Patent Description:
The torque sensor of this type comprises a first structure to which a torque is applied, a second structure from which a torque is output, and a plurality of strain generation parts that connects the first structure and the second structure, and a strain sensor is disposed in these strain generation parts (see, for example, Patent Literatures <NUM>, <NUM>, and <NUM>).

Patent literature <NUM> discloses a multiple axis load cell or controller in which axial and torsion measurements are decoupled while maximizing the outputs of both measurements. Similarly, patent literature <NUM> deals with a force measurement device having integrated force sensors to provide accurate measurements of force components. Patent literature <NUM> deals with a torque sensor dedicated to determine torque independent of various loads.

In the torque sensor, it has been difficult to set independently the sensitivity and the allowable torque (maximum torque) of the strain sensor, or the mechanical strength of the torque sensor.

Embodiments of the present invention provide a torque sensor capable of independently setting the sensitivity and the allowable torque of the strain sensor or the mechanical strength of the torque sensor.

The problem is solved by a torque sensor according to claim <NUM>.

The present invention can provide a torque sensor capable of independently setting the sensitivity and allowable torque of the strain sensor or the mechanical strength of the torque sensor.

Mode for Carrying Out the Invention Embodiments will be described hereinafter with reference to the accompanying drawings. In the drawings, the same parts are denoted by the same reference numerals.

In <FIG>, a torque sensor <NUM> comprises a first structure (first region) <NUM>, a second structure <NUM> (second region), a plurality of beams (third regions) <NUM>, a first strain generation part <NUM>, and a second strain generation part <NUM>. The first structure <NUM>, the second structure <NUM>, the plurality of beams <NUM>, the first strain generation part <NUM>, and the second strain generation part <NUM> are formed of, for example, metal, but can be formed by using materials other than metal if a mechanical strength can be sufficiently obtained to the applied torque.

The first structure <NUM> to which the torque is applied and the second structure <NUM> from which the torque is output have an annular shape. A diameter of the second structure <NUM> is smaller than a diameter of the first structure <NUM>. The second structure <NUM> is disposed concentrically with the first structure <NUM>, and the first structure <NUM> and the second structure <NUM> are connected by the plurality of beams <NUM> radially arranged, the first strain generation part <NUM>, and the second strain generation part <NUM>. In addition, the second structure <NUM> also includes a hollow portion 12a.

The first strain generation part <NUM> and the second strain generation part <NUM> are arranged at positions symmetrical with respect to the centers of the first structure <NUM> and the second structure <NUM> (the center of action of the torque).

As shown in <FIG>, the first strain generation part <NUM> comprises a first protrusion 14a, a second protrusion 14b, and a first strain body <NUM>. The first protrusion 14a protrudes from the first structure <NUM>, and the second protrusion 14b protrudes from the second structure <NUM>. A first gap is provided between the first protrusion 14a and the second protrusion 14b, and the first protrusion 14a and the second protrusion 14b are connected by the first strain body <NUM>. The first strain body <NUM> comprises, for example, a plurality of strain sensors (hereinafter, referred to as strain gauges) as resistors to be described later.

The second strain generation part <NUM> comprises a third protrusion 15a, a fourth protrusion 15b, and a second strain body <NUM>. The third protrusion 15a protrudes from the first structure <NUM>, and the fourth protrusion 15b protrudes from the second structure <NUM>. A second gap is provided between the third protrusion 15a and the fourth protrusion 15b, and the third protrusion 15a and the fourth protrusion 15b are connected by the second strain body <NUM>. The second strain body <NUM> comprises, for example, a plurality of strain gauges as resistors to be described later.

The first structure <NUM>, the second structure <NUM>, and the beams <NUM> have a first thickness T1, and the first strain generation part <NUM> and the second strain generation part <NUM> have a second thickness T2 smaller than the first thickness T1. The substantial thicknesses (second thickness T2) for obtaining the rigidity of the first strain generation part <NUM> and the second strain generation part <NUM> correspond to the thicknesses of the first strain body <NUM> and the second strain body <NUM>, respectively. More specifically, when the first thickness T1 is, for example, <NUM>, the second thickness T1 is, for example, approximately <NUM>.

