Acceleration sensor

Details Acceleration sensor Acceleration sensor

2003-03-05

2004-10-26

Chapman, John E. (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

C310S332000

Reexamination Certificate

active

06807859

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates an acceleration sensor including a piezoelectric element.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 6-273439 (corresponding to counterpart U.S. Pat. Nos. 5,515,725 and 5,490,422) discloses an acceleration sensor including a piezoelectric ceramic. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in series.
FIG. 8A
illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and
FIG. 8B
is a circuit diagram of the acceleration sensor.
Japanese Unexamined Patent Application Publication No. 2000-121661 (corresponding to counterpart U.S. Pat. No. 6,360,603) discloses another acceleration sensor. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions separated at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in parallel.
FIG. 9A
illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and
FIG. 9B
is a circuit diagram of the acceleration sensor.
In the acceleration sensor shown in
FIGS. 8A and 8B
, cells (
1
) through (
3
) in one layer are connected in parallel and cells (
4
) through (
6
) in the other layer are connected in parallel. In the acceleration sensor shown in
FIGS. 9A and 9B
, cells (
1
) through (
6
) in the two layers are connected in parallel. In these arrangements, voltages generated in the cells are not summed. Sensitivity to the voltage generated in each layer is low, and the performance of these acceleration sensors is not high enough in applications where high sensitivity is required. Since all six cells (
1
) through (
6
) are connected in parallel in the acceleration sensor shown in
FIGS. 9A and 9B
, the voltage sensitivity thereof is half the voltage sensitivity provided by the acceleration sensor shown in
FIGS. 8A and 8B
.
To increase the voltage sensitivity, the gain of a voltage amplifier connected to the sensor must be increased. If the gain of the amplifier is increased, a noise component applied to the amplifier is also amplified. Also, the S/N ratio is degraded. Increasing the gain of the voltage amplifier does not serve the purpose of measuring a small acceleration.
In an acceleration sensor including a piezoelectric ceramic, no noise is generated from the piezoelectric ceramic itself. The sources of noise include external noise ingressing between the sensor and the input terminal of a circuit thereof, and internal noise responsive to input conversion voltage noise generated at an input stage of an operational amplifier defining the amplifier.
These noises will now be discussed with reference to a charge amplifier shown in
FIG. 10 and a
voltage amplifier shown in FIG.
11
.
The sensor S here has voltage sensitivity Vs, charge sensitivity Qs, and capacitance Cs. It is well known that the relationship of Qs=Vs×Cs holds. Overall gains G of the two amplifiers are listed in Table 1. In the charge amplifier, the gain G is proportional to the charge sensitivity Qs. In the voltage amplifier, the gain is proportional to the voltage sensitivity Vs.
TABLE 1
External
Internal
G converted
G converted
noise
noise
external
internal
Circuit
voltage
voltage
noise
noise
type
Gain G
(x Vn)
(x En)
(x Vn)
(x En)
Charge
Qs/C1
Cc/C1
(Cs + C1)/C1
Cc/Qs
(Cs + C1)/Qs =
amplifier
{(Cs + C1)/Cs}/Vs
Voltage
Vs = Qs/Cs
Cc/Cs
1
Cc/Qs
1/Vs
amplifier
The external noise Vn is now considered. The external noise Vn is electrostatically coupled through capacitance Cc to a line running between a sensor S and an input terminal of an amplifier OP, and the magnitude of noise is thus represented by Vn. The external noise Vn appears as Von after being amplified through the amplifier. Since the polarity of noise is not important, all noises are expressed in absolute values in Table 1.
In the charge amplifier, a coupling capacitance Cc and a feedback capacitance C1 form an inverting amplifier. The external noise voltage has a magnitude that is obtained by multiplying Vn by gain (Cc/C1). In the voltage amplifier, noise is voltage divided by a coupling capacitance Cc and capacitance Cs of a sensor, and is then output through a voltage follower.
If G converted noise that represents the magnitude of noise with reference to the level G is defined by the reciprocal of the S/N ratio, the G converted noise becomes a noise voltage/G gain. For example, if the G converted noise is 100 mG, noise as large as 100 mG is continuously generated even if no acceleration appears in the output of the circuit. An acceleration below that level cannot be measured. The G converted noise in each circuit is listed in Table 1. The G converted external noise is not related to the circuit type. The larger the charge sensitivity Qs, the smaller the G converted external noise, and the smaller acceleration is measured.
Table 1 lists internal noise voltage. Internal noise En is generated at a positive input terminal of an operational amplifier OP. A non-inverting amplifier is defined by a capacitance Cs of the sensor S and the feedback capacitance C1, and the output thereof is obtained by multiplying En by the resulting gain. Since the voltage amplifier is a voltage follower of gain one, En is directly output. The G converted internal noise is obtained by dividing noise voltage Von by G gain Vos as listed in Table 1. The G converted internal noise is not related to the circuit type. The larger the voltage sensitivity Vs, the smaller the G converted internal noise, and the smaller acceleration that is measured.
A sensor having a large charge sensitivity and a large voltage sensitivity is a good sensor. The product Qs·Vs/2 of the charge sensitivity and voltage sensitivity corresponds to energy Es generated by acceleration. To increase the energy Es, the sensor must be large in size. The external noise here depends on the layout on a printed circuit board, and the internal noise depends on an amplifier characteristic of an operational amplifier in use. Depending on the status of a host apparatus of the sensor, more weight is attached to one of the charge sensitivity Qs and the voltage sensitivity Vs than the other while the energy Es, which is the product of the charge sensitivity and the voltage sensitivity, is maintained. The above-disclosed sensors with the large charge sensitivity thereof are appropriate for use in applications where external noise level is high, but are not appropriate for use in applications where internal noise level is high because of the small voltage sensitivity thereof.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide an acceleration sensor which increases a voltage sensitivity without reducing energy, which is equal to the product of charge sensitivity and voltage sensitivity, as much as possible, and which is appropriate in an operating environment where internal noise is high in level.
An acceleration sensor of a preferred embodiment of the present invention includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of at least two piezoelectric layers. Each of the least two external piezoelectric layers of the piezoelectric element includes three longitudinally aligned regions separated at two borders where stress is inverted in the longitudinal direction of the piezoelectric element when acceleration is applied. Each of a plurality of cells defined by a respective region i

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