GB2232489A

GB2232489A - Silicon acceleration sensor

Info

Publication number: GB2232489A
Application number: GB9012398A
Authority: GB
Inventors: Louis Pasqual Farace; John Dunning Titus
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1989-06-05
Filing date: 1990-06-04
Publication date: 1990-12-12
Anticipated expiration: 2010-06-04
Also published as: DE4015464A1; GB9012398D0; GB2232489B

Description

:2:2:3:2 1 j-1 E.) DOUBLEINTEGRATING SILICON ACCELERATION SENSING DEVICE

BACKGROUND OF THE INVENTION

This invention relates, in general, to acceleration sensing devices, and more specifically, to micromachined acceleration sensing devices which determine the distance travelled by an object.

Acceleration sensing devices such as accelerometers are used in a variety of commercial and military applications. The automobile industry, for example, uses accelerometers to activate certain safety devices, such as air bags, during accidents. In military applications, accelerometers aid in measuring the velocity or distance travelled by a missile or projectile during flight. This allows the missile's or projectile's fuze to be armed at the appropriate distance along the missile's or projectile's trajectory. Such accelerometers require sophisticated electronic circuitry to provide a measure of time. The time and the output from the accelerometer are combined in a microcomputer which integrate the output over time to yield velocity or distance travelled.

In the military arena, acceleration driven devices conventionally comprise a mechanical runaway escapement coupled to an eccentric gear. This type of accelerometer is double integrating, or in other words, measures a force caused by acceleration of the accelerometer and estimates the distance travelled by the projectile or missile to which it is attached. The major problem with the mechanical runaway escapement/eccentric gear accelerometers is the low accuracy and high probability of mechanical failure due to the high number of moving parts.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an acceleration sensing device which can determine the distance travelled by an object from the object's acceleration by mechanical double integration.

2 Another object of a preferred embodiment of the present invention is to provide an inexpensive double integrating acceleration sensing device which is miniaturized and solid state.

An object of another embodiment of the present invention is provide an improved flexible silicon diaphragm According to one aspect of the invention there is provided an acceleration sensing device comprising a housing defining a sealed cavity; a flexible diaphragm dividing the sealed cavity into two cavities; a generally incompressible fluid filling the cavities; fluid coupling means for providing a predetermined flow rate fluid coupling between the cavities in response to movement of the diaphragm due to acceleration of the device and means responsive to movement of the diaphragm for providing an indication of the acceleration of the device.

According to a second aspect of the invention there is provided a flexible micromachined diaphragm comprising:

silicon membrane means; a plurality of dimple means for increasing the flexibility of said silicon membrane means; and said plurality of dimple means embedded within at least one surface of said silicon membrane means.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a cut-away side view of a double integrating accelerometer according to the present invention.

Figure 2 is top view of a "dimpled" diaphragm according 30 to the present invention.

Figure 3 is a side view of the "dimpled" diaphragm of Figure 2.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a cut-away side view of a double integrating silicon acceleration sensing device (ASD) 10. ASD 10, in its preferred embodiment, is micromachined and comprises diaphragm 12, dynamic plate 14, static plate 16, orifice 18, and rigid housing 20.

Rigid housing 20 is hermetically sealed container which forms a housing cavity 22. Housing cavity 22 is divided by diaphragm 12 into two cavities, shallow cavity 24 and deep cavity 26. Shallow cavity 24 and deep cavity 26 are filled with a fluid that is relatively incompressible over a given range of operating pressure. By hermetically sealing rigid housing 20, the properties and performance of the fluid are not affected by changes in temperature.

Dynamic plate 14 is secured to a central portion of a face 36 of diaphragm 12 (also shown in Figure 2.). Face 36 defines one wall of shallow cavity 24. Static plate 16 is secured to face 28 of rigid housing 20 and opposite of and adjacent to dynamic plate 14. Face 28 of rigid housing 20 defines a second wall of shallow cavity 24. The other walls of shallow cavity 24 are further defined by rigid housing 20. Dynamic capacitance plate 14 and static capacitance plate 16 form a capacitor 32 which is electronically coupled to a external control circuit (not shown).

