JPH0814918A - Pot-shaped optical fiber gyro sensor coil - Google Patents

Abstract

PURPOSE: To provide a gyroscopic sensor coil for use in a severe vibratory and thermal environment by forming a coil in a pot shape from a silicon material containing a filler of carbon black, enhancing the hardness of polymer within the elasticity region, and decreasing the bias sensitivity of a gyroscope for vibrations. CONSTITUTION: The winding of an optical fiber 12 is robotized within the base material consisting of adhesive 16. Generally, the existence of adhesive 16 provide many advantages because of gyroscopic effect, including the promotion of preciseness in the coil windings. That is, the adhesive 16 is turned in pot shape applied layer by layer and hardened, so that smooth surfaces are generated among the windings of the subsequent layers. Such windings encourage the control of the coil structure resulting therefrom including essential factors such as the fiber spacing, the number of turns per layer, and the number of layers per coil and minimizes the winding defects such as shortage of turns.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to fiber optic gyroscopes. In particular, the present invention relates to a pot-shaped sensor coil design used in a severe vibration environment or a rapidly changing temperature environment.

[0002]

2. Description of the Related Art An optical fiber gyroscope for an interferometer comprises the following main components. That is, (1) light source and (2) “minimum reciprocal form” (S. Ezekiel and MJ Arditty, Fiber Optic
Rotation Sensors , New York, Springer-Verlag p. 2-2
6 1982) beam splitter (optical fiber directional coupler and / or integrated optical Y-branch) to meet requirements
And (3) a fiber optic sensing coil made of either polarization-maintaining (PM) fibers or low birefringence (standard telecommunications) fibers, and (4) polarizers (and sometimes one or more depolarizations). Container) and (5) detector. The light from the light source is split by the loop beam splitter into counter-propagating waves traveling through the sensing coil. The associated electronics measure the phase relationship between two interfering counter-propagating light beams that originate from both ends of the coil. The difference in phase shift experienced by the two beams is proportional to the rotational speed of the workbench where the instrument is fixed due to the well-known Sagnac effect.

Environmental factors can affect the phase shift difference between the measured back-propagating beams, which can introduce bias errors. The environmental factors include variables such as temperature, vibration (acoustic and mechanical) and magnetic field. Generally, the factors change over time,
It is distributed unevenly throughout the coil. These environmental factors cause a change in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts caused by the two waves are not equal, resulting in a net undesired phase shift indistinguishable from the rotation induced signal.

One approach to reducing the sensitivity resulting from environmental factors has required the use of various symmetrical coil winding configurations. In the coil, the windings are arranged such that the structural center of the coil is located in the innermost layer while the two ends of the coil are located in the outermost layer. N. Frigo says “Compensation of Li
near Sources of Non-reciprocity in Signal Interfer
ometers ", Fiber Optics and Laser Sensors I , Prc
s. SPIE, Vol.412, p.268 (1983), proposed the use of specific winding patterns to compensate for non-reciprocity. Further, Bednarz U.S. Pat. No. 4,793,708 entitled "Fiber Optic Sensing Coil" teaches a symmetrical fiber optic sensing coil formed by two or four pole windings. The coil disclosed in this patent is
It shows enhanced performance over conventional spiral wound windings.

US Pat. No. 4,856,900 to Ivancevic, entitled "Quadrupole Fiber Optic Sensing Coil and Method of Making It," discloses fiber tightening and microminiaturization due to the presence of pop-up fiber segments adjacent the end flange. Bending teaches an improved 4-pole wound coil that is overcome by replacing the pop-up segment with a concentrically wound turn wall that rises up between the connecting layers. Both of the US patents referenced above are the property of their respective assignees. Proper coil winding techniques minimize some of the bias error in the output of fiber optic gyros, but do not eliminate all of the bias. In particular, the design of the gyro sensor coil, random steps of the gyro, bias stability, bias temperature sensitivity, bias temperature gradient sensitivity, bias vibration sensitivity, bias vibration sensitivity, bias magnetic sensitivity, scale factor temperature sensitivity, scale factor linearity, And, it may have a strong influence on the input shaft temperature sensitivity.

