JPH0748046B2 - Vibrating gyro - Google Patents

Description

The present invention relates to a vibrating gyro that detects Coriolis force for the purpose of detecting angular velocity.

[Conventional technology]

As a conventional vibration gyro of this type, for example, there is a piezoelectric type as shown in FIG. 21, which has a three-dimensional coordinate system in which each surface of the fixing means 4 orthogonal to the Y axis is Two vibrators 5 made of a piezoelectric material, such as a bimorph, a unimorph and the like, are fixed in a tuning fork shape, and the free ends of the respective vibrators 5 are provided with respective detection means 6 also made of a piezoelectric material. Each wide surface is a transducer 5
It is configured by fixing in a state of facing in the direction orthogonal to that.

When using such a vibration gyro, first,
An alternating voltage is applied to the vibrator 5 to cause the vibrator 5 to vibrate symmetrically in the direction of the solid arrow (Y-axis direction). As a method of producing such symmetrical vibration, in addition to a method of applying an AC voltage to both vibrators 5, an AC voltage is applied to only one vibrator 5 and the other vibrator 5 is used as a vibration monitor. There is a method of stabilizing the vibration by controlling the vibration state. In any of these methods, in order to increase the Coriolis force described later, the vibrator 5 is vibrated in the resonance state to increase the vibration amplitude. I have decided.

Then, under the vibrating state of the vibrator 5, the vibrating gyro is rotated about the Z axis at an angular velocity ω so that the detecting means 6 bends the vibrating gyro in the direction of the broken line arrow (X-axis direction). A Coriolis force Fc that acts is generated, and as a result, a voltage is generated between the electrodes provided in the detection means 6.

Here, since this generated voltage is proportional to the magnitude of the Coriolis force Fc, the voltage corresponding to the magnitude of the angular velocity ω can be obtained by measuring the generated voltage.

Incidentally, in general, the fixing means 4 is provided with a support rod 7 to support the entire apparatus to improve the efficiency.

[Problems to be Solved by the Invention] However, in such a conventional technique, since the detection unit 6 is connected to the tip of the vibrator 5, the size of the device is increased, and The wiring for supplying an alternating current, the wiring for taking out a signal voltage from the detection means 6, and the like are complicated, and in particular, the wiring for the detection means 6 has a great deal of difficulty in routing the wire rod. It was

That is, the detection means 6 is always under vibration having an amplitude of approximately several μm to 100 μm, and the mass and elastic modulus of the wire rod for extracting the signal voltage, and further the deformation state and the like are mainly Since it has a great influence on the vibration of the vibrator 5 and causes a change in the detection sensitivity, the wire is adhered to the side surface 5 ′ of the vibrator 5 and extended to the vicinity of the fixing means 4 with small vibration. Means for pulling out from there to a predetermined connection terminal, extending the predetermined connection terminal to a position in the vicinity of the detection means 6, and shortening the length of the wire rod to reduce the influence of the wire rod. Has been taken.

However, according to this, a significant reduction in the manufacturing work efficiency of the vibration gyro was inevitable.

On the other hand, when the wide surface of the vibrator 5 and the wide surface of the detecting means 6 are not exactly orthogonal to each other, a vibration component in the Y-axis direction leaks into the detection signal of the detecting means 6. At the same time, the detection accuracy itself decreases. By the way, as shown in the figure, under the structure in which the end face of the transducer 5 and the end face of the detection means 6 are directly connected, it is possible to obtain sufficient connection accuracy by simply fixing them with an adhesive. Absent.

Therefore, there has been proposed a method of connecting the vibrator 5 and the detection means 6 via a connecting member 8 as shown in FIG.
According to the method using the connecting member 8, a part of each electrode of the vibrator 5 and the detecting means 6 is provided on the connecting member 8 and adhered to each of the surfaces facing in the directions orthogonal to each other. The vibrator 5 and the detection means 6 can be connected relatively easily with a high perpendicularity. However, in this case, since the vibrator bonding surface and the detecting means bonding surface of the connecting member 8 are oriented in directions orthogonal to each other, the bonding of the vibrator 5 and the detecting means 6 to the connecting member 8 is performed. In order to carry out simultaneously, each of the vibrator 5 and the detecting means 6 is connected to the connecting member 8 until the adhesive is cured.
In addition to the complicated jig structure required for accurate positioning and holding under a predetermined relative relationship, the jig becomes large and the workability deteriorates, and such bonding work is performed in two steps. When the steps are performed separately, the work man-hours increase significantly.

