Acceleration detector

Abstract: An acceleration detector (1) having a movable magnetic body (4) movable under acceleration and a differential transformer having primary coils (6a,6b) and secondary coils (7a,7b) and being arranged so that an output differential is produced between the secondary coils (7a,7b) when the movable magnetic body (4) displaces. The movable magnetic body (4) extends through the primary coils (6a,6b) and the secondary coils (7a,7b) and has its both ends protruding from the coils. Leaf springs (3a,3b) are provided to support the protruding ends of the movable magnetic body (4). The case (2) comprises a cylindrical body (2-1) surrounding the outer periphery of the differential transformer and lids (2-2) closing openings of the body (2-1) at both ends thereof. Between the lids (2-2) and both ends of the movable body (4) predetermined gaps (g) are provided through which a magnetic flux can flow. The leaf springs (3a,3b) are secured in position between end faces of the body and the lids.

Description: This invention relates to an acceleration detector for detecting the acceleration of a moving body such as an automobile. Among the known acceleration detectors, there is one in which the motion of a magnetic body moving under acceleration is detected by use of a differential transformer. Fig. 5 shows one example. If acceleration is applied to this acceleration detector 41 in the direction A in this figure, a magnetic body 44 supported by leaf springs 43a and 43b moves in the direction B while resiliently deforming the leaf springs. By this movement, the portion of the magnetic body 44 that is present inside a righthand secondary coil 47b will become longer than its portion located inside a lefthand secondary coil 47a. This produces a difference in induced voltage between the secondary coils 47a and 47b. The acceleration is detected by this difference. In Fig. 5, 46 is a primary coil and 42 is a case. One example of this type of detectors is disclosed in Japanese Utility Model Unexamined Publication 59-95266. With this type of detectors, since an induced voltage difference is produced between the right and left secondary coils in response to the relative displacement between the secondary coils and the magnetic body, the magnetic reluctance between the primary coil and the secondary coils is rather large and thus the sensitivity of detection tends to be low. Further, in order to stabilize its performance, the parts have to be positioned relative to each other with high accuracy. This requires time-consuming adjustment work and thus the efficiency of assembly is low and the assembling cost high. Moreover, since there is no standard for positioning, accurate adjustment is difficult, which will make it difficult to attain the performance stability. Objects of this invention are to obviate these shortcomings, to improve the detecting sensitivity of the detector, to make it possible to improve the relative positioning accuracy of the parts easily, and to stabilize the performance, simplify the assembly and reduce the assembling cost. In order to solve the above problems, the present invention provides an acceleration detector comprising a case made of a magnetic material, a movable magnetic body mounted in the case to be movable under acceleration, a differential transformer having primary coils and secondary coils and being arranged so that an output differential is produced between the secondary coils when the movable magnetic body displaces, the movable magnetic body extending through the primary coils and the secondary coils and having both ends thereof protruding from the coils, leaf springs supporting the protruding ends of the movable magnetic body, and the case comprising a cylindrical body surrounding the outer periphery of the differential transformer and a pair of lids closing openings of the body at both ends thereof, the lids and the both ends of the movable body defining predetermined gaps therebetween through which a magnetic flux can flow, the leaf springs being secured in position between end faces of the cylindrical body and the lids. In the acceleration detector according to the present invention, after inserting the lids of the case into the openings of the body, they may be secured to the body by caulking the body along the edges of the openings. Magnetic pieces may be press-fitted to both ends of the movable magnetic body with free ends of the leaf springs sandwiched and fixed in position therebetween. The differential transformer may be formed of two bobbins made of resin and provided at both sides and two coil pairs. Each pair comprise one each primary and secondary coils and are wound concentrically around the respective bobbins. Further, a yoke is integrally provided betweeen the bobbins and secured to the case. The bobbins may be provided at both ends thereof with protrusions pressed against the lids of the case to hold the bobbins in a fixed position. Otherwise, the bobbins may have their outer peripheral