TILT SENSOR

Abstract: There is proposed a tilt sensor in which a permanent magnet having a surface coated with a magnetic fluid is housed in a hollow portion of a nonmagnetic case, and a magnetic sensing element is disposed outside a bottom portion of the case, When the case is tilted at an angle exceeding a predetermined angle, the permanent magnet is greatly moved and the magnetic sensing element detects the movement of the magnet. Furthermore, adverse influences due to magnetic attraction between the permanent magnet and the magnetic sensing element or a surface tension of the magnetic fluid are compensated.

Description: Background of the Invention: I. Field of the Invention The present invention relates to a magnetic tilt sensor for detecting a predetermined tilt of an object when the object is tilted. II. Description of the Prior Art Conventional tilt measuring devices of this type vary from simple devices such as a level and a device used in an oilstove to visually show horizontality by suspending a counterweight through a chain to a sophisticated one such as a gyroscopic horizon. Although a simple device such as a level is inexpensive, a tilt can be checked by only a visual observation. Therefore, it is not designed to be used in a control system for resetting a tilted object to a horizontal position. Use of a sophisticated device such as a gyroscopic horizon is limited to an expensive machine such as in aircraft. From GB-A-2 158 239 a magnetic angular position sensor is already known comprising a ferrofluid filled spherical chamber in a cylindrical housing, within which floats centrally a body, the body being provided with at least one pair of magnetic poles and having a substantially neutral buoyancy, and a magnetic flux detection means attached to the exterior of the chamber responsive to changes in the magnetic flux caused by rotation of the body within the chamber. In case the above described sensor is tilted, the permanent magnet is continuously and gradually, i.e. linearly, moved so that the intensity of magnetism around the magnetic sensing element continuously and gradually changes. This continuous and gradual change is in many cases not desirable because there the main aspect is to detect with high precision and reliability whether a tilt of an object is below or exceeds a predetermined angle. In view of this requirement, the intensity of the magnetism around a magneting sensing element should only change significantly at the time the tilt exceeds this angle. In addition, this feature should be achieved by a simple arrangement with good vibration and acceleration characteristics. The invention as claimed is intended to unite these requirements. To solve the above problem a simple constructional embodiment achieves that the magnetism detected by magnetic sensing elements scarcely changes when a tilt of a case is below a predetermined angle, and when it exceeds the predetermined angle, a permanent magnet inside the case is significantly moved so that the magnetic field detected by the magnetic sensing element abruptly, i.e. nonlinearly, changes, thereby accurately detecting that the tilt of the case exceeds the predetermined angle. In order to achieve the above object, a bottom portion of a tilt sensor of the present invention is designed such that a first bottom surface located at the central position of the case, and an inverted frustoconical surface as a a second bottom surface formed around the first bottom surface cross each other to define a predetermined angle. As a result, it is only when a tilt exceeds the pretermined angle that the permanent magnet is greatly and significantly displaced. A tilt-detecting apparatus is also known from GB-A-2 072 838. According thereto, the tilt sensor comprises an opaque rigid ball (13) located within a two-part housing (11, 21) and mounted on concave surface (12) to roll relative to the housing as the housing is tilted. The housing supports a light source (22) and a light responsive detector (15), the ball being arranged in relation to the source and detector so as to vary the amount of light reaching the detector in relation to the tilt of the housing. The main drawback of this conventional nonmagnetic tilt-measuring device is that it is only operable with electrical inputs to the light source and the light responsive detector and that its embodiments are very complex and complicated. Furthermore, the opaque rigid ball is very sensitive with respect to vibration and acceleration. Summary of the Invention It is a first object of the present invention to provide a tilt sensor wherein unlike the proposed tilt sensor in which as a case is tilted, a permanent magnet is continuously and gradually, i.e., linearly moved so that an intensity of the magnetic field around the magnetic sensing element continuously and gradually changes, when a tilt of a case is below a predetermined angle, the magnetic field detected by a magnetic sensing element scarcely changes, and when it exceeds the predetermined angle, a permanent magnet inside the case is greatly moved so that the magnetic field detected by the magnetic sensing element abruptly, i.e., nonlinearly changes, thereby accurately detecting that the tilt of the case exceeds the predetermined angle. It is a second object to solve a problem, i.e., that the magnetic sensing element responds at different angles when the case