[Background technology]

In general, when a piezoelectric bimorph element whose one end is cantilevered is flexed by applying a force F as shown in FIG. 23, taking the series type bimorph 13 as an example, approximately _{= (3g 31 · l · F} ) / (2t · w) g 31: voltage output factor l: length t: thickness w: generating a voltage V represented by width. On the other hand, as shown in FIG. 24, while polarizing in the direction shown by the white arrow,
When the piezoelectric material 14 having upper and lower surfaces provided with electrodes (not shown) and acting as a slip oscillator is subjected to shear deformation by applying force F, similarly, V ′ = (t ′ · g ₁₅ · F) / ( l ′ · w ′) g ₁₅ : voltage output coefficient l ′: length t ′: thickness w ′: width to generate a voltage V ′ represented by:

Here, taking a piezoelectric ceramic material typified by lead zirconate titanate (PZT) and the like as an example, the voltage output coefficients g ₃₁ and g ₁₅ are about g ₁₅ / g ₃₁ 3 in terms of the ratio and are added. Although the slip oscillator is more advantageous in terms of voltage output coefficient than the force, the thickness t,
Considering t ', width w, w', and length l, l ', it is clear from the above two equations that the slip oscillator is not always advantageous as the output voltage.

However, as shown in FIG. 25, one end of the driving vibrator 5 is connected to the fixing means 4, and an AC voltage is applied to the vibrator 5 to vibrate in the Y-axis direction while the fixing means 4 is moved to Z direction. The Coriolis force generated when rotating at an angular velocity ω around the axis
Fc applies a torsional moment M indicated by a solid arrow to the connecting portion 9 of the fixing means 4 with the vibrator 5 in addition to the shear stress indicated by a dashed arrow to generate a force in the direction of twisting the fixing means 4. Let Therefore, by positively utilizing this twisting moment M to measure the Coriolis force Fc, it is possible to improve the workability in manufacturing the vibration gyro and to realize its miniaturization. Moreover, high measurement accuracy can be brought about.

The present invention was made with attention to such points,
The present invention provides a novel vibration gyro that has never existed before.

More specifically, as shown in FIG. 26, the base 10
And the arm 12 as shown in FIG. 27, the long side length is a,
When a force F in the X-axis direction is applied to the tip of the arm 12 when the intermediate member 11 having a rectangular contour with a short side length of b is connected, the shear stress τ = F / a is applied to the intermediate member 11.・ Along with b, the torsional moment with the Y axis as the axis of rotation M = F ・ L '
Acts, and this torsional moment M causes the intermediate member
At 11, a torsional shear stress τ'is generated centering on the intersection O of the coordinate axes X, Y, Z. This torsional shear stress τ ′ becomes maximum at the midpoint A of the long side of the intermediate member 11 shown in FIG. 27,
The value is a constant determined by τ′max = F · L ′ / α · a · b ² α: the ratio a / b of the lengths of the long side and the short side.

Since τ'max has a relative relationship of τ'max / τ = L '/ a · b with respect to τ, it is extremely effective to measure the torsional shear stress τ'.

In addition, here, in order to facilitate the explanation, the intermediate member
Although 11 has a rectangular outline shape of a> b, if a <b in FIG. 27, the maximum torsional shear stress acts on the midpoint B, and if a = b, both midpoints are applied. The maximum torsional shear stress acts on A and B. By the way, with the torsional shear stress τ ′, the distribution becomes zero at the center O and the four corners of the intermediate member 11 and becomes high at the midpoint of the side.

As described above, the intermediate member 11 is subjected to not only the shear stress τ but also the torsional shear stress τ ′, so that
As shown in FIG. 28, 11 is the shear strain γ due to the shear stress τ.
And a shear strain γ ′ due to the torsional shear stress τ ′.

Here, regarding the relationship between the tensile and shear stresses and the electric displacement, when the electric displacement D generated when the stress T and the electric field E are applied to the piezoelectric material is expressed by an equation, If lead zirconate titanate is used as an example of the piezoelectric material, the electric displacement when only the stress T is applied is It is represented by. In this case, the applied stresses T _{1 to} T ₆
Is acting in the direction shown in FIG. 29 and FIG. 30, and the piezoelectric material is assumed to be polarized in the direction of the third axis as indicated by the white arrow.

