WO2015133903A1

WO2015133903A1 - Transducer for use in a capacitive vibration sensor

Info

Publication number: WO2015133903A1
Application number: PCT/NL2015/050142
Authority: WO
Inventors: René Ignaz Peter SEDMIK
Original assignee: Stichting Vu-Vumc
Priority date: 2014-03-06
Filing date: 2015-03-06
Publication date: 2015-09-11

Abstract

A transducer for use in a capacitive vibration sensor comprises a mass- spring system (3) and a first flat plate. The mass-spring system comprises a flat central proof mass (4), a surrounding support structure (5), and at least three interleaved spiral-shaped spring arms (6, 7, 8). The first flat plate is mounted at a first fixed position relative to the surrounding support structure, spaced from, and parallel to a first flat side (41) of the flat central proof mass. The flat central proof mass and the first flat plate are forming a first electric capacitance. Externally caused accelerations of the surrounding support structure cause parallel displacement of the flat central proof mass relative to the first flat plate, resulting in a changing first capacity of the first electric capacitance.

Description

Title: Transducer for use in a capacitive vibration sensor.

FIELD OF THE INVENTION

The invention relates to a transducer for use in a capacitive vibration sensor. The invention also relates to a capacitive vibration sensor comprising such a transducer. More in particular, it concerns a high precision sensor for low-frequent low-amplitude accelerations, such as seismic movements.

BACKGROUND OF THE INVENTION

Vibration sensors are used in different industrial applications and for research purposes in various forms, with diverse principles of operation and, most important, sensitivity. In industrial processes accelerations and vibrations of machine parts and structures are mostly monitored using piezo-electric or piezo-resistive sensors which offer robustness and stabihty but are not very sensitive. At the other end of the scale, for the detection of seismic and other low frequent accelerations, mainly two types of sensors are available. First, electro-mechanical mass-spring systems (geophones), which detect the voltage induced in a coil due to the movement (velocity) of a proof-mass, which is made out of magnetic material and is supported by a spring. Second, micro-electromechanical systems (MEMS), which are micro- machined from a solid block of (semiconductor) material and consist of a movable shuttle supported by springs. Interleaving comb-like structures on the shuttle and the surrounding fixed housing form an electric capacity which may be evaluated by a dedicated electronic circuit. The capacity varies with the motion exerted by the shuttle with respect to the housing, which in turn is (for small signal amplitudes) inversely proportional to the acceleration experienced by the sensor. While geophone sensors offer superior sensitivity they are expensive, heavy, and give a signal relative to velocity, thereby reducing their sensitivity at lowest frequencies. MEMS devices are capable of measuring constant accelerations and are much cheaper than other sensors but inherently suffer from larger mechanical noise due to the small mass of the movable element. For this reason, the sensitivity of MEMS devices is normally insufficient to capture the low seismic background level.

SUMMARY OF THE INVENTION

It is an object of the invention to provide at least an alternative solution which allows a sensor to reach the sensitivity level of current high- end devices in a smaller, lighter, and cheaper construction.

For that purpose, the invention provides a transducer according to the appended independent claim 1, while specific embodiments of the invention are set forth in the appended dependent claims 2-9.

Hence, the invention provides a transducer for use in a capacitive vibration sensor, the transducer comprising:

- a mass-spring system, which comprises a flat central proof mass, a surrounding support structure, and at least three interleaved spiral- shaped spring arms, wherein the flat central proof mass is suspended from the surrounding support structure by said at least three interleaved spiral- shaped spring arms, in that each of said at least three interleaved spiral - shaped spring arms at its inner end is connected tangentially to the flat central proof mass, and at its outer end is connected to the surrounding support structure; and

- a first flat plate, which is mounted at a first fixed position relative to the surrounding support structure, spaced from, and parallel to a first flat side of the flat central proof mass, in such a way:

- that the flat central proof mass and the first flat plate form a first electric capacitance, and

- that externally caused accelerations of the surrounding support structure cause parallel displacement of the flat central proof mass relative to the first flat plate, resulting in a changing first capacity of the first electric capacitance.

