GB2255387A - Hydraulically actuated vibrator - Google Patents

Abstract

A hydraulically actuated vibrator (8), particularly for use with noise cancellation systems, comprises a movable mass (10) located within a cylinder (9). Hydraulic power is applied via lines (24 and 25) to move the mass (10) axially to and fro producing a noise cancelling vibration. <IMAGE>

Description

HYDRAULICALLY ACTUATED VIBRATOR This invention relates to hydraulically actuated vibrators particularly, but not exclusively, to vibrators used in noise and vibration cancellation systems for use in the reduction of noise and/or vibration in vehicles.

In the past sound deadening materials have been used to attenuate noise and vibration within vehicles. However such materials are generally heavy and add to the vehicle's weight. For some vehicles such as aircraft, this addition of weight is highly undesirable. Further the ability of the materials to absorb sound varies with the frequency. To reduce these problems so called active systems have been used which act not to absorb the sound or vibration but to cancel it.

This is achieved by producing a vibration with a hydraulically actuated vibrator which corresponds in amplitude to the vibration to be cancelled but is in antiphase to it. The two vibrations then combine and cancel.

A typical prior art hydraulically actuated vibrator is shown in Figure 1 and comprises a piston and cylinder 3, 4 connected via a piston rod 2 to a mass 1 externally of the cylinder. The piston 3 is made to oscillate by applying cyclically varying hydraulic pressure to it via ports 5 and 6. A seal 7 is provided between the piston rod 2 and the cylinder.

A problem with these existing vibrators is that the mass must be oscillated at a high frequency in order to cancel high frequency vibrations, and this can cause a rapid deterioration of the seal 7, giving rise to leakage of fluid from the vibrator.

Accordingly the present invention seeks to reduce or overcome this problem by providing a hydraulically actuated vibrator comprising a cylinder, a mass movably located within the cylinder and means for displacing the mass within the cylinder by application of hydraulic power.

By locating the mass within the cylinder, the need for a mechanical connection to the mass externally of the cylinder is eliminated together with the need for a seal and the risk of leaks associated with such a seal. A further advantage in eliminating the seal is that a larger amplitude vibration may be generated permitting a smaller mass to be used, thereby reducing the weight of the vibrator.

Preferably, the mass is supported by at least one bearing. The bearing may comprise a roller or ball bearing or a plane bearing composed of low friction material such as PTFE. Alternatively, the bearing may be a hydrostatic bearing in which the mass is supported by a flow of hydraulic fluid.

Preferably, the vibrator includes a releasable lock to prevent movement of the mass as a safety feature in the event of a malfunction.

Preferably, the vibrator is provided with means whereby it can be mounted on to a chassis. The chassis may itself be part of the vehicle to which the vibrator is to be attached or a separate component which is then bolted, riveted, glued or otherwise fixed to the vehicle.

Preferably, the vibrator includes a position sensor for producing an output characteristic of the position of the mass. By including a position sensor drift of the mass to one end of the vibrator can be corrected for by applying hydraulic power to recentre the mass.

The invention further provides a noise cancellation system comprising a vibrator as described previously and control means for causing the vibrator to vibrate to produce a noise cancelling vibration.

The control means will preferably include a control valve for controlling the application of hydraulic power to the vibrator.

Preferably, the system will include at least one vibration sensor for producing an output in response to a sensed undesired vibration and wherein the control means responds to the output to drive the vibrator to produce a vibration cancelling the undesired vibration.

Specific embodiments of the invention will now be described by way of example only, with reference to the drawings in which: Figure 2 shows a sectional view of a vibrator in accordance with a first embodiment of the invention; Figure 3 shows, schematically, the vibrator of Figure 2 connected to a servo control valve; Figure 4 shows a sectional view of a vibrator in accordance with a second embodiment of the invention; Figure 5 shows, schematically, the vibrator of Figure 4 connected to a servo control valve; and Figure 6 shows a noise cancellation system in accordance with the invention.

As shown in Figure 2, a vibrator 8 comprises a cylinder 9 having longitudinally displaceable therein a mass 10. The mass 10 effectively separates the inside of the cylinder 9 into two cavities 11 and 12 either end which communicate via a drilling 13 made in the mass 10. Attached to the mass 10 by a rod 14 is a sensor probe 15 which projects into a sensing coil 17 located within an axially extending bore 16 in an end wall of the cylinder. The mass 10 is formed with a central bore 18a, 18b which is closed at one end and has a wall formed with a gland 19 at the other. A pillar 20 extends axially from an end wall of the cylinder through the gland 19 and carries at its end an enlarged head 21 which cooperates with the bore 18a, 18b.

Annular PTFE bearing sleeves 22 and 23 are interposed between the head 21 and bore 18a, 18b, and the pillar 20 and the gland 19, to provide bearing surfaces.

Formed through an end of the cylinder 9 and along the pillar 20 are hydraulic supply lines 24 and 25. Line 25 communicates with bore 18a whilst line 25 communicates with bore 18b. A return port 26 passes through the cylinder 9 into cavity 11.