portions fixed to the inner wall of the body of the case. These arrangements will improve accuracy and simplify the assembling work and further stabilize the performance. When coupling the magnetic pieces to both ends of the movable magnetic body, since the pieces are considered as parts of the movable magnetic body, the predetermined gaps are provided between the pieces and the side walls of the magnetic case. The magnetic flux produced by the primary coils is transmitted through the movable magnetic body to the secondary coils, thus inducing a voltage in the secondary coils. The magnitude of the induced voltage is determined by the amount of the magnetic flux passing through the secondary coils. The amount of the magnetic flux passing through the secondary coils is in turn determined by the magnetic reluctance of the magnetic circuit present between the primary coils and the secondary coils. According to this invention, the gaps provided between the movable magnetic body and the side walls of the case are adapted to change. This serves to increase the width of change in the magnetic flux and thus the detecting sensitivity. Namely, when the movable magnetic body is moved under acceleration toward one end, one of the gaps between the movable magnetic body and the side walls of the magnetic case reduces and the other increases. As a result, the magnetic reluctance due to the gap reduces at the one end, so that the amount of magnetic flux passing through this portion (that is, the amount of magnetic flux passing through the magnetic case toward the secondary coils) increases compared with the prior art structure. At the other end, the magnetic reluctance due to the gap increases, so that a phenomenon opposite to the above occurs. Since the flow rate of magnetic flux increases at one end and reduces at the other end, the difference between the voltages of the secondary coils induced by the flow rate of magnetic flux is large compared with the prior art. By measuring this difference, even a very small acceleration can be detected. According to this invention, unlike a prior art detector in which an induced voltage differential is produced between the two secondary coils according to the relative position between the movable magnetic body and the secondary coils, a voltage differential (detector output) is produced according to the difference of gaps. This eliminates the need to set the relative position between the coils and the movable magnetic body with high accuracy (which is very difficult because there is no reference surface). On the other hand, the relative position between the yoke and the coils as well as the gap sizes at both ends of the movable magnetic body have to be set with high accuracy. According to this invention, the movable magnetic body has its both ends supported by the leaf springs, which is in turn secured between the end faces of the body of the magnetic case and the end faces of the lids. Thus, the sizes of the gaps at both ends of the movable magnetic body can be determined with high accuracy simply by setting the dimensions from the end faces of the movable magnetic body to the mounting portions of the leaf springs with high accuracy. In another arrangement wherein the magnetic pieces are coupled to both ends of the movable magnetic body with the free ends of the leaf springs secured therebetween, the distance between the two leaf springs and the dimensions from the leaf springs to the end faces of the pieces can be set with high accuracy, using both ends of the movable magnetic body as references. Higher gap accuracy means higher sensitivity. By integrally forming the yoke and the bobbins on which the primary and secondary coils are wound concentrically, the relative position between the yoke and the coils can be set accurately. Further, by fixing the lids and body of the case together by caulking, the leaf springs can be fixed simultaneously. This simplifies the structure and the assembling work. In the arrangement in which the pieces are press-fitted to the movable magnetic body, no stress (such as due to welding heat) will occur. This leads to performance stability. In the arrangement in which the bobbins are secured to both ends or outer periphery of the case, the bobbins will never displace in the case. This not only leads to stability of performance but simpler structure because no additional fixing parts are necessary. In the acceleration detector according to this invention, the movable magnetic body has its both ends supported by the leaf springs which are held between the end faces of the case body and the lids. This improves the dimensional accuracy between the leaf springs and the inner surface of the lids of the case and thus the accuracy of the gaps provided at both ends of the movable magnetic body. Furhter, by fixing the case body and the lids together by caulking, the assembling steps as well as the entire structure can be simplified since the leaf springs can be fixed in position when connecting the