A Z-axis capacitive accelerometer includes a substrate, a capacitance sensing plate, a proof mass and at least one pair of spring beams. The capacitance sensing plate includes two symmetrical sense areas to create differential capacitive measurement. A decoupling structure separates the proof mass and the capacitance sensing plate and their rotational motions from each other. In the proposed Z axis capacitive accelerometer, the distance of the capacitance sensing plate relative to its rotation axis is considerably increased, thereby effectively enhancing the sensitivity when measuring the Z-axis acceleration.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Chinese Patent Application No. 201010552669.3, filed on Nov. 19, 2010, entitled “Z-Axis Capacitive Accelerometer” by Leyue JIANG, Hanqin ZHOU, and Yang ZHAO, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a Micro-Electro-Mechanical System (MEMS) capacitive accelerometer, and more particularly to a capacitive accelerometer for measuring the Z-axis (out-of-plane) acceleration.

BACKGROUND OF THE INVENTION

With the development of micro-electro-mechanical system (MEMS) technology, micro-size, reliable, and low-cost silicon accelerometers have been in volume production and widely adopted in automotive, consumer electronics, and industrial applications.

The operation principle of the differential capacitive micro-accelerometer is as follows. A proof mass is suspended above the substrate by spring beams/tethers and deflects in the plant of the substrate in the present of an acceleration field. When the proof mass moves, the gap between capacitors would decrease on one side while increasing on the other, resulting in a differential capacitance variation for measurement of acceleration.

To detect an out-of-plane (Z-axis) acceleration, the capacitive accelerometer is usually designed with torsional parallel-plates. Since the plate weight (proof mass) is asymmetrically distributed with respect to the rotational axis, acceleration along the normal axis to the substrate will make the top plate rock in one direction or another and thus cause differential change of capacitance on the two sides of the rotational axis.

However, in the conventional design of Z-axis accelerometer, a significant part of the poof mass to the far tip cannot be utilized for sensing, but only for creating imbalanced motion in response to inertial force, and the unused sensing area would have the largest signal displacement. This substantially limits the die area efficiency for electrical sensitivity.

SUMMARY OF THE INVENTION

The present invention relates to a Z-axis micro-machined capacitive accelerometer design of higher sensitivity than that of existing design, given the same substrate area.

In one embodiment, the Z-axis micro-machined accelerometer separates the mechanical structure into a parallel-plate capacitance sensing portion and a proof mass portion using 3 pairs of elastic torsion springs and 3 anchor areas, allowing the sensing areas to be at the ends of the structure with the largest displacement. In the presence of an out-of-plane acceleration, the sensing plate and proof mass will rotate in different directions (one clockwise while the other anti-clockwise). The sensing areas are placed at the right and left ends, respectively, and then would experience maximum vertical displacement which results in a larger capacitance sensitivity.

In one embodiment, the capacitance sensing plate has the rotation axis1, the proof mass has the rotation axis2, and these two rotation axes are parallel to each other.

In one embodiment, the differential capacitance sensing plate areas are symmetrical with respect to the first rotation axis.

In one embodiment, the second rotation axis is placed at one side of the proof mass.

In one embodiment, the spring beams are parallel to the first and the second rotation axes.

In one embodiment, the first rotation axis consists of a pair of spring beams (upper and lower) which are of the same length. There are two anchor points fix the beams to the substrate.

In one embodiment, the second rotation axis consists of a pair of spring beams (upper and lower) which are of the same length. There is one anchor point fix the two beams to the substrate.

In one embodiment, the spring beam pair consists of an upper and a lower spring beams of the same length.

In one embodiment, the rotation direction of the capacitance sensing plate is opposite to the rotation direction of the proof mass.

Compared with the embodiments of related art, the present invention has the following advantages. In the Z-axis capacitive accelerometer according to the present invention, a mechanical structure in which the proof mass and the capacitance sensing plate are decoupled from each other is adopted, and the differential capacitance sensing plates are placed opposite at the two sides of the rotational axis and outside the proof mass, wherein the opposite means the plates can be placed either symmetrical and nonsymmetrical to the rotational axis. The average distance of the capacitance sensing plate to the rotation axis is thus increased. Given the same angular displacement, the capacitance change of the sensing plate will be increased significantly, thereby effectively improving the sensitivity of acceleration measurement.

DETAILED DESCRIPTION OF THE INVENTION

The description will be made as to the embodiments of the present invention in conjunction with the accompanying Figures.

Referring toFIG. 1, a Z-axis capacitive accelerometer according to the present invention has a proof mass and a capacitance sensing plate whose rotational movements can be mechanically decoupled.

One embodiment of the present invention comprises of at least a pair of spring beams, 3 anchor points, and 2 rotational axes as shown inFIG. 1.

The outer sensing plate is nearly symmetric from left to right around anchor points (15inFIG. 1), except for an attachment to the proof plate at spring beam pair3.

Spring beam pair1(14&16inFIG. 1) is the sensing plate symmetric torsion axis having anchor points (15inFIG. 1).

Spring beam pair2(22&24inFIG. 1) is the proof mass asymmetric torsion axis having the anchor point (23inFIG. 1), providing the torque for displacement in response to acceleration.

Spring beam pair3(31&32inFIG. 1) provides mechanical decoupling between the proof mass and the capacitance sensing plate with mechanical amplification and separation of rotation directions.

The length of beam pair1is h1(assuming upper and lower of equal length).

The length of beam pair2is h2(assuming upper and lower of equal length).

The length of beam pair3is h3(assuming upper and lower of equal length). The horizontal distance from the centerline of beam pair1to the centerline of beam pair3is l1.

The horizontal distance from the centerline of beam pair3to the centerline of beam pair2is l2.

The horizontal distance from the centerline of beam pair1to the centerline of beam pair2is l=l1+l2.

The overall dimension of the mechanical structure is 2L (x-axis dimension) by H (y-axis dimension).

The spring constants for the beam pairs1,2,3can be calculated as follows:

k1=2⁢⁢J1⁢Gh1=2⁢⁢tw13⁢G3⁢⁢h1k2=2⁢⁢J2⁢Gh2=2⁢⁢tw23⁢G3⁢⁢h2k3=2⁢⁢J3⁢Gh3=2⁢⁢tw33⁢G3⁢⁢h3,
where Jxis the corresponding moment of inertia, G is the shear modulus of elasticity, t is the beam thickness, wxis the corresponding beam width, and hxis the corresponding beam length.

When an external acceleration a is applied, the angular displacement will be

θ1=2⁢⁢ρ⁢⁢tHl2⁢rk1+k2⁢r2+k3⁡(1+r)2⁢a,
where r=l1/l2, ρ is the plate density, and t is the plate thickness.

The resultant capacitance change is

In the present invention of Z-axis capacitive accelerometer, a decoupling structure which separates rotational movement of the capacitance sensing plate and the proof mass is employed.

The differential capacitance change in the present invention is greater than the differential capacitance change of prior art for the same angular displacement. In case of the same substrate area and mechanical resonance frequency, larger sensitivity in response to Z-axis acceleration can be achieved than conventional Z-axis capacitive accelerometers with a single torsion axis and asymmetrical capacitance sensing plate/proof mass.