is tilted and when the case is returned to a horizontal position, due to surface tension of the magnetic fluid. In order to achieve the above object, the case is arranged such that the bottom surface is continuous with its side wall at a relatively large radius R of curvature. The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description of embodiments in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS : Fig. 1 is a sectional view of a first embodiment of the present invention; Figs. 2(a) and 2(b), Fig. 3, and Figs. 4(a), 4(b), and 4(c) are views illustrating an operation of the embodiment; Fig. 5 is a view showing an arrangement of a second embodiment of the present invention; Fig. 6 is a sectional view of a third embodiment; Figs. 7(a) and 7(b), and Figs. 8(a) and 3(b) are views illustrating an operation of the third embodiment; Figs. 9(a) and 9(b) are views showing influences of surface tension of a magnetic fluid; Fig. 10 is a sectional view of a fourth embodiment of the present invention; Fig. 11 is an exploded perspective view thereof; and Figs. 12(a) and 12(b) are views showing an indented bottom surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS : Fig. 1 shows a first embodiment of the present invention. Reference numeral 1 denotes a nonmagnetic case made of a material such as aluminum. As shown in Fig. 2, a hollow portion of the case 1 includes a bottom portion 2 which is constituted by a round flat surface 2a as a first bottom surface located at the central position of the hollow portion, and an inverted frustoconical surface 2b as a second bottom surface formed around the flat surface 2a to have a predetermined angle ϑ with respect thereto. Fig. 2(a) is a sectional view taken along the line X - X in Fig. 2(b). Reference numeral 3 denotes a permanent magnet a surface of which is coated with a magnetic fluid 4. The magnetic fluid 4 is prepared by dispersing magnetic metal powder (e.g., cobalt, iron, nickel, or the like) particles having a diameter of 50 Å to 150 Å in a liquid (e.g., kerosine or water) having a relatively low viscosity. In general, a surface-active agent is added. Reference numeral 5 denotes a cover located at a predetermined height above the bottom portion 2 so that the permanent magnet 3 is not turned upside down; 6, a mounting base; and 7, a lead switch serving as a nonlinear magnetic sensing element arranged outside the bottom portion of the case 1. One end of one lead 7a is located near the central portion of the bottom portion 2 and one end of the other lead 7b is located apart from the bottom portion. An operation of the embodiment will be described below. Fig. 2 shows a state wherein the case 1 is not tilted and the flat surface 2a located at the central portion of the bottom portion is horizontal. In this case, since the permanent magnet 3 is positioned on the flat surface 2a, the end portion of one lead 7a of the lead switch is positioned near the permanent magnet 3. As a result, one end portion of the lead 7a is influenced by the magnetic field of the permanent magnet 3 and is magnetized with the N pole, and the other end (a central portion of the lead switch) of the lead 7a becomes the S pole. In addition, since one end portion of the other lead 7b located near the S pole of the lead 7a is magnetized with the N pole, both the leads attract each other and are electrically connected to each other. In this case, if a circuit is connected to the leads 7a and 7b, the circuit is closed. Assume that the case is tilted, as shown in Fig. 3. When a tilt is small, since the frustoconical surface 2b is still defining an upwardly inclined surface at a certain angle with respect to a horizontal surface L, the permanent magnet 3 is positioned near the flat surface of the central portion, and hence the lead 7a of the lead switch 7 is influenced by the the magnetic field of the permanent magnet, thereby keeping the both leads connected to each other. As shown in Figs. 4(a) and 4(b), when the case 1 is further tilted at an angle ϑ (in practice, a tilt angle is larger than ϑ due to viscosity of the magnetic fluid, as shown in Fig. 4(c)) defined by the flat surface 2a of the substantially central portion and the frustoconical surface 2b, the permanent magnet 3 is quickly and completely moved from the flat surface 2a of the central porion to the frustoconical surface 2b by its weight. That is, a position of the permanent magnet with respect to the flat surface 2a of the central portion is changed in a nonlinear manner. When the permanent magnet is moved, the magnetic fluid 4 serves as a lubricant so that the permanent magnet can be moved with a very small resistance, and hence it can be easily moved to the lowest position of the bottom portion. In this case, the end portion of the lead 7a of the lead switch 7 is quickly, i.e., nonlinearly positioned out of substantially the range of magnetic influences, so that the lead 7a is substantially demagnetized. As a result, a magnetic attraction between the leads 7a and 7b substantially disappears to open the switch. As is apparent from the above description, in the embodiment, when the case is tilted at an angle exceeding a predetermined angle, the lead switch is quickly, i.e, nonlinearly opened, thereby accurately detecting that the case is tilted at an angle exceeding the predetermined angle. If the case is returned to the horizontal position, the