A z-axis micromachined accelerometer and method of controlling the same

Abstract

This invention discloses a Z-axis micromechanical accelerometer and its control method, belonging to the field of acceleration measurement technology. An asymmetric seesaw sensing structure is constructed using mass blocks and a torsional elastic beam. Two symmetrical multifunctional capacitors are formed by designing electrodes below the two mass blocks. The torsion generated by the seesaw structure under Z-axis acceleration can be detected by the change in the differential capacitance of the capacitors, where the capacitors serve as the detection mechanism. Based on the detected change in torsion angle and the PID controller, a PWM voltage for force balance control can be calculated and applied to the symmetrical capacitors via a push-pull method, together with a DC tuned voltage. This generates a constant electrostatic negative stiffness, thereby reducing the equivalent stiffness of the seesaw sensing element, and also generates an electrostatic resultant force that can be modulated by the PWM, maintaining the position of the seesaw sensing structure constant, achieving closed-loop detection. The PWM voltage reflects the magnitude of the external acceleration.

Description

Technical Field

[0001] This invention belongs to the field of acceleration measurement technology, and more specifically, relates to a micromechanical accelerometer and its control method. Background Technology

[0002] Most Z-axis micro accelerometers employ a sandwich structure, requiring detection / drive electrodes to be designed above and below the mass block to form a three-layer structure. This results in complex manufacturing processes and low electrode integration. To ensure manufacturing reliability, the Z-axis elastic beam typically has high stiffness, thus limiting the improvement of detection accuracy. Seesaw-structured micro accelerometers are structurally simpler, with only two layers, reducing manufacturing complexity. However, they also require separate design of detection / drive electrodes and increased torsional stiffness, limiting the integration and accuracy of this type of micro accelerometer. Summary of the Invention

[0003] To address the shortcomings and deficiencies of existing Z-axis micromechanical accelerometers, this invention provides a Z-axis micromechanical accelerometer and its control method. It utilizes an asymmetrically arranged mass block and a torsional elastic beam to achieve Z-axis rotation sensing. A multifunctional multiplexed capacitor formed between a multifunctional multiplexed electrode and the mass block simultaneously achieves electrostatic adjustment, angle detection, and force balance. Not only does it employ a closed-loop control method to achieve closed-loop acceleration detection along the Z-axis, but the electrostatic adjustment method within the control method can also adjust the equivalent stiffness of the accelerometer to zero, thereby improving the accelerometer's detection accuracy and avoiding interference from various signals. This accelerometer and its control method are simpler and more practical.

[0004] The technical solution adopted in this invention is as follows:

[0005] In a first aspect, the present invention provides a Z-axis micromechanical accelerometer, including a seesaw structure and a multifunctional multiplexed electrode;

[0006] The seesaw structure includes a torsional elastic beam and mass blocks asymmetrically distributed on both sides of the torsional elastic beam. The mass blocks are rigid structures and are fixedly connected to the torsional elastic beam. The two ends of the torsional elastic beam are fixedly supported on the anchor area structure. The plane of the mass blocks is defined as the XY plane. The asymmetrical mass block refers to the rigid mass block whose center of gravity does not coincide with the torsional center axis in the Y direction. That is, the center of gravity and the torsional center axis are a certain distance apart in the XY horizontal direction. In other words, there is a distance between the center of gravity of the mass block and the torsional center axis of the torsional elastic beam.

[0007] The multifunctional multiplexed electrode is fixed on a substrate on one side of the mass block in the Z direction, and includes a first electrode and a second electrode with equal distances from the torsion center axis.

[0008] The seesaw structure and multifunctional multiplexed electrode constitute the sensitive element, which can be equivalent to a second-order mass-spring system. Its equivalent stiffness includes torsional elastic beam stiffness and electrostatic negative stiffness.

[0009] Furthermore, the multifunctional multiplexed electrode and the mass block in the seesaw structure constitute a set of differential capacitors. The set of differential capacitors can reuse the angle detection capacitor, stiffness tuning capacitor, and force balance capacitor. Mutual interference of signals can be avoided by electrostatic adjustment and force balance control voltage based on PWM.

[0010] Furthermore, the aforementioned angle detection capacitor is used to detect changes in the torsional angle of the seesaw structure, by applying a high-frequency carrier voltage V to the electrodes of the differential capacitor. c and -V c This enables carrier modulation of the signal caused by capacitance change due to the variable gap. After the signal on the mass block is processed by the CV circuit, AD conversion circuit, multiplication demodulation and low-pass filtering, it becomes a digital angle signal.

