A MEMS micro current sensor

Abstract

The present invention discloses a MEMS micro current sensor, belonging to the technical field of sensors. The micro current sensor mainly includes a thermal driver part Ⅰ and a resonator part Ⅱ. The thermal driver part Ⅰ mainly includes an anchor point 1 and a V-shaped beam 2; the two open ends of the V-shaped beam 2 are fixed on the anchor point 1, and the other end, i.e., the tip, is connected to the resonator part Ⅱ. The present invention utilizes the thermal effect of current to cause the V-shaped beam 2 to expand and generate thermal stress, change the stiffness of the first resonator 3, and change the amplitude ratio of the first resonator 3 and the second resonator 6. Compared with the prior art, it avoids external electromagnetic interference and can meet the measurement requirements of miniaturization, high precision, and high robustness of the sensor in a specific application scenario within the preferred range of the structural parameter θ.

Description

Technical Field

[0001] The present invention relates to a MEMS micro current sensor, belonging to the technical field of sensors. Background Art

[0002] A current sensor is a component for current measurement and is widely used in fields such as scientific research and industrial production. According to different current sensing mechanisms, commercial current sensors can be divided into Hall current sensors, current transformers, and resistor shunts, etc. The above sensors have been applied in fields such as real-time monitoring of electrical systems, overload protection of expensive devices, and efficient management of electric power energy, but they are relatively large in size and high in cost, and it is difficult to meet the requirements for miniaturization of current sensors in application scenarios.

[0003] Micro-Electro-Mechanical System (MEMS) sensors have the advantages of small size, low cost, and mass production, etc., and have become the mainstream development trend of industrial sensors. According to different current sensing mechanisms, existing MEMS current sensors are divided into optical and resonant types. Since optical current sensors mainly detect signals based on complex optical paths and have disadvantages such as complex optical structures and strict installation accuracy requirements, resonant current sensors have begun to be studied. The patent "A quartz resonant current sensor" uses the Ampere force generated by a current in a magnetic field to disturb the stiffness of a quartz tuning fork and change the resonant frequency of the tuning fork. The above current sensor has a complex structure, poor stability, and is vulnerable to external electromagnetic interference.

[0004] To solve the above problems, the present invention proposes a MEMS micro current sensor based on the current thermal effect. In the present invention, when the current to be measured flows through the V-shaped beam, the Joule heat causes it to expand, and the structural thermal stress interferes with the weakly coupled resonant system, causing a change in the amplitude ratio output by the resonator. Compared with the prior art, the sensing mechanism of the present invention makes the system more compact, avoids interference from external magnetic fields, and can meet the measurement requirements for miniaturization, high precision, and high robustness of sensors in specific application scenarios. Summary of the Invention

[0005] The object of the present invention is to propose a high-precision miniaturized current sensor based on the current thermal effect, which can meet the measurement requirements for miniaturization, high precision, and high robustness of sensors in specific application scenarios.

[0006] The present invention proposes a MEMS micro current sensor, see Figure 1 , characterized in that: the micro current sensor mainly includes a thermal driver part I and a resonator part II.

[0007] The thermal driver part I mainly includes an anchor point 1 and a V-shaped beam 2; both open ends of the V-shaped beam 2 are fixed to the anchor point 1, and the other end, i.e., the tip, is connected to the resonator part II.

[0008] The resonator part II mainly includes a resonator I 3, a resonator II 6, a coupling structure 11, a middle anchor point 5, and a right anchor point 8. The resonator I 3 mainly includes a resonant beam 12, a driving electrode 4, and a detection electrode 13. The resonator II 6 mainly includes a resonant beam 10, a driving electrode 7, and a detection electrode 9. The resonator I 3 and the resonator II 6 achieve weak coupling through the coupling structure 13. The coupling structure 13 is fixed to the middle anchor point 5. The coupling structure 13 has a very weak influence on the stiffness of the system and can achieve weak coupling between the two resonators. The resonator I 3 is connected to the V-shaped beam 2, and the resonator II 6 is connected to the right anchor point 8.

[0009] See Figure 1 , the main working process of the micro current sensor proposed by the present invention is as follows: The current to be measured flows through the V-shaped beam 2. Due to the thermal effect of the current, the temperature change amount ΔT of the V-shaped beam 2 is obtained. Simplify the heat transfer process of the V-shaped beam 2 and list the following equation:

[0010]

[0011] Among them, ρ is the resistivity of the material, h is the equivalent heat transfer coefficient of the system to the outside world, l1 is the length of the V-shaped beam arm, A is the equivalent heat transfer area of the system to the outside world, and A1 is the effective cross-sectional area of the V-shaped beam.

