JP3098045B2 - Electrodynamic accelerometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to an accelerometer for measuring vibration, and more particularly to an electrodynamic accelerometer. More specifically, the present invention provides
The present invention relates to an electrokinetic accelerometer suitable for detecting signals propagating through various formations.

[0002]

2. Description of the Related Art Earthquake detectors called geophones are widely used in seismic surveys. This is because geophones are small, sensitive, and low cost. In a geophone, a moving coil is suspended in a magnetic field by a pair of springs. Mass and spring of the moving coil to determine the natural frequency f _0. Typically, a geophone has an attenuation coefficient of 0.7,
Thus, it operates in a mass-controlled area. That is, a geophone can make meaningful measurements on signals higher than the natural frequency. Geophone frequency bands are typically 10Hz and 200H
z or 300 Hz (for a well designed geophone). In this regard, Maurice
"Handbook of Geophysical Exploration (Handbook of Geophysical)" by Pieuchot
EXPLORATION) ", Section 1, Seismic Exploration, Vo
l. 2. Seismic instruments (Seismic Instrumentet)
ation), Geophysical Press Publisher, 1984
See the year literature.

[0003] The seismic signal frequency is usually 10 Hz and 10 Hz.
Although between 0 Hz, this will vary depending on the conditions and type of exploration. In areas of very high attenuation or in the case of very deep reflections, only low frequency energy is transmitted. In such a case, the available frequency band is
5 Hz to 50 Hz. In the case of shallow seismic survey, it is possible to transmit signals up to 500 Hz. High frequencies are desirable as long as the formation transmits signals, as higher frequencies provide finer resolution. Recently, tomographic exploration has become a powerful way to provide new graphical displays. In this case, the well from the surface (ie,
Drilling) or seismic exploration from well to well
In addition, it is possible to observe a signal up to 2 KHz. Conventional geophones are not satisfactory for collecting such a wide range of seismic signals.

[0004] In borehole drilling, a geophone is mounted in a pressure-tight housing of a downhole seismic probe that is lowered into a borehole drilled deep into the earth. Such a downhole seismic device can be oriented at any angle depending on how wells or boreholes deviate. When the geophone is tilted, its responsiveness deteriorates, so a geophone with a high natural frequency or a gimbal-mounted geophone is used. When using a high frequency geophone, low frequency information cannot be sampled. Figure 1
Figure 3 shows an airgun seismic signal measured in a perforation using a 14 Hz geophone. In this regard, reference may be made to JP-A-62-58184 which discloses a perforated seismic survey system. In FIG. 1, the dotted line shows the frequency response of the geophone used. As can be seen from this figure, the seismic energy is concentrated near the natural frequency of the geophone, and the measured spectrum has lost the low frequency information. For this reason, a gimbal mount type is often desired, but it is expensive and the complexity of the mechanism may cause unexpected resonance in the seismic frequency band.

[0005] The geophone bandwidth is limited for two reasons. That is, the lowest frequency is limited by its natural frequency, and the highest frequency is limited by accidental effects. First, the effect of the natural frequency will be described. A major problem associated with geophones is that seismic energy has a lower frequency than the geophone's natural frequency. The response of a geophone below the natural frequency has a low response amplitude and inverted phase. From an engineering viewpoint, it is difficult to design a geophone having a natural frequency of 10 Hz or less for the following reasons.

[0006] The displacement of the moving coil is large for low frequency signals. Low frequency geophones require a large space for the moving coil to move. When the natural frequency is low, the geophone operates only in a small range of tilt angles. This is because the moving coil is displaced by gravitational acceleration when it is tilted. This is more important in drilling seismic surveys. This is because the geophone in a perforated seismic probe is tilted by the angle at which the well deviates. Geophones are often mounted on mechanical gimbal mounts to avoid tilt effects in drilling seismic surveys.

[0007] The balance of force between the spring and gravity can be expressed as follows. Thus, the above equations (1) and (2) can be rewritten as follows as a function of natural frequency only:

For a typical geophone of 1 inch diameter, the maximum stroke of the coil is ± 2 mm. With a natural frequency of 10 Hz, 7 for a vertical geophone.
Nine degrees and 54 degrees for horizontal geophones are absolute limits for zero dynamic range. 45 degrees for vertical geophones and 60 degrees for horizontal geophones are practical engineering limits with reduced dynamic range. Thus, there is no way to avoid this physical constraint in the conventional geophone configuration to lower the natural frequency. Of course, it is expected that the spring constant itself will be high for such a large displacement. This makes the geophone response non-linear and manifests itself as a shift in natural frequency.

