US3498138A - Accelerometer - Google Patents

Description

Mal-c113, 1970 R. a. STEWART :'..49s,13a

ACCELBROIETER Filed Sept. 9, 1966 y 4 Sheets-Sheet 1 $1 zil F 2 ROBERT E. STEM/427' INVENTOR.

R. E ST EW nounm 1 4 Sheets-Sheet 2 Filed Sept. 9, 1956 R R W 8mm% w m f M 0 W I. x m m M 6 05 w m R 8 l w/%@ a: 0 1 "z 1 3/ m R. E. STEWART ACCELBROIIBIBR 4 Sheets-Sheet 4 v 'rned Sept. 9, 1966 United States Patent U.S. Cl. 73-517 19 Claims ABSTRACT OF THE DISCLOSURE An accelerometer having a sensing member which includes a nonconductive wafer divided, by a slot formed therein, into radially interior and exterior portions. The exterior portion is supported for rotation relative to the interior portion by a pair of flexible hinge members connected such that translation of the outer portion results in restoring forces tending to produce only translational displacement thereof. A servo system for positioning the exterior portion to a neutral position, includes magnetic means for super-imposing a permanent magnets unidirectional field with a field produced by an electromagnet, to apply a restoring torquing force couple to the sensing member. Means are provided for varying the working air gap of the permanent magnet circuit to compensate for changes in the characteristics of the magnet due to temperature variations.

This invention pertains to an accelerometer, and more particularly to an accelerometer in which the sensing mass is servoed into a neutral position.

The state of the accelerometer art is represented by the device shown and described in US. Patent No. 3,229,530 issued Jan. 18, 1966 to D. E. Wilcox et al. The sensing mass of the device of that patent is cantilevered on a flexible hinge relative to a supporting body whose acceleration is to be measured.

One of the effects that is displayed in prior art accelerometers using flexible hinges is that accelerations change the characteristic of the hinge member material in a non-linear manner, thereby changing the scale factor of the accelerometer.

Further, accelerations perpendicular to the sensing axis of the accelerometer tend to unbalance the accelerometer and indicate an acceleration along the sensing axis. In one embodiment of the device contemplated by this inven tion, however, those efiects are eliminated.

The device contemplated by this invention supports the angularly rotatable deflectable sensing member on flexible beams which are oppositely sprung so that the non-linear changes in characteristics are compensated.

In prior known accelerometers, the sensing mass is translated relative to the magnetic forcer, whereby at least a pair of opposing magnetic fields are required to ensure linearity between the restoring force and the current in the coil of the magnetic forcers.

The device contemplated by this invention, however, uses an electromagnet coil and a permanent magnet which are arranged so that the magnetic field of the electromagnet coil and the magnetic field of the permanent magnet are maintained at right angles to each other to apply a pure torque to the sensing mass, whereby only one magnet system is required to generate a linear relation between the current applied to the restoring electromagnet coil and the restoring torque.

In prior art devices, when permanent magnets are used to interact with a restoring magnetic field, the magnets are temperature sensitive which causes the scale factor of the accelerometer to change with temperature.

The device contemplated by this invention uses a novel "ice magnetic return path in which the working air gap in the permanent magnet circuit is varied with temperature to compensate for changes in the magnetic characteristics of the permanent magnet.

The device of this invention uses a ceramic wafer which is divided into two portions, one of which is attached to the housing of the accelerometer and the other forming the major portion of the angularly deflectable member. Capacitors or pick-off plates for sensing angular deflection of the deflectable portion relative to the housing fixed portion are formed in a film on the surface of the deflectable portion. Electrical connections are made between the deflectable and housing fixed portions through a pair of conductive hinge elements.

It is therefore an object of this invention accurately to sense acceleration of a supporting member.

It is also an object of this invention, in an accelerometer, to reduce the non-linear effects in the supporting hinges caused by the application of force and acceleration to the accelerometer.

It is another object of this invention to use a wafer of ceramic material which is divided into two portions, one of which is attached to the accelerometer housing and the other of which forms an angularly deflectable member, and in Which metallic films are formed in a predetermined pattern on the surface of the ceramic material, including the forming of a thin film metallic hinge member connecting the portions of the wafer.

It is likewise an object of this invention to use a novel temperature-compensated permanent magnet in a torquer for an accelerometer.

