WO2018232779A1 - 一种mems重力仪 - Google Patents

一种mems重力仪 Download PDF

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Publication number
WO2018232779A1
WO2018232779A1 PCT/CN2017/091111 CN2017091111W WO2018232779A1 WO 2018232779 A1 WO2018232779 A1 WO 2018232779A1 CN 2017091111 W CN2017091111 W CN 2017091111W WO 2018232779 A1 WO2018232779 A1 WO 2018232779A1
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WIPO (PCT)
Prior art keywords
stiffness spring
vibrator unit
cavity
gravimeter
mems
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PCT/CN2017/091111
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English (en)
French (fr)
Inventor
涂良成
唐世豪
刘金全
范继
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华中科技大学
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Priority to US16/308,239 priority Critical patent/US11262474B2/en
Publication of WO2018232779A1 publication Critical patent/WO2018232779A1/zh

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
    • G01V7/02—Details
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
    • G01V7/02—Details
    • G01V7/04—Electric, photoelectric, or magnetic indicating or recording means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V7/00—Measuring gravitational fields or waves; Gravimetric prospecting or detecting
    • G01V7/08—Measuring gravitational fields or waves; Gravimetric prospecting or detecting using balances

Definitions

  • the invention belongs to the technical field of gravity measurement, and more particularly to a MEMS gravimeter.
  • Gravimeter is an instrument for measuring gravity acceleration, which is of great significance in geophysical and underground resource exploration.
  • gravimeters can be mainly divided into two categories: one is an absolute gravimeter and the other is a relative gravimeter.
  • the relative gravimeter does not measure the absolute value of the gravitational acceleration, but only the amount of change in gravitational acceleration.
  • the relative gravimeter is usually based on a vibrator unit composed of a proof mass, a spring, and an outer frame to detect changes in gravitational acceleration.
  • the gravimeter is in the gravity environment, the gravity of the inspection quality is balanced with the elastic force generated by the spring deformation, and the inspection quality is at a certain equilibrium position; when the gravity acceleration of the environment where the gravimeter is placed changes, the spring deformation will follow The change causes the spring to produce a new balance between the spring force and the gravity, causing the proof mass to shift to a new equilibrium position.
  • This displacement of the test quality can detect changes in gravitational acceleration.
  • the displacement relationship between the acceleration change and the proof mass can be expressed as:
  • ⁇ x is the proof mass displacement
  • ⁇ a is the gravitational acceleration change
  • ⁇ 0 is the eigenfrequency of the vibrator unit.
  • the eigenfrequency of the vibrator unit needs to be as low as possible. This means that the spring that requires the vibrator unit is as soft as possible, or that the inspection quality is as large as possible.
  • MEMS Microelectromechanical Systems
  • INOVA's ML21 MEMS accelerometers have an accuracy of 42 ng/ ⁇ Hz
  • SERCEL's DSU1 508 MEMS accelerometers have an accuracy of 15 ng/ ⁇ Hz.
  • the measurement accuracy and stability of existing MEMS accelerometers still do not meet the requirements of gravimeters.
  • the present invention provides a MEMS gravimeter capable of achieving high-precision, high-stability gravity measurement while effectively reducing the volume and mass of the gravimeter. Cost can also be effectively limited.
  • the present invention provides a MEMS gravimeter comprising: a vibrator unit, a displacement sensing structure, a displacement detecting circuit, a cavity and a horizontal adjustment base; the vibrator unit is disposed inside the cavity,
  • the vibrator unit includes: a negative stiffness spring, a positive stiffness spring, a proof mass, and an outer frame; the proof mass is coupled to the outer frame by the positive stiffness spring and the negative stiffness spring, and the positive stiffness spring
  • the negative stiffness spring is symmetrically disposed about the proof mass, the outer frame is fixed to the cavity; the displacement sensing structure is disposed on a surface of the proof mass, and the displacement detecting circuit is used And detecting a displacement signal of the displacement sensing structure;
  • the horizontal adjustment base is disposed at a bottom of the cavity for adjusting a level of the cavity.
  • the negative stiffness spring is a curved beam, the ends of the beam being fixed, and the displacement of the proof mass during operation causes the beam to axially expand and contract and generate an axial force inside the beam.
  • the vibrator unit effectively reduces the equivalent stiffness by the stiffness matching of the positive and negative stiffness springs under 1 g of gravity such that the eigenfrequency is less than 5 Hz.
  • the MEMS gravimeter further includes: a support structure fixed inside the cavity, and an outer frame of the vibrator unit is fixed on a surface of the support structure.
  • the support structure is a gantry structure, and the material thereof is a material having a thermal expansion coefficient of less than 2.5 ppm/° C.
  • the MEMS gravimeter further includes: a temperature control module disposed inside the cavity for maintaining a stable temperature inside the cavity
  • the MEMS gravimeter further includes a signal interface disposed on a surface of the cavity for transmitting a signal of the displacement sensing structure to the displacement detecting circuit.
  • the present invention also provides a MEMS processing method for a vibrator unit, the vibrator unit comprising: a negative stiffness spring, a positive stiffness spring, a proof mass and an outer frame; the proof mass passing through the positive stiffness spring and the negative stiffness spring Connected to the outer frame, and the positive stiffness spring and the negative stiffness spring are symmetrically disposed with respect to the proof mass; wherein the MEMS processing method comprises the following steps:
  • the invention also provides a three-axis gravimeter comprising three vibrator units, wherein the planes of the three vibrator units are at an angle to the horizontal plane, for example 60° or other angles, and the planes of the three vibrator units are two The two are at an angle to each other, such as 120° or other angles; each vibrator unit includes: a negative stiffness spring, a positive stiffness spring, a proof mass, and an outer frame; the proof mass passes the positive stiffness spring and the negative A stiffness spring is coupled to the outer frame, and the positive stiffness spring and the negative stiffness spring are symmetrically disposed about the proof mass.
  • the invention also provides a gravity gradiometer, wherein two identical gravimeters are spatially separated by a certain distance, and the gravitational acceleration of the gravimeter is measured, and the measured gravitational acceleration is divided by two points in the space.
  • Distance which measures the gravity gradient of the gravitational field, forms a gravity gradiometer.