From the above, when the electrode is provided on the surface orthogonal to the first axis, the electric displacement D _{1 in} the first axis direction due to stress is D ₁ = d ₁₅ · T ₅ , which is It becomes a sliding oscillator that generates electrical displacement with respect to the surrounding shear stress T ₅ .

Next, as shown in FIG. 31, planes x ₁ and x ₂ orthogonal to the X axis
And the planes y ₁ and y ₂ orthogonal to the Y axis and the plane z orthogonal to the Z axis.
It occurs when a torsional shearing stress τ'acts in the plane orthogonal to the Y-axis due to the torsional moment M about the Y-axis passing through the center of the rectangular parallelepiped piezoelectric material 14 formed by ₁ and z _2. Consider electrical displacement.

When the torsional shearing stress τ ′ is generated in the plane orthogonal to the Y-axis by the torsional moment M, the orthogonal shearing stress having the same magnitude and the opposite moment coexists on the surface orthogonal to the torsional shearing stress. .

That is, acting on the point P ₂ on the plane y ₂ of the piezoelectric material 14 is polarized in the Y-axis direction, the direction of the shear stress Tau'p ₂ torsion indicated by a dotted arrow in the figure an angle of β with respect to the X axis Then,
On the plane y _1, the point P ₁ against the point P _2, the torsional shear stress Tau'p ₂ and equal and direction acts the opposite torsional shear stress, connecting the points P _1, P ₂ Region, torsional shear stress τ′p
_The orthogonal shear stress τ'p ₂ as shown by the solid arrow in the figure acts on the plane orthogonal to _{2 or} in other words, the plane that includes the Y axis and makes an angle of (β + π / 2) with the X axis. .

Therefore, in the regions P ₁ and P ₂ , the Y axis is orthogonal to the X axis and the angle α
The electric displacement in the direction of is Dp = d ₁₅ τ'p ₂ cos (α-β). Further, at a point P ₂ ′ that is axially symmetrical to the point P ₂ with respect to the Y-axis, a torsional shear stress acting on P ₂ is equal in magnitude and opposite in direction, and
The 'P ₁ against the' P _2, 'since the torsional shear stress shear stress and magnitude equal direction opposite that acts on acts respectively, the point P _1' the point P _2, the point P ₂ ' the versa orthogonal shear stress Tau'p ₂ acts in the region connecting the regions P ₁ ', P _2'
The electric displacement in the direction orthogonal to the Y-axis at an angle of α with the X-axis is Dp ′ = d ₁₅ τ′p ₂ cos (α−β−π) = −d ₁₅ τ′p ₂ cos (Α-β).

Thus, since electric displacements having the same magnitude and different polarities are generated in the regions P ₁ and P ₂ and the regions P ₁ ′ and P ₂ ′, for example, the angle α is zero, that is, the plane x ₁ and the plane Even if electrodes are provided on the entire surface of each x ₂ , the generated charges cancel each other out, so that no voltage based on the action of the torsion moment M is generated between the electrodes. Even when the intermediate member 11 is used as a sliding oscillator having such a configuration, the torsional shear stress τ ′ is not detected and only the simple shear stress τ is detected as a voltage, so the detected stress is small and the detection sensitivity is high. Can't get

Therefore, according to the present invention, the configuration in which the specific electric displacement is preferentially taken out enables the detection of the torsion moment M with high sensitivity, and is small in size and highly vibrating with high accuracy. I will provide a.

[Means for Solving the Problems]

The vibrating gyroscope of the present invention, in particular, polarizes the piezoelectric material in the Y-axis direction of the three-dimensional coordinate system, and forms electrodes on at least two opposing surfaces of the piezoelectric material parallel to the Y-axis. The slip oscillator is arranged as at least a part of the fixing means for fixing one end of the driving oscillator, and at least one of the electrodes opposed to each other of the slide oscillator is electrically displaced by a torsion moment. The Coriolis force is detected based on the differential output between the electrodes formed on the opposing surface of the piezoelectric material so that the Coriolis force cannot be canceled out.