[0011] Furthermore, the stiffness-tuning capacitor is used to generate electrostatic negative stiffness by applying a push-pull control voltage V1+V2+V to the electrodes of the differential capacitor. c and V1-V2-V c Electrostatic negative stiffness is generated, where V1 is the electrostatic adjustment voltage and V2 is the force balance PWM voltage. The electrostatic negative stiffness value is related to V1. 2 +V2 2 The equivalent stiffness can be changed by adjusting the size of V1 or V2.

[0012] Furthermore, the force balancing capacitor is used to generate electrostatic torque to counteract the inertial force generated by external acceleration. Electrostatic torque is generated by applying push-pull control voltages V1+V2 and V1-V2 to the electrodes of the differential capacitor. The electrostatic torque is proportional to V1·V2. The magnitude of the electrostatic torque can be changed by adjusting the duty cycle of the PWM voltage with an amplitude of V2.

[0013] Secondly, the present invention provides a control method for the aforementioned Z-axis micromechanical accelerometer, which stabilizes the position of the mass block at a reference angle through force balancing, and simultaneously reduces its equivalent stiffness to a preset value by applying electrostatic negative stiffness, including the following steps:

[0014] Step 1: Calculate the tuned electrostatic negative stiffness based on the calibrated torsional elastic beam stiffness and the preset equivalent stiffness value, and calculate the electrostatic adjustment voltage V1 according to the following formula:

[0015]

[0016] Where ε is the dielectric constant, S is the capacitor overlap area, d0 is the capacitor gap, and km and k eff These are the calibrated torsional elastic beam stiffness value and the preset equivalent stiffness value, respectively, and |V2| is the force balance PWM voltage amplitude.

[0017] Step 2: When the mass block is subjected to external acceleration, a pair of carrier voltages V with the same amplitude but opposite signs are applied to the first electrode 2 and the second electrode 5. c That is, applying a high-frequency carrier voltage V to the two sets of electrodes respectively. c and -V c At this time, the two sets of capacitors act as angle detection capacitors to achieve carrier modulation of the capacitance change signal caused by the variable gap. After the mass block signal is processed by CV circuit, AD conversion circuit, multiplication demodulation and low-pass filtering, digital angle signals are obtained respectively.

[0018] Step 3: The digital rotation angle signal is used by the PID controller to calculate the duty cycle of the Z-axis force balance PWM voltage V2; through the push-pull circuit, voltages V1+V2 and V1-V2 are applied to the first electrode 2 and the second electrode 5 respectively. At this time, the two sets of capacitors act as Z-axis driving capacitors, so that the mass block is maintained at a constant horizontal reference position, and the equivalent stiffness can be adjusted to a preset value; where V1 and V2 are the Z-axis electrostatic adjustment voltage and the Z-axis force balance PWM voltage, respectively.

[0019] The detected acceleration can be expressed as:

[0020]

[0021] Where ε is the dielectric constant, m is the mass of the mass block, S and d0 are the overlapping area and gap of the capacitors, respectively, L1 and L2 are the lever arms corresponding to the inertial torque generated by the asymmetric mass block and the electrostatic torque generated by the force balancing capacitor around the torsion center axis, respectively, V1 and |V2| are the amplitudes of the electrostatic adjustment voltage and the force balancing PWM voltage, respectively, and b is the duty cycle of the force balancing PWM voltage.

[0022] In summary, compared with the prior art, the technical solution conceived by the present invention has the following beneficial effects:

[0023] (1) This invention achieves closed-loop detection of Z-axis acceleration by designing a double-layer structure of Z-axis micromechanical acceleration, including an asymmetric mass block, a torsional elastic beam and a multifunctional multiplex capacitor. It has the advantages of simple processing technology, high integration and miniaturization.

[0024] (2) In this invention, the rotation angle detection, electrostatic adjustment and force balance are realized simultaneously by using a multiplexed capacitor, and a PWM-based electrostatic adjustment and force balance control method is proposed, which not only realizes the electrode integrated design, but also avoids mutual interference of signals.

[0025] (3) The present invention applies force balance closed-loop control and electrostatic adjustment technology at the same time, which can improve the linearity and accuracy of micromechanical accelerometer. The proposed force balance and equivalent stiffness control method is easy to implement in digital controller.