  • a partial tensor gravity gradiometer or a full tensor gravity gradiometer may be separately constructed by using one or more pairs of the above-mentioned single-axis gravimeter.
  • the eigenfrequency can reach 5Hz and below, and on the basis of the same displacement detection accuracy, higher precision acceleration detection can be realized;
  • the vibrator unit uses monocrystalline silicon with a nearly perfect lattice structure to be integrally processed by MEMS technology, which avoids problems such as creep of metal materials, and avoids the problem of unstable connection points between different devices in conventional processing, so that the vibrator Units can achieve better stability;
  • the vibrator unit can be made up to 20 ⁇ 20 ⁇ 0.5mm, which makes the volume and weight of the whole instrument have the potential to shrink, making the instrument more portable and reducing the difficulty of research and development of the dynamic base gravimeter;
  • the vibrator unit can effectively reduce the production cost of the instrument by means of the mass production capacity of MEMS technology.
  • FIG. 1 is a schematic structural diagram of a MEMS gravimeter according to an embodiment of the present invention.
  • FIG. 2 is a schematic structural diagram of a low eigenfrequency oscillator unit according to an embodiment of the present invention
  • FIG. 3 is a force-displacement curve of a structure of a low eigenfrequency oscillator unit according to an embodiment of the present invention
  • Figure 4 is a schematic view of a three-axis gravimeter fixed in an umbrella shape
  • Figure 5 is a plot of the eigenfrequency as a function of load for an accelerometer fabricated according to design.
  • Figure 6 is a gravity gradiometer consisting of two identical triaxial gravimeters as described above.
  • the x-axis represents the horizontal direction
  • the y-axis represents the direction of gravity
  • g represents the gravitational acceleration
  • the same reference numerals are used to denote the same elements or structures, where: 1 is the first vibrator unit, 2 is the displacement Sense structure, 3 is the signal interface, 4 is the displacement detection circuit, 5 is the vacuum interface, 6 is the vacuum module, 7 is the cavity, 8 is the temperature control module, 9 is the first support structure, 10 is the horizontal adjustment base, 11
  • 12 is a negative stiffness spring
  • 13 is the proof mass
  • 14 is a positive stiffness spring
  • 15, 16, 17 are the same second vibrator unit
  • 18 is the second support structure
  • 19 is the bracket.
  • the invention provides a MEMS gravimeter comprising a vibrator unit, a displacement sensing structure, a displacement detecting circuit, a cavity and a horizontal adjusting base; wherein the vibrator unit comprises: a negative stiffness spring, a positive stiffness spring, a proof mass and a frame
  • the displacement sensing structure is located on the surface of the above-mentioned inspection quality, and its signal is detected and recorded by the displacement detecting circuit; the vibrator unit is installed inside the cavity, and the outer frame is fixed with the cavity; the horizontal adjustment base is installed at the bottom of the cavity, and the adjustment cavity is The level of the body.
  • the gravimeter In the work, the gravimeter is in the gravity environment, the gravity of the inspection mass is balanced with the elastic force generated by the spring deformation, and the inspection quality is at a certain equilibrium position; when the gravity acceleration of the environment where the gravimeter is changed, the spring deformation will follow The change balances the spring force generated by the spring with the gravity, causing the proof mass to shift to a new equilibrium position.
  • the displacement sensing structure converts this displacement of the proof mass into a certain detectable physical quantity (such as capacitance, light intensity, etc.), and the displacement detecting circuit detects this
  • the irradiance detection obtains the displacement of the proof mass and detects the change in the gravitational acceleration.
  • the proof mass in the vibrator unit is connected to the outer frame by a positive stiffness spring and a negative stiffness spring, and the positive and negative stiffness springs are symmetrically arranged with respect to the proof mass.
  • the positive stiffness spring can effectively reduce the equivalent stiffness by matching with the negative stiffness spring, thereby reducing the eigenfrequency of the vibrator unit, making the vibrator unit more sensitive to changes in acceleration.
  • the eigenfrequency can be effectively reduced to 5 Hz and below, and theoretically can be reduced to any frequency.
  • the MEMS gravimeter in order to reduce the influence of the external temperature change of the gravimeter on the vibrator unit, the MEMS gravimeter further includes: a support structure fixed in the cavity, and the outer frame of the vibrator unit is fixed on the surface of the support structure. .
  • a material with a thermal expansion coefficient of less than 2.5 ppm/° C. is selected to form a gantry structure to provide support for the vibrator unit while reducing the influence of external temperature changes on the vibrator unit.
  • the MEMS gravimeter in order to reduce the influence of the airflow in the cavity on the vibrator unit, the MEMS gravimeter further includes: a vacuum interface and a vacuum module, the vacuum interface is on the surface of the cavity, and the cavity and the vacuum module are connected.
  • the vacuum environment of the chamber is maintained by the vacuum module, and the degree of vacuum is maintained below 1 Pa, and the degree of vacuum changes by less than 1%.
  • the vacuum environment not only reduces the influence of the airflow on the vibrator unit, but also acts as a passive heat insulation, which is conducive to the stability of the internal temperature of the cavity.
  • the MEMS gravimeter in order to influence the temperature change on the vibrator unit, further includes: a temperature control module located inside the cavity to maintain the stability of the internal temperature of the cavity.
  • the closed-loop control maintains the chamber at a fixed temperature with a temperature change of less than 10 -2 °C.
  • the MEMS gravimeter when the displacement sensing structure needs to be wiredly connected with the displacement detecting circuit, the MEMS gravimeter further includes: a signal interface located on the surface of the cavity. The main purpose is to transmit the signal of the displacement sensing structure to the displacement detecting circuit.
  • the triaxial gravimeter can be constructed by three vibrating units of the same low eigenfrequency as above.
  • the whole gravimeter is placed on the stable platform, and the gravity measurement of the moving base can be realized.
  • two identical gravimeters can be placed in two positions, respectively The gravitational acceleration at two locations is measured to measure the gravity gradient. Furthermore, by using multiple pairs of the same gravimeter combination, a full tensor gravity gradiometer can be constructed to achieve all tensor component measurements of the gravity gradient.