[0012] As is known, when the temperature changes by ΔT, the free expansion amount of the V-shaped beam arm is:

[0013] Δl1 = αΔTl1 (2)

[0014] Among them, α is the linear expansion coefficient of the material. Since the V-shaped beam 2 is connected to the resonant beam 12, when the V-shaped beam 2 expands thermally by Δl1, the V-shaped beam 2 and the resonant beam 12 interact with each other and bear pressures F1 and F2 respectively. The above pressures cause the V-shaped beam 2 and the resonant beam 12 to generate deformations Δl1′ and Δl2′ respectively, satisfying the relationship:

[0015] F1sinθ = F2 (3)

[0016] (Δl1 - Δl1′)sinθ = Δl2′ (4)

[0017] Among them, θ is the angle between the anchor point 1 and the V-shaped beam 2. According to Hooke's law, under the action of forces F1 and F2, the deformations of the V-shaped beam Δl1′ and the resonant beam Δl2′ are respectively:

[0018]

[0019]

[0020] Wherein, E is the Young's modulus of the material, A2 is the effective cross-sectional area of ​​the resonant beam, and l2 is the length of the resonant beam.

[0021] Combining formulas (1)-(6) yields:

[0022]

[0023] It is known that when the resonance beam 12 is subjected to the axial pressure F2, the stiffness change is:

[0024]

[0025] See also Figure 2 , the equivalent schematic diagram of the resonator part II of the present invention, lists the following vibration equation according to Newton's second law:

[0026]

[0027] Where x1 and x2 are the displacements of the mass equivalent 4 of resonator 1 and the mass equivalent 6 of resonator 2, respectively, k is the equivalent stiffness 3 of resonator 1 and the equivalent stiffness 7 of resonator 2, and k c is the equivalent stiffness of the coupled structure 5. Solving the equations, we can obtain that when there is a stiffness change Δk, the amplitude ratio of resonator 1 to resonator 2 is:

[0028]

[0029] Combining equations (7), (8) and (10), we can obtain the relationship between the measured current and the amplitude ratio:

[0030]

[0031] In engineering practice, the ultimate stress σ of the resonant beam 12 cannot exceed the strength limit of the material, that is:

[0032]

[0033] Where [σ] is the allowable stress of the material, n b is the safety factor. In engineering practice, the stiffness disturbance of the resonant beam 12 is greater than the sensitive resolution, that is:

[0034]

[0035] Combining formulas (7), (12) and (13) and simplifying them, the preferred range of the structural parameter sinθ of the thermal driver part I can be obtained, namely:

[0036]

[0037] Among them, I min represents the resolution of the current sensor, and I max represents the range of the current sensor. According to formula (14), the preferred range of parameter θ can be obtained.

[0038] Therefore, the present invention can obtain the value of the current to be measured I by detecting the amplitude ratio AR, realizing miniaturization and high-precision current detection.

[0039] Beneficial effects of the present invention: The present invention utilizes the thermal effect of the current to cause the V-shaped beam 2 to expand and generate thermal stress, changing the stiffness of the resonator one 3, and changing the amplitude ratio between the resonator one 3 and the resonator two 6. Compared with the prior art, it avoids external electromagnetic interference and can meet the measurement requirements of miniaturization, high precision, and high robustness of the sensor in a specific application scenario within the preferred range of the structural parameter θ. Description of the Drawings

[0040] Figure 1 is a schematic structural diagram of the current sensor proposed by the present invention.

[0041] In the figure, 1 - anchor point, 2 - V-shaped cantilever beam, 3 - resonator one, 4 - driving electrode, 5 - middle anchor point, 6 - resonator two, 7 - driving electrode, 8 - right anchor point, 9 - detection electrode, 10 - resonator beam two, 11 - coupling structure, 12 - resonator beam one, 13 - detection electrode.

[0042] Figure 2 is a schematic simplified model diagram of the resonator part in the current sensor proposed by the present invention.

[0043] In the figure, 1 - equivalent of resonator one, 2 - equivalent of resonator two, 3 - equivalent stiffness of resonator one, 4 - equivalent mass of resonator one, 5 - equivalent stiffness of coupling structure, 6 - equivalent mass of resonator two, 7 - equivalent stiffness of resonator two.