In a preferable embodiment of the invention, the transducer further comprises:

- a second flat plate, which is mounted at a second fixed position relative to the surrounding support structure, spaced from, and parallel to a second flat side of the flat central proof mass, wherein the first flat side and the second flat side are mutually opposite flat sides of the flat central proof mass, in such a way:

- that the flat central proof mass and the second flat plate form a second electric capacitance, and

- that externally caused accelerations of the surrounding support structure cause parallel displacement of the flat central proof mass relative to the second flat plate, resulting in a changing second capacity of the second electric capacitance.

In a further preferable embodiment of the invention, the flat central proof mass, the surrounding support structure, and the at least three interleaved spiral-shaped spring arms are machined from one single block of material.

In a yet further preferable embodiment of the invention, each of said at least three interleaved spiral-shaped spring arms at its outer end is connected tangentially to the surrounding support structure.

In a yet further preferable embodiment of the invention, the thickness of each of said at least three interleaved spiral-shaped spring arms of the mass-spring system is held constant over its entire length, while its width scales with the third exponent of its length, wherein said thickness is taken orthogonal to the planform of the mass-spring system, while said width is taken as extending in radial direction of the spiral-shaped spring arms. In a yet further preferable embodiment of the invention, the transducer further comprises a rigid structure, which mechanically connects the first flat plate to the surrounding support structure, wherein said rigid structure encloses the mass-spring system to form a pressure-tight entity.

In a yet further preferable embodiment of the invention, said rigid structure also mechanically connects said second flat plate to the

surrounding support structure.

In a yet further preferable embodiment of the invention, the flat central proof mass is large enough to suppress mechanical Brownian noise to below the level of motion caused by seismic accelerations at frequencies around or below 1 Hz.

The invention may also be embodied in a capacitive vibration sensor comprising:

- a transducer according to the invention; and

- electrical circuitry configured to read out said changing first capacity of the first electric capacitance and to transform the read-out changing first capacity into electrical signals representing accelerations sensed by the sensor.

In a preferable embodiment of a capacitive vibration sensor according to the invention, said electrical circuitry is further configured to read out said changing second capacity of the second electric capacitance and to transform the read-out changing first capacity and the read-out changing second capacity into electrical signals representing accelerations sensed by the sensor.

Hence, the capacitive sensor according to the invention distinguishes over the abovementioned known MEMS devices by the geometry of the transducer. The invention allows for a comparably larger surface area, and a higher mass which translates to an inherently low noise level. The proof mass and the spring system can be made (but are not limited to) a membrane or plate which is cut in such a way that the connection between the fixed frame or outer boundary and a central disc (or related geometry) forming the proof mass, is made by the said at least three interleaved spiral-shaped arms. The latter extend tangentially from the central disc in order to maximally limit the degrees of freedom of the proof mass in directions other than normal to its surface. The flat central (disclike) proof mass is opposed on either one or both of its flat sides by a flat plate of similar surface area as the proof mass. Accelerations acting on the frame or housing of the sensing element are translated to a relative displacement of the central proof mass with respect to the plate(s) which results in a variation of the electric capacity between these elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The abovementioned aspects and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter by way of non-limiting examples only and with reference to the schematic figures in the enclosed drawing.

Fig. 1 shows in planform (view from above) an example of an embodiment of a mass-spring system 3 for use in a transducer according to the invention.

Figs. 2A, 2B illustrate, in cross-sectional side views through part of a transducer according to the invention, the capacitive working principle of transducers according to the invention.

Fig. 3 shows, in cross-sectional side view, an example of an embodiment of a capacitive vibration sensor 1 according to the invention, wherein the sensor 1 comprises an example of an embodiment of a transducer 2 according to the invention, wherein the transducer 2 comprises the mass-spring system 3 of Fig. 1.

Fig. 4 shows, in cross-sectional side view, an example of another embodiment of a capacitive vibration sensor 101 according to the invention, wherein the sensor 101 comprises an example of another embodiment of a transducer 102 according to the invention, wherein also the transducer 102 comprises the mass-spring system 3 of Fig. 1.