Next, the problem of the accidental influence of the conventional geophone will be described. At high frequencies, as shown in FIG. 2, a geophone always exhibits a so-called accidental response. This accidental effect can be caused by various things, for example, the lateral movement of the movable coil based on the buckling of the suspension spring, the harmonic vibration of the spring, the rotational vibration of the movable coil, and the movable coil itself. Mode etc. If the geophone is tilted and its moving coil is displaced from the neutral position, the lateral movement of the moving coil is particularly significantly increased. The frequency of this accidental effect is typically 20 times the natural frequency. If the accidental frequency is greater than 20 times the natural frequency, the geophone is well designed. Therefore,
The frequency range of the geophone is between natural frequencies and accidental frequencies which are typically at most 20 times (best 30 times) the natural frequency. If the stroke of the moving coil is 2 mm and the dynamic range at low frequencies is considered, the practical lower limit of the natural frequency is 10 Hz.

Under these circumstances, accelerometers are considered as more desirable sensors. However, current piezo accelerometers are expensive and require special electronics, charge amplifiers. Charge amplifiers generate more electronic noise than geophone amplifiers, especially when the temperature is high. JP 61
No. 239,164 proposes an optical servo accelerometer, which also requires a relatively high quality servo amplifier and is relatively large in size.

An electrokinetic accelerometer suitable for use in seismic exploration is disclosed in US Pat. No. 4,412,317 issued Oct. 25, 1983 (Asjes et a.
l. ), And K. B. Klaassen and J.A.
C. L. Co-authored by Van Peppen, Geophysical Prospectin 31
g 31) ", EAEG, 1983, pp. 457-
480 documents. However, the electrokinetic accelerometer has a stationary coil assembly and a moving magnet assembly, which is rather large and heavy. As a result, the sensitivity is relatively low and the overall size is limited, which imposes limitations on the application. Furthermore, because of the use of a feedback system, two circuits must be provided, each associated with its own magnet,
Thus, the overall system is rather complex.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide an improved accelerometer which is small in size and high in sensitivity.
It is another object of the present invention to provide an improved electrokinetic accelerometer with a reduced number of parts and therefore lower manufacturing costs. It is yet another object of the present invention to provide an improved electrodynamic accelerometer having a very wide frequency range, and particularly having a high frequency limit.
It is yet another object of the present invention to provide an improved electrokinetic accelerometer particularly suitable for use in seismic exploration.

[0013]

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided an improved electrokinetic accelerometer of small size and high sensitivity particularly adapted for use in seismic sounding. This electrodynamic accelerometer is
It has a single moving coil located in a strong magnetic field formed by a pair of opposing magnets and supported by elastic means, and an operational amplifier, wherein the operational amplifier has a motion of the moving coil. Operatively coupled to the moving coil so as to be attenuated by the virtual ground of the operational amplifier. Thus, according to the present invention, the virtual grounding of the operational amplifier is used to impart a significantly greater damping force to the movement of the moving coil. The moving coil is electrokinetically damped by the virtual ground of the operational amplifier, so that the frequency response is obtained over a wide range of interest. Because the strong grounding of the operational amplifier is used to provide strong electrokinetic damping, the response characteristics of the present electrokinetic accelerometer are proportional to the acceleration of the applied vibration in its resistance control region. Thus, the frequency response of the device is governed by a damping factor, which can be set to a value which is preferably higher than one. For example, if the damping factor is set to 8, the frequency response is 0.1 to 10 times the natural frequency. By adjusting the natural frequency to the middle of the frequency band of interest, it is possible to minimize phase distortion.

A pair of permanent magnets, preferably made of rare earth magnets, such as, for example, cobalt magnets, are fixed to the housing with their same polarity facing each other, so that at a predetermined position between the magnets a respective permanent magnet is provided. The combined magnetic flux is defined from the respective magnetic flux from the magnet. This combined magnetic flux effectively doubles its magnetic flux density and intersects the moving coil movably positioned in the space between the two magnets. The moving coil is connected in series in a circuit that defines a loop between a pair of input terminals of the operational amplifier (one of which has a virtual ground set).