Here, instead of omitting a part of at least one of the electrodes, at least one of the opposing electrodes is set to Y.
It is also possible to divide it into a plurality of planes including the axis, or to divide the sliding oscillator itself into a plurality of planes including the Y axis.

Further, preferably, two vibrators are attached to one fixing means in a tuning fork shape.

[Action]

This vibration gyro detects the Coriolis force by both the simple shear stress acting on the slip oscillator and the torsion shear stress of the slip oscillator based on the torsion moment generated by the Coriolis force. The detection sensitivity can be greatly improved as compared with the case where the bending force of the detection means 6 is detected as in the technology.
This is the same in the case where at least one of the opposing electrodes is partially omitted, or in the case where at least one of the opposing electrodes or the slip oscillator itself is divided into a plurality of parts.

Further, in the vibration gyro in which the two vibrators for driving are attached to the fixing means in a tuning fork shape, the detection sensitivity can be doubled and the S / N ratio with respect to external vibration noise can be improved.

On the other hand, in this vibrating gyro, the slide oscillator is provided as at least a part of the fixing means of the driving oscillator, so that the detecting means 6 described in the prior art is not necessary and the apparatus is sufficiently miniaturized. In addition, you can
By concentrating the electrodes in the vicinity of the fixing means, it is possible to effectively eliminate the inconvenience associated with the wiring of the wire.

Moreover, here, when assembling the vibrating gyro, one end of the driving vibrator can be surface-bonded to the fixing means, which makes it extremely easy to position and accurately maintain both of them, and therefore variations in performance are caused. It is possible to supply a large amount of vibration gyro with small and stable performance at low cost.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view illustrating a slide oscillator which is a main part of the present invention.

This is to polarize the rectangular parallelepiped piezoelectric material 14 composed of the planes x ₁ , x ₂ , y ₁ , y ₂ , z ₁ , z ₂ in a direction parallel to the Y axis, and at the same time the planes x ₁ , x ₂ Electrodes 15a and 16a are formed on each of the lines as indicated by the diagonal lines in the figure, and an arbitrary point on the plane x ₁ is Q ₁ and the plane x
Over ₂ therewith points to counter when the Q _2, at least one electrode, where shown in the figures, the point of the electrode 16a Q ₂
1 shows a slip oscillator 1 in which a portion is locally missing.

Such a sliding oscillator 1 constitutes the fixing means 3 by attaching it to one surface of the base 2 orthogonal to the Y axis, as shown in FIG. 4, for example. The vibrator 5 can be connected to the fixing means 3 by fixing one end of the vibrator 5, which is the lower end in the figure, to the sliding vibrator 1 with an adhesive or the like.

Here, when an AC voltage is applied to the vibrator 5 to force it to vibrate in the Y-axis direction and the fixing means 3 is rotated around the Z-axis at an angular velocity ω to generate the Coriolis force Fc, Due to the torsion moment M, an in-plane torsional shear stress and an orthogonal shearing stress having a magnitude corresponding to this torsional shearing stress act on the piezoelectric material 14, and the electrodes 15a, 16
An electrical displacement that is not canceled by the electrical displacement generated between points Q ₁ and Q ₂ is generated between points a and is detected as a voltage between both electrodes. The configuration with high detection sensitivity and high practicality is for planes x ₁ and x ₂
In which electrodes on at least one of the surfaces are formed only on half of those surfaces, and the missing edges of the half of the electrodes are parallel to the Y axis, or a flat surface
The electrodes on at least one of the surfaces z ₁ and z ₂ are formed only on half of those surfaces, and the missing edges of the half electrodes are parallel to the Y axis. An example of the configuration is shown in FIGS.

In the configuration shown in FIG. ₂ , the electrode 16b on the plane x ₂ is replaced by the plane x
With the _two half areas are those missing edge 17b of the electrode 16b and parallel to the Y axis, also, the configuration shown in FIG. 3, the both planes x _1, x ₂ on the electrodes 15c, 16c together mutually to oppose the area of half of their plane x _1, x _2, together in which each of the missing edge 17c, 17d, and parallel to the Y axis.

5, 6 and 7 are other application examples of the sliding oscillator,
In other words, it is a perspective view showing another embodiment of the present invention.

