Electrostatic capacitance tilt angle sensor

An electrostatic capacitance tilt angle sensor having a size that is reduced without lowering detection capacity. The sensor is provided with a case including a first wall and a second wall facing each other. A common electrode is arranged on the first wall and a differential electrode is arranged on the second wall. A liquid electrostatic capacitance medium is accommodated in the case and contacts the common electrode and the differential electrode. The electrostatic capacitance medium includes an insulative liquid base and fine particles mixed in the base. The fine particles have a dielectric constant that is higher than that of the base.

BACKGROUND OF THE INVENTION

The present invention relates to sensors, more specifically to an electrostatic capacitance tilt angle sensor, installed in a measuring instrument or a vehicle to detect the tilt angle.

Japanese Laid-Open Patent Publication No. 08-261757 describes a prior art example of an electrostatic capacitance tilt angle sensor. The electrostatic capacitance tilt angle sensor includes an oil case, a liquid electrostatic capacitance medium contained in the oil case, and two differential electrodes and two common electrodes arranged in the oil case. The parts of the two differential electrodes and the two common electrodes immersed in the electrostatic capacitance medium each configure two capacitors. When the electrostatic capacitance tilt angle sensor is in a horizontal state, the parts of each differential electrode immersed in the electrostatic capacitance medium have substantially the same area (immersed area). Thus, the capacitors have substantially the same electrostatic capacitances. When the electrostatic capacitance tilt angle sensor is tilted, the immersed area of one of the differential electrode increases and the immersed area of the other one of the differential electrodes decreases. This produces a difference between the electrostatic capacitances of the capacitors. The electrostatic capacitance tilt angle sensor calculates the tilt angle based on the difference between the electrostatic capacitances.

The conventional electrostatic capacitance tilt angle sensor has a volume of several cubic centimeters to more than a hundred cubic centimeters and is thus relatively large. There has been a recent demand for a smaller electrostatic capacitance tilt angle sensor that occupies less space. However, when the electrostatic capacitance tilt angle sensor is simply made smaller, the detection reliability and detection resolution fall due to the decrease in electrostatic capacitance and the influence of the surface tension of the electrostatic capacitance medium on the inner wall surface of the oil case.

SUMMARY OF THE INVENTION

The present invention provides a compact electrostatic capacitance tilt angle sensor without lowering the capacity of the sensor.

One aspect of the present invention provides a sensor for detecting tilt angle. The sensor is provided with a case including a first wall and a second wall facing each other. A common electrode is arranged on the first wall. A differential electrode is arranged on the second wall. A liquid electrostatic capacitance medium is accommodated in the case contacting the common electrode and the differential electrode. The electrostatic capacitance medium includes an insulative liquid base and fine particles that are mixed in the base and have a higher dielectric constant than the base.

A further aspect of the present invention is a sensor for detecting tilt angle. The sensor is provided with a case including a first wall and a second wall, which face each other, and a central portion. A common electrode is arranged on the first wall. A differential electrode is arranged on the second wall. A liquid electrostatic capacitance medium is accommodated in the case contacting the common electrode and the differential electrode. A projection is arranged on at least one of the common electrode and the differential electrode in at least the vicinity of the central portion of the case.

Another aspect of the present invention is a sensor for detecting tilt angle. The sensor is provided with a case including a first wall and a second wall, which face each other, and a central portion. A common electrode is arranged on the first wall. A differential electrode is arranged on the second wall. A liquid electrostatic capacitance medium is accommodated in the case contacting the common electrode and the differential electrode. A cylindrical first boss is arranged in the central portion of the case. A plurality of second bosses are arranged symmetrically with respect to the center of the case about the first boss.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrostatic capacitance tilt angle sensor1according to a first embodiment of the present invention will now be described in detail with reference toFIGS. 1A to 7.

FIG. 1Ais a partial cross sectional front view showing the electrostatic capacitance tilt angle sensor (hereinafter referred simply as “tilt angle sensor”)1of the first embodiment.FIG. 1Bis a partial cross sectional side view of the tilt angle sensor1. The cross sectional position inFIG. 1Bis taken along line1B—1B inFIG. 1A, and the cross sectional position inFIG. 1Ais taken along line1A—1A inFIG. 1B.

