Assembly of a magnetic tilt sensor

Abstract: A magnetic tilt sensor has a permanent magnet (53) having a surface coated with a magnetic fluid (54) and housed in a hollow portion of a nonmagnetic case (51), and a magnetic sensing element (57) arranged outside the bottom portion of the case. When the case is tilted or is abruptly moved, the permanent magnet in the case is relatively displaced. The magnetic sensing element detects this displacement. A cap (55) having a predetermined weight covers the permanent magnet so that a response characteristic is improved.

Description: BACKGROUND OF THE INVENTION: I. Field of the Invention The present invention relates to a method for assembling a tilt sensor according to claim 1 and to an assembly for use in the manufacture of a magnetic tilt sensor according to claim 3. II. Description of the Prior Art According to a proposed tilt sensor as it is disclosed within EP-A-0 296 565 there is provided a non-magnetic case, a permanent magnet movable housed in said case, a magnetic fluid adhering to at least a portion of said permanent magnet which is located between said permanent magnet and a bottom surface of said case, and a magnetic sensing element, arranged and fixed under a bottom portion of said case, for detecting changes in the magnetic flux due to movements of said permanent magnet, wherein a cap having a predetermined wight covers the permanent magnet. GB-A-2 158 239 discloses a magnetic angular position sensor having a ferrofluid field spherical chamber in a cylindrical housing within which floats a body comprising a buoyant core carrying a magnet. Hall effect devices are located in recesses in the ends of the housing. The housing also carries a calibration solenoid which comprises a bobbin which is arranged within recesses provided on the outside of the housing. This known magnetic angular position sensor must be kept free from vibrations, tilts and also from accelerations since such sensor should measure a magnetic field of the earth and since this magnetic field of the earth is only very small such a sensor is not applicable for measuring tilts and the like. According to a further proposed tilt sensor, a permanent magnet having a surface coated with a magnetic fluid is housed in a hollow case, and a magnetic sensing element is arranged outside the case (U.S. Serial No. 112,626 and E.P.C. Application No. 87115618.8). With this arrangement, the permanent magnet is moved to a lower position by its own weight when the case is tilted. As a result, magnetism detected by the magnetic sensing element is changed, and hence the tilt sensor detects that the case is tilted. Fig. 10 shows a tilt sensor of this type. Reference numeral 1 denotes a nonmagnetic case made of aluminum. A cup-like case main body 1a and a cover 1b of the case 1 are bonded to each other by an adhesive applied between overlapped portions thereof, so that the case 1 has a sealed hollow portion 2. A substantially disk-like permanent magnet 4 having a surface coated with a magnetic fluid 3 is housed in the hollow portion 2. Note that reference numeral 1c denotes a small hole through which the magnetic fluid 3 is injected into the case 1 after the permanent magnet 4 is placed in the case 1 and the case main body 1a and the cover 1b are bonded to each other by the adhesive. After the injection of the magnetic fluid 3, the small hole 1c is sealed. Reference numeral 5 denotes a sensor mounting base consisting of a plastic molded member. The case 1 is inserted in an upper portion of the mounting base 5 and is fixed thereto by a resin 6. Reference numeral 7 denotes a reed switch as a magnetic sensing element arranged on a printed board 8 and having soldered lead terminals. Leads 9 extend from the printed board 8. Reference numeral 10 denotes a lower cover fixed to the mounting base 5 by engaging pawls 10a protruding from a pair of opposite sides of the cover 10 with recesses formed in the inner wall of the mounting base 5; and 11, an amorphous magnetic piece bonded to the top portion of the mounting base 5. The response angle of the tilt sensor is adjusted by a position where the amorphous magnetic piece 11 is bonded. Reference numberal 12 denotes a label on which the name, a model number, the name of a manufacturer, and the like of a tilt sensor are displayed. In such an arrangement, while the case 1 is not tilted, the permanent magnet 4 is located at a central portion of the case 1. Since a pair of reeds of the reed switch 7 are equally influenced by the magnetism of the permanent magnet 4, they are polarized to the same pole at a central portion of the switch 7 and repulse each other. As a result, the switch 7 is set in an open state. When the case 1 is tilted, and the permanent magnet 4 is moved to a lower position, and only one reed is influenced by the magnet 4. As a result, the reeds are polarized to the opposite poles at the central portion of the switch 7, and hence the switch 7 is set in a connected state. Note that the magnetic fluid 3 serves as a lubricating oil when the permanent magnet 4 is moved. In this manner, a tilt of the case 1 can be detected by an opening/closing operation of the reed switch 7. In the prior art having the above-described arrangement, however, the following problems are posed. (1) As