VIBRATION CONTROL SYSTEM, ESPECIALLY FOR CIRCUIT BOARDS

Abstract: An active vibration control system for controlling flexure of a printed circuit board (18; 24) comprises an actuator (10; 20) bonded to the board (18; 24). The actuator is not bonded directly to the board (18; 24) but is bonded to two alumina feet or strips (12; 14; 16; 22) which are themselves bonded to the board (18; 24). In this way the actuator (10; 20) can span regions of the board (18; 24) populated with electronic components, connections and tracks.

Description: VIBRATION CONTROL SYSTEM, ESPECIALLY FOR CIRCUIT BOARDS The present invention relates to vibration control and is particularly, but not exclusively, related to vibration control of planar structures such as printed circuit boards (PCBs). A known method of controlling vibration uses an active system which typically comprises a sensor (which is normally piezoelectric) to detect vibration to be suppressed and control electronics to drive piezoelectric actuators. It is conventional for the sensor to be mounted on an upper surface of the actuator. The actuators are configured so as to oppose the natural bending of the structure thereby mechanically stiffening it against vibration. Active vibration control (AVC) offers the advantages of providing systems of low mass and small size which can be effective at low frequencies (several hundred hertz) through to frequencies of several kHz. To control vibration in essentially planar structures the actuators must be optimally situated. An optimum position is the anti-node of flexure. In order to get the best performance of the AVC system the actuator should be bonded directly onto the planar structure using a thin glue layer. However, if the AVC system is to be used on a PCB then locating the actuator of the system may prove difficult since there is often little space available due to a high population of electronic tracks and components. This problem also occurs in boards which are single sided since an upper side carries components and an underside carries solder connections for the components. Locating the actuator on either side is therefore difficult. One way of overcoming this problem is to design the PCB to have clear areas (which are not populated by tracks or components) onto which the actuator can be bonded directly. However, if it is desired to retrofit an AVC system to a PCB which has not been designed to have such clear areas, difficulties can arise in finding sites which are suitable to locate the actuator. This can result in the actuator being located at sites which are not optimal. According to a first aspect of the invention is provided an active vibration control system comprising an actuator which is mountable onto a sheet member in which the actuator is spaced apart from a surface of the sheet member by at least one support element such that clearance is provided between the surface and the actuator. Preferably clearance is provided such that features of surface topography on the surface of the sheet member may be accommodated between the surface and the actuator. The clearance may be an air gap. Alternatively it may be filled by a solid material. According to a second aspect of the invention is provided an active vibration control system comprising an actuator which is mountable onto a sheet member in which the actuator is adapted to span areas of the sheet member occupied by features of surface topography. The term "sheet" refers to a member which is thin relative to its width and height. This means that it is prone to flexure. Although the term "sheet" primarily covers members which are planar it also covers non-planar members, for example, members which may be curved or may be undulated. Preferably the features of surface topography are tracks and/or components present on the sheet member. Most preferably the features of surface topography are electronic tracks and/or components. Preferably the sheet member is a PCB. Conveniently the actuator is mounted onto the sheet member by the or each support element. Preferably the or each support element is bonded on one side to the sheet member and on the other side to the actuator the support element serving to space apart the actuator and the sheet member to provide clearance therebetween. The or each support element may comprise a glue layer. Such a glue layer may occupy substantially the space between the actuator and the sheet member which is not filled with features of surface topography. Alternatively a plurality of support elements may each join a relatively small area of the actuator to a relatively small area of the sheet member to provide at least one air gap between the sheet member and the actuator. An advantage of using an air gap arrangement is that heat dissipation from features of surface topography such as tracks or components is not inhibited. In an embodiment having a plurality of support elements, a first of a pair of support elements may be located at a first edge of the actuator and a second of the pair may be located at an opposite edge of the actuator. There may be a plurality of such pairs. A plurality of support elements may be disposed around the periphery of the actuator. However the locations of the support elements may simply be dictated by being located in those areas of the sheet member which are clear of features of surface topography and available to accommodate support elements. The support elements may be in the form of cylinders or pieces of sheet material in the shape of circles, squares or rectangles. One preferred embodiment comprises a pair of strips disposed