FIG. 5 shows that the slide oscillator 1 is attached to two surfaces of the base 2 which are orthogonal to the Y-axis to form the fixing means 3, and two vibrators 5 are attached at the lower end portions of the fixing means 3. It is attached to the sliding oscillator 1 like a tuning fork, and the example shown in FIG.
The fixing means 3 is configured by sandwiching one slide vibrator 1 between the bases, and the lower ends of the two vibrators 5 are respectively arranged on the respective surfaces of the bases 2 orthogonal to the Y axis. It is fixed in a shape. In the example shown in FIG. 7, the fixing means 3 is composed of only the sliding oscillator 1.

Here, for example, in the vibration gyro shown in FIG. 5, the output voltage of the two piezoelectric materials 14 can be taken out differentially by configuring the detection circuit as illustrated in FIG. , The S / N ratio with respect to external vibration noise can be improved, and the detection sensitivity of the Coriolis force Fc can be doubled.

Further, as shown in FIG. 9, when the whole is supported by the support rod 7 attached to the fixing means 3, the forced vibration of the vibrator 5 is stabilized and the efficient operation under resonance is ensured. be able to.

When the slip oscillator 1 as shown in FIG. 3 is used, the portion where electrodes are not formed is not always necessary because the electric displacement generated there cannot be used as a signal. As shown, the portion not sandwiched by the electrodes 15c, 16c can be configured by another spacing member 18,
The spacing member 18 may be integrally formed with the base 2.

FIG. 11 is a diagram illustrating another slip oscillator, and FIG.
In the sliding oscillator shown in the figure, in order to effectively use the electric displacement generated in the portion where the electrodes 15c and 16c are not formed,
The polarization direction is the Y-axis direction, and each of the electrodes on the plane x ₁ and the plane x ₂ is divided by the punching patterns 19a and 19b parallel to the Y-axis to form the electrodes 15d.
16d and electrodes 15e and 16e are opposed to each other, and similarly, the electrode on plane z ₁ is punched out to form electrodes 15f and 15g by pattern 19c.
Then, the electrode on the plane z ₂ is removed and the electrode 16 is formed by the pattern 19d.
f and 16g are divided into electrodes 15f and 16f and electrodes 15e and
It is a mutual opposition to 16e.

At the corners 20, 21, 22, 23 of the piezoelectric material 14, adjacent electrodes are insulated from each other.

In this sliding oscillator 1, when a torsional moment M acts on the sliding oscillator 1, the electrodes 15e, 15f, 16 are placed between the opposing electrodes.
Since the electric displacements toward d and 16g have the same polarity, it is possible to increase the amount of electric charge generated by the torsion moment M by configuring a detection circuit as shown in FIG. The / N ratio can be improved. Therefore, by constructing a vibrating gyro as shown in FIGS. 4 to 7 with this slide oscillator, excellent performance can be exhibited.

It should be noted that the positions and the number of the extraction patterns are merely examples of good detection efficiency, and needless to say, can be variously changed without being limited to the illustrated examples.
Also, either plane x ₁ or plane x ₂ , or /
It is also possible to leave the electrodes on either the plane z ₁ or the plane z ₂ undivided.

FIG. 13 is a perspective view exemplifying still another sliding oscillator. This is a polarization oscillator having a Y-axis as a polarization direction, and at a coordinate origin position, it is divided into two parts on a plane orthogonal to the Z-axis. Piezoelectric material
Electrodes 15h, 15i and 16h, 16i are formed on the respective planes x ₁ and x ₂ of 14.

The slip oscillator 1 increases the amount of electric charge generated by the torsion moment M by effectively taking out the electric displacement of the opposite polarity under the circuit configuration as shown in FIGS. A good S / N ratio for noise can be achieved.

By the way, the position and angle of division of the slide oscillator 1 can be appropriately changed as required. In addition, it is necessary to perform the division in a plane orthogonal to the Y axis with a plane crossing both planes y ₁ and y ₂ .

FIG. 16 is a diagram showing an example thereof, in which the piezoelectric material 14 polarized in the Y-axis direction includes the Y-axis and the respective surfaces forming + π / 4 and −π / 4 with respect to the X-axis, It is divided into four in a plane orthogonal to the Y axis.