It is still another object of this invention to apply a restoring torque to a sensing mass in an accelerometer.

It is a more specific object of this invention to support a sensing mass, relative to an element whose acceleration is to be measured, for pure rotation in response to applied accelerations.

It is also a specific object of this invention to support a sensing mass having rotational freedom upon oppositely sprung flexures relative to the element whose acceleration is to be measured.

It is yet another object of this invention to restore, magnetically, a rotatable member into a neutral position by sensing the angular rotation of said member from said neutral position and by applying a torque to said rotatable member from a pair of crossed magnetic fields, one of said fields being generated by apparatus attached to said rotatable member.

It is still a more specific object of this invention to provide an accelerometer having a sensing mass which is impelled into rotation in response to applied acceleration, to sense angular displacement of said sensing mass from a neutral position, and magnetically to drive said sensing mass, against its tendency to rotate, by a pair of crossed magnetic fields, one of which is generated upon said sensing mass and the other of which is generated from a permanent magnet, said permanent magnet being forced, in response to changes in temperature, to modify its gap in a sense and magnitude to compensate for changes in magnetic field strengths caused by changes of temperature.

It is yet another object of this invention to provide a permanent magnet and a magnetic return path, said magnetic return path being substantially circular in shape and spaced apart from said magnet to generate a specified magnetic field strength between said magnet and said return path, and a forcing member, responsive to temperature changes and adapted to expand and contract with temperature, engaging said return path to distort.

said return path in a fashion to modify the width of the gap between said magnet and said return path to maintain the magnetic field strengths within said gap substantially constant over a wide temperature range.

Other objects will become apparent from the following description, taken in connection with the accompanying drawings, in which:

FIGURES 1 and 2 are schematic diagrams, partly in section, of first and second embodiments of the angularly deflectable sensing element of the invention;

FIGURE 3 is a plan view of an angularly deflectable sensing member and a housing-fixed member showing hinge-beams connecting the two members in a non-compensating manner;

FIGURE 4 is a plan view of an angularly deflectable sensing member and a housing-fixed member in which the hinge-beams connect the two members in a compensating manner;

FIGURE 5 is a sectional view taken at 55 in FIG- URES 3 and 4;

FIGURE 6 is a sectional view taken at 6-6 in FIG- URE 4;

FIGURE 7 is an oblique view, partly in section, of a typical device in accordance with this invention;

FIGURE 8 is a sectional view taken at 8-8 in FIG- URE 7;

FIGURES 9 and 10 are views taken at 9--9' and 10- 10 in FIGURE 8;

FIGURE 11 is a view taken at 11-11 in FIGURE 9;

FIGURE 12 is an expanded view of the hinge region between the angularly deflectable and housing-fixed members of the invention, taken at 12-12 in FIGURE 9;

FIGURE 13 is a view taken at 1313 in FIGURE 8; and

FIGURE 14 is a typical schematic electrical diagram showing the interrelation between the angular displacement sensor and the electromagnetic torque-producing member.

FIGURES 1 and 2 are presented to demonstrate two preferred conditions of mass unbalance (eccentric weighting) of an angularly-defiectable-sensing member to adapt the deflectable member to sense accelerations in the directions indicated by arrows 20 and 26. The entire deflectable portion of FIGURES 1 and 2 is adapted to tilt slightly about an axis 10. Capacitance plates (not shown) may be placed on the web 12 to be used, in conjunction with stationary plates, to sense angular displacement of the deflectable portion about the axis 10. An electromagnet coil 14 may be attached to the edge of the web 12 and adapted to generate a magnetic field to interact with a second magnetic field which is generated by a permanent magnet (not shown) to restore the angular position of the device about axis 10 into a predetermined neutral position.

In the device of FIGURE 1, the center of gravity 16 is raised to a position directly above the axis of rotation 10 by the addition of appropriate symmetrical weights 18 to cause the device to be sensitive to accelerations in the direction shown by arrows 20. In place of separate weights 18, a symmetrical ring or rim (not shown) may extend around the entire periphery of the web 12.

In the device of FIGURE 2, a single unbalanced weight 22 is positioned on the edge of the web 12 to cause the center of gravity to move to a position 24 to cause the angularly deflectable portion to be sensitive to accelerations in the directions shown by arrow 26.

