Acceleration detector

Abstract: An acceleration detector has a case and a movable magnetic body mounted in the case and supported by a leaf spring secured to the case so as to be movable in two opposite directions in response to acceleration. Stoppers made of a magnetic material are secured to the inner wall of the case at both ends of the magenetic body, with predetermined gaps left therebetween. Two pairs of primary and secondary coils are provided around the magnetic body for producing a difference in output between the secondary coils according to the displacement of the magnetic body.

Description: This invention relates to an acceleration detector for detecting the acceleration and deceleration of a moving body such as an automobile. Among known acceleration detectors, there is a type having a magnetic body mounted so as to move when subjected to acceleration and a differential transformer for detecting its movement. One example of this type is shown in Fig. 8. With this differential transformer type acceleration detector 71, when it is subjected to acceleration in the direction of arrow A in the figure, a magnetic body 74 supported by leaf springs 73a and 73b moves in the direction of arrow B in the figure while resiliently deforming the leaf springs. With this movement, the portion of the magnetic body 74 located inside a secondary coil 77b at the righthand side of the figure becomes longer than its portion located inside a secondary coil 77a at the lefthand side, thus producing a difference in induced voltage between the secondary coils 77a and 77b. The acceleration is detected by this difference. In the figure, numeral 76 designates a primary coil, and 72 is a case. One prior art example of this type is disclosed in Japanese Unexamined Utility Model Publication 59-95266. With this type of detector, if the acceleration is small, the magnetic body moves little, producing too small a voltage difference to be detected clearly. Thus, its detection sensitivity is bad. With the known detector, the leaf springs for suspending the magnetic body might suffer a permanent deformation or might be destroyed if an excessive acceleration acts. Further, because a difference in voltage is produced by the relative movement between the secondary coil and the magnetic body, they have to be positioned with high accuracy. This will lower the assembling efficiency, take a long time and thus increase the assembling cost. An acceleration detector according to the preamble of claim 1 is disclosed in US-A-3267740. In this detector the secondary coils are provided at positions directly facing the end faces of the movable body such that the magnectic flux is transferred directly to the secondary coils. Thus, in order to improve the sensitivity to acceleration it is necessary to considerably increase the number of turns of the coils and their sectional area. It is an object of the present invention to provide a differential transformer type acceleration detector which obviates the above said shortcomings and to improve their sensitivity to acceleration. This object is achieved by an acceleration detector as defined in claim 1; the dependent claims are related to further development of the invention . The magnetic field generated by the primary coils is propagated to the movable magnetic body through the stoppers or to the stoppers through the movable magnetic member (the order of propagation depends on the winding direction of the primary coils) and then to the secondary coils, inducing a voltage in the secondary coils. The magnitude of the induced voltage depends on the amount of the magnetic flux passing through the secondary coils. The amount of the magnetic flux is determined by the magnetic body. According to the present invention, the stoppers for the magnetic member serve to reduce the entire magnetic reluctance in the circuits and to amplify the fluctuation of the magnetic flux with the amount of displacement of the movable magnetic body. Thus the sensitivity of detection will improve. Namely, when acceleration acts on the movable magnetic body, moving it toward one of the stoppers, the gap between the movable magnetic body and the one of the stoppers will reduce while the gap at the other side of the magnetic body will increase. Thus the magnetic reluctance will reduce at the side of the one of the stoppers because of the reduction in size of the gap, thus allowing a larger amount of magnetic flux to pass therethrough. At the other side, the magnetic reluctance will increase owing to the increase in size of the gap, developing a phenomenon contrary to the above. By the increase and decrease of the amount of magnetic flux passing at both sides of the magnetic body, the difference between the voltages induced in the secondary coils by the magnetic flux will be amplified. Thus, by measuring this difference, even a very low acceleration can be detected. The detector according to this invention does not produce a difference in induced voltage between two secondary coils when there is a relative displacement between the movable magnetic body and the secondary coils as with prior art detectors, but produces a voltage difference (the detector output) when there is a difference between the gaps defined by the two stoppers and the movable magnetic body. This arrangement eliminates the necessary of adjusting the relative position between the coils and the movable magnetic body with high accuracy. Further, since the stoppers serve as magnetic paths, the entire magnetic reluctance in the two magnetic circuits will decrease, thus increasing the sensitivity of detection to very small fluctuations in the magnetic reluctance. Thus the output voltage corresponding to the acceleration will increase greatly. Further, since the stoppers serve to restrict any excessive movement of the movable magnetic body, the leaf spring is prevented from being subjected to unduly large loads. Further, as described above, since the voltage difference is produced utilizing the difference between