Angular velocity sensor and its fabricating method

Abstract: The angular velocity sensor of the present invention has a tuning-fork structure comprising a first vibrator (4) including a first actuator (2) and a first detector (3) joined orthogonally to each other, and a second vibrator (8) including a second actuator (6) and a second detector (7) joined orthogonally to each other, the two vibrators being coupled by an electrode block (9) to each other in parallel relationship along the axis of detection. Also, at least one of the first and second detectors has a beveled surface (11a-11b) where a part of a corner edge of the four-sided plate of the detector is removed.

Description: BACKGROUND OF THE INVENTION The present invention relates to an angular velocity sensor known as a gyroscopic instrument and more particularly, to a high-performance angular velocity sensor having a tuning-fork construction where two vibrator units containing piezoelectric elements are coupled to each other and a method of fabricating the same. A conventional gyroscopic inertia navigation system includes mechanical rotor gyros for determining the direction of a moving object, e.g. an airplane or ship. Such a mechanical gyroscopic system is steady in the performance but bulky in the size thus increasing the cost of production and hardly permitting the application to any small-sized pertinent apparatus. Also, an oscillator-type angular velocity sensor is known for detecting a "Coriolis force" with its detector while it is vibrating but not rotating. Such a sensor commonly employs a piezoelectric or electromagnetic oscillation mechanism. The detection of an angular velocity in the sensor is implemented by sensing a vibration torque of a frequency equal to that of the mass of a gyro which is not rotating but vibrating at a constant rate. The vibration torque is known as the Coriolis force generated when an angular velocity is involved. The oscillator-type angular velocity sensor can detect the amplitude of the vibration torque to determine an angular velocity. Particularly, a variety of the oscillator-type angular velocity sensors employing piezoelectric elements have been introduced for example, as depicted in the Proceeding of Japanese Institute of Aviation and Space, Vol.23, No.257, pp. 339-350 and JP-A-3/084412. One of the conventional tuning-fork structure oscillator-type angular velocity sensors will now be described referring to Figs. 11 to 13. The arrangement of the angular velocity sensor is best illustrated in Fig. 11 which consists mainly of four piezoelectric bimorphous elements serving as an actuator, a monitor, and a first and a second detectors. The actuator 101 is orthogonally coupled by a joiner 105 to the first detector 103 constituting a first vibrator 109 while the monitor 102 is orthogonally coupled by another joiner 106 to the second detector 104 constituting a second vibrator 110. The first 109 and the second vibrators 110 are coupled to each other by a connector 107 which is supported at a point by a support 108, thus constructing a tuning-fork structure. When the actuator 101 of piezoelectric bimorphous element is loaded with a sine-wave voltage signal, its inverse piezoelectric effect causes the first vibrator 109 to vibrate. Then, the action of the tuning-fork structure results in vibration of the second vibrator 110. Accordingly, the monitor 102 of piezoelectric bimorphous element generates a charge on its surface through its piezoelectric action. The charge is proportional to the sine-wave voltage signal applied to the actuator 101. Hence, a constant, continuous action of vibration is developed by controlling the sine-wave voltage signal to the actuator 101 so that the charge generated by the monitor 102 remains uniform in the amplitude. The action of the angular velocity sensor for producing an output corresponding to an angular velocity involved will be explained referring to Figs. 12 and 13. Fig.12 is a top view of the angular velocity sensor of Fig. 11. As shown, the turning movement at an angular velocity ofω produces a Coriolis force on the first detector 103 which vibrates at a speed of v. The Coriolis force is at a right angle to the speed v and its magnitude is 2mvω (where m is the equivalent mass at the distal end of the first detector 103). As the first detector 103 vibrates at the speed v, the second detector 104 is responsive to vibrate at -v and a Coriolis force on the second detector 104 is -2mvω. The two detectors 103 and 104 are stressed in opposite directions by their respective Coriolis forces thus producing charges on the surface through their piezoelectric actions. When the speed v of vibration created by fork oscillation is expressed by: v = a · sin ω ⊘ t where a is the amplitude of the vibration and ω o is the period of the vibration, the Coriolis force is: F c = a · ω · sin ω ⊘ t While the angular velocityω is proportional to the vibration amplitude a, the Coriolis force causes either of the two detectors 103 and 104 to deflect in one direction. Hence, the surface charge Q on the detectors 103 and 104 is expressed by: Q ∝ a · ω · sin ω ⊘ t When the vibration amplitude a is controlled to a constant, Q ∝ ω · sin ω ⊘ t As understood, the surface charge Q is found proportional to the angular velocityω and can be converted to a direct current signal through synchronous transaction at ω o t. In theory, if the angular velocity sensor is subjected to a translational movement rather than the rotation, its two detectors 103 and 104 produce two charges of the same polarity and their resultant DC signals are offset each other generating no output. However, the two signals derived from the unwanted charges are not always canceled to zero because of a symmetrical error