described above, the nonmagnetic case 1 is designed such that the permanent manget 4 is placed in the cup-like case main body 1a, and the case main body 1a and the cover 1b coated with the adhesive, which is applied at the overlapped portions in advance, are engaged with each other to be bonded. Then, after the magnetic fluid 3 is injected through the small hole 1c formed in the cover 1b, the small hole 1c is sealed. An adhesive is also used at this time. The case 1 sealed in this maanner is inserted from an opening of a bottom portion of the mounting base 5. The resultant structure is then turned upside down, and the resin 6 is supplied therein, so that the case 1 is fixed to the base 5 by curing the resin 6. These adhesive and resin must be thermally curred upon each operation. In addition, a two-part liquid type adhesive which is not affected by the magnetic fluid 3 is used as the adhesive. Therefore, the manufacture of the sensor requires a long total manufactured time including a time required for preparation of the adhesive, and the yield is greatly decreased. (2) After the case 1 is fixed to the upper portion of the mounting base 5, the printed board 8 having the reed switch 7 mounted thereon is inserted in the base 5. The printed board 8 is then positioned by a stopper (not shown) and is fixed by fitting the lower cover 10 in the base 5. A distance between the permanent magnet 4 and the reed switch 7 formed at this time greatly influences the response angle of the tilt sensor. In the arrangement shown in Fig. 10, a mounting position of the case 1 varies depending on the bonded state of the cup-like case main body 1a and the cover 1b, the sealed state of the small hole 1c, the fixed state of the case 1 by the resin 6, and the like. In additon, if the adhesive overflows and a projection is formed, a gap is formed between the cover 1b and the ceiling of the mounting base 5. Hence, the response angle tends to vary. (3) The permanent magnet 4 housed in the case 1 must have a certain weight so as to increase the response speed with respect to a tilt. This is because as a weight W is increased, a component of force Wsinα for moving the magnet 4 along a tilted bottom surface of the case 1 when the case 1 is tilted at an angle a is increased. As a result, the component of force Wsina can overcome various forces for blocking the movement of the magnet 4, such as an attraction between the magnet 4 and the lead terminals of the read switch 7 and the viscosity and surface tension of the magnetic fluid 3. In addition, the magnetic fluid 3 serves as a lubricating oil upon movement of the permanent magnet 4. More specifically, since the magnet 4 attracts the magnetic fluid 3, the fluid 3 flows into a space between the magnet 4 and the bottom surface of the case 1 and adheres thereto. This adhesive force serves as a force for flowing the permanent magnet 4. Consequently, the magnet 4 is moved in response to a tilt or vibrations of the case 1 while it is floated from the bottom surface of the case 1. Since a frictional resistance upon movement of the magnet 4 is generated only between the fluid 3 and the bottom surface of the case 1, it is very small. Although the magnetic fluid 3 is required only between the magnet 4 and the bottom surface of the case 1, the fluid 3 acutally adheres to the permanent magnet 4 so as to cover its entire surface. The fluid 3 adhering to upper and side surfaces of the magnet 4 is unnecessary. (4) When the reed switch 7 is arranged to be perpendicular to the bottom surface of the case 1, a distance between the bottom surface of the case 1 and the lead terminal of the reed switch 7 sensitively affects the sensitivity of the tilt sensor or a tilt angle at which the reed switch 7 responds. Therefore, by adjusting this distance, the sensitivity and the response angle can be adjusted. In the prior art, threads are formed in the outer circumference of a hold member for holding the reed switch and in the inner circumferential surface of the mounting base, which is brought into contact with the outer circumferential surface of the hold member, so as to be meshed with each other, and the hold member is pivoted to be vertically moved, thereby performing the adjustment. Such a thread arrangement, however, undesirably increases the manufacturing cost. (5) Since the tilt sensor incorporates the permanent magnet and the magnetic sensing element, if an instrument or a circuit susceptible to magnetic influences is present around the tilt sensor, it is influenced by a magnetic field generated by the tilt sensor. Furthermore, in contrast with this, if an external magnetic field is present near the tilt sensor, the magnetic sensing element may be erroneously operated. Therefore, the tilt sensor must be magnetically shielded from its surroundings. SUMMARY OF THE INVENTION: The present invention has been made to solve the above described problems. It is an object of the present invention to provide an assembling method and an assembly in which a distance between a case and a reed switch is adjusted without employing a thread arrangement and the cost can be decreased. According to the present invention this object is solved by the method of claim 1 and by the assembly of claim 3. Further improved embodiments result