As shown inFIG. 1A and 1B, the tilt angle sensor1includes a case11. The case11includes a first wall12, a second wall13, and a third wall14, which are made of synthetic resin. As shown inFIG. 1B, the first wall12and the second wall13are arranged facing each other, and the third wall14is arranged between the first wall12and the second wall13. The first wall12and the second wall13are plates having sides with lengths of approximately four to six millimeters.

As shown inFIG. 1BandFIG. 2, common electrodes15aand15bare arranged on the surface of the first wall12facing the second wall13. Differential electrodes16aand16bare arranged on the surface of the second wall13facing the first wall12. The common electrodes15aand15band the differential electrodes16aand16bare each substantially semicircular. The common electrode15afaces towards the differential electrode16a. The common electrode15bfaces towards the differential electrode16b. The electrodes15a,15b,16a, and16bare formed through processing techniques such as, hot embossment processing, printing, and vapor deposition. Further, as shown inFIGS. 1A,1B, and2, terminals15cand15dare defined on the common electrodes15a,15b, respectively. Terminals16cand16dare formed on the differential electrodes16aand16b, respectively.

The third wall14is annular and arranged between the common electrodes15aand15band the differential electrodes16aand16b. The periphery of the third wall14is flush with the peripheries of the common electrodes15aand15band the differential electrodes16aand16b. The third wall14is formed so that a predetermined distance H (30 to 40 μm in the first embodiment) is provided from the common electrodes15aand15bto the differential electrodes16aand16b. Further, referring toFIG. 1A, the third wall14has an inner diameter R of about 3 to 5 mm. The case11is cylindrical. An accommodating space C is defined in the case11.

A first boss21(projection), which is cylindrical, is arranged at the central portion of the accommodating space C. The axis of the first boss21lies along the axis O of the accommodating space C. A first end face of the first boss21is connected to the common electrodes15aand15b. In other words, the first end face of the first boss21is indirectly connected to the inner surface of the first wall12facing the second wall13by the common electrodes15aand15b. The second end face of the first boss21is connected to the differential electrodes16aand16b. In other words, the second end face of the first boss21is indirectly connected to the inner surface of the second wall13facing the first wall12by the differential electrodes16aand16b. A plurality of equally spaced hypothetic circles that are concentric to the first boss21are defined in the space between the peripheral surface of the first boss21and the inner surface of the third wall14so as to equally divide the space into a plurality of sections (four in the first embodiment). A plurality of second bosses22(projections), which are cylindrical and have a diameter smaller than the diameter of the first boss21, are arranged along the circles. A first end face of each second boss22is connected to the common electrodes15aand15b. In other words, the first end face of the second boss22is indirectly connected to the inner surface of the first wall12facing the second wall13by the common electrodes15aand15b. The second end face of the second boss22is connected to the differential electrodes16aand16b. In other words, the second end face of the second boss22is indirectly connected to the inner wall surface of the second wall13facing the first wall12by the differential electrodes16aand16b. In the first embodiment, the second bosses22are equally spaced along each hypothetical circle. Each second boss22is arranged so that a line connecting it to the closest second boss22of a different circle (e.g., line L shown inFIG. 1A) extends through the axis O. In each circle, the second bosses22are arranged at every 1° about the axis O. Therefore, the second bosses22are arranged point symmetrically with respect to one another about the axis O. InFIG. 1AandFIG. 7, only some of the second bosses22are shown to simplify the drawings.

