KR20040007170A - Accelerometer for low frequency vibration - Google Patents

Abstract

PURPOSE: An acceleration sensor for measuring a low frequency vibration is provided to measure a fine amount of acceleration speed, to simplify the structure thereof and to secure the reliability thereof during a long time by removing the structural operation unit. CONSTITUTION: An acceleration sensor for measuring a low frequency vibration includes a shield case(1), a plurality of piezoelectric devices(5), a pair of top and bottom printed circuit boards(3a,3b) and a basic structure(4). The shield case(1) isolates the piezoelectric devices(5) and the inertial mass from outside and shields the electromagnetic wave. The plurality of piezoelectric devices(5) supports the inertial mass. The pair of top and bottom printed circuit boards(3a,3b) are formed between the inertial mass and the piezoelectric devices(5). And, the piezoelectric devices(5) is positioned in symmetric with respect to the center line of gravity and connected in serial to draw a signal line to outside.

Description

Accelerometer for Low Frequency Vibration Measurement {ACCELEROMETER FOR LOW FREQUENCY VIBRATION}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric acceleration detector that detects the acceleration of a vibrating body using a piezoelectric ceramic element. In particular, the present invention aims at high sensitivity and high output in detecting acceleration in a low speed low frequency region, and improves reliability and stability.

Acceleration, which is a physical quantity, is expressed by Equation 1 below.

F = ma (where F: force, m: mass, a: acceleration) [Equation 1]

As shown in Equation 1, acceleration is proportional to the applied force.

An acceleration detector is a device that converts the mechanical energy of the above-mentioned force into electrical energy and detects it. Acceleration detectors are divided into piezoelectric, servo, and torsion gauge types according to the method of converting mechanical energy into electrical energy. Among them, piezoelectric type is the most widely used method for acceleration sensors.

The piezoelectric acceleration detector utilizes a piezoelectric effect in which electrical energy is generated in proportion to the magnitude of the force when an external force is applied to the piezoelectric element located in the detector. The main structure of the piezoelectric acceleration sensor is composed of a basic structure, an inertial mass, a piezoelectric body for detection, and the like, and there are about three types according to the deformation generation method of the piezoelectric element located in the detection unit. This is briefly described with reference to FIG. 1 as follows.

1) Compression Type:

As shown in Fig. 1A, when the force F is applied to the inertial mass M, the piezoelectric element P between the inertial mass M and the base structure B is compressed and deformation occurs in the same direction as the polarization direction of the piezoelectric element.

2) Shear type:

As shown in Fig. 1B, if a force F is applied to the weight M mounted through the piezoelectric element P, the piezoelectric element P receives a shearing force and deformation occurs as a shearing force on the same plane as the polarization direction of the piezoelectric element P. .

3) Work prevention support type:

As shown in Fig. 1C, when a piezoelectric element P is mounted in a supported shape on one side and a force F is applied to the weight M mounted on the tip thereof, twist occurs at right angles to the polarization axis direction of the piezoelectric element.

These piezoelectric acceleration sensors of various types are classified and used according to conditions such as frequency, acceleration amount, and measurement range.

For example, the compression type and the shear type are used for the detection of the medium frequency, and the one-precision support type that can detect the micro vibrations has a higher detection sensitivity than these methods to detect the low frequency vibration.

However, the work prevention support type used in the low frequency region acceleration detection to be tried in the present invention is easy to measure the acceleration of low frequency, but it is difficult to easily implement the supporting structure of the piezoelectric element and precise measurement due to structural difficulties. However, there is a vulnerability that it is difficult to secure stable sensitivity. In particular, since the beam of the sensing part should be made of piezoelectric ceramics that are mechanically weak, it is difficult to manufacture and weak in impact, making it difficult to secure reliability.

On the other hand, a representative sensor used for low frequency acceleration detection is a servo type acceleration detector. There are two types of servo detectors: moving coil type and moving mass type. These sensors are structurally characterized by a mechanical drive part and circuit feedback and tuning are particularly important because they use signal feedback. Therefore, the servo acceleration detector is not suitable for long-term use without adjustment.

Recently, the collapse of large structures has emerged as a big social problem, increasing the interest in safety diagnosis. Accordingly, recently, more objective and scientific methods using measurement and data analysis using various sensors have been gradually introduced from methods such as member evaluation and visual inspection, which are conventionally used. In this field of measurement, the acceleration detector is one of the most basic detectors used to detect the behavior of the structure by detecting the macroscopic behavior by detecting the vibration of the structure.

