ANTI-VIBRATION MOUNT ASSEMBLY

Abstract: An assembly (Figure l) including an electronic device (3) and package body (9) mounted upon a block (l5). Bending effects are minimised by locating the package body (9) and block (l5) upon a suspension system (l7,l9) that is located symmetrically with respect to the body centre and that lies within the body periphery. Point, bar hollow rectangle or ring suspension may be adopted. Where ring suspension is used, the ring radius can be chosen to minimise bending effect in the plane of the device (3). An array of devices can be substituted for the single device mentioned. The device (3) is preferably located at or near the neutral bending plane of the assembly. To this end the lid (2l) of the package (9) may be of significantly large mass.

Description: Technical Field The present invention concerns improvements in or relating to anti-vibration mount assemblies, especially mount assemblies including electronic devices wherein device optimum performance deteriorates with bending effects, essentially those introduced by oscillatory motion caused by vibration. Pyroelectric infra-red detecting devices, especially, are susceptible to microphony voltages generated by stretching of detector elements in the plane of the device, as may be caused by bending of the substrate or package upon which they are mounted. Background Art Hitherto bending effects have been reduced by means of mounting the device package upon a solid massive mount - see for example United Kingdom Patent Application No: GB 2l48596A. It is however a problem that this type of assembly results in a heavy and bulky construction, which is generally inconvenient in use. Disclosure of the Invention This invention is intended as an alternative and comparatively light-weight assembly to reduce or even eliminate package bending at the position of the detector and in consequence to reduce microphony effects to a minimum. In accordance with the invention thus there is provided an anti-vibration mount assembly comprising:- one or more electronic devices; a package body supporting said one or more devices; and, a block member of matched cross-section area adjacent to the package body; the assembly being characterised by:- a suspension system comprised of one or more suspension members mounted on a frame, each member being disposed to contact the block member within the peripheral boundary thereof, the system being arranged symmetrically relative to the centre of the package body. It is desirable that the suspension system should match or emulate the planar symmetry of a detector or other device which it is required to mount. Thus it may consist of parallel bars or equivalent to mount a linear array of devices or an elongate device, or four suspension points or a rectangular support to mount a rectangular array of devices or a rectangular device. For many purposes, especially for devices of four-fold symmetry or higher, circularly symmetric mounting is adequate. This may also lead to easier manufacture. Member spacing, - or in the case of a single hollow suspension member, the diameter or rectangle dimensions, may be chosen to optimise bending effect reduction. It is advantageous, furthermore, to locate the one or more devices so that the plane in which they lie is co-incident with, (or at least nearly so) the neutral bending plane of the assembly. This may be achieved by using as package lid, one of a significantly increased mass. Brief Introduction of the Drawings In the drawings accompanying this specification:- Figure l is a cross-section view of an anti-vibration mount assembly, constructed in accordance with this invention; Figure 2 is a graph showing experimental plots for measured microphony voltage as a function of the diameter of a ring support, the form of support member shown in Figure l; Figure 3 and 4 show in cross-section and in plan-view, respectively, an assembly modified to afford device mounting in the neutral bending plane thereof; Figure 5 is a graph showing measured microphony voltage as a function of detector distance from the package centre, for different values of support ring radius; and, Figure 6 is a graph showing measured microphony voltage, for a detector at package centre, as a function of support-ring radius for two sizes of package lid, and for a standard size package. Description of a Preferred Embodiment So that this invention may be better understood, embodiments thereof will now be described with reference to the drawings. The description that follows is given by way of example only. It is intended that this invention should be applicable to a wide variety of devices, packages, and applications, but for the purposes of illustration an application to a multi-element square pyroelectric detector array mounted centrally in a large package is described. An assembly l including a multi-element rectangular pyroelectric array 3 is shown in Figure l. The detector array 3 is mounted on a silicon signal processing slice 5 which is in turn mounted on a ceramic substrate 7 the surface of which is covered with metallised radiating tracks to bring the electric signals generated by the detector 3 to the periphery of a package 9 where they can be connected via flexible connections ll to following signal