case parts. Also, in the arrangement where the pieces made of magnetic material are connected as by press-fitting to both ends of the movable magnetic body to secure the free ends of the leaf springs between the pieces and the movable magnetic body, the dimension between the leaf springs and the end faces of the pieces can be set with high accuracy and thus the distance between the leaf springs and the size of the gaps can be set with high accuracy. Further the leaf springs can be kept free of any unnecessary stress. This leads to higher sensitivity and stabler performance. In the arragement where the primary and secondary coils are wound concentrically around bobbins made of resin and integral with the yoke, the relative positioning accuracy between the bobbins and the yoke can be improved easily. The entire structure and the assembling steps can be simplified. Accordingly, this invention provides a high-performance acceleration detector which is easy to manufacture and inexpensive. Other features and objects of the present invention will become apparent from the following description taken with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of one embodiment of the acceleration detector according to this invention; Fig. 2 is a schematic view of the second embodiment; Fig. 3 is a schematic view of the third embodiment; Fig. 4 is a schematic view of the fourth embodiment; and Fig. 5 is a view showing a prior art differential transformer type acceleration detector. Fig. 1 shows schematically the first embodiment. An acceleration detector 1 has a case 2 made of a magnetic material. The case 2 comprises a cylindrical body 2-1 and two cup-shaped lids 2-2 closing the openings at both ends of the body. Leaf springs 3a and 3b have their both ends sandwiched (as shown at c of Fig. 1) between the axial abutting surfaces of the lids and the body. The right and left lids 2-2 are inserted in the openings of the body 2-1. The body has their thin edge portions around the openings bent inward (or caulked) to fix together the body and the lids and thus the leaf springs 3a and 3b. A movable magnetic body 4 extends axially between the central parts of the leaf springs 3a and 3b. The movable magnetic body is formed with holes in both ends into which are press-fitted protrusions of magnetic pieces 5 to fix the free ends of the leaf springs 3a and 3b to the movable magnetic body 4. In this state, there are predetermined gaps g provided between the outer ends of the magnetic pieces 5 and the inner walls of the side walls (or lids 2-2) of the case. Concentrically provided around the movable magnetic body 4 are primary coils 6a and 6b for generating a magnetic flux and secondary coils 7a and 7b for detecting change in magnetic flux. As is apparent from the figure, the coils 6a and 7a and the coils 6b and 7b are provided in pairs at left and right, each pair of coils being stacked one on the other. A yoke 8 is provided between these pairs of coils. In the first embodiment, when the moving body is accelerated or decelerated, the movable magnetic body 4 (having a mass M) is subjected to a force F = M.G. The leaf springs 3a and 3b are deformed by the force F till this force balances with the force of the leaf springs 3a and 3b and thus the movable magnetic body 4 displaces. Such a displacement is proportional to the acceleration G if the spring constant of the springs 3a, 3b and the mass M are constant. Thus the gaps g between the inner surfaces of the lids 2-2 and the end faces of the pieces 5 change in proportion to the acceleration G. If the acceleration detector 1 is accelerated in the direction A in the figure, the movable magnetic body 4 will move in the direction B. Thus, the gap at the righthand side decreases while the gap at the left increases. This allows a smoother flow of magnetic flux between the righthand lid 2-2 and the movable magnetic body 4, so that the magnetic flux transmitted from the primary coil 6b to the secondary coil 7b increases. Thus a larger voltage is induced at the secondary coil than when the acceleration is zero. In contrast, on the lefthand side, due to the increased gap size, the magnetic reluctance increases. This reduces the magnetic flux passing through the secondary coil 7a and thus the output voltage. It will be apparent from the foregoing that in this state, the output of the secondary coil 7b is larger and the output of the coil 7a is smaller compared with the prior art arrangements. Thus, the difference in output between the coils to the acceleration is greater than with the conventional arrangements. This serves to increase the sensitivity of detection. If the acceleration applied in the direction A in the figure is excessive, the movable magnetic body 4 will abut the inner surface of the lid, so that any further movement is prevented. Thus, even if excessive acceleration is applied, the leaf springs 3a and 3b are protected against excessive deflection, permanent deformation or breakage. With this arrangement, the accuracy