A liquid electrostatic capacitance medium23is accommodated in the accommodating space C. The liquid electrostatic capacitance medium23occupies about half of the accommodating space C. Thus, in a state in which the tilt angle sensor1is horizontal (state shown inFIG. 1A), the common electrodes15aand15band the differential electrodes16aand16bare about half immersed in the electrostatic medium23. The immersed parts of the electrodes15a,15b,16a, and16bfunction as capacitors. More specifically, the parts of the common electrode15aand the differential electrode16aimmersed in the electrostatic capacitance medium23configure a first capacitor. The parts of the common electrode15band the differential electrode16bimmersed in the electrostatic capacitance medium23configure a second capacitor. When the tilt angle sensor1is horizontal, the parts of the common electrode15aand the differential electrode16aimmersed in the electrostatic capacitance medium23have substantially the same area as that of the parts of the common electrode15band the differential electrode16bimmersed in the electrostatic capacitance medium23. Therefore, in this state, the electrostatic capacitances of the first and the second capacitor are substantially the same.

As shown inFIG. 3, the electrostatic capacitance medium23includes a liquid base23a, which is made of an insulating material (e.g., silicon oil (dielectric constant εa=2.7)), and fine particles23b, which is made of an insulating material (e.g., barium titanate (dielectric constant εb=100 or greater)) mixed in the base23a. In the first embodiment, modified silicon oil is used as the base23a. More specifically, the base23ais made of reactive silicon oil or non-reactive silicon oil, and has a modified structure of side-chain type, double-end type, single-end type, or side-chain-double-end type. Reactive silicon oil includes amino modified, epoxy modified, carboxyl modified, methacryl modified, mercapto modified, phenol modified, or heterogeneous functional group modified silicon oil. The non-reactive silicon oil includes polyether modified, methyl styryl modified, alkyl-modified, higher fatty acid ester-modified, hydrophilic specially modified, higher alkoxy modified, higher fatty acid containing or fluorine modified silicon oil. The fine particles23bare nanoparticles having a diameter of several tens of nanometers. The fine particles23bare mixed in the base23aand occupy about 10% to 15% of the electrostatic capacitance medium23. The fine particles23bare extremely small, and thus perform Brownian motion in the base23a. The fine particles23bare thus equally distributed throughout the base23a. Particularly, since the base23ais modified silicon oil, the fine particles23bare easily and equally mixed in the base23a. The dielectric constant εc of the electrostatic capacitance medium23of the first embodiment is approximately 135 and about 50 times of that compared to when the electrostatic capacitance medium23includes only of the base23a.

As shown inFIGS. 1A and 4, the surface tension of the liquid level of the electrostatic capacitance medium23accommodated in the accommodating space C acts on the inner wall surface of the case11, the first boss21, and the second bosses22closest to the liquid level. Thus, the liquid level of the electrostatic capacitance medium23is substantially horizontal. If, for example, the first boss21and the second bosses22were not arranged in the accommodating space C, the surface tension of the liquid level of the electrostatic capacitance medium23would only act on the inner wall surface of the case11. The accommodating space C is an extremely small space having an inner diameter R of 3 to 5 mm and a clearance H (predetermined gap) of 30 to 40 μm. Therefore, if the surface tension of the liquid level of the electrostatic capacitance medium23were to act only on the inner wall surface of the case11, the liquid level would not be horizontal due to the surface tension, as shown inFIG. 5. In this case, the change in the difference between the electrostatic capacitance of the first capacitor and the electrostatic capacitance of the second capacitor would not necessarily correspond to changes in the tilt of the tilt angle sensor1.

As shown inFIG. 1, a processor board31is arranged on the outer wall surface of the second wall13in the case11. First to third terminals32ato32care arranged on the surface facing the case11of the processor board31. The first terminal32ais electrically connected to the terminal16cof the differential electrode16aby a wire33a. The second terminal32bis electrically connected to the terminal16dof the differential electrode16bby a wire33b. The third terminal32cis electrically connected to the terminals15cand15dof the common electrodes15aand15bby a wire33c.