from the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a sectional view of an embodiment of the present invention; Fig. 2A is a plan view of the embodiment in Fig. 1; Figs. 2B and 2C are side views of the embodiment in Fig. 1; Fig. 2D is a bottom view of the embodiment in Fig. 1; Fig. 3 is an exploded perspective view of the embodiment in Fig. 1; Fig. 4 is a view illustrating functions of a cap; Figs. 5 and 6 are graphs respectively showing intensities of an external magnetic field with and without the cap; Fig. 7 is a graph showing intensities of an external magnetic field with and without a shield case; Fig. 8 is a graph showing intensities of an external magnetic field with and without a shield plate; Fig. 9 is a perspective view of a shield band; and Fig. 10 is a sectional view showing a conventional magnetic tilt sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Fig. 1 is a sectional view of an embodiment of the present invention. Fig. 2A, Figs. 2B and 2C, and Fig.2D are a plan view, side views, and a bottom view of the embodiment in Fig. 1, respectively. Fig. 3 shows each component. Referring to Figs. 1 to 3, reference numeral 51 denotes a nonmagnetic case which is made of, e.g., aluminum, and comprises a case main body 51a and a cover 51b respectively having flanges air-tightly coupled to each other by so-called cold welding method ; 52, a case bottom portion; and 53, a disk-like permanent magnet housed in the case 51. A cap 55 is mounted on the magnet 53. The cap 55 is made of a magnetic material and has a conical projection 55a at its top. A disk-like weight 55b may be arranged inside the cap 55 as needed. Reference numeral 54 denotes a magnetic fluid adhering to the bottom surface of the permanent magnet 53; and 56, a mounting base constituted by a resin molded member. Reference numeral 57 denotes a reed switch, arranged and fixed outside the case bottom portion 52, for detecting the magnetism of the permanent magnet 53. The reed switch 57 is held by a switch holder 58, which is constituted by a resin molded member. In addition to the reed switch 57, the switch holder 58 holds an annular permanent magnet 59 which is arranged coaxially with the reed switch 57 and is magnetized so as to have a polarity direction opposite to that of the permanent magnet 53 in the case (the functions and effects of the permanent magnet 59 are described in U.S. Serial No. 209,064 and European Application No. EP-A-0 296 565. The reed switch 57 and the annular permanent magnet 59 are assembled in the switch holder 58 such that an upper terminal 60 is inserted in the switch holder 58 from the above, the reed switch 57, the annular permanent magnet 59, and a lower holder 61 are inserted in the switch holder 58 from below, and then end portions of the upper terminal 60 are bent to be fixed. One lead terminal of the reed switch 57 is soldered to an upper hole 60a of the upper terminal 60, whereas the other lead terminal of the reed switch 57 is soldered to a lower terminal 62 arranged at a central portion of the bottom surface of the lower holder 61. Note that extraction leads 63a and 63b are respectively soldered to the bent portions of the upper terminal 60 and the lower terminal 62. Reference numeral 64 denotes a shield case fitted on the mounting base 56 from the above so as to cover the overall case 51; and 65, a shield plate mounted on a bottom portion of the mounting base 56 so as to cover a lower portion of the reed switch 57. As shown in Fig. 4, the cap 55 mounted on the permanent magnet 53 has a shape covering most of the portions from the upper surface to the side surfaces near the bottom surface of the magnet 53. A slight gap 55c is preferably formed at the lower end portion of the cap 55 opposing the permanent magnet 53. Functions and effects of the cap 55 will be described below. Firstly, the cap 55 functions as a weight. When a weight is added to the permanent magnet 53 to set a desired weight W, a component of force Wsinα for moving the permanent magnet 53 along the tilted bottom surface when the case 51 is tilted at an angle α is also increased. Since the increased component of force can easily overcome various forces for blocking the movement of the magnet 53, i.e., an attraction between the magnet and the lead terminals of the reed switch, the viscosity and surface tension of the magnetic fluid, and the like, the magnet can immediately respond to a tilt of the case, i.e., sensitivity can be increased. Secondly, the magnetic fluid 54 can be saved. Most of the upper and side surfaces of the permanent magnet 53 are covered with the cap 55, and only the bottom surface and portions of the side surfaces near the bottom surface of the magnet 53 are exposed. Therefore, the magnetic fluid 54 adheres only to the exposed surfaces of the magnet 53. Since the tilt sensor of the present invention satisfactorily functions in such a condition, a required amount of magnetic fluid can be decreased. Thirdly, assuming that the cap 55 is made of a magnetic material, the cap 55 functions as a yoke and constitutes the magnetic path of magnetic fluxes. With this arrangement, a concentrated magnetic field is formed between the lower end portion of the cap 55 and the lower pole (S pole in this case) of the permanent magnet 53. That