The frequency band is very different from the normal acceleration detection in the safety diagnosis of large structures. That is, the natural frequency of a large building has a very low frequency band of about 1Hz-0.1Hz, it is important to apply an appropriate detector to it.

In order to detect such low frequency vibration, there is a method using a first prevention support type detector, or a second method using a piezoelectric sensor detector in which a through hole is formed in a piezoelectric element or a punching part is designed. (Japanese Patent No. 61-99026 "Piezoelectric Sensor", Domestic Patent Publication 93-6644 "Piezoelectric Acceleration Sensor", etc.)

However, as described above, the work prevention support sensor and the piezoelectric sensor in Japanese Patent Application Laid-Open No. 61-99062 have structural weaknesses and are difficult to have sufficient reliability against temperature changes, and in the case of Korean Patent Publication No. 93-6644, By using the piezoelectric element, there is a problem in securing the stability of detection when continuous measurement is performed outside for a long time. In addition, in the low frequency vibration that generates a very low force in most of the above schemes, the detected signal is also very weak so that a high level of detection power can be secured only when a high performance amplifier and a filter circuit are added.

Thus, the need to design new accelerometers for low frequency vibration measurements has increased.

In addition, a sensor using a piezoelectric member uses a charge amplifier because its charge sensitivity is low. A high pass filter is formed between the charge amplifier and the piezoelectric member to generate a cut-off frequency at a low frequency. In this case, the impedance of the charge amplifier must be sufficiently increased to detect the low frequency band. However, when the input impedance increases, the noise of the signal increases, and the S / N ratio decreases.

In general, when the acceleration is to be measured at a remote location, a signal conditioner and a transmission conversion unit for amplifying a signal, etc., which are not used only as the detector itself, are generally included. However, these signal processing units are generally configured for indoor use, and it is difficult to secure measurement accuracy during the four seasons due to exposure to the external environment.

As mentioned above, the necessity of the development of the technology for the acceleration sensor for low frequency vibration which fully improved the conventional system has rapidly increased, and the development of the signal processing technology accompanying this has become an urgent problem.

Accordingly, an object of the present invention is to provide an acceleration detector having a novel structure having sufficient measurement sensitivity in a frequency band of 1 Hz or less.

Still another object of the present invention is to provide an acceleration detector having excellent environmental resistance, which is installed outdoors and has sufficient measurement accuracy for four seasons.

Still another object of the present invention is to provide a piezoelectric acceleration detector capable of ensuring long term reliability.

It is another object of the present invention to provide a piezoelectric acceleration detector with a built-in signal transmission system to enable stable long-distance transmission of the detected acceleration signal.

1 is a classification of the piezoelectric acceleration sensor according to the deformation generation method of the piezoelectric element

1a: Compression type

Figure 1b: Shear

Figure 1c: work prevention support

2 is a conceptual diagram of the acceleration sensor of the present invention

3 is a perspective view from the inertial mass

4 is a perspective view showing the structure of the acceleration sensor of the present invention

5 is a vertical cross-sectional view showing the structure of the acceleration sensor of the present invention

6 is a perspective view of a printed circuit board positioned above and below the piezoelectric body.

6a: lower printed circuit board

6b: upper printed circuit board

7 is a conceptual diagram of an electronic circuit unit located inside the sensor

7A: Circuit diagram conceptual diagram of an existing sensor

Figure 7b: conceptual diagram of the sensor circuit portion of the present invention

8 is a signal example of a sensor detected by the present invention.

〈Code Description for Main Parts〉

1: Shield case

2: inertial mass

3a: upper printed circuit board

3b: bottom printed circuit board

4: Foundation Structure

41: main body of the foundation structure

42: lower cover of the foundation structure

43: space inside the foundation

5: piezoelectric

6: signal leader

Hereinafter, the present invention will be described in detail.

The basic configuration of the present invention of the present invention will be described with reference to FIGS. 2 and 3. The basic structure of this invention consists of the inertial mass 2 which generate | occur | produces a force by acceleration, the piezoelectric body 5 which supports this inertial mass, and the foundation structure 4. As shown in FIG. At this time, the piezoelectric body 5 supporting the inertial mass 2 is located symmetrically with respect to the center of gravity line of the inertial mass 2 as shown in FIG. Piezoelectric supports positioned on both sides have a structure in which they are polarized in the same direction and are connected to each other in series and are drawn out to the external signal line 6.