processing circuits. The package 9 incorporates an infra-red transmitting window l3. To increase rigidity it is normal practice to mount the package 9 firmly on a thicker support l5 having the same cross-sectional area as the package 9. The suspension system consists of a rectangular or annular ring l7 which supports the package 9, of such dimensions, and positioned centrally relative to the package 9, so that the bending moments at the centre of the package 9 are zero when the package is vibrated in a vertical direction normal to the plane of the detector. The supporting ring l7 has itself to be mounted on a frame l9 such that it is not distorted with vibratory movements. It must be sufficiently compliant to isolate the complete assembly from external shock, but rigid enough so as not to produce a resonant frequency within the operating frequency bandwidth of the detector. Since the suspension system is applicable to a very wide range of package designs and applications it is not possible here to give definitive dimensions for either the ring l7 or the frame l9. Simple calculations are not in themselves accurate enough to define the parameters, and must be supplemented by experiment designed to optimise calculated critical dimensions which can only be approximate. Simple calculations using a circular thin disc approximation show that if the package 9 illustrated in Figure l is suspended on a circular ring of radius 0.62 R where 'R' is an effective radius of the package or supporting block (ie. πR²∼A, where 'A' is the area of the package), zero bending occurs at the centre. The effective area 'A' is however dependent upon many factors such as non-uniformity of construction, the effect of connecting cables, the position of the detector relative to the neutral bending axis of the complete assembly, and must at present be finally determined by experimental measurements using ring diameters based on an effective area 'A' equal to that of the horizontal linear dimensions of the package and assembly. For most purposes it is important that the residual microphony voltages generated by bending of the detector, which varies across its surface as ∼ B + Kr² (where 'K' and 'B' are constants and r is the distance from the centre), are in phase with the centre as this makes subsequent noise compensation easier to accomplish. Thus it may be necessary to not quite eliminate the bending effect at the centre in order to ensure that with manufacturing tolerances all residual bending effects are in phase with each other. Experimental results with pyroelectric detectors mounted in 24mm. sq. flat ceramic packages 9 supported on equal area blocks l5 of different thicknesses and different diameter ring supports l7 and subjected to 2g sinusoidal vibration over the frequency range 50 HZ to l000 HZ have shown that it is possible to reduce mean peak microphony voltages to 20 µV/g at the centre of the package, and 50 µV/g in the corner of the package, with a support ring l7 of annular thickness lmm. and mean diameter of l7mm (see Figure 2). The measured microphony voltages varied approximately linearly and could have been reduced to zero and 40 µV/g for the centre and corner of the package if the mean support ring diameter had been increased to l7.5mm. These minimum measured results are about an order of magnitude lower than those usually expected for these types of device in this sort of package. If the neutral bending axis of the package is close to the plane of the detector, then not only are the microphony voltages caused by stretching the detector in the plane of the detector low, (resulting from vibration in the direction normal to the plane of the detector), but the increase of microphony as the radius of the supporting annulus is varied from ideal is also low. In order to verify this, a thick ceramic lid 2l (Figure 3) was designed for a 50mm sq. ceramic package 9 and was used to obtain preliminary measurements of the microphony voltages developed in a pyroelectric detector array 3 when the package 9 was vibrated in a direction perpendicular to the plane of the detector array 3. The lid 2l, manufactured from ceramic of the same type as the 6mm thick base 23, was made l0mm high with an fl, l0mm square aperture for the window l3, and recessed to accommodate the detector 3. Approximate calculations, using bending moments of the lid 2l, the base 23 and supporting system l5, l7 and l9 showed that the neutral plane would lie about lmm below the plane of the detector array, i.e. at about l0mm below the top surface of the lid 2l. The whole package 9 was supported on an annular ring l7 of 39mm internal diameter, and 43mm external diameter, chosen so that the microphony signals from the centre and the edge of the pyroelectric array 3 were of the same phase. This annular ring l7 was fixed to a cruciform support l9 by which it was bolted to a test vibrator. The assembly was oscillated in a direction normal to the plane of the detectors 3 at 2g over the frequency range 0 to lKHz. Measured microphony voltages (Figure 5) show that with the supporting annulus l7 of mean diameter 4lmm the resulting microphony varied from l0 µV/g at the centre of the package to 35 µV/g at 5, from the centre. With a supporting ring l7 of mean diameter 46mm the