[0006] Potting the coil windings of the sensor coil within the adhesive matrix has been found to be beneficial because it promotes coil coil precision. In addition, the composition of the potting material depends on the non-reciprocal phase shift between the light waves propagating in opposite directions in the coil as a result of changes in fiber length and refractive index caused by vibrational dynamic strain. Co-inventors Amado Cordova, Donald J. Bilinski, and Samuel N. Ferscht (“Sensor Coils for Low-Bias Fiber Optic Gyroscopes”) can have a considerable effect on vibration bias sensitivity. Samuel N. Fersht), Glenn Emslavian (Glenn
M. Surabian), John D. Wil
de) and Paul A. Hinman in pending US patent application Ser. No. 07 / 938,294. The cited US patent application discloses a sensor coil whose design includes many features that minimize the aforementioned environmental factors.
Among the issues identified and addressed in this patent application is the relationship between the elastic modulus of the potting material composition of the capsule sensor coil and the vibration-induced bias.

In general, gyro performance with respect to vibration-induced bias is a result of high modulus or Young's modulus (other problems associated with gyro operation at temperatures well away from the potting material curing temperature, such as temperature-related coil cracking and This application is significantly improved by using a potting material with h-parameter (polarization cross-coupling) degradation when the coil is made of PM fiber composition and not so high as to cause large bias temperature sensitivity. Is disclosed in. Polymers are attractive candidates for tacky potting materials due to their common properties such as substantial impermeability such as moisture. The sensor coil is constructed of a polymer embodying the teachings of the cited patent application. For example, the trademark "NO
The coil encapsulated with UV curable acrylic adhesive commercially available on RLAND 65 "showed sufficient vibration bias characteristics. However, when it was cycled through the temperature range including the operating range of the gyro, the coil was It has shown some temperature-related transformations that are agitated, including so-called “bias spiking” and “bias crossing.” Each of the above phenomena can significantly hinder the good operation of the gyro. It may be large enough to be removed from the specifications.
Bias crossings can effectively demonstrate the ability to represent impractical or impractical bias errors.

[0008]

SUMMARY OF THE INVENTION The present invention addresses the above and additional shortcomings and disadvantages of the prior art and provides a sensor coil for a fiber optic gyroscope. Such coils include optical fibers. The fiber is arranged in concentric cylindrical layers. Each layer consists of multiple turns of the fiber, each turn arranged in a predetermined winding pattern. The turns are encapsulated with potting material whose glass transition temperature is outside the operating range of the gyro. The above and other features and advantages of the present invention will become more apparent from the detailed description below. The above description is accompanied by a set of drawings. The numbers in the figures refer to the various features of the invention that correspond to the numbers in the description and the same numbers refer to the same features in each.

[0009]

1 is a perspective view of a sensor coil 10 according to the present invention. As mentioned above, the sensor coil 10 provides an important element of a fiber optic gyroscope system. In use, the sensor coil 10 is fixed to the workbench whose rotational speed is to be measured. The sensor coil 10 consists of an optical fiber 12 wound on a support spool 14 and serves as a light guide for counter-propagating beam pairs emanating from a common light source (not shown). The support spool 14 of FIG. 1 includes end flanges, the presence or absence of which does not form part of the claimed invention.

Spool 14 is preferably a mixture of carbon hybrids or the Amoco Corporatio under the trademarks "P-25", "P-55" or "P-105".
n) pending US patent application Ser. No. 07/9, including woven carbon fibers such as those commercially available from sources such as
It consists of other materials with the same processing thermal properties, in particular hardened materials with a low coefficient of thermal expansion, as disclosed in 38,294. The spool 14 is formed of fibers made into a multiplicity of fiber layer tubes or sheets with a splice substrate, eg, of a phenolic material. Spool 1
4 may be formed from the tube or sheet by many known methods including, for example, cuts therefrom. Alternately, the woven fiber can be trimmed in a predetermined direction to give it a tint and shape the mold of bonding material surrounding it. Another method uses transfer molding in which chopped fibers are mixed with transfer molding material and transferred or pressed into a transfer mold. The fibers are preferably oriented longitudinally and circumferentially at right angles to the axis of rotation 22 of the spool within the bond matrix material. With such fiber alignment, spool 14 will expand longitudinally and radially symmetrically with temperature.