In this state, when all the components of the sensor including the inertial mass are subjected to the acceleration movement in the horizontal direction, the inertial mass generates a vertical force due to the instability of the support point. In the opposite direction, tension is generated. As a result of the applied force, the piezoelectric support generates voltage twice as long as voltages in the same direction are induced and have a series connection structure. The improvement of the voltage sensitivity appearing at this time may be represented by Equations 2 and 3 below.

Voltage Sensitivity ∝ Piezoelectric Coefficient (d33) * Force (F) [Equation 2]

Voltage Sensitivity = V Tension + V Compression = 2 V [Equation 3]

In the above equation, it can be seen that the voltage sensitivity is improved when the piezoelectric coefficient is large and the applied force is large, and when the same acceleration occurs, a larger force is generated when the inertial mass is large.

In addition, the piezoelectric support generates an ultra-voltage due to temperature change. Since the support polarized in the same direction generates voltages in opposite directions, the voltage induced to the outside becomes zero. This error correction phenomenon is represented by the following equation (4).

Pyroelectric voltage (temperature error) = V tension-V compression = 0 [Equation 4]

In addition, even when the force in the vertical direction is the support polarized in the same direction generates a voltage in the opposite direction to each other, the voltage induced to the outside becomes 0, expressed as follows.

Compression sensitivity (vertical force error) = V tensile-V compression = 0 [Equation 5]

This effect can be installed outdoors to block the error factors caused by temperature changes that occur during long-term operation, and can also eliminate the error caused by the vertical vibration.

As described above, as in the present invention, the inertial mass 2, the piezoelectric body 5 supporting the inertial mass, and the base structure 4, wherein the piezoelectric body 5 supporting the inertial mass 2 is inertial. The piezoelectric acceleration detectors, which are positioned symmetrically with respect to the center of mass of the mass 2 and positioned on both sides, are polarized in the same direction and connected in series with each other and drawn out to the external signal line 6. From the above theoretical basis, it can be seen that it is an acceleration detector with a novel structure that exhibits very excellent performance for low frequency acceleration detection. As will be described later, the performance of the acceleration detector as seen in the embodiment and the like will be verified.

In addition, an accelerometer is designed to secure a space in the base structure 4 of the structure configured as described above so as to incorporate circuits such as a charge amplifier, a signal amplifier, a signal transmission system, and the like.

The built-in signal processing unit not only improves the reliability of the overall system but also minimizes the size of the entire detector.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the drawings.

Example 1

The accelerator detector of the present invention is described with reference to the attached FIGS. 4 and 5. The acceleration detector includes an inertial mass (2), a printed circuit board (3), a foundation structure (4), four piezoelectric elements (5) and a shield case (1) made of metal, and supports the piezoelectric elements (5). Printed circuit boards 3a and 3b constituting the circuit, a base structure 4 and two conductive leads 6 are included.

The piezoelectric element 5 used in the present invention is a cylindrical piezoelectric element, which is formed by baking silver electrodes on the upper and lower surfaces to form electrodes and polarizing them to impart piezoelectricity. As shown in FIG. 6, the piezoelectric elements 5 are arranged at right angles to each other with the center symmetrical to the printed circuit board on which the conductive pattern is formed. FIG. 6A shows a perspective view of the lower printed circuit board and FIG. 6B shows a perspective view of the upper printed circuit board, where the four piezoelectric elements are located in the hatched circle.

The conductive patterns of the printed circuit boards 3a and 3b disposed above and below the piezoelectric element 5 form a network in a series structure by connecting the piezoelectric elements arranged symmetrically to each other, and conducting signal extraction leads to the opposite printed circuit board. 6) Connect so that the signal can be drawn out. As for the printed circuit board, it is preferable that the glass transition temperature of FR-4 series is 130 degreeC or more.

Bonding of the piezoelectric element and the printed circuit boards positioned above and below the element is performed using a conductive thermosetting epoxy adhesive. The conductive epoxy adhesive may disperse the silver powder on the epoxy resin to secure good adhesion stability and impart conductivity. In this case, it is preferable to use a curing temperature of 120 ° C. or less because the deformation of the printed circuit board and the degradation of the piezoelectric element may occur when the temperature is higher.

Piezoelectric element pairs positioned at right angles to each other are electrically isolated from each other. A pair of two piezoelectric elements positioned symmetrically in a straight line with respect to the center serves as a detector in one direction. In this embodiment, two pairs of piezoelectric elements positioned at right angles to each other are configured to simultaneously detect accelerations in X and Y biaxial directions. In the configuration as described above, the piezoelectric element is structured to support four places at equal intervals on the concentric circle, resulting in a structurally stable structure. The radius of the concentric circle in which the piezoelectric element is located is directly related to the detection force of the sensor, and the determination of the position needs to be determined as an appropriate compromise position between the detection force and the stability of the sensor structure. In other words, as the concentric radius becomes smaller than the radius of the inertial mass, the instability of the inertial mass increases, and when the detector performs the acceleration movement, the force in the vertical direction applied to the piezoelectric element increases and the detection force tends to increase. On the other hand, since the instability of the inertial mass is structurally problematic, it is preferable that the concentric circles be 2/3-1/2 compared to the radius of the inertial mass.