gaps formed between the stoppers and the movable magnetic body, positioning of the parts is easy. Namely, because coils are usually wound around resin bobbins, dimensional accuracy is low. Thus, with the prior art method, the accuracy of relative positioning between the movable magnetic body and the coils was poor. With the structure according to this invention, the accuracy of gaps is determined by the combination of metallic works, which can be machined with high precision. The output is determined by the gaps. Thus the positioning at the time of assembling will be easy. Further, by using a magnetic material at least for the parts constituting the magnetic paths in the case body or by providing a yoke between the coils, the magnetic reluctance in the magnetic circuits will reduce, thus increasing the output. Though small in size, the detector according to the present invention has a high sensitivity of detection and operates reliably even if subjected to excessive acceleration and shocks. Further, it can be manufactured easily and thus at low cost. 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 the first embodiment of the acceleration detector according to this invention; Figs. 2 to 7 are schematic views of the other embodiments; and Fig. 8 is a view showing a prior art differential transformer type acceleration detector. Fig. 1 shows the first embodiment. This acceleration detector 1 comprises a case 2, a leaf spring 3 having one end thereof fixed to the case 2 and the other end secured to a movable magnetic body 4 of a predetermined mass. Stoppers 5a and 5b made of a magnetic material are supported by the case 2 so as to be located at both ends of the movable magnetic body 4. A gap is formed between each stopper and the movable magnetic body 4. Around the movable magnetic body 4 and the stoppers 5a and 5b, there are provided in coaxial relation with each other primary coils 6a and 6b for generating magnetic flux and secondary coils 7a and 7b for detecting fluctuations of the magnetic flux. The coils may be supported by the stoppers or by the case. In the operation of the detector of the first embodiment, when the moving body is accelerated or decelerated, a force F = M x G (M is the mass of the movable magnetic body 4 and G is the acceleration) will act on the movable magnetic body 4. The force F will resiliently deform the leaf spring 3 while displacing the movable magnetic body until it balances the resilient force of the leaf spring 3. The amount of displacement is proportional to the acceleration provided the spring constant of the leaf spring 3 and the mass M of the movable magnetic body 4 are constant. Thus, the gaps between the stoppers 5a and 5b and the movable magnetic body 4 will change in proportion to the acceleration G. Suppose now that the acceleration detector 1 is subjected to acceleration in the direction of arrow A in the figure. Then the movable magnetic body 4 will move in the direction B, reducing the gap with the stopper 5b and increasing the gap with the stopper 5a. This provides smoother flow of magnetic flux from the stopper 5b to the magnetic body 4, thus increasing the amount of magnetic flux transmitted from the primary coil 6b to the secondary coil 7b. Thus a voltage larger than when the acceleration is zero will be induced in the secondary coil 7b. On the other hand, at the stopper 5a, the magnetic reluctance increases due to the increased gap, thus reducing the magnetic flux passing through the secondary coil 7a. Thus the output voltage will reduce, too. Thus, the output of the secondary coil 7b will increase and that of the secondary coil 7a will decrease due to the above-described action. This will expand the difference in output between the secondary coils for the acceleration and thus increase the sensitivity of detection compared with any prior art detector. When an excessive acceleration acts in the direction A, the movable magnetic body 4 will butt the stopper 5b and be prevented from moving any further. This prevents excessive distortion of the leaf spring 3 due to excessive acceleration, thus protecting it from permanent deformation and breakage. Fig. 2 shows the outline of the second embodiment. This acceleration detector 11 is similar in construction and function to the first embodiment. But it differs from the first embodiment in that the magnetic body 14 is supported by a pair of the leaf springs 13 having one end thereof secured to the case, that the primary coils 16a and 16b are provided around and coaxially with the movable magnetic body 14 with the secondary coils 17a and 17b disposed outside the primary coils (i.e. in front and rear with respect to the direction of movement), and that stoppers 15a and 15b are integral with the case 12. By providing the primary coils inside (near the leaf spring) and the secondary coils outside (near the case), the movable magnetic body 14 can be increased both in size and weight without increasing the size of the detector. This will be advantageous for further improvement in sensitivity. There is a fear that if the movable magnetic body 14 has a large mass, it may move not only in the desired directions but also move up and down due to vertical acceleration, disturbing the output voltage. In this embodiment, because the magnetic body 14 is supported by a pair of leaf springs, this problem can be prevented. Also, in this embodiment, because not only the stoppers but also the entire case is made of a magnetic material, the magnetic reluctance in the magnetic circuits around the coils can be reduced. This increases the ratio of the magnetic reluctance at the gaps to the entire magnetic reluctance and thus further increases the sensitivity of detection. Fig. 3 shows the outline of the third embodiment. This acceleration detector 21 is basically the same in construction and function as the first and second embodiments. Only differences are that a movable magnetic body 24 is supported by two cantilever leaf springs 23a and 23b extending parallel to each other and