TORSIONAL OSCILLATOR MICRO ELECTRO MECHANICAL SYSTEMS ACCELEROMETER

Abstract

A resonant mechanical structure, such as one for use in a torsional oscillator MEMS accelerometer that includes a mounting substrate and a reference mass configured to move within a reference mass plane, the resonant mechanical structure being connected to the mounting structure and the reference mass, and the resonant mechanical structure including a body, a center of mass, and an aperture, wherein the aperture is surrounded and defined by the body, and wherein the body includes a first mass portion and a second mass portion that are configured to oscillate about an oscillation axis located within the reference mass plane, wherein the center of mass is located on the oscillation axis, and wherein a movement of the reference mass within the reference mass plane varies a moment of inertia of the body while the center of mass of the body remains located on the oscillation axis.

Claims

1. A sensor comprising: a mounting structure; a reference mass configured to move within a reference mass plane; at least one resonant mechanical structure connected to the mounting structure and the reference mass, the resonant mechanical structure comprising a body and an aperture, wherein the aperture is surrounded and defined by the body, and wherein the body comprises: a first mass portion and a second mass portion that are configured to oscillate about an oscillation axis located within the reference mass plane; and a center of mass located on the oscillation axis; and wherein a movement of the reference mass within the reference mass plane varies a moment of inertia of the body while the center of mass of the body remains located on the oscillation axis.

2. The sensor of claim 1, wherein the movement of the reference mass is at least one of a pendulous movement or a linear movement.

3. The sensor of claim 1, wherein the movement of the reference mass causes the first and second mass portions to move toward or away from the oscillation axis.

4. The sensor of claim 1, wherein the aperture is positioned on the oscillation axis.

5. The sensor of claim 1, wherein the center of mass is positioned on or in the aperture.

6. The sensor of claim 1, further comprising a second resonant mechanical structure connected to the mounting structure and the reference mass, the second resonant mechanical structure comprising a second body and a second aperture, wherein the second aperture is surrounded and defined by the second body, and wherein the second body comprises: a third mass portion and a fourth mass portion that are configured to oscillate about a second oscillation axis located within the reference mass plane; and a second center of mass located on the second oscillation axis; and wherein the movement of the reference mass within the reference mass plane varies a moment of inertia of the second body while the second center of mass remains located on the second oscillation axis.

7. The sensor of claim 6, wherein the movement of the reference mass causes the first and second mass portions to move toward the oscillation axis and the third and fourth mass portions to move away from the second oscillation axis, or the first and second mass portions to move away from the oscillation axis and the third and fourth mass portions to move toward the second oscillation axis.

8. The sensor of claim 6, wherein the second oscillation axis intersects the oscillation axis, or is collinear, parallel, or orthogonal to the oscillation axis.

9. A resonant mechanical structure comprising: a body and an aperture, wherein the aperture is surrounded and defined by the body, and wherein the body comprises: a first mass portion and a second mass portion that are configured to oscillate about an oscillation axis; and a center of mass located on the oscillation axis.

10. The resonant mechanical structure of claim 9, wherein the body is configured to be directly or indirectly connected to a reference mass, and wherein the first and second mass portions are configured to move toward or away from the oscillation axis upon a movement of a reference mass connected to the body.

11. The resonant mechanical structure of claim 9, wherein the body is configured to be compressibly or tensibly connected, directly or indirectly, to a reference mass.

12. The resonant mechanical structure of claim 9, wherein the body is configured to directly or indirectly connect to a reference mass in a manner such that a movement of the reference mass within a reference mass plane would vary a moment of inertia of the body while the center of mass of the body would remain located on the oscillation axis, and wherein the oscillation axis would be located within the reference mass plane.

13. The resonant mechanical structure of claim 9, wherein the body is configured to directly or indirectly connect to a reference mass, and wherein the first and second mass portions are configured to move toward the oscillation axis upon a movement of the reference mass causing a compression of the body, or away from the oscillation axis upon a movement of the reference mass causing a tension of the body.

14. The resonant mechanical structure of claim 9, wherein the aperture is positioned on the oscillation axis.

15. The resonant mechanical structure of claim 14, wherein the center of mass is positioned on or in the aperture.

16. A method of measuring a force, comprising: using a resonant mechanical structure to measure a movement of a reference mass within a reference mass plane, wherein the resonant mechanical structure is connected to the reference mass, and wherein the resonant mechanical structure comprises: a body and an aperture, wherein the aperture is surrounded and defined by the body, and wherein the body comprises: a first mass portion and a second mass portion that are configured to oscillate about an oscillation axis located within the reference mass plane; and a center of mass located on the oscillation axis.