[0044] Figure 3 is a curve diagram of the current to be measured and the amplitude ratio in the current sensor in the embodiment.

[0045] Figure 4 is a schematic principle diagram of the current sensor proposed by the present invention and the existing resonant current sensor. Detailed Embodiments

[0046] Refer to Figure 1, the miniaturized current sensor described in this embodiment mainly includes: a thermal driver part I and a resonator part II. The thermal driver part I mainly includes an anchor 1 and a V-shaped beam 2; one end of the V-shaped beam 2 is fixed on the anchor 1, and the other end is connected to the resonator part II. In this embodiment, the V-shaped beam is made of polysilicon material, with an elastic modulus E of 160 GPa, a resistivity ρ of 0.01 Ω·cm, a linear expansion coefficient α of 2.6×10 -6 / °C, and an allowable stress [σ] of 0.5 GPa. The equivalent heat transfer coefficient h of the system to the outside is 20000 W / (m 2 ·K), the angle θ between the anchor 1 and the V-shaped beam 2 is 45°, the length l1 of the V-shaped beam arm is 420 μm, and the equivalent heat transfer area A of the system to the outside is 600000 μm 2 , and the effective cross-sectional area A1 of the V-shaped beam is equal to the product of the beam width and height, 80 μm×50 μm.

[0047] The resonator part II mainly includes a resonator one 3, a resonator two 6, a coupling structure 11, a middle anchor 5, and a right anchor 8. The resonator one 3 mainly includes a resonator beam 12, a driving electrode 4, and a detection electrode 13, and the resonator two 6 mainly includes a resonator beam 10, a driving electrode 7, and a detection electrode 9; the resonator one 3 and the resonator two 6 are weakly coupled through the coupling structure 13, and the coupling structure 13 is fixed on the middle anchor 5; the coupling structure 13 has a very weak influence on the stiffness of the system, realizing weak coupling between the two resonators; the resonator one 3 is connected to the V-shaped beam 2, and the resonator two 6 is connected to the right anchor 8. In this embodiment, the resonator part II is made of polysilicon material, the length l2 of the resonator beam is 1400 μm, the effective cross-sectional area A2 of the resonator beam is equal to the product of the beam width and height, 14 μm×50 μm, and the stiffness k c of the coupling structure is 0.0075 N / m.

[0048] See Figure 1 , the main working process of the miniaturized current sensor proposed by the present invention is as follows: The current to be measured flows through the V-shaped beam. Due to the thermal effect of the current, the temperature change amount ΔT of the V-shaped beam. Simplify the heat transfer process of the V-shaped beam and list the following equation:

[0049]

[0050] where ρ is the resistivity of the material, h is the equivalent heat transfer coefficient of the system to the outside, l1 is the length of the V-shaped beam arm, A is the equivalent heat transfer area of the system to the outside, and A1 is the effective cross-sectional area of the V-shaped beam.

[0051] As is known, when the temperature changes by ΔT, the free expansion amount of the V-shaped beam arm is:

[0052] Δl1 = αΔTl1 (16)

[0053] Among them, α is the coefficient of thermal expansion of the material. Since the V-shaped beam 2 is connected to the resonant beam 12, when the V-shaped beam 2 expands thermally by Δl1, the V-shaped beam 2 and the resonant beam 12 interact and bear pressures F1 and F2 respectively. The above pressures cause the V-shaped beam 2 and the resonant beam 12 to generate deformations Δl1′ and Δl2′ respectively, satisfying the relationship:

[0054] F1sinθ = F2 (17)

[0055] (Δl1 - Δl1′)sinθ = Δl2′ (18)

[0056] Among them, θ is the angle between the anchor point 1 and the V-shaped beam 2. According to Hooke's law, under the action of forces F1 and F2, the deformations of the V-shaped beam Δl1′ and the resonant beam Δl2′ are respectively:

[0057]

[0058]

[0059] Among them, E is the Young's modulus of the material, A2 is the effective cross-sectional area of the resonant beam, and l2 is the length of the resonant beam.