According to the present invention, since a single moving coil is used, a coil assembly comprising a moving mass, ie, the moving coil and a bobbin around which the moving coil is wound, is extremely small and Lightweight. Furthermore, because of the use of two opposed magnets, the combined flux passing through the moving coil is effectively double in density, which increases sensitivity and extends the frequency range, especially at the upper end. Enables expansion.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described in detail with reference to the accompanying drawings. First, referring to FIG. 3, an overall electrical configuration of an electrodynamic accelerometer 1 configured according to an embodiment of the present invention is schematically shown. As shown in FIG. 3, the electrodynamic accelerometer 1 generally includes a movable coil 13 which is arranged in a magnetic field and is movably supported as described later, and an operational amplifier 20. In the illustrated example, the movable coil 13 has a coil resistance r (individually shown in FIG. 3 for clarity), and the coil 13 has one end connected to the inverting input terminal of the operational amplifier 20. The other end is connected to the non-inverting input terminal of the operational amplifier 20 which is grounded. The line connecting the coil 13 to the operational amplifier 20 has a line resistance R1, and the inverting input terminal of the operational amplifier 20 is connected to its output terminal via a feedback resistance Rf. As is known, in FIG. 3, node P is called a virtual ground, which is always set to ground level. This is because the inverting and non-inverting input terminals of the operational amplifier 20 define a virtual ground. In this configuration, when the coil 13 moves in the magnetic field, the virtual grounding of the operational amplifier 20 provides strong damping, that is, attenuation, to the movement of the coil 13.

Since node P is maintained at ground level,
All of the current flowing from the coil 13 to the node P flows to the output terminal V _out via the feedback resistor R _f ,
Therefore, the movement of the coil 13 can be detected in the form of an electric signal. It should be noted that the configuration shown in FIG. 3 is merely an example of connecting the coil 13 to the operational amplifier 20 and that the virtual grounding of the operational amplifier will It is understood that other forms of connection may be used by those skilled in the art without departing from the scope of the invention, as long as damping is applied.

FIG. 4 shows the mechanical configuration of an electrodynamic accelerometer constructed according to one embodiment of the present invention. In this embodiment, the top and bottom end plates 3 and 4 are made of a magnetically permeable material and act as part of the yoke. A pair of lower and upper permanent magnets 11a and 11b are provided apart from each other, and a central pole member 14 made of a magnetically permeable material is located substantially at an interface between the upper and lower magnets 11a and 11b. . The lower support spring 7 includes a lower magnet 11 a and a center pole member 14.
And the upper support spring 8 is held substantially at the interface between the center pole member 14 and the upper magnet 11b. A bobbin 12 is supported between the lower and upper support springs 7 and 8, and a coil 13 is wound around and fixed to the bobbin 12. The center pole member 14 is provided with a circumferential projection 14 a in the center region in relation to the coil 13. Thus, the magnetic flux flowing from the north pole of each of the magnets 11a and 11b flows through the central pole member 14, the coil 13, the housing 2 and the end plate 3 or 4, and returns to the south pole of the corresponding magnet 11a or 11b. . Accordingly, a fixed magnetic field is defined, and the coil 13 is movably supported by the pair of springs 7 and 8, so that the coil 13 can move relative to the fixed magnetic field.

The detailed construction of the lower and upper support springs 7 and 8 is shown in FIG. As shown, the support spring 7 or 8 is formed with a plurality of slots having a particular shape, thereby defining an inner and outer peripheral shape, whereby the inner and outer peripheral mounting sections 7a are formed. (8a) and 7b (8b). The inner peripheral mounting section 7a (8a) is held between the corresponding magnet 11a or 11b and the central pole member, and the outer peripheral mounting section 7b
(8b) is fixed to the corresponding end of the bobbin 12, so that the bobbin 12 and the coil 13 carried by the bobbin 12 can move relative to the magnets 11a and 11b.