The strength of the beams <NUM> is defined by the width of the beams <NUM> if the thicknesses of the first structure <NUM> and the second structure <NUM> are assumed to be equal. A substantial rotation angle of the first structure <NUM> to the second structure <NUM> is determined based on the plurality of beams <NUM> in accordance with the torque applied to the first structure <NUM>.

In addition, strain generated in the first strain generation part <NUM> and the second strain generation part <NUM> in accordance with the rotation angle of the first structure <NUM> to the second structure <NUM> is detected by the plurality of strain gauges provided in the first strain body <NUM> and the second strain body <NUM>.

The thickness of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b are set to, for example, a third thickness T3 which is smaller than the first thickness T1 and larger than the second thickness T2. The thickness of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b to the thickness T1 of the first structure <NUM> and the second structure <NUM> is variable. The sensitivity of the torque sensor <NUM> can be adjusted by adjusting the thicknesses T1, T2, and T3.

Each of the length of the first protrusion 14a and the second protrusion 14b of the first strain generation part <NUM> and the length of the third protrusion 15a and the fourth protrusion 15b of the second strain generating part <NUM> is set to L1, and each of length L2 of a first gap provided between the first protrusion 14a and the second protrusion 14b and length L2 of a second gap provided between the third protrusion 15a and the fourth protrusion 15b of the second strain generation part <NUM> is set to be shorter than L1. Furthermore, the total length of the first protrusion 14a and the second protrusion 14b and the total length of the third protrusion 15a and the fourth protrusion 15b, that is, <NUM>×L1 are shorter than length L3 of each of the plurality of beams <NUM> (<FIG> shows only the lengths L1 and L2 on the first strain generation part <NUM> side, but L3 is not shown).

When the torque is applied to the first structure <NUM>, the amount of strain generated in the first strain generation part <NUM> and the second strain generation part <NUM> can be adjusted by adjusting these lengths L1, L2, and L3. More specifically, the length L2 of the first gap and the length L2 of the second gap are shorter than the length L1 of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b, and the first length L1 of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b is shorter than the length L3 of the plurality of beams <NUM>. For this reason, when the torque is applied to the first structure <NUM>, the amount of strain of the first strain generation part <NUM> and the second strain generation part <NUM> becomes larger than the amount of strain of the beams <NUM>. Therefore, a bridge circuit to be described later can obtain a large gain.

In addition, the allowable torque (maximum torque) and mechanical strength of the torque sensor <NUM> can be set based on, for example, the thickness and width of the first structure <NUM>, the second structure <NUM> and the plurality of beams <NUM>, independently of the first strain generation part <NUM> and the second strain generation part <NUM>.

Furthermore, the sensitivity of the torque sensor <NUM> can be set by the thickness of the first strain body <NUM> and the second strain body <NUM>.

<FIG> specifically show the first strain generation part <NUM> and the second strain generation part <NUM>. The first strain generation part <NUM> includes a first accommodation part 14c for accommodating the first strain body <NUM>, and the second strain generation part <NUM> includes a second accommodation part 15c for accommodating the second strain body <NUM>. The first accommodation part 14c positions the first strain body <NUM> with respect to the first strain generation part <NUM>, and the second accommodation part 15c positions the second strain body <NUM> with respect to the second strain generation part <NUM>. The first accommodation part 14c is constituted by a substantially frame-shaped projection provided on the first projection 14a and the second projection 14b, and the second accommodation part 15c is constituted by a substantially frame-shaped projection provided on the third protrusion 15a and the fourth protrusion 15b. The first accommodation part 14c includes a gap corresponding to the gap between the first projection 14a and the second projection 14b, and the second accommodation part 15c includes a gap corresponding to the gap between the third protrusion 15a and the fourth protrusion 15b.

