An apparatus and method for suspending a movable structure form a support structure wherein first and second flat and thin arcuately shaped flexures are formed having spaced apart substantially planar and parallel opposing surfaces, each of the first and second flexures being structured for connection between a support structure and a movable structure to be suspended from the support structure and being aligned along a common axis of rotation between the support structure and the movable structure.

FIELD OF THE INVENTION

The present invention relates generally to micro-machined electromechanical sensor (MEMS) devices and methods for manufacturing the same, and in particular to suspension devices and methods for mounting rotational masses in MEMS devices.

BACKGROUND OF THE INVENTION

Many micro-machined electromechanical sensor (MEMS) devices and methods for manufacturing the same are generally well-known. See, for example, U.S. Pat. No. 6,642,067, entitled METHOD OFTRIMMINGMICRO-MACHINEDELECTROMECHANICALSENSORS(MEMS) DEVICES, issued Nov. 4, 2003, to Paul W. Dwyer, which is assigned to the assignee of the present application and the complete disclosure of which is incorporated herein by reference, that describes a MEMS acceleration sensor and method for manufacturing the same. In another example, U.S. Pat. No. 6,428,713, MEMS SENSORSTRUCTURE ANDMICROFABRICATIONPROCESSTHEREFORE, issued to Christenson, et al. on Aug. 6, 2002, which is incorporated herein by reference, describes a capacitive acceleration sensor formed in a semiconductor layer as a MEMS device. Other known MEMS devices include, for example, micro-mechanical filters, pressure sensors, gyroscopes, resonators, actuators, and rate sensors, as described in U.S. Pat. No. 6,428,713.

MEMS accelerometer devices generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of MEMS accelerometer is the capacitive accelerometer. As disclosed in U.S. Pat. No. 4,435,737, LOWCOSTCAPACITIVEACCELEROMETER, issued to Colton on Mar. 6, 1984, which is incorporated herein by reference, capacitive accelerometers are generally well known in the art. In a closed-loop capacitive accelerometer, the acceleration sensor is a proof mass suspended by flexures or hinges for rotation relative to an outer frame portion. The acceleration sensor is bonded between glass plates with the proof mass forming a differential capacitor with the glass plates.

The proof mass rotates about the flexures according to the principle of Newton's law: F=ma, when subjected to acceleration along the input or “sensitive” axis which is normal to the plane of the proof mass. An electrical drive and sense circuit measures applied acceleration force as a function of the displacement of the proof mass and the resulting differential capacitance.

Accelerometers of the type that are based on a rotating mass often need to be firmly constrained relative to the in-plane axes while being permitted movement in the third input axis. In devices having a rotating mass, the position of the axis of rotation also needs to be constrained. Recently, the proof mass and flexures have been fabricated in an active epitaxial or layer grown on a silicon substrate. The proof mass and flexures are structured using Reactive Ion Etching (RIE) or Deep Reactive Ion Etching (DRIE), which permits etching of very narrow slots between nearly vertical walls. DRIE permits the width, length, and thickness of the flexures to be closely controlled so that desirable bending characteristics are obtained. The flexures define a linear axis of rotation or “hinge” axis about which the proof mass moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other accelerated body having the accelerometer mounted thereon. Traditionally, the flexures are substantially rectangularly shaped with a substantially constant cross-sectional area. The substantially rectangular shape gives the flexures greater in-plane stiffness along the major axis, i.e., the accelerometer hinge axis, and substantially less in-plane stiffness along its minor axis.

Prior art micromachined accelerometers have effectively used the substantially rectangular flexures for pliantly suspending a rotating or translating proof mass. However, the flexures of some prior art devices, such as those fabricated using RIE or DRIE, are essentially two-dimensional designs that do not permit changes in material thickness that can be used to control flexure stiffness. The rectangular flexures operate as a beam having a constant area moment of inertia, I, which is defined as the integral of the area of the cross-section times the square of the distance of the incremental area from the neutral axis. See, e.g., Shigley, MECHANICALENGINEERINGDESIGN,3rdedition, page 45, which is incorporated herein by reference. The rectangular flexures bend along their entire lengths, similarly to a beam of constant cross-section that is supported at both ends. The rectangular flexures therefore lack a well-defined hinge axis.