Figure 2 shows a top view of diaphragm 12 in its preferred embodiment. Diaphragm-12 is a flexible membrane, and is preferably comprised of silicon. Silicon has been used for micromachined diaphragms, see, Micromechanics: The Eyes and Ears of Tomorrow's Computers, Business Week, March 17 1986, pg. 88; Young's Modulus Measurements of Thin Films Using Micromechanics, Kurt E Peterson and C R Guarnierei J. App. Phys., 50(11), November 1979; Silicon as a Mechanical Material, Proceedings of the IEEE, Vol. 70, No. 5, May 1982; but still maintains some brittle properties.

Non-micromachined silicon diaphragms have been corrugated to increase the diaphragm's flexibility. See, Design of Corrugated Diaphragms, ASME Transactions, Vol. 79, 1957. However, corrugating the diaphragms creates points of high stress in the silicon at each corrugation point. This high stress increases the possibility of failure in the diaphragm.

- 4 Diaphragm 12 increases the flexibility of the silicon without adding points of high stress by "dimpling", rather than corrugating, face 26. Specifically diaphragm 12 comprises a number of semi-spherical dimples 32 arranged in a hexagonal close pack arrangement as shown in Fig. 2. Figure 3 shows dimples 32 in a cut-away side view of diaphragm 12 showing the semicircular nature of dimples 32. By placing dimples 32 in a hexagonal close pack arrangement, a maximum number of dimples 32 can be incorporated within diaphragm 12.

Side 34 of diaphragm 12 in Fig. 2 are secured to, and contained within, rigid housing 20 to completely dissect housing cavity 22.

Orifice 18 is coupled between shallow cavity 24 and deep cavity 26 to allow fluid f - low between deep cavity 26 and shallow cavity 24. Because of the restricting shape or orifice 18, fluid flow can be controlled to facilitate double integration of acceleration as discussed below.

Referring again to Fig. 1, when ASD 10 is initially at rest, dynamic plate 14 and static plate 16 form gap 30 having gap Xg. Capacitor 32, at gap Xg, has an initial capacitance Cl. When ASD 10 is subjected to any given acceleration in a direction generally along the axis of arrow 40, the mass of diaphragm 12 and dynamic plate 14 cause diaphragm 12 to deflect into deep cavity 26. As diaphragm 12 deflects into deep cavity 26, a pressure differential is created between shallow cavity 24 and deep cavity 26. This differential forces the fluid within deep cavity 26 to flow towards shallow cavity 24. The rate of movement of the fluid is proportional to the hydraulic diameter of orifice 18 and the square root of the pressure differential.

The deflection of diaphragm 12 cause Xg to increase changing the capacitance of capacitor 32. As Xg continues to increase, the capacitance of capacitor 32 continues to change yielding analog outputs. When ASD 10 is used as a switch, ASD 10 activates an external switch when a predetermined threshold capacitance is reached.

ASD 10 can further comprise capacitor plates in deep cavity 26. As shown in Figure 1, ASD 10 may comprise plate 42 coupled opposite to dynamic plate 14 on diaphragm 12, and plate 44 coupled to housing 22 opposite plate 42 in deep cavity 26. As with dynamic plate 14 and static plate 16, plates 42 and 44 form a capacitor 46. As diaphragm 12 deflects into deep cavity 26, a change in the capacitance of capacitor 46 may be combined with the change in capacitance of capacitor 32 to produce a differential capacitance.

The distance travelled by an object initially at rest and subject to a acceleration is the double integral of acceleration with respect to time. ASD 10 is mechanical device which double integrates constant acceleration and pseudo-double integrates non-constant acceleration measured over time to give the distance travelled by an object to which ASD 10 is attached. The following mathematical discussion illustrates the double integrating property of ASD for constant acceleration. Constant acceleration is illustrated due to ease of calculation. However, it should be recognised that distance may be determined by ASD 10 for all applications where acceleration versus time is well behaved and does not contain significant steps.