Ina typical operation of the devices of FIGURES 1 and 2, the entire device is free to deflect about axis 10 in response either to accelerations in the direction 20 (in FIG. 1) or accelerations in the direction 26 (in FIG. 2). The deflecting of the device about axis 10' away from a neutral position is sensedfor exampleby means of a capacitance bridge with the plates of the capacitors (not shown) positioned on the web 12 and on the device whose acceleration is being sensed (not shown) to detect the angular displacement of the shown device away from its neutral or zero acceleration position. The sensed displacement is then amplified and a current is supplied to the coil 14 to generate a magnetic field which interacts with the field of a permanent magnet to return and maintain the shown device in its neutral position.

The deflectable portion of FIGURES 1 and 2 may-for examplebe supported about axis 10 relative to a nondeflectable portion whose acceleration is being sensed, on low or non-friction bearings, on torsion pivots, or the like. The device may be externally or internally supported, i.e. from a surrounding structure (not shown) or from an internal anchor (not shown in FIGURES 1 and 2, but shown in FIGURES 3 through 12).

Typical configurations using an internally positioned housing fixed portions in which the angularly deflectable acceleration sensing member is supported upon flexible beams as shown' in FIGURES 3 and 4. Of the two conflgurations, the configuration in FIGURE 4 is the preferred one. FIGURES 5 and 6 are views taken at 55 and 6-6, respectively, in FIGURES 3 and 4, and are introduced for purposes of explaining the manner in which the configuration of FIGURE 4 excels the configuration of FIGURE 3.

In FIGURE 3, the acceleration sensing member 30 is supported for limited angular rotation relative to housingfixed member 28 by a pair of flexible beams 32 and 34. The beams 32 and 34 may be said to be sprung in the same direction, i.e. when deflected as shown in FIGURE 5, bias torques delivered by springs 32 and 34 are in the same direction, shown by arrow 36, thereby tending to tilt the angularly deflectable member 3 0 relative to the anchor member 28.

In the configuration of FIGURE 4, the acceleration sensing member 30 is supported for limited angular rotation, relative to housing-fixed member 28-, by a pair of beam springs 32 and 38 which are oppositely sprung, i.e. when angularly deflectable member 30 is deflected, spring 32 applies a bias torque in a first direction about the axis of relative rotation, as shown by arrow 36 in FIGURE 5, and spring 38 applies a bias torque in the opposite direction, as shown by arrow 40 in FIGURE 6. Thus, in the configuration of FIGURE 4 the bias torques represented by arrows 36 and 40 cancel each other. It is this cancelling effect which is described herein when it is stated that the supporting beam-springs are oppositely sprung.

In the configuration of FIGURES 3 through 6, particularly as shown in FIGURES 5 and 6, the members 28 and 30 are shown as ceramic-like members while the beam- springs 32, 34 and 38 are shown as metal beams. Actually, in a preferred embodiment, as shown in FIG- URES 7 through 13, the beam would be made of a metal such as electroless nickel which is bonded in a film to the surfaces of the ceramic members 28 and 30*.

The preferred embodiment of the invention is shown in FIGURES 7 through 13, with a circuit used with the invention shown in FIGURE 14. In the FIGURES 7 through 13, the central housing-fixed member 42 is anchored between the top 43 and the bottom 44 of an outer housing 45 by means of an O-ring 46, a pressure ring 47, a wiring-harness ring 48, a permanent magnet member 50, and a magnet temperature compensating member 52. The metal film sections which are bonded to the ceramic anchor member 42 at 54, 56, 58, 60, 62 and 64 on one surface and 66 on the other surface are used as electrical conductors. So, too, the conductive plating 68 on the surface of the magnet 50 is a conductive surface adapted to carry electrical potential from electrode 66 to the magnet 50. The film '70 on the surface of member 52 is an insulating layer which may-for examplebe anodized aluminum. A centering pin 72 and a centering collar 74 are used to center the various components, with the centering pin 72 recessed into the bottom 44 at 74 (FIG. 8). The entire assembly is held in the outer housing 45 by a snap ring 76. A magnet fluxreturn ring, or return path 78 surrounds magnet 50- and is held in place by magnet retaining member 52, as shown at 72 and 80 in FIG. 13. Space is made between magnet 50 and return path 78 to form a gap into which an electromagnet coil 82, atached to sensing mass 84 is inserted.