  • the first embodiment of the present invention provides a structure of a single-axis MEMS gravimeter as shown in FIG. 1;
  • the single-axis MEMS gravimeter includes a first vibrator unit 1, a displacement sensing structure 2, a signal interface 3, and a displacement detecting circuit 4,
  • the outer frame 11 of the first vibrator unit 1 is fixed to the cavity 7 by the first support structure 9.
  • the displacement sensing structure 2 fixed to the surface of the first vibrator unit 1 for inspecting the mass 13 is connected to the displacement detecting circuit 4 via the signal interface 3.
  • the temperature control module 8 is mounted on the surface of the first support structure 9 in the vicinity of the first vibrator unit 1; the cavity 7 is connected to the vacuum module 6 via the vacuum interface 5.
  • the leveling base 10 is mounted on the bottom surface of the cavity 7.
  • the first vibrator unit 1 can reach a very small eigenfrequency under 1 g of gravity, less than 5 Hz, and thus constitute a single-axis MEMS gravimeter.
  • the schematic diagram of the structure of the first vibrator unit 1 is as shown in FIG. 2.
  • the proof mass 13 of the first vibrator unit 1 is connected to the outer frame 11 by a negative stiffness spring 12 and a positive stiffness spring 14.
  • the sensitive axis is along the y-axis direction, ie the g direction of gravity g.
  • the negative stiffness spring 12 is a curved beam that is fixed at both ends when there is no load.
  • the shape of the negative stiffness spring 12 is a symmetrical curve. Its curved shape has many designs, such as the form of cosine curve, "V" shape and so on. Taking the form of the cosine curve as an example, its elastic-displacement curve is shown by the dotted line in Fig. 3, the horizontal axis represents the normalized displacement of the proof mass, and the positive value is along the +y direction; the vertical axis represents the normalization of the spring. The elasticity is positive along the -y direction.
  • the negative stiffness spring 12 in the form of a cosine curve produces an elastic force along the -y direction, as indicated by the region I: the displacement increases and the spring force increases until the forward direction Maximum value.
  • the displacement of the proof mass 13 continues to increase, and the spring force will decrease from the positive maximum value to 0, and even begin to increase in the +y direction until the negative maximum value, as shown in the region II, which is utilized in the present invention.
  • Negative stiffness region The reason for this phenomenon is that the negative stiffness spring 12 is fixed at both ends, so that the displacement of the inspection mass 13 causes the axial expansion and contraction of the beam to generate an axial force inside the beam.
  • the displacement of the inspection mass 13 reaches a certain position, it will start to outward. Do work and produce negative stiffness.
  • the displacement of the proof mass 13 continues to increase, and the spring force increases from the negative maximum to the -y direction, as indicated by region III.
  • the positive stiffness spring 14 is a commonly used folding beam whose elastic-displacement curve is shown by a chain line in FIG. As the proof mass 13 displacement increases, the spring force of the positive stiffness spring 14 increases linearly. The equal-efficiency-displacement curves of the two spring combinations are shown by the solid lines in Figure 3, and their equivalent stiffness is effectively reduced in Zone II.
  • the negative stiffness of the negative stiffness spring 12 and the positive stiffness of the positive stiffness spring 14 can be made by adjusting the shape or geometrical parameters of the negative stiffness spring 12 and the positive stiffness spring 14, such as the length, width of the beam, and the magnitude of the bending of the negative stiffness spring 12. The absolute values are close, and the positive and negative stiffness cancellations give a structure with very low equivalent stiffness.
  • the vibrator unit When the equivalent stiffness is positive, the vibrator unit is still a stable system and can perform open loop detection. When the equivalent stiffness is negative, the system is unstable, and the inspection quality 13 is prone to jump. It is necessary to increase the feedback actuator.
  • the closed loop control is such that the proof mass 13 is stabilized at a fixed position in the area II. At the same time, the quality of the proof mass 13 is adjusted such that the first vibrator unit 1 is under the action of 1 g of gravitational acceleration, and the equilibrium position is located in the region II of Fig. 3. In addition, this symmetrical design is better for other vibration modes of the vibrator unit. Inhibition.
  • the above-mentioned vibrator unit 1 can be fabricated by using a single crystal silicon by a MEMS processing process, and the main processes required are: pattern transfer, etching, release, and the like.
  • the processing steps are as follows: (1) transferring the outer frame 11, the negative stiffness spring 12, the proof mass 13 and the positive stiffness spring 14 pattern mask of the designed vibrator unit 1 to the surface of the silicon wafer by photolithography; (2) using deep reactive ions Etching (DRIE) etches the silicon wafer and etches the oscillator unit 1 in one piece; (3) removes the unnecessary structure by wet etching, and releases the vibrator unit.
  • DRIE deep reactive ions Etching
  • the displacement sensing structure 2 is formed on the surface of the proof mass 13 of the first vibrator unit 1 by a certain processing method.
  • the displacement sensing structure 2 is connected to the displacement detecting circuit 4 via a signal interface 3.
  • One possible solution is to make a capacitive displacement sensor. Making a surface on the surface of the proof mass 13 of the first vibrator unit 1 Some of the capacitor plates act as a moving plate, and a fixed plate is fixed directly above the moving plate. When the proof mass 13 is displaced, the displacement of the moving plate with the displacement of the moving plate and the plate is changed.
  • the signal interface 3 uses a vacuum matched ceramic electrode.
  • the displacement detecting circuit 4 is correspondingly designed as a capacitance detecting circuit, and detecting the change in the capacitance, the displacement of the proof mass 13 can be detected. The entire displacement detection accuracy can reach the order of nm or even pm.
  • the first support structure 9 is a structure in which the outer frame 11 in the first vibrator unit 1 is fixed to the cavity 7 and can be made into a gantry structure using glass ceramics having a particularly small thermal expansion coefficient.
  • the first supporting structure 9 provides a heat-insensitive support, so that the first vibrator unit 1 and the cavity 7 are less likely to be displaced, and at the same time have a certain isolation effect on the temperature change of the cavity.
  • the temperature control module 8 is mounted on the surface of the first support structure 9 to control the temperature of the first vibrator unit 1.
  • the closed-loop feedback control loop is formed by the thermistor and the heater so that the temperature variation of the first transducer unit is controlled within 10 -3 °C.
  • the cavity 7 is connected to the vacuum module 6 through the vacuum interface 5 to form a vacuum system, which provides a stable vacuum environment for the first vibrator unit 1, the displacement sensing structure 2 and the like.