SUMMARY OF THE INVENTION

The present invention overcomes limitations of the prior art for providing proof mass suspension in a force sensor, which is critical to isolating the accelerometer mechanism from in-plane force inputs during operation while responding to out-of-plane force inputs.

The arcuate suspension apparatus of the invention is stiffer when loaded in the plane of the arcuate shape than out-of-plane. The arcuate suspension apparatus transfers in-plane loads along the included arch, which causes the arcuate suspension apparatus to be strong and rigid in opposing in-plane loads, and flexes or bends when subjected to out-of-plane loads, which causes the arcuate suspension apparatus to be flexibly compliant when loaded out-of-plane.

According to one aspect of the invention, the arcuate suspension apparatus includes a first substantially arcuately shaped flexure having a cross-section that is relatively substantially extended in the plane of the arcuate shape, the first flexure having a first end structured for connection to a support structure and a second end structured for connection to a movable structure to be suspended from the support structure; and a second substantially flexure that is arcuately shaped similarly to the first flexure and having a cross-section that is relatively substantially extended in the plane of the arcuate shape and having a first end structured for connection to the support structure and a second end structured for connection to the movable structure, the arcuate shape of the second flexure being aligned with the arcuate shape of the first flexure and with the first flexure forming a hinge having an axis of rotation that extends through the first and second flexures. The arcuate shape of the first and second flexures are aligned either facing oppositely from one another, or facing toward one another.

According to another aspect of the invention, the invention includes a structure to be suspended and a support structure spaced away from the structure to be suspended. The first ends of the first and second flexures are connected to the support structure, and the second ends of the first and second flexures are connected to the structure to be suspended.

According to another aspect of the invention, the structure to be suspended is an accelerometer sensor mechanism structured using micro-machined electromechanical sensor (MEMS) techniques in an epitaxial growth of semiconductor material on a silicon substrate.

According to still other aspects of the invention, methods for suspending an acceleration apparatus are provided.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In the Figures, like numerals indicate like elements.

The present invention is an apparatus and method for suspending a relatively movable structure of an acceleration sensor, i.e., a proof mass, from a support structure, i.e., an inner sensor frame. The suspension apparatus of the invention includes at least first and second arcuately shaped flexures each having similar cross-sections that are relatively substantially extended in a plane in which the arcuate shape lies. The first and second arcuately shaped flexures each have a first end structured for connection to the support structure and a second end structured for connection to the movable structure that is to be suspended from the support structure. The first and second flexures both lie in the same plane and are aligned with their respective outer arcuate shapes facing oppositely each from the other and forming a hinge having an axis of rotation in the plane of the first and second arcuate shapes that extends linearly through respective portions of the first and second flexure that are aligned substantially cross-wise to the axis of rotation.

FIG. 1is not to scale but clearly illustrates the apparatus and method of the invention embodied by example and without limitation in a suspension structure for pliantly suspending a single cantilevered proof mass in a micro-machined electromechanical sensor (MEMS) accelerometer10. For clarity, the top and bottom cover plates are shown spaced apart from the sensor mechanism12. The sensor mechanism12is structured in an epitaxial growth or “active” semiconductor layer grown on a silicon substrate (not shown). Structuring of the sensor mechanism12includes structuring a cantilevered proof mass14operable as a rotating or “moveable” electrode. Opposing cover plates16are formed in glass wafers and provided with fixed surface electrodes18. The sensor mechanism12is bonded, for example, by electrostatic bonding, to one of the two cover plates16with the fixed electrode18aligned with one planar surface of the proof mass14. The silicon substrate is removed, leaving the sensor mechanism12, including the second surface of the proof mass14, exposed. Using electrostatic bonding, the second of the two cover plates16is bonded to the sensor mechanism12opposite the first cover plate16and having its fixed electrode18aligned with the proof mass14. The proof mass14thus operates as a moveable electrode between the two fixed electrodes18on either side. An electrical drive and sense circuit20measures differential capacitance between the proof mass14and each of the opposing fixed electrodes18as a function of the displacement of the proof mass14resulting from an acceleration force applied along an input axis I, and outputs a resulting acceleration signal.