is, leaking magnetic fluxes are greatly reduced, and the magnetic influence on the reed switch 57 is increased. In this case, if the gap 55c is formed at the lower end portion of the cap 55, magnetic fluxes directly flowing from the cap 55 to the permanent magnet 53 are decreased, and magnetic fluxes flowing to the S pole through the space are increased. As a result, magnetic fluxes can be effectively used. Figs. 5 and 6 show results obtained by measuring intensities of magnetic fields above (Fig. 5) and at a side (Fig. 6) of the permanent magnet 53 with and without the cap 55. It is apparent from Figs. 5 and 6 that leaking magnetic fluxes are greatly reduced by covering the magnet with the cap. Note that the conical projection 55a formed on the top of the cap 55 is brought into contact with the inner surface of the cover 51b of the case 51 so as to prevent the permanent magnet 53 and the cap 55 from being toppled or upset, and to prevent the cap 55 from being brought into contact with the inner surface of the cover 51b. Referring to Fig. 4, a distance H from the bottom surface of the case 51 to a lead terminal 57a of the reed switch 57 influences the sensitivity of the tilt sensor or a tilt angle at which the reed switch responds with high sensitivity. Therefore, by adjusting the distance H, the sensitivity and the response angle can be adjusted. In the conventional technique, threads are formed in the outer circumferential surface of a hold member for holding the reed switch and in the inner circumferential surface of the mounting base which is brought into contact with the hold member so as to be meshed with each other, and the hold member is pivoted to be vertically moved, thereby performing the adjustment. In this embodiment, the outer circumferential surface of the switch holder 58 and the inner circumferential surface of the mounting base 56 have sizes to be fitted, and a groove 58a is formed in the switch holder 58 in its circumferential direction. In addition, an adjustment window 56a is formed in a body portion of the mounting base 56, as shown in Fig. 2B. With this arrangement, after the distance H is adjusted by hooking a pin or the like on the groove 58a through the window and vertically moving the switch holder 58, the switch holder 58 is fusion-bonded and fixed to the mounting base 56 through a hole 56b formed in the body portion of the mounting base 56. According to the tilt sensor of the present invention, influences of an external magnetic field must be minimized. At the same time, it is preferable to prevent magnetic influences on surroundings due to leaking magnetic fluxes. In this embodiment, therefore, the shield case 64 is fitted on the mounting base 56 from the above so as to cover the entire case 51. The shield plate 65 is arranged on the bottom portion of the mounting base 56 so as to cover the lower portion of the reed switch 57. In addition to a magnetic shield, the shield case 64 functions as a means for fixing the aluminum case 51 to the mounting base 56. A hole 64a portion is finally fixed to the mounting base 56 by fusion bonding. The shield plate 65 is fixed to the mounting base 56 through the holes 65a. Fig. 7 shows intensities of magnetic fields measured in cases where the shield case 64 is present (resin case plated with Fe + Ni) and not present (Al case), respectively. Curves 1 ○ and 2 ○ in Fig. 7 respectively represent measurement results in the cases wherein the shield case 64 is and is not present. Fig. 8 shows intensities of magnetic fields measured in cases wherein the shield plate 65 is present (resin plate plated with Fe + Ni) and not present (Al plate), respectively. Curves 1 ○ and 2 ○ in Fig. 8 respectively represent measurement results in the cases wherein the shield plate 65 is and is not present. It is apparent from these results that leaking magnetic fluxes are greatly decreased when the shield case 64 and the shield plate 65 are present. In order to further improve the magnetic shield effect, a shield band 67 is fitted on a side portion of the mounting base adjacent to the switch holder 58, as shown in Fig. 9. With this arrangement, a perfect shield effect can be obtained. This shield band 67 is made of, e.g., a soft magnetic material. A portion of the shield band 67 is cut off. After the shield band 67 is fitted on the side portion of the mounting base by spreading both the ends of the band 67, the ends are locked to each other. Instead of using the resin molded member plated with a magnetic metal and the soft magnetic material as the shield member as described above, a molded member obtained by mixing flakes of iron, ferrite, amorphous metal, and the like with a resin may be used as a shield material. For example, the mounting base may be constituted by a molded member of this type. As described above, according to the embodiment, when the cap is covered on the permanent magnet housed in the case, a necessary weight is added to the magnet. As a result, a component of force for moving the permanent magnet along the bottom surface when the case is tilted is increased, and the magnet can quickly respond to a tilt or vibrations of the case, thereby increasing the sensitivity of the tilt sensor. Since the magnetic fluid adheres only to an exposed portion of the permanent