  • the vacuum port 5 uses a CF16 type interface, and the vacuum module 6 uses an ion pump for vacuum maintenance so that the cavity 7 is maintained at a vacuum of less than 10 -4 Pa.
  • the level adjusting base 10 is mounted on the bottom surface of the cavity 7 for adjusting the level such that the sensitive axes of the first vibrator unit 1 have the same gravity direction.
  • the gravimeter sensitive shaft coincides with the direction of gravity by the horizontal adjustment base 10.
  • the cavity 7, the vacuum port 5, and the vacuum module 6 maintain the vacuum of the cavity 7 at less than 10 -4 Pa.
  • the temperature control module 8 controls the temperature through a closed loop such that the internal temperature of the chamber 7 is maintained at a certain temperature with a temperature change of less than 10 -3 °C.
  • the entire instrument is in the gravitational field, the proof mass 13 of the first vibrator unit 1 is subjected to gravity, and the negative stiffness spring 12 of the first vibrator unit 1 and the positive stiffness spring 14 of the first vibrator unit 1 are deformed to generate an elastic force, so that the test is performed.
  • the mass 13 is balanced at a position in the region II where the equivalent stiffness is small; when the gravitational acceleration changes, the deformation of the negative stiffness spring 12 and the positive stiffness spring 14 changes accordingly to balance the spring force generated by the spring with the gravity, so that the quality of the test 13 occurs and the displacement reaches a new equilibrium position.
  • the proof mass 13 displacement causes the capacitance of the above capacitive displacement sensing structure 2 to change, and this capacitance change is transmitted to the displacement detecting circuit 4 through the signal interface 3, and the displacement detecting circuit 4 detects this capacitance change, thereby detecting the proof quality 13
  • the displacement which in turn detects the change in gravitational acceleration of the gravitational field.
  • the matching of the positive and negative stiffness springs effectively reduces the eigenfrequency of the vibrator unit, and increases the coefficient of acceleration conversion into displacement.
  • the detection accuracy of the acceleration is effectively improved.
  • the vibrator unit is obtained by integral processing using single crystal silicon, which reduces the mechanical connection point and effectively improves the structural stability.
  • the vacuum module, the support structure and the temperature control module provide a stable gas pressure and stable temperature environment for the detection unit such as the vibrator unit and the displacement sensing structure, so that the accuracy and stability of the instrument can be effectively improved.
  • FIG. 1 A three-axis gravimeter sensitive structure provided by the second embodiment of the present invention is shown in FIG.
  • the same three second transducer units 15, 16, 17 are mounted on the surface of the second support structure 18.
  • the second support structure 18 is a boss made of glass-ceramic having a small coefficient of thermal expansion.
  • the planes of the same three second vibrator units 15, 16, 17 are at an angle to the horizontal plane, for example 60°, and the planes of the same three second vibrator units 15, 16, 17 form a certain clip with each other. Angle, for example 120°.
  • the component of gravity in the direction of the sensitive axis of the second transducer unit 15, 16, 17 is g ⁇ sin 60°.
  • Such a design can measure the components of the gravitational acceleration in three directions by the three second vibrator units 15, 16, 17 to realize the vector measurement of the gravitational acceleration.
  • the structure of the second transducer unit 15, 16, 17 is similar in structure to the first transducer unit 1.
  • the negative stiffness spring 12 can be made by adjusting the shape or geometric parameters of the negative stiffness spring 12 and the positive stiffness spring 14, such as the length and width of the beam and the magnitude of the bending of the negative stiffness spring 12.
  • the negative stiffness is close to the absolute value of the negative stiffness of the positive stiffness spring 14, and the positive and negative stiffness cancellation can result in a structure with a very small equivalent stiffness; at the same time, the quality of the proof mass 13 is adjusted such that the first transducer unit 1 is at g ⁇ sin60 Under the action of gravity acceleration, the equilibrium position is located in the area II in Fig. 3, and the second vibrator unit 15, 16, 17 can be obtained by such design.
  • the proof mass surfaces of the second vibrator units 15, 16, 17 respectively make the displacement sensing structure 2 to perform displacement detection on the proof mass.
  • the first vibrator unit 1 and the first support structure 9 are transposed into the second vibrator unit 15, 16, 17 and the second support structure 18, the signal interface 3, and the displacement detecting circuit 4 A corresponding increase can be made into a three-axis gravimeter.
  • Fig. 5 is a diagram showing changes in the eigenfrequency of the MEMS vibrator unit according to the above design concept as the load acceleration changes. It can be seen from the figure that as the load acceleration increases, the eigenfrequency of the vibrator unit continuously decreases, and after reaching the lowest point, it starts to rise again. This is very consistent with the trend expected in Figure 3. It has been proved that the vibrator unit design method of the present invention is feasible.
  • two identical three-axis gravimeters are spatially separated by a certain distance and placed on the surface of a bracket 19 as shown in FIG. 6, and a gravity gradiometer can be formed.
  • Gravity gradient is the derivative of gravity acceleration with respect to space. By measuring the gravitational acceleration of two points in space, the difference can be used to detect the gravity gradient.
  • Gravity gradient ⁇ (g 1 - g 2 ) / d, g 1 , g 2 are the gravitational accelerations of the two locations measured by the above three-axis gravimeter, and d is the spacing between the two points.
  • the holder 19 is fabricated using a glass ceramic or ULE material having a particularly small coefficient of thermal expansion.