As disclosed in the cited patent application, the theoretical model of bias non-reciprocity of a fiber optic gyroscope revealed by the inventor is that the gyro bias error in a dynamic thermal environment is due to thermal stress. It discloses that there is something to do. This effect is "Therm
ally Induced Non-Reciprocity in the Fiber Optic In
terferometer ", DMShupe, Applied Optics , Vol. 19,
This is the same as the standard temperature-shoop effect published on p.654 (1980). Spool 14 based on carbon hybrid
To address the thermal mismatch that exists between one of the bias sources, a glass optical fiber and a conventional metal spool. Other sources of thermal stress induced bias are
Thermal stress due to expansion / contraction (described below) of the coil pot material. The difference between the standard temperature Shupe effect and the thermal stress-induced Shupe effect is clearly noticeable when the coil is exposed to a steady temperature gradient. The bias error due to the standard Shupe effect becomes less constant as the temperature gradient becomes constant over time, but the bias error due to the thermal stress effect does not become zero while the coil temperature changes, and the effect is It remains even after the gradient reaches steady state. Contrasting these effects, the standard Shupe effect is primarily a function of the rate of change of the temperature gradient across the coil, while the thermal stress-induced Shupe effect is primarily the rate of change of the average temperature of the coil. Is a function of.

FIG. 2 is an enlarged cross-sectional view of a representative portion of layered windings of optical fiber 12. As can be seen, the windings of optical fiber 12 are potted in a substrate of adhesive 16. In general, the presence of the adhesive 16 offers many advantages for the gyro. These include facilitating coil winding precision. That is, the potting adhesive 16 may be applied layer by layer and cured so that a smooth surface is present between the windings of subsequent layers.
Such winding conditions enhance control of the resulting coil structure, including essential factors such as fiber spacing, number of turns per layer, number of layers per coil, and windings such as "insufficient turns". Minimize line defects.

Various manufacturing methods can be used to make a coil in which the turns or windings are embedded in the substrate of the potting adhesive. The method includes, for example, applying and curing the adhesive with a dropper-type dispenser. The method ensures that a smooth surface is provided for subsequent image windings. UV-curable adhesives that can be rapidly cured are most suitable for the method. Other manufacturing methods include dry coil winding with vacuum injection with a very low viscosity adhesive. Another wet winding technique uses a thermosetting adhesive that is applied while winding the coil. The adhesive remains uncured (in flow form) during winding. The finished (rolled) coil is then heat set.

While potting the coil offers many advantages, the choice of potting material and the method of application can itself significantly affect gyro performance. In particular, careful selection of potting adhesive 16 can significantly reduce the sensitivity of sensor coil 10 to vibration induced bias errors and temperature effects. The bias vibration sensitivity of the sensor coil results from the action in the coil that introduces into its output a non-reciprocal phase error that cannot be distinguished from the rotational speed signal. Said sensitivity is caused by the non-reciprocal phase difference of the waves propagating in opposite directions, which in turn results from the changes in fiber length and refractive index carried over by the oscillatory dynamic strain. This bias error is similar in nature to the previously mentioned Shuep bias error, the main difference being that environmental disturbances are oscillatory strains rather than temperature changes.

The inventor has found that the gyroscope open-loop response to a sinusoidal oscillatory sweep is a linear function of the oscillatory frequency when the resonant frequency deviates significantly from the instrument performance bandwidth (and the noise factor is negligible). This was observed experimentally. This is true if the vibration direction is either parallel to the coil input shaft (vibrating axially) or perpendicular (vibrating laterally). Is. That is, the bias vibration sensitivity of the optical fiber gyro becomes a linear function of the vibration frequency, and the inventor clarified the result predicted by the bias vibration sensitivity model. Further, under lateral vibration, the gyro output exhibits a nearly sinusoidal (ie, varying as SIN of azimuth angle) azimuth dependence.

