Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2003-05-21
2004-07-27
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06766690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acceleration sensor using piezoelectric ceramic material.
2. Description of the Related Art
Various types of acceleration sensors using piezoelectric ceramic material have been proposed. Generally, in acceleration sensors, two types of sensitivities are used: voltage sensitivity and charge sensitivity. By increasing the charge sensitivity, the S/N ratio can be increased with respect to electromagnetic noise caused by external apparatuses and the circuit which affects the junction between the acceleration sensor and an amplifier, which is connected to the subsequent stage. On the other hand, by increasing the voltage sensitivity, the S/N ratio can be increased with respect to the voltage noise caused by the amplifier itself. Therefore, in order to increase the S/N ratio with respect to both external electromagnetic noise and noise inside the amplifier, both charge sensitivity and voltage sensitivity must be increased. That is, in a high S/N or highly sensitive sensor, a large amount of energy is generated, the energy being represented as ½ of the product of charge sensitivity and voltage sensitivity.
Japanese Unexamined Patent Application Publication No. 10-62445 discloses an acceleration sensor including three or more laminated piezoelectric ceramic layers, each layer having substantially the same thickness, and the layers being electrically connected in parallel. In this acceleration sensor, charge sensitivity can be increased by increasing the number of layers. In this acceleration sensor, however, by increasing the number of layers while the entire thickness of a sensing element is kept the same, the thickness of each layer is reduced and capacitance increases. Thus, the voltage sensitivity at each layer decreases accordingly. Also, the potential at an inner layer is lower than that at an outer layer. Therefore, by connecting these layers in parallel, the voltage sensitivity of the entire sensing element is the average of the voltage of each layer, and the entire voltage sensitivity is decreased as the number of laminated layers is increased. As a result, generated energy does not significantly increase.
Table 1 shows the sensitivity of two types of acceleration sensors: a two-layered type and a four-layered type. Herein, each acceleration sensor has a one-end-supported structure, the thickness of the entire sensing element is 0.42 mm, the free length except a supporting portion is 3.0 mm, and the width of the sensing element is 0.4 mm. As can be seen, the four-layered sensor has much higher charge sensitivity than that of the two-layered sensor, but voltage sensitivity is low, and thus the amount of generated energy in both sensors is the same. This table shows the characteristic in a state where each layer is electrically connected in parallel to each other, although the state is not shown in a figure.
TABLE 1
Characteristic
Two-layered
Four-layered
Voltage sensitivity (mV/G)
3.45
1.00
Charge sensitivity (pC/G)
0.245
0.841
Capacitance (pF)
70.9
837
Energy (10
−18
J/G)
422
422
In the above-shown characteristic, the entire thickness and free length of the sensing element are the same in the two-types of sensors, in order to compare the two sensors. In the characteristic of the shape including the thickness and free length, the following equations are satisfied:
charge sensitivity
Q=kd·WL
3
/L; and
voltage sensitivity
V=kg·L
2
.
If this shape is changed, the characteristic is also changed. Herein, L represents free length; W represents the width of the sensing element; T represents the entire thickness of the sensing element; g and d represent a piezoelectric constant; and k represents another coefficient. According to the above equations, the free length L should be increased in order to increase charge sensitivity and voltage sensitivity. Also, if the thickness T is decreased, the charge sensitivity increases and the voltage sensitivity is not changed, and thus, generated energy increases. However, the size increases by increasing L, and strength decreases by decreasing T. In this case, resonance frequency at a detecting portion decreases so that the acceleration at high frequency cannot be precisely measured. Thus, the size is limited.
Japanese Unexamined Patent Application Publication No. 9-26433 discloses a two-layered acceleration sensor having a one-end-supported structure. In this acceleration sensor, an intermediate electrode and a surface electrode are formed over the entire length thereof. When acceleration is applied to the sensor, stress is generated due to the acceleration and also a charge is generated so that an output signal is generated. In a one-end-supported structure, the stress is large at the vicinity of the supported portion of the sensing element and becomes smaller toward the free end thereof. In this state, the free end does not contribute so much to generation of a charge, and this state is equivalent to a state where only capacitance components are electrically connected in parallel. Therefore, voltage sensitivity of the entire sensing element is the average along the entire length. Also, the entire voltage sensitivity decreases compared to a case where an electrode is provided only near the supported portion, and thus, generated energy cannot be increased.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a highly sensitive acceleration sensor in which generated energy can be significantly increased without changing the free length and thickness thereof.
According to a first preferred embodiment of the present invention, an acceleration sensor includes a sensing element, and a pair of supporting members for supporting the sensing element at one end, both ends, or a central portion in the longitudinal direction thereof. The sensing element includes at least four laminated piezoelectric layers, each layer including a piezoelectric ceramic material. The four piezoelectric layers include a pair of first layers positioned at the center in the thickness direction and a pair of second layers sandwiching the pair of first layers. Electrodes are provided at the center in the thickness direction of the sensing element, between the pair of first layers and the pair of second layers, and on the outer surfaces of the pair of second layers. Cells formed by the first and second layers located on the same side with respect to the center in the thickness direction are electrically connected in parallel. The pair of first layers preferably have substantially the same thickness and the pair of second layers preferably have substantially the same thickness. The ratio of the thickness of each first layer to the total thickness of each first and second layer is about 62% to about 76%.
According to a second preferred embodiment of the present invention, an acceleration sensor includes a sensing element, and a pair of supporting members for supporting the sensing element at one end, both ends, or a central portion in the longitudinal direction thereof. The sensing element includes six laminated piezoelectric layers, each layer including a piezoelectric ceramic material. The six piezoelectric layers include a pair of first layers positioned at the center in the thickness direction, a pair of second layers sandwiching the pair of first layers, and a pair of third layers positioned on the outer side. Electrodes are provided at the center in the thickness direction of the sensing element, between the pair of first layers and the pair of second layers, between the pair of second layers and the pair of third layers, and on the outer surfaces of the pair of third layers. Cells formed by the first, second, and third layers located on the same side with respect to the center in the thickness direction are electrically connected in parallel. The pair of first layers preferably have substantially the same thickness, the pair of second layers preferably have substantially the same
Keating & Bennett LLP
Kwok Helen
Murata Manufacturing Co. Ltd.
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