Diaphragm 12 can be analagised to a damped spring having the mathematical relationship:

(1) F = ma + pv2 - kx where F is the force applied to diaphragm 12, m is the mass of diaphragm 12 and dynamic plate 14, a is the acceleration of diaphragm 12, p is the hydraulic diameter of orifice 18, v is the velocity of the fluid flowing th rough orifice 18 (velocity of the deflection of diaphragm 12 and the fluid flow are equal), k is the spring constant of diaphragm 12, and x is the deflection of diaphragm 12. After a short period, pv2>>ma and kx Therefore, the equation becomes:

(2) F = pv2 For an object having a constant acceleration, the relationship between distance and acceleration is given by:

(3) x = 1/2amt2 where t is time and am is the acceleration ofthe object.

The force exerted on diaphragm 12 by the object is:

(4) F = m am Combining equations (2) and (4), and solving for v yields:

(5) v = [(c/0)mam]0.5 Furthermore, the deflection Xgl of diaphragm 12 can be found by (6) Xgl = Vt Combining equations (3), (5), and (6), and combining terms yields:

(7) x =- C1Xgl where c' is a proportionality constant dependent upon the hydraulic diameter of orifice 18. The value Xg represents the distance travelled by the object. Thus it can be seen that the displacement of diaphragm 12 (and therefore the change in capacitance) relative to an object to which ASD 10 is attached is directly related to the displacement of the accelerating object. ASD 10 in this way double integrates acceleration to produce the distance travelled by the object.

Accelerometer 10 may be used to determine the distance travelled by a spinning object having a near constant rotational velocity. The distance travelled by the object along its trajectory is directly related to its rotation. Therefore, the centrifugal force on diaphragm 12 of accelerometer 10 can combine with the double integrating action to produce a turns counting effect.

To illustrate, equation (3) is combined with the centrifugal force field (8) F = cmC02 where 0) is the rotation of the object in radians per second, to yield:

(9) v = CCO Since deflection speed of diaphragm 12 is directly proportional to the spin rate of the object, the number of rotations in any -ime 'It" will likewise be proportional to 1 - 7 the displacement of diaphragm 12 in the same time "t". Further, since the distance travelled by the object is directly related to the object's rotation, ASD 10 determine distance travelled from rotations of the object.

Thus there has been provided, in accordance with the present invention, a double integrating silicon accelerometer that fully satisfies the objects, aims, and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.

Claims

1. An acceleration sensing device comprising a housing defining a sealed cavity; a flexible diaphragm dividing the sealed cavity into two cavities; a generally incompressible fluid filling the cavities; fluid coupling means for providing a predetermined flow rate fluid coupling between the cavities in response to movement of the diaphragm due to acceleration of the device and means responsive to movement of the diaphragm for providing an indication of the acceleration of the device.

2. The device of claim 1 wherein the flexible diaphragm has a plurality of semi-spherical dimples in at least one face thereof.

3. The device of claim 2 wherein the semi-spherical dimples are arranged in a hexagonal close pack pattern.

4. The device of any preceding claim wherein the flexible diaphragms is formed of silicon material.

5. The device of any preceding claim wherein the means responsive to movement of the diaphragm comprises capacitive means.

6. The device of claim 5 wherein the capacitive means comprises a first capacitor plate located in a first of the two cavities and coupled to one side of the flexible diaphragm and a second capacitor plate facing the first capacitor plate and coupled to the housing of the first cavity.

1 9

7. The device of claim 6 wherein the capacitive means comprises a third capacitor plate located in a second of the two cavities and coupled to the flexible diaphragm on the opposite face of the diaphragm to the first capacitor plate and a fourth capacitor plate spaced from the third capacitor plate and coupled to the housing of the second cavity.

8. The device of any preceding claim wherein the fluid coupling means is an orifice for controlling the rate of flow of fluid.

9. A flexible micromachined diaphragm comprising: silicon membrane means; a plurality of dimple means for increasing the flexibility of said silicon membrane means; and said plurality of dimple means embedded within at least one surface of said silicon membrane means.

10. A flexible micromachined diaphragm according to claim 9 wherein said plurality of dimple means are arranged in a hexagonal close- pack pattern.

Published 1990 at The Patent Office. State House, 66 71 High Holborn, London WClR4TP. Further copies maybe obtained from The Patent Office. Sales Branch. St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent, Con. 1187