The sensor mechanism12is structured having one or more pairs of flexures, indicated generally at22, that are structured according to the invention for pliantly suspending the proof mass14from an inner sensor frame or plate24for movement of the proof mass14along the input axis I normal to the proof mass14. Each pair of flexures22defines a linear hinge axis H about which the proof mass14moves in response to an applied force, such as the acceleration of the accelerated body having the accelerometer10mounted thereon. As illustrated inFIG. 1, the flexures22are identical in thickness to the remainder of the sensor mechanism12, because they are similarly formed in the epitaxial growth or active layer grown on the silicon substrate and are similarly structured using one of the RIE or DRIE processes that do not permit changes in material thickness. The flexures22therefore are structured according to the invention using an arcuate shape, shown inFIG. 2, that is used to control flexure stiffness and to accurately position the hinge axis H.

FIG. 2illustrates the apparatus and method of the invention embodied by example and without limitation in a suspension structure for pliantly suspending a proof mass in another single proof mass micro-machined electromechanical sensor (MEMS) accelerometer100, commonly referred to as a Tee design. The invention is similarly practicable as a suspension structure for pliantly suspending a proof mass in another pendulous mass accelerometer such as one of the different accelerometer designs illustrated in each of U.S. Pat. No. 5,287,744, ACCELEROMETER WITHFLEXUREISOLATION, issued to Norling, et al. on Feb. 22, 1994; U.S. Pat. No. 4,944,184, ASYMMETRICFLEXURE FORPENDULOUSACCELEROMETER, issued to Blake, et al. on Jul. 31, 1990; and U.S. Pat. No. 6,282,959, COMPENSATION OFSECOND-ORDERNON-LINEARITY INSENSORSEMPLOYINGDOUBLE-ENDEDTUNINGFORKS, issued to Blake, et al. on Sep. 4, 2001, which are all incorporated herein by reference.

The accelerometer100inFIG. 2is formed, as described above, in an epitaxial or “active” layer102of semiconductor material grown on a silicon substrate (not shown). The accelerometer100includes an acceleration sensor mechanism110having one or more pairs of flexures112structured according to the invention for pliantly suspending a proof mass114from an inner sensor frame or plate116for movement of the proof mass114along an input axis12normal to the proof mass114. Each of the two pairs of flexures112define coincident linear hinge axes H2about which the proof mass114moves in response to an applied force, such as the acceleration of the accelerated body having the accelerometer100mounted thereon. The proof mass114is thus operable as a rotatable or “moveable” electrode. As described above, top and bottom cover plates (not shown) are structured with opposing fixed electrodes and are bonded to opposite sides of the sensor mechanism100to form capacitive force sensors in combination with the moveable electrode114.

The sensitive acceleration sensor mechanism110is supported by mechanical coupling of the accelerometer sensor frame116to a pair of cover plates16, as shown inFIG. 1, one of which in turn is typically connected to a ceramic or metal mounting plate (not shown). The sensor frame116may be suspended from a second outer or external frame portion124by flexures126formed by overlapping slots128and130through the layer102. The sensor frame116is thus able to move relative to the outer frame124, as described above and in U.S. Pat. No. 5,948,981, Vibrating Beam Accelerometer, issued to Woodruff on Sep. 7, 1999, and assigned to the Assignee of the present application, which is incorporated herein by reference. Such isolation minimizes the distortion of the sensor frame116, and thereby decreases the effects of external stresses and strains on the sensor mechanism.

The flexures112are structured in the epitaxial semiconductor layer102using one of the RIE or DRIE processes. The flexures112are arcuately shaped having first and second curved or rounded arch members112aand112bthat lie in the plane of the epitaxial semiconductor layer102. each of the flexures112a,112bare formed with first and second ends, the first ends are structured for connection to the support structure, i.e., the sensor frame116, and the second ends are structured for connection to the movable structure, i.e., the proof mass114. The arcuate shape of the first and second flexures112a,112bare aligned with one another, but the arcuate shape of each flexure faces oppositely each from the arcuate shape of the other flexure, as illustrated in FIG.2.

The first and second flexures112a,12btogether form a hinge having an axis of rotation H2in the plane of the first and second arcuate shapes that extends linearly through the first and second flexures. In cross-section, the first and second flexures112a,12bare substantially rectangularly shaped, each having its major axis lying in a plane that is substantially parallel with the plane of the epitaxial semiconductor layer102and the axis of rotation H2, and its minor axis lying substantially cross-wise to the plane of the epitaxial layer102or through it thickness. Thus, as illustrated inFIG. 3, the cross-section of the flexures112a,12bis substantially extended in the plane of the arcuate shape relative to the cross-section taken cross-wise to the plane of the arcuate shape.

