both ends of the sensitive element. (2) a preamplifier circuit, since the excitation signal of the sensitive element should not be too large, only a few millivolts, resulting in an output signal of only a few millivolts, a preamplifier circuit is needed to amplify the output signal of the sensitive element. (3) a peak detection circuit, which extracts the peak value of the signal output by the sensitive element, since the output signal is a sinusoidal signal. (4) a low-pass filter circuit, which causes some high-frequency signals to be mixed in the DC signal output by the sensitive element due to interference from external electromagnetic fields and noise generated by components such as the circuit's own resistance and semiconductor diodes. To address these issues, the present invention uses a low-pass filter circuit to smooth the output signal of the peak detection circuit. (5) a differential amplifier circuit, since the output signal of the sensitive element is not 0 when no pressure is applied, the reference voltage of the differential amplifier circuit is set to be the same as the output signal of the sensitive element when no pressure is applied. When the signal passes through the differential amplifier circuit, the reference voltage of the differential amplifier circuit will be subtracted to ensure that the sensor output signal is 0 when no pressure is applied.

[0014] The signal processing circuit also includes a nonlinear correction circuit, which includes an AD sampling module, a microcontroller, and a display screen. The AD sampling module collects the signal from the differential amplifier circuit and transmits it to the microcontroller. The microcontroller performs nonlinear correction on the collected signal according to the built-in nonlinear correction algorithm, and the corrected data is displayed on the display screen.

[0015] Preferably, the sensitive element is made of cobalt-based amorphous ribbon material, 20 mm long, 1.5 mm wide, and 34 μm thick, with electrodes made of thin copper sheets wrapped around both ends.

[0016] Preferably, the small magnet is made of a small ferrite and serves as the magnetic field signal source.

[0017] Preferably, the sensitive element is placed vertically relative to the small magnet, so that it is parallel to the direction of the magnetic field lines.

[0018] Preferably, the signal generation circuit uses the MAX038 chip, which can generate high-frequency AC signals above 1MHz.

[0019] Preferably, the operational amplifier in the signal processing circuit is an LM318 amplifier.

[0020] Nonlinear errors in sensors can be corrected through software, making the sensor's output curve approximate an ideal straight line. This invention employs a polynomial curve fitting correction algorithm. The basic idea of ​​curve fitting correction is to design a polynomial with the sensor's direct output as the variable, so that the output data after polynomial calculation approximates an ideal straight line. The polynomial for correcting nonlinear errors can be expressed as:

[0021]

[0022] This is the corrected data; denoted as polynomial coefficients, r as the sensor output, and n as the order (in this invention, n is 3).

[0023] The beneficial effects of this invention are:

[0024] 1. The giant magnetoresistance effect of the cobalt-based amorphous ribbon after heat treatment is most obvious when the excitation signal is 1MHz. Its impedance changes with the magnetic field at a rate of over 80%, while the rate of change before annealing is only about 20%.

[0025] 2. After selecting a fourth-order Butterworth low-pass filter, the original sawtooth waveform is almost approximated as a smooth straight line.

[0026] 3. Winding an excitation coil around the sensitive element will make the giant magnetoresistance effect in the positive magnetic field more obvious, which can increase the rate of change by more than 10%. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a pressure sensor structure based on the giant magnetoresistance effect proposed in this invention;

[0028] Figure 2 This is a schematic diagram of the structure of the sensitive element proposed in this invention;

[0029] Figure 3 This is a schematic diagram of the heat treatment circuit principle proposed in this invention;

[0030] Figure 4 This is a schematic diagram of the signal processing circuit proposed in this invention;

[0031] Figure 1 In the middle: 1. Lower housing; 2. Upper housing; 3. First elastic diaphragm; 4. Semi-rigid diaphragm; 5. Incompressible cavity; 6. Second elastic diaphragm; 7. Small magnet; 8. Adhesive plate; 9. Excitation coil; 10. Sensing element

[0032] Figure 2 In the middle: 11. Copper sheet; 12. Input terminal; 13. Ground terminal Detailed Implementation

[0033] This invention proposes a pressure sensor based on the giant magnetostatic effect, comprising an upper housing and a lower housing, which are fixed together by a threaded structure. An adhesive plate is fixed to the bottom of the lower housing, and a sensitive element wound with an excitation coil is attached to the adhesive plate. A first elastic diaphragm and a second elastic diaphragm are respectively fixed to the upper and lower ends of the upper housing. An incompressible cavity in the shape of an inverted cone is fixed between the first and second elastic diaphragms. A semi-rigid diaphragm is fixed above the first elastic diaphragm to receive external pressure, and a small magnet is fixed below the second elastic diaphragm. Signal processing circuitry is connected to both ends of the sensitive element. When pressure is applied to the semi-rigid diaphragm, the displacement of the small magnet and the sensitive element changes, and the sensitive element outputs a corresponding voltage signal to the signal processing circuitry. The signal processing circuitry calculates the relationship between the pressure and the output voltage, and then obtains the corresponding pressure value based on the voltage value. The invention will be further described below with reference to the accompanying drawings.