It is noted that, where the same reference signs are used throughout different ones of the abovementioned Figs. 1, 2A, 2B, 3, 4, these reference signs denote the same or similar parts or aspects.

The reference signs used in Figs. 1, 2A, 2B, 3, 4 refer in the following manner to the various parts and aspects of the invention, already mentioned above, and/or (further) discussed in the detailed description below.

1, 101 capacitive vibration sensor

2, 102 transducer

3 mass-spring system

4 flat central proof mass

5 surrounding support structure

6, 7, 8 three spiral-shaped spring arms

9 rigid structure

10 electrical circuitry

11 first flat plate

12 second flat plate

14 electrical connection

41 first flat side

42 second flat side

Ci first capacity

C₂ second capacity

a external acceleration

d distance at zero position

Ad displacement on acceleration

AC change of capacity DETAILED DESCRIPTION OF EMBODIMENTS

In the shown example of the mass-spring system 3, the flat central proof mass 4 has a circular, disc-like shape, while the number of the at least three spiral-shaped spring arms is three. These three spring arms 6, 7, 8 are mutually identical, and have equal angular spacing as seen in the planform of Fig. 1.

As mentioned, the transducer 2 of Fig. 3 and the transducer 102 of Fig. 4 have the same mass-spring system 3 in common. However, the difference between the two transducers 2 and 102 is that the transducer 102 of Fig. 4 has both the first flat plate 11 and the second flat plate 12, whereas the transducer 2 of Fig. 3 only has the first flat plate 11, not the second flat plate 12. Accordingly, a further difference between the sensor 1 of Fig. 3 and the sensor 101 of Fig. 4 is, that the sensor 101 of Fig. 4 has three electrical connections 14 with the electrical circuitry 10, whereas the sensor 1 of Fig. 3 has only two electrical connections 14 with the electrical circuitry 10. The reason is that sensor 1 does not need electrical connection for a second flat plate, since a second flat plate is absent for sensor 1.

Fig. 1 clearly shows that the arms 6, 7, 8 are shaped such that they form interleaved spirals which, at their outer end, are connected rigidly to the surrounding support structure 5 (outer spring fixture). In particular embodiments, the flat central proof mass 4 may be of a shape other than circular disc-like. In the same sense, the outer support structure 5 may have an arbitrary shape and may either be a separate part which is clamped, glued, welded, soldered, or in another way rigidly attached to the outer end of the springs. In particular embodiments of the invention, all three parts (the central disc 4, the springs 6, 7, 8, and the outer support structure 5) may be made from a single plate or membrane by excavating the free space in Fig. 1 between adjacent springs 6, 7, 8 by means of etching, laser cutting or stamping. In particular embodiments the central disc 4 may be of different thickness (referring to the extension of the disc material in the direction orthogonal to the disc 4) than the springs 6, 7, 8 in order to increase or decrease the proof mass. The thickness of the central disc 4 in various embodiments must be sufficient in the sense that the structural flexing of the disc under all conditions is negligible with respect to the parallel displacement of the central disc due to the deformation of the springs.

The mechanical frequency response of the mass-spring system 3 has a direct influence on the sensitivity and can be tuned in a wide range by shaping the cross-section, number, and length of the springs 6, 7, 8. The illustration in Fig. 1 shows a particular embodiment where the thickness of the springs 6, 7, 8 is held constant over their entire length, while the width (extension in radial direction) scales with the third exponent of the length of the spring, which causes an almost linear radial bending of the spring structures under the weight of the proof mass 4 and is beneficial in terms of the space used by the spring structure. Also, the latter geometry results in an optimum (maximum) of the ratio of lateral (in the plane of the figure) to normal (orthogonal to the figure plane) stiffness for a given size of the entire spring structure. Particular embodiments of the invention may utilize various materials for the actual implementation of the transducers according to the invention. For proper operation without large errors due to finite conductance or surface charge effects, a metallic or highly doped semiconductor material, or any other material having sufficient

conductivity, has to be used as a coating of the surfaces, if the conductivity of the bulk material of the springs 6, 7, 8 and the proof mass 4 is too low.