magnet and does not adhere to an unnecessary portion unlike in the conventional technique, a required amount of magnetic fluid can be reduced. As a result, the cost can be decreased. In addition, if the cap is constituted by a magnetic member, the cap functions as a yoke. As a result, a magnetic field is concentrated between the lower end portion of the cap and the bottom portion of the permanent magnet. Therefore, leaking magnetic fluxes can be minimized, and magnetic influences on the reed switch are increased. As a result, the size of the tilt sensor can be reduced. Furthermore, the outer circumferential surface of the hold member for holding the magnetic sensing element and the inner circumferential surface of the mounting base have sizes to allow fitting and sliding movement between the two members, the groove is formed in the hold member in its circumferential direction, and the adjustment window is formed in the body portion of the mounting base. With this arrangement, the hold member is vertically moved by inserting a pin or the like through the adjustment window so as to adjust a distance between the hold' member and the case housing the permanent magnet. As a result, the manufacturing cost can be decreased compared with the conventional thread arrangement. In addition, fixing of the hold member upon adjustment can be easily performed by fusion bonding without using a screw and the like. Moreover, the tilt sensor is magnetically shielded from its surroundings. This arrangement can prevent an internal magnetic field from leaking out, and can block influences of an external magnetic field on the tilt sensor. Therefore, no limitation is posed in terms of an installation place of the tilt sensor, and its detection precision can be increased.

Claims: Method for assembling a magnetic tilt sensor by means of assembling: a) a nonmagnetic case (51), b) a permanent magnet (53) movably housed in said case (51), c) a magnetic fluid (54) adhering to at least a portion of said permanent magnet (53), which is located between said permanent magnet (53) and a bottom surface of said case (51), d) a magnetic sensing element (57), arranged under a bottom portion of said case (51), for detecting changes in the magnetic flux due to movement of said permanent magnet (53), e) a cap (55) having a predetermined weight for covering said permanent magnet (53), f) a mounting base (56) having an adjustment window (56a) formed in a lower hollow portion of a body portion of said mounting base (56), g) a hold member (58) having a groove (58a) formed about its circumference in the outer circumferential surface, h) the groove (58a) for hooking a pin or the like for adjustment of the distance between said hold member (58) and said case (51), wherein i) said hold member (58) being accessible via said adjustment window (56a) and slidably movable inside said mounting base (56) when said hold member (58) is fitted into said lower hollow portion of said mounting base (56), comprising the following steps: j) said pin or the like is inserted from the adjustment window and is put through said groove (58a) so as to adjust the hold member (58), k) said hold member (58) being vertically adjusted, l) said hold member (58) being fixed after adjustment. A method according to claim 1, characterized in that a magnetic shield is arranged so as to prevent leakage of a magnetic flux from said permanent magnet (53) and to prevent an external magnetic field from influencing the inside of said tilt sensor. An assembly for use in the manufacture of a magnetic tilt sensor, said assembly comprising, a) a nonmagnetic case (51), b) a permanent magnet (53) movably housed in said case (51), c) a magnetic fluid (54) adhering to at least a portion of said permanent magnet (53), which is located between said permanent magnet (53) and a bottom surface of said case (51), and d) a magnetic sensing element (57), arranged and fixed under a bottom portion of said case (51), for detecting changes in the magnetic flux due to movement of said permanent magnet (53), wherein e) a cap (55) having a predetermined weight covers said permanent magnet (53), f) a mounting base (56) having an adjustment window (56a) formed in a lower hollow portion of a body portion of said mounting base (56), g) a hold member (58) having a groove (58a) formed about its circumference in the outer circumferential surface, h) the groove (58a) for hooking a pin and the like for adjustment of the distance between said hold member (58) and said case (51), i) said hold member (58) being accessible via said adjustment window (56a) and slidably movable inside said mounting base (56) when said hold member (58) is fitted into said lower hollow portion of said mounting base (56). An assembly according to claim 3, characterized in that a magnetic shield is arranged so as to prevent leakage of a magnetic flux from said permanent magnet (53) and to prevent an external magnetic field from influencing the inside of said tilt sensor.

Inventors: IMAIZUMI HIRAKU, MORITA AKIRA, ODAGAWA YOSHIMOTO, SHIOURA TAKASHI, YANAGISAWA YASUSHI, IMAIZUMI, HIRAKU, MORITA, AKIRA, ODAGAWA, YOSHIMOTO, SHIOURA, TAKASHI, YANAGISAWA, YASUSHI
Applicants: TDK CORP, TDK CORPORATION
Application year: 1988
Main classification: G01C9
IPC codes: G01C9