[0060] Combining formulas (15)-(20) can obtain:

[0061]

[0062] As is known, when the resonant beam 12 is subjected to an axial pressure F2, the stiffness change amount is:

[0063]

[0064] See Figure 2 , the equivalent schematic diagram of the resonator part II of the present invention, and list the following vibration equations according to Newton's second law:

[0065]

[0066] Among them, x1 and x2 are the displacements of the mass equivalent 4 of the first resonator and the mass equivalent 6 of the second resonator respectively, k is the equivalent stiffness 3 and 7 of the first resonator and the second resonator, and k c is the equivalent stiffness 5 of the coupling structure. Solving the system of equations, when there is a stiffness change Δk, the amplitude ratio of the first resonator and the second resonator is:

[0067]

[0068] Combining formulas (21), (22) and (24) can obtain the relationship between the current to be measured and the amplitude ratio as:

[0069]

[0070] In engineering practice, the ultimate stress σ on the resonant beam - 12 shall not exceed the strength limit of the material, i.e.:

[0071]

[0072] where [σ] is the allowable stress of the material, and n b is the safety factor. In engineering practice, the stiffness perturbation of the resonant beam - 12 is greater than the sensitivity resolution, i.e.:

[0073]

[0074] Combining formulas (21), (26) and (27) and simplifying, the preferred range of the structural parameter sinθ of the thermal actuator part I can be obtained, i.e.:

[0075]

[0076] where I min represents the resolution of the current sensor, and I max represents the range of the current sensor. According to formula (28), the preferred range of the parameter θ can be obtained.

[0077] Since the material in this embodiment is polysilicon, the elastic modulus E is 160 GPa, the resistivity ρ is 0.01 Ω·cm, the linear expansion coefficient α is 2.6×10 -6 / °C, the allowable stress [σ] is 0.5 GPa, and the safety factor n b is 2. The equivalent heat transfer area A of the system to the outside is 600000 μm 2 , the equivalent heat transfer coefficient h of the system to the outside is 20000 W / (m 2 ·K), the angle θ between the anchor point 1 and the V - beam 2 is 45°, the length l1 of the V - beam arm is 420 μm, the effective cross - sectional area A1 of the V - beam is 80 μm×50 μm, the length l2 of the resonant beam is 1400 μm, the effective cross - sectional area A2 of the resonant beam is 14 μm×50 μm, and the coupling structure stiffness k c is 0.0075 N / m. Therefore, the amplitude ratio AR between resonator one and resonator two is:[[]]

[0078] AR = 2642236.3I 2 (A) (29)

[0079] When the designed resolution of the current sensor is 0.1 mA and the range is 30 mA, the preferred range of the structural parameter θ of the thermal actuator part I can be obtained as:[[]]

[0080] 15° ≤ θ ≤ 80° (30)

[0081] Since the above analysis simplifies the heat transfer process of the V-shaped beam, the present embodiment is reproduced below by finite element simulation. Refer to Figure 3 , which is a curve graph of the current to be measured and the amplitude ratio in this embodiment.

[0082] The above content is only to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited thereby. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A MEMS micro current sensor, characterized in that: It mainly includes a thermal driver part Ⅰ and a resonator part Ⅱ; The thermal driver part Ⅰ mainly includes an anchor (1) and a V-shaped beam (2); both open ends of the V-shaped beam (2) are fixed to the anchor (1), and the other end, i.e., the tip, is connected to the resonator part Ⅱ; The resonator part Ⅱ mainly includes a resonator one (3), a resonator two (6), a coupling structure (11), a middle anchor (5), and a right anchor (8); the resonator one (3) mainly includes a resonant beam (12), a driving electrode (4), and a detection electrode (13), and the resonator two (6) mainly includes a resonant beam (10), a driving electrode (7), and a detection electrode (9); the resonator one (3) and the resonator two (6) achieve weak coupling through the coupling structure (11), the coupling structure (11) is fixed to the middle anchor (5), and the influence of the coupling structure (11) on the stiffness of the system is very weak, enabling weak coupling between the two resonators; the resonator one (3) is connected to the V-shaped beam (2), and the resonator two (6) is connected to the right anchor (8); the angle θ between the anchor (1) and the V-shaped beam (2) is determined according to the following formula: Among them, I min represents the resolution of the current sensor, and I max represents the range of the current sensor; ρ is the resistivity of the material, h is the equivalent heat transfer coefficient of the system to the outside world, A is the equivalent heat transfer area of the system to the outside world, A1 is the effective cross-sectional area of the V-shaped beam, l1 is the length of the V-shaped beam arm, A2 is the effective cross-sectional area of the resonant beam, l2 is the length of the resonant beam, k min is the sensitive resolution of the resonant beam to stiffness perturbation, α is the linear expansion coefficient of the material, E is the Young's modulus of the material, [σ] is the allowable stress of the material, and n b is the safety factor.

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