substantially parallel to each other in which opposite sides of the actuator are bonded to respective strips. In another preferred embodiment the support elements are cross-shaped or L-shaped. Alternatively there may be a single support element in the form of a frame which has a periphery shaped to correspond to that of the actuator. The frame may be circular such as a ring, it may be rectangular or it may be some other shape. By providing a mounting between the actuator and sheet member which removes the need for the actuator to be in close physical proximity to the sheet member, the actuator can be located over areas which have non-planar surface topography, particularly those areas which are populated by tracks, components or both. In this way AVC systems may be retrofitted readily to sheet members which have not had clear areas especially designed to accommodate actuators. The locations of the support elements are determined by the need to transmit force adequately from the actuator to the sheet member. Therefore whilst a regular arrangement of two support elements one located on each side of the actuator is preferred, it is not essential if other configurations are able to transmit sufficient force. Preferably the material of the or each support element is elastic. In addition to its conventional technical meaning, the term elastic means that the or each support element transfers substantially all of the force in the plane of the sheet member applied to the region of the or each support element adjacent the sheet member to the region of the or each support element adjacent the actuator. Preferably the or each support element is substantially wholly elastic. Preferably the or each support element comprises metal, glass, ceramic or a composite material. Most preferably the material is alumina. However other materials may be used so long as they are sufficiently rigid to transmit force from the actuator to the sheet member. Preferably the system has a damping capability which is wholly active. That is it does not use visco-elastic properties of materials or inertial damping. Hitherto it has been understood that bonding between the actuator and the sheet member is necessary over the whole undersurface of the actuator in order for the actuator to transmit sufficient force to the sheet member to control vibration. Surprisingly it has been found that full contact is not required and that the actuator can perform sufficiently well if there is bonding between the actuator and the sheet member at at least two points over which the actuator forms a bridge. Contact between the "bridged" portion of the sheet member and the actuator is not required. Although it is preferred that the distance between these two points be substantially the same distance as the length of the actuator, it may be less. A lesser distance may simply compromise performance. According to a third aspect of the invention is provided a sheet member incorporating an active vibration control system in accordance with the first or second aspects of the invention. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows an arrangement of an actuator and a support element; Figure 2 shows an alternative arrangement of an actuator and support elements; Figures 3a and 3b show further arrangements of an actuator and support elements; Figures 4a and 4b show the arrangement of Figure 3 a mounted on a printed circuit board; and Figures 5a and 5b show yet further arrangements of an actuator and support elements mounted on a printed circuit board. Figure 1 shows an actuator 10 bonded to a support element 12 which comprises a ring frame. The support element 12 is made of alumina which is a rigid material thus providing a good medium for transmitting force exerted by the actuator to a PCB to which the support element is bonded. Other rigid materials such as other ceramics or metals could serve equally well. Figure 2 shows an alternative arrangement in which the actuator 10 is bonded to support elements comprising four pads, or feet, 14 which are made of alumina. Figure 3 a shows the actuator 10 bonded to support elements comprising alumina strips 16 such that a side part of each strip extends beyond the actuator 10. Figure 3b shows the actuator 10 bonded to two alumina strips 16 such that the edges of the actuator extend as far as the edges of the sides of the strips 16. This provides a more compact arrangement. Figure 4a shows the arrangement of Figure 3 a in which the actuator 10 is bonded to the alumina strips 16 which are in turn bonded longitudinally to a PCB 18. The actuator 10 straddles the strips 16 in such a way as to form a bridge arrangement in which components on the PCB 18 can be accommodated between the strips 16 and beneath the actuator 10. Figure 4b shows a similar arrangement to Figure 4a in which the strips are bonded transversely to the PCB 18. Figure 5 a shows an arrangement using an elongate actuator 20 which is supported by support elements comprising alumina strips 22 which are similar to the strips 16. The actuator 20 is bonded longitudinally to a PCB 24. Figure 5b shows an arrangement in which an elongate actuator 20 is bonded transversely to a PCB 24. In the embodiments of Figures 5a and 5b the actuator 20 has dimensions of 30 x 15 x 1mm. In the foregoing embodiments the support elements generally have a thickness of about lmm. In one preferred embodiment the thickness is about 3mm. Longitudinal or transverse orientation of the actuator on the PCB depends on the vibration mode or modes present in the PCB and will be chosen to optimise vibration control. In an alternative