permanent magnet is also quickly returned to the flat surface of the central portion, thereby quickly, i.e., nonlinearly closing the lead switch. Although the first bottom surface is a flat surface and the second bottom surface is a frustoconical surface in this embodiment, the surfaces may be slightly concaved. What is essential is that the first and second bottom surfaces cross each other at an angle in a nonlinear manner. Fig. 5 shows an embodiment (a second embodiment) similar to that of Fig. 1. In the embodiment in Fig. 1, when the case 1 is in the state shown in Fig. 2, the lead switch 7 is closed. In the embodiment in Fig. 5, however, the switch is kept open by arranging an auxiliary magnet 8 near one end portion of the other lead 7b of the lead switch. More specifically, if the polarities of the two magnets and disposition of the lead switch are arranged as shown in Fig. 5, the ends of both leads have the same polarity at the central portion of the lead switch 7, and hence they repel each other and are not connected to each other. When the case is tilted and one end portion of the lead 7a is not substantially subjected to magnetic influences of the permanent magnet 3, the lead 7a is influenced only by the auxiliary magnet 8, so that the ends of the both leads have the different polarities and are connected to each other. Fig. 6 shows a third embodiment of the present invention, wherein the lead switch is laid on its side to be substantially parallel to the first bottom surface of the case 1. In this case, when the case is not tilted, as shown in Figs. 7(a) and 7(b), the both leads 7a and 7b of the lead switch 7 receive the same influences from the permanent magnet 3. Since the both leads have the same polarity at the central portion of the switch, they repel each other. Therefore, the switch is opened. When the case is tilted and the permanent magnet 3 is moved to a position shown in Fig. 8, i.e., the second surface is positioned slightly below the horizontal plane, only one lead 7b is influenced by the influences of the permanent magnet 3. As a result, the both leads have the different polarities at the central portion of the switch and are connected to each other. When the case 1 is tilted such that the permanent magnet is moved in a direction M, the both leads are not subjected to the influences of the permanent magnet, or are equally subjected to them, and hence the switch is not closed. Therefore, in this embodiment, directivity of a tilt can also be detected. In this embodiment, the relationship between a tilt angle of the case and movement of the permanent magnet can be defined in the same manner as in the first embodiment. In order to optimize the magnetic influences of the permanent magnet with respect to the lead switch, it is preferable that a mounting position of the lead switch can be adjusted. Furthermore, the permanent magnet may have a ring-like shape. Furthermore, the permanent magnet may have any shape as long as it is not turned upside down in relation with the cover 5. Although the lead switch is used as a magnetic sensing element in this embodiments, other elements, e.g., a Hall element, may be used. In addition, an oxide magnetic fluid prepared by dispersing a ferrite magnetic powder in a liquid as well as the magnetic metal fluid can be used as the magnetic fluid to obtain the same effect. As has been described above, according to the present invention: (1) since the first and second bottom surfaces nonlinearly cross each other, when the case is tilted, and the tilt angle exceeds a predetermined angle, the switch is opened/closed in a nonlinear manner, so that whether the case, i.e, the tilt sensor is tilted at a predetermined angle or not can be accurately detected, while an erroneous operation of the tilt sensor due to small vibrations or the like can be prevented, (2) since the lead switches in the first and second embodiments are vertically mounted, they are especially suitable to a case wherein a size in a lateral direction is required to be small, (3) since the auxiliary magnet is provided in the second embodiment, the magnetism around the lead switch is enhanced so that the operation of the lead switch is stabilized, and the erroneous operation of the lead switch by small vibrations can be prevented, (4) since the lead switch is laterally arranged in the third embodiment, it is especially suitable to a case wherein a size in a vertical direction is required to be small, and directivity of a tilt can also be detected, and (5) since each of the lead switches used in the first, second, and third embodiments has a simple structure, and hence can be manufactured at low cost, and moreover, a nonlinear open/close operation can be easily performed. When the permanent magnet is moved, the permanent magnet is influenced by the viscosity and surface tension of the magnetic fluid in addition to the gravity and magnetic attraction between the magnet and leads of the lead switch. For example, as shown in Figs. 9(a) and 9(b), when the case is tilted and the permanent magnet 3 is moved, a relatively large area of the side wall of the case is wet with the magnetic fluid 4 due to the surface tension thereof. As a result, when the case is returned to a normal state from a state wherein it is titled, the movement of the permanent magnet is restricted by the wet state of the side wall. More