FIGS. 17 to 20 are views each showing an application example of the above-mentioned split slip oscillator, here the oscillator shown in FIG. 13, and excellent performance can be obtained by any of these vibration gyros. Can bring

Incidentally, in the above description, the basic shape of the piezoelectric material 14 is a rectangular parallelepiped shape for the sake of simplicity of explanation, but other shapes can achieve the intended effect of the present invention. Of course. Further, even if lead titanate, barium titanate or the like is used as the piezoelectric material, the same effect can be obtained.

〔The invention's effect〕

Thus, according to the present invention, the detection sensitivity can be greatly improved, the vibration gyro can be sufficiently miniaturized and the productivity thereof can be improved, and further, the quality can be stabilized and the accuracy can be improved. And can bring.

[Brief description of drawings]

FIGS. 1 to 3 are perspective views illustrating a slide oscillator according to the present invention, FIGS. 4 to 7 and FIGS. 9 and 10 are perspective views illustrating a vibration gyro of the present invention, and FIG. , A diagram illustrating a detection circuit, FIG. 11 is a perspective view illustrating another sliding oscillator, FIG. 12 is a perspective view illustrating another detection circuit of Coriolis force, and FIG. 14 is a perspective view illustrating another detection circuit, FIG. 16 is a perspective view illustrating an example of division of the piezoelectric material, and FIGS. Each is a perspective view illustrating a vibrating gyroscope using a split type sliding oscillator, FIG. 21 is a perspective view illustrating a conventional technique, FIG. 22 is a perspective view illustrating a conventional connecting member, and FIG. FIG. 31 is a diagram for explaining the operation of the present invention. 1 ... Slip vibrator, 2 ... base, 3 ... fixing means, 5
…… Vibrator, 14 …… Piezoelectric material, 15a to 15i, 16a to 16i ……
Electrodes, 17b to 17d ... missing edges, 19a to 19d ... punching pattern.

Claims (4)

[Claims] 1. A vibrator, wherein one end of the vibrator is fixed to a surface of a fixing means orthogonal to the Y axis in a three-dimensional coordinate system, and the vibrator is vibrated in the Y axis direction. In a vibrating gyroscope that detects the Coriolis force generated in the X-axis direction by the rotational movement of the fixing means around the Z-axis, the piezoelectric material is polarized in the Y-axis direction and parallel to the Y-axis of the piezoelectric material. A sliding oscillator formed by forming electrodes on at least two opposing surfaces, which is formed as at least a part of the fixing means, and at least one of the opposing electrodes of the sliding oscillator is twisted. In order to prevent the electric displacement due to the moment from being canceled, a part of it is omitted, and the Coriolis force is detected based on the differential output between the electrodes formed on both surfaces of the piezoelectric material. Features and Vibrating gyro to do. 2. One end of the vibrator is fixed to a plane of a fixing means orthogonal to the Y-axis in a three-dimensional coordinate system, and the vibrator is vibrated in the Y-axis direction. In a vibrating gyroscope that detects the Coriolis force generated in the X-axis direction by the rotational movement of the fixing means around the Z-axis, the piezoelectric material is polarized in the Y-axis direction and parallel to the Y-axis of the piezoelectric material. A sliding oscillator formed by forming electrodes on at least two opposing surfaces of the sliding oscillator is disposed as at least a part of the fixing means, and at least one electrode of the opposing electrodes of the sliding oscillator is A vibrating gyroscope characterized in that the Coriolis force is detected on the basis of a differential output between electrodes formed respectively on opposing surfaces of a piezoelectric material by dividing the surface into a plurality of surfaces including an axis. 3. One end of a vibrator is fixed to a plane of a fixing means orthogonal to the Y-axis in a three-dimensional coordinate system, and the vibrator is vibrated in the Y-axis direction. In a vibrating gyroscope that detects the Coriolis force generated in the X-axis direction by the rotational movement of the fixing means around the Z-axis, the piezoelectric material is polarized in the Y-axis direction and parallel to the Y-axis of the piezoelectric material. A sliding oscillator formed by forming electrodes on at least two surfaces opposed to each other, is provided as at least a part of the fixing means, and the sliding oscillator is divided into a plurality of planes including the Y axis, A vibrating gyroscope characterized in that the Coriolis force is detected based on a differential output between electrodes formed on the opposing surface of each divided body. 4. A vibrating gyroscope according to claim 1, wherein two of said vibrators are attached to a fixing means in a tuning fork shape.