FIG. 6 shows the housing 2 and the end plate 3
5 shows the configuration of the electrokinetic accelerometer of FIG. In this configuration, only one coil 1
3 is provided, and a pair of permanent magnets 11a and 1a are provided.
1b are provided with their polarities facing each other. Therefore, it is possible to give a very high magnetic flux density and to move the movable parts, namely the coil 13 and the bobbin 12
Can be extremely small and light as compared with the prior art. Thus, the electrokinetic accelerometer of the present invention provides extremely high sensitivity and can provide a wide operating range with an extended upper limit of over 500 Hz and up to approximately 2 kHz.

FIG. 7 shows another embodiment of the electrodynamic accelerometer according to the present invention. As shown in the figure, this embodiment uses the disk-shaped magnet 1 having the configuration shown in FIG.
It is similar in many respects to the embodiment shown in FIG. 4, except that a pair of ring-shaped permanent magnets 11a and 11b are provided outside the movable coil 13 instead of 1a and 11b. In this embodiment, the movable coil 13
Since the ring-shaped magnets 11a and 11b are provided on the outside of the coil, it is possible to determine a magnetic field having an increased magnetic field strength, and it is possible to further reduce the size of the coil 13 and, particularly, to use a coil having a small diameter. And therefore can be lighter. In FIG. 7, a pair of bottom and upper ring-shaped magnets 11a and 11b are shown.
A yoke 15 having a cylindrical shape is provided between the yoke 15.
A central pole member 14 is provided between the bottom and upper plates 3 and 4 acting as a yoke. A pair of upper and lower springs 7 and 8 are fixed to the central pole member 14 at a distance from each other, and the bobbin 12 is supported by the pair of springs 7 and 8, so that the bobbin carrying the coil 13 is It is possible to move axially relative to the center pole member 14.

FIG. 8 shows a state in which the upper and lower support springs 7 and 8 are fixed to the bobbin 12.
That is, the bobbin 12 has a substantially cylindrical shape, and has a plurality of mounting projections 12a at each end. As shown in FIG. 9, the support spring 7 or 8 in this embodiment has an axial section 7c or 8c and a radial section 7d or 8d. FIG.
9 shows a spring blank to be bent as appropriate to form the spring 7 or 8. That is, the radial section 7d or 8d is bent in one predetermined direction at a connection to the axial section 7c or 8c which is bent to form a ring. Accordingly, the free ends of the radial sections 7d or 8d are secured to the corresponding mounting projections 12a of the bobbin 12, giving an arrangement as shown in FIG. Thus, the axial sections 8c and 7c are secured to the center pole member 14 after the center pole member 14 has been inserted into the bobbin 12. In this configuration, the bobbin 12, and thus the coil 12 carried by the bobbin 12, is connected to the central pole member 14 by bending of the upper and lower springs 7 and 8 when an external force is applied to the coil 13 and the bobbin 12. It is possible to move relatively axially. Since the magnetic field is fixed at a predetermined position in the accelerometer, the coil 13 can move relative to the magnetic field when external vibration is applied to the accelerometer.

A specific example constructed in the form of a prototype of the present invention will be described. The electrodynamic accelerometer shown in FIG. 4 was manufactured by using the rare-earth cobalt magnets 11a and 11b and sandwiching the pole member 14 therebetween so as to face each other. In the pole member 14,
Magnetic flux from both magnets 11a and 11b flows in, and the magnetic flux density is twice that of a single magnet.
As described above, the magnetic flux density of the pole member 14 was 9000 G for the movable coil 13 having an outer shape of 1 inch (2.54 cm). The movable coil 13 was formed by winding a 35-micron copper wire 2,000 times around an aluminum bobbin 12 having a diameter of 20 mm to a thickness of 0.1 mm. The overall weight and coil resistance of the moving coil 13 was 2.7 grams and 2700Ω. The support springs 7 and 8 were designed such that the natural frequency was 28 Hz. The characteristics of the prototype thus configured were as follows when no electronic circuit was provided.

From these measured parameters, the total damping factor for maximum damping was calculated to be 4.14. Therefore, the flat acceleration response frequency range at -3 dB amplitude response was calculated to be between 3.2 Hz and 225 Hz. On the other hand, the flat speed response frequency range extends from 225 Hz to a maximum of 2 kHz. It is expected that better response characteristics will be obtained if the copper wire is replaced with a more suitable material, such as an anodized aluminum wire.