The consequences of this vibration sensitivity are significant. Even if a DC DC bias effect called "DC rectification" is not observed, vibration-induced saturation of electronic components can prevent closed-loop electronics from monitoring rotational speed at constant vibration frequencies. This can appear as apparent DC rectification. Also, angular velocity noise results from the vibrations as well as system level pseudo-cone formation. The problems associated with the vibrations mentioned above are
It can be minimized or eliminated by aligning the substrate of potting adhesive with the fiber windings to minimize the oscillatory dynamic stresses that occur in the fiber windings. High stresses and strains in the fiber core result from dynamic amplification. This deleterious dynamic amplification effect can be traced to the use of potted adhesives that have insufficient elastic hardness. However, the use of very high Young's modulus potting material is limited or limited to some extent by the constant temperature-related effect of the potting material becoming too stiff. Among these effects are coil cracking, h-parameter (polarization cross-coupling) degradation when the coil is made of PM fiber, and large bias temperature gradient sensitivity. A satisfactory solution to vibration-induced bias has been found by potting the sensor coil with various polymers that are highly desirable due to their viscosity and encapsulation properties. However, when potting the coil with polymer, the gyro has been found to exhibit other significant bias errors that appear independent of vibration. Most importantly, the inventor has found that bias spiking and / or bias crossing disturbance phenomena occur uniformly in gyros using coil potting materials such as UV cured acrylic adhesives marketed as NORLAND 65. I found that. The transformation appears when the sensor coil circulates above the operating temperature range.

FIG. 3 is a graph of coil temperature (over about -10 ° C. to 70 ° C.) versus bias error (degrees per hour) for a gyro containing a sensor coil potted with NORLAND 65 acrylic adhesive. . The test coil is a 200 meter Corning
ning corporation) 165 micron fiber. It was wound into a 20-layer form on a mandrel of carbon composite material. The coil temperature was ramped up and down. The data as plotted was corrected for both the linear relationship between gyro output and temperature and temperature rate dependence. The remaining residual bias has coil temperatures of -10 ° C and 7
It is characterized by a standard deviation of 0.62 degrees per hour as it cycles between 0 ° C. As can be seen in the figure, a steep and extreme deviation of the data (bias spike) occurs around 50 ° C. The bias spike in the figure exceeds 5 degrees per hour.

FIG. 4 is a similar graph of data obtained from another NORLAND 65 potted sensor coil cycled between about -25 ° C and 70 ° C. The data from this coil represents the bias crossing phenomenon. The standard deviation was 0.61 with a peak-to-peak bias excursion of more than 4 degrees per hour. A plot of the data obtained from different directions of the temperature gradient is
It intersects with each other at two points corresponding to about 5 ° C and 50 ° C. The crossing shows that the bias dependence (that is, the Shupe coefficient) on the temperature change rate also becomes temperature dependent. The dependence is, in turn, very complicated for bias modeling and for bias analysis to model bias errors outside the gyro output signal which is impractical.

The inventor has linked the phenomenon described above to the physical action of the polymer used as the potting material. All polymers are characterized by their so-called glass transition region, ie the temperature range over which a considerable change in the Young's modulus of the material is observed. In this region, a transition from a glassy state to an elastic state occurs as the temperature increases. The polymer has 1 over its glass transition region.
50,000 p. s. i. From the above, 400 p. s. i.
Can show changes in hardness. In FIG. 5, the temperature is -10.
Temperature and cured NOR when increasing from 0 ℃ to 100 ℃
It is a graph of the relationship of Young's modulus of LND 65. As can be seen, the sharp decrease in the Young's modulus of the material begins when the acrylic adhesive cools to approximately 0 ° C and the transition ends at approximately 50 ° C. This corresponds to the physical transition from the vitreous state to the elastic state. The center of the transition region almost coincides with the peak 18 of the dotted line graph of Young's modulus that occurs at 28 ° C. NORLAN
The Young's modulus of D 65 is about 22 over the transition region.
10,000 p. s. i. From about 400 p. s. i. The hardness is reduced by 500 times.

The inventors have noted that the adverse phenomena of bias spiking and bias crossing occur at temperatures in or near the glass transition region of the potted material. In fact, the inventor has found that "bias spiking" and "bias crossing" are both two "edges" of the transition region.
It was found to occur in the vicinity of 0 ° C., that is, around 0 ° C. and around 50 ° C. This led the inventor to infer a correlation between the temperature dependence of the bias phenomenon and the glass transition region action of the polymer potting material, and to make a new potting material based on it. Accordingly, the inventor has (1) a glass transition temperature outside the operating range of the gyro and (2) (coil form factor and expected vibrations) in order to effectively reduce the bias vibration sensitivity to acceptable levels. A polymer-based potting material for the sensor coil was sought, which was characterized by both a rather large elastic modulus (based on resonance).