A detection circuit is arranged in the processor board31to convert the difference of the electrostatic capacitances of the first and the second capacitor (“electrostatic capacitance of first capacitor”—“electrostatic capacitance of second capacitor”) to a voltage difference and externally output a detection voltage Voutbased on the voltage difference. More specifically, as shown inFIG. 6, the detection circuit outputs a detection voltage Voutthat is equal to a predetermined reference voltage Vs when the tilt angle sensor1is horizontal, that is, when the voltage difference is “0”. When the tilt angle sensor1is tilted in one direction, for example, the electrostatic capacitance of the first capacitor increases and the electrostatic capacitance of the second capacitor decreases. In this case, the voltage difference is shifted to the positive side, and the detection circuit adds the shifted voltage ΔV to the reference voltage Vs and outputs the detection voltage Voutthat is equal to voltage Vs+ΔV. Further, when the tilt angle sensor1is tilted in the opposite direction, the electrostatic capacitance of the first capacitor decreases and the electrostatic capacitance of the second capacitor increases. In this case, the voltage difference is shifted to the negative side, and the detection circuit subtracts the shifted voltage ΔV from the reference voltage Vs and outputs the detection voltage Voutthat is equal to voltage Vs−ΔV.

As shown inFIG. 1, the case11and the processor board31are sealed in a package34made of a synthetic resin or ceramic.

The operation of the tilt angle sensor1will now be described.

When the tilt angle sensor1is held horizontally, the area of the parts of the common electrode15aand the differential electrode16aimmersed in the electrostatic capacitance medium23is substantially the same as the area of the parts of the common electrode15band the differential electrode16bimmersed in the electrostatic capacitance medium23, as mentioned above. Thus, the electrostatic capacitance of the first capacitor and the electrostatic capacitance of the second capacitor are substantially the same, and the voltage difference based on the difference between the electrostatic capacitances of the first and the second capacitors is substantially “0”. In this case, the tilt angle sensor1outputs a detection voltage Voutthat is equal to the reference voltage Vs.

As shown inFIG. 7, for example, if the tilt angle sensor1is tilted by a predetermined angle θ (inFIG. 7, θ=30°) in the left direction as viewed inFIG. 7from the horizontal state, the electrostatic capacitance of the first capacitor increases and the electrostatic capacitance of the second capacitor decreases. The difference between the electrostatic capacitance of the first capacitor and the electrostatic capacitance of the second capacitor is thus shifted to the positive side by a shifted amount proportional to the tilt angle θ. The voltage difference based on the difference of the electrostatic capacitances of the two capacitors is thus shifted to the positive side. As a result, the tilt angle sensor1outputs a detection voltage Voutthat is equal to voltage Vs+ΔVθ obtained by adding the shifted voltage ΔVθ, which is proportional to the tilt angle θ, to the reference voltage Vs. If the tilt angle sensor1is tilted by a predetermined angle θ in the right direction from the horizontal state, the voltage difference based on the difference of the electrostatic capacitances of the capacitors is shifted to the negative side. As a result, the tilt angle sensor1outputs a detection voltage Voutequal to voltage Vs−ΔVθ obtained by subtracting the shifted voltage ΔVθ, which is proportional to the tilt angle θ, from the reference voltage Vs.

In this way, the detection voltage Voutoutput from the tilt angle sensor1is shifted in accordance with the tilt angle. The tilt angle of the tilt angle sensor1is obtained from the detection voltage Vout. That is, the tilt angle sensor1outputs the tilt angle as the detection voltage Vout.

The tilt angle sensor1of the first embodiment has the following advantages.

(1) The electrostatic capacitance medium23includes the liquid base23aand the fine particle23b, which are mixed in the base23a, and has a dielectric constant that is about 50 times greater than the dielectric constant of the base23a. That is, by mixing the fine particles23bwith the base23a, the dielectric constant of the electrostatic capacitance medium23is reliably and easily increased. Thus, the tilt angle sensor1is made small but still has a high electrostatic capacitance. Thus, the tilt angle sensor1prevents the reliability and resolution of the tilt angle detection from being decreased. Since the dielectric constant of the electrostatic capacitance medium23is high, the distance (clearance H) between the common electrodes15aand15band the differential electrodes16aand16bmay be increased. This decreases the influence of the surface roughness of the electrodes15a,15b,16a, and16bon the electrostatic capacitance of the tilt angle sensor1, which, in turn, reduces manufacturing error in the electrostatic capacitance.

