CAPACITANCE ACCELEROMETER

• US 3,709,042 A
• Filed: 05/14/1969
• Issued: 01/09/1973
• Est. Priority Date: 05/14/1969
• Status: Expired due to Term

• Current Assignee: Shih-Ying Lee, Yao Tzu Li
• Original Assignee: Shih-Ying Lee, Yao Tzu Li
• Inventors: Lee, Shih-Ying, Li, Yao Tzu
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Goldstein, Herbert
• Application Number:
  US04/824,566
• Time in Patent Office: 1,336 Days
• Field of Search: 73/71.2,515,516,517
• US Class Current: 73/514.13
• CPC Class Codes: G01P 15/125 by capacitive pick-up

Abstract

The mass of a seismic suspension serves as the center plate of a balanced differential capacitor suitable for use in an open loop accelerometer system. The flexures constrain the seismic mass to motion in one direction. The trapped gas between the plates of the differential capacitor provides squeezed film damping. The differential capacitor modulates circuitry such as a differential pulse width modulation system to provide a high-level output.

Claims

1. A unitary accelerometer for measurement of acceleration along a predetermined section comprising a seismic mass, said seismic mass being made of a flexible member, said seismic mass being bent to a domed configuration, said domed seismic mass having two major surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby the changes in the capacitance of said differential capacitor are a function of the displacement of the seismic mass.

2. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major planar surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means constraining motion of said seismic mass along a predetermined direction with respect to a casing, said flexure means comprising, an element having a longitudinal axis, said element having at least two regions of substantially lower stiffness spaced along said axis, each of said regions of reduced stiffness comprising two portions of reduced cross-section, said portions of reduced cross-section lying on opposite sides of the longitudinal axis of said element, whereby said flexure means serves as a linkage with four pivot points, one end of said element being connected to said casing, and the other end of said element being connected to said seismic mass.

3. Apparatus according to claim 2 employing at least three of said elements, each of said elements having one end attached to said casing and the other end attached to said seismic mass, whereby said elements provide rectilinear motion of said seismic mass with respect to said casing.

4. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby changes in the capacitances of said differential capacitor are a function of the displacement of the seismic mass, and electronic measurement system, said differential capacitor serving to control the output of said electronic system, whereby the output of said system is linearly proportional to the displacement of said seismic mass, said differential capacitor having four surfaces, said surfaces being arranged in facing pairs, and the space between said seismic mass in the capacitor plates being filled with gas, the resulting film of gas between facing pairs of the surfaces serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate which it approaches when an acceleration force is applied to the transducer, at least one of said surfaces being provided with grooves, saId grooves serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate.

5. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a stationary capacitor plate adjacent each of said surfaces, said seismic and stationary capacitor plates serving to provide a differential capacitor, said seismic mass being formed as a central portion of a disc of material, said central portion being surrounded by an area containing perforations, said perforations being formed in the shape of slots producing a reduction in stiffness, whereby said area containing perforations serves to provide supporting flexure means for said seismic mass, said disc of material having two major planar surfaces, said major surfaces being parallel and equidistant throughout the entire area of the disc, and each of said capacitor plates having a planar surface adjacent a planar surface of said disc, whereby the major planar surfaces of said disc and the planar surfaces of said adjacent capacitor plates may be accurately and precisely aligned in parallel planar relationship to provide a differential capacitor of very accurately predetermined characteristics.

6. Apparatus according to claim 5 wherein said perforations are formed as spirals.

7. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby changes in the capacitance of said differential capacitor are a function of the displacement of said seismic mass, said differential capacitor having four surfaces, said surfaces being arranged in facing pairs, the space between said seismic mass and the capacitor plates being filled with gas, the resulting film of gas between facing pairs of surfaces serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate which it approaches when an acceleration force is applied to the transducer, the damping being provided by the energy dissipated by the motion of said gas between the seismic mass and the approached capacitor plate, at least one capacitor plate being provided with at least one hole, said hole providing an effective cross-section substantially larger than the effective cross-section provided by the spacing between the facing pairs of surfaces through which the film of gas is squeezed, said larger cross-section providing substantially lower resistance to the passage of the gas than the resistance presented by the path between facing pairs of surfaces, said hole serving to reduce the length of the path over which gas travels as said seismic mass approaches the adjacent capacitor plate, said reduced path serving to reduce the elastic effect of compressed gas which would otherwise be provided by said film of gas, said reduced elastic effect permitting operation at higher frequencies without degraded performance due to the spring effect of compressed gas, and said reduced path serving to reduce the viscous damping caused by motion of said film of gas between said diaphragm and the adjacent capacitor plate, whereby the effect of damping ratio is maintained constant over an extended useful high-frequency range, and whereby said damping ratio is maintained at a desired damping ratio as the gas is squeezed between the seismic mass and the capacitor Plate toward which the seismic mass is deflected by an applied acceleration force.

8. Apparatus according to claim 7 wherein at least one of said capacitor plates is provided with a plurality of holes, the number of holes being chosen to reduce the length of the paths over which the gas travels as said seismic mass approaches the adjacent capacitor plate, said reduced paths serving to reduce the elastic effect of the compressed gas which would otherwise be provided by said film of gas, said reduced elastic effect permitting operation at very high frequencies without degraded performance due to the spring effect of compressed gas, and said reduced paths serving to reduce the viscous damping caused by motion of said film of gas between said diaphragm and the adjacent capacitor plate until it reaches the nearest hole, whereby the effective damping ratio is maintained constant over an extended useful high frequency range, and whereby said damping ratio is maintained at a desired damping ratio as the gas is squeezed between the seismic mass and the capacitor plate toward which the seismic mass is deflected by an applied acceleration force.

9. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby changes in the capacitance of said differential capacitor are a function of the displacement of said seismic mass, said differential capacitor having four surfaces, said surfaces being arranged in facing pairs, the space between said seismic mass and the capacitor plates being filled with gas, the resulting film of gas between facing pairs of surfaces serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate which it approaches when an acceleration force is applied to the transducer, the damping being provided by the energy dissipated by the motion of said gas between the seismic mass and the approached capacitor plate, said seismic mass being provided with at least one hole, said hole providing an effective cross-section substantially larger than the effective cross-section provided by the spacing between the facing pairs of surfaces through which the film of gas is squeezed, said larger cross-section providing substantially lower resistance to the passage of the gas than the resistance presented by the path between facing pairs of surfaces, said hole serving to reduce the length of the path over which gas travels as said seismic mass approaches the adjacent capacitor plate, said reduced path serving to reduce the elastic effect of compressed gas which would otherwise be provided by said film of gas, said reduced elastic effect permitting operation at higher frequencies without degraded performance due to the spring effect of compressed gas, and said reduced path serving to reduce the viscous damping caused by motion of said film of gas between said diaphragm and the adjacent capacitor plate, whereby the effect of damping ratio is matintained constant over an extended useful high-frequency range, and whereby said damping ratio is maintained at a desired damping ratio as the gas is squeezed between the seismic mass and the capacitor plate toward which the seismic mass is deflected by an applied acceleration force.

10. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a capacitor plaTe adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby changes in the capacitance of said differential capacitor are a function of the displacement of said seismic mass, said differential capacitor having four surfaces, said surfaces being arranged in facing pairs, the space between said seismic mass and the capacitor plates being filled with gas, the resulting film of gas between facing pairs of surfaces serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate which it approaches when an acceleration force is applied to the transducer, the damping being provided by the energy dissipated by the motion of said gas between the seismic mass and the approached capacitor plate, said seismic mass being formed as a thin diaphragm, said diaphragm being secured to a casing around the perimeter, the energy dissipated as said gas moves between said diaphragm and the approached capacitor plates serving to provide damping, and said diaphragm serving as the spring restraint to motion of the seismic mass, said diaphragm containing at least one hole, said hole providing an effective cross-section substantially larger than the effective cross-section provided by the spacing between the facing pairs of surfaces through which the film of gas is squeezed, said larger cross-section providing substantially lower resistance to the passage of the gas than the resistance presented by the path between facing pairs of surfaces, said hole serving to reduce the length of the path over which said gas travels as said diaphragm approaches the adjacent capacitor plate, said reduced path serving to reduce the elastic effect of compressed gas which would otherwise be provided by said film of gas, said reduced elastic effect permitting operation at higher frequencies without degraded performance due to the spring effect of compressed gas, and said reduced path serving to reduce the viscous damping caused by motion of said film of gas between said diaphragm and the adjacent capacitor plate, whereby the effective damping ratio is maintained substantially constant over an extended useful frequency range, and whereby said damping ratio is maintained at a desired damping ratio as the gas is squeezed between the seismic mass and the capacitor plate toward which the seismic mass is deflected by an applied acceleration force.

11. A unitary accelerometer for measurement of acceleration along a predetermined direction comprising a seismic mass, said seismic mass having two major surfaces, a capacitor plate adjacent each of said surfaces, flexure means providing a predetermined spring resistance to motion of said seismic mass along said predetermined direction, said flexure means having substantially higher spring resistance to displacement in other directions, said seismic mass and capacitor plates serving to provide a differential capacitor, whereby changes in the capacitance of said differential capacitor are a function of the displacement of said seismic mass, said differential capacitor having four surfaces, said surfaces being arranged in facing pairs, and the space between said seismic mass and the capacitor plates being filled with gas, the resulting film of gas between facing pairs of surfaces serving to provide a predetermined damping ratio as the film of gas is squeezed between the seismic mass and the capacitor plate which it approaches when an acceleration force is applied to the transducer, the damping being provided by the energy dissipated by the motion of said gas betweeN the seismic mass and the approached capacitor plate, said seismic mass including a central body, said central body having two major planar surfaces, said planar surfaces being parallel to and equidistant from each other, two discs of material, each of said discs having two major parallel planar surfaces, one of said discs being affixed to each major planar surface of said central body, the central portions of said disc and said body serving as a seismic mass, a casing, the outer portions of said discs being affixed to the casing, and the portions of said discs between said central body and the casing serving to provide the supporting flexure means, the surface of the central portions of said discs not affixed to said central body serving as capacitor plates, said uniform planar central body and discs serving to permit extremely accurate alignment of said seismic mass in parallel planar relationship with adjacent planar capacitor plates in a casing, whereby transducer characteristics of extreme accuracy and predetermined characteristics may be obtained.

12. Apparatus according to claim 11 wherein said central portion of each of said discs is surrounded by an area containing perforations, said perforations serving to reduce stiffness, said areas of reduced stiffness serving to provide the supporting flexure means.