1. Field of the Invention
The present invention relates generally to a fluid-filled elastic mount used as a vibration damper such as an engine mount for a motor vehicle, and more particularly to such 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 electrically control the damping characteristics of the mount.
2. Discussion of the Prior Art
As a vibration damper such as an engine mount or a suspension bushing for a motor vehicle, 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 of bushing type interposed between the two members of the vibration system. The bushing type elastic mount has an elastic body interposed between an inner and an outer sleeve, which are radially spaced apart from each other. The inner and outer sleeves are elastically connected by the elastic body and are fixed to one and the other of the two members of the vibration system.
Keeping pace with recent tendency of upgrading motor vehicles, such elastic mounts of modern vintage for motor vehicles are increasingly required to exhibit improved damping characteristics over a wide frequency range of the vibrations to be damped, and various attempts have been made to develop elastic mounts which meet such a requirement.
In view of the above requirement, there have been proposed various fluid-filled elastic mounts of non-bushing type wherein a first and a second support, which are spaced apart from each other in the load-receiving direction, are elastically connected by an elastic body interposed therebetween. Examples of such non-bushing type elastic mounts are disclosed in JP-A-60-8540, JP-A-59-1828 and Publication No. 3-73741 of unexamined Japanese Utility Model Application, wherein the elastic body partially defines a fluid chamber filled with a non-compressible fluid. The fluid chamber is further partially defined by an oscillating plate which is oscillated by a magnetic or electromagnetic force produced by a solenoid, so as to regulate the fluid pressure within the chamber. Thus, the non-bushing type elastic mount is electrically controlled for adjusting the damping characteristics so as to effectively damp the input vibrations over a wide frequency range.
However, the construction of the known bushing type elastic mount described above is not suitable to incorporate an oscillating plate partially defining a fluid chamber and an electromagnetic drive device such as a solenoid to actuate the oscillating plate, as disclosed in the above-identified publications. Up to the present, no effective measures for improving the damping characteristics of the bushing type elastic mount have been proposed or developed.
In the known electrically controllable fluid-filled elastic mount of the non-bushing type constructed as disclosed in the above-identified publications, the operation of the oscillating plate cannot be easily or suitably regulated so as to enable the elastic mount to exhibit intended damping characteristics, since it is difficult to obtain a sufficient drive force to effectively and stably actuate the oscillating plate. Thus, the known electrically controllable non-bushing type elastic mount is not practically satisfactory in its damping characteristics.
Described more specifically, the above fluid-filled elastic mount suffers from insufficiency of the magnetic flux density in the magnetic field in which the oscillating plate or solenoid coil is placed, because the magnetic path or circuit formed by the solenoid or 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, when the oscillating plate is actuated or displaced in the oscillating manner, the open magnetic circuit formed by the solenoid or permanent magnet inevitably causes a large variation in the magnetic flux density in the field in which the oscillating plate or coil is placed. 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.
Although the oscillating plate can be suitably controlled to regulate the fluid pressure within the fluid chamber so as to exhibit an intended damping effect with respect to the vibrations in a specific frequency band, the distortion of the pulsation waveform of the fluid would result in adversely amplifying the vibrations in another frequency band. Thus, the known fluid-filled elastic mount as a whole is incapable of exhibiting the intended damping characteristics over a wide frequency range of the input vibrations.