Force sensor and robot hand system using force sensor

Abstract

To provide a force sensor capable of achieving a wide measurement range and miniaturization, and a robot hand system using the force sensor.SOLUTION: A vibrator member including a cubic metal material and having a through hole, a first piezoelectric element and a second piezoelectric element which are respectively mounted on a third surface and a fourth surface which do not have the through hole and are opposed to each other, and a contact which does not have the through hole and is in contact with a contact surface on which the piezoelectric elements are not mounted. An AC voltage is applied between the first piezoelectric element and the second piezoelectric element, and the amount of change in the natural frequency of the vibrator member according to the magnitude of the pressure applied to the contact surface by the contact is measured, thereby measuring the pressure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a force sensor and a robot hand system using a force sensor. [Background technology]

[0002] Currently, robotic hands are used in a variety of industries and applications. Patent Document 1 and Non-Patent Document 1 disclose a robotic hand that uses a multi-fingered hand that mimics a human hand. Robotic hands used for body care applications come into direct contact with the human body and therefore must operate accurately and safely, making it important to know the forces acting on the fingertips of the robotic hand.

[0003] Robotic hands that mimic human hands require small force sensors, measuring only a few millimeters, to detect the force applied to the fingertips. It is desirable for the measurable force to cover a wide range, from 1 gf to several thousand gf. Furthermore, during shiatsu massage, for example, large forces of tens of kgf may be applied to the fingertips. Conventional force sensors for robots widely use semiconductor strain gauges. While small and highly sensitive, they cannot withstand large loads. Piezoelectric force sensors detect the amount of load by converting the strain generated by the application of a load to a piezoelectric material into an electric charge based on the piezoelectric effect. Their high sensitivity and simple structure make them easy to miniaturize.

[0004] Non-Patent Document 1 discloses a technique for applying a force sensor using a piezoelectric element to the fingertips of a massage robot hand. Non-Patent Document 2 discloses a technique for measuring a person's pulse using a force sensor using a piezoelectric element. Force sensors using piezoelectric elements are easy to miniaturize and have a wide range of detectable loads, making them suitable for application to robot hands for body care purposes. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent Publication No. 2021-018806 [Non-patent literature]

[0006] [Non-Patent Document 1] Ryosuke Tazaki et al., Systems / Control / Information, Vol. 66, No. 2, pp. 50-55, DOI: https: / / doi.org / 10.11509 / isciesci.66.2_50 [Non-patent document 2] Tatsuya Bando et al., Proceedings of the 37th Annual Conference of the Robotics Society of Japan, Paper ID: RSJ2019AC3C3-07, 2019 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when a piezoelectric element is used as a load detection element, its output value is an electric charge and is therefore susceptible to noise. Furthermore, if a constant load is continuously applied to a piezoelectric element, the output of the piezoelectric element decreases over time. Therefore, while it is suitable for detecting dynamically changing loads, it cannot detect static loads. Furthermore, if a load is applied directly to a piezoelectric element, the piezoelectric element may be damaged if a large load is applied, or it may not be able to demonstrate its inherent piezoelectric properties if a stress greater than the maximum induced strain is applied.

[0008] The present invention has been made in consideration of the above points, and aims to provide a force sensor that enables a wide measurement range and miniaturization, and a robot hand system that uses a force sensor. [Means for solving the problem]

[0009] A first aspect of the present invention is a force sensor comprising: a cubic-shaped vibrator member made of a metal material and having a through hole that passes vertically from a first surface to a second surface opposite the first surface; a first piezoelectric element and a second piezoelectric element that do not have a through hole and are attached to a third surface and a fourth surface that face each other, respectively, of the vibrator member; and a contactor that comes into contact with the contact surface, which is a fifth surface that does not have a through hole and has no piezoelectric elements attached thereto, and measures pressure by applying an AC voltage between the first piezoelectric element and the second piezoelectric element and measuring the amount of change in the natural frequency of the vibrator member that corresponds to the magnitude of the pressure that the contactor applies to the contact surface.

