GB2165667A

GB2165667A - Method of reducing the transmission of vibrations

Info

Publication number: GB2165667A
Application number: GB08418605A
Authority: GB
Inventors: William Redman-White; Philip Arthur Nelson
Original assignee: University of Southampton
Current assignee: University of Southampton
Priority date: 1984-07-20
Filing date: 1984-07-20
Publication date: 1986-04-16
Also published as: GB2165667B

Abstract

A method and apparatus for reducing the transmission of vibrations from a source of vibration 22 through a body 24 coupled thereto, in which the vibrational motion of the body is monitored, preferably, at two or more positions 26 on the body. Control forces 28 are applied to the body in dependence on the vibrational motion at the said monitoring positions; the control forces being such that together they act to minimise both the linear and rotational motion of the body at the monitoring positions. <IMAGE>

Description

SPECIFICATION Method of reducing the transmission of vibrations The present invention relates to a method of reducing the transmission of vibrations, in particular, vibrations resulting from the operation of machinery.

It is frequently desirable to isolate vibrating machinery from the foundation on which it is mounted so as to prevent the transmission of such vibrations to other machinery or to the fabric of the building in which the machinery is housed. Equally, it is in some cases necessary to isolate machinery from foundations or mountings which are themselves subject to vibration.

Existing methods for isolating machinery vibrations involve the positioning of resilient mountings between the machinery and its foundation so as to absorb at least some of the vibration. The resilient mountings may be in the form of coil springs or blocks of rubber or other suitably resilient material and their stiffness is carefully chosen to suit the particular machinery under consideration. Resilient mounts of this kind allow the machinery to move so that the vibrational forces generated by the machine are balanced by the inertia forces of the machine motion.

The use of resilient mountings, although effective in isolating vibration to a certain extent, is subject to limitations. Their performance is limited by the flexibility of the foundation on which the machinery is supported. If the foundation is relatively flexible, as is the case, for example, with machinery mounted on board ship, the degree of isolation provided by resilient mountings is often inadequate and some vibrational power is transmitted through the foundation.

The effectiveness of resilient mounting arrangements is also limited by a further factor.

Considering, by way of example a uniform beam 10 of the kind shown in Fig. 1, which is undergoing flexural wave motion, the mechanical power transmitted along the beam 10 has two components: (i) the product of the bending moment M, and the rotational velocity de dt of the beam; and (ii) the product of the shear force F and the transverse velocity dw dt It therefore, follows that: Flexural Wave Power=M.dB+F. dw dt dt We have appreciated that a single control force applied to the body by, for example, a coil spring or by an electrodynamically or hydraulically operated actuator can only ever influence the wave power component dw F. dt related to the shear force.Far from any boundaries or discontinuties in the beam 10, the components of the wave power related to the shear force and to the bending moment are equal to one another and, as a result, the provision of a single externally-applied force can only influence half the total flexural wave power transmitted through the beam. We have appreciated from this that, in order to control the total flexural wave power, externally applied forces must be able to control both the transverse velocity and the rotational velocity of the beam.

The invention provides a method of reducing the transmission of vibrations from a source of vibration through a body coupled thereto, the method comprising monitoring the vibrational motion of the body by means of a plurality of transducers coupled thereto; and applying to the body control forces which vary in dependence on the vibrational motion detected by the transducers; the control forces being such that together they act to minimise both the linear and the rotational motion of the body at the positions at which the vibrational motion is monitored.

The control force necessary to counteract the rotational motion of the body may either be applied directly by imparting a rotational motion to the body or by applying two control forces a small distance apart so as to provide the required controlling torque.