17. The method of claim 16, wherein the aperture is positioned on the oscillation axis, or the center of mass is positioned on or in the aperture.

18. The method of claim 16, wherein the resonant mechanical structure is connected to the reference mass in a manner such that a movement of the reference mass within the reference mass plane varies a moment of inertia of the body while the center of mass of the body remains located on the oscillation axis.

19. The method of claim 16, wherein the first and second mass portions are configured to move toward or away from the oscillation axis upon a movement of the reference mass.

20. The method of claim 19, wherein the first and second mass portions are configured to move toward the oscillation axis upon a movement of the reference mass causing a compression of the body, or away from the oscillation axis upon a movement of the reference mass causing a tension of the body.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] For a better understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:

[0019] FIG. 1 is a schematic representation of a spring and mass accelerometer model.

[0020] FIG. 2 is a schematic representation of a resonant beam pick-off with pendulous mass.

[0021] FIG. 3 illustrates a MEMS inertial sensor in accordance with the present disclosure, the MEMS inertial sensor having an in-plane proof mass capable of moving inpendulous motion.

[0022] FIG. 4a illustrates a top view of a resonant mechanical structure in accordance with the present disclosure.

[0023] FIG. 4b illustrates an isometric view of the resonant mechanical structure shown in FIG. 4a.

[0024] FIG. 5 illustrates a MEMS inertial sensor similar to the one shown in FIG. 3, the inertial sensor including a driving electrode and a sensing mechanism.

[0025] FIG. 6 illustrates the MEMS inertial sensor shown in FIG. 3, the proof mass being rotated in a clockwise position from the position shown in FIG. 3.

[0026] FIG. 7 illustrates the MEMS inertial sensor shown in FIG. 3, the proof mass being rotated in a counter-clockwise position from the position shown in FIG. 3.

[0027] FIG. 8A shows the resonant mechanical structure illustrated in FIGS. 4a-4b in a state of tension.

[0028] FIG. 8B shows the resonant mechanical structure illustrated in FIGS. 4a-4b in a state of compression.

[0029] FIGS. 9a-9g illustrate a various alternative embodiments of a resonant mechanical structure in accordance with the present disclosure.

[0030] FIG. 10 illustrates an alternative embodiment a MEMS inertial sensor in accordance with the present disclosure, the MEMS inertial sensor having an in-plane proof mass capable of moving in a linear motion.

[0031] Like reference numerals refer to corresponding parts throughout the drawings.

DETAILED DESCRIPTION

[0032] Referring now to the drawings, a MEMS inertial sensor with a resonant pickoff in accordance with the present disclosure is generally indicated by reference numeral 20 in FIG. 3. Specifically, the MEMS inertial sensor 20 illustrated in FIG. 3 may be an accelerometer. The inertial sensor 20 may include a mounting structure 22, such as a substrate, a reference mass 24, and a pair of resonant mechanical structures 26a, 26b. Mounting structure 22 may be fabricated of several types of material, including but not limited, to silicon, glass, quartz, and crystalline GE. Sensor 30 may include only a single resonant mechanical structure, or may include two, three or more resonant mechanical structures.

[0033] Reference mass 24 may be connected to mounting structure 22 via an anchor point 28. In a further embodiment, reference mass 24 may be attached to a hinge 30 that is connected to the anchor point 28. The hinge 30 enables the reference mass 24 to move in a pendulous motion PM within a reference mass plane XY. Reference mass plane XV may lie in a coordinate system defined by coordinate X and coordinate Y. In the embodiment shown in FIG. 3, the reference mass plane XY is substantially parallel to the mounting structure 22. The resonant mechanical structures 26a, 26b may also connected to the mounting structure 22 via a plurality of anchor points 28. Additionally, each of the resonant mechanical structures 26a, 26b may be connected to the reference mass 24. The resonant mechanical structures 26a, 26b may be oriented relative to the reference mass 24 in a manner such that movement of the reference mass in a clockwise direction places resonant mechanical structure 26a in a state of compression and resonant mechanical structure 26b in a state of tension. Conversely, movement of the reference mass 24 in a counter-clockwise direction places resonant mechanical structure 26a in a state of tension and resonant mechanical structure 26b in a state of compression.