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Abstract

一种MEMS重力仪；包括：振子单元（1,15,16,17），位移传感结构（2），位移检测电路（4），腔体（7）和水平调节基座（10）；振子单元（1,15,16,17）设置在腔体（7）的内部，振子单元（1,15,16,17）包括：负刚度弹簧（12）、正刚度弹簧（14）、检验质量（13）和外框（11）；检验质量（13）通过正刚度弹簧（14）和负刚度弹簧（12）与外框（11）相连，且正刚度弹簧（14）与负刚度弹簧（12）关于检验质量（13）对称设置，外框（11）与腔体（7）固联；位移传感结构（2）设置在检验质量（13）的表面，位移检测电路（4）用于检测位移传感结构（2）的位移信号；振子单元（1,15,16,17）通过正负刚度弹簧（12，14）的匹配实现本征频率的降低；通过检测检验质量（13）的位移进而检测重力加速度的变化。MEMS重力仪稳定性高，体积小，质量轻，能够有效地降低生产成本，并且能有效降低信号检测单元和稳定平台的研发难度。

Description

一种MEMS重力仪 【技术领域】
本发明属于重力测量技术领域，更具体地，涉及一种MEMS重力仪。
【背景技术】
重力仪是一种测量重力加速度的仪器，在地球物理、地下资源勘探等领域具有十分重要的意义。目前重力仪主要可以分为两类：一类是绝对重力仪，一类是相对重力仪。绝对重力仪以Micro-g&LaCoste公司的FG5-X为代表，它通过非常精确地测量真空中物体下落的位置与时间关系来测量绝对的重力加速度大小。它的精度非常高，准确度可以达到2μGal/√Hz，精确度达到15μGal/√Hz(1Gal＝1cm/s2)。但是整个仪器也十分巨大，总重量达到150kg，总体积达到1.5m3。它的价格更是十分昂贵，达到几百万人民币。
相对重力仪相对于绝对重力仪而言，它不测量重力加速度的绝对值，只是测量重力加速度的变化量。目前相对重力仪通常是基于由检验质量、弹簧、外框构成的振子单元来检测重力加速度变化。工作中，重力仪处在重力环境中，检验质量受到的重力与弹簧形变产生的弹力达到平衡，检验质量处于某一个平衡位置；当重力仪所处环境的重力加速度发生变化时，弹簧形变会随之变化使弹簧产生的弹力与重力达到新的平衡，使检验质量发生位移达到一个新的平衡位置。检测检验质量的这一位移就可以检测到重力加速度的变化。加速度变化和检验质量的位移关系可以表示为：
Δx＝Δa/ω0 2
Δx为检验质量位移，Δa为重力加速度变化，ω0为振子单元的本征频率。为了使振子单元对于加速度变化更加敏感，振子单元的本征频率需要尽可能地低。这就意味着需要振子单元的弹簧尽量软，或者检验质量尽量大。
为了实现在重力场(重力加速度记为g，大约为9.8m/s2)中，测到百ng量级(1ng＝10-9g)的重力加速度变化，“零长弹簧”的结构被广泛应用到相对重 力仪中，使得振子单元能够在承受1g重力作用的同时对于重力加速度变化很敏感。目前，商用的相对重力仪基本上都是基于这一思想设计制作的。例如Scintrex公司的CG6型相对重力仪，它就利用了熔融石英制作的零长弹簧进行重力测量，它的质量达到5.2kg，测量的标准重复度达到5μGal。但是，它的价格依然十分昂贵，达到上百万人民币。
微机电系统(MicroelectromechanicalSystem，MEMS)随着微电子技术的发展越来越成熟，不断地被应用到各个领域。在仪器小型化、制作成本控制等方面，有着独特的优势。目前，商用的MEMS加速度计已经被广泛应用在汽车、智能手机、机器人等领域。商用MEMS加速度计也优化到了较高的精度，例如INOVA公司的ML21型的MEMS加速度计精度可以达到42ng/√Hz，SERCEL公司的DSU1 508型号MEMS加速度计精度为15ng/√Hz。但是目前已有的MEMS加速度计的测量精确度和稳定度仍然达不到重力仪的需求。
英国的Glasgow大学曾经报道过一种基于“geometricanti-spring”的MEMS重力仪。他们借助“geometricanti-spring”的设计，将MEMS振子单元的本征频率做到很低，使得它对于加速度的变化十分敏感。根据报道，它的测量精度达到40μGal/√Hz，零漂达到140μGal/day。
【发明内容】
针对现有重力仪上的缺陷或改进需求，本发明提供了一种MEMS重力仪，能够实现高精度、高稳定性的重力测量，同时使重力仪的体积和质量都得到有效地减小，制作成本也能得到有效地限制。
为实现上述目的，本发明提供了一种MEMS重力仪，包括：振子单元，位移传感结构，位移检测电路，腔体和水平调节基座；所述振子单元设置在所述腔体的内部，所述振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置，所述外框与所述腔体固联；所述位移传感结构设置在所述检验质量的表面，所述位移检测电路用 于检测所述位移传感结构的位移信号；所述水平调节基座设置在所述腔体底部，用于调节腔体的水平。
更进一步地，所述负刚度弹簧为弯曲的梁，所述梁的两端固定，工作中检验质量的位移使得梁产生轴向伸缩且在所述梁的内部产生轴向力。
更进一步地，所述振子单元在1g重力作用下通过正、负刚度弹簧的刚度匹配有效地降低了等效刚度使得本征频率小于5Hz。
更进一步地，所述MEMS重力仪还包括：支撑结构，固定在所述腔体内部，且所述振子单元的外框固定在支撑结构的表面。
更进一步地，所述支撑结构为龙门结构，其材料为热膨胀系数小于2.5ppm/℃的材料。
更进一步地，所述MEMS重力仪还包括：真空接口和真空模块，所述真空接口设置在所述腔体表面，用于连接所述腔体与设置在所述腔体外部的所述真空模块。
更进一步地，所述MEMS重力仪还包括：温度控制模块，设置于所述腔体内部，用于维持所述腔体内部温度的稳定
更进一步地，所述MEMS重力仪还包括：信号接口，设置于所述腔体的表面，用于将所述位移传感结构的信号传导至所述位移检测电路。