In a further aspect, the invention provides apparatus for reducing the transmission of vibrations from a source of vibrations through a body coupled thereto, the apparatus comprising a plurality of transducers for monitoring the motion of the body and means responsive to the motion of the body at the monitoring positions for applying to the body control forces which vary in dependence on the vibrational motion at the monitoring positions; the said means being such that the control forces together act to minimise both the linear and the rotational motion of the body at the monitoring positions The monitoring transducers may be coupled indirectly to the body, for example, by means of a rigid lever fixed to the body. However, it is preferred that the transducers are mounted on the body at two positions spaced along the direction of propagation of the vibrations.

The method and apparatus of the invention may be used either alone or in combination with conventional resilient mountings. Using the method and apparatus of the invention it is thus possible to influence both components of the flexural wave power so as to isolate machinery from its foundations with far greater effectiveness than has heretofore been possible.

Although the discussion above relates only to a uniform beam, the method of the invention can be applied to a support or foundation of any type in which flexural wave motion is transmitted in a well-defined manner. Whilst it may not be possible to apply the method of the invention directly to existing foundations, such foundations may, in most cases, be easily replaced by a support or foundation in which vibration is transmitted in a well-understood way.

Embodiments of the invention will be described in detail, by way of example, with reference to the drawings, in which: Figure 2 is a schematic diagram illustrating the application of the method of the invention to isolate random vibrations; Figure 3 is a schematic diagram illustrating an alternative way of applying the method of the invention to isolate random vibrations; Figure 4 is a schematic diagram illustrating the application of the method of the invention to isolate periodic vibrations; and Figure 5 is a schematic diagram illustrating a second alternative way of applying the method of the invention to isolate random vibrations.

Figs. 2, 3 and 5 show a beam 20 of uniform cross-section which is mechanically coupled to a source 22 of random vibrations. The source 22 of vibrations may be, for example, a machine mounted on the beam 20 or a vibrating foundation from which the beam 20 serves to isolate vibration-sensitive equipment or instruments. In either case, it is desired to isolate the portion 24 of the beam 20 remote from the source 22 from the vibrations it generates.

In Fig. 2, this is achieved by creating in the beam 20 an approximation to a clamped boundary. Two motion-detecting sensors 26 are mounted on the beam 20 a short distance apart. The electrical signals from each of the sensors 26 are amplified and applied to an associated force-generating actuator 28 coupled to the beam 20. The actuators 28 are such that they apply a force to constrain the velocity of the beam 20 at the two points at which the sensors 26 are mounted to a minimum value. The portion of the beam 20 between the sensors 26 thus behaves like a ciamped boundary and prevents the transmission of vibrations from the source 22 of the portion 24 of the beam remote from it.

In Fig. 3, two accelerometers 30 are mounted on the beam 20 to detect travelling wave vibrations generated at the source 22. Their output is fed to a microprocessor 32 which analyses the vibrations generated at the source 22. The microprocessor 32 also controls two actuators 34 mounted on the beam 20 at a distance from the accelerometers 30. The microprocessor 32 operates the actuators 34 so as to generate in the beam 20 a pair of waveforms which combine to synthesise a perfect refiection of the vibrations generated by the source 22, thus preventing their transmission to the beam portion 24.

Fig. 5 illustrates how the necessary controlling torque can be applied directly to the beam 20.

The beam 20 is provided with two transversely-extending lever arms 50 of equal length. the arms 50 may either be formed integrally with the beam 20 or they may be formed separately and mounted on the beam 20. In either case, the arms 50 must, however, be secured to the beam 20 sufficiently rigidly to enable them to transmit bending forces to the beam.

Three force actuators 52, 54 and 56 are arranged to act on the lever arms 50. Two of these, 52 and 54, are positioned so as to exert a force in directions parallel to the direction of propagation of the vibrations but in opposite directions to one another. The third force actuator 56 is arranged to exert a force on one of the lever arms 50 transversely of the beam 20. All three force actuators 52, 54 and 56 are controlled by a microprocessor 58 in response to the output signals of sensors 60 which are mounted on the beam 20.