[0010] A second aspect of the present invention is a force sensor comprising: a vibrator member made of a cubic metal material and having a through hole that passes perpendicularly from a first surface to a second surface opposite the first surface; a first piezoelectric element and a second piezoelectric element that do not have the through hole and are attached to a third surface and a fourth surface that face each other; and a plurality of contactors, one in contact with each of the plurality of contact surfaces, with two or more of the first surface, the second surface, the third surface, the fourth surface, and the fifth surface of the vibrator member as contact surfaces; and the pressure sensor measures pressure by applying an AC voltage between the first piezoelectric element and the second piezoelectric element and measuring the amount of change in the natural frequency of the vibrator member in response to the magnitude of pressure applied to the contact surfaces with which the plurality of contactors are in contact.

[0011] In the first or second aspect of the present invention, the contact may be a sphere, a spherical cap, or a semicircular shape.

[0012] In the first or second aspect of the present invention, the contactor may make elastic contact with the contact surface by line contact or point contact.

[0013] In the first or second aspect of the present invention, the portion of the contactor that comes into contact with the contact surface may have a spherical shape.

[0014] In the first or second aspect of the present invention, the natural frequency may be obtained by configuring a self-oscillating circuit using a first piezoelectric element for excitation and a second piezoelectric element for detection, and measuring the oscillation frequency of the self-oscillating circuit.

[0015] In the first or second aspect of the present invention, the natural frequency may be obtained by measuring the anti-resonance frequency in the impedance frequency characteristics of the first piezoelectric element and the second piezoelectric element.

[0016] In the first or second aspect of the present invention, the contact may be made of a material having a Young's modulus equal to or greater than that of the metal material that constitutes the vibrator member.

[0017] In the first or second aspect of the present invention, the magnitude of pressure W and the change in natural frequency Δf satisfy the relationship of the following formula (1), and the magnitude of pressure may be calculated using formula (1).

[0018]

number

[0019] (where a and b are constants.)

[0020] In the third aspect of the present invention, the contact surfaces of two or three force sensors of the first aspect may be arranged so as to be perpendicular to each other and so as to be in contact with one common contactor that is in contact with the contact surfaces of the two or three force sensors.

[0021] In a third aspect of the present invention, the force sensor is configured such that two or three vibrator members of the force sensor according to the first aspect are made of different types of metal.

[0022] A fourth aspect of the present invention is a robot hand system, which may include the force sensor according to the first or second aspect and / or the force sensor according to the third aspect at the fingertips. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a force sensor that allows for a wide measurement range and miniaturization, and a robot hand system that uses a force sensor. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a perspective view showing the structure of an example of a composite vibrator configured from a vibrator member and a piezoelectric element used in the force sensor according to the first embodiment. FIG. [Figure 2] 3 is a schematic diagram showing an example in which a voltage supply device is connected to a composite vibrator used in the force sensor according to the first embodiment. FIG. [Figure 3] 3A to 3C are diagrams illustrating vibration modes of a composite vibrator used in the force sensor according to the first embodiment. [Figure 4] 2 is a perspective view showing a state in which a contactor is brought into contact with the composite vibrator shown in FIG. 1. FIG. [Figure 5] 5A and 5B are graphs showing the frequency characteristics of the current I when a load is applied to the force sensor according to the embodiment, where FIG. 5A is a graph when the contact material is high carbon steel, and FIG. 5B is a graph when the contact material is silicon nitride ceramic. [Figure 6] 6A and 6B are graphs showing the measurement results of the change in resonant frequency when a load is applied to the force sensor according to the first embodiment, where FIG. 6A is a graph when the contact material is high carbon steel, and FIG. 6B is a graph when the contact material is silicon nitride ceramic. [Figure 7] 7(a) and 7(d) are graphs showing the measurement results of the amount of change in resonant frequency when a load is applied to the force sensor according to the first embodiment, where the size of the vibrator member is 1 mm square in FIG. 7(a), 2 mm square in FIG. 7(b), 3 mm square in FIG. 7(c), and 4 mm square in FIG. [Figure 8] 8(a) and 8(b) are schematic diagrams illustrating the shapes of the contacts, where FIG. 8(a) shows a long convex shape, FIG. 8(b) shows a short convex shape, FIG. 8(c) shows no convex portion, FIG. 8(d) is a perspective view of a semicircular contact, FIG. 8(e) shows a long convex shape, FIG. 8(f) shows a short convex shape, FIG. 8(g) shows no convex portion, and FIG. 8(h) is a front view of a semicircular contact. [Figure 9]9 is a graph showing the measurement results of the amount of change in resonance frequency of the force sensor when contact pieces of each shape shown in FIG. 8 are used. [Figure 10] FIG. 10 is a perspective view showing an example of a force sensor according to a second embodiment. [Figure 11] FIG. 10 is a perspective view showing an example of a force sensor according to a third embodiment. [Figure 12] 12A and 12B are graphs showing the rate of change in resonant frequency and an approximate plane when a load is applied to the force sensor according to the third embodiment, where FIG. 12A is a graph showing the results for Mode 1 and FIG. 12B is a graph showing the results for Mode 2. DETAILED DESCRIPTION OF THE INVENTION