(2) The size of the fine particles23bis set so as enable the Brownian motion to occur in the base23a. The size of the fine particles23bis in the scale of nanometers in the first embodiment. This ensures that the fine particles23bperform Brownian motion in the base23a. The fine particles23bare thereby evenly dispersed throughout the base23aeven if an external force for dispersing the fine particles23bin the base23ais not applied. Thus, differences in the dielectric constant at different portions of the electrostatic capacitance medium23do not occur. This ensures detection reliability of the tilt angle.

(3) The first boss21and the second bosses22are arranged on the opposing inner wall surfaces of the case11. Thus, the surface tension of the liquid level of the electrostatic capacitance medium23acts on the first and the second bosses21and22. For this reason, even if the case11is compact and the volume of the accommodating space C is small, the liquid level of the electrostatic capacitance medium23is easily maintained in the horizontal state. Further, the liquid level of the electrostatic capacitance medium23remains horizontal even if the tilt angle sensor1is tilted. When the tilt angle sensor1is tilted, the electrostatic capacitances of the first capacitor and the second capacitor change, and the tilt angle sensor1detects the tilt angle based on the change in the electrostatic capacitances. If the bosses21and22were not arranged in the accommodating space C, the surface tension of the liquid level of the electrostatic capacitance medium23would act only on the inner wall surface of the case11. In this case, if the case11were small, the surface tension would curve and deform the liquid level. Thus, the electrostatic capacitances of the first capacitor and the second capacitor would change at a different rate when tilted, and the tilt angle sensor1may not output detection voltage that is in correspondence with the tilt angle. However, in the first embodiment, the tilt angle sensor1has the bosses21and22arranged in the accommodating space C. This improves the detection reliability of the tilt angle.

(4) The first boss21is arranged at the central portion of the accommodating space C. The second bosses22are equally spaced along concentric circles of different diameters about the axis O of the first boss21. Thus, irrespective of how the tilt angle sensor1is tilted, the surface tension of the liquid level of the electrostatic capacitance medium23always acts on the bosses21and22.

A tilt angle sensor1according to a second embodiment of the present invention will now be described with reference toFIG. 8AtoFIG. 9C.

As shown inFIG. 8AtoFIG. 9C, in the tilt angle sensor1of the second embodiment, the first boss21is arranged in the case11at the central portion of the accommodating space C in the same manner as the first embodiment. The diameter of the first boss21of the second embodiment is smaller than the diameter of the first boss21of the first embodiment. Rectifying walls41(projections) are arranged along hypothetic circles concentric to the first boss21to equally divide the space between the peripheral surface of the first boss21and the inner surface of the third wall14into a plurality of sections (in this embodiment, three sections). First ends of the rectifying wall41are connected to the common electrodes15aand15b. In other words, the first ends of the rectifying walls41are indirectly connected to the inner surface of the first wall12facing the second wall13by the common electrodes15aand15b. Second ends of the rectifying walls41are connected to the differential electrodes16aand16b. In other words, the second ends of the rectifying walls41are indirectly connected to the inner surface of the second wall13facing the first wall12by the differential electrodes16aand16b. That is, the rectifying walls41are coupled to two opposing inner wall surfaces of the case11. Each rectifying wall41extends along an arc of the associated circle. Two rectifying walls41are arranged on each circle. More specifically, the rectifying walls41formed along the same circuit are symmetrical to each other with respect to line8B—8B, as shown inFIG. 8A, and extend along arcs of center angle θw, as shown inFIG. 9A.FIGS. 9A to 9Conly show elements in the accommodating space C of the case11to simplify the drawings.

As shown inFIGS. 9A to 9C, a medium charge port42is arranged at the lower part of the case11, and an air release hole43is arranged at the upper part of the case11. The medium charge port42and the air release hole43are formed in the third wall14that forms the accommodating space C. The electrostatic capacitance medium23is charged into the accommodating space C from the medium charge port42. When the electrostatic capacitance medium23is injected, the air in the accommodating space C is released from the air release hole43.

