Accelerometer With Reduced Extraneous Vibrations Owing To Improved Return Movement

• US 20080092651A1
• Filed: 08/11/2004
• Published: 04/24/2008
• Est. Priority Date: 08/13/2003
• Status: Active Grant

• Current Assignee: Sercel (Cggveritas), Shell Oil Company (Shell Plc)
• Original Assignee: Sercel (Cggveritas)
• Inventors: Menard, Jean-Paul, Moreau, Maurice
• Granted Patent: US 7,552,638 B2
• Time in Patent Office: Days
• US Class Current: 73/488
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

The invention relates to an accelerometer comprising a moving mass (5) and a fixed part (2), which uses variations in capacitance (3, 4) in order to detect the movement of the mass (5). The inventive accelerometer comprises a first series of electrodes (3, 4) which are solidly connected to the mass (5) and which are interdigitated with a series of electrodes (3, 4) which are solidly connected to the fixed part (2). According to the invention, each moving electrode (4), together with an adjacent fixed electrode (3), forms a capacitance which varies according to the position of the moving mass (5). The accelerometer further comprises an electronic circuit which is used to: (i) detect the variation in at least one capacitance between the moving mass (5) and the fixed part (2), in the form of a moving mass displacement indicator; and also (ii) generate an electrostatic stress in order to return the moving mass (5) to the initial position thereof. Moreover, the aforementioned electronic circuit is provided in order to generate the electrostatic return stress in such a way that it is automatically controlled by a preceding displacement measurement. The invention is characterized in that the recurring return stress thus generated is specifically selected such that the mechanical power frequency spectrum (10, 20) thereof has an essentially zero power zone at the mechanical resonance frequency of the electrodes (4) of the moving mass (5) and/or of the fixed part (2).

Claims

1. An accelerometer with a moving mass (5) and a fixed part (2) using variations in capacitance (3, 4) in order to detect the movement of the mass (5), wherein a first series of electrodes (3, 4) solidly connected to the mass (5) is interdigitated with a series of electrodes (3, 4) solidly connected to the fixed part (2), each moving electrode (4) forming, with an adjacent fixed electrode (3), a variable capacitance depending on the position of the moving mass (5), the accelerometer further comprising an electronic circuit provided for detecting the variation of at least one capacitance between the moving mass (5) and the fixed part (2) as an indicator of the displacement of the moving mass, and to also generate an electrostatic force for returning the moving mass (b) to its initial position, the electronic circuit being provided for generating the electrostatic return force in a way controlled by a previous displacement measurement, characterized in that the thus generated repeated return force is specifically selected so that its frequency mechanical power spectrum (10, 20) has a substantially zero power zone at the mechanical resonance frequency of the electrodes (4) of the moving mass (5) and/or of the fixed part (2).

2. The accelerometer according to claim 1, characterized in that the control means perform repeated return of the moving mass in time slots (Ta), and in that the frequency transform (20) of the corresponding square wave signal (20) has a return to substantially zero at the resonance frequency of the electrodes (3, 4) of the moving part (5) and/or of the fixed part (2).

3. The accelerometer according to claim 2, characterized in that the average slot time is predetermined so that the frequency transform (20) of the square wave signal (20) has a return to substantially zero (1/Ta) at the resonance frequency of the electrodes (3, 4) of the moving part (5) and/or of the fixed part (2).

4. The accelerometer according to claim 3, characterized in that the average slot time (Ta) is selected so that its frequency transform (20) has a first return to zero (1/Ta) at the resonance frequency of the electrodes (4) of the moving part (5) and/or the fixed part (2).

5. The accelerometer according to any of claims 1 to 4, characterized in that the electrostatic return force is applied in time slots with an average width Ta, and in that Ta is selected according to the relationship 1/Ta=F where F is the resonance frequency of the electrodes (4) of the moving part (5) and/or the fixed part (2).

6. The accelerometer according to the preceding claim, characterized in that the frequency transform of the square wave signal has the shape of a cardinal sine.

7. The accelerometer according to any of the preceding claims, characterized in that electrode(s) (3, 4) of the moving mass (5) and/or the fixed part (2) each have a section, the width of which varies towards their free end.

8. The accelerometer according to claim 7, characterized in that the electrodes (3, 4) of the moving mass (5) and/or of the fixed part (2) have at least one part with a continuous change in section width.

9. The accelerometer according to any of claims 7 or 8, characterized in that the electrodes (3, 4) of the moving mass (5) and/or the fixed part (2) have at least one part, the section width of which has steep changes.

10. The accelerometer according to any of claims 7 to 9, characterized in that the electrodes (3, 4) of the moving mass (5) and/or the fixed part (2) have at least one part, the section width of which decreases towards the free end of the relevant electrode.

11. The accelerometer according to any of claims 7 to 10, characterized in that the electrodes (3, 4) of the moving mass (5) and/or the fixed part (2) include a trapezoidal shape with decreasing width (L,I) towards their free end.

12. The accelerometer according to any of claims 7 to 11, characterized in that the electrodes (3, 4) of the moving mass (5) and/or the fixed part (2) include a part, the section width of which varies with steps of constant width, the width (L,I) steeply changing between each step.

13. The accelerometer according to any of claims 7 to 12, characterized in that the electrodes (3, 4) of the moving mass (5) and/or the fixed part (2) include a part, the width of which gradually decreases towards the free end of the electrode by forming at least one rounded side edge.

14. The accelerometer according to any of claims 7 to 13, characterized in that the electrodes (3) of the fixed part (2) have a complementary shape to the gap located between two adjacent electrodes (4) of the moving mass (5).