Patent Application
20110179870
The present invention relates to a dual-axis acceleration detection element and particularly to a capacitive dual-axis acceleration detection element.
Micro-electromechanical system (MEMS in short) adopts semiconductor manufacturing process and other micro mechanical fabrication methods to fabricate and integrate various types of sensors, actuators, optical elements and the like. Through MEMS technique, elements can be miniaturized to achieve a lot of benefits such as lower cost, lower power loss, faster response speed and higher precision.
The conventional micro-sensor adopts a principle by transforming a targeted physical quantity to an electric signal through a sensing element, then analyzing the electric signal to get the targeted physical quantity indirectly. An acceleration sensor detects alterations of physical state caused by acceleration through a detection element to generate a corresponding electric signal such as voltage, resistance, inductance. It is widely used in applications such as vehicle safety detection, handsets, computers, electronic game machines and the like.
Frobenius made a detection element in 1972 through a cantilever structure of varying lengths. When the detection element is interfered by an external force the cantilever structure moves due to inertia to make a corresponding conductor to generate a signal to detect acceleration. Roylance made a piezoresistive micro-accelerator in 1979 by coupling a cantilever with a mass block and incorporating piezoresistive characteristics of silicon. Rudolf proposed in 1983 a capacitive micro-acceleration sensor that includes a mass block with a cantilever structure at two sides for support. When the mass block is subject to an external force and swings, the cantilever is driven and twisted to generate a capacitance alteration to get a corresponding electric signal.
The capacitive micro-acceleration sensor detects alteration of capacitance to derive acceleration. Compared with the conventional acceleration sensors that adopt piezoelectric, piezoresistive, and tunneling current, the capacitive acceleration sensor provides a higher sensitivity, lower temperature effect, lower electric power consumption, simpler structure and higher output. Hence a lot of efforts have been devoted to its research and applications. R.O.C. patent No. I284203 entitled “Accelerator” discloses a capacitive accelerator which comprises a stationary unit and a movable unit that contain respectively a plurality of detection electrodes arranged in an interdigitated fashion. When the movable unit is moved by an external force, the distance between the detection electrodes changes and results in alteration of capacitance. Thereby acceleration alteration can be detected.
According to capacitance equation of parallel electrode plates: C=∈A/d (where ∈ is dielectric coefficient, A is overlapped area of two electrode plates, and d is the distance between the two capacitor plates), capacitance alteration can be obtained by detection of distance (d) change. Alteration values of the capacitance and the distance alterations form a nonlinear relationship, hence estimate and operation of the acceleration are more difficult, and errors are prone to occur. Thus the present invention aims to provide a dual-axis acceleration detection device to get an improved linear relationship on acceleration by detecting capacitance alteration caused by area change.
Therefore, the primary object of the present invention is to provide a dual-axis acceleration detection element that has a high sensitivity and improved linear relationship.
Another object of the present invention is to provide a dual-axis acceleration detection element to detect capacitance difference caused by alteration of electrode area to detect acceleration amount and direction.
To achieve the foregoing objects, the dual-axis acceleration detection element according to the present invention comprises a first detection element, a second detection element and a stationary unit. The first detection element is movable relative to the second detection element. The second detection element is movable relative to the stationary unit. The relative movements take place on different axes. Hence accelerations on two different axes can be detected. Furthermore, the first detection element and the second detection element are interposed by corresponding detection electrodes, and the second detection element and the stationary unit also are interposed by other corresponding detection electrodes. When a relative movement takes place among the first detection element, second detection element and stationary unit, the overlapped area of the detection electrodes changes, therefore a capacitance difference is generated and output. Thereby acceleration alteration can be detected.
In an embodiment of the present invention, the detection electrodes form an elevation difference between them and include an overlapped area to form differential capacitor detection electrodes.
The dual-axis acceleration detection element according to the present invention can be fabricated through a micro-electromechanical fabrication process at a smaller size and lower cost. It provides improved acceleration linear relationship, higher sensitivity, and smaller detection errors in non-detection axes.
The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
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The first detection element 10 can be held in the housing space 22 and connected to the annular portion 21 through the first axis 12 as shown in
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When external forces are absent, the first detection element 10 is supported by the first axis 12 in a suspended manner and remains still relative to the second detection element 20; similarly, the second detection element 20 is supported by the second axis 25 in a suspended manner and remains still relative to the stationary unit 30. When the dual-axis acceleration detection element 1 of the present invention receives an acceleration on an X-Y plane, the mass body 11 outputs an inertial force and generates a torque through a pendulum structure, and transmits the force to the first axis 12 and second axis 25, hence the first axis 12 and/or second axis 25 are decoupled so that the mass body 11 outputs respectively a corresponding torque to the first axis 12 and second axis 25 to drive the detection platform swinging.
According to the capacitance equation C=∈A/d previously discussed, when two parallel electrode area changes, capacitance also alters. Hence when the first detection element 10 swings (twists) about the first axis 12, the first detection electrodes 13 at two sides of the first axis 12 corresponding to the second detection electrodes 23 generate area alterations and incur changes of capacitance values of +ΔC and −ΔC at two ends. Through output of capacitance difference at two sides, measurement by differential capacitance can be accomplished to detect acceleration parallel with direction of the second axis 25 (X axis). Similarly, when the second detection element 20 swings about the second axis 25, the acceleration parallel with direction of the first axis 12 (Y axis) also can be detected. It is to be noted that different accelerations cause the first detection element 10 or second detection element 20 to generate corresponding swing amounts, and different swing amounts correspond to different capacitances at the final detection, therefore can be used to detect the amount of acceleration.
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It is to be noted that in the present invention the first detection element 10, second detection element 20 and stationary unit 30 are defined separately. Such a division merely aims to facilitate discussion. In practice, they can be independent and separated and assembled together, or be directly fabricated through micro-electromechanical or semiconductor manufacturing processes, such as etching, photolithography, refill and the like. These techniques are known in the art. For instance, the dual-axis acceleration detection element 1 of the invention can be made by adopting a MOSBE micro-electromechanical platform fabrication process. Reference of this platform technique can be found in “The Molded Surface-micromachining and Bulk Etching Release (MOSBE) Fabrication Platform on (111) Si for MOEMS┘ (Journal of Micromechanics and Microengineering, vol. 15, pp. 260-265” published in 2005. Details are omitted herein. Thus the detection electrodes and the first axis 12 and second axis 25 can be made through the technique of trench-refill with material of polycrystalline silicon. The mass body 11 can be formed by backside etching with material of silicon or the like. The acceleration detection element made through the micro-electromechanical fabrication process has many advantages, such as smaller size, lower cost and higher sensitivity.
While the preferred embodiments of the invention have been set forth for the purpose of disclosure, modifications of the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.