As shown inFIGS. 8A to 9C, a plurality of medium guides44are formed between the first boss21and the medium charge port42in the accommodating space C. Each medium guide44has a cylindrical shape. First ends of the medium guides44are connected to the common electrodes15aand15b. In other words, the first ends of the medium guide44are indirectly connected to the inner surface of the first wall12facing the second wall13by the common electrodes15aand15b. Second ends of the medium guides44are connected to the differential electrodes16aand16b. In other words, the second ends of the medium guides44are indirectly connected to the inner surface of the second wall13facing the first wall12by the differential electrodes16aand16b. Further, the medium guides44are arranged along two rows extending from the medium charge port42towards the first boss21. More specifically, as shown in the enlarged view ofFIG. 9A, the medium guides44are arranged along two lines L1and L2extending from the medium charge port42to the first boss21. In the two lines L1and L2, the distance C1between the two medium guides44closest to the medium charge port42is greater than the distance C2between the two medium guides44closest to the first boss21. That is, the distance between the lines L1, L2decreases as the first boss21becomes closer.

Further, the distance between two adjacent medium guides44on each of the lines L1and L2, or the distance between two adjacent medium guides44in the direction from the medium charge port42towards the first boss21, decreases as the first boss21becomes closer. As shown in the enlarged view ofFIG. 9A, there are twelve medium guides44in the second embodiment. In this case, the distances T1to T5between two adjacent medium guides44in each line L1or L2satisfy the relationship of “T1>T2>T3>T4>T5”. The distances between adjacent medium guides44is set so that the medium guides44satisfy the relationship of “sparse→dense” from the medium charge port42towards the first boss21. Further, the distance T1is narrower than  the distance between the rectifying wall41and the third wall14. In the second embodiment, the distance T1is about twice the diameter of the medium guide44. The distance T5is about 0.2 times the diameter of the medium guide44.

As shown inFIGS. 8A to 9C, third bosses45, which are cylindrical, are arranged between the medium guides44and the rectifying walls41. A plurality of fourth bosses46, which are cylindrical, are arranged between the air release hole43and the first boss21in the accommodating space C. First ends of the third bosses45and the fourth bosses46are connected to the common electrodes15aand15b. In other words, the first ends of the third bosses45and the fourth bosses46are indirectly connected to the inner surface of the first wall12facing the second wall13by the common electrodes15aand15b. Second ends of the third bosses45and the fourth bosses46are connected to the differential electrodes16aand16b. In other words, the second ends of the third bosses45and the fourth bosses46are indirectly connected to the inner surface of the second wall13facing the first wall12by the differential electrodes16a,16b. The fourth bosses46are arranged at a predetermined interval along the circles that the rectifying walls41extend.

The flow of the electrostatic capacitance medium23when charged into the case11will now be described.

As shown inFIGS. 9A to 9C, the electrostatic capacitance medium23is charged through the medium charge port42into the accommodating space C of the case11in the direction of arrow F (upward). The surface tension of the electrostatic capacitance medium23then acts on the medium guides44so that, as shown inFIG. 9B, the medium guides44guide the electrostatic capacitance medium23toward the first boss21. More specifically, the electrostatic capacitance medium23first flows through a flow path (guide path) surrounded by the medium guides44and reaches the first boss21. Subsequently, as shown inFIG. 9C, the electrostatic capacitance medium23spreads between the first boss21and the inner rectifying wall41. Then, the electrostatic capacitance medium23spreads into the gap between the two rectifying walls41and then into the gap between the outer rectifying wall41and the third wall14. Thus, the electrostatic capacitance medium23is stably charged into the central portion of the case11. In addition, the surface tension of the liquid level of the electrostatic capacitance medium23acts on the peripheral surface of the first boss21, the outer surfaces of the rectifying walls41, and the inner surface of the third wall14. The liquid level of the electrostatic capacitance medium23is thus substantially horizontal, as shown inFIG. 8A.