The working principle of the sensor is illustrated by means of the example in Figs. 2A, 2B. In vertical direction (orthogonal to the plane of Fig. 1), the proof mass 4 is placed above or below a fixed plate, for a particular embodiment incorporating only one fixed plate. In an embodiment with two fixed plates, the proof mass 4 is placed between the two fixed plates (for example, but not necessarily, in the vertical center between the two fixed plates), as is the case with the two fixed plates 11 and 12 in Figs. 2A, 2B.

In the absence of external accelerations, which is the case in Fig. 2A, the proof mass 4 remains at its "zero position" of Fig. 2A. At this zero position, the distance between the proof mass 4 and the first flat plate 11 has been indicated by d in Fig. 2A.

In the event of a small external acceleration a, the proof mass 4 is displaced by Ad resulting in a change AC, or differential change 2 AC, of the capacities between the proof mass 4 and one or two of the first and second flat plates 11 and/or 12, respectively. This is illustrated by Fig. 2B as follows. If an external acceleration a acts onto the sensor (respectively the structure connected to the outer support structure and the fixed plate(s), the springs will flex and the proof mass 4 will be displaced in opposite direction to the acceleration by a distance Ad. This displacement is directly

proportional to the amplitude of a but can be different at various

frequencies in dependence on the spectral response of the mechanical mass- spring system. The electric capacity (Ci or C_? in a particular embodiment with ony one plate 11 or 12; or Ci and C∑ if there are two plates 11 and 12 present) between the proof mass 4 and the fixed plate(s) 11 and/or 12 can be measured by means of a dedicated electronic circuit. A displacement Ad is reflected by an inverse change AC ~ C_x Adld (where x stands for 1 or 2) of the capacitance. Using two fixed plates instead of one gives the advantage of an increase in the sensitivity by a factor 2, but does not change the principle of operation.

Fig. 3 shows a particular embodiment of the invention with one fixed plate 11. The housing 9 is closed in order to allow for protection of the device from dust, humidity, or other harsh environmental conditions on the outside. Encapsulation also offers the opportunity to tune the damping coefficient of the mechanical system by introducing a gas at a specific pressure into the housing. The particular embodiment in Fig. 3 also features included electronics 10 for the measurement of the capacity and

amplification of the resulting signal. In other particular embodiments, this may not be the case, as the capacity can also be measured by an external apparatus such as a capacitive bridge. The two electrical wires 14 shown schematically in Fig. 3 connect the electronics 10 of the sensor 1 with the transducer 2. In Fig. 3 it is seen that there is an electrical wire 14, which connects the electrical circuitry 10 with the mass-spring system 3 (to eventually obtain electrical connection with the central disc 4 of system 3), while there is another electrical wire 14, which connects the electrical circuitry 10 with the first flat plate 11.

As mentioned, Fig. 4 illustrates another particular embodiment of the invention, with two fixed plates 11 and 12. The same considerations as above for Fig. 3 apply. However, in this case of two fixed plates 11 and 12, the electronics 10 must be connected to both fixed plates 11 and 12, as well as to the proof mass 4. As compared to Fig. 3, it is seen that in Fig. 4 there is a further electrical wire 14, which connects the electrical circuitry 10 with the second flat plate 12.

Finally, the following more or less summarizing notes are made as a yet further explanation, being perhaps more or less superfluous. The present invention concerns a capacitive acceleration sensor with a novel type of geometry of the transducer. The latter may comprise a flat, disc-like proof mass suspended by three or more concentric spiral-shaped springs which are connected tangentially to the proof mass as well as to a fixed support structure. Either one or two fixed plates are mounted opposite to the flat surface(s) of the proof mass in such a way that they form an electric capacitance. Small external accelerations are translated by the transducer to a parallel displacement of the proof mass which causes a proportional (for small signals only) change in the capacities which can be detected by dedicated electronic circuits. The design allows for relatively large proof masses while maintaining very low force constants (sensitive mechanical response) in a compact and small form and is therefore suited for seismic and other high-sensitivity measurements at low frequencies.

While in the foregoing detailed description and drawing figures the present invention has been described in terms of particular

embodiments and applications, in both summarized and detailed forms, it is not intended that these descriptions in any way limit its scope to any such embodiments and applications, and it will be understood that many substitutions, changes and variations in the described embodiments, applications and details of the methods illustrated herein can be made by those skilled in the art without departing from the scope of the appended claims.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single unit may fulfill the functions of several items recited in the claims. For the purpose of clarity and a concise description, features are disclosed herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

embodiments having combinations of all or some of the features disclosed. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. Transducer for use in a capacitive vibration sensor (1; 101), the transducer (2; 102) comprising:

- a mass-spring system (3), which comprises a flat central proof mass (4), a surrounding support structure (5), and at least three interleaved spiral-shaped spring arms (6, 7, 8), wherein the flat central proof mass is suspended from the surrounding support structure by said at least three interleaved spiral-shaped spring arms, in that each of said at least three interleaved spiral-shaped spring arms at its inner end is connected tangentially to the flat central proof mass, and at its outer end is connected to the surrounding support structure; and

- a first flat plate (11), which is mounted at a first fixed position relative to the surrounding support structure, spaced from, and parallel to a first flat side (41) of the flat central proof mass, in such a way:

- that the flat central proof mass (4) and the first flat plate (11) form a first electric capacitance (4, 11), and

2. Transducer according to claim 1, further comprising:

- a second flat plate (12), which is mounted at a second fixed position relative to the surrounding support structure (5), spaced from, and parallel to a second flat side (42) of the flat central proof mass (4), wherein the first flat side and the second flat side are mutually opposite flat sides of the flat central proof mass, in such a way: - that the flat central proof mass (4) and the second flat plate (12) form a second electric capacitance (4, 12), and

3. Transducer according to claim 1 or 2, wherein the flat central proof mass (4), the surrounding support structure (5), and the at least three interleaved spiral-shaped spring arms (6, 7, 8) are machined from one single block of material.

4. Transducer according to any one of the preceding claims, wherein each of said at least three interleaved spiral-shaped spring arms (6, 7, 8) at its outer end is connected tangentially to the surrounding support structure (5).

5. Transducer according to any one of the preceding claims, wherein the thickness of each of said at least three interleaved spiral-shaped spring arms (6, 7, 8) of the mass-spring system (3) is held constant over its entire length, while its width scales with the third exponent of its length, wherein said thickness is taken orthogonal to the planform of the mass-spring system (3), while said width is taken as extending in radial direction of the spiral-shaped spring arms (6, 7, 8).

6. Transducer according to any one of the preceding claims, further comprising a rigid structure (9), which mechanically connects the first flat plate (11) to the surrounding support structure (5), wherein said rigid structure encloses the mass-spring system (3) to form a pressure-tight entity.

7. Transducer according to claim 6, wherein, more specifically, the transducer is at least according to claim 2, and wherein said rigid structure (9) also mechanically connects the second flat plate (12) to the surrounding support structure (5).

8. Transducer according to any one of the preceding claims, wherein the flat central proof mass (4) is large enough to suppress mechanical Brownian noise to below the level of motion caused by seismic accelerations at frequencies around or below 1 Hz.

9. Capacitive vibration sensor (1; 101) comprising:

- a transducer (2; 102) according to any one of the preceding claims; and

- electrical circuitry (10) configured to read out said changing first capacity of the first electric capacitance (4, 11) and to transform the readout changing first capacity into electrical signals representing accelerations sensed by the sensor.

10. Capacitive vibration sensor (101) according to claim 9, wherein, more specifically, the transducer (102) is at least according to claim 2, and wherein said electrical circuitry (10) is further configured to read out said changing second capacity of the second electric capacitance (4, 12) and to transform the read-out changing first capacity and the read-out changing second capacity into electrical signals representing accelerations sensed by the sensor.