[0034] Referring now to FIGS. 4a and 4b, each of the resonant mechanical structures 26a, 26b may comprise a body 34 and an opening 32. In some embodiments, the body 34 may surround and define the openings 32. The body 34 may include a first mass portion 36, such as an upper mass, and second mass portion 38, such as a lower mass. The body 34 also has a center of mass 40. The resonant mechanical structure 26 is configured such that the first/upper mass 36 and the second/lower mass 38 oscillate about an oscillation axis OX. The oscillation axis OX may be located within the reference mass plane XY. The resonant mechanical structure 26 is designed such that the resonant mechanical structure can be excited into an oscillatory motion having a resonant frequency such that the first/upper mass 36 and the second/lower mass 38 oscillate about the oscillation axis OX and the body's center of mass 40 is located on the oscillation axis. When excited into the oscillatory motion, the first/upper mass 36 will follow an upper oscillation path 42 and the second/lower mass 38 will follow a lower oscillation path 44.

[0035] Resonant mechanical structure 26 may be driven into oscillatory motion in various ways. For example, as shown in FIG. 5, the MEMS inertial sensor 20 may include a driving electrode 45. Driving electrode 45 may be positioned relative to the first/upper mass 36 in a manner such that the driving electrode can pull and/or push the first/upper mass 36 with a certain constant force, thereby setting the resonant mechanical structure 26 into oscillatory motion. Sensor 20 may further include a sensing mechanism 46 for detecting resonant frequency. As shown in FIG. 5 5, sensing mechanism 46 may be, for example, a capacitive gap sensor positioned relative to the second/lower mass 38 in a manner such that the capacitive gap sensor may sense the resonant frequency of oscillation of the second/lower mass 38. Using this type of an arrangement, the MEMS inertial sensor 20 can detect the resonant frequency of the resonant mechanical structure 26. It should be noted that in an alternative embodiment driving electrode 45 may positioned on second/lower mass 38, and sensing mechanism 46 may sense the oscillation of the first/upper mass 36. In embodiments where sensor 20 may include multiple resonant mechanical structures, such additional structure may feature bodies which are similar or identical to that of resonant mechanical structure 26.

[0036] The concept of operation for the resonant mechanical structure 26 is that it functions like a torsional pendulum. To calculate the resonant frequency f.sub.n of a torsional pendulum, the following equation (Equation 1) is used:

[00001]$\begin{array}{cc}{f}_n=\frac{1}{2.Math.}.Math.\sqrt{\frac{k}{I}}& \left(1\right)\end{array}$

[0037] In Equation 1, k is the torsional spring constant and I is the moment of inertia. Accordingly, Equation 1 implies an inverse proportionality of the resonant frequency f.sub.n to the square root of the moment of inertia I. Based on Equation 1, if the torsional spring constant k remains the same and the moment of inertia I increases, the resonant frequency f.sub.n will decrease. Conversely, if the torsional spring constant k remains the same and the moment of inertia I decreases, the resonant frequency f.sub.n will increase. The resonant mechanical structure 26 of the present disclosure uses the relationship between moment of inertia I and resonant frequency f.sub.n to enable sensor 20 to determine whether sensor 20 is exposed to acceleration along the sensing axis. As used throughout this disclosure, the term sensing axis refers to the axis (or axes) along which sensor 20 is designed to detect acceleration.

[0038] Specifically, resonant mechanical structure 26 is driven into oscillatory motion (e.g. via a driving electrode or other method known to a person of ordinary skill in the art) when reference mass 24 is in a first neutral position, for example, as shown in FIG. 3. When sensor 20 is exposed to acceleration along the sensing axis, the reference mass 24 moves to a second position, as shown in FIG. 6. When the reference mass 24 is in the second position, the body 32 of resonant mechanical structure 26b is in tension and the body 32 of resonant mechanical structure 26a is in compression. When the reference mass 24 is in the third position (as shown in FIG. 7), the body 32 of resonant mechanical structure 26b is in compression and the body 32 of resonant mechanical structure 26a is in tension. FIG. 8a shows resonant mechanical structure 26 in tension and FIG. 8b shows resonant mechanical structure 26 in compression.

[0039] The geometric shape of the body 32 of each resonant mechanical structure 26a, 26b changes depending on the position of the reference mass 24; however, the center of mass 40 for each body 32 remains located on the respective oscillation axis OX. Changing the geometric shape of each body 32 as its upper and lower masses 36, 38 are oscillating about the oscillation axis OX changes the moment of inertia for each body 32.