When the tilt angle sensor1is tilted, the electrostatic capacitance medium23smoothly moves along the rectifying wall41. Thus, the liquid level of the electrostatic capacitance medium23is constantly maintained in a horizontal state. This ensures that the electrostatic capacitances of the first capacitor and the second capacitor change so that the tilt angle sensor1accurately detects the tilt angle based on the change of the electrostatic capacitance. The medium guides44are arranged so to satisfy the relationship of “sparse→dense” from the medium charge port42towards the first boss21. Further, when the electrostatic capacitance medium23moves within the case11as the tilt angle sensor1tilts, the medium guides44do not inhibit the movement of the electrostatic capacitance medium23.

Accordingly, the tilt angle sensor1of the second embodiment has the following advantages in addition to advantages (1) and (2) of the first embodiment.

(5) The first bosses21and the rectifying walls41are arranged on the opposing inner wall surfaces of the case11. Thus, the surface tension of the liquid level of the electrostatic capacitance medium23acts on the bosses21and the rectifying wall41. For this reason, even if the case11is compact and the volume of the accommodating space C is small, the liquid level is easily maintained in a horizontal state, and the liquid level is reliably maintained in a horizontal state even when the tilt angle sensor1is tilted. This ensures that the electrostatic capacitances of the first capacitor and the second capacitor changes when the tilt angle sensor1is tilted so that the tilt angle sensor1accurately detects the tilt angle based on the change of the electrostatic capacitances. In addition, the electrostatic capacitance medium23smoothly moves along the rectifying walls41when the tilt angle sensor1is tilted. Thus, the tilt angle sensor1further reliably detects the tilt angle based on the change of the electrostatic capacitances.

(6) When charged into the case11, the electrostatic capacitance medium23is smoothly guided by the medium guides44toward the central portion of the case11. This ensures the charging of the electrostatic capacitance medium23into the case11.

(7) The distances T1to T5between the adjacent medium guide44in the direction from the medium charge port42towards the first boss21decreases as the first boss21becomes closer. Thus, even if the electrostatic capacitance medium23is charged into the case11by a weak charging pressure, the medium guides44guide the electrostatic capacitance medium23to the central portion of the case11. Further, when the tilt angle sensor1is tilted, the medium guides44do not inhibit the movement of the electrostatic capacitance medium23. This prevents the responsiveness of the detection voltage Vout, which corresponds to the tilt angle of the tilt angle sensor1, from being lowered.

As shown inFIG. 1A, in the first embodiment, the second bosses22are equally spaced along each circle. In addition, the line L connecting the closest second bosses22of different circles extends through the axis O. Thus, as also shown inFIG. 4, the distance between adjacent second bosses22decreases as the circle on which the second bosses22are arranged becomes smaller. However, the second boss22does not have to be arranged with such regularity. As shown inFIG. 10, for example, the second bosses22may be arranged so that the distance T1between adjacent second bosses22is always the same regardless of the diameter of each circle. The second bosses22may also be arranged so that the distance between adjacent bosses22increases as the diameter of the circle becomes greater. Further, the second bosses22may be arranged so that the distance T1is equal to the distance T2between the closest second bosses22of different circles. In addition, the second bosses22do not necessarily have to be arranged along a circle of which center is the axis O. That is, the second bosses22may be arranged in any way as long as gaps are provided between the bosses22.

In the first embodiment, the first boss21arranged in the central portion of the case11(accommodating space C) has a greater diameter than the second bosses22. However, as shown inFIG. 11for example, a plurality of equally spaced second bosses22may be arranged in the central portion of the case11in place of the first boss21. That is, all the bosses arranged in the case11may have the same shape and size. Alternatively, bosses of three or more different sizes may be arranged in the case11.

In the first embodiment, the bosses21and22do not have to be cylindrical and may be polygonal.

As shown inFIG. 12A, for example, in the second embodiment, the rectifying walls41arranged in the case11may be omitted. In other words, as shown inFIG. 12A, in this case, only the first boss21and the medium guides44are arranged in the case11. In this case, the surface tension of the liquid level of the electrostatic capacitance medium23charged into the case11acts on the inner wall surface of the case11and the first boss21, and thus the liquid level of the electrostatic capacitance medium23is substantially horizontal. When the electrostatic capacitance medium23is charged into the case11, the electrostatic capacitance medium23is guided by the medium guides44and reliably reaches the first boss21. This avoids the problem shown inFIG. 5in which the electrostatic capacitance medium23does not reach the axis O. Thus, in the structure ofFIG. 12A, the tilt angle sensor1has a simple and compact structure without lowering the tilt angle detection capacity.