As shown in <FIG>, the first strain body <NUM> and the second strain body <NUM> are accommodated in the first accommodation part 14c and the second accommodation part 15c from the upper sides of the first accommodation part 14c and the second accommodation part 15c, respectively. As shown in <FIG>, the first straining body <NUM> is fixed to the first projection 14a and the second projection 14b by, for example, welding, in a state in which the first strain body <NUM> and the second strain body <NUM> are accommodated in the first accommodation part 14c and the second accommodation part 15c, respectively. In addition, the second strain body <NUM> is fixed to the third protrusion 15a and the fourth protrusion 15b by, for example, welding. The method of fixing the first strain body <NUM> and the second strain body <NUM> is not limited to welding, but may be a method of fixing the first strain body <NUM> and the second strain body <NUM> to the first to fourth protrusions 14a to 15b with strength sufficient to the torque applied to the first strain body <NUM> and the second strain body <NUM>. Wirings (not shown) of the first strain body <NUM> and the second strain body <NUM> are covered with an insulating member <NUM> (shown in <FIG>).

<FIG> shows an example of a strain gauge <NUM> provided on the first strain body <NUM> and the second strain body <NUM>, and shows a cross-section of an end portion of the strain gage <NUM>. The strain gauge <NUM> comprises, for example, an insulating film 21a, a thin film resistor (strain sensitive film) 21b, an adhesive film 21c, a wiring 21d, an adhesive film 21e, and a glass film 21f serving as a protective film. For example, the insulating film 21a is provided on the first strain body <NUM> (second strain body <NUM>) formed of metal, and the thin film resistor 21b composed of, for example, a Cr-N resistor is provided on the insulating film 21a. The thin film resistor 21b may have a linear shape, a shape bent at plural times, etc. A wiring 21d serving as an electrode lead formed of, for example, copper (Cu) is provided on the end of the thin film resistor 21b via an adhesive film 21c. The adhesive film 21e is provided on the wiring 21d. The insulating film 21a, the thin film resistor 21b, and the adhesive film 21e are covered with the glass film 21f. The adhesive film 21c enhances the adhesion between the wiring 21d and the thin film resistor 21b, and the adhesive film 21e enhances the adhesion between the wiring 21d and the glass film 21f. The adhesive films 21c and 21e are films containing, for example, chromium (Cr). The configuration of the strain gauge <NUM> is not limited to this.

Each of the first strain body <NUM> and the second strain body <NUM> comprises, for example, two strain gauges <NUM> shown in <FIG>, and a bridge circuit to be described later is constituted by four strain gauges <NUM>.

<FIG> shows the relationship between the torque sensor <NUM> and, for example, a speed reducer <NUM> provided at one of the joints of the robot. The first structure <NUM> of the torque sensor <NUM> is attached to the speed reducer <NUM> by bolts 31a, 31b, 31c, and 31d. The speed reducer <NUM> is connected to a motor (not shown). The insulating member <NUM> is attached to the second structure <NUM> of the torque sensor <NUM> by the bolts 31e and 31f. The insulating member <NUM> covers lead wirings of a plurality of strain gauges <NUM> (not shown). The insulating member <NUM>, the first strain generation part <NUM>, and the second strain generation part <NUM> are covered with a lid <NUM>. The lid <NUM> is attached to the second structure <NUM> by bolts <NUM> and <NUM>. Furthermore, the second structure <NUM> is attached to, for example, the other of joints of a robot (not shown).

<FIG> show the operation of the torque sensor <NUM>, and <FIG> shows a case where the torque is applied to the first structure <NUM>, <FIG> shows a case where a thrust force is applied to the first structure <NUM> in the X-axis direction in the figure, and <FIG> shows a case where a thrust force is applied to the first structure <NUM> in the Y-axis direction in the figure.

As shown to <FIG>, when the torque is applied to the first structure <NUM>, the plurality of beams <NUM>, the first strain generation part <NUM> and the second strain generation part <NUM> are elastically deformed, and the first structure <NUM> is pivoted relative to the second structure <NUM>. The balance of a bridge circuit to be described later is lost and the torque is detected in accordance with the elastic deformation of the first strain generation part <NUM> and the second strain generation part <NUM>.