[0040] For example, in FIG. 3, wherein the reference mass 24 is in the first neutral position, body 32 of resonant mechanical structure 26a has a first moment of inertia and a first resonant frequency. When the reference mass 24 is in the second position, body 32 of resonant mechanical structure 26a has a second moment of inertia and a second resonant frequency. As shown in FIG. 6, the second moment of inertia would be greater than the first moment of inertia because upper and lower masses 36, 38 are spaced a greater radial distance from the oscillation axis OX than in FIG. 3. Because the second moment of inertia is greater than the first moment of inertia, the second resonant frequency will be less than the first resonant frequency. When the reference mass is in the third position, body 32 of resonant mechanical structure 26a has a third moment of inertia and a third resonant frequency. As shown in FIG. 7, the third moment of inertia would be less than the first moment of inertia because upper and lower masses 36, 38 are a spaced a lesser radial distance from the oscillation axis OX than in FIG. 3. Because the third moment of inertia is less than the first moment of inertia, the third resonant frequency will be greater than the first resonant frequency. The sensing mechanism 46 of sensor 20 may configured to detect the difference in the resonant frequency of each body 32 as the reference mass 24 moves from the first neutral position to either the second or third positions. Calculating the difference between, for example, the first resonant frequency and the second resonant frequency enables sensor 20 to determine acceleration along the sensing axis.

[0041] Using a resonant pick-off as described in this disclosure, the moment of inertia of body 32 within sensor 20 is linearized such that the varying moment of inertia, which is based on a linear displacement of the upper and lower masses 36, 38, is the primary variable responsible for varying the resonant frequency. This differs from the MEMS accelerometer discussed in the Background Section, which relies solely on the force exerted by a reference mass 18 on the oscillatory beam 16 to vary the resonant frequency. By relying on the moment of inertia which varies based on a linear displacement of the upper and lower masses 36, 38, the resonant mechanical structure 26 removes many of the non-linear effects present in beam deformation. Consequently, the resonant mechanical structure 26 reduces the scale factor error associated with sensor 20 relative to the MEMS accelerometers discussed in the Background Section.

[0042] FIGS. 9a through 9g show various alternative embodiments of a resonant mechanical structure in accordance with the present disclosure. A person of ordinary skill in the art will understand that there are other potential embodiments of the resonant mechanical structure with varying geometric shapes.

[0043] FIG. 10 shows an alternative embodiment of a sensor 48, such as a MEMS inertial sensor, with a resonant pick-off in accordance with the present disclosure. In FIG. 10, reference mass 50 is configured to move in a linear motion within the reference mass plane XY rather than a pendulous motion. Sensor 48 further includes a plurality of flexures 52 to connect the reference mass 50 to a plurality of anchor points 54. The anchor points 54 in turn connect the reference mass 50 to a mounting structure 56, such as a mounting substrate. The resonant mechanical structures 58a, 58b of sensor 48 are structured and operate in a similar manner as described above with reference to sensor 20.

[0044] Although the embodiment of sensor 20 in FIG. 3 and sensor 48 in FIG. 10 are in-plane inertial sensors (i.e. a sensor with an input axis that is substantially parallel to the mounting structure/substrate), a person of ordinary skill in the art will understand that the resonant mechanical structure could be incorporated into an out-of-plane inertial sensor (e.g. a sensor having an input axis perpendicular to the mounting structure/substrate). Furthermore, a person of ordinary skill in the art would recognize that the embodiments of the MEMS inertial sensors presented herein could be fabricated using any material or process suitable for a MEMS device. A person of ordinary skill in the art would also recognize that the concepts and designs presented herein could be extended to other fabrication scales (e.g. Nano Electro Mechanical Systems (NEMS)). Given the present disclosure, a person of ordinary skill in the art could conceive a number of electronic or opto-electronic interfaces for the generation of a useful output signal or message.

[0045] The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. Indeed, while certain features of this invention have been shown, described and/or claimed, it is not intended to be limited to the details above, since it will be understood that various omissions, modifications, substitutions and changes in the apparatuses, forms, method, steps and system illustrated and in its operation can be made by those skilled in the art without departing in any way from the spirit of the present invention.

[0046] Further, the foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, many modifications and variations are possible in view of the above teachings. For example, multi-axis resonant accelerometers may be formed in connection with the teachings of the invention. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.