corresponding microphonic voltages were 55 µV/g and 90 µV/g at the centre and at 5mm from the centre, and with a supporting annulus l7 of mean diameter 36mm the corresponding microphony voltages were l8 µV/g at the centre and 5 µV/g at 5mm from the centre. A change of phase of the microphony voltages relative to the acceleration sense occured between the results obtained for supporting rings l7 of mean diameters 4lmm and 36mm indicating that the strain had gone from compressive in the first to tensile in the second. At an intermediate position between these two support ring diameters the microphony voltage at the centre would be reduced to zero, and to about 20 µV/g at 5mm from the centre. To verify the position of the neutral bending axis, measurements were repeated with lmm of material ground off the top surface of the package lid 2l, reducing the overall height of the lid 2l above the detectors 3 to 8.5mm and lowering the neutral bending axis to an estimated l.5mm below the plane of the detectors 3. The microphony voltages (Figure 6) at the centre and further out showed a corresponding increase for both support annulus rings 36, and 4lmm diameter Figure 2, indicating that by reducing the thickness of the lid 2l, the neutral axis had moved further away from the plane of the detectors 3 compared with the package 9 as originally designed. These results show that designing this particular package (Figure 3) to have the neutral bending axis within lmm of the plane of the pyroelectric detectors 3, produces improvements in the positioning tolerance of the supporting annulus l7 to achieve low microphony voltages. Thus to achieve less than l0 µV/g at the centre of the package 9 a supporting ring tolerance of +/-0.75mm is indicated (Figure 5) as against a tolerance of 0.25mm indicated by the results shown in Figure 2. Since for small bending distortions of the package 9 the effects are linear, it would be possible to double the tolerance in ring diameter if the distance between the neutral bending axis of the package 9 and the detectors 3 were halved. It may be necessary, for optimum results, to correct the calculated dimensions by experimental determinations. For example it has been found that the optimum support ring diameter is usually greater than 0.7R (where R is the effective radius for the package), rather than 0.62R as shown by simple calculation. It is also necessary to take into account any stiffening effect on the package 9 of the supporting annulus l7. Though the positioning of the detector 3 at the neutral bending plane of the package 9 can be used effectively by itself to produce a low microphony package, when used with the annular ring support system l7 and l9, it produces a robust low microphony system with a large manufacturing tolerance.

Claims: 1. An anti-vibration mount assembly l comprising:- one or more electronic devices 3; a package body 9 supporting said one or more devices 3; and, a block member l5 of matched cross-section area adjacent to the package body 9; the assembly l being characterised by:- a suspension system l7, l9 comprised of one or more suspension members l7 mounted on a frame l9, each member l7 being disposed to contact the block member l5 within the peripheral boundary thereof. 2. An assembly l, as claimed in claim l, the assembly l including a single elongate device 3, and, a spaced pair of parallel bar suspension members l7 located symmetrically each side of the package 9 centre. 3. An assembly l, as claimed in claim l, this assembly l including a plurality of devices 3 arranged in a linear array, and a spaced pair of parallel bar suspension members l7 located symmetrically each side of the package 9 centre. 4. An assembly l, as claimed in claim l, this assembly l comprising a single rectangular device 3 and a suspension system l7, l9 comprising four point members l7. 5. An assembly l, as claimed in claim l, this assembly l comprising a rectangular array of devices 3 and a suspension system l7, l9 comprising four point members l7. 6. An assembly l, as claimed in claim l, this assembly l comprising a single rectangular device 3 and a suspension system l7, l9 comprising a hollow rectangular suspension member l7. 7. An assembly l, as claimed in claim l, this assembly l comprising a rectangular array of devices 3 and a suspension system l7, l9 comprising a hollow rectangular suspension member l7. 8. An assembly l, as claimed in claim l, this assembly l comprising a single square device 3 and a suspension system l7, l9 comprising a ring suspension member l7. 9. An assembly l, as claimed in claim l, this assembly l comprising a four-fold symmetric array of devices 3 and a suspension system l7, l9 comprising a ring suspension member l7. l0. An assembly l, as claimed in any one of the preceding claims wherein the one or more devices 3 are located in, or substantially near to, the neutral bending plane of the assembly l. 11. An assembly l, as claimed in claim l0, wherein a lid 2l of the package is of a massive structure such that the neutral bending plane of the assembly l lies above the package 9 base.

Inventors: BICKNELL, RONALD WILLIAM, SHORROCKS, NICHOLAS MARTYN
Applicants: PLESSEY OVERSEAS LIMITED
Application year: 1986
Main classification: G12B17
IPC codes: G12B5, G01J5, F16F15