[0025] Next, an embodiment will be described with reference to the drawings. In the description of the drawings relating to the embodiment, the same or similar parts are denoted by the same or similar reference numerals. Of course, there are parts whose relationships with each other differ between the drawings.

[0026] Furthermore, the embodiments are merely examples of devices and methods for realizing the technical idea, and the technical idea of ​​the present invention does not limit the configuration of each component to those described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0027] The force sensor of the embodiment has a simple structure, enabling a wide load measurement range and miniaturization. Furthermore, the force sensor of the embodiment achieves high load measurement sensitivity and high stability by applying a load to the composite vibrator via a spherical, hemispherical, or semicircular contact made of a metal material.

[0028] (First embodiment) The force sensor according to this embodiment will be outlined with reference to Figures 1 to 4. The force sensor according to this embodiment is made of a metal material and is made up of a composite vibrator, which is made up of a cubic vibrator member having a hole in the center of the metal body, combined with a piezoelectric element. The load applied to the composite vibrator is detected by detecting changes in the natural frequency of the composite vibrator. First, the composite vibrator used in the force sensor according to this embodiment will be described below.

[0029] Fig. 1 shows a perspective view of a composite vibrator 10 used in the force sensor according to this embodiment. The composite vibrator 10 shown in Fig. 1 is composed of a vibrator member 11, and a first piezoelectric element 12 and a second piezoelectric element 13 attached to the vibrator member 11.

[0030] The vibrator member 11 is made of a metal material and has a cubic shape, such as stainless steel or phosphor bronze.

[0031] The oscillator member 11 has a through hole 113 that penetrates the oscillator member 11 perpendicularly from a first surface 111, which is the outer peripheral surface of the cube, to a second surface 112 that faces the first surface 111, i.e., in a direction parallel to the z-axis in FIG. 1. The through hole 113 shown in FIG. 1 is cylindrical as an example, but is not limited to this and may be, for example, a square prism shape. The through hole 113 shown in FIG. 1 is cylindrical and is arranged so that the axis of the cylinder passes through the center of the oscillator member 11.

[0032] The first piezoelectric element 12 and the second piezoelectric element 13 are plate-shaped and are bonded to two opposing surfaces of the outer peripheral surface of the vibrator member 11 that do not come into contact with the through hole 113, i.e., in the example shown in Figure 1, the third surface 114 and the fourth surface 115 that faces the third surface 114, using a conductive adhesive.

[0033] The first piezoelectric element 12 and the second piezoelectric element 13 are made of a piezoelectric material having 31 modes that vibrate in a direction perpendicular to the polarization direction. Examples of piezoelectric materials include barium titanate, lead zirconate titanate (PZT), lithium niobate, lithium tantalate, quartz, and aluminum nitride. PZT is particularly suitable for use in the force sensor according to this embodiment for reasons such as its relative ease of molding into various shapes, its ability to produce materials with high piezoelectric constants, its high Curie temperature at which it loses piezoelectricity being around 310°C, its resistance to temperature changes, and its mass production. Furthermore, as will be described later, the force sensor according to this embodiment utilizes the resonant frequency of a composite vibrator, and therefore has a mechanical quality factor Q m It is desirable to use a material that has a high resistance and is prone to vibrating near the resonance frequency.

[0034] As shown in FIG. 2, electrodes 119 and 120 made of thin metal films are provided on the first piezoelectric element surface 117 and the second piezoelectric element surface 118, which are the outer surfaces of the first piezoelectric element 12 and the second piezoelectric element 13 that do not contact the vibrator member 11. A voltage supply device 15 applies an AC voltage between the thin metal films 119 and 120, i.e., to the composite vibrator 10. The applied AC voltage causes the first piezoelectric element 12 and the second piezoelectric element 13 to vibrate ultrasonically, causing the entire composite vibrator 10 to vibrate in bending. FIG. 3 shows an example of a fundamental vibration mode of the composite vibrator 10, which is generated by applying an AC voltage to the composite vibrator 10, as obtained by a simulation using FEM mode analysis. The shades of gray in FIG. 3 represent the intensity of the vibration mode.