As shown in <FIG>, when the thrust force is applied to the first structure <NUM> in the X-axis direction, the plurality of beams <NUM>, the first strain generation part <NUM> and the second strain generation part <NUM> are elastically deformed, and the first structure <NUM> is moved in the X-axis direction with respect to the second structure <NUM>. The balance of the bridge circuit is lost due to the elastic deformation of the first strain generation part <NUM> and the second strain generation part <NUM>. As described later, however, the torque and the thrust force are not detected.

As shown in <FIG>, when the thrust force is applied to the first structure <NUM> in the Y-axis direction shown in the figure, the plurality of beams <NUM>, the first strain generation part <NUM>, and the second strain generation part <NUM> are elastically deformed, and the first structure <NUM> is moved in the Y-axis direction with respect to the second structure <NUM>. The balance of the bridge circuit is lost due to the elastic deformation of the first strain generation part <NUM> and the second strain generation part <NUM>. As described later, however, the torque and the thrust force are not detected.

<FIG> schematically shows a bridge circuit <NUM> provided in the present torque sensor <NUM>. As described above, each of the first strain body <NUM> of the first strain generation part <NUM> and the second strain body <NUM> of the second strain generation part <NUM> comprises two strain gauges <NUM>. More specifically, the first strain body <NUM> comprises strain gauges <NUM>-<NUM> and <NUM>-<NUM>, and the second strain body <NUM> comprises strain gauges <NUM>-<NUM> and <NUM>-<NUM>. The first strain body <NUM> and the second strain body <NUM> are arranged symmetrically with respect to the centers of the first structure <NUM> and the second structure <NUM>, and the strain gauges <NUM>-<NUM> and <NUM>-<NUM> and the strain gauges <NUM>-<NUM> and <NUM>-<NUM> are also arranged symmetrically with respect to the centers of the first structure <NUM> and the second structure <NUM>.

In the bridge circuit <NUM>, the strain gauges <NUM>-<NUM> and <NUM>-<NUM> are connected in series, and the strain gauges <NUM>-<NUM> and <NUM>-<NUM> are connected in series. The strain gauges <NUM>-<NUM> and <NUM>-<NUM> connected in series are connected in parallel to the strain gauges <NUM>-<NUM> and <NUM>-<NUM> connected in series. A power source Vo, for example, 5V, is supplied to a connection point of the strain gauges <NUM>-<NUM> and <NUM>-<NUM>, and a connection point of the strain gauges <NUM>-<NUM> and <NUM>-<NUM> is, for example, grounded. An output voltage Vout+ is output from a connection point of the strain gauges <NUM>-<NUM> and <NUM>-<NUM>, and an output voltage Vout- is output from a connection point of the strain gauges <NUM>-<NUM> and <NUM>-<NUM>. An output voltage Vout of the torque sensor <NUM> represented by equation (<NUM>) is obtained from the output voltage Vout+ and the output voltage Vout-. <MAT> where.

<FIG> shows the variation in resistance value of the bridge circuit <NUM> in a case where the torque is applied to the torque sensor <NUM> as shown in <FIG>, and <FIG> shows the variation in resistance value of the bridge circuit <NUM> in a case where, for example, the thrust force in the X-axis direction is applied to the torque sensor <NUM> as shown in <FIG>. In <FIG>, ΔR is the value of variation in the resistance.

<FIG> shows results of obtaining the output voltage Vout of the torque sensor <NUM> under different conditions (<NUM>) to (<NUM>) from the equation (<NUM>).

In <FIG>, R·(<NUM>+α-ΔT) indicates the resistance value at the time when the temperature coefficient of resistance is α and the temperature variation is ΔT.

Under each of the conditions represented in (<NUM>), (<NUM>), (<NUM>), and (<NUM>), the output voltage Vout of the torque sensor <NUM> is 0V. That is, when the thrust force is applied to the first structure <NUM> and the second structure <NUM>, and/or when a temperature variation is applied to the strain gauges <NUM>-<NUM> and <NUM>-<NUM>, the thrust force and the temperature variation are canceled and each output voltage Vout of the torque sensor <NUM> is 0V.