本发明还提供了一种振子单元的MEMS加工方法，所述振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置；其特征在于，所述MEMS加工方法包括下述步骤：
(1)通过光刻工艺将振子单元的外框、负刚度弹簧、检验质量和正刚度弹簧的图形掩膜转移至硅片表面；
(2)利用深反应离子刻蚀(DRIE)工艺对所述硅片进行一体刻蚀并获得所述振子单元中间件；
(3)利用湿法腐蚀工艺将所述振子单元中间件中不需要的结构去除后获得 所述振子单元。
本发明还提供了一种三轴重力仪，包括三个振子单元，三个振子单元所处的平面均与水平面成一定夹角，例如60°或者其它角度，三个振子单元所处的平面两两相互之间成一定夹角，例如120°或者其它角度；每个振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置。
本发明还提供了一种重力梯度仪，两个相同的上述重力仪在空间上分开一定距离放置，测量重力仪所在的重力加速度，对所测得的重力加速度进行差分除以上述空间两点的距离，可以测量重力场的重力梯度，构成一个重力梯度仪。采用一对或者多对上述单轴重力仪组合可分别构成部分张量重力梯度仪或者全张量重力梯度仪。
总体而言，通过本发明所构思的以上技术方案与现有技术相比，具有以下
有益效果：
(1)振子单元在承受一定重力作用下，本征频率可以达到5Hz及以下，在相同的位移检测精度的基础上，可以实现更高精度的加速度检测；
(2)振子单元使用近乎完美晶格结构的单晶硅通过MEMS技术一体加工得到，避免了金属材料的蠕变等问题，同时避免了传统加工中不同器件间连接点不稳定的问题，使得振子单元可以实现更好的稳定性；
(3)振子单元可以制作小到20×20×0.5mm以内，使得整个仪器的体积与重量都有了缩小的潜力，使得仪器可以更加便携，同时降低了动基座重力仪的研发难度；
(4)振子单元借助MEMS技术的批量生产能力，可以有效地降低仪器的生产成本。
【附图说明】
图1是本发明实施例提供的MEMS重力仪的结构示意图；
图2是本发明实施例提供的低本征频率振子单元结构示意图；
图3是本发明实施例提供的低本征频率振子单元结构的力-位移曲线；
图4是伞状固联的三轴重力仪示意图；
图5是根据设计加工出来的加速度计的本征频率随着载荷的变化曲线。
图6是通过两个相同的上述三轴重力仪组成的重力梯度仪。
在所有附图中，x轴表示水平方向，y轴表示重力方向，g表示重力加速度，相同的附图标记用来表示相同的元件或结构，其中：1为第一振子单元，2为位移传感结构，3为信号接口，4为位移检测电路，5为真空接口，6为真空模块，7为腔体，8为温度控制模块，9为第一支撑结构，10为水平调节基座，11为外框，12为负刚度弹簧，13为检验质量，14为正刚度弹簧，15、16、17为结构相同的第二振子单元，18为第二支撑结构，19为支架。
【具体实施方式】
为了使本发明的目的、技术方案及优点更加清楚明白，以下结合附图及两个实施例，对本发明进行进一步详细说明。应当理解，此处所描述的具体实施例仅仅用以解释本发明，并不用于限定本发明。此外，下面所描述的本发明各个实施方式中所涉及到的技术特征只要彼此之间未构成冲突就可以相互组合。
本发明提供了一种MEMS重力仪，包括振子单元，位移传感结构，位移检测电路，腔体和水平调节基座；其中，振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；位移传感结构位于上述检验质量表面，它的信号由位移检测电路检测记录；振子单元安装在腔体内部，其外框与腔体固联；水平调节基座安装在腔体底部，调节腔体的水平。
工作中，重力仪处在重力环境中，检验质量受到的重力与弹簧形变产生的弹力达到平衡，检验质量处于某一个平衡位置；重力仪所处环境的重力加速度发生变化时，弹簧形变会随之变化使弹簧产生的弹力与重力平衡，使检验质量发生位移达到一个新的平衡位置。位移传感结构将检验质量的这一位移转化为某种可以检测的物理量(例如电容、光强等)，位移检测电路通过检测这一物 理量检测得到检验质量的位移进而检测到重力加速度的变化。
在本发明实施例中，振子单元中检验质量通过正刚度弹簧和负刚度弹簧与外框相连，正、负刚度弹簧关于检验质量对称设置。正刚度弹簧通过与负刚度弹簧的匹配可以有效地减小等效刚度，进而使振子单元的本征频率降低，使得振子单元对于加速度的变化更加敏感。本征频率可以有效地降低到5Hz及以下，理论上可以降低到任意频率。
在本发明实施例中，为了减小重力仪外界的温度变化对振子单元的影响，MEMS重力仪还包括：支撑结构，它固定在腔体内部，同时振子单元的外框固定在支撑结构的表面。选择热膨胀系数小于2.5ppm/℃的材料，制作成龙门结构，为振子单元提供支撑的同时，减小外界温度变化对于振子单元的影响。
在本发明实施例中，为了减小腔体内气流对振子单元的影响，MEMS重力仪还包括：真空接口和真空模块，真空接口在腔体表面，连接腔体与真空模块。通过真空模块维持腔体真空环境，真空度维持在1Pa以下，真空度变化小于1％。真空环境不仅减小了气流对振子单元的影响，同时起到被动隔热作用，有利于腔体内部温度的稳定。
在本发明实施例中，为了温度变化对振子单元的影响，MEMS重力仪还包括：温度控制模块，位于腔体内部，维持腔体内部温度的稳定。通过闭环控制维持腔体稳定在一个固定的温度，温度变化小于10-2℃。
在本发明实施例中，当位移传感结构需要与位移检测电路进行有线连接时；MEMS重力仪还包括：信号接口，位于腔体表面。主要是将位移传感结构的信号传导到位移检测电路。
在本发明实施例中，可以通过三个相同上述低本征频率的振子单元通过伞状形式固联构成三轴重力仪。
在本发明实施例中，将整个重力仪安置在稳定平台上，可以实现动基座重力测量。
在本发明实施例中，可以把两个相同的上述重力仪放置在两个位置，分别 测量两个位置的重力加速度，进而测量重力梯度。更进一步，采用多对相同的重力仪组合放置，可以构成全张量重力梯度仪，进而实现重力梯度的所有张量分量测量。