[0035] The length of one side of the cubic shape of the vibrator member 11 constituting the composite vibrator 10 is defined as L. The third surface 114 and the fourth surface 115, to which the first piezoelectric element 12 and the second piezoelectric element 13 are bonded, are each regarded as a metal beam of length L. When a load W is applied in the vertical direction to the outer peripheral surface of the vibrator member 11 that is not in contact with the through-hole 113 and to which neither the first nor second piezoelectric element is attached (fifth surface 116 in the example shown in FIG. 1), each of the third surface 114 and the fourth surface 115 becomes a metal beam of length L' due to distortion caused by the load W. Here, L>L'.

[0036] Generally, the frequency of the lateral vibration of a metal beam is inversely proportional to the length L of the beam and proportional to the cross-sectional area of ​​the beam. Therefore, if the natural frequency of the unloaded composite vibrator 10 before a load is applied is f0, the natural frequency f when a vertical load is applied will be f>f0. Since the length L' of the beam changes according to the load W, that is, the natural frequency f also changes, the magnitude of the load W applied to the composite vibrator 10 can be estimated by finding the relationship between the load W and the amount of change in f according to the load W, Δf=f-f0.

[0037] The natural frequency of the composite vibrator 10 can be obtained by configuring a self-oscillating circuit using one of the first piezoelectric element 12 and the second piezoelectric element 13 for excitation and the other for detection, and measuring the oscillation frequency with a frequency counter. Alternatively, the natural frequency of the composite vibrator 10 can be obtained by measuring the anti-resonance frequency in the impedance frequency characteristics of the first piezoelectric element 12 and the second piezoelectric element 13 with an impedance analyzer or the like.

[0038] Fig. 4 shows a state in which the contactor 14 is in contact with the composite vibrator 10 shown in Fig. 1. The contactor 14 is not in contact with the through-hole 113 of the vibrator member 11, and is in contact with the fifth surface 116 on which neither the first piezoelectric element nor the second piezoelectric element is mounted. The contactor 14 shown in Fig. 4 is, for example, spherical.

[0039] In this embodiment, when a load is applied to the composite vibrator 10, the load is applied to the composite vibrator 10 via a contactor whose contact portion with the composite vibrator 10 is a spherical cap including a sphere, a hemisphere, or a semicircular shape. This reduces the contact area between the vibrator member 11 and the contactor 14, does not inhibit the vibration of the composite vibrator 10, and suppresses leakage of vibration energy of the vibration of the composite vibrator 10 to the outside of the vibrator member 11. As a result, it becomes possible to detect the change Δf in the natural frequency of the composite vibrator 10 with higher sensitivity. Furthermore, if the material of the contactor 14 is a material having a Young's modulus equal to or larger than that of the metal material constituting the vibrator member 11 of the composite vibrator 10, it becomes possible to more stably detect the change Δf in the natural frequency of the composite vibrator 10.

[0040] (First Experimental Example) A force sensor according to this embodiment was actually fabricated, and the change in resonance frequency when a load W was applied to the fabricated force sensor was measured. The structure of the vibrator member of the fabricated force sensor was the same as that of the vibrator member 11 shown in FIG. 1. Two types of material were used for the vibrator member 11: stainless steel and phosphor bronze, and force sensors were fabricated using each metal material. The material constants of the stainless steel used in fabrication were Young's modulus 195 GPa and density 8.0×10 3 kg / m 3 , and Poisson's ratio is 0.29. The material constants of phosphor bronze are Young's modulus 110 GPa, density 8.78×10 3 kg / m 3 , and Poisson's ratio is 0.341.