[0034] like Figure 1 As shown, the sensor structure consists of a cylindrical upper shell 2 and a lower shell 1. The shells are 3D-printed plastic structures or metal structures. The two shells are tightened together by a threaded structure. The first elastic diaphragm 3 and the second elastic diaphragm 6 are fixed to the shells with screws, ensuring the diaphragms are wrinkle-free. The elastic coefficient of the elastic diaphragms is relatively small because this invention measures minute pressures (such as the weak pressure generated by heart rate and pulse). The semi-rigid diaphragm 4 is cut from a semi-rigid clip diaphragm and is circular in shape, with the same radius as the upper circle radius of the incompressible cavity 5, both being 20mm. The incompressible cavity is an inverted conical structure with a top radius of 20mm and a bottom radius of 10mm. Its function is to make the center displacement of the second elastic diaphragm 6 more obvious. This incompressible cavity 5 is also a 3D-printed plastic structure (aluminum alloy or titanium alloy can also be used), requiring only low mass and high hardness. The small magnet 7 is made of ferrite. Because the small magnet is close to the sensitive element, but the magnetic field it generates cannot be too large, the magnetic field generated by ferrite is smaller than that of neodymium iron boron. The sensing element 10 is a cobalt-based amorphous strip. When the external magnetic field sensed by the sensing element (10), which receives AC signals at both ends, changes slightly, the impedance of the sensing element (10) will change dramatically, causing a significant change in the output signal of the sensing element (10). The sensing element 10 must be placed vertically, horizontally aligned with the direction of the magnetic field lines of the small magnet 7, to increase the effective range of the sensor.

[0035] The sensitive element is cut from cobalt-based amorphous ribbon. The wires cannot be directly soldered to both ends of the amorphous ribbon. The two ends are made of thick copper sheets that are folded and pressed to serve as electrodes. The excitation coil 9, made of a skeleton and wires outside the amorphous ribbon, can make the giant magnetoresistance effect generated by the positive magnetic field more obvious.

[0036] Figure 2 In the middle, port 12 and port 13 are connected to the signal input terminal and the ground terminal, respectively.

[0037] More specifically, the skeleton length is 15mm and the excitation coil has 120 turns.

[0038] Sensitive element 10 requires heat treatment before use. Before treatment, its giant magnetoresistance characteristics are relatively weak. After heat treatment, its internal microstructure changes, and the internal pressure is fully released. In this invention, pulse annealing is used for the heat treatment of the cobalt-based amorphous ribbon. The annealing circuit is as follows: Figure 3 As shown. After setting the pulse parameters in the function generator, the probe is connected to both ends of the relay and then to the annealing circuit. The DC power supply is adjusted to a suitable voltage value, and the voltmeter can detect the specific current value during annealing. Optimal material magnetic properties can be obtained through multiple experiments.

[0039] More specifically, the annealing parameters are: pulse frequency of 1Hz, annealing time of 50s, pulse density of 127A / mm², duty cycle of 50%, and pulse width of 0.5s.

[0040] The signal generation circuit uses the MAX038 high-frequency precision function signal generator integrated chip, which can generate sine wave signals above 1MHz with high accuracy (low waveform distortion, sine wave distortion less than 0.75%).

[0041] The preamplifier circuit consists of two stages. The first stage is a non-inverting proportional amplifier, characterized by high input impedance and low output impedance, commonly used in signal processing circuits. The second stage is an inverting proportional amplifier. This is because the signal output from the amorphous bandgap is not only a high-frequency signal with the same frequency as the excitation signal, but also contains a negative DC signal. For ease of subsequent detection, an inverting amplifier circuit is included to amplify the negative DC signal into a positive DC signal. The operational amplifier used in this invention is the LM318 operational amplifier chip, which has a unity-gain bandwidth of 15MHz.

[0042] More specifically, the amplification factor of the preamplifier circuit is approximately 10, the amplification factor of the postamplifier circuit is approximately 9, and the total amplification factor is approximately 90.

[0043] The peak detection circuit used in this invention employs a detector diode-based peak detection circuit, which is simple in structure and easy to debug. The diode used is a germanium diode from the 2AP series. It features low forward resistance, high reverse resistance, and low junction capacitance, thus entering the linear detection region at voltages lower than silicon diodes, and is widely used for detection. The diode's forward resistance is set to 320Ω, and its reverse resistance to 8×10Ω. 5 Ω, the carrier sinusoidal excitation signal frequency is 1MHz, and the highest frequency of the modulation signal is set to 10KHz.

[0044] Considering that the Butterworth filter has a flat amplitude-frequency response curve with no ripples or peaks, exhibits maximum flatness within the passband, and shows a monotonically decreasing trend with rapid attenuation near the cutoff frequency, this invention ultimately chose the Butterworth filter. The low-pass filter circuit is a fourth-order Butterworth low-pass filter composed of two operational amplifiers. The peak detection circuit detects the peak value of the AC voltage signal across the magnetometer probe, obtaining a DC-dominated signal reflecting the magnitude of the applied magnetic field. However, due to the influence of circuit components and load, detection time constant, and interference from the external magnetic field, the detected signal inevitably contains a certain amount of high-frequency noise, with a cutoff frequency of 10kHz.