A servo control valve 27 connects the lines 24, 25 to a pressure supply port 28 and reservoir return port 29 as shown in Figure 3.

The servo control valve 27 is provided with three ports 30, 31 and 32 and a spool valve 33 having two lands 34 and 35. The spool valve 33 is moved by an electromechanical servo driver 36 by a control rod 38 such that the lands 34r 35 open or close the ports 31, 32.

By closing port 32 hydraulic pressure is applied to bore 18a via ports 31, 30 and line 24 from the supply port 28. This causes the mass 10 to move to the left as shown in the figure.

To move the mass 10 to the right, the spool valve 33 is moved to the right by the servo driver 36 to close port 31 and open port 32. The relative exposed areas of the ports 31, 32 change varying the pressure available at port 30 causing a reduction of pressure in bore 18a. The mass 10 therefore moves to the right to equalise the pressures in the bore 18a, 18b.

The drilling 13 permits fluid to flow between the cavities 11 and 12 as the mass 10 vibrates.

It should be noted that the PTFE sleeves 22 and 23 whilst reducing the clearance between the mass 10 and the head 21 and pillar 20, do not provide a seal. Fluid passing into the bore 18a, 18b leaks past the sleeves 22 and 23 into cavity 11 and returns via port 26 to the return port 29.

The sensing coil 17 provides an electrical output representative of the position of the mass 10 which is used as a feedback signal in a control system (not shown) which recenters the mass 10 to compensate for drift.

A second embodiment of the invention is shown in Figure 4 and comprises a cylinder 38 within which a mass 39 is provided with two blind bores 40 and 41. Within the bores 40 and 41 are located pistons 42 and 43 by coaxial ball bearing arrangements 44a and 44b. Thus the mass 39 is supported within the cylinder 38 by the pistons 42 and 43.

A peripherally extending groove 45 is formed in the mass 39 into which a pawl 46 of a lock actuator 47 releasably engages.

A position sensor 48 is connected to piston 42 by a rod 49 to sense the position of the mass 39 and produce a position feedback signal.

To move the mass 39, pressure is applied to one piston and then to the other. To move the mass 39 to the left in the figure, fluid pressure is applied to piston 42 via a port 54 (not shown) causing it to move to the left pushing the mass 39 before it. To move the mass 39 in the other direction, fluid pressure is applied to piston 43 via port 53. Fluid leaking out of the pistons is returned via a return port 50 to a servo control valve 55 as shown in Figure 5.

The servo control valve 55 is a spool valve having three lands 56, 57 and 58. Fluid is supplied to the valve 53 from a supply 59 via supply ports 60 and 61 and leaves via ports 62 and 63 and a drain port 64. The drain port 64 and return port 50 returns the hydraulic fluid to the supply via pipe 65.

The servo control valve 55 is actuated by an electromechanical servo driver 66 linked to it by a control rod 67.

By uncovering port 61 by moving the land 58 to the right in the figure the pressure at port 60 drops creating a pressure imbalance between ports 63 and 62 which causes a corresponding movement of pistons 43 and 45 to equalize the pressure in similar manner to the first embodiment.

To lock the mass 39 to prevent unwanted movement, the lock actuator 47 is depressurised allowing a spring 68 to force the pawl 46 downwards into the groove 45. To release the lock, pressure is applied in annular area 69 to override the spring 68 and withdraw the pawl 46.

In both embodiments, by rapidly moving the mass one way and then the other a vibration can be produced to cancel unwanted vibrations.

Outputs from noise/vibration sensors 69, fixed to an aircraft airframe, are passed to a control system computer 70, as shown schematically in Figure 6. The computer 70 is programmed to determine the way in which vibrators 71 must be driven in order to produce a cancelling vibration.

The output of the computer 70 drives the vibrators 71 by means of a servo drive 72 and servo valve 73.

In alternative embodiments of the invention the servo control valve may be actuated by other types of servo drivers such as pneumatical or hydraulic servo drivers.

Claims (9)

1. A hydraulically actuated vibrator comprising a cylinder, and a mass movably located within the cylinder and movable by application of hydraulic power.

2. A vibrator as claimed in claim 1 wherein the mass is supported by at least one bearing.

3. A vibrator as claimed in claim 1 or claim 2 further comprising a releasable lock to prevent unwanted movement of the mass.

4. A vibrator as claimed in claims 1, 2 or 3 comprising means to mount the vibrator to a chassis.

5. A vibrator as claimed in any preceding claim comprising a position sensor for producing an output characteristic of the position of the mass.

6. A vibrator as claimed in claim 5 including means to recentre the mass in response to the output from the position sensor.

7. A hydraulically actuated vibrator substantially as hereinbefore described and as illustrated by Figures 2 or 3 or 4 of the drawings.

8. A noise cancellation system comprising a vibrator as claimed in any preceding claim and control means for causing the vibrator to vibrate to produce a noise cancelling vibration.

9. A system as claimed in claim 8 including at least one vibration sensor for producing an output in response to a sensed undesired vibration and wherein the control means responds to the output to drive the vibrator to produce a vibration cancelling the undesired vibration.