The sensing mass 84 has metal films 86, 87, 88 and 89 bonded to one surface and metal films 91, 92, 93 and 94 bonded to the other surface. Films 92 and 93 bridge the radial hinge gap 96 at '97 and 98, and are attached to housing fixed member 42. Films 91 and 94 bridge the radial hinge gap 100 at 101 and 102 and are attached to housing-fixed member 42. The beams formed at 97, 98, 101 and 102 by films 92, 93, 91 and 94 form hinges between the angularly deflectable member 84 and the housing-fixed member 42 to allow limited rotation of rotator 84 about an axis of rotation 170 formed by these hinges. In the preferred embodiment the housing-fixed member 42 and the deflectable member 84 are fabricated from a piece of ceramic material and the conducting films on the two surfaces thereof are fabricated of electroless nickel. To make the housing-fixed member 42 and the fabricated together out of one blank, wafer, or sheet of ceramic. The outer edge of the sensing member 84 the arcuate groves 104, 106, and the radial hinge grooves 96 and 100 are first cut out of the blank. The cutting may befor exampleby ultrasonic means, such as by a Cavitron. Copper is then plated into the slots or grooves by using first a flash of electroless copper than electroforming the copper into the grooves to completely fill the grooves. The copper is then lapped flush with the faces of the base ceramic material. Electroless nickel is then plated onto the faces of the blank comprising the members 42 and 84, and the copper material filling the gaps 104, 106, 96 and 100. A photoresistive material is than applied to the nickel in a predetermined pattern to define the electrodes and the hinge beams which are shown particularly in FIGURES 9 and 10. The nickel is then deplated or etched with the photo-resistive material resisting the deplating or etching process at the places where it is desired to leave the electroless nickel. The material is then etchedfor example-with hot chromic acid, or the like, to remove the copper material from the grooves 96, 100, 104 and 106. During the period when the copper was deposited in the grooves 96, 100, 104 and 106, copper was also deposited in holes, punched in the ceramic base material for that purpose, to form conducting pins 108, 110, 112, 114, 116, 118 and 120 which connect films on opposite faces of the members 42 and 84. The films 89 and 87 are connected to opposite ends of the electromagnet coil 82 to conduct current to and from coil 82. One of the connections is shown in FIGURE 9 at 122.

There are, feeding through the top 43 of the housing, six insulated conducting pins, two of which are shown in FIGURE 8 and five of which are shown in FIGURE 7 at 124, 126, 128, 130 and 132. Each of the conducting pins connects to a separate conducting buss of the wiring harness ring 48. Typical conducting busses are shown in FIGURES 7 and 8 at 134 and 136. The various busses carry electrical voltage and/or current to or from the conductive films 54, 85, 60, 62, and 64. Voltage applied to the pin connected to film 62 is conducted through the conducting member 114 to film 66 which is in contact (see FIG. 8) with film 68. Film 68 is electrically in contact with the magnet 50. Voltage applied to film 64 is carried through a conducting member 116 to conducting film 93 which act as one capacitor plate for the pick off for detecting the displacement of member 84. Voltage applied to film 58 is carried through conducting member 112 to conducting film 91 which acts as a second capacitor plate for the pick off of member 84. Magnet current is carried from conducting film 54, through conductive member 118, across gap 96 on conducting hinge member and film 97, through conducting member 120, through conducting film 89, through wire 122 to coil 82. The current is returned from magnet coil 82 through a conductor (not shown) to conductive film 87, through conductive member 108, through film 94, across gap 100, through conductive member 110, and through conductive film 60.

In the shown invention of FIGURES 7 through 13; an unbalanced weight is placed on one side at to cause the deflectable member 84 to sense accelerations in the fashion described in connection with the device of FIGURE 2. Should it be desired to sense acceleration in the direction specified in the device of FIGURE 1, the weight 150 could be extended completely around the outer periphery of member 84, or an additional Weight could be placed diametrically opposite the position of balance weight 150. Alternatively, in either situation, the mass of the member 84 could be adjusted to achieve the desired result by distributing the masses to create the desired amount and position of mass unbalance.

A pair of stops 152 and 154 are placed on the top of the magnet 50 to snub the member 84, should it angularly displace through that distance.