and a difference in weight between the two, left and right, prongs of the tuning-fork structure of the conventional angular velocity sensor in which a plurality of piezoelectric bimorphous elements are assembled in a relatively complex manner and may not be identical in the quality. For overcoming the disadvantages, best care is taken to assemble the tuning-fork structure to ensure the symmetry and balance of the fork structure. So far, such efforts are found unsuccessful and fail to cancel both undesired signals. The two unwanted signals cause the sensor to deteriorate the thermal characteristics and become oversensitive to external interruption or vibration. The piezoelectric element essentially includes two electrodes at both sides regardless of bimorphous or unimorphous type. The electrodes are commonly formed of silver materials printed onto both surfaces of an piezoelectric substance and thus, become rarely identical in weight, size, and quality due to the presence of unavoidable burrs impairing the symmetry of the tuning-fork structure. From JP-A-61/181913 is known to cut and grind the free end of a piezo-electric bimorph element to keep the driving frequency of an angular velocity sensor constant with the regulation of the resonance frequency of the vibrators. SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved angular velocity sensor in which unwanted signals derived from a balancing error of its tuning-fork structure are cancelled and thus, the thermal characteristics will be enhanced and the generation of noise caused by external interruption or vibration will be avoided. It is another object of the present invention to provide a method of manufacturing an improved angular velocity sensor by eliminating unwanted factors which impair the balance of the sensor during the assembly to ensure lifelong reliability of the performance. The angular velocity sensor according to the present invention as set forth in claim 1 has a tuning-fork structure comprising a first vibrator including a first actuator and a first detector joined orthogonally to each other, and a second vibrator including a second actuator and a second detector joined orthogonally to each other, the two vibrators being coupled by an electrode block to each other in parallel relationship along the axis of detection. The first and second detectors have a cross-sectionally four-sided plate shape accompanied with a piezoelectric element. Also, at least one of the first and second detectors has a beveled surface where a part of a corner edge of the four-sided plate of the detector is removed. A method of manufacturing the angular velocity sensor comprises a step of grinding at least one of the first and second detectors for balancing between the two, first and second, vibrators to eliminate the generation of unwanted signals derived from a balancing error between the two vibrators. For the purpose, the method also includes a step of detecting and applying the unwanted signals to determine whether one or two of the detectors are to be trimmed in the grinding process. In accordance with claim 2, the piezoelectric element has two electrodes applied to both surfaces of a piezoelectric substance so that at least one of the two electrodes is disposed not to overlap the outer edge of the piezoelectric substance. Accordingly, the manufacture of the piezoelectric elements will be carried out with readiness and the fork assembly of the two vibrators will be balanced with ease. Also, the first vibrator is formed by bonding a piezoelectric detector element and a piezoelectric actuator element to two, upper and lower, parts of a metal base plate respectively which serve as the first detector and the first actuator and the second vibrator is formed by bonding a piezoelectric detector element and a piezoelectric actuator element to two, upper and lower, parts of a metal base plate respectively which serve as the second detector and the second actuator. Furthermore, in accordance with claim 3, the metal base plate is shaped by turning its two, upper and lower, parts in opposite directions through 90 degrees along a center line defined by two slits extending from both ends inwardly and crosswisely of said base plate so that the two, upper and lower, parts intersect at a right angle to each other. According to claim 7, the grinding process is preferably executed by grinding off a part of one corner edge of the four-sided plate of the detector to form a beveled surface. As the two unwanted signals produced by the first and second detectors are offset by each other, the angular velocity sensor provides improved thermal characteristics and prevents the generation of noise caused by external interruption or vibration. As the vibrators are formed by bonding the piezoelectric element to the metal base plate of a particular shape and the electrodes of the piezoelectric element are arranged in a specific manner, the manufacture of the angular velocity sensor will be effected accurately and readily without creating any burrs and dust of the electrodes. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of an angular velocity sensor manufactured by a method of a first embodiment of the present invention; Fig. 2 is a flow chart explaining the manufacture method of the first embodiment; Fig. 3 is a schematic view showing trimming locations in the angular velocity sensor viewed from top; Fig. 4(a) is a perspective view showing an action of the trimming process on the angular velocity sensor of the present invention; Fig. 4(b) is a perspective view of a part of the angular velocity sensor when the trimming process is completed; Fig. 5 is a perspective view of two vibrators of the angular velocity sensor explaining the invention; Fig. 6 is a characteristic diagram showing the relation between an unwanted signal level and a trimming size at a given location in the present invention; Fig. 7 is a perspective view of an angular velocity sensor manufactred by a method of a second embodiment of the present invention; Figs. 8(a) and 8(b) are a perspective and a cross sectional views of a part of the method of manufacture of the angular velocity sensor of the second embodiment; Figs. 9(a) and 9(b) are a perspective and a cross sectional views of another part of the method of manufacture of the angular velocity sensor of the second embodiment; Figs. 10(a) and 10(b) are a perspective and a cross sectional views of a further part of the method of manufacture of the angular velocity sensor of the second embodiment; Fig. 11 is a perspective view of a conventional angular velocity sensor; Fig. 12 is an explanatory view showing the principles of detecting an angular velocity with the conventional angular velocity sensor; and Fig. 13 is a perspective schematic view explaining the detection of an angular velocity with the conventional velocity sensor. DETAILED DESCRIPTION OF THE INVENTION Preferred embodiments of the present invention will be described in the form of an angular velocity sensor and its method of manufacture referring to the accompanying drawings. First Embodiment Fig. 1 illustrates an angular velocity sensor of the present invention having a tuning-fork structure which comprises a first vibrator 4, a second vibrator 8, an electrode block 9 coupling the first 4 and the second vibrators 8 to each other, and a support pin 10 supporting at one point the electrode block 9. The first vibrator 4 includes a first eternally elastic metal plate 1 formed by turning its two, upper and lower, parts in opposite directions at center to intersect each other at a right angle, and a piezoelectric actuator 2 and a first piezoelectric detector 3 which are bonded to the two parts of the first eternally elastic metal plate 1 respectively. Similarly, the second vibrator 8 includes a second eternally elastic metal plate 5 formed by turning its two regions parts in opposite directions at center to intersect each other at a right angle, and a piezoelectric monitor 6 and a second piezoelectric detector 7 which are bonded to the two parts of the second eternally elastic metal plate 5 respectively. The eternally elastic metal of this embodiment is elinvar alloy. The principles of actuation and detection of an angular velocity of the angular velocity sensor of the first embodiment are identical to those of the prior art and will no more be explained. The specific arrangement of the embodiment is such that the upper part of the eternally elastic metal plate 5 of the second vibrator 8 carrying the second piezoelectric detector 7 has at least one beveled surface extending between lla and llb where the corner edge is eliminated. The method of fabricating the angular velocity sensor will now be described referring to the flow chart of Fig. 2. As depicted previously in the prior art, the first vibrator 4 and the second vibrator 8 assembled in the tuning-fork structure, precisely or not, tend to produce their respective unwanted signals which cannot be offset to zero by each other because of a symmetrical error and a difference in weight between the two vibrators 4 and 8 of which piezoelectric elements are rarely identical in the quality. It is thus provided a trimming process for forming a beveled surface on the upper part or detector component of the vibrator 8 carrying the second piezoelectric detector 7. The procedure of the method starts with an inspection process (1) for detecting unwanted signals representing the unbalancing factors of symmetrical error in the tuning-fork assembly (Step a). The unwanted signals detected in the inspection process are compared with a reference value L (Step b) and if the signals are smaller than L, the assembly is transferred to the next inspection process (2). If not, it is subjected to the trimming process for rough modification of its detector components. The detector component of the assembly is modified by grinding a part of its corner edge in the trimming process (Step c). Then, unwanted signals on the modified detector elements are detected again (Step d) and compared with a reference value M (M>L) (Step e). If the unwanted signals are smaller than M, the assembly is transferred to a finish process. If not, it is subjected to the trimming process for fine modification of its detector component (Step f). The assembly is returned to the inspection process (1) for reexamining the balance. The assembly accepted at the two inspection processes (1) and (2) only is transferred to the finish process. The detector component of the first embodiment includes the piezoelectric detector bonded to the eternally elastic metal plate and thus, the trimming on the eternally elastic metal plate will be most effective for balancing the assembly. For example, two of the four corners of the second eternally elastic metal plate 5 are partially removed by grinding in the embodiment. The trimming process is designed for keeping the balance in weight and performance between the two detector components of the tuning-fork assembly so that the two resultant unwanted signals are offset each other. Fig. 3 illustrates trimming locations in the angular velocity sensor viewed from top. As apparent, the four corner edges A, B, C, and D of the two four-sided detector components, each comprising a piezoelectric detector and an eternally elastic metal plate, are removed. The trimming can be carried out with precision by controlling the size and/or location of the removed edges. The two unwanted signals can thus be offset each other by an