Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2002-03-06
2003-12-09
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C361S283200, C361S283400, C307S400000
Reexamination Certificate
active
06658938
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates capacitive transducers self-biased by a thin-film electret. More specifically, it relates to wide dynamic range force, pressure, and displacement sensors; electrostatic actuators; and acoustic and ultrasonic transceivers with multiple capacitor elements.
BACKGROUND OF THE INVENTION
Simple electrostatic transducers comprise a variable capacitor with two, substantially parallel-plate electrodes: a flexible electrode responsive to a physical effect and a rigid counter-electrode. A change in pressure or force applied to the flexible electrode causes it to deflect. Displacement of the flexible electrode is electrically sensed by detecting a change in capacitance between cooperating electrodes. When a polarization voltage V
0
is applied across the electrodes, the voltage v
c
(t) across the capacitor for a small diaphragm displacement &dgr;(t) can be approximated by,
v
c
(
t
)
≈
δ
(
t
)
s
0
V
0
≈
Δ
C
(
t
)
C
0
V
0
(
δ
<<
s
0
)
(
1
)
where C
0
is the quiescent capacitance and s
0
is the equilibrium spacing between the capacitor plates biased with a static potential V
o
.
Conventional capacitive microphones utilize a high polarization voltage V
0
to transduce the amplitude of a time varying acoustic pressure p
a
(t) to an open circuit voltage v
c
(t) across the capacitor electrodes. The polarization voltage is applied across the microphone capacitor through a high-value charging resistor R that maintains a substantially constant charge Q
o
=C
0
V
0
on the variable capacitor. The sensitivity S of a microphone in terms of its open-circuit voltage divided by the pressure amplitude of an incident acoustic wave of angular frequency w can be expressed as
S
=
v
c
(
t
)
Δ
p
(
t
)
=
Δ
C
(
t
)
V
0
C
0
·
C
0
C
0
+
C
s
·
jω
R
(
C
0
+
C
s
)
1
+
jω
R
(
C
0
+
C
s
)
(
2
)
where C
s
is the total parasitic capacitance including the input capacitance of amplification electronics. Eq. 2 can be further refined to include additional frequency dependent factors to account for specific electrode geometry and the effective mass and compliance of the diaphragm and surrounding fluids.
Electret capacitive transducers operate without an external dc polarization voltage. A thin-film electret is affixed to or formed on one capacitor electrode. An electret has a permanent state of electrical polarization that provides an electric field to self-bias a variable capacitor. This permits small electret microphones to be manufactured in high volume for hearing aids and communications equipment by avoiding the cost and complexity of providing a low-noise source of high voltage.
Many methods are known to provide electrets and electret capacitance microphones. Four references of note are: 1) G. M. Sessler and J. E. West, “Self-Biased Condenser Microphone with High Capacitance,” Acoust. Soc. of Am. J. 34: 1787-1788, 1962; 2) U.S. Pat. No. 3,740,496 of Carlson et al.; U.S. Pat. No. 5,536,982 of Mino et al.; and 4) U.S. Pat. No. 2001/0033670 A1 of Tai et al.
Prior-art electret transducers are constructed with substantially parallel-plate electrodes. The sensitivity, linearity, and dynamic range of these capacitive transducers are limited by the geometric constraints of parallel-plate construction. The capacitance-displacement sensitivity of a gap-varying capacitor at mid-range signal frequencies is substantially:
&Dgr;
C/&Dgr;s=−&egr;A/s
2
(3)
The dependency on s
2
results in a non-linear capacitance sensitivity with plate spacing.
Other disadvantages of prior-art, variable capacitors result from the minimum spacing that can be reliably maintained between parallel spaced electrodes and associated low values of quiescent capacitance. A transducer with low quiescent capacitance has a high source impedance 1/j&ohgr;C at acoustic frequencies. This generally requires the capacitor voltage v
c
(t) to be detected by a JFET amplifier. The noise of a FET and high value bias resistors further limit dynamic range at low frequencies. Another disadvantage of small quiescent capacitance is a loss in sensitivity S due to stray capacitance. The total parasitic capacitance C
s
of fringing fields, support structure, electrodes, and inputs of electronic circuitry reduces sensitivity S in Eq. 2 by the factor C
o
/(C
o
+C
s
).
The spacing between capacitor electrodes limits the maximum displacement of a movable electrode. This displacement is further restricted by the well-known “pull-in” instability occurring at a critical voltage at which the movable electrode deflects by about ⅓ of the undeflected capacitor gap. Precision capacitance accelerometers use electrostatic force-rebalanced feedback to maintain an inertial mass suspended on a flexible electrode at a substantially fixed position to minimize non-linear capacitