Elastic mount and method of manufacturing the elastic mount
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a fluid-filled elastic mount having a fluid chamber partially defined by an oscillating plate which is actuated by an electromagnetic drive device so as to change the fluid pressure in the fluid chamber to thereby control the damping characteristics of the mount. More particularly, the present invention is concerned with such a fluid-filled elastic mount which can be easily fabricated and which has an electromagnetic drive device capable of stably producing a large drive force to actuate the oscillating plate. The present invention is also concerned with a method of manufacturing the fluid-filled elastic mount. 
     2. Discussion of the Prior Art 
     As a vibration damper for flexibly connecting two members in a vibration system or mounting one of the two members on the other member in a vibration damping fashion, there is known an elastic mount interposed between the two members of the vibration system. The elastic mount has an elastic body interposed between and elastically connecting a first and a second support which are respectively fixed to one and the other of the two members of the vibration system. This type of elastic mount may be used as an engine mount or a suspension bushing for a motor vehicle, for example. 
     Recently, there have been proposed various types of fluid-filled elastic mounts adapted to exhibit sophisticated damping characteristics, wherein the elastic body which elastically connects the first and second supports partially defines a fluid chamber filled with a non-compressible fluid. Some of these fluid-filled elastic mounts are adapted to electrically control the fluid pressure within the fluid chamber, depending upon the type of the input vibrations received, so that the specific vibrations can be suitably damped or isolated. 
     Examples of such electrically controllable fluid-filled elastic mount are disclosed in JP-A-59-1828, JP-A-59-1829 and JP-U-3-73741, wherein the fluid chamber is partially defined by an oscillating plate, which is oscillated by an electromagnetic force produced by a permanent magnet and a coil, so as to suitably control the fluid pressure within the fluid chamber, to thereby enable the elastic mount to exhibit different vibration damping characteristics depending upon the type of the input vibrations received. 
     In the known electrically controllable fluid-filled elastic mount constructed as described above, the operation of the oscillating plate cannot be suitably regulated so as to enable the elastic mount to exhibit satisfactory damping characteristics, since it is difficult to obtain a sufficiently large drive force to effectively actuate the oscillating plate. 
     Described more specifically, the above fluid-filled elastic mount suffers from insufficiency of the magnetic flux density in the magnetic field in which the coil is placed, because the magnetic path or circuit formed by the permanent magnet is open. In particular, the open magnetic circuit leads to insufficiency of the drive force to actuate the oscillating plate so as to effectively regulate the fluid pressure within the fluid chamber, when the elastic mount receives a vibrational load of medium to low frequencies having a relatively large amplitude. 
     Moreover, the open magnetic circuit formed by the permanent magnet is unable to provide a substantially constant magnetic flux density in the field in which the coil is placed, and therefore inevitably causes a large variation in the magnetic flux density applied to the coil when the oscillating plate is actuated or displaced in the oscillating manner. As a result, the drive force which acts on the oscillating plate tends to be unstable, making it difficult to effectively control the oscillating plate, whereby the waveform of the pulsation induced within the fluid chamber is distorted, causing a fluid pressure control distortion of the fluid chamber. Thus, the known electrically controllable fluid-filled elastic mount is not satisfactory in terms of its damping characteristics. 
     Furthermore, the permanent magnet which is conventionally made of a magnetic steel is fragile and difficult to be formed into a desired shape by molding. Accordingly, the conventional permanent magnet suffers from a cumbersome procedure for producing the same, low shock resistance, and difficulty in securing the magnet to other members. Consequently, the vibration damper using the permanent magnet is difficult to fabricate and is not satisfactory in its durability. 
     SUMMARY OF THE INVENTION 
     It is therefore a first object of the present invention to provide an electrically controllable fluid-filled elastic mount having an oscillating plate partially defining a fluid chamber, which elastic mount can be easily fabricated assuring excellent durability, and is provided with an electromagnetic drive device capable of effectively actuating the oscillating plate with a sufficiently large and stable drive force. 
     A second object of the invention is to provide a method of manufacturing such electrically controllable fluid-filled elastic mount. 
     The first object may be achieved according to one aspect of the present invention, which provides a fluid-filled elastic mount comprising: a first and a second support which are spaced apart from each other; an elastic body which is interposed between the first and second supports for elastically connecting the first and second supports and which partially defines a fluid chamber filled with a non-compressible fluid; an oscillating plate which partially defines the fluid chamber and which is displaceable to change a pressure of the fluid in the fluid chamber; a permanent magnet disposed on one of opposite sides of the oscillating plate remote from the fluid chamber, the permanent magnet being made of a plastic magnetic material including a magnetic material and a plastic material as a binder for the magnetic material; a first and a second yoke member which are connected to respective opposite magnetic pole faces of the permanent magnet and which cooperate with the permanent magnet to define a closed magnetic circuit, the first and second yoke members defining therebetween an annular gap in the magnetic circuit; and an annular moving coil received in the annular gap and fixed to the oscillating plate, the moving coil being displaced in the annular gap in an axial direction thereof, to oscillate the oscillating plate upon energization of the moving coil. 
     In the fluid-filled elastic mount of the present invention constructed as described above, the annular moving coil is disposed in the annular gap defined by and between the first and second yoke members, which cooperate with the magnet to define a closed magnetic circuit or path. Accordingly, the present arrangement is effective to minimize the amount of leakage of the magnetic flux, thereby increasing the magnetic flux density at the annular gap and improving the uniformity of the magnetic flux density. 
     Consequently, the moving coil is exposed to the sufficiently high density of magnetic flux, whereby an accordingly large magnetic force is produced to move the coil upon energization of the coil, irrespective of the axial position of the coil which varies over a predetermined range of operating stroke. This means a large drive force to oscillate the oscillating plate, assuring increased operating stability of the oscillating plate. 
     The increased operating stability of the oscillating plate with a large drive force assures improved accuracy and stability of regulation of the fluid pressure in the fluid chamber, and enhanced damping characteristics of the elastic mount. 
     Further, the use of the plastic magnetic material for the permanent magnet leads to increased freedom in selecting the shape of the magnet and improved dimensional accuracy, compared to the conventional permanent magnet made of a magnetic steel. Since the permanent magnet can be easily formed by molding and secured to the first and second yoke members without requiring any means for fastening or bonding the magnet and yoke members together, the process of fabricating the present fluid-filled elastic mount is considerably simplified. 
     Moreover, the permanent magnet made of the plastic magnetic material exhibits a remarkably improved shock resistance compared to the conventional permanent magnet made of a magnetic steel, thereby assuring excellent durability of the elastic mount. 
     The second object of the invention may be achieved according another aspect of the present invention, which provides a method of producing a fluid-filled elastic mount including (a) a first and a second support which are spaced apart from each other; (b) an elastic body which is interposed between the first and second supports for elastically connecting the first and second supports and which partially defines a fluid chamber filled with a non-compressible fluid; (c) an oscillating plate which partially defines the fluid chamber and which is displaceable to change a pressure of the fluid in the fluid chamber; (d) a permanent magnet disposed on one of opposite sides of the oscillating plate remote from the fluid chamber; (e) a first and a second yoke member which are connected to respective opposite magnetic pole faces of the permanent magnet and which cooperate with the permanent magnet to define a closed magnetic circuit, the first and second yoke members defining therebetween an annular gap in the magnetic circuit; and (f) an annular moving coil received in the annular gap and fixed to the oscillating plate, the moving coil being displaced in the annular gap in an axial direction thereof, to oscillate the oscillating plate upon energization of the moving coil, the method comprising the steps of: preparing a plastic magnetic material which gives the permanent magnet, by mixing a magnetic material with a plastic material; positioning the first and second yoke members in a mold with a spacing left therebetween; filling the spacing between the first and second yoke members with the plastic magnetic material as it is plasticized; and solidifying the plastic magnetic material to form the permanent magnet such that the respective opposite magnetic pole faces of the permanent magnet are in contact with the first and second yoke members. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be better understood by reading the following detailed description of the preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
     FIG. 1 is an elevational view in axial cross section of one embodiment of a fluid-filled elastic mount of this invention in the form of an engine mount for a motor vehicle; and 
     FIG. 2 is an elevational view in axial cross section of a magnet assembly prepared as an intermediate product during manufacture of the elastic mount of FIG. 1, including a plastic permanent magnet and two yoke members which define a magnetic circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1 showing an electrically controllable fluid-filled elastic engine mount for a motor vehicle, reference numerals 10 and 12 denote a first and a second support which are made of metals and are spaced apart from each other by a suitable distance in a load receiving direction in which the engine mount receives input vibrations. These two supports 10, 12 are elastically connected to each other by an elastic body 14 interposed therebetween. The engine mount is used to mount a power unit (including an engine) on the body of the vehicle, in a vibration damping fashion, such that the first and second supports 10, 12 are fixed to one and the other of the power unit and the vehicle body. 
     The first support 10 consists of an upper member 16 and a lower member 18 which are generally hat-shaped and have respective outward flanges 20, 22. The upper member 16 has a cylindrical portion defining a cylindrical recess 23, and the outward flange 20 extends radially outwardly from the open end of the cylindrical portion. The lower member 18 has a frusto-conical portion defining a frusto-conical recess 25, and the outward flange 22 extends radially outwardly from the open end of the frusto-conical portion. The upper and lower members 16, 18 are butted together at the outward flanges 20, 22 such that the cylindrical and frusto-conical recesses 23, 25 cooperate to define an enclosed space. The two members 16, 18 are bolted together to form the first support 10. 
     Within the enclosed space 23, 25 of the first support 10, there is disposed a generally frusto-conical flexible diaphragm 24. This diaphragm 24 is fixed with its peripheral portion being gripped by and between the opposed surfaces of the outward flanges 20, 22 of the upper and lower members 16, 18. The enclosed space 23, 25 is divided by the flexible diaphragm 24 into two fluid-tight sections corresponding to the two recesses 23, 25. 
     On the other hand, the second support 12 is a generally annular member having a relatively large diameter. The second support 12 is spaced by a suitable distance from the lower member 18 of the first support 10, in the axial direction which is parallel to the load receiving direction. The elastic body 14 is interposed between the first and second supports 12, 14 for elastic connection therebetween. This elastic body 14 has a generally frusto-conical shape and a generally frusto-conical bore, and is formed such that it is bonded at its small end to the frusto-conical outer surface of the lower member 18 of the first support 10, and at its large end to the axial end face of the second support 12. Thus, there is prepared an intermediate product consisting of the second support 12, the lower member 18 of the first support 10, and the elastic body 14 formed therebetween. 
     In the intermediate product 12, 14, 18, the frusto-conical bore of the elastic body 14 is closed at its small end by the bottom wall of the lower member 18. Thus, there is formed a frusto-conical recess 26 which is open at its large end to the bore of the second support 12. 
     Within the bore of the second support 12 communicating with the frusto-conical recess 26, there is disposed an oscillating plate 27 in the form of a circular disk having a relative small thickness, such that the oscillating plate 27 is coaxial with the support 12 and is located at the open end of the recess 26. This oscillating plate 27 is elastically supported by the second support 12, through an annular elastic support 28, which is secured to the second support 12 through an annular retainer ring 30 fixed to the inner surface of the support 12 by a plurality of bolts 31. That is, the annular elastic support 28 is interposed between and secured to the oscillating plate 27 and the retainer ring 30, so that the plate 27 is displaceable in the axial direction relative to the support 12, due to elastic deformation of the elastic support 28. 
     With the oscillating plate 27 elastically fixed to the second support 12 by the elastic support 28, the frusto-conical recess 26 is fluid-tightly enclosed to form a fluid chamber in the form of a pressure-receiving chamber 32 filled with a suitable non-compressible fluid, preferably water, alkylene glycol, polyalkylene glycol and silicone oil. 
     Since the pressure-receiving chamber 32 is partially defined by the wall of the elastic body 14, the pressure of the fluid in the chamber 32 changes due to elastic deformation of the elastic body 14 upon application of a vibrational load between the first and second support 10, 12 in the load receiving direction, i.e., in the axial direction of the engine mount. 
     The pressure-receiving chamber 32 communicates, through an orifice passage 46, with an equilibrium chamber 34 also filled with the non-compressible fluid. Namely, the equilibrium chamber 34 is defined by the flexible diaphragm 24 and a disk 38 accommodated in the frusto-conical recess 25 of the lower member 18 of the first support 10. The disk 38 is bolted to the bottom wall of the lower member 18, which functions as a partition which separates the pressure-receiving and equilibrium chambers 32, 34. The disk 38 has a circumferential groove 40 in the surface which contacts the bottom wall of the lower member 18. The circumferential groove 40 communicates with the pressure-receiving chamber 32 through a communication hole 42 formed through the bottom wall of the lower member 18, and with the equilibrium chamber 34 through a communication hole 44 formed through the disk 38. Thus, the groove 40 cooperates with the communication holes 42, 44 to define the orifice passage 46 for fluid communication between the two fluid chambers 32, 34. 
     The flexible diaphragm 24 elastically yields to permit a each volumetric change of the equilibrium chamber 34 when the fluid flows into and from the equilibrium chamber 34 upon application of a vibrational load to the engine mount. Thus, the flexible diaphragm 24 absorbs a pressure change in the equilibrium chamber 34. The flexible diaphragm 24 and the upper member 16 of the first support 10 define an air chamber 36, which permits elastic deformation or displacement of the flexible diaphragm 24. 
     When a pressure change of the fluid occurs in the pressure-receiving chamber 32 due to the input vibration, the fluid is forced to flow through the orifice passage 46, between the two fluid chambers 32, 34, whereby the input vibration is damped based on the resonance of the fluid flowing through the orifice passage 46, as well known in the art. The orifice passage 46 is tuned, that is, the length and cross sectional area of the passage 46 are determined, so as to effectively damp low-frequency vibrations such as shake, based on the resonance of the fluid flowing through the orifice passage 46. 
     The engine mount is equipped with an electromagnetic drive device 48 for actuating the oscillating plate 27, which partially defines the pressure-receiving chamber 32. The drive device 48 is disposed on the side of the plate 27 remote from the pressure-receiving chamber 32. 
     The electromagnetic drive device 48 includes a cylindrical permanent magnet 50 having opposite magnetic poles or pole faces at its axially opposite ends. A ferromagnetic circular end disk 56 having a relatively large thickness is disposed in contact with the upper pole face of the permanent magnet 50. 
     The permanent magnet 50 is accommodated in a ferromagnetic base member 54 including a bottom wall portion 54a and a cylindrical wall portion 54b, such that the lower pole face of the magnet 50 is in contact with a radially central portion of the bottom wall portion 54a of the base member 54. The base member 54 has an outward flange 52 at the open end of its cylindrical wall portion 54b, and is attached at its outward flange 52 to the second support 12, by screws 76, such that the cylindrical wall portion 54b is open toward the oscillating plate 27. The base member 54 further has at the same open end an annular protrusion 58 which protrudes radially inwardly toward the periphery of the end disk 56. This annular protrusion 58 has an inner circumferential surface which is spaced a suitable distance from the outer circumferential surface of the end disk 56 in the radial direction. 
     The base member 54 and end disk 56 are both made of an iron or other ferromagnetic material, so that there is formed a closed magnetic path or circuit including the permanent magnet 50. An annular or cylindrical gap 62 is defined between radially opposed faces of the end disk 56 and the annular protrusion 58 of the base member 54 which partially define the closed magnetic circuit. In the present embodiment, the end disk 56 functions as a first yoke member connected to the upper pole face of the permanent magnet 50, while the base member 54 functions as a second yoke member connected to the upper pole face of the magnet 50. 
     The permanent magnet 50 consists of a plastic magnet which may be formed by mixing a magnetic material such as ferrite, or a rare earth element alloy (including Nd-Fe-B alloy, for example, with a plastic material such as thermoplastic resin including nylon, as known in the art. The permanent magnet 50 in the form of the plastic magnet can be formed using a mold into an intended shape with high accuracy, by injection molding or other molding technique. 
     To form the permanent magnet 50 of the present embodiment, a plasticized plastic magnetic material is injected to fill a spacing between the end disk 56 and base member 54 which are held in place within a suitable mold, and the injected material is then solidified. The base member 54 and end disk 56 are provided with respective through-holes 59, 60 adapted to permit injection of the plastic magnetic material into the mold and bleeding of the air out of the mold. 
     The end disk 56 and the bottom wall portion 54a of the base member 54 have respective engaging portions 61 having an undercut shape, at their mutually facing surfaces which are in contact with the opposite magnetic pole faces of the permanent magnet 50. The engaging portion 61 formed in the relevant surface may have a circumferentially continuous dovetail recess or a plurality of non-continuous dovetail recesses which is/are tapered so as to have a smaller diameter of the side of the permanent magnet 50. With the plastic magnetic material intruding into the engaging portions 61 and being solidified therein, the permanent magnet 50 is fixedly held in engagement with the end disk 56 and base member 54. Thus, the permanent magnet 50, end disk 56 and base member 54 are formed upon molding into an integral assembly as shown in FIG. 2. 
     The permanent magnet 50 is prepared by magnetizing an appropriate unmagnetized mass of the above-described magnetic material. This magnetization may be effected first, followed by assembling of the thus obtained magnet 50 with the end disk 56 and base member 43, or may be effected upon injection molding by which the plastic magnet 50 is formed between the end disk 56 and base member 54. However, the magnetization for the permanent magnet 50 is desirably effected after the molding of the plastic magnetic material into the magnet 50, and concurrent or subsequent assembling of the solidified mass of the plastic magnetic material with the end disk 56 and base member 54 for defining the closed magnetic circuit. In this case, the material for the permanent magnet 50 is molded while being exposed to a weak magnetic field, that is, being slightly polarized, to form an appropriate magnetic domain. The thus obtained assembly of the permanent magnet 50, end disk 56 and base member 54 is given an improved permeance coefficient upon the magnetization of the assembly as a whole, and can therefore be magnetized to a greater extent. 
     Above the end disk 56 of the thus formed magnetic assembly as shown in FIG. 2, there is disposed with a suitable spacing a cylindrical movable member 64 made of a non-magnetic material such as a synthetic resin or aluminum alloy, as shown in FIG. 1. The movable member 64 has a cylindrical portion 66 extending through the above-indicated annular gap 62 between the end disk 56 and the annular protrusion 58 of the base member 54. To permit the movable member 64 to be axially movable over a suitable distance relative to the end disk 56, a small clearance is provided between the outer circumferential surface of the end disk 56 and the inner circumferential surface of the cylindrical portion 66. 
     Within the annular gap 62 in which the cylindrical portion 66 of the movable member 64 is positioned, there is disposed an annular moving coil 68 which is axially movable within the annular gap 62. The moving coil 68 is secured to the outer circumferential surface of the cylindrical portion 66 of the movable member 64, so that the movable member 64 is moved with the coil 68 when the coil 68 is moved with an electric current applied thereto as described below, through conductor wire 70 extending through hole 72 formed through the base member 54. 
     The axial length of the annular coil 68 is selected to be smaller than the axial length of the annular protrusion 58 of the base member 54 partially defining the annular gap 62, so that the coil 68 axially displaced within the gap 62 is always positioned within the axial length of the annular protrusion 58, in order to assure a substantially constant magnetic flux density applied to the coil 68, irrespective of the axial position of the coil 68. 
     The electromagnetic drive device 48 thus obtained is attached, at the outward flange 52 of the base member 54, to the lower end of the second support 12 by means of screws 76. At the same time, the base wall of the movable member 64 is secured to the underside of the oscillating plate 27 by a bolt 74. In this condition, the annular moving coil 68 is - located at an axially middle portion of the annular gap 62. 
     In operation of the engine mount constructed as described above, the moving coil 68 is energized by a controlled alternating current, whereby the coil 68 is subject to an electromagnetic force (Lorentz force) produced according to the Fleming&#39;s left-hand rule, so that the coil 68 is moved with the movable member 64. As a result, the oscillating plate 27 is displaced with a force proportional to the amount of electric current applied to the coil 68. The oscillating plate 27 is oscillated by controlling the current applied to the coil 68, depending upon the pressure change in the pressure-receiving chamber 32 due to the input vibrational load. Thus, the fluid pressure in the chamber 32 can be effectively regulated so as to change the damping characteristic of the engine mount, depending upon the type of vibration received. 
     Described in detail, when the frequency of the input vibration is relatively low, the oscillating plate 27 is oscillated in the same phase as the input vibration, so as to positively cause a fluid pressure change in the pressure-receiving chamber 32, for increasing the amount of the fluid which flows through the orifice passage 46, and thereby improving the damping effect based on the fluid flow through the orifice passage 46. When the frequency of the input vibration is in a medium or low band, the phase of oscillation of the oscillating plate 27 is reversed with respect to that of the input vibration, to thereby absorb the fluid pressure change in the chamber 32 or reduce the amount of the fluid pressure change, so that the engine mount exhibits an effectively reduced dynamic spring constant with respect to the medium to low frequency vibration. 
     In the electromagnetic drive device 48 of the present engine mount, the magnetic field to which the moving coil 68 is exposed has a sufficiently high magnetic flux density, with a reduced amount of magnetic flux leakage from the permanent magnet 50, since the magnetic field is produced at the annular gap 62 provided in the closed magnetic circuit or path. Consequently, upon energization of the moving coil 68, a sufficiently large magnetic force is produced to actuate the oscillating plate 27 so as to suitably regulate the fluid pressure in the pressure-receiving chamber 32, and thereby exhibit optimum damping characteristics depending upon the type of the input vibration, without increasing the complexity and size of the electromagnetic drive device 48. 
     As the magnetic field in which the moving coil 68 is placed is produced at the annular gap 62 in the closed magnetic circuit or path defined by the components 50, 54, 56, the magnetic flux density in the magnetic field and the magnetic force produced are made uniform throughout the field, irrespective of the axial position of the coil 68 which is axially moved within the gap 62. This arrangement permits the produced magnetic force to be substantially proportional to the amount of electric current to be applied to the moving coil 68, whereby the oscillation of the oscillating plate 27 can be comparatively easily controlled, with an effectively reduced amount of distortion of waveform of the fluid pressure pulsation in the pressure-receiving chamber 32. Thus, the present engine mount is capable of intricately and precisely controlling the fluid pressure within the chamber 32, so as to exhibit improved vibration damping characteristics. 
     In the present engine mount, the permanent magnet 50 of the electromagnetic drive device 48 for actuating the oscillating plate 27 is made of a plastic magnetic material which can be easily molded into a magnet having a desired shape, by injection molding or other molding technique, which cannot be adopted for forming a conventional permanent magnet made of a magnetic steel. Accordingly, the use of the plastic magnetic material leads to increased freedom in selecting the shape of the permanent magnet 50 and improved dimensional accuracy of the magnet 50 thus produced, while achieving a considerably simplified process of fabricating the engine mount as well as the permanent magnet 50. 
     Further, the permanent magnet 50 formed of a plastic magnetic material has a remarkably enhanced shock resistance, compared to the conventional permanent magnet made of a magnetic steel. This is particularly advantageous when the plastic magnet is used in the automobile engine mount which is likely to receive a shock load. In this case, the durability of the engine mount is greatly enhanced. 
     In the instant embodiment, the permanent magnet 50 is molded between the mutually facing surfaces of the base member 54 and end disk 56, so that the magnet 50 is fixedly held in engagement with these members 54, 56 upon completion of the molding operation. Thus, the permanent magnet 50, base member 54 and end disk 56 can be assembled together to define the closed magnetic circuit, without requiring adhesives, bolts or other fastening or bonding means. This means further simplification of the process of fabricating the engine mount and excellent production efficiency. 
     Since the electromagnetic drive device 48 is prepared separately from a mount body of the engine mount including the first and second supports 10, 12 and elastic body 14, the injection molding and magnetization for the permanent magnet 50 may be effected with ease before the device 48 is attached to the mount body. 
     While the present invention has been described in detail in its presently preferred embodiment with a certain degree of particularity, it is to be understood that the invention is not limited to the details of the illustrated embodiment, but may be otherwise embodied. 
     In the illustrated embodiment, the fluid chamber (pressure-receiving chamber 32) is held in communication with the equilibrium chamber 34 through the orifice passage 46. However, the present invention is applicable to a fluid-filled elastic mount which does not have such equilibrium chamber and orifice passage and which damps input vibrations by suitably regulating the fluid pressure in the fluid chamber (32) by the oscillating plate 27. 
     The permanent magnet (plastic magnet) and yoke members which cooperate to provide a closed magnetic circuit are not limited to the members 50, 54, 56 used in the illustrated embodiment, but may be suitably modified, in the configuration and construction, provided an annular gap accommodating an annular coil is formed behind the oscillating plate 27. For instance, the magnetic circuit may be defined by an annular permanent magnet having magnetic pole faces at its axially opposite ends, a first yoke member connected to one of the opposite magnetic pole faces of the magnet and extending through a center bore of the annular magnet toward the other magnetic pole face, and a second yoke member connected to the other magnetic pole face of the permanent magnet and cooperating with the first yoke member to define an annular gap therebetween. 
     While all of the members 50, 54, 56 defining the closed magnetic circuit are prepared separately from the mount body in the illustrated embodiment, a part of the second support 12 may be utilized to partially define the magnetic circuit. 
     In the illustrated embodiment, the axial length of the annular moving coil 68 positioned within the annular gap 62 is selected to be smaller than that of the annular protrusion 58 of the base member 54 partially defining the gap 62. However, the annular moving coil may have a larger axial length than the annular protrusion 58 so as to protrude from the annular gap 62. In this case, too, the annular gap 62 is surely given a substantially constant magnetic flux density, whereby the magnetic flux density applied to the annular coil is less likely to change even when the coil is displaced relative to the annular protrusion 58. 
     In the illustrated elastic mount, the first and second supports 10, 12 are secured to the opposite ends of the elastic body 14 such that these supports 10, 12 are opposed to each other with a spacing therebetween in the load-receiving direction. However, the present invention may be equally applicable to a so-called cylindrical elastic mount wherein inner and outer sleeves are disposed with a radial spacing therebetween, and are connected to each other by an elastic body interposed therebetween. 
     While the illustrated fluid-filled elastic mount is an engine mount for a motor vehicle, the principle of the present invention is equally applicable to other types of vehicle damping devices such as vehicle body mounts and differential gear mounts, and even to vibration dampers or elastic mounts used in various equipment or systems other than those for motor vehicles. 
     It is to be understood that the present invention may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the following claims.