Generally, for commercial use, -40 ° C to 60 ° C.
The operating range is ℃, but the military specification is -5.
It requires operation over the range of 5 ° C to 105 ° C. Of course, it is noted that the glass transition temperature of the NORLAND 65 potted sensor coil and the resulting detrimental bias spikes and crossings are unfortunately well within both military and commercial operating temperature specifications. . The inventor has developed a polymer-based potting material that does not expose potted coils and gyros to the type of errors experienced with polymers such as NORLAND 65. This was accomplished in two ways. First, the coil is potted with a polymer adhesive whose glass transition region is outside the operating temperature range of the gyro sensor coil. Secondly, a suitable "filler" can be added to the polymer to allow for the dependence of the vibration-induced bias on the stiffness and to solidify the polymer in the elastic region to obtain the required Young's modulus. .

In particular, the inventor has discovered silicon which provides a good candidate material. Their glass transition temperature is -5.
Below 5 ° C and therefore out of commercial and military specifications. Therefore, the above-mentioned materials are satisfactory in ensuring that no significant changes in the Young's modulus of the material, which lead to bias spikes and bias crossings, occur during normal gyro operation, but beyond the glass transition region ( The Young's modulus of the material in the (more positive) temperature region has a much lower modulus than in the transition region and below. This is, of course, a symptom of all polymers. For example, in FIG. 5, note that the Young's modulus becomes very stable when the transition temperature exceeds a limit, but at a significantly reduced stiffness that cannot be large enough to provide the desired resistance to vibration-induced bias. be able to. Similarly, FIG.
Has a Young's modulus of about 370 p.s. s. i. It is therefore not as stiff as necessary for the gyroscope required to operate in a severe vibration environment.

The inventor has discovered that some "fillers" of varying material composition can be added to silicon to enhance material performance with respect to vibration-induced bias. Therefore, the addition of said filler makes it possible to obtain the desired vibration resistance despite the relatively low Young's modulus of the silicon material "only" when the glass transition temperature is exceeded. In summary, the addition of filler increases the hardness of the silicone to the elastic region and reduces the gyro vibration sensitivity to the desired level. Among the fillers that make pot-shaped sensor coils with excellent bias properties is carbon black. This material is known to chemically react with rubber to increase tensile strength and Young's modulus. Therefore, carbon black is known as a reinforcing filler for rubber. The inventor has found that the reinforcing properties of carbon black are applicable to the various problems mentioned above. Carbon black also increases the cohesive strength of the silicon composition so that silicon potted coils containing carbon black fillers are more resistant to thermally induced cracking and leaf clefts due to reduced cohesion. If various fillers are added to the silicon only, care must be taken with respect to possible degradation of adhesion or bond strength. Fiber "firing" above 75 ° C
Improves the bonding strength between the fiber and the potting material. Other satisfactory fillers for silicon potting materials not included by the inventor include glass particles, quartz, silicon carbide, graphite and alumina (aluminum oxide) powder.

FIG. 6A shows the trademark "Mastersyl 1".
Commercially available at 51 "Master Bond, Hackensack, NJ
3 is a graph of Young's modulus vs. temperature for silicon composition of Inc.). FIG. 6B is a graph of Young's modulus versus temperature for the same silicon material filled with carbon black. The carbon black filler used was Alberta, Canada.
Marketed by Cancarb Limited of Medicine Hat, Alberta
It was THERMAX Medium Thermal Black N-991. Figure 6A
As can be seen in Fig.
0p. s. i. The glass transition of silicon materials is centered around −66 ° C., where the hardness of the material is detrimentally reduced as it increases. This effect should be contrasted with the carbon black filled silicon material of FIG. 6B where the glass transition region is better centered at -72 ° C. below the gyro operating range. It can be seen that the Young's modulus of the material gradually decreases because the filler is heated above the glass transition temperature. In contrast to the silicone material lacking filler, the sample of Figure 6B
About 1,500 p.s. s. i. The value of is approaching the stable Young's modulus. Thus, experimental data demonstrate the effectiveness of a particular filler, carbon black, as it hardens a silicon material when heated above its glass transition temperature.