[0026] (4) The micromechanical accelerometer in this invention does not rely on the high difficulty of silicon processing technology. It can easily achieve preset low equivalent stiffness or even quasi-zero equivalent stiffness by using electrostatic adjustment technology. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the Z-axis micromechanical accelerometer provided in an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the control method for the Z-axis micromechanical accelerometer provided in an embodiment of the present invention;

[0029] In all the accompanying drawings, the same reference numerals are used to denote the same structures, wherein: 1-asymmetric mass block, 2-first electrode, 3-torsional elastic beam, 4-anchor region, 5-second electrode, V1-electrostatic adjustment voltage, V2-force balance PWM voltage, V c - Carrier voltage, C / V - Capacitor / Voltage Converter, AD - Analog-to-Digital Converter, LPF - Low-Pass Filter, PID - Proportional-Integral-Derivative Controller. Detailed Implementation

[0030] To more clearly illustrate the objectives, technical solutions, and advantages of this invention, further explanations are provided below in conjunction with the accompanying drawings and formula derivations. It should be understood that the principles described herein are used to explain this invention, but are not intended to limit it.

[0031] The present invention includes structures such as an asymmetric mass block, a torsional elastic beam, electrodes, and an anchor zone. Figure 1 This is a schematic diagram of the structure of the Z-axis micromechanical accelerometer provided in an embodiment of the present invention, including a mass block 1, a first electrode 2, a torsional elastic beam 3, an anchor region 4, and a second electrode 5. The first electrode 2 and the second electrode 5, together with the mass block, form a set of capacitors, which can be reused as an angle detection capacitor, an electrostatic adjustment capacitor, and a force balance capacitor.

[0032] The torsional elastic beam and the mass blocks asymmetrically distributed on both sides of the torsional elastic beam constitute a seesaw structure. The mass blocks are rigid structures, fixedly connected to the torsional elastic beam. The two ends of the torsional elastic beam are fixed to the anchor structure, and there is a distance between the center of gravity of the mass blocks and the torsional central axis of the torsional elastic beam. The first electrode 2 and the second electrode 5 are fixed to a substrate on the Z-direction side of the mass blocks. In this embodiment, the mass blocks are symmetrical about an axis perpendicular to the torsional central axis in the XY plane, i.e., the axis of symmetry is the Y-direction. The asymmetrical mass blocks here refer to the mass blocks being asymmetrically arranged with respect to the torsional elastic beam in the X-direction; the mass blocks distributed on both sides of the torsional elastic beam have different lengths in the X-direction, but the same width in the Y-direction and thickness in the Z-direction. Figure 2 The cross-sectional view of the seesaw structure shown shows that, in order to ensure the elasticity of the torsional elastic beam, the thickness of the elastic beam is less than the thickness of the mass block.

[0033] The Z-axis micromechanical accelerometer designed in this invention has a simple structure, integrates detection, tuning and driving functions, and avoids mutual interference, which helps to improve the miniaturization, integration and accuracy of the Z-axis accelerometer.

[0034] When there is external acceleration along the Z-axis, the center of gravity of the asymmetric mass block does not coincide with the torsional axis, so the inertial force generates torque, causing rotation around the axis. Since the capacitors are symmetrically distributed on both sides of the torsional axis, if the gap between one set of capacitors increases, the gap between the other set decreases. When the rotation angle is minimal (which holds true under force balance), the capacitors can be assumed to still be parallel-plate capacitors. After differential conversion between the two sets of capacitors, a C / V converter and an AD converter are used, followed by demodulation and low-pass filtering to output a voltage signal representing the rotation angle. This voltage signal is then processed by a PID controller to obtain the force balance PWM control voltage V2. The voltages applied to the electrodes of the two sets of capacitors via the push-pull circuit are V1 + V2 + V... c and V1-V2-V c Where V1 is the DC tuning voltage, and since the carrier voltage is a high-frequency signal, it can be omitted in the calculation of electrostatic force and electrostatic negative stiffness. Meanwhile, V2 is the PWM control voltage, and only the duty cycle changes during the force balance process; therefore, V1... 2 +V2 2 This keeps the electrostatic negative stiffness constant, thus avoiding mutual interference between force balance and electrostatic adjustment.

[0035] Based on the electrostatic adjustment capacitor, the equivalent stiffness of the system can be adjusted, and the corresponding DC tuning voltage V1 can be expressed as:

[0036]

[0037] Where ε is the dielectric constant, S is the capacitor overlap area, d0 is the capacitor gap, and km and k eff These are the calibrated torsional elastic beam stiffness value and the preset equivalent stiffness value, respectively, and |V2| is the force balance PWM voltage amplitude.