[0045] The differential circuit consists of a reference voltage source and a differential circuit. Since the sensor's output voltage is not zero when the pressure input is 0, a differential circuit is needed for zero calibration to ensure that the output is 0 when the sensor input is 0.

[0046] Due to temperature drift and zero drift, sensors inevitably exhibit nonlinear errors. Software-based nonlinear error correction is effective and cost-efficient. This invention uses a microcontroller coupled with a polynomial curve fitting algorithm to calculate the polynomial coefficients.

[0047] In this invention, pressure is applied to the semi-rigid membrane, causing the first elastic film 3 and the second elastic film 6 to undergo elastic deformation. This displacement of the small magnet relative to the sensing element results in a change in the output voltage of the sensing element. Subsequent signal processing yields voltage data linearly related to the pressure, and the pressure-voltage functional relationship U=kP+M is calculated. The pressure value can then be deduced from the voltage value. Here, U is the sensor output voltage signal value, P is the applied pressure value, k is the proportionality coefficient, and M is the acceptable error.

[0048] The detailed descriptions listed above are merely specific descriptions of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent methods or modifications that do not depart from the technology of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pressure sensor based on the giant magnetoresistance effect, characterized in that, The system includes an upper housing (2) and a lower housing (1). A bonding plate (8) is fixed to the bottom of the lower housing. A sensitive element (10) with an excitation coil (9) wound around its surface is bonded to the bonding plate (8). One end of the excitation coil (9) is connected to one end of the sensitive element (10) to improve the sensitivity of the sensitive element (10) to the magnetic field. A first elastic film (3) and a second elastic film (6) are fixed to the upper and lower end faces of the upper housing, respectively. The surfaces of the first elastic film (3) and the second elastic film (6) are wrinkle-free. An incompressible cavity (5) in the shape of an inverted cone is fixed between the first elastic film and the second elastic film. A semi-rigid membrane (4) is fixed above the first elastic film to receive external pressure. The semi-rigid membrane (4) is circular in shape, and its radius is equal to or less than that of the first elastic film. The upper radius of the compressible cavity (5) is the same; a magnet (7) is fixed below the second elastic film. Copper plates are provided at both ends of the sensitive element (10). Input and grounding terminals are provided on the copper plates for connecting the signal processing circuit. The sensitive element is placed vertically. The material of the sensitive element is cobalt-based amorphous ribbon material. When pressure is applied to the semi-rigid membrane (4), the magnet (7) is displaced relative to the sensitive element (10), causing the output voltage of the sensitive element (10) to change. After processing by the signal processing circuit, a voltage value that is linearly related to the pressure is obtained. The pressure-voltage function relationship U=kP+M is calculated, where U is the sensor output voltage signal value, P is the applied pressure value, k is the proportional coefficient, and M is the acceptable error. The pressure value can be deduced from the voltage value.

2. A pressure sensor based on giant magnetoresistance effect according to claim 1, characterized in that, The sensitive element is 20mm long, 1.5mm wide, and 34μm thick, with copper electrodes wrapped around both ends; the magnet is made of ferrite and serves as the magnetic field signal source; the excitation coil has 120 turns; the semi-rigid membrane (4) has a radius of 20mm; the incompressible cavity has a top surface radius of 20mm and a bottom surface radius of 10mm.

3. A pressure sensor based on giant magnetoresistance effect according to claim 1, characterized in that, The adhesive plate (8) and the sensitive element (10) for winding the excitation coil are both located in the center of the lower housing; The incompressible cavity (5) is located in the center of the upper shell.

4. A pressure sensor based on giant magnetoresistance effect according to claim 1, characterized in that, Both the upper and lower shells are cylindrical and are fixed together by screws.

5. A pressure sensor based on giant magnetoresistance effect according to claim 1, characterized in that, The upper and lower housings are 3D-printed plastic structures or metal structures.

6. A pressure sensor based on giant magnetoresistance effect according to claim 1, characterized in that, The signal processing circuit includes: The signal generation circuit provides an excitation signal to both ends of the sensitive element; A preamplifier circuit is used to amplify the signal output by the sensitive element. Peak detection circuit, used to extract the peak value of the output signal of the sensitive element; A low-pass filter circuit is used to smooth the output signal of the peak detector circuit; The differential amplifier circuit ensures that the sensor output signal is 0 when no pressure is applied.

7. A pressure sensor based on giant magnetoresistance effect according to claim 6, characterized in that, It also includes a nonlinear correction circuit, which includes an AD sampling module, a microcontroller, and a display screen. The AD sampling module collects the signal processed by the above signal processing circuit and sends it to the microcontroller. The microcontroller performs nonlinear correction on the collected signal according to the built-in nonlinear correction algorithm and displays it on the display screen.

8. A pressure sensor based on giant magnetoresistance effect according to claim 7, characterized in that, The nonlinear correction algorithm employs a polynomial curve fitting correction algorithm. For the corrected data, denoted as polynomial coefficients, r as the sensor output, and n as the order, where n is 3.

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