FIG. 3shows a cross-section taken through the first flexure112aalong the hinge axis H2, which illustrates that the major axis X of the substantially rectangular cross-section is substantially longer than its minor axis Y that is aligned along the hinge axis H2. The arcuate shape of each of the flexures112a,12bincludes a respective portion, shown inFIG. 3, that is aligned substantially cross-wise to the axis of rotation H2. The axis of rotation H2, i.e., the hinge axis, extends through these respective cross-wise portions of the flexures112a,112b.

One drawback to the prior art rectangular-style flexures is that they are essentially two-dimensional designs that do not permit changes in material thickness that can be used to control flexure stiffness. This inability to control flexure stiffness by varying material thickness is avoided by the feature of the arcuate flexure geometry of the present invention that provides a cross-section having a major axis that can be varied by changing the relative inside and outside radii, r and R, respectively, of the arcuate forms. For example, when the inside and outside radii are concentric, increasing the difference between the inside and outside radii increases the major axis X of the rectangular cross-section relative to the minor axis Y, while decreasing the difference decreases the major axis relative to the minor axis. The arcuate beam geometry of the present invention thus provides a distinct advantage over the rectangular-style flexures of the prior art.

Furthermore, when the flexures112a,112bare formed with concentric inner and outer radii r and R, respectively, the curvature of the flexures112a,12bcauses the length of the major axis X of the cross-section to increase to a length X1along the direction of the axis of rotation H2as the a function of increasing distance from the axis of rotation, while the length of the minor axis Y remains substantially constant. In other words, the width of the flexures112a,112bis a minimum at the axis of rotation H2, and the flexures112a,112bbecome wider in cross-section as cross-sections are taken further from the axis of rotation H2. At the same time the thickness of the epitaxial semiconductor layer102remains substantially constant, so that the length of the minor axis Y does not change.

FIG. 4illustrates that this tendency for the major axis X to increase as a function of increasing distance from the axis of rotation is enhanced by increasing the outer radius R and positioning its centerline offset at a distance from the centerline of the inner radius r along the axis of rotation H2in the concave direction of the arcuate shape. Thus, the inner and outer radii r, R are in closest proximity where the cross-section intersects the axis of rotation H2, and the distance X between the inner and outer radii increases rapidly to X2as distance from the axis of rotation increases.

The arcuate shaped suspension apparatus of the present invention is stiffer when loaded in-plane, i.e., in the plane containing the arcuate shape, than when loaded out-of-plane, i.e., along the accelerometer input axis I. The arcuate shape and relatively wide in-plane cross-section of the suspension apparatus provide a high modulus of rigidity in the plane of the arcuate shape, and the flat, planar shape and relatively thin out-of-plane cross-section cause it to have a very low out-of-plane modulus of rigidity. The suspension apparatus thus transfers in-plane loads along the arch of the wide, arcuate shape of the suspension apparatus causing it to be strong and stiff. In contrast, the suspension apparatus flexes or bends when subjected to out-of-plane loads, which causes it to be very flexible. This flexing or bending feature allows sensitivity to acceleration applied along input axis I, while the suspension apparatus remains rigid in the plane of the arcuate shape. The arcuate shaped suspension apparatus is thus useful as a flexure in micromachined accelerometers that are designed to be sensitive to out-of-plane force or acceleration inputs. The arcuate shaped suspension apparatus of the present invention is particularly useful in controlling rotating masses in structures where depth of structure is limited, such as structures formed using RIE and DRIE processing.

FIG. 5illustrates that, while the arcuate shaped suspension apparatus of the invention is embodied inFIG. 2as two flexures112each formed of pairs of semi-circular flexures112a,112bthat combine form substantially circular flexures112, the individual flexure members112a,112bof the arcuate shaped suspension apparatus of the invention do not need to be semi-circular flexure members, nor do the pairs of the individual flexure members112a,112bneed to form completely circular flexures112. Rather, the arcuate shaped suspension apparatus can be formed in any curved or rounded shape. The completely circular arcuate shaped suspension apparatus functions well as a hinge axis because the individual semi-circular flexure members112a,112bflex or bend at a predictable place, i.e., across the center at hinge axis H3, in response to a force or acceleration applied along the input axis12, while remaining rigid in the two in-plane axes. In the exemplary embodiment of the arcuate shaped suspension apparatus of the invention illustrated inFIG. 5, two individual semi-circular flexure members112c,112dlie with their respective outer arcuate shapes facing toward each other and are interconnected at the peaks of their respective arches in an “X” configuration that exhibits many of the characteristics of a circular suspension apparatus.