为了跟进一步的说明本发明实施例提供的MEMS重力仪，现结合附图以及具体实例详述如下：
本发明第一实施例提供了一种单轴MEMS重力仪的结构如图1所示；单轴MEMS重力仪包括第一振子单元1、位移传感结构2、信号接口3、位移检测电路4、真空接口5、真空模块6、腔体7、温度控制模块8、第一支撑结构9和水平调节基座10。第一振子单元1的外框11通过第一支撑结构9与腔体7固联。固联在第一振子单元1检验质量13表面的位移传感结构2通过信号接口3与位移检测电路4相连。温度控制模块8安装在第一支撑结构9表面，在第一振子单元1的附近；腔体7通过真空接口5和真空模块6相连。水平调节基座10安装在腔体7的底面。
第一振子单元1在1g重力作用下可以达到非常小的本征频率，小于5Hz，进而构成一个单轴的MEMS重力仪。第一振子单元1结构示意图如图2所示，第一振子单元1中检验质量13通过负刚度弹簧12、正刚度弹簧14与外框11相连。敏感轴沿着y轴方向，即重力g方向。
负刚度弹簧12在没有载荷时是一种弯曲的两端固定的梁。第一振子单元1处于水平状态时，负刚度弹簧12的形状是一种对称的曲线。它的弯曲形状有很多种设计，例如余弦曲线形式、“V”字形等。以余弦曲线形式为例，它的弹力-位移曲线如图3中虚线所示，横轴表示检验质量的归一化的位移，沿着+y方向为正值；纵轴表示弹簧的归一化的弹力，沿着-y方向为正值。当检验质量13沿着+y方向发生位移时，余弦曲线形式的负刚度弹簧12会产生沿着-y方向的弹力，即如区域I所示：位移增大，弹力也会增大直至正向最大值。检验质量13的位移继续增大，弹力会从正向最大值开始下降直至为0，甚至开始沿着+y方向增大直至负向最大值，即如区域II所示，这就是本发明中利用到的负刚度 区域。这一现象产生的原因是负刚度弹簧12由于两端固定使得检验质量13的位移会导致梁的轴向伸缩在梁的内部产生轴向力，检验质量13的位移到一定位置后会开始向外做功，产生负刚度。检验质量13的位移继续增大，弹力会从负向最大值开始沿着-y方向增加，即如区域III所示。
正刚度弹簧14是一种常用的折叠梁，它的弹力-位移曲线如图3点划线所示。随着检验质量13位移增大，正刚度弹簧14弹力会线性增大。两个弹簧组合的等效力-位移曲线如图3中实线所示，它们的等效刚度在区域II实现了有效地降低。通过调节负刚度弹簧12和正刚度弹簧14的形状或者几何参数，例如梁的长度、宽度以及负刚度弹簧12弯曲的幅度等，可以使得负刚度弹簧12的负刚度与正刚度弹簧14的正刚度的绝对值接近，正、负刚度抵消可以得到一个等效刚度非常小的结构。等效刚度为正值时，振子单元仍然是一个稳定的系统，可以进行开环检测；等效刚度为负值时，系统不稳定，检验质量13容易发生跳变，需要增加反馈执行机，进行闭环控制，使得检验质量13稳定在区域II中一个固定的位置。同时，调节检验质量13的质量，使得第一振子单元1在1g重力加速度作用下，平衡位置位于图3中的区域II.此外，这一种对称设计对于振子单元的其他振动模态有较好的抑制作用。
上述振子单元1可以使用单晶硅利用MEMS加工工艺制作得到，主要需要用到的工艺有：图形化转移、刻蚀、释放等。加工步骤如下：(1)通过光刻，将设计的振子单元1的外框11、负刚度弹簧12、检验质量13和正刚度弹簧14图形掩膜转移到硅片表面；(2)利用深反应离子刻蚀(DRIE)对硅片进行刻蚀，一体刻蚀出振子单元1；(3)利用湿法腐蚀去除不需要的结构，把振子单元释放出来。通过这样的一体加工方式，避免了机械连接结构，消除了连接结构不稳定的问题。
位移传感结构2通过一定的加工方法，制作在第一振子单元1的检验质量13的表面。位移传感结构2通过信号接口3与位移检测电路4相连。一种可行的方案是制作出电容位移传感器。在第一振子单元1的检验质量13表面制作一 些电容极板作为动极板，动极板的正上方固定有定极板。检验质量13产生位移时，动极板随着位移，动极板与定极板间的电容变化。信号接口3使用真空匹配的陶瓷电极。位移检测电路4相应的设计为电容检测电路，检测这一电容变化，可以检测到检验质量13的位移。整个位移检测精度可以达到nm量级甚至pm量级。
第一支撑结构9是将第一振子单元1中的外框11与腔体7固联的结构，可以使用热膨胀系数特别小的微晶玻璃制作成龙门结构。第一支撑结构9提供一个热不敏感的支撑，使得第一振子单元1与腔体7之间不易产生位移，同时对腔体的温度变化有一定的隔离作用。
温度控制模块8安装在第一支撑结构9的表面，对第一振子单元1进行温度控制。通过热敏电阻和加热器构成闭环反馈控制环路，使得第一振子单元的温度变化控制在10-3℃以内。
腔体7通过真空接口5与真空模块6相连，构成一个真空系统，为第一振子单元1、位移传感结构2等提供一个稳定的真空环境。真空接口5使用CF16型接口，真空模块6使用离子泵进行真空维持，使得腔体7真空度维持在小于10-4Pa。
水平调节基座10安装在腔体7的底面，用于调节水平，使得第一振子单元1的敏感轴重力方向相同。
工作时，通过水平调节基座10使重力仪敏感轴与重力方向重合。腔体7、真空接口5、真空模块6使得腔体7真空度维持在小于10-4Pa。温度控制模块8通过闭环控制温度，使得腔体7内部温度维持在某一温度，温度变化小于10-3℃。整个仪器处于重力场中，第一振子单元1的检验质量13受到重力作用，第一振子单元1的负刚度弹簧12和第一振子单元1的正刚度弹簧14发生形变，产生弹性力，使得检验质量13在上述等效刚度很小的区域II中某一位置平衡；当重力加速度变化时，负刚度弹簧12和正刚度弹簧14的形变会随之变化使弹簧产生的弹力与重力平衡，使检验质量13发生位移达到一个新的平衡位置。检验 质量13位移使得上面的电容式位移传感结构2的电容发生变化，这一电容变化通过信号接口3传到位移检测电路4,位移检测电路4检测这一电容变化，进而检测到检验质量13的位移，进而检测到重力场的重力加速度变化。