In addition, when the torque is applied to the torque sensor <NUM> represented in (<NUM>), and when the torque is applied to the torque sensor <NUM> represented in (<NUM>) and the temperature variation is given to the strain gauges <NUM>-<NUM> and <NUM>-<NUM>, -ΔR/R·Vo is output as the output voltage Vout of the torque sensor <NUM>. The output voltage Vout is a value which does not include temperature coefficient α or temperature change ΔT of the resistance. Therefore, the torque sensor <NUM> can offset the thrust force and the temperature variation and detect only the torque.

According to the present embodiments, the first structure <NUM> and the second structure <NUM> are connected by the plurality of beams <NUM> and, furthermore, the first structure <NUM> and the second structure <NUM> are connected by the first strain generation part <NUM> and the second strain generation part <NUM>. The thickness T1 of the plurality of beams <NUM> is set to be larger than the substantial thickness (thickness of the first strain body <NUM> and second strain body <NUM>) T2 for obtaining the rigidity of the first strain generation part <NUM> and the second strain generation part <NUM>. For this reason, the allowable torque of the torque sensor <NUM> and the mechanical strength of the torque sensor <NUM> are defined by the first structure <NUM>, the second structure <NUM>, and the beams <NUM>. Therefore, the allowable torque of the torque sensor <NUM> and the mechanical force of the torque sensor <NUM> can be freely set as needed by changing the thickness T1 of the first structure <NUM>, the second structure <NUM> and the beams <NUM>, or changing the number of the beams <NUM>.

In addition, the first strain generation part <NUM> is configured by the first protrusion 14a and the second protrusion 14b provided at the first structure <NUM> and the second structure body <NUM>, respectively, and the first strain generation part <NUM> including the strain gauges <NUM>-<NUM> and <NUM>-<NUM> which connect the first protrusion 14a and the second protrusion 14b, and the second strain generation part <NUM> is configured by the third protrusion 15a and the fourth protrusion 15b provided at the first structure <NUM> and the second structure body <NUM>, respectively, and the second strain generation part <NUM> provided with the strain gauges <NUM>-<NUM> and <NUM>-<NUM> which connect the third protrusion 15a and the fourth protrusion 15b. The first strain body <NUM> and the second strain body <NUM> are independent of the first structure <NUM>, the second structure <NUM>, the plurality of beams <NUM>, the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b. For this reason, the size including the shape, thickness and/or width of the first strain body <NUM> and the second strain body <NUM> can be set freely.

Furthermore, the first strain body <NUM> and the second strain body <NUM> are independent of the first structure <NUM>, the second structure <NUM>, the plurality of beams <NUM>, the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b. For this reason, the sensitivity and size of the strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> provided on the first strain body <NUM> and the second strain body <NUM> can be set in accordance with the size of the first strain body <NUM> and the second strain body <NUM>. Therefore, the sensitivity and the size of the strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> can be set easily.

In addition, the length L1 of the first gap provided between the first protrusion 14a and the second protrusion 14b of the first strain generation part <NUM>, and the length L1 of the second gap provided between the third protrusion 15a and the fourth protrusion 15b of the second strain generation part <NUM> are shorter than the length L2 of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a, and the fourth protrusion 15b, and the length L2 of the first protrusion 14a, the second protrusion 14b, the third protrusion 15a and the fourth protrusion 15b is shorter than the length L3 of the plurality of beams <NUM>. For this reason, the first strain generating part <NUM> and the second strain generating part <NUM> can generate strain larger than the strain of the beams <NUM>.

Moreover, since the first straining body <NUM> and the second straining body <NUM> can generate a large strain as compared with the beams <NUM>, the gains of the strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> provided at the first straining body <NUM> and the second straining body <NUM> can be made larger. Therefore, resistance to noise and detection accuracy of torque can be improved.

In addition, the first strain body <NUM> is configured separately from the first protrusion 14a and the second protrusion 14b, and the second strain body <NUM> is configured separately from the third protrusion 15a and the fourth protrusion 15b. For this reason, the fine strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> can easily be formed on the first strain body <NUM> and the second strain body <NUM>.