[0041] Four sizes of the vibrator member 11 were prepared: 1 mm, 2 mm, 3 mm, and 4 mm in terms of the length of one side of the cube. The through-hole 113 was cylindrical as shown in FIG. 1, with a diameter equal to half the length of one side of the cube of the vibrator member 11. The first piezoelectric element 12 and the second piezoelectric element 13 were made of PZT, with the height, depth, and thickness shown in FIG. 1 being 4 / 5, 1, and 1 / 10 of the length of one side of the cube of the vibrator member 11, respectively. The first piezoelectric element 12 and the second piezoelectric element 13 were bonded to the vibrator member 11 using a conductive adhesive. Electrodes 119 and 120 made of gold thin film were formed by sputtering on the first piezoelectric element surface 117 and the second piezoelectric element surface 118 of the first piezoelectric element 12 and the second piezoelectric element 13, respectively, which are the surfaces of the first piezoelectric element 12 and the second piezoelectric element 13 that are not in contact with the vibrator member 11. The metal material of the vibrator member 11 is cut out from a metal block by wire electric discharge machining, and the first piezoelectric element 12 and the second piezoelectric element 13 are cut out from a large PZT by dicing, so no special processing steps are required, and the sensor can be made smaller with a simple structure.

[0042] A normal load W was applied to the fabricated force sensor via the contact 14, and an AC voltage was applied to the composite vibrator 10 between the electrodes 119 and 120 by the voltage supply device 15, and impedance measurements were performed. Figure 5 shows the frequency characteristics of the current I flowing through the fabricated force sensor, obtained by the impedance measurement. The frequency at which the current I shown in Figure 5 is at its maximum is the resonant frequency of the force sensor. The load W was in the range of 0 N to 500 N. The contact 14 was spherical with a diameter of 8 mm, and two types of material were used: high carbon steel (SUJ2) and silicon nitride ceramic, and measurements were performed for each. The material constants of the high carbon steel were Young's modulus 207 GPa and density 7.8 x 10 3 kg / m 3 , and Poisson's ratio is 0.30. The material constants of silicon nitride ceramic are Young's modulus 310 GPa, density 3.2×10 3 kg / m 3 , and Poisson's ratio is 0.27.

[0043] Figure 5(a) shows the measurement results when the contactor 14 is made of high-carbon steel, and Figure 5(b) shows the measurement results when the contactor 14 is made of silicon nitride ceramic. Whether high-carbon steel or silicon nitride ceramic contactor 14 is used, the resonant frequency of the force sensor increases as the load W increases. Meanwhile, comparing the sharpness of the peak of the current I, the use of silicon nitride ceramic results in a higher sharpness than the use of high-carbon steel. This indicates that the higher the sharpness of the peak of the current I, the less the vibrational energy of the composite vibrator 10 leaks outside the composite vibrator 10. In other words, applying a load via contactor with a high Young's modulus makes it less likely for the vibrational energy of the composite vibrator 10 to leak outside, resulting in a force sensor with a high Q value.

[0044] Figure 6 shows the measurement results of the change in the resonant frequency of the force sensor when a load W is applied to the composite vibrator 10 of the fabricated force sensor. Figure 6(a) shows the measurement results when the contactor 14 is made of high-carbon steel, and Figure 6(b) shows the measurement results when the contactor 14 is made of silicon nitride ceramic. As with the measurement shown in Figure 5, the load W was applied to the composite vibrator 10 via a spherical contactor 14. Loads W were applied to the composite vibrator 10 in 5-N increments from 0 N to 500 N, and six measurements were performed at each load W. The data shown in Figure 6 are the average values ​​of the six measurements at each load W, and the error bars represent the standard deviation. Whether high-carbon steel or silicon nitride ceramic contactor 14 was used, the change in the resonant frequency of the force sensor increased with increasing load W. However, comparing the data variance, the standard deviation was smaller when using silicon nitride ceramic compared to when using high-carbon steel. In the case of high carbon steel, there is a variation of about 5% from the average value of each data, whereas in the case of silicon nitride ceramic, the variation is suppressed to about 0.6%. In other words, by applying a load via a contact 14 with a high Young's modulus, the stability of the force sensor can be improved.

