JPH08166302A - External-force sensor - Google Patents

Abstract

PURPOSE: To obtain an external-force sensor by which an acceleration is detected precisely without being affected by a temperature change by a method wherein the pattern of a first surface acoustic wave resonator(SAWR) formed on the first face of a beam is made similar to the pattern of a second SAWR formed on the second face of the beam. CONSTITUTION: Even when resonant cavity lengths L1 , L2 of two SAWRs 21A, 21B are different, temperature characteristics of the SAWRs 21A, 21B become identical when ratios L1 /λ10 , L2 /λ20 of the resonant cavity lengths L1 , L2 with reference to resonance wavelengths λ10 , λ20 are equal, and the difference Δf between transmission frequencies as output signals is not affected by a temperature. When the SAWRs 21A, 21B are formed to be mutually similar shapes, a condition that the ratios of the resonant cavity lengths with reference to the resonance wavelengths become L1 /λ10 =L2 /λ20 is satisfied, and a bias frequency ΔfB is set. When the SAWRs 21A, 21B are made to be similar shapes by a shrink method, a desired resonance frequency ratio f10 /f20 is obtained, and the desired bias frequency ΔfB is obtained easily.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a surface acoustic wave resonator (SURF
ACE ACOUSTIC WAVE RESONATOR: SAWR)

[0002]

2. Description of the Related Art An example of a conventional external force sensor using a surface acoustic wave resonator (SAWR) will be described with reference to FIG. The external force sensor has an elastic body that is cantilevered on a fixed wall, for example, a beam 11 and SAWRs 21A and 21B arranged on an upper surface 11A and a lower surface 11B of the beam 11. The beam 11 is made of, for example, a piezoelectric single crystal of quartz, lithium niobate, lithium tantalate, or the like, and an ST cut quartz single crystal is used particularly when temperature characteristics are emphasized.

A support member 13 is mounted on the fixed side of the beam 11, and a weight 15 is mounted on the free end side. Upper SA
WR21A and lower SAWR21B are amplifier 2 respectively
5A, 25B, and thereby the oscillator circuit 20
A and 20B are configured. The output signals of the oscillation circuits 20A and 20B are supplied to the buffer amplifiers 27A and 27B, respectively, and the output signals of the buffer amplifiers 27A and 27B are supplied to the filter 33 via the mixer 31. Filter 33
Takes out the difference Δf of the oscillation frequency as an output signal.

[0004]

[Formula 1] Δf = f ₁ −f ₂

Here, f ₁ is the oscillation frequency of the upper oscillation circuit 20A, and f ₂ is the oscillation frequency of the lower oscillation circuit 20B.

SAWR21A on the upper side and SAWR2 on the lower side
When the configuration of 1B is the same and the two oscillation frequencies f ₁ and f ₂ are the same, the difference Δf in oscillation frequency becomes zero when the acceleration A or the external force F does not act. However, the external force F
The two oscillation frequencies f ₁ and f ₂ are designed to be different in advance for the purpose of discriminating the direction of the two and the purpose of avoiding the phenomenon of sucking the two oscillations.

Therefore, even when the acceleration A or the external force F does not act, the difference Δf in oscillation frequency does not become zero, and a constant frequency output is generated. Such an output Δf _B is called a bias frequency.

When an acceleration A or an external force F acts on the external force sensor in the direction of the arrow, the beam 11 bends and the two surfaces 11A, 11
One side of B extends and the other side contracts. As a result, the oscillation frequency output from each of the oscillation circuits 20A and 20B changes. Therefore, the difference Δf between the oscillation frequencies output from the filter 33 changes. The acceleration A or the external force F is a function of the difference Δf in the oscillation frequency.

[0009]

## EQU2 ## A = K × (Δf−Δf _B )

Where A is the input acceleration, K is the proportional constant,
Δf _B is the bias frequency. The direction of the external force F can be determined by the sign of the acceleration A.

The relationship between the bias frequency Δf _B and the operating range of the external force sensor will be described with reference to FIG. FIG. 6 shows the relationship between the input acceleration A and the oscillation frequency difference Δf. When the input acceleration A is in the range of -maxA to + maxA, the minimum value Δfmin of the oscillation frequency difference Δf must be set so as not to be close to zero.

Next, a conventional method of providing the bias frequency Δf _B will be described with reference to FIG. FIG. 6A shows beam 1
1 shows the SAWR21A of the upper surface 11A of this 1.
21A is an input interdigital transducer (I
DT) 21-1A and output interdigital transducer (IDT) 21-2A and two reflectors 21-3
A, 21-4A. FIG. 6B shows the lower surface 11 of the beam 11.
B SAWR21B is shown, and this SAWR21B is also shown.
Is the input IDT 21-1B and the output IDT 21-2B and 2
It has two reflectors 21-3B and 21-4B.

In order to provide the bias frequency Δf _B , the two SAWRs 21A and 21B are usually arranged so that the resonance cavity lengths L ₁ and L _{2 of the} two SAWRs 21A and 21B are different.
B is designed.

The simplest method is as shown in FIG.
Input IDT 21-1A and output IDT 21- of surface 11A
2A and reflector 21-3A, 21-4A and lower surface 11B
Force IDT 21-1B and output IDT 21-2B and reflector
21-3B and 21-4B have the same shape, and
Distance L between reflectors 21-3A and 21-4A on surface 11A ₁
And the distance between the reflectors 21-3B and 21-4B on the lower surface 11B
L₂Design to be different from.

The temperature characteristics of the SAWR formed by such a method will be described with reference to FIG. The curve in FIG. 7A shows the temperature characteristics of the SAWR 21A on the upper surface 11A.
The curve B shows the temperature characteristics of the SAWR 21B on the lower surface 11B. However, this example assumes the case of an ST-cut crystal having excellent temperature characteristics.

In the graph of FIG. 7, the horizontal axis indicates temperature T, and the vertical axis indicates oscillation circuit 20A using SAWRs 21A and 21B.
20B oscillation frequencies f ₁ and f ₂ . In general, the oscillation frequency of the SAWR oscillation circuit using the ST-cut crystal changes with temperature and changes in a parabolic shape as illustrated.
The temperature at which the oscillation frequency is the maximum f ₀ is referred to as the peak temperature T _0.

The peak temperature changes depending on the shape, size, relative positional relationship and the like of the IDT and the reflector that constitute the SAWR. Therefore, the SAWR 21A on the upper surface 11A and the lower surface 11
When the resonant cavity lengths L ₁ and L _{2 of the} B SAWR 21B are different, two SAWRs 21A,
The peak temperatures T ₁₀ and T ₂₀ of 21B are different. The temperature characteristic of SAWR is typically represented by the peak temperature.

The oscillation frequency f ₁ generated on the upper surface 11A of the beam 11 is expressed as a function of the surface strain ε (extension is +, contraction is −) and the temperature T as follows.

[0019]

F ₁ = f ₁₀ {1-a (T−T ₁₀ ) ² } (1-kε)

Where f ₁ is the oscillation frequency, T is the temperature, and ε
Is strain, f ₁₀ is an oscillation frequency at the apex temperature T ₁₀ , a is a constant representing temperature characteristics, and k is a constant.

Similarly, the oscillation frequency f ₂ generated on the lower surface 11B of the beam 11 is a function of the surface strain ε and the temperature T.
It is expressed as follows.

[0022]

Equation 4] _{f 2 = f 20 {1-} a (T-T 20) 2} (1-kε)

The difference Δf in oscillation frequency is expressed as follows.

[0024]

Δf = f ₁ −f ₂ = f ₁₀ {1-a (T−T ₁₀ )
² } (1-kε) -f ₂₀ {1-a (T-T ₂₀ ) ² } (1
-Kε) = _{[f 10 {1-a (T} -T 10) 2} -f 20 {1
^{_{-A (T-T 20) 2}} } ] (1-kε)

Since the bias frequency Δf _B is the difference Δf in oscillation frequency when the external force is zero, that is, when the strain ε = 0, it can be obtained by ε = 0 in the equation (5).

[0026]

Δf _B = f ₁₀ {1-a (T−T ₁₀ ) ² } −f ₂₀
{1-a (T-T ₂₀ ) ² }

Here, the equation (5) can be rewritten as follows using the bias frequency Δf _B of the equation (6).

[0028]

[Formula 7] Δf = f ₁ −f ₂ = Δf _B (1-kε)

The oscillation frequency difference Δf expressed by the equation (7) is the output signal of the oscillation circuit 20A, 20B or the external force sensor. Here, the bias frequency Δf _B , which is the coefficient of the equation (7), is modified as follows.

[0030]

Δf _B = f ₁₀ {1-a (T−T ₁₀ ) ² } −f ₂₀
{1-a (T-T 20) 2} = f 10 -f 20 -a (f 10 -f
_{20) (T-T 20)} 2 + 2af 10 (T 10 -T 20) (T-T
^{_{10) 2 -af 10 (T 10}} -T 20) 2

As is clear from the equations (7) and (8), the output signal Δf is a function of the external force, that is, the strain ε, but the resonance frequency f _{10 at the} temperature T, the apex temperatures T ₁₀ , T ₂₀ , and the apex temperature. , F _{20 as} a function.

[0032]

Conventionally, in the surface acoustic wave type external force sensor, when the bias frequency Δf _B is provided, the SAWR 21A and the lower surface 11B of the upper surface 11A of the beam 11 are provided.
The SAWRs 21B are formed to have different shapes. Therefore, a difference ΔT ₀ = (T ₁₀ −T ₂₀ ) occurs in the temperature characteristics of the two SAWRs 21A and 21B, that is, the peak temperature, and the output signal Δf changes from the peak temperature as shown in the equations (7) and (8). It was affected by the difference ΔT ₀ .

In the conventional surface acoustic wave type external force sensor, when the bias frequency Δf _B is provided, the output signal Δf changes when the temperature T changes as shown in the equations (7) and (8), and an accurate acceleration is obtained. There was a drawback that could not be done.

In view of the above point, an object of the present invention is to provide an external force sensor capable of accurately detecting acceleration without being affected by temperature change.

[0035]

According to the present invention, for example, as shown in FIG. 1, a cantilever-supported beam formed of a single crystal piezoelectric plate for converting an external force into a strain, and two beams of the beam are provided. An external force sensor having a surface acoustic wave resonator mounted on a surface for converting a strain generated in the beam into a change in resonance frequency, the first force formed on the first surface of the beam;
The pattern of the surface acoustic wave resonator and the pattern of the second surface acoustic wave resonator formed on the second surface of the beam are similar to each other.

According to the present invention, in the external force sensor, the pattern of the first surface acoustic wave resonator and the pattern of the second surface acoustic wave resonator are formed similar to each other by using the shrink technique at the time of making the photomask. It is characterized by being done.

[0037]

In general, in order to set the bias frequency Δf _B ,
Make sure that the two SAWRs 21A and 21B have different shapes.
That is, the two SAWR21A, resonant cavity length L ₁ of 21B, L ₂ is configured differently. Two SAWR
When the resonance cavity lengths L ₁ and L _{2 of} 21A and 21B are different, the temperature characteristics of the two SAWRs 21A and 21B are different. That is, the peak temperatures T ₁₀ and T ₂₀ are different. As a result, the difference Δf in oscillation frequency, which is an output signal, is affected by temperature.

However, the two SAWRs 21A, 2
The shape of 1B is different, so two SAWR21A,
Resonant cavity length L ₁ of 21B, even if the L ₂ are different, the resonance wavelength lambda _10, the resonant cavity length L ₁ with respect to lambda _20, the ratio L ₁ / lambda ₁₀ of L _2, L ₂ / lambda when ₂₀ is equal to , The temperature characteristics of the two SAWRs 21A and 21B are the same, and the peak temperatures T ₁₀ and T ₂₀ are the same. That is, the difference Δf in oscillation frequency, which is an output signal, is not affected by temperature. The resonance wavelengths λ ₁₀ and λ ₂₀ are determined by the SAWR design rule.

Therefore, the ratio L ₁ / λ ₁₀ of the resonant cavity lengths L ₁ and L _{2 to} the resonant wavelengths λ ₁₀ and λ ₂₀ is the same L ₁ / λ.
_{Under the} condition of ₁₀ = L ₂ / λ ₂₀ , two SAWRs 21A,
When the shape of 21B is set, the difference Δf in the oscillation frequency which is the output signal is not influenced by the temperature.

According to the invention, two SAWRs 21A,
21B are formed similar to each other. Thereby, the resonant cavity lengths L ₁ and L ₂ for the resonant wavelengths λ ₁₀ and λ ₂₀ are obtained.
The condition that the ratios of L ₁ / λ ₁₀ = L ₂ / λ ₂₀ are the same is satisfied, and the bias frequency Δf _B is provided.

According to the invention, two SAWR21A,
21B is formed by the photolithography method, and a similar shape can be easily obtained by the shrink method. Moreover, in the shrink method, the desired resonance frequency ratio f ₁₀ / f ₂₀ can be obtained by setting the shrink rate to a predetermined value, so that the desired bias frequency Δf _B can be easily obtained.

[0042]

First, the concept of the present invention will be described. In the surface acoustic wave type external force sensor, when the bias frequency Δf _B is provided by the conventional method, the output signal Δf becomes a function of the temperature T as shown in the equations (7) and (8). Among the respective terms on the right side of the equation (8), the third term that most affects the output signal Δf is the third term including the difference ΔT ₀ = (T ₁₀ −T ₂₀ ). Therefore, if the peak temperature difference ΔT ₀ is zero, the output signal Δf is not affected by the temperature T and is a function of only the strain ε.

However, if the SAWRs 21A and 21B on both sides of the beam 11 are formed to have different shapes as described above, the difference in temperature characteristics between the two SAWRs 21A and 21B, that is, the vertex temperature ΔT ₀ = (T ₁₀ -T _20). ) Occurs.

The peak temperature changes depending on the shape of SAWR
However, in detail, the ratio L of the resonant wavelength to the resonant cavity is₁
/ Λ_Ten, L₂/ Λ₂₀Is a function of. Resonance wavelength λ_Ten, Λ₂₀
Is determined by the design rules of SAWR21A and 21B
Maru Therefore, even if the shape of the SAWR changes, the resonance frequency
Ratio of resonant cavity to length L₁/ Λ_Ten, L₂/ Λ ₂₀
If does not change, the peak temperature does not change.

Therefore, even if the two SAWRs have different shapes in order to provide the bias frequency Δf _B , the ratio of the resonant cavity length to the resonant wavelength L ₁ /
If λ ₁₀ and L ₂ / λ ₂₀ are the same, the peak temperatures of the two SAWRs will be equal.

According to the present invention, the S of the upper surface 11A of the beam 11 is
The AWR 21A and the SAWR 21B on the lower surface 11B are formed in a similar shape to each other. As a result, the bias frequency Δf _B
And two SAWRs 21A, 2
In 1B, the ratio of the resonant cavity length to the resonant wavelength is the same L ₁ / λ ₁₀ = L ₂ / λ ₂₀ . Therefore, two S
The peak temperatures of the AWRs 21A and 21B are equal. Substituting ΔT ₀ = (T ₁₀ −T ₂₀ ) = 0 into the formula of the formula 8, the following formula is obtained.

[0047]

Δf _B = f ₁₀ −f ₂₀ −a (f ₁₀ −f ₂₀ ) (T−T ₂₀ ) ²

Since the second term on the right side is minute, it can be ignored to obtain the following approximate expression.

[0049]

[Formula 10] Δf _B = f ₁₀ −f ₂₀

By substituting this into the equation (7), the output signal Δf can be obtained.

[0051]

Δf = (f ₁₀ −f ₂₀ ) (1−kε)

An example of the external force sensor according to the present invention will be described with reference to FIG. FIG. 1 shows an example of a surface acoustic wave resonator (SAWR) according to the present invention. Similar to FIG. 6, FIG. 1A shows beam 1
1B shows the first SAWR 21A on the top surface 11A of FIG.
Shows a second SAWR 21B on the lower surface 11B of the beam 11.
The first SAWR 21A has an input interdigital transducer (IDT) 21-1A and an output interdigital transducer (IDT) 21-2A and two reflectors 21-3A, 21-4A, and a second SAWR 21A.
R21B is an input IDT 21-1B and an output IDT 21-
2B and two reflectors 21-3B and 21-4B.

According to this example, the first SAWR 21A and the second SAWR 21B are formed in similar shapes to each other.
That is, the two SAWRs 21A and 21B have different sizes but the same shape. According to this example, two S
The two SAWRs 21A and 2B shown in FIG. 6 except that the AWRs 21A and 21B are formed in a similar shape to each other.
It may be similar to 1B. Further, according to this example, the external force sensor may be the same as the external force sensor shown in FIG. 4, except for the two SAWRs 21A and 21B.

According to this example, two SAWRs 21A, 21
B is formed by a suitable method. For example, it may be manufactured by forming a predetermined pattern on both surfaces 11A and 11B of the beam 11 by a photolithography method.

Preferably two SAWRs 21A, 21
Bs are formed in a similar shape to each other by the shrink method.
According to the shrink method, the same photomask data is used, one SAWR is drawn at 100%, and the other SAWR is drawn.
WR is drawn at an appropriate reduction ratio so as to obtain a desired bias frequency. The shrink method can easily form the SAWR having the same shape as the mask pattern at a desired enlargement ratio or reduction ratio.

Next, an actual calculation example will be shown. 2 as above
When the two SAWRs 21A and 21B are formed in a similar shape, the ratio of the resonance wavelength and the resonance cavity length does not change, but the resonance wavelength and the resonance cavity length change by the same ratio.
The ratio of the resonance wavelengths of the two SAWRs 21A and 21B, that is,
The ratio of the resonance frequencies f ₁₀ and f ₂₀ is equal to the enlargement or reduction ratio of the SAWR pattern.

[0057]

F ₂₀ = f ₁₀ × r

Here, r is an enlargement ratio or a reduction ratio. The bias frequency Δf _B is obtained by substituting this into the expression of Equation 10.

[0059]

Δf _B = f ₁₀ −f ₂₀ = f ₁₀ −f ₁₀ × r = f ₁₀ (1-r)

For example, if the resonance frequency f ₁₀ of the first SAWR 21A is designed to be f ₁₀ = 400 MHz and the bias frequency Δf _B is Δf _B = 400 KHz, the second resonance frequency f ₂₀ is f ₂₀ = 399. It becomes 6 MHz, and the reduction ratio is r = 0.999. That is, 99.9
You just need to make a% shrink.

Another configuration example of the beam 11 of the external force sensor according to the present invention will be described with reference to FIG. According to this example, the beam 11 has two
It is manufactured by joining the thin beams 11-1 and 11-2. Two SAWR21s that are similar to each other
A and 21B are respectively formed on both surfaces 11A and 11B of one beam 11 formed by joining.

SAWRs 21A and 21B are two thin beams 1
It may be formed after joining 1-1 and 11-2, but is preferably formed before joining the two thin beams 11-1 and 11-2. For example, two thin beams 11-1, 11-
The two surfaces 11A and 11B may be sequentially subjected to SAWR 21A and 21B by a lithography method, and then two thin beams 11-1 and 11-2 may be joined. Thereby, the SAWRs 21A and 21B can be efficiently formed.

FIG. 3 shows another structural example of the beam 11 of the external force sensor according to the present invention. In this example, two thin beams 11-1 and 11-2 similar to those shown in FIG. 2 are used. The two thin beams 11-1 and 11-2 are joined to the support member 13 and the weight 15 with the thin beams 11-1 and 11-2 sandwiched therebetween.

The SAWRs 21A and 21B are the upper surface 11A of the upper thin beam 11-1 and the lower surface 1 of the lower thin beam 11-2.
1B, respectively.

Similarly, in the SAWRs 21A and 21B, two thin beams 11-1 and 11-2 are attached to the support member 13 and the weight 15.
Although it may be formed after the two thin beams 11-1 and 11-2 are joined together, it is preferably formed before the two thin beams 11-1 and 11-2 are joined together. For example, each surface 11A of two thin beams 11-1 and 11-2,
11B, SAWR21 by lithography method sequentially.
A, 21B and then may be joined. Thereby,
The SAWRs 21A and 21B can be efficiently formed.

Although the embodiments of the present invention have been described in detail above, those skilled in the art will understand that the present invention is not limited to the above-mentioned embodiments and various other configurations can be adopted without departing from the gist of the present invention. Easy to understand.

[0067]

According to the present invention, in the surface acoustic wave type external force sensor, even if the bias frequency Δf _B is provided, the output signal is not affected by the temperature.

According to the present invention, in the surface acoustic wave type external force sensor, there is an advantage that the temperature characteristics of the two SAWRs can be made the same by an extremely simple method.

According to the present invention, in the surface acoustic wave type external force sensor, the bias frequency Δf can be obtained by an extremely simple method.
There is an advantage that _B can be set to a desired value.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of SAWR of an external force sensor according to the present invention.

FIG. 2 is an explanatory diagram for explaining a configuration example of a beam of the external force sensor according to the present invention.

FIG. 3 is an explanatory diagram for explaining another configuration example of the beam of the external force sensor according to the present invention.

FIG. 4 is a diagram showing a configuration example of a conventional external force sensor.

FIG. 5 is an explanatory diagram for explaining a function of a bias frequency Δf _B of a conventional external force sensor.

FIG. 6 is an explanatory diagram showing an example of a SAWR of a conventional external force sensor.

FIG. 7 is an explanatory diagram illustrating a temperature characteristic of a SAWR of a conventional external force sensor.

[Explanation of symbols]

11 elastic body, beam 11-1, 11-2 beam 11A upper surface 11B lower surface 13 support member 15 weights 20A, 20B oscillation circuit 21A, 21B surface acoustic wave resonator (SAWR) 21-1, 21-1A, 21-1B input Interdigital Transducer (IDT) 21-2, 21-2A, 21-2B Output Interdigital Transducer (IDT) 21-3A, 21-3B Reflector 21-4A, 21-4B Reflector 25A, 25B Amplifier 27A, 27B Buffer Amplifier 31 Mixer 33 Filter

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] February 15, 1996

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0011

[Correction method] Change

[Correction content]

The relationship between the bias frequency Δf _B and the operating range of the external force sensor will be described with reference to FIG. FIG. 5 shows the relationship between the input acceleration A and the difference Δf between the oscillation frequencies. When the input acceleration A is in the range of -maxA to + maxA, the minimum value Δfmin of the oscillation frequency difference Δf must be set so as not to be close to zero.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideki Ouchi 2-16-46 Minami Kamata, Ota-ku, Tokyo Within Tokimec Co., Ltd.

Claims (2)

[Claims] 1. A beam, which is formed of a single crystal piezoelectric plate for converting an external force into strain and is supported by a cantilever, and a strain which is attached to two surfaces of the beam and which is generated in the beam is converted into a change in resonance frequency. And a surface acoustic wave resonator for forming a first surface acoustic wave resonator pattern formed on the first surface of the beam and a second surface formed on the second surface of the beam. An external force sensor characterized in that the pattern of the surface acoustic wave resonator of 2 is similar to each other. 2. The external force sensor according to claim 2, wherein the pattern of the first surface acoustic wave resonator and the pattern of the second surface acoustic wave resonator are similar to each other by using a shrink technique at the time of making a photomask. An external force sensor characterized by being formed on the.