specifically, the tilt angles of the case are different when the permanent magnet is moved from the first bottom surface to the second bottom surface and when it is moved from the second bottom surface to the first bottom surface. Accordingly, the response angle of the lead switch varies. Therefore, a fourth embodiment will be described wherein a magnetic sensing element is reliably operated at a substantially given response angle when the case is tilted and returned to the horizontal position. In Fig. 10, a nonmagnetic case 11 is constituted by a bottom portion 12 consisting of a first bottom surface 12a located at a central portion of the case, and a second bottom surface 12b continuous with the first bottom surface, a side wall 13 continuous with the bottom portion, and a lid 14, and includes a hollow portion a. This embodiment can be characterized in that the second bottom surface 12b is continuous with the side wall at a relatively large radius R of curvature. Fig. 11 is an exploded perspective view of Fig. 10. Reference numeral 22 denotes a spacer. Normally, a printed board 17 is stopped by a lower portion of a cylindrical portion of a mounting base 15, and hence the printed board cannot be further inserted therein. As a result, a predetermined gap is formed between the the bottom portion of the case 11 and the upper surface of a lead switch 7. If the predetermined gap cannot be assured due to a mounting state of the case 11, an appropriate number of spacers 22 can be stacked and inserted to assure the gap. In the embodiment having the above-described arrangement, when the case is tilted at a predetermined angle, the permanent magnet 3 is moved from the first bottom surface 12a to the second bottom surface 12b by its weight, thereby operating the lead switch 7. Furthermore, when the case is returned to the normal state from the tilted state, since a portion R rises at a considerably steep angle, the permanent magnet can be easily moved from the second bottom surface to the first bottom surface. As a result, the permanent magnet is moved at the predetermined angle despite the state wherein a considerably large area of the side wall is wet with the magnetic fluid, thereby operating the lead switch. The lead switch can be reliably operated at the substantially same response angle when the case is tilted as well as when the case is returned to the horizontal position, in the above-described manner. Figs. 12(a) and 12(b) are views wherein the first bottom surface 12a is indented, and portions 23 extending toward the permanent magnet are formed to be substantially small. This arrangement allows smooth movement of the permanent magnet. In this arrangement, even if the sensor is kept unused for a long period of time, the magnetic fluid can be smoothly moved when detection is performed, thereby improving detection precision.

Claims: A tilt sensor comprising a nonmagnetic case (1) having a hollow portion defined by a bottom portion (2) and a side wall, a permanent magnet (3), the surface of said magnet being at least partially coated with a magnetic fluid (4), said permanent magnet (3) being housed in the hollow portion, and a magnetic sensing element disposed and fixed outside the bottom portion (2) of said case, for detecting the magnetic field of said permanent magnet (3), characterized in that (a) said bottom portion (2) consists of a first bottom portion (2a) comprising a round flat surface and located at the central position of the hollow portion, and an inverted frustoconical surface as a second bottom portion (26) formed around the flat surface to have a predetermined angle (ϑ) with respect to said flat surface, and (b) said side wall is formed continuous with said bottom portion (2). A sensor according to claim 1, wherein said magnetic sensing element is mounted such that a distance between said magnetic sensing element and said permanent magnet (3) located in a first bottom surface area can be adjusted. A sensor according to claim 1, wherein said magnetic sensing element includes a nonlinear magnetic sensing element. A sensor according to claim 3, wherein said nonlinear magnetic sensing element comprises a lead switch (7). A sensor according to claim 4, wherein said lead switch (7) is mounted in a direction perpendicular or substantially perpendicular to the first bottom surface. A sensor according to claim 5, wherein said lead switch is disposed such that one end portion of one lead is located near said permanent magnet located in the first bottom surface area, and one end portion of the other lead is located apart from said permanent magnet, while an auxiliary magnet is disposed near the one end portion of the other lead. A sensor according to claim 4, wherein said lead switch (7) is mounted in a direction parallel or substantially parallel to the first bottom surface. A sensor according to claim 1, wherein said second bottom portion (12b) is continuous with the side wall (13) at a relatively large radius R of curvature. A sensor according to claim 8, wherein said first bottom surface (12a) is indented, said indentations being substantially smaller than said surface.

Inventors: IMAIZUMI, HIRAKU, MORI, TERUO, ODAGAWA, YOSHIMOTO, OKADA, KAZUHIRO, SHIOURA, TAKASHI TDK NARITA-RYO, YANAGISAWA, KIYOSHI, YANAGISAWA, YASUSHI
Applicants: TDK CORPORATION
Application year: 1987
Main classification: G01C9
IPC codes: G01C9, H01H35