of various dimensions including the size of the gaps provided at both ends of the movable magnetic body can be improved easily and the assembling step can be simplified. Fig. 2 shows schematically the second embodiment. This acceleration detector 11 is similar to the first embodiment in structure and function. Only differences are that two leaf springs 13a and 13b are in the form of cantilevers and that the movable magnetic body 14 is provided at both ends with projections onto which are press-fitted ring-shaped pieces 15 to fix the free ends of the leaf springs to the movable magnetic body 14. By use of the ring-shaped pieces 15 the parts can be machined easily. Fig. 3 shows schematically the third embodiment. An acceleration detector 21 has primary coils 26a and 26b and secondary coils 27a and 27b. The coils 26a and 27a and the coils 26b and 27b are provided in pairs, put one on the other concentrically around bobbins 29 made of resin to form coil units provided around a magnetic movable body 24 on both sides thereof. A yoke 28 is provided between the bobbins 29 and thus between the coil units so as to be integral therewith. The bobbins 29 are provided at both ends thereof with protrusions 29a and 29b which are held by side walls 22-2 to fix the bobbins 29 in position in the case 22. This serves to simplify the assembling work furthermore. Fig. 4 shows schematically the fourth embodiment. This acceleration detector 31 has substantially the same structure as the third embodiment and functions in a similar manner. This embodiment differs from the third embodiment in that the yoke 38 is inserted between the bobbins 39 when forming bobbins so as to be integral therewith and in that the bobbins 39 have their outer peripheral portions fixed to the inner periphery of the case 32-1 instead of fixing the bobbins at their ends as in the third embodiment. With this arrangement, the yoke 38 and the bobbins 39 can be connected more firmly and thus their relative positioning accuracy can be increased more easily. It will be understood from the above description that the best form of the acceleration detector according to this invention will have the coil and yoke of the detector in the first or second embodiment mounted in such a manner as in the third or fourth embodiment.

Claims: An acceleration detector comprising a case made of a magnetic material, a movable magnetic body mounted in said case to be movable under acceleration, a differential transformer having primary coils and secondary coils and being arranged so that an output differential is produced between said secondary coils when said movable magnetic body displaces, said movable magnetic body extending through said primary coils and said secondary coils and having both ends thereof protruding from said coils, leaf springs supporting the protruding ends of said movable magnetic body, and said case comprising a cylindrical body surrounding the outer periphery of said differential transformer and a pair of lids closing openings of said body at both ends thereof, said lids and said both ends of said movable body defining predetermined gaps therebetween through which a magnetic flux can flow, said leaf springs being secured in position between end faces of said cylindrical body and said lids. An acceleration detector as claimed in claim 1, wherein said lids of said case are inserted in said openings of said cylindrical body and secured to said body by caulking said body and said lids together. An acceleration detector as claimed in claim 1 or 2, wherein magnetic pieces are coupled to the ends of said movable magnetic body with ends of said leaf springs fixed in position therebetween. An acceleration detector as claimed in claim 3, wherein said movable magnetic body is provided on the end faces thereof with holes or projections in or on which said pieces are press-fitted. An acceleration detector as claimed in any of claims 1 - 4, wherein said differential transformer comprises a pair of bobbins made of resin, said one primary coil and said one secondary coil being wound concentrically around said each bobbin and wherein a yoke is integrally provided betweeen said bobbins, said yoke being secured to said case. An acceleration detector as claimed in claim 5, wherein said yoke is inserted between said bobbins when forming said bobbins so as to be integral therewith. An acceleration detector as claimed in claim 5 or 6, wherein an assembly comprising said bobbins and said yoke is provided at both ends thereof with protrusions which are pressed against said lids of said case to hold said bobbins in a fixed position. An acceleration detector as claimed in claim 5 or 8, wherein said assembly has an outer peripheral portion thereof fixed to the inner wall of said cylindrical body of said case.

Inventors: KUME MASAHIRO C O ITAMI WORKS, SAITO HIDETOSHI C O ITAMI WORK, KUME, MASAHIRO, C/O ITAMI WORKS OF SUMITOMO, SAITO, HIDETOSHI, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: G01P15
IPC codes: G01P15