that secondary coils 27a and 27b are provided coaxially on the outer periphery of primary coils 26a and 26b. In this embodiment, by the provision of the two cantilever leaf springs, the movable magnetic body can not only be supported as stably as in the second embodiment but also it can move as large an amount as it is supported by a single cantilever leaf spring. In other words, this arrangement satisfies the two requirements contradictory to each other, i.e. the requirements for stability and increased output. Also, by disposing the primary coils and the secondary coils one upon the other, the width of winding of the coils can be reduced. Thus this structure will be of advantage in a case where the mounting space is limited with respect to the direction in which acceleration acts. Fig. 4 shows the fourth embodiment of this invention. This acceleration detector 31 has the same construction as the first embodiment except that a yoke 8 made of a magnetic material is provided between the secondary coils 7a and 7b to reduce the magnetic reluctance. By the provision of the yoke 8, the magnetic flux produced in the primary coil 6a or 6b can flow through the movable magnetic body 4, the yoke 8 and the case 2 made of a magnetic material back into the respective stoppers 5a and 5b. With this arrangement, only at the gaps formed between the stoppers and the movable magnetic body, the magnetic reluctance is high in the magnetic circuits, thus making more conspicuous the changes in output with fluctuations in the size of the gaps. As a result, the sensitivity of detection will improve. Fig. 5 shows the outline of the fifth embodiment. This acceleration detector 41 comprises a case 12 made of a magnetic material, leaf springs 3a and 3b having one end thereof fixed to the case 2 and a movable magnetic body 14 having a predetermined mass and secured to the other or free end of the leaf springs 3a and 3b. The case 12 is integrally formed with projections 15a and 15b so as to oppose to both ends of the movable magnetic body 14 with gaps formed therebetween. A yoke 18 integral with the case is provided between primary coils 16a and 16b. The primary coils 16a and 16b for generating magnetic flux and the secondary coils 17a and 17b for detecting changes in the magnetic flux are supported by the case so as to be located coaxially around the movable magnetic body 14. Fig. 6 shows the outline of the sixth embodiment. This acceleration detector 51 has the same construction and function as the fifth embodiment except that the case 2 is made of a non-magnetic material, that magnetic bodies 5a and 5b are fixedly mounted on the case, and that a yoke 28 provided between the coils 16a and 16b is supported by the case but not integral with the case. The case made of a non-magnetic material serves to reduce the entire weight of the detector though the magnetic reluctance in the entire circuits may increase even if the yoke is provided. Fig. 7 shows the outline of the seventh embodiment. This detector 61 has the same construction and function as the fifth embodiment except that the primary coils 26a and 26b are placed on the outer periphery of the secondary coils 27a and 27b, that the yoke between the coils is omitted, that a single leaf spring 3 is used instead of two, and that the projections on the inner wall of the case are omitted so that the movable magnetic body 34 will directly face the wall surfaces of the case. In this embodiment, though the output tends to be lower than in the fifth embodiment, the amount of displacement of the movable magnetic body can be increased easily. Because the width of winding of coil can be reduced, this embodiment is advantageously applicable to a situation where the mounting space is limited with respect to the direction in which acceleration acts.

Claims: An acceleration detector comprising a case (2), a movable magnetic body (4) mounted in said case (2), and a leaf spring (3,13,23) having one end thereof secured to said case (2) and the other end secured to said magnetic body (4) for supporting said magnetic body (4), said magnetic body (4) being supported so as to be movable in two opposite directions in response to acceleration, characterized in that said acceleration detector further comprises stoppers (5,15,25) made of a magnetic material and secured to the wall of said case disposed on both sides of said magnetic body along the direction of movement of said magnetic body with gaps formed therebetween, said gaps being of such a width that magnetic flux can be transmitted between said movable magnetic body (4,14,34) and said stoppers (5,15,25), and a differential transformer mounted in said case and comprising two pairs of primary and secondary coils (6,7;26,27;16,17), said primary coils and said secondary coils being arranged coaxially around said movable magnetic body (4), said coils being positioned so that a magnetic flux produced in said movable body by said primary coils is transmitted to said stoppers through said respective gaps and then from said stoppers to said secondary coils, thereby inducing an electric current in said secondary coils and detecting the acceleration in terms of a difference in output between said secondary coils which is determined by the size of said gaps. An acceleration detector as claimed in claim 1, wherein said case (2) is at least partially made of a magnetic material. An acceleration detector as claimed in claim 1, wherein said stoppers (15,25) are integral with said case. An acceleration detector as claimed in claim 1, further comprising a yoke (18,28) disposed in the center of said case and between said two pairs of primary and secondary coils. An acceleration detector as claimed in claim 1, wherein said stoppers (15,25) have their ends protruding into said differential transformer.

Inventors: KUME MASAHIRO C O ITAMI WORKS, SAITO HIDETOSHI C O ITAMI WORK, KUME, MASAHIRO, C/O ITAMI WORKS, SAITO, HIDETOSHI, C/O ITAMI WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: G01P15
IPC codes: G01P15