Technical Field The present invention relates to mounting systems for precision transducers and, in particular, to a stress-free mounting system for a transducer such as an accelerometer.

Background of the Invention

It is often necessary to isolate a precision transducer from external stress. Such stress may be caused by mechanical distortion of the case or other structure to which the transducer is mounted, or by differential thermal expansion or contraction between the transducer and the case. Isolation from external stress can in principle be achieved by using a compliant mounting system. However, a compliant mounting system will not in general provide precise and stable alignment of the transducer with respect to its case. For many transducers, such alignment is critical for achieving proper operation. A compliant mounting system may also result in unwanted mechanical oscillation of the transducer when the case is exposed to vibration.

One type of precision transducer that is especially susceptible to external stress is an accelerometer. An accelerometer is an example of an instrument that must not be allowed to change position or vibrate with respect to its case. One prior accelerometer mounting technique has been to connect the accelerometer to the case by means of a metal ring or by means of a structural adhesive such as an epoxy resin. These prior noncompliant mounting techniques result in stress being transmitted to the accelerometer due to differential thermal expansion between the accelerometer and the mounting ring and case. These prior techniques also transmit stress to the accelerometer when the case is subjected to mechanical distortion. Distortion can be induced by mounting the case to a surrounding support, or by differential thermal expansion between the case and the support. All such stresses may affect the output of α precision accelerometer, and may result in reduced stability. The temperature induced stresses also may lead to increased variation of accelerometer output

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with temperature, and may create thermally induced errors in the accelerometer output.

Summary of the Invention The present invention provides a mounting system for a precision transducer such as an accelerometer. The mounting system is compliant to differential volumetric expansion but rigid against rotation or translation of the transducer with respect to the case.

In one preferred embodiment, the mounting system of the present invention is adapted to support a precision transducer in spaced alignment with a supporting case. The mounting system comprises a plurality of mounting means, each mounting means having first and second ends and a resilient intermediate portion. The first end of each mounting means is connected to the transducer, and the second end of each mounting means is connected to the case. The first ends of adjacent mounting means are preferably joined to one another by bridge sections to form a mounting ring having a continuous, inwardly facing mounting surface, and the second ends of adjacent mounting means are preferably separated by gaps. At least the first ends and bridge sections are composed of a substance that has a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of the transducer. The intermediate portion of each mounting means is adapted to provide a low resistance to relative movement between the transducer and the case in a radial direction, and a high resistance to relative movement between the transducer and case in directions normal to the radial direction. Differential thermal expansion between the transducer and the case therefore does not apply stress to the transducer, or cause misalignment between the transducer and the case.

Brief Description of the Drawings FIGURE 1 is a perspective view of an accelerometer mounted in a case by the mounting system of the present invention;

FIGURE 2 is a perspective view of the mounting ring of FIGURE 1; FIGURE 3 is a cross-sectional view showing the connection of one mounting element between the transducer and the case;

FIGURE 4 is a side-elevational view of a portion of the mounting ring.

Detailed Description of the Invention FIGURE 1 shows an accelerometer mounted by means of the mounting system of the present invention. The accelerometer of FIGURE 1 includes case 12, transducer 14 and mounting ring 16. Case 12 includes cylindri¬ cal sidewall 18, bottom wall 20 and flange 22. Flange 22 includes mounting

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holes 24 that are used to mount the case and accelerometer to a supporting structure.

Transducer 12 has a cylindrical overall shape and comprises excita¬ tion rings 26 and 27 joined by bellyband 28. The transducer is adapted to respond to accelerations along sensitive axis S by producing an electrical signal that indicates the direction and magnitude of such acceleration. The transducer is mounted to the case at excitation ring 27 by mounting ring 16. As described below, the mounting ring provides precise and stable alignment of the trans¬ ducer, such that the transducer is not free to undergo translational or rotational movement with respect to the case. However, the mounting ring does permit differential radial or volumetric thermal expansion or contraction between the transducer and the case, and also serves to isolate the transducer from stresses that would otherwise result from distortion of the case. Distortion of the case may be caused by mounting the flange to a surface that is not perfectly flat, or by differential thermal expansion between the flange and the support.

Mounting ring 16 is illustrated in greater detail in FIGURE 2. The mounting ring comprises a plurality of mounting elements 30, each mounting element comprising upper end 32 and lower end 34 joined by resilient beam 36. As described below, the upper ends of the mounting elements are attached to the case, and the lower ends are attached to the transducer. The lower end of each mounting element is joined to the lower ends of adjacent mounting elements by bridge sections 38. The bridge sections thereby join the mounting elements into a single, cylindrical mounting ring, as illustrated in FIGURE 2. It is not required for the practice of the present invention that the mounting elements be joined to one another by bridge sections 38. However the use of bridge sections is preferred because it significantly facilitates manufacturing and assembly of the accelerometers.