The inventor has applied the observed desirable properties of silicon and silicon-filled potting material to the fabrication of gyro sensor coils and has achieved remarkable results with regard to improved performance. FIG. 7 is a graph of bias error as a function of temperature for a gyro that includes a sensor coil potted with the carbon black hardened silicon composition of FIG. 6B. Bias data was obtained from a 40-layer wound sensor coil made of Corning's 200 meter 165 micron optical fiber. 3 and 4
As in the case of the data plotted in the graph of Figure 7,
Data have been corrected for both gyro output and temperature linear relationships and temperature rate dependence. The remaining residual bias is due to sensor coil temperatures of -50 ° C and 95 ° C.
It can be seen that as it cycles between ° C it has a standard deviation of 0.19 degrees per hour, which is negligible. Bias spikes and bias crossings are not present in the data of FIG. A comparison of the data of FIG. 7 with the data of FIGS. 3 and 4 is dramatic and substantiates the inventor's insights and hypotheses regarding the nature of the problems associated with potting sensor coils with polymers.

FIG. 8 is a graph of vibration frequency versus AC bias due to sinusoidal vibration for a carbon black filled silicon potted coil. This bias data was also obtained from a 40-layer wound coil formed of 200 meter 165 micron optical fiber from Corning. As this graph shows, the AC bias output is virtually negligible at this measurand. The acceleration level was kept constant at 1 g. Thus, it can be seen that the teachings of the present invention provide a substantially improved sensor coil with respect to minimizing bias sensitivity due to dynamic thermal and vibrational environments. Using the teachings of the present invention, it is possible to obtain gyro performance that is substantially free of environmentally-induced bias errors that were not previously recognized or addressed in the prior art. Although the present invention has been described with respect to the presently preferred embodiment, it is not so limited. Rather, the invention is limited only insofar as defined by the appended claims and includes all equivalents thereto.

[Brief description of drawings]

FIG. 1 is a perspective view of a sensor coil for an optical fiber gyroscope according to the present invention.

FIG. 2 is an enlarged cross-sectional view of an exemplary portion of a layered winding of a sensor coil according to the present invention.

FIG. 3 is a graph showing bias as a function of temperature for a gyro that includes a sensor coil potted with a NORCAND 65 UV cured acrylic adhesive.

FIG. 4 is a graphical representation of bias as a function of temperature for a gyro that includes a sensor coil potted with a NORLAND 65 UV curable acrylic adhesive.

FIG. 5: Cured NORLAN as a function of temperature
It is a figure of a Young's modulus graph of an acrylic resin adhesive called D65.

FIG. 6A is a graphical representation of the Young's modulus of a silicon composition without carbon black filler as a function of temperature.

FIG. 6B is a graphical representation of Young's modulus of silicon composition with carbon black filler as a function of temperature.

FIG. 7 relates to a gyro including a sensor coil potted with a silicon composition containing carbon black filler,
FIG. 5 is a graph of bias error as a function of temperature.

FIG. 8: A by vibration frequency vs. vibration for a carbon pot filled silicon potted coil.
It is a figure of the graph of C bias.

[Explanation of symbols]

 Optical fiber 12 Support spool 14 Adhesive 16

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Glenn M. Slavian United States, 91304 California, West Hills, Jonathan Street 23106

Claims (8)

[Claims] 1. A sensor coil for a fiber optic gyroscope, comprising: a) an optical fiber; b) the fiber being arranged in a plurality of concentric cylindrical layers; and c) each layer being a plurality of layers of the fiber. D) each turn is arranged in a predetermined winding pattern, and e) each turn is encapsulated with a potting material whose glass transition temperature is outside the operating range of the gyroscope. A sensor coil characterized by. 2. The sensor coil according to claim 1, wherein
Furthermore, the potting material contains silicon, The sensor coil characterized by the above-mentioned. 3. The sensor coil according to claim 2, wherein
Furthermore, the said potting material contains the filler of a predetermined composition, The sensor coil characterized by the above-mentioned. 4. The sensor coil according to claim 3,
The sensor coil in which the filler is carbon black. 5. The sensor coil according to claim 3,
The filler is a sensor coil made of glass particles. 6. The sensor coil according to claim 3,
The filler is a sensor coil made of silicon carbide. 7. The sensor coil according to claim 3,
The filler is a sensor coil made of graphite. 8. The sensor coil according to claim 3,
The filler is a sensor coil made of aluminum oxide powder.