In FIGURE 14 is a typical electrical circuit which is adapted to sense the angular displacement of member 84 and to apply restoring current to the torquing coil 82. The capacitors 156 and 158 are part of the mechanism itself. The common plate of the two capacitors is the surface of the magnet 50. The other two plates are the films 91 and 93. Electrical connection to magnet 50 and to films 91 and 93 has been described above. The remaining electrical components are external to the housing 45. Capacitors 156 and 158 together with resistors 160 and 162 form a Wheatstone Bridge, the input of which is connected to a source of alternating current 164, and the output of which is connected to the input of a balanced differential amplifier 166. The output of amplifier 166 is connected through a demodulator 168 to the torquing coil 82. The connections to the torquing coil 82 through certain films on the member 42 and the member 84 have been described.

The interior of the housing is filledfor examplewith a gas such as dry nitrogen. The movable member 84 is damped by the cushion of gas squeezed between the member 84 and the top of the magnet 50. The damping is symmetrical to cause both sides of the member 84 to clamp equally deflections in either direction.

It should also be noted that the separation slot between members 84 and 42 are formed by a pair of hinge slots 96 and 100 which are radially directed in opposite directions along a common diameter. The separation slot 106 is an arcuate slot which connects the radially outward end of slot 96 to the radially inward end of slot 100. Separation slot 104 is an arcuate slot which connects the radially outward end of slot 100 to the radically inward end of slot 96.

Plate 86 and 88 are present to oppose distortions which might result in member 84 from differential expansion between plates 91 and 93 and member 84.

The magnet 50 is shown as an annular shaped member, and is fabricated-for exampleof an aluminum nickel alloy such as Alnico, in which north and south poles appear at opposite ends of a preferred diameter because of the processing of the magnet in the presence of a magnet field. The diameter defining the north and south poles is positioned at right angles to the hinge line 170 between the member 84 and the member 42, The extreme ends of the section of magnet 50 in sectional view of FIG- URE 8 is substantially along the diameter between the north and south poles of the magnet 50. The return path for the magnetic field generated by the magnet 50 is through the gap in which electromagnetic coil 82 is positioned and through the flux ring 78. Thus, the magnet field direction of the permanent magnet within the gap between the magnet 50 and the return of the flux ring 78 is in a direction across the diameter of the magnet 50, between the north and south poles. The magnet field generated by the current in the electromagnetic coil 82 is perpendicular to that line and to the hinge line 170, thereby generating a maximum amount of torque by interaction between the two magnetic fields for a given amount of applied current, the torque (at least for the small angles involved) being precisely linear with the amount of current applied to the electromagnet coil 82.

The magnetic field strength of the permanent magnet tends to decrease as the temperature increases. To maintain the field strength constant Within the gap between magnet 50 and magnetic flux ring 78, in the device of this invention, the gap is varied inversely with the temerature, i.e. as the temperature increases the gap is reduced, and as the temperature decreases the gap is increase-d, along the axis between the north and south poles of the magnet. The temperature compensation is accomplished in this invention by means of a magnet compensation member 52. The member 52 mayfor example-be fabricated of aluminum with the anodized surface 70 acting as an electrical insulator between the aluminum and the magnet. Two bearing members 79 and 80 contact the flux ring 78 symmetrically on an axis perpendicular to the axis between the north and south poles of the magnet '0 and parallel to the hinge axis 170, as shown in FIGURE 13. As the temperature increases, the member 52 expands, whereby the bearing members 79 and 80 elongate the axis of the flux ring 78 in the direction parallel to axis 170, as shown by the dash line at 180. Extension of the axis of flux ring 78 in a direction along the axis 170 decreases the diameter of the flux ring in the direction perpendicular to axis 170 and parallel to the diameter of magnet 50 between the north and south poles, as shown by the dashed line at 182. Thus, as the temperature increases, the gap between the magnet 50 and the flux ring 78 in the region,of the north and south poles of the magnet 50, is decreased, thereby compensating for a decrease in the magnetization of the permanent magnet caused by increased temperatures.

In operation, the device shown in FIGURES 7 through 14 senses acceleration in the direction indicated by arrow 200 in FIGURE 8.