[0045] Figure 7 shows the measured change in resonant frequency of the force sensor when a load W is applied to the composite vibrator 10 of the fabricated force sensor. Figure 7(a) shows the graph for a 1 mm square, Figure 7(b) shows the graph for a 2 mm square, Figure 7(c) shows the graph for a 3 mm square, and Figure 7(d) shows the graph for a 4 mm square. Force sensors were fabricated using stainless steel and phosphor bronze as the metallic material for the vibrator member 11, and measurements were taken by applying a load W to the composite vibrator 10 for each force sensor via a spherical contactor 14 made of silicon nitride ceramic. The load W was varied in 5 N increments from 0 N to 500 N for force sensors with a side length of the vibrator member 11 of 4 mm, in 5 N increments from 0 N to 400 N for force sensors with a side length of 3 mm, in 1 N increments from 0 N to 100 N for force sensors with a side length of 2 mm, and in 1 N increments from 0 N to 20 N for force sensors with a side length of 1 mm. A total of six measurements were performed, with one trial consisting of measurements performed with the load W changed from 0 N to the maximum load for each force sensor.

[0046] The amount of change in resonance frequency shown in Figure 7 represents the average value of the measurement results for six trials. It can be seen that the amount of change in resonance frequency tends to increase as the load W increases. As shown in Figure 7, the change in resonance frequency with the increase in load W is not a linear change, so the relationship between the load W and the amount of change in resonance frequency Δf was approximated by a power function shown in the following equation.

[0047]

number

[0048] where Δf [Hz] is the amount of change in resonant frequency, W [N] is the load, and a and b are constants. The approximation results are shown in Figure 7 by the solid and dashed lines. As shown in Figure 7, a good relationship was obtained between the measurement results and the approximation results for each force sensor. This means that the force sensor can estimate the load W applied to the vibrator member 11 of the force sensor from the change in resonant frequency Δf.

[0049] As shown in FIG. 7, the smaller the size of the oscillator member 11, the greater the change in resonant frequency when a load W is applied, resulting in a higher sensitivity as a force sensor. This is thought to be because the smaller the size of the oscillator member 11, the greater the strain that occurs when a load W is applied. On the other hand, the larger the size of the oscillator member 11, the higher the allowable stress for damage and deformation of the force sensor, thereby expanding the measurement range as a force sensor. Therefore, the size of the oscillator member 11 can be selected depending on the target and application of the force sensor according to this embodiment. As an example, as a force sensor to be mounted on the fingertips of a human-sized robot hand for body care purposes, a force sensor with an oscillator member 11 of 2 mm square size is suitable for measuring loads of up to about 100 N.

[0050] (Second Experimental Example) To investigate the relationship between the shape of the contactor 14 used in the force sensor according to the embodiment and the resonant frequency change of the force sensor, four different shapes of contactor 14 were prepared: a long convex shape (FIG. 8(a)), a short convex shape (FIG. 8(b)), no convex portion (FIG. 8(c)), and a semicircular shape (FIG. 8(d)). A portion of the vibrator member 11 to which the first piezoelectric element 12 was attached was removed, and a load W was applied to the removed portion via the contactor 14 of each shape. FIG. 9 shows the measurement results of the resonant frequency change of the force sensor when using the contactor 14 of each shape. Three trials were conducted for each of the four types of contactor, with one trial consisting of measurements with loads ranging from 0 N to 30 N. The resonant frequency change data shown in FIG. 9 represent the average of the three trials for each contactor. As shown in FIG. 9, the slope of the resonant frequency change with respect to the load W was larger when the semicircular contactor was used among the four types of contactor. In other words, by making the force sensor and the contactor 14 in elastic contact through line contact or point contact rather than surface contact when applying a load, it is possible to prevent the vibration of the composite vibrator 10 from being suppressed by the contactor 14, and as a result, the sensitivity of the force sensor can be increased.

[0051] (Second embodiment) The force sensor according to the first embodiment was only capable of detecting loads in the direction perpendicular to the contact surface of the vibrator member 11 that the contactor 14 contacts. The force sensor according to this embodiment uses a plurality of force sensors according to the first embodiment, and is capable of detecting loads in multiple axial directions.

[0052] FIG. 10 shows an example of a force sensor 20 according to this embodiment. The force sensor 20 shown in FIG. 10 is composed of a first composite vibrator 21, a second composite vibrator 22, and a single contactor 23. The force sensor shown in FIG. 10 is connected to an impedance analyzer 28, for example. The configurations of the first composite vibrator 21 and the second composite vibrator 22 are similar to that of the composite vibrator 10 of the force sensor according to the first embodiment. The first composite vibrator 21 and the second composite vibrator 22 are arranged so that the first contact surface 24 and the second contact surface 25, which are contact surfaces with the contactor 23 of the first composite vibrator 21 and the second composite vibrator 22, are perpendicular to each other. The contactor 23 is in contact with both the first contact surface 24 and the second contact surface 25, and each of the first composite vibrator 21 and the second composite vibrator 22 detects a component of the load W applied via the contactor 23 in a direction perpendicular to the first contact surface 24 and the second contact surface 25.