embodiment alumina supports are not used. Instead the actuator is mounted on a glue layer which is thick enough for the actuator to be supported above the tracks, components or both on the PCB. In such an embodiment the glue material must be sufficiently rigid in order to transmit enough force from the actuator to control vibration. Suitable glues are cyanoacrylate or epoxy resin. Reinforced epoxy resin, such as that reinforced with glass or carbon fibre, is preferred. The actuators described in the foregoing description may be piezoelectric, such as PZT (lead zirconium tantalate) or electrostrictive, such as PMN (lead manganese niobate). In a preferred embodiment the actuator, sensor, electronics and support element or members are packaged as an assembly in an enclosure. The enclosure comprises a carbon fibre or glass fibre composite material. In a variant of this embodiment the actuator and sensor are packaged in a first enclosure and the electronics are packaged in a second enclosure. A two enclosure arrangement can be more squat occupying a relatively greater plan area of a PCB but a relatively lesser height. This is important because PCBs are usually stacked one above another and minimising the height of AVC systems can be useful. The system operates over a frequency range from several hertz (for example 1 to 10 Hz) to several kHz (for example 1 , 2, 3, 4 or 5 kHz). The lower end of the range extends as far down as DC (that is 0Hz) In the embodiments according to the invention described above one or more support elements are provided between the actuator and the PCB. The presence of one or more support elements can provide a convenient location to locate the sensor since it may be located between the actuator and the PCB or on a support element. A sensor is shown in Figures 3a and 3b and designated by numeral 26. If the sensor is of the accelerometer type it may be mounted beneath the actuator, perhaps bonded to its underside, on the PCB or on (or even in) a support element. If the sensor is a strain sensor it may be mounted on the PCB to detect strain in the PCB. An advantage of locating the sensor in a location other than on top of the actuator is that a more squat arrangement may be obtained.

Claims: CLAIMS 1. An active vibration control system comprising an actuator which is mountable onto a sheet member in which the actuator is spaced apart from a surface of the sheet member by at least one support element such that clearance is provided between the surface and the actuator. 2. A system according to claim 1 characterised in that the support element comprises elastic material. 3. A system according to claim 1 or claim 2 characterised in that the actuator is adapted to span areas of the sheet member occupied by features of surface topography. 4. A system according to claim 3 characterised in that the features of surface topography are electronic tracks and/or components. 5. A system according to any preceding claim characterised in that the sheet member is a printed circuit board. 6. A system according to any preceding claim characterised in that the or each support element is bonded on one side to the sheet member and on the other side to the actuator the support element serving to space apart the actuator and the sheet member to provide the clearance therebetween. 7. A system according to claim 5 or claim 6 characterised in that there are a plurality of support elements each joining a relatively small area of the actuator to a relatively small area of the sheet member to provide at least one air gap between the sheet member and the actuator. 8. A system according to claim 7 characterised in that a first of a pair of support elements is located at a first edge of the actuator and a second of the pair is located at an opposite edge of the actuator. 9. A system according to claim 7 or claim 8 characterised in that the support elements comprise a pair of strips disposed substantially parallel to each other in which opposite sides of the actuator are bonded to respective strips. 10. A system according to claim 8 or claim 9 characterised in that there is a plurality of such pairs. 11. A system according to claim 5 or claim 6 characterised in that there is a single support element in the form of a frame which has a periphery shaped to correspond to that of the actuator. 12. A system according any of claims 5 to 11 characterised in that the or each support element comprises a material selected from metal, glass, ceramic or a composite material. 13. A system according to claim 12 characterised in that the material is alumina. 14. A system according to any preceding claim characterised in that the or each support element comprises a glue layer. 15. A system according to any preceding claim characterised in that a sensor is provided between the actuator and the surface. 16. A system substantially as described herein with reference to the Figures of the accompanying drawings. 17. A sheet member incorporating an active vibration control system in accordance with the any preceding claim. 18. A sheet member substantially as described herein with reference to the Figures of the accompanying drawings.

Inventors: MILLAR CAROLINE ELIZABETH, OWEN CHRISTOPHER EDWARD, SALLOWAY ANTHONY JOHN, MILLAR, CAROLINE, ELIZABETH, OWEN, CHRISTOPHER, EDWARD, SALLOWAY, ANTHONY, JOHN
Applicants: MARCONI GEC LTD, MILLAR CAROLINE ELIZABETH, OWEN CHRISTOPHER EDWARD, SALLOWAY ANTHONY JOHN, GEC-MARCONI LIMITED, MILLAR, CAROLINE, ELIZABETH, OWEN, CHRISTOPHER, EDWARD, SALLOWAY, ANTHONY, JOHN
Application year: 1998
Main classification: F16F15
IPC codes: F16F15, H05K1