Acceleration in the direction of arrow 200 due to the eccentric loading of member 84, by weight or mass 150, causes member 84 to tend to tilt about the hinge axis 170. Slight tilting of the member 84 unbalances the capacitance bridge of capacitors 156 and 158 (FIG. 14) to apply a signal to the input of amplifier 166 which is demodulated by demodulator 168, which applies a current of appropriate amplitude and direction to torquing coil 82 to maintain member 84 in its predetermined neutral position. The current amplitude applied to torquing coil 82 is a measureof the amplitude of the applied acceleration, and the direction of current is a measure of the sense of the applied acceleration.

On the other hand, if member 84 is weighted in the manner shown in FIGURE 1, acceleration in the direction of arrow 200 tends to translate member 84 slightly from its no-acceleration position. The translation of member 84 is not sensed by the circuit of FIGURE 14 because the capacitors 156 and 158 are not unbalanced by the translation. Further, because the member 84 is oppositely sprung relative to the member 42 in a fashion similar to that shown and described in connection with the device of FIGURE 4, the translation of the member 84 does not introduce a net torque about the axis 170.

The restoring torque caused by the. interaction between the magnetic fields of electromagnet 82 and permanent magnet 50 is a pure torque. The current applied to magnet 82 is linearly related to the restoring torque because the field of the electromagnet 82 is maintained at right angles to the field of the permanent magnet 50.

Further, because of the compensating device of FIG- URE 13, whose operation has been explained above, the scale factor between the applied torque or the applied acceleration and the current in electromagnet 82 is not disturbed by changes of temperature over a relatively wide range.

The structure of the member 84 and the member 42 are relatively inexpensive to manufacture and, in fact can be manufac ured in m ss. p oduc ion with extreme accuracy because of the process of manufacture which is described herein.

Thus, the device of this invention is a very compact, and relatively inexpensive, accelerometer having extreme precision over relatively wide temperature ranges, in which the relation between the applied acceleration and the restoring current is exceptionally linear.

Although the invention has been described in detail above, it is not intended that the invention should be limited to the device described but only in accordance with the spirit and scope of the appended claims:

What is claimed is:

1. A device for sensing acceleration comprising:

an accelerometer housing;

an acceleration sensing member having separate radially exterior and radially interior portions joined by at least a pair of flexible hinge members, and supported in said housing for rotation of one of said portions relative to the other portion about a hinge line which passes through a central section of said radially interior portion, said one portion being eccentrically weighted;

a capacitance pick-off, including capacitance plates on said one rotatable portion of said sensing member, for detecting rotation of said one rotatable portion about said hinge line;

electromagnet means, carried by said rotatable portion;

permanent magnet means, positioned relative to said electromagnet means to cause interaction between the magnetic fields of said electromagnet means and said permanent magnet means, said interaction causing oppositely directed forces to be applied simultaneously to different segments of said rotatable portion, located on opposite sides of said hinge line, said forces being effective for rotation of said rotatable portion about said hinge line; and electronic means for providing a restoring signal to said electromagnectic means, in response to said detected rotation, whereby said forces reposition said rotatable portion to a predetermined neutral position.

2. A device as recited in claim 1 in which said hinge members are aflixed to opposite radial sides of the interior portion of said sensing member; whereby when said rotatable portion is translated, said hinge members apply restoring forces tending to result in only translational displacement of the rotatable portion.

3. A device as recited in claim 1 and further comprising:

a magnetic flux ring, associated with said permanent and electromagnets, positioned to create a working gap in the field of said permanent magnet into which the coil of said electromagnet is inserted.

4. A device as recited in claim 3 and further comprising:

temperature responsive forcing means engaging said magnetic flux ring to vary the width of said working gap in response to changes in temperature.

5. A device as recited in claim 1 in which said sensing member comprises a ceramic wafer having thin metallic films on the face thereof to form capacitance plates for said pick-off and to form hinge members connecting a housing-fixed ceramic portion and said movable portion.

6. A device as recited in claim 4 in which said sensing member comprises a ceramic wafer having thin metallic films on the face thereof to form capacitance plates for said pick-off and to form hinge members connecting a housing-fixed ceramic portion and said movable portion.

7. A device as recited in claim 6 in which said hinge members are beam springs.

8. A device as recited in claim 7 adapted to provide an output signal which is a function of said acceleration and in which said beam springs are affixed to opposite radial sides, on opposite ends of said interior portion of said sensing member.