[0053] Since the axial direction of the load that the force sensor 20 can detect is determined by the angle between the first contact surface 24 and the second contact surface 25, the angle between the first contact surface 24 and the second contact surface 25 may be changed depending on the application of the force sensor 20 of this embodiment.

[0054] The force sensor 20 according to this embodiment uses two composite vibrators, but by combining three composite vibrators in the same manner as the force sensor 20, it becomes possible to detect loads in three axial directions.

[0055] In the force sensor 20 according to this embodiment, when the first oscillator member 26 of the first composite oscillator 21 and the second oscillator member 27 of the second composite oscillator 22 are made of the same metal material, if the first composite oscillator 21 and the second composite oscillator 22 are the same size, the resonant frequencies of the first composite oscillator 21 and the second composite oscillator 22 will be the same. If the first oscillator member 26 and the second oscillator member 27 are made of different metal materials, the resonant frequencies of the first composite oscillator 21 and the second composite oscillator 22 will be different from each other. Therefore, even if the resonant frequencies of the force sensor 20 are measured as a single data set using a single measuring device, the resonant frequencies can be obtained separately.

[0056] (Third embodiment) The force sensor according to the first embodiment was only capable of detecting loads in the direction perpendicular to the contact surface of the vibrator member 11 with which the contactor 14 made contact. The force sensor according to the second embodiment uses a plurality of force sensors according to the first embodiment to enable load detection in multiple axial directions. Both the force sensors according to the first and second embodiments were configured to detect loads only in the direction perpendicular to the contact surface of the vibrator member 11.

[0057] The force sensor of the present invention can measure the load W when a contactor is brought into contact with not only the surface of the vibrator member 11 that does not have a through hole and does not have a piezoelectric element attached, but also the surface that has a through hole and / or the surface that has a piezoelectric element attached, and a load is applied via the contactor to the surface that has a through hole and / or the surface that has a piezoelectric element attached.

[0058] To measure loads in two axial directions, two or more vibration modes in which the piezoelectric element expands and contracts are required. Modal frequency analysis confirmed that, in force sensor 30 according to this embodiment, multiple vibration modes that significantly expand and contract the piezoelectric element can be observed, which shows that force sensor 30 can measure loads in at least two directions.

[0059] In the first embodiment, the resonance frequency of a single vibration mode is measured, and the load is detected using the amount of change in the resonance frequency. In this embodiment, the resonance frequencies of multiple vibration modes are used, so the rate of change in the resonance frequency Δf i The load is detected using

[0060]

number

[0061] where: i f is the rate of change of the resonant frequency in vibration mode i at each load, i f0 is the resonance frequency at the reference load. Here, the reference load is F x =F y =1.

[0062] A force sensor 30 according to this embodiment is shown in Fig. 11. The force sensor 30 shown in Fig. 11 does not have a through-hole but is made of a piezoelectric element. A load Fx is applied perpendicularly to the surface where no piezoelectric element 33 is attached, and a load Fy is applied to the surface that does not have a through-hole but has a first piezoelectric element 33 attached. The force sensor 30 shown in FIG. 11 has a 10 mm × 10 mm outer periphery on the surface with the through-hole and a depth of 5 mm. The vibrator member 36 is made of a 1 mm thick aluminum alloy plate, and the opening of the through-hole 35 is a square measuring 8 mm × 8 mm. Loads Fx and Fy were applied to the force sensor 30 shown in FIG. 11 in 1 N increments over the range from 1 N to 20 N, and the resonant frequency was measured.

[0063] The relationship between the rate of change of the resonant frequency obtained by measurement and the load in the two axial directions is shown in three-dimensional space. The rate of change of the resonant frequency Δf i and the two-axial load F x、 F y The relationship between is approximated by the following formula:

[0064]

number

[0065] Here, the constants a, b, c, and d were calculated using the least squares method. i is the number of vibration modes, where i = 1, 2.