[0025]

According to the present invention, the following effects can be obtained. That is, according to the present invention, a pair of magnets are arranged to face each other to define a combined magnetic field having a high magnetic flux density, and a single movable coil is movably positioned within the magnetic field. . Therefore, the movable output defined by the movable coil is minimized, and the magnetic flux passing through the movable coil is maximized. This makes it possible to obtain an electrodynamic accelerometer that is very small but very sensitive. Furthermore, this configuration allows the use of a virtual ground for the operational amplifier.
Therefore, the movable coil is connected in series in a circuit loop coupled to the pair of input terminals of the operational amplifier whose virtual ground is set at one of the pair of input terminals.
Since the magnetic flux density is high and the movable output is very small, there is no problem with using virtual grounding of the operational amplifier.

The electrodynamic accelerometer of the present invention has a small number of parts and is therefore simple to manufacture and low cost. Further, the overall dimensions of the electrokinetic accelerometer of the present invention are significantly smaller than conventional devices. The oscillating movement of the coil assembly is minimized because the coil assembly with a single moving coil is very small, especially in the axial direction, which may lead to accidental effects, especially at high frequencies. Has contributed to reducing the likelihood.

Although the specific embodiments of the present invention have been described in detail, the present invention should not be limited to these specific examples, but may be variously modified without departing from the technical scope of the present invention. Of course is possible.

[Brief description of the drawings]

FIG. 1 is a graph showing typical characteristics of a conventional geophone.

FIG. 2 is a graph showing typical characteristics of a conventional geophone.

FIG. 3 is a schematic circuit diagram showing an electric configuration of an electrodynamic accelerometer configured based on one embodiment of the present invention.

FIG. 4 is a schematic diagram showing the overall mechanical configuration of an electrodynamic accelerometer configured according to one embodiment of the present invention.

FIG. 5 is a schematic plan view showing an annular leaf spring used in the configuration shown in FIG. 4;

FIG. 6 is a schematic perspective view showing the internal configuration of the electrodynamic accelerometer shown in FIG. 4 with some parts removed.

FIG. 7 is a schematic diagram illustrating an electrodynamic accelerometer configured according to another embodiment of the present invention.

FIG. 8 is a schematic perspective view showing the internal structure of the accelerometer shown in FIG. 7 in a somewhat enlarged manner.

FIG. 9 is a schematic diagram showing an expanded configuration of a leaf spring used in the accelerometer shown in FIGS. 7 and 8.

[Explanation of symbols]

 2 Housing 3,4 End plate 5,6 Spacer 7,8 Support spring 9,10 Pole member 11 Magnet 12 Bobbin 13 Coil 20 Operational amplifier

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masahiro Kamada Texas, 77479, Sugar Land, Austin Parkway 1800, No. 1806 (56) References JP-A-61-17959 (JP, A) JP-A-2- 254366 (JP, A) JP-A-3-71060 (JP, A) U.S. Pat. No. 4,412,317 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01P 15/11

Claims (6)

(57) [Claims] 1. A pair of magnets are fixed to a housing separated from each other and opposed to have the same polarity to define a combined magnetic flux at a predetermined position, and a single movable coil is substantially disposed between the pair of magnets. And a support means for movably supporting the movable coil with respect to the pair of magnets, and having a pair of input terminals and an output terminal. An operational amplifier connected to provide virtual ground at one of the pair of input terminals, and a circuit coupled to define a loop between the operational amplifier and the pair of input terminals; An electrokinetic accelerometer comprising the movable coil in series therein. 2. The electrodynamic accelerometer according to claim 1, wherein said pair of magnets are disk-shaped permanent magnets. 3. The electrodynamic accelerometer according to claim 1, wherein said pair of magnets are ring-shaped permanent magnets. 4. The electrodynamic accelerometer according to claim 1, wherein a yoke member is interposed between the pair of magnets. 5. The electrodynamic device according to claim 1, wherein said support means has a bobbin, and said movable coil is wound around said bobbin. Type accelerometer. 6. The electrodynamic accelerometer according to claim 5, wherein said support means has a pair of substantially annular leaf springs, and said bobbin is fixed to said springs.