FIGS. 6A and 6Billustrate the arcuate shaped suspension apparatus of the invention embodied in other accelerometers200aand200b, respectively, as two pair of flexures212pliantly suspending an epitaxial semiconductor proof mass214from a support structure embodied as an inner sensor plate216in a “teeter-totter” arrangement between fixed electrodes (not shown) for measuring forces applied to the proof mass214.

Each of the flexures includes first and second arcuately shaped flexure members212a,212beach having a first end structured for connection to the inner sensor plate216and a second end structured for connection to the movable proof mass214that is to be suspended from the inner sensor plate216. The first and second flexure members212a,212bboth lie in the same plane and are aligned, as illustrated inFIG. 6A, with their respective outer arcuate shapes facing oppositely each from the other and forming a hinge having an axis of rotation or hinge axis H4in the plane of the first and second arcuate shapes that extends linearly through respective portions of the first and second flexure that are aligned substantially cross-wise to the axis of rotation.

Alternatively, as illustrated inFIG. 6B, the flexure members212a,212bof each pair of flexures212are aligned with their respective outer arcuate shapes facing toward each other and similarly forming a hinge having an axis of rotation or hinge axis H4in the plane of the first and second arcuate shapes that extends linearly through respective portions of the first and second flexure that are aligned substantially cross-wise to the axis of rotation.

InFIGS. 6A and 6Bthe arcuate shaped suspension apparatus of the invention are embodied in a pair of quarter-round flexure members212a,212bof substantially identical shape each being aligned with the interior of their respective arcuate shape facing one toward the other. As illustrated by example and without limitation inFIGS. 6A,6B, each of the flexure members212a,212bof the two flexures212may be structured with their inner and outer radii being substantially concentric, as shown inFIG. 3, and each of the two different pairs of flexure members212a,212bmay share substantially the same center point which also lies on the hinge axis H4. The two flexures212formed of quarter-round flexure members212a,212bresult in a suspension apparatus that is much stiffer in the rotational axis than that formed of two pair of semicircular flexure members112a,112b, as illustrated inFIG. 2, while providing the in-plane rigidity that is desirable in force/displacement sensors.

FIG. 7illustrates the arcuate shaped suspension apparatus of the invention alternately embodied by example and without limitation in a configuration that employs corner arches to selectively stabilize an interior mass. The arcuate shaped suspension apparatus of the intention is, for example, embodied in a capacitive accelerometer300as four flexures312pliantly suspending an interior proof mass314from a support structure embodied as an outer sensor frame or plate316in a “trampoline” arrangement for measuring forces applied to the epitaxial proof mass314. The capacitive accelerometer300illustrated inFIG. 7is constructed in an epitaxial semiconductor layer using conventional microcircuit techniques as described herein and includes a pair of glass plates318(bottom plate shown) having opposed parallel planar faces. The plates318are spaced from one another and each has a metal layer electrode320of predetermined configuration deposited on one surface to form a capacitor plate. The interior epitaxial semiconductor layer proof mass314suspended from the outer sensor frame316is positioned between the metal layer electrodes320to form a common capacitor plate which moves in response to an applied force or acceleration. The outer sensor frame316is attached to the opposed faces of the top and bottom glass plates318with the interior proof mass314suspended therebetween by the four flexible flexures312forming two pairs of capacitor plates with the metal layer electrodes320on the opposed top and bottom glass plates318. Movement of the interior semiconductor proof mass314in response to an applied force or acceleration changes the spacing between the interior semiconductor proof mass314and the metal layer electrodes320, thereby causing a change in capacitance which is indicative of the applied force or acceleration.

The metal layer electrodes320and the interior semiconductor proof mass314suspended therebetween are coupled to electrical circuits capable of measuring the capacitance between the plates formed by the respective metal layer electrodes320and the interior semiconductor proof mass314. Such electrical circuits are generally well known in the art. By example and without limitation, one of such an electrical circuit is disclosed in U.S. Pat. No. 4,077,132, DIGITALLEVELINDICATOR, issued to Erickson on Mar. 7, 1978, which is incorporated herein by reference.