[0066] Figure 12 shows the obtained resonant frequency change rate and approximation plane. Figure 12(a) shows the results for mode 1, and Figure 12(b) shows the results for mode 2. Figures 12(a) and 12(b) also show schematic diagrams of the force sensor 30 shown in Figure 11 vibrating in mode 1 and mode 2. In mode 1, the resonant frequency change rate increases as the load Fx increases, and in mode 2, as the load Fx and load Fy increase. It can also be seen that the load direction that is most likely to change the resonant frequency differs for each mode. The equation for the approximation plane is as follows:

[0067]

number

[0068]

number

[0069] From these equations, the load Fx and the load Fy can be calculated.

[0070] (Application example) The force sensors according to the first and second embodiments are small and have a wide load measurement range, and are therefore suitable for applications such as massage robot hand systems that require a wide load measurement range in a small area such as a fingertip.

[0071] As mentioned above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description. [Explanation of symbols]

[0072] 10 Composite vibrator 11. Oscillator member 12 First piezoelectric element 13 Second piezoelectric element 14, 23 Contacts 111 Page 1 112 Side 2 113 Through Hole 114 Page 3 115 Page 4 116 Page 5 117 First piezoelectric element surface 118 Second piezoelectric element surface 119, 120 electrode 15 Voltage supply device 20, 30 Force sensor 21 First composite oscillator 22 Second composite oscillator 24 1st contact surface 25 Second contact surface 26 First vibrator member 27 Second vibrator member 28 Impedance Analyzer

Claims

1. a cubic-shaped vibrator member made of a metal material and having a through hole that passes perpendicularly from a first surface to a second surface opposite the first surface; a first piezoelectric element and a second piezoelectric element attached to a third surface and a fourth surface of the vibrator member, respectively, which do not have the through-hole and which face each other; a fifth surface that does not have the through hole and to which neither the first piezoelectric element nor the second piezoelectric element is attached is used as a contact surface; and a contactor that comes into contact with the contact surface. and applying an AC voltage between the first piezoelectric element and the second piezoelectric element, and measuring a change in the natural frequency of the vibrator member according to the magnitude of the pressure applied by the contactor to the contact surface, thereby measuring the pressure.

2. a cubic-shaped vibrator member made of a metal material and having a through hole that passes perpendicularly from a first surface to a second surface opposite the first surface; a first piezoelectric element and a second piezoelectric element attached to a third surface and a fourth surface of the vibrator member, respectively, which do not have the through-hole and which face each other; a plurality of contacts, each contacting one of the first, second, third, fourth, and fifth surfaces of the vibrator member as contact surfaces; and applying an AC voltage between the first piezoelectric element and the second piezoelectric element, and measuring a change in the natural frequency of the vibrator member according to the magnitude of pressure applied to the contact surfaces with which the plurality of contacts respectively come into contact, thereby measuring the pressure.

3. 3. The force sensor according to claim 1, wherein the contacts are spherical, spherical cap, or semicircular.

4. 3. The force sensor according to claim 1, wherein the contactor is in elastic contact with the contact surface by line contact or point contact.

5. 3. The force sensor according to claim 1, wherein the contactor has a spherical portion that comes into contact with the contact surface.

6. 3. The force sensor according to claim 1, wherein the natural frequency is obtained by forming a self-oscillating circuit using the first piezoelectric element for excitation and the second piezoelectric element for detection, and measuring the oscillation frequency of the self-oscillating circuit.

7. 3. The force sensor according to claim 1, wherein the natural frequency is obtained by measuring an anti-resonance frequency in the impedance frequency characteristics of the first piezoelectric element and the second piezoelectric element.

8. 3. The force sensor according to claim 1, wherein the contacts are made of a material having a Young's modulus equal to or greater than that of the metallic material constituting the vibrator member.

9. 3. The force sensor according to claim 1, wherein the magnitude of the pressure W and the change in the natural frequency Δf satisfy the relationship of the following equation (1), and the magnitude of the pressure is calculated using the following equation (1). [Equation 1] (where a and b are constants.)

10. A force sensor characterized in that the contact surfaces of two or three of the force sensors described in claim 1 are arranged so as to be perpendicular to each other and so that one contactor common to the contact surfaces of the two or three force sensors is in contact with the contact surfaces.

11. 11. The force sensor according to claim 10, wherein two or three vibrator members of the force sensor according to claim 1 are made of different kinds of metals.

12. 3. A robot hand system comprising the force sensor according to claim 1 or 2 at a fingertip.

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