Micro-miniature accelerometer

• US 4,736,629 A
• Filed: 12/20/1985
• Issued: 04/12/1988
• Est. Priority Date: 12/20/1985
• Status: Expired due to Term

• Current Assignee: Silicon Designs Incorporated
• Original Assignee: Silicon Designs Incorporated
• Inventors: Cole, John C.
• Primary Examiner(s): CHAPMAN JR, JOHN E
• Application Number:
  US06/812,137
• Time in Patent Office: 844 Days
• Field of Search: 73/514, 73/517 R, 73/517 B, 324/60 C, 324/60 CD, 324/61 R, 361/280
• US Class Current: 73/514.32
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: G01P 2015/0831 the mass being of the paddle type having the pivot axis between the longitudinal ends of the mass, e.g. see-saw configuration

Abstract

The accelerometer comprises a substrate (52), a metallic movable plate (50), and a mounting system (54) for mounting the movable plate such that it is positioned above the substrate and can rotate about a flexure axis (64) that is above and is substantially parallel to the substrate. The flexure axis divides the sensing element into first (66) and second (68) sections. The total moments of the first and second sections about the flexure axis are different, such that acceleration normal to the substrate tends to rotate the sensing element about the flexure axis. A first electrode (74) is mounted by the substrate adjacent the first section to form a first capacitor (C_A), and a second electrode (76) is mounted by the substrate adjacent the second section to form a second capacitor (C_B). A detector for measuring the relative capacitances of the first and second capacitors is provided comprising an integrator (166), an inverting amplifier (168), and switches (160, 162, 164) for periodically charging and discharging the capacitors in response to a clock signal. In one embodiment, the sensing element includes an internal opening (16, 56, 126) and the mounting system is positioned within the opening and includes a pedestal mounted to the substrate, and torsion bars (20, 22, 118, 120) and/or a beam (60, 114) connecting the pedestal to the sensing element. The sensing element preferably comprises a metallic plate, and the substrate preferably comprises a semiconductor upon which the detector and electrodes are fabricated in a single step process requiring no final assembly of components. In an embodiment adapted for high g applications, a pedestal (146) divides a plate member into first (148) and second (150) cantilevered beams that flex in the same direction in response to a given acceleration.

Claims

1. An accelerometer, comprising:

a substrate;

a sensing element comprising a metallic member, the sensing element including an internal opening;

mounting means positioned within said opening for mounting the sensing element such that the sensing element is positioned above the substrate and can rotate about a flexure axis that is above and substantially parallel to the substrate, the mounting means comprising a pedestal mounted to the substrate and flexible support means connected between the sensing element and the pedestal, the support means comprising a single beam extending between the sensing element and pedestal in a direction parallel to the sensing element and substrate and perpendicular to the flexure axis, the beam forming the only connection between the sensing element and pedestal, the flexure axis dividing the sensing element into a first section on one side of the flexure axis and a second section on the opposite side of the flexure axis from the first section, the total moments of the first and second sections about the flexure axis being different from one another, whereby acceleration normal to the substrate tends to rotate the sensing element about the flexure axis; and

means for sensing rotation of the sensing element about the flexure axis.

2. The accelerometer of claim 1, wherein the sensing element has a center of mass that is located approximately at the point where the beam is connected to the sensing element, whereby the flexure axis intersects the beam at a location spaced from the pedestal by a distance equal to one third the length of the beam.

3. An accelerometer, comprising:

a substrate;

a sensing element comprising a metallic member;

mounting means for mounting the sensing element above the substrate, the mounting means comprising a pedestal connected between the substrate and the sensing element, the pedestal dividing the sensing element into a first beam to one side of the pedestal and a second beam on the opposite side of the pedestal, each beam being adapted to flex towards and away from the substrate in response to acceleration normal to the substrate;

first and second electrodes mounted by the substrate adjacent the first and second beams respectively, the first electrode and first beam together forming a first capacitor, the second electrode and second beam together forming a second capacitor, and the first and second beams and first and second electrodes being arranged such that the capacitances of the first and second capacitors change by different amounts in response to acceleration normal to the substrate; and

detection means coupled to the first and second capacitors for sensing differential change in said capacitors.

4. The accelerometer of claim 3, wherein each beam comprises a metal plate mounted to the pedestal such that the plate is substantially parallel to the substrate.

5. The accelerometer of claim 3, wherein the substrate comprises a semiconductor.

6. The accelerometer of claim 3, wherein the first and second electrodes are identical in size and shape, and symmetrically positioned on opposite sides of the pedestal, and wherein the total moment of the first beam about the pedestal is different from the total moment of the second beam about the pedestal.

7. An accelerometer for measuring acceleration along a sensitive axis, comprising:

(a) an inertial mass;

(b) mounting means for mounting the inertial mass such that the inertial mass tends to move in response to acceleration along the sensitive axis;

(c) first and second capacitors associated with the inertial mass such that when the inertial mass moves, the capacitances of the first and second capacitors change by unequal amounts, each of the first and second capacitors comprising first and second plates; and

(d) detection means for measuring the relative capacitances of the first and second capacitors and providing a corresponding output signal, the detection means comprising;

(i) means for providing first and second reference voltages;

(ii) an output terminal at which the output signal is produced;

(iii) an inverting amplifier having its input connected to the output terminal;

(iv) an integrator having its output connected to the output terminal;

(v) a first switch having a common terminal and A and B terminals, the common terminal of the first switch being connected to the first plates of the first and second capacitors, the A terminal of the first switch being connected to receive the first reference voltage and the B terminal of the first switch being connected to the input of the integrator;

(vi) a second switch having a common terminal and A and B terminals, the common terminal of the second switch being connected to the second plate of the first capacitor, the A terminal of the second switch being connected to the output of the inverting amplifier, and the B terminal of the second switch being connected to receive the second reference voltage;

(vii) a third switch having a common terminal and A and B terminals, the common terminal of the third switch being connected to the second plate of the second capacitor, the A terminal of the third switch being connected to receive the second reference voltage, and the B terminal of the third switch being connected to the output terminal; and

(viii) means for causing the first, second and third switches to periodically change in concert between state A wherein all switches connect their common terminals to their A terminals and state B wherein all switches connect their common terminals to their B terminals.

8. An accelerometer, comprising:

a substrate;

a sensing element comprising a metallic member, the sensing element including an internal opening;

mounting means positioned within said opening for mounting the sensing element such that the sensing element is positioned above the substrate and can rotate about a flexure axis that is above and substantially parallel to the substrate, the mounting means comprising a pedestal mounted to the substrate and flexible support means connected between the sensing element and the pedestal, the support means comprising an element surrounding the pedestal, a beam extending between the pedestal and the element, and a pair of torsion members extending in opposite directions from the element to the sensing element, the flexure axis dividing the sensing element into a first section on one side of the flexure axis and a second section on the opposite side of the flexure axis from the first section, the total moments of the first and second sections about the flexure axis being different from one another, whereby acceleration normal to the substrate tends to rotate the sensing element about the flexure axis; and

means for sensing rotation of the sensing element about the flexure axis.