Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Patent
1996-08-07
1998-06-09
Chapman, John E.
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
G01P 1511
Patent
active
057637830
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an acceleration sensor, and in particular to an acceleration sensor which can be easily miniaturized and made thin, with high accuracy and low production cost.
BACKGROUND ART
Acceleration sensors wherein a semiconductor substrate is processed using micro-machining technology to achieve miniaturization and thin shape are known. With these acceleration sensors, devices such as capacitance type, piezoelectric-resistance type, and induced current type devices have been proposed.
A capacitance type device is disclosed for example in Japanese Unexamined Patent Publication No. 57-125355.
With this device, a V-shaped groove is formed in a substrate, and a cantilever beam supported at one end, is positioned in the V-shaped groove. Respective electrodes are positioned facing each other on mutually opposed inclined faces of the V-shaped groove and inclined faces of the beam. When subjected to acceleration, the beam is displaced corresponding to the magnitude of the acceleration, the amount of displacement being detected as a change .DELTA.C in electrostatic capacity C between the electrodes, to thereby detect the acceleration.
With the device of the abovementioned disclosure however, due to the cantilever construction the beam can move in two directions, namely up and down and sideways, thus giving two axes which are sensitive to the acceleration. Hence with acceleration in the up/down direction for example there is the possibility of sideways displacement of the beam. In this case, the output change component due to the sideways direction change must be removed, and hence a compensating circuit is required, resulting in a complicated circuit structure. Moreover, there are problems when the displacement amount is large, due for example to the displacement amount of the portion towards the non supported end of the beam being greater than that of the portion towards the supported end, the gap between the electrodes is no longer uniform.
In the case of the capacitance type acceleration sensor, to increase detection sensitivity, the change amount .DELTA.C of the electrostatic capacity C can be increased, and to achieve this the original electrostatic capacity C can be increased. Since the electrostatic capacity C is given by C=.epsilon..multidot.A/d (where .epsilon. is the dielectric constant), with A as the surface area of the electrode and d as the electrode gap, then one method of increasing the electrostatic capacity C, is to increase the electrode surface area A or decrease the electrode gap d.
However, increasing the electrode surface area A results in an increase in sensor size. Hence from the point of sensor miniaturization, it is better to reduce the gap d between the electrodes. However, if the actual gap d between the electrodes is made very narrow of the order of 2-3 .mu.m, then production yield is extremely poor due for example to dust becoming attached in the gap between the electrodes during the manufacturing process. Moreover, if the actual gap between the electrodes is reduced, then the dynamic range of the sensor is reduced. There are also problems due for example to the difficulty in making the narrow gap uniform. Furthermore, once an electrode pair becomes stuck together, since the sticking force by the electrostatic attraction is strong, the device becomes inoperable.
As an improved form of the capacitance type device which avoids inoperability due to sticking and gives an increase in the dynamic range, there is the electrostatic servo type device disclosed for example in Japanese Unexamined Patent Publication No. 5-26902.
With this device, a movable electrode is provided between two fixed electrodes, the arrangement being such that when the movable electrode is displaced due to acceleration, a voltage corresponding to the difference in electrostatic capacity caused by the displacement of the movable electrode, is fed back to the electrode section to control the movable electrode so that the difference in electrostatic capacity becomes zero. The vo
REFERENCES:
patent: 5524490 (1996-06-01), Lautzenhiser et al.
Chapman John E.
The Nippon Signal Co. Ltd.
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