The structure consisting of a piezoelectric element and a printed circuit board is bonded again using an inertial mass and an adhesive, and the bottom is bonded using a base structure and an adhesive. The adhesive for bonding the structure uses a non-conductive adhesive, it is preferable to use an epoxy-based adhesive having a glass transition temperature of 100 ℃ or more in order to ensure long-term reliability and temperature stability.

The inertial mass needs to have a small volume for miniaturization of the detector and a large mass for large detection power. This conflicting purpose can be solved by using a material having a high specific gravity. In addition, in order to secure long-term reliability, it is necessary to use a stable material in the air. In this embodiment, a mass was formed using tungsten carbide (WC) cemented carbide. In this case, the material showed good results with the material that meets the needs of the acceleration sensor. Of course, it is not necessary to limit the inertial mass to tungsten carbide alloy. Any material can be used as long as it has a high specific gravity and stability.

Since the alignment direction of the piezoelectric element and the sensing axis of the acceleration are directly related, the bonding position or the alignment state of the element affects the accuracy of the detector. Therefore, it is desirable to construct a guide for alignment on the outer circumferential surface of the printed circuit board for accurate assembly.

The signal line drawn from the piezoelectric elements is led to a space 43 formed inside the structure through a hole formed in the center of the foundation structure and is connected to an internal circuit part located in the space.

As shown in FIG. 7, the internal circuit is composed of a charge amplifier located in the signal input unit and a non-inverting amplifier for amplifying the detected signal.

In the case of a long-distance signal transmission, the signal may be distorted due to external radiation noise, resistance in the signal line, etc., and it is difficult to transmit more than several tens of meters due to this effect. In the case of transmitting the current signal is significantly reduced the effect on the above factors and can be transmitted up to several Km, it can be said that the transmission of the current signal in the case of long distance transmission.

Since the detector is sensitive to electrical noise, it is enclosed in a metal shield case 3. The shield case 3 has a circular cross section formed by an opening that can be connected to the foundation structure. It is also desirable to use a conductive material to protect the circuitry located therein from noise and the underlying structure to be completely inside the shield case using a cover made of a conductive material.

Next, Fig. 8 shows a detection signal by the sensor of the embodiment. As shown in the figure, it can be seen that low frequency acceleration below 1Hz can be detected.

The acceleration detector according to the present invention is an acceleration detector that provides good detection performance in the low frequency acceleration region of 1 Hz or less, and can measure minute acceleration amounts of 10 ^-4 g (1g = 9.8 m / sec ² ) and is structurally simple. Since there is no mechanical moving part, reliability can be secured over a long period of time.

Another effect of the present invention is to detect the acceleration amount in the two axial direction at the same time in one detector and by comparing these values it is possible to know the direction angle of the acceleration motion.

Yet another effect of the present invention is to have a structure that can fundamentally eliminate the error factors due to the temperature difference or the vertical motion error can detect a highly reliable signal in the outdoor environment.

Another effect of the present invention is to embed the circuit portion related to the signal processing in close proximity to the detection portion, thereby increasing the reliability of the detection signal and achieving miniaturization as a whole.

Another effect of the present invention is to enable the long-distance signal transmission by adopting the current signal method as the signal transmission method.

Claims (3)

A shield case (1) which blocks the piezoelectric body and the photonic mass from the outside and blocks electromagnetic waves; Inertial mass (2); Piezoelectric elements 5 consisting of two pairs each supporting an inertial mass; Upper and lower printed circuit boards 3a and 3b formed between the inertial mass and the piezoelectric body; Foundation structure (4); Made up of The piezoelectric body 5 is positioned symmetrically with respect to the center of gravity line of the inertial mass and is connected in series with each other to be drawn out to the external signal line 6. The method of claim 1, Piezoelectric acceleration detector, characterized in that each piezoelectric element pair has the same polarization direction The method of claim 1, In the space 43 inside the foundation 4 of the detector, Piezoelectric acceleration detector further comprises a built-in sensor circuit consisting of a charge amplifier circuit, a signal amplifier circuit, a signal transmission system