optimum action of the trimming. Fig. 4(a) shows an action of the trimming process in which a part of the corner edge of the eternally elastic metal plate 7 carrying the piezoelectric detector 7 is being ground with a rotary disk grinder 12 known as a luter to form a beveled surface 11. Fig. 4(b) is a perspective view of a part of the angular velocity sensor in a trimmed state. Fig. 6(a) is an experimental data diagram showing the relation between the unwanted signal level and the trimming variations defined by a discrete size of the removed edge at a different location. The size of the removed edge is classified in three levels, assigning the four corner edges A, B, C, and D to A-1, A-2, A-3, B-1,..., C-1, C-2, and C-3 respectively. As apparent, the control in a positive direction is effected by trimming of A and C and in a negative direction by trimming of B and D. It is found that the trimming on the outside one of the two corner edges of each detector component results in a steep change in the signal level. Accordingly, it is a good idea for controlling the signal level towards the positive direction to perform the rough modification by removing the corner edge A and then, the fine modification by eliminating the corner edge C. If the control towards the positive direction is overperformed, it may be corrected by trimming on the B or D corner edge. As set forth above, the angular velocity sensor of the first embodiment of the present invention comprises the first vibrator composed of a first actuator component and a first detector component coupled orthogonally to each other and the second vibrator composed of a second actuator component and a second detector component coupled orthogonally to each other. Also, the two, first and second, vibrators are arranged parallel to each other along the axis of detection to form a tuning-fork structure. In particular, the angular velocity sensor can be controlled in the operating characteristics by grinding off a part(s) of the corner edges of each detector component so that two unwanted signals derived from the symmetrical error are offset to zero. Accordingly, the angular velocity sensor of the first embodiment of the present invention is specifically improved in the thermal characteristics and will avoid the generation of noise caused by external interruption or vibration. Second Embodiment In the first embodiment, each of the first 1 and the second eternally elastic metal plates 5 is formed by turning two, upper and lower, parts of a metal plate through 90 degrees in opposite directions along a center folding line defined by two crosswise slits extending inwardly from opposite ends to the center so that they intersect each other at a right angle. The folding lines la and 5a are located at a center of the width and extends longitudinally from the center thus contributing to the symmetry of the two vibrators. Second Embodiment A second embodiment of the present invention will be described referring to Figs. 7 to 10. Fig. 7 shows an angular velocity sensor of the third embodiment having a tuning-fork structure which comprises a first vibrator 4, a second vibrator 8, an electrode block 9 coupling the first 4 and second vibrators 8 to each other, and a support pin 10 supporting at one point the electrode block 9. The first vibrator 4 includes a first eternally elastic metal plate 1 formed by turning its two, upper and lower, parts in opposite directions to intersect each other at a right angle, and a piezoelectric actuator 2 and a first piezoelectric detector 3 which are bonded to the two parts of the first eternally elastic metal plate 1 respectively. Similarly, the second vibrator 8 includes a second eternally elastic metal plate 5 formed by turning two parts in opposite directions to intersect each other at a right angle, and a piezoelectric monitor 6 and a second piezoelectric detector 7 which are bonded to the two parts of the second eternally elastic metal plate 5 respectively. Each of the piezoelectric actuator 2, the piezoelectric monitor 6, the first piezoelectric detector 3, and the second piezoelectric detector 7 is formed of a piezoelectric substance 13a sandwiched between two electrodes 14 which are smaller in size than the piezoelectric substance 13a. The upper region of the eternally elastic metal plate 5 carrying the second piezoelectric detector 7 has a beveled surface 11 extending between 11a and llb where the corner edge is removed. As set forth above, the angular velocity sensor of the second embodiment of the present invention comprises the first vibrator composed of a first actuator element and a first detector element coupled orthogonally to each other and the second vibrator composed of a second actuator element and a second detector element coupled orthogonally to each other. Also, the two, first and second, vibrators are arranged parallel to each other along the axis of detection to form a tuning-fork structure. The angular velocity sensor can be controlled in the operating characteristics by grinding off a part(s) of the corner edges of each detector element so that two unwanted signals derived from the symmetrical error are offset to zero. In particular, at least one or two electrodes of the detector element are spaced from the corner edge of the piezoelectric substance. Accordingly, the angular velocity sensor of the third embodiment is specifically improved in the thermal characteristics and will avoid generation of noise caused by external interruption or vibration. Furthermore, the electrodes formed by printing process are not sources of producing burrs and metallic dust thus contributing to the high reliability of the angular velocity sensor which is free from an electrical shortcircuit between the electrodes.