Furthermore, the torque sensor <NUM> can be configured by attaching the first strain body <NUM> provided with the strain gauges <NUM>-<NUM> and <NUM>-<NUM> to the first protrusion 14a and the second protrusion 14b of the first strain generation part <NUM>, and attaching the second strain body <NUM> provided with the strain gauges <NUM>-<NUM> and <NUM>-<NUM> to the third protrusion 15a and the fourth protrusion 15b of the second strain generation part <NUM>. For this reason, the torque sensor <NUM> can be manufactured easily.

Furthermore, the first strain generation part <NUM> provided with the first strain body <NUM> and the second strain generation part <NUM> provided with the second strain body <NUM> are arranged at positions symmetrical with respect to the centers of the first structure <NUM> and the second structure <NUM>. For this reason, the thrust force can be offset and the only torque can be detected.

Moreover, a pair of strain gauges <NUM>-<NUM> and <NUM>-<NUM> are provided at the first strain body <NUM>, and a pair of strain gauges <NUM>-<NUM> and <NUM>-<NUM> are provided at the second strain body <NUM>, and a bridge circuit <NUM> is composed of strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM>. For this reason, the influence of the temperature coefficient of the strain gauges <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> can be offset.

In addition, the second structure <NUM> includes a hollow portion 12a in the first structure <NUM> and the second structure <NUM> arranged concentrically. For this reason, a plurality of strain gauge wirings, and wirings necessary for control of a robot can be passed through the hollow portion 12a, and space can be used effectively.

In the present embodiment, the first structure <NUM> and the second structure <NUM> are arranged concentrically, and the first structure <NUM> and the second structure <NUM> are connected by the plurality of beams <NUM>. However, the present invention is not limited to this, but can employ the following configuration.

For example, the first structure and the second structure are configured linearly, and the first structure and the second structure are arranged in parallel. The first structure and the second structure are connected by the plurality of beams. Furthermore, a first sensor unit having a strain body provided with a resistor, and a second sensor unit having the same configuration as the first sensor unit are disposed at central portions in the longitudinal direction of the first structure and the second structure, and the first structure and the second structure are connected by the first sensor unit and the second sensor unit. The first sensor unit and the second sensor unit are arranged at positions where the central portions in the longitudinal direction of the second structure of the first sensor unit and the second structure of the second sensor unit are located at an equal distance from the action center of the torque, and the first sensor unit and the second sensor unit are parallel to each other. That is, the strain body of the first sensor unit and the strain body of the second sensor unit are arranged at symmetrical positions with respect to the action center of the torque. Also in this configuration, the same effects as the above embodiment can be obtained.

The torque sensor according to the embodiments of the present invention can be applied to, for example, a joint of a robot arm.

Claim 1:
A torque sensor (<NUM>) equipped with a first region (<NUM>), a second region (<NUM>), and a plurality of third regions (<NUM>) connecting the first region (<NUM>) and the second region (<NUM>), and having a torque to be measured transmitted between the first region (<NUM>) and the second region (<NUM>) via the third region (<NUM>), the torque sensor (<NUM>) comprising:
a first strain generation part (<NUM>) provided between the first region (<NUM>) and the second region (<NUM>) and provided with a first resistor; and
a second strain generation part (<NUM>) provided at a position separate from the first strain generation part (<NUM>), between the first region (<NUM>) and the second region (<NUM>), and provided with a second resistor, wherein,
the first strain generation part (<NUM>) comprises a first protrusion (14a) protruding from the first region (<NUM>), a second protrusion (14b) protruding from the second region (<NUM>), and a first strain body (<NUM>) provided with the first resistor connecting the first protrusion (14a) and the second protrusion (14b); and
the second strain generation part (<NUM>) comprises a third protrusion (15a) protruding from the first region (<NUM>), a fourth protrusion (15b) protruding from the second region (<NUM>), and a second strain body (<NUM>) provided with the second resistor connecting the third protrusion (15a) and the fourth protrusion (15b), wherein
a first gap is provided between the first protrusion (14a) and the second protrusion (14b), and
a second gap is provided between the third protrusion 15a and the fourth protrusion 15b, characterized in that
a length of the first gap is shorter than a length of the first protrusion (14a) and the second protrusion (14b) and a length of the second gap is shorter than a length of the third protrusion (15a) and the fourth protrusion (15b).