Each of the flexures312suspending the interior proof mass314includes first and second arcuately shaped flexure members312a,312beach having a first end structured for connection to the outer sensor frame316and a second end structured for connection to the movable proof mass314that is to be suspended from the outer sensor frame316. The first and second flexure members312a,312bboth lie in the same plane and are aligned with their respective outer arcuate shapes facing each toward the other, in contrast to the first and second arcuately shaped flexure members previously illustrated. Each of the different pairs of first and second flexure members312a,312bare spaced apart along one edge of the interior proof mass314in a gap formed between it and the outer sensor frame316. The flexure members312a,312bform a hinge having an axis of rotation or hinge axis H5in the plane of the first and second arcuate shapes and that extends linearly through respective portions of the first and second flexure that are aligned substantially cross-wise to the axis of rotation.

InFIG. 7the arcuate shaped suspension apparatus of the invention are embodied in a pair of quarter-round flexure members312a,312bof substantially identical shape each being aligned with the interior of their respective arcuate shape facing outwardly, i.e., facing away one another. As illustrated by example and without limitation inFIG. 7, each of the flexure members312a,312bof the four flexures312may be structured with their inner and outer radii being substantially concentric, as shown inFIG. 3, with their respective centerlines lying on the hinge axis H5. The four flexures312formed of approximately quarter-round flexure members312a,312bresult in a suspension apparatus that permits the interior proof mass314to move up and down in a “trampoline” manner relative to the outer sensor frame or plate316along an input axis13normal to the plane of the sensor300, while providing the in-plane rigidity that is desirable in force/displacement sensors.

FIG. 8illustrates the arcuate shaped suspension apparatus of the intention alternately embodied by another example and still without limitation in a configuration that employs two pair of side arches to selectively stabilize an interior mass. The arcuate shaped suspension apparatus of the intention is, for example, embodied in another capacitive accelerometer400as two flexures412pliantly suspending an interior proof mass414from a support structure embodied as an outer sensor frame or plate416in another “trampoline” arrangement for measuring forces applied to the proof mass414similar to the configuration illustrated in FIG.7.

Each of the side flexures412suspending the interior proof mass414includes first and second arcuately shaped flexure members412a,412beach having a first end structured for connection to the outer sensor frame416and a second end structured for connection to the movable proof mass414that is to be suspended from the outer sensor frame416. The first and second flexure members412a,412bboth lie in the same plane and, in contrast to the embodiment illustrated inFIG. 7, are aligned with their respective inner arcuate shapes facing each toward the other similarly to the first and second arcuately shaped flexure members112a,112billustrated in FIG.2. Each of the different pairs of first and second flexure members412a,412bare spaced apart along one edge of the interior proof mass414in a gap formed between it and the outer sensor frame416. The flexure members412a,412bform a hinge having an axis of rotation or hinge axis H6in the plane of the first and second arcuate shapes that extends linearly through respective portions of the first and second flexure that are aligned substantially cross-wise to the axis of rotation.

InFIG. 8the arcuate shaped suspension apparatus of the invention are embodied in a pair of quarter-round flexure members412a,412bof substantially identical shape each being aligned with the interior of their respective arcuate shape facing inwardly, i.e., facing toward one another. As illustrated by example and without limitation inFIG. 8, each of the flexure members412a,412bof the two flexures412may be structured with their inner and outer radii being substantially concentric, as shown inFIG. 3, with their respective centerlines being coincident and lying on the hinge axis H6. The two flexures412formed of quarter-round flexure members412a,412bresult in a suspension apparatus that permits the interior proof mass414to move up and down in a “trampoline” manner along an input axis14normal to the plane of the sensor400relative to the outer sensor frame or plate416, while providing the in-plane rigidity that is desirable in force/displacement sensors.

FIG. 8also illustrates the arcuate shaped suspension apparatus of the invention embodied in an optional configuration wherein a single pair of half-round flexure members412a,412bof substantially identical shape extend between opposing inner walls in a space418within the outer sensor frame416and share a common centerline. Accordingly, pair of half-round flexure members412a,412bform a circularly shaped suspension apparatus412with the interior proof mass414suspended on top.