Micromechanical inertial sensor having reduced sensitivity to the influence of drifting surface charges, and method suited for operation thereof

• US 20080134785A1
• Filed: 11/20/2007
• Published: 06/12/2008
• Est. Priority Date: 12/08/2006
• Status: Active Grant

• Current Assignee: Robert Bosch GmbH
• Original Assignee: Robert Bosch GmbH
• Inventors: Pruetz, Odd-Axel
• Granted Patent: US 7,721,604 B2
• Time in Patent Office: Days
• US Class Current: 73/514.32
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• 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

A micromechanical inertial sensor having at least one seismic mass which may be deflected relative to a substrate, and at least one electrode surface which in terms of circuitry, together with at least portions of the seismic mass forms at least one capacitor having a capacitance which is dependent on the deflection of the seismic mass. At least one additional auxiliary electrode is included which is located outside the region which forms the capacitor and which may be set at a potential that deviates from the potential of the seismic mass.

Claims

1. A micromechanical inertial sensor, comprising:

at least one seismic mass which may be deflected relative to a substrate;

at least one electrode surface which in terms of circuitry, together with at least portions of the seismic mass, forms at least one capacitor having a capacitance which is dependent on a deflection of the seismic mass; and

at least one additional auxiliary electrode located outside a region which forms the capacitor and which may be set at a potential that deviates from a potential of the seismic mass.

2. The sensor as recited in claim 1, wherein the sensor is designed as a Z sensor and has a seismic mass in the form of an asymmetric rocker which may be rotated about a torsion spring, surface regions of the rocker which are symmetrical with respect to the rotational axis extending parallel to at least one symmetrical electrode pair which is fixedly connected to the substrate, and in terms of circuitry, together with the electrode pair, forming capacitors having a capacitance which is dependent on the deflection of the seismic mass, at least one additional surface region of the rocker being situated opposite an auxiliary electrode which is located in a fixed position with respect to the substrate outside a region of the electrodes which are part of the capacitors, and which in terms of circuitry may be set at a potential that deviates from the potential of the seismic mass.

3. The sensor as recited in claim 1, wherein in regions of the micromechanical sensor which may act as a stop for the seismic mass, additional electrodes are provided which support the seismic mass in an event of impact and which in terms of circuitry are at a same potential as the seismic mass.

4. The sensor as recited in claim 2, wherein the surface of the auxiliary electrode is at least 80% of the substrate-side surface of the seismic mass which does not extend opposite the electrodes that are part of the capacitors.

5. A method for reducing an influence of drifting surface charges on the output signal from a micromechanical Z sensor, comprising:

using a seismic mass in a form of an asymmetric rocker which may be rotated about a torsion spring, surface regions of the rocker which are symmetrical with respect to the rotational axis extending parallel to at least one symmetrical electrode pair, and in terms of circuitry together with the electrode pair forming capacitors having a capacitance that is dependent on the deflection of the seismic mass, and at least one additional surface region of the rocker being situated opposite an auxiliary electrode which is located in a fixed position with respect to the substrate outside the region of the electrodes that are part of the capacitors;

measuring an acceleration in a Z direction by evaluating the capacitance of at least one of the capacitors; and

setting, at least during the measurement of acceleration, the auxiliary electrode at a potential that deviates from a potential of the seismic mass.

6. The method as recited in claim 5, wherein an acceleration in the Z direction is measured by evaluating the capacitance of the two symmetrically positioned capacitors, the capacitors being connected in such a way that they form a differential capacitor, and the evaluation being performed according to a differential capacitive measuring principle.

7. The method as recited in claim 5, wherein a potential is continuously applied to the auxiliary electrode, or is held constant at least during measurement of the acceleration.

8. The method as recited in claim 7, wherein the potential applied to the auxiliary electrode is set in such a way that it differs by at least 50 mV from the potential of the seismic mass.

9. The method as recited in claim 8, wherein the potential applied to the auxiliary electrode is set in such a way that when there is no effect from an acceleration to be measured a minimum offset value results at the output of the Z sensor.

10. The method as recited in claim 5, wherein at least during measurement of the acceleration, the electrodes of the symmetrical electrode pair which is part of the capacitors are acted on by potentials clocked according to a differential capacitive measuring principle, the clock sequence including measurement clock pulses and compensation clock pulses.