[0038] Based on the force-balancing capacitor, Z-axis acceleration detection can be achieved, and the acceleration can be expressed as:

[0039]

[0040] Where ε is the dielectric constant, m is the mass, S and d0 are the overlapping area and gap of the capacitors, respectively, L1 is the lever arm corresponding to the inertial torque generated by the asymmetrically distributed mass block about the torsion center axis, L2 is the lever arm corresponding to the electrostatic torque generated by the force-balancing capacitor about the torsion center axis, V1 and |V2| are the amplitudes of the DC tuning voltage and the force-balancing PWM voltage, respectively, and b is the duty cycle of the force-balancing PWM voltage.

[0041] Those skilled in the art should understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A Z-axis micromechanical accelerometer, characterized in that, Includes a seesaw structure and a multi-functional reusable electrode; The seesaw structure includes a torsional elastic beam and mass blocks asymmetrically distributed on both sides of the torsional elastic beam. The mass blocks are rigid structures and are fixedly connected to the torsional elastic beam. The two ends of the torsional elastic beam are fixedly supported on the anchor area structure. There is a distance between the center of gravity of the mass blocks and the torsional center axis of the torsional elastic beam. The multifunctional multiplexed electrode is fixed on a substrate on one side of the mass block in the Z direction, and includes a first electrode and a second electrode with equal distances from the torsion center axis; The multifunctional multiplexed electrode and the mass block in the seesaw structure constitute a set of differential capacitors. The set of differential capacitors can reuse the angle detection capacitor, stiffness tuning capacitor and force balance capacitor.

2. The Z-axis micromechanical accelerometer according to claim 1, characterized in that, The mass block is symmetrical about an axis perpendicular to the torsion center axis in the XY plane.

3. The Z-axis micromechanical accelerometer according to claim 1, characterized in that, The aforementioned angle detection capacitor is used to detect changes in the torsional angle of the seesaw structure. This is achieved by applying a high-frequency carrier voltage V to the electrodes of the differential capacitor. c and -V c This enables carrier modulation of the capacitance change signal caused by the variable gap, resulting in a digital angle signal.

4. The Z-axis micromechanical accelerometer according to claim 1, characterized in that, The stiffness-tuning capacitor is used to generate electrostatic negative stiffness by applying a push-pull control voltage V1+V2+V to the electrodes of the differential capacitor. c and V1-V2-V c Electrostatic negative stiffness is generated, where V1 is the electrostatic adjustment voltage and V2 is the force balance PWM voltage. The electrostatic negative stiffness value is related to V1. 2 +V2 2 Proportional relationship.

5. The Z-axis micromechanical accelerometer according to claim 1, characterized in that, The force-balancing capacitor is used to generate electrostatic torque to counteract the inertial force generated by external acceleration. Electrostatic torque is generated by applying push-pull control voltages V1+V2 and V1-V2 to the electrodes of the differential capacitor. The electrostatic torque is related to V1... V2 is proportional.

6. A control method based on the Z-axis micromechanical accelerometer as described in claim 1, characterized in that, Includes the following steps: Step 1: Calculate the tuned electrostatic negative stiffness based on the calibrated torsional elastic beam stiffness and the preset equivalent stiffness value, and calculate the electrostatic adjustment voltage V1 according to the following formula: ； Where ɛ is the dielectric constant, S is the overlapping area of ​​the capacitors, and d0 is the capacitor gap. and These are the calibrated torsional elastic beam stiffness value and the preset equivalent stiffness value, respectively, where |V2| is the force balance PWM voltage amplitude; Step 2: When the mass block is subjected to external acceleration, a pair of carrier voltages with the same amplitude but opposite signs are applied to the multifunctional multiplexed electrodes to achieve carrier modulation of the capacitance change signal caused by the variable gap. After the mass block signal is processed by the CV circuit, AD conversion circuit, multiplication demodulation and low-pass filtering signal, digital angle signals are obtained respectively. Step 3: The digital rotation angle signal is used by the PID controller to calculate the duty cycle of the Z-axis force balance PWM voltage V2; Through the push-pull circuit, voltages V1+V2 and V1-V2 are applied to the multifunctional multiplexed electrode respectively, so that the mass block is maintained at a constant horizontal reference position, the acceleration is calculated, and the equivalent stiffness is adjusted to the preset value; where V1 and V2 are the electrostatic adjustment voltage and the force balance PWM voltage, respectively.

7. The control method for the Z-axis micromechanical accelerometer according to claim 6, characterized in that, The acceleration calculation formula is as follows: ； Where m is the mass of the mass block, L1 is the lever arm corresponding to the inertial torque generated by the asymmetrically distributed mass block about the torsion center axis, L2 is the lever arm corresponding to the electrostatic torque generated by the force balancing capacitor about the torsion center axis, and b is the duty cycle of the force balancing PWM voltage.

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