FIGURES 3 and 4 illustrate further details of the mounting elements and of the connection between the mounting elements and the transducer and case. As best illustrated in FIGURE 3, upper end 32 of mounting element 30 includes pad 40 that includes outwardly facing surface 42. Sur¬ face 42 preferably has a cylindrical contour that matches the contour of the adjacent inner wall of side wall 18 of case 12. Pad 40 is joined to sidewall 18 by adhesive layer 44. A suitable material for adhesive layer 44 is a structural adhesive such as an epoxy resin. Lower end 34 of mounting element 30 includes inwardly projecting flange 46, flange 46 having a cylindrical inner surface 48 that has a contour that matches the contour of the adjacent outer surface of excitation ring 27 of transducer 14. The cross sections of bridge sections 38

(FIGURE 4) may be similar to the cross sections of flanges 46, such that the bridge sections together with the lower ends of the mounting elements form a ring having a continuous, cylindrical inner surface. The radius of such inner surface is dimensioned to match the radius of the adjacent outer surface of excitation ring 27. The mounting ring is joined to the excitation ring by a process, such as welding or brazing, that produces a rigid and integral bond between the mounting ring and the transducer. FIGURE 3 illustrates the use of weld joint 50 to create the bond between flange 46 and excitation ring 27. The point of attachment of the transducer to the mounting ring is preferably spaced as far as possible from flange 22 of case 12, in order to minimize the transmis¬ sion of stress from the flange to the transducer.

It is an important aspect of the present invention that the mounting ring is attached to the transducer in such a way that minimal stress is produced when the transducer and mounting ring undergo thermal expansion or contraction. This result is achieved by fabricating at least lower ends 34 and bridge sections 38 from a material that has a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of the transducer, and in particular of excitation ring 27. An intervening layer of material between the mounting ring and the transducer (e.g., an adhesive layer) should generally not be used unless the intervening layer has a coefficient of thermal expansion approximately equal to that of the transducer and mounting ring. Similarly, where a welding or brazing process is used to join the mounting ring to the transducer, any filler metal or brazing material used should have a coefficient of thermal expansion matched to the coefficient of thermal expansion of the transducer and mounting ring. In a preferred embodiment, mounting ring 16 is entirely fabricated from a metal identical to the metal forming excitation ring 27, and is welded to excitation ring 27 without the use of a filler metal. Because of its low coefficient of thermal expansion, Invar, a 36% nickel-iron alloy, is a particularly suitable metal with which to form excitation ring 27 and mounting ring 16.

In general, it will not be practical to match the coefficient of thermal expansion of the mounting ring to the coefficient of thermal expansion of case 12 or adhesive layer 44. Upper ends 32 of mounting elements 30 are therefore preferably not abutting or joined to one another, but are instead spaced apart by gaps 56 (FIGURE 4). Such gaps eliminate or greatly reduce the high hoop stress that would otherwise occur due to differential thermal expan

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sion or contraction between the mounting ring and the adhesive layer and case. Similar gaps are not required between lower ends 34 of mounting elements 30, because the coefficient of thermal expansion of the mounting ring is matched to that of the excitation ring to which the mounting ring is attached. Each beam 36 is dimensioned such that the beam has α compliant axis oriented in the radial direction indicated by arrows 52 and 54 of FIGURE 3. The compliant axis of each beam preferably intersects the centerline of the transducer. However, the beam is dimensioned such that it is rigid in directions normal to arrows 52 and 54, i.e, along the length L of the beam and in the direc- tions into and out of the plane of the drawing in FIGURE 3. Differential radial or volumetric thermal expansion (or contraction) between the transducer, mount- • ing ring and case therefore results in differential movement between the trans¬ ducer and case along the compliant axes of the beams. The beams therefore flex to take up the differential movement without transmitting significant stress to the transducer. However, the rigidity of beams 36 normal to their compliant axes results in a mounting system in which the transducer is not free to rotate or to undergo overall translational movement with respect to the case.

The required compliant characteristics of beam 36 are preferably achieved by making the width and length L of each beam substantially greater than the thickness T of that beam. The width W of each beam must, of course, be limited (with respect to the circumference of the mounting ring) such that each beam is essentially planar and compliant in a radial direction. In general, width-to-thickness ratios between about 10:1 and 20:1 are most suitable, although other ratios may be used, depending on the nature of the transducer and the mounting ring materials. One preferred mounting ring comprises 24 mounting elements, the beam of each mounting element having a length-to- thickness ratio of about 21:1, and a width-to-thickness ratio of about 12:1. The distance between adjacent mounting elements, i.e., the width of gaps 56, should be large enough to avoid interference between the beams due to thermal expansion or seismic inputs. Referring to FIGURE 3, the distances that pad 40 and flange 46 extend from the plane of beam 36 should similarly be large enough to avoid interference between the beams and the transducer and case.

While the preferred embodiments of the invention have been illustrated and described, it should be understood that variations will be apparent to those skilled in the art. Accordingly, the invention is not to be limited to the specific embodiments illustrated and described, and the true scope and spirit of the invention are to be determined by reference to the following claims.