The first boss21does not necessarily have to be arranged on the axis O of the case11. For instance, as shown inFIG. 12B, a plurality of (two inFIG. 12B) first bosses21may be arranged in the vicinity of the axis O of the case11. This would also keep the liquid level of the electrostatic capacitance medium23substantially horizontal. Thus, the size of the tilt angle sensor1may be reduced without lowering the tilt angle detection capacity.

In the second embodiment, the medium guides44are arranged along two rows. However, the medium guides44do not have to be arranged in two rows and may be arranged in, for example, three or four rows.

In the second embodiment, the medium guides44may be omitted. Further, the medium guides44and the rectifying wall41may be omitted.

In the second embodiment, the medium guides44do not have to be cylindrical and may be, for example, polygonal.

The medium guides44of the second embodiment may be arranged in the case11for the tilt angle sensor1of the first embodiment.

The tilt angle sensor1of the first embodiment includes the electrostatic capacitance medium23, which contains fine particles23b, and the bosses21and22. However, the tilt angle sensor1may include the electrostatic capacitance medium23, which contains fine particles23b, and only either one of the bosses21and22. The tilt angle sensor1of the second embodiment may include, in addition to the electrostatic capacitance medium23containing fine particles23b, only one of the first boss21, the rectifying walls41, and the medium guides44. In such structures, the size of the tilt angle sensor1may also be reduced without lowering the detection capacity.

In each of the above embodiments, the diameter of the fine particles23bis not limited to several tens of nanometers and may be of any size as long as the Brownian motion is enabled in the base23a. The diameter of the fine particles23bmay be in the scale of nanometers in the range of, for example, several nanometers to several hundred nanometers or in the scale of micrometers, for example, several micrometers.

In each of the above embodiments, the base23aof the electrostatic capacitance medium23is not limited to silicon oil and may be a liquid having a dielectric constant εa between about 20 to 30, for example, a liquid organic compound, such as acetone, ethanol, methanol, and the like. The fine particles23bare not limited to barium titanate, and may be, for example, alumina (dielectric constant: 8.9) or zirconia (dielectric constant: 50). The electrostatic capacitance medium23may include any combination of the above substances. For instance, if silicon oil is used as the base23a, alumina is used as the fine particle23b, and the mix ratio of the fine particles23bwith respect to the base23ais 8%, the dielectric constant εc of the electrostatic capacitance medium23is 3.1. That is, compared to a case in which the electrostatic capacitance medium23contains only the base23a, the dielectric constant εc is higher by about 15%. This also easily ensures that the dielectric constant of the electrostatic capacitance medium is high.

Changes in the property change of silicon oil with respect to temperature change are smaller than a liquid organic compound. Thus, when the tilt angle sensor1is used in a high temperature atmosphere such as, for example, when the tilt angle sensor1is installed in a vehicle, the use of silicon oil as the base23ais more suitable. The mix ratio of the fine particle23bwith respect to the base23ais preferably between about 5% and 15% and more preferably between 10% and 15%.

In each of the above embodiments, the common electrodes15aand15band the differential electrodes16aand16bhave a semicircular shape. However, they may have any shape, such as a rectangular shape.

In each of the above embodiments, the semicircular common electrodes15aand15band the semicircular differential electrodes16aand16bform the first capacitor and the second capacitor. Instead, a circular common electrode and the semicircular differential electrode16aand16bmay be used to form each capacitor.

In each of the above embodiments, the first boss21, the second bosses22, the rectifying walls41, the medium guides44, the third bosses45, and the fourth bosses46do not necessarily have to be connected to the opposing inner wall surfaces of the case11. That is, the first boss21, the second bosses22, the rectifying walls41, the medium guides44, the third bosses45, and the fourth bosses46may be connected to only one of the opposing inner wall surfaces of the case11.

It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.