本实施例，通过正、负刚度弹簧的匹配有效降低振子单元的本征频率，增大了加速度转换为位移的系数，在位移检测精度一定的情况下，有效地提高了加速度的检测精度。振子单元利用单晶硅通过一体加工得到，减少了机械连接点，有效提高了结构的稳定性。同时，真空模块、支撑结构、温度控制模块为振子单元和位移传感结构等检测单元提供了一个气压稳定、温度稳定的环境，使得仪器精度、稳定性都可以有效地提升。
本发明第二实施例提供的一种三轴重力仪敏感结构如图4所示。相同的三个第二振子单元15、16、17安装在第二支撑结构18的表面。第二支撑结构18是一个由热膨胀系数很小的微晶玻璃制成的凸台。相同的三个第二振子单元15、16、17所处的平面与水平面成一定的夹角，例如60°，相同的三个第二振子单元15、16、17所处的平面相互构成一定夹角，例如120°。这样在第二振子单元15、16、17敏感轴方向重力的分量为g·sin60°。这样的设计可以通过三个第二振子单元15、16、17测得重力加速度在三个方向的分量，实现重力加速度的矢量测量。
第二振子单元15、16、17结构与第一振子单元1结构类似。以第一振子单元1的结构为例说明，通过调节负刚度弹簧12和正刚度弹簧14的形状或者几何参数，例如梁的长度、宽度以及负刚度弹簧12弯曲的幅度等，可以使得负刚度弹簧12的负刚度与正刚度弹簧14的负刚度的绝对值接近，正负刚度抵消可以得到一个等效刚度非常小的结构；同时，调节检验质量13的质量，使得第一振子单元1在g·sin60°重力加速度作用下，平衡位置位于图3中的区域II，通过这样的设计可以得到第二振子单元15、16、17。
相应地，第二振子单元15、16、17的检验质量表面分别制作位移传感结构2对检验质量进行位移检测。
在第一实施例的基础上，将其中的第一振子单元1以及第一支撑结构9换位为第二振子单元15、16、17和第二支撑结构18，信号接口3，位移检测电路4相应地增加，可以制成一个三轴重力仪。
图5根据上述设计思想制作的MEMS振子单元本征频率随着载荷加速度变化而变化的图。从图中可以看出，随着载荷加速度的增加，振子单元的本征频率不断的下降，达到最低点后，又开始上升。这与图3预期的趋势非常一致。证明了本发明中振子单元设计方法是可行的。
在本发明实施例中，两个相同的上述三轴重力仪空间上分开一定距离放置在一个支架19表面如图6所示，可以组成一个重力梯度仪。重力梯度是重力加速度关于空间的导数，通过测量空间中两个点的重力加速度，进行差分可以检测得到重力梯度。重力梯度Γ＝(g1-g2)/d，g1、g2分别是两个上述三轴重力仪测到的所在位置的重力加速度，d是两点的间距。同样地，支架19使用热膨胀系数特别小的微晶玻璃或者ULE材料加工制作。
本领域的技术人员容易理解，以上所述仅为本发明的较佳实施例而已，并不用以限制本发明，凡在本发明的精神和原则之内所作的任何修改、等同替换和改进等，均应包含在本发明的保护范围之内。

Claims (11)

  1. 一种MEMS重力仪，其特征在于，包括：振子单元，位移传感结构，位移检测电路，腔体和水平调节基座；
    所述振子单元设置在所述腔体的内部，所述振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置，所述外框与所述腔体固联；
    所述位移传感结构设置在所述检验质量的表面，所述位移检测电路用于检测所述位移传感结构的位移信号；
    所述水平调节基座设置在所述腔体底部，用于调节腔体的水平。
  2. 如权利要求1所述的MEMS重力仪，其特征在于，所述负刚度弹簧为弯曲的梁，所述梁的两端固定，工作中检验质量的位移使得梁产生轴向伸缩且在所述梁的内部产生轴向力。
  3. 如权利要求1所述的MEMS重力仪，其特征在于，所述振子单元在1g重力作用下通过正、负刚度弹簧的刚度匹配有效地降低了等效刚度进而使本征频率有效地减小。
  4. 如权利要求1所述的MEMS重力仪，其特征在于，所述MEMS重力仪还包括：支撑结构，固定在所述腔体内部，且所述振子单元的外框固定在支撑结构的表面。
  5. 如权利要求4所述的MEMS重力仪，其特征在于，所述支撑结构为龙门结构，其材料为热膨胀系数小于2.5ppm/℃的材料。
  6. 如权利要求1-5任一项所述的MEMS重力仪，其特征在于，所述MEMS重力仪还包括：真空接口和真空模块，
    所述真空接口设置在所述腔体表面，用于连接所述腔体与设置在所述腔体外部的所述真空模块。
  7. 如权利要求1-6任一项所述的MEMS重力仪，其特征在于，所述MEMS 重力仪还包括：温度控制模块，设置于所述腔体内部，用于维持所述腔体内部温度的稳定。
  8. 如权利要求1-7任一项所述的MEMS重力仪，其特征在于，所述MEMS重力仪还包括：信号接口，设置于所述腔体的表面，用于将所述位移传感结构的信号传导至所述位移检测电路。
  9. 一种振子单元的MEMS加工方法，所述振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置；其特征在于，所述MEMS加工方法包括下述步骤：
    (1)通过光刻工艺将振子单元的外框、负刚度弹簧、检验质量和正刚度弹簧的图形掩膜转移至硅片表面；
    (2)利用深反应离子刻蚀(DRIE)工艺对所述硅片进行一体刻蚀并获得所述振子单元中间件；
    (3)利用湿法腐蚀工艺将所述振子单元中间件中不需要的结构去除后获得所述振子单元。
  10. 一种三轴重力仪，其特征在于，包括三个振子单元，三个振子单元所处的平面均与水平面成某个夹角，三个振子单元所处的平面两两相互之间成某个夹角；每个振子单元包括：负刚度弹簧、正刚度弹簧、检验质量和外框；所述检验质量通过所述正刚度弹簧和所述负刚度弹簧与所述外框相连，且所述正刚度弹簧与所述负刚度弹簧关于所述检验质量对称设置。
  11. 一种重力梯度仪，其特征在于，包括：两个如权利要求1-8任一项所述的重力仪在空间中分开一定距离放置，测量上述重力仪所在空间两点的重力加速度，将测量的重力加速度进行差分后除以上述空间两点的距离获得重力梯度。
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