9. A device for sensing acceleration comprising:

a substantially circular non-conducting wafer divided into two portions, a radially exterior portion and a radially interior portion, by a dividing slot formed in said Wafer, said slot being defined by a pair of hinge slots which are radially directed along opposite radii of the same diameter and each extending from a first radius to a second radius of the circular shape of said wafer and by a pair of substantially identical arcuate separation slots symmetrically disposed on said wafer connecting the outer end of said first hinge slot to the inner end of said second hinge slot and the outer end of said second hinge slot to the inner end of said first hinge slot;

at least one thin film metallic hinge bridging said first hinge slot and at least one thin film metallic hinge bridging said second hinge slot to allow relative angular deflection between said portions;

a housing attached to said inner portion and enclosing said inner and outer portions;

means for sensing the relative rotation between said outer and inner portions;

magnetic means for applying rotational forces to said outer portion; and

electrical means, responsive to said sensing means, for providing a restoring signal to said magnetic means, whereby said outer portion is repositioned to a predetermined neutral angular position and said restoring signal is a function of the acceleration applied to the device.

10. A device as recited in claim 9 in which said hinges are beam springs.

11. A device as recited in claim 10 in which said beam springs are afiixed to opposite radial sides on opposite ends of said inner portion, whereby when said outer portion is translated said hinge members apply restoring forces tending to produce only translational displacement of said outer portion.

12. The apparatus of claim 9 wherein said magnetic means includes a plate shaped permanent magnet having its axis of magnetism extending transversely therethrough; and an electromagnet coil attached to said outer portion and encompassing said permanent magnet so as to extend into the field of said permanent magnet; whereby the coil, upon being energized, produces a magnetic field which interacts with the unidirectional field of the permanent magnet so that forces, experienced on opposite sides of the coil, are oriented in opposite directions and the two magnetic fields cooperate to apply restoring torquing forces to said outer portion.

13. The device as recited in claim 12 wherein:

said sensing means includes at least two substantially identical electrical capacitance plates formed on said outer portion symmetrically displaced with respect to the hinge line of said hinge members and said hinge slots; and stationary capacitor plates, adjacent to said capacitance plates formed on said outer portion; and

said electrical means is connected between said capacitance plates and said electromagnet coil to cause said electromagnet coil to generate a magnetic field in response to deflection of said outer portion relative to said inner portion to hold said outer portion at a predetermined angular neutral position.

14. A device as recited in claim 13 in which said permanent magnet means is a substantially circular metallic disc permanent magnet attached to said housing and having its face substantially parallel to the capacitive plates upon said outer portion when said outer portion is in its neutral position, the face of said electromagnet forming a housing-fixed electrode, and a ferromagnetic flux return member for conducting a magnetic flux between the poles of said permanent magnet externally to said permanent magnet; and

in which said electromagnet coil is positioned in a gap between said flux return member and said permanent magnet.

15. A device as recited in claim 14 in which said flux return member is a flux ring; and further comprising a temperature sensitive member, bearing against said flux ring, adapted to deform said flux ring in response to temperature changes to vary the gap between said flux ring and said permanent magnet in the region of the poles of said permanent magnet.

16. A device as recited in claim 15 and further comprising an alignment pin for aligning said ceramic inner portion, said permanent magnet, said flux ring, and said temperature sensitive member.

17. A device as recited in claim .16 in which said temperature sensitive member is centered on said alignment pin and has radial projections therefrom, bearing against said flux ring along the hinge line of said hinge members to shorten the gap between said flux ring and said permanent magnet on an axis in the plane of said permanent magnet and perpendicular to said hinge line.

1-8. A device as recited in claim 17 in which said temperature sensitive member is made of aluminum and formed into a disc having bearing projections thereform, contacting said flux ring along said hinge line.

19. A device as recited in claim 9 and further comprising additional metal films on said wafer to oppose stresses in said wafer due to differential expansion.

References Cited UNITED STATES PATENTS 3,339,419 9/1967 Wilcox 73517 2,959,459 1l/l960 Ryan 3467 RICHARD G. QUEISSER, Primary Examiner J. W. WHALEN, Assistant Examiner US. Cl. X-IR- 735 18, 519

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