Claims: An angular velocity sensor comprising: a first vibrator (4) including a first actuator (2) and a first detector (3) joined orthogonally to each other; a second vibrator (8) including a second actuator (6) and a second detector (7) joined orthogonally to each other; and an electrode block (9) between said first and second vibrators (4,8); wherein said first vibrator (4) is formed so that two piezoelectric elements for actuation and detection are bonded on a first metal plate (1) as a first actuator (2) and a first detector (3); respectively; said second vibrator (8) is formed so that the two piezoelectric elements for actuation and detection are bonded on a second metal plate (5) as a second actuator (6) and a second detector (7), respectively; said first and second vibrators (4,8) form a tuning-fork structure coupled by said electrode block (9) at said first and second actuators (2,6) so that said first and second vibrators (4,8) are parallel to each other along a detection axis and the surfaces on which the piezoelectric elements of said first and second detectors (3,7) are bonded direct in the same direction; said first and second metal plates (1,5) having at least one bevelled region where a lengthwise edge (A,B,C,D) is removed. An angular velocity sensor comprising: a first vibrator (4) including a first actuator (2) and a first detector (3) joined orthogonally to each other; a second vibrator (8) including a second actuator (6) and a second detector (7) joined orthogonally to each other; an electrode block (9) coupled between the first and second vibrators (4,8); said first and second vibrators (4,8) being arranged parallel to each other along the axis of detection and coupled to each other by said electrode block (9) at the lowermost ends of their actuators (2,6) forming a tuning-fork structure; said first and second detectors (3,7) having a cross-sectionally four-sided plate shape accompanied with a piezoelectric element; and at least one of said first and second detectors (3,7) having a beveled surface where a part of a corner edge (A,B,C,D) of the four-sided plate of said detector (3,7) is removed, wherein each of said actuators (2,6) and detectors (3,7) is formed of a piezoelectric substance (13a) sandwiched between two electrodes (14), at least one of the two electrodes (14) being sized smaller than the piezoelectric substance (13a) not to overlap the outer edge of the piezoelectric substance (13a). An angular velocity sensor according to Claim 1, wherein said metal base plate is shaped by turning its two, upper and lower, parts (1,5) in opposite directions through 90 degrees along a center line defined by two slits extending from both ends inwardly and crosswisely of said base plate so that the two, upper and lower, parts intersect at a right angle to each other. An angular velocity sensor according to Claim 3, wherein each of said piezoelectric actuator and detector elements is formed of a piezoelectric substance (13a) sandwiched between two electrodes (14), at least one of the two electrodes (14) being sized smaller than the piezoelectric substance (13a) not to overlap the outer edge of the piezoelectric substance (13a). A method of manufacturing an angular velocity sensor which has a tuning-fork structure comprising a first vibrator including a first actuator and a first detector joined orthogonally to each other, a second vibrator including a second actuator and a second detector joined orthogonally to each other, the two vibrators being coupled to each other in parallel relationship along the axis of detection, said method comprising: bonding two piezoelectric elements for actuation and detection on a first metal plate as a first actuator and a first detector, respectively; bonding two piezoelectric elements for actuation and detection on a second metal plate as a second actuator and a second detector, respectively; grinding a portion of the corner edge of at least one of the two, said first and second metal plate for controlling the operating balance between the two vibrators. A method of fabricating an angular velocity sensor according to Claim 5, further comprising a step of detecting a balancing error between the two, first and second, vibrators and utilizing the balancing error to control the grinding step. A method of manufacturing an angular velocity sensor, according to Claim 5, wherein said grinding step is executed by grinding a corner edge of the metal base plate lengthwisely of the same. A method of manufacturing an angular velocity sensor, according to Claim 5, wherein said grinding step is executed by grinding lengthwisely an outer corner edge of the metal base plate for rough modification and an inner corner edge for fine modification. A method of manufacturing an angular velocity sensor, according to Claim 5, wherein the balance control in said grinding step is implemented by grinding off a given length of the corner edge of the metal base plate. A method of manufacturing an angular velocity sensor, according to Claim 5, wherein the two, upper and lower, parts of the metal base plate are turned in opposite direction through 90 degrees along a center line defined by two slit extending from both ends inwardly and crosswisely of the metal base plate so that they intersect each other at a right angle.

Inventors: KAINO KIKUO, NISHIMURA HITOSHI, TAKENAKA HIROSHI, TAMURA MASAMI, TERADA JIRO, USHIHARA MASAHARU, KAINO, KIKUO, NISHIMURA, HITOSHI, TAKENAKA, HIROSHI, TAMURA, MASAMI, TERADA, JIRO, USHIHARA, MASAHARU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: G01C19
IPC codes: G01C19