The microprocessor 58 controls the force acutators so that the couple exerted on the beam 20 by the actuators 52 and 54 through the lever arms 50 counteracts rotational motion of the beam 20 while the transverse force exerted on the beam 20 by the actuator 58 counteracts the linear vibrational motion of the beam in a transverse direction.

Fig. 4 shows a beam 20 coupled to a source 23 of periodic vibrations. A pair of sensors 40 are mounted on the section 42 of the beam which is to be isolated from the source 23 of the vibration. Each of the sensors 40 generates an output signal dependent on the r.m.s. velocity of the beam 20 at the point at which the sensor 40 is mounted. The output signals from the sensors 40 are fed to a signal processor 44. The fundamental frequency of the vibrations from the source 23 is also input into the signal processor 44 which controls a pair of force actuators46 mounted on the beam 20 between the source 23 and the section 42 to be isolated. The signal processor 44 operates the actuators 46 so as to minimise the r.m.s. velocity of the beam 20 at the sensors 40.

Whilst the four examples described relate to the isolation of vibrations in a uniform beam, the method can be applied, using suitably modified control forces, to a support or foundation of any configuration provided it is possible to analyse the vibrations transmitted therethrough into a number of simple components which can be counteracted by the application of suitable control forces.

Claims

1. A method of reducing the transmission of vibrations from a source of vibration through a body coupled thereto, the method comprising: monitoring the vibrational motion of the body by means of a plurality of transducers coupled thereto; and applying to the body control forces which vary in dependence on the vibrational motion detected by the transducers; the control forces being such that together they act to minimise both the linear and the rotational motion of the body at the positions at which the vibrational motion is monitored.

2. A method according to claim 1 wherein the transducers are mounted on the body at two positions spaced from one another along the direction of propagation of the vibrations.

3. A method according to claim 1, wherein each control force is varied independently of the control force or forces in response to the vibrational motion of the body at a respective one of the monitoring positions.

4. A method according to claim 2 or 3, wherein the velocity of the body is monitored at the said two positions, the control being varied to minimise the said velocities.

5. A method according to claim 2 or 3, wherein the acceleration of the body is monitored at the said two positions, the control forces being varied to minimise the said accelerations.

6. A method according to any preceding claim, wherein the control forces are applied to the body at points spaced from the monitoring positions.

7. A method of reducing the transmission of vibrations from a source of vibrations through a body coupled thereto, the method being substantially as hereinbefore described with reference to Figs. 2, 3, 4 or 5 of the drawings.

8. Apparatus for reducing the transmission of vibrations from a source of vibrations through a body coupled thereto, the apparatus comprising a plurality of transducers for monitoring the motion of the body and means responsive to the motion of the body at the monitoring positions for applying to the body control forces which vary in dependence on the vibrational motion at the monitoring positions; the said means being such that the control forces together act to minimise both the linear and the rotational motion of the body at the monitoring positions.

9. Apparatus according to claim 8, wherein the transducers are mounted on the body at two positions spaced along the direction of propagation of the vibrations.

10. Apparatus according to claim 9, wherein the means for applying the control forces is arranged to vary each control force independently of the other control force or forces in response to the vibrational motion of the body at a respective one of the monitoring positions.

11. Apparatus according to claim 9 or 10, wherein the means for monitoring the motion of the body includes a velocimeter and means for applying the control forces is such as to vary the control forces to minimise the velocities at the monitoring positions.

12. Apparatus according to claim 9 or 10, wherein the means for monitoring the motion of the body includes an accelerometer and the means for applying the control forces is such as to vary the control forces to minimise the accelerations at the monitoring positions.

13. Apparatus according to any of claims 7 to 12, wherein the means for applying the control forces is arranged so that the control forces are applied to the body at points spaced from the monitoring positions.

14. Apparatus for reducing the transmission of vibrations from the source of vibration through a body coupled thereto, the apparatus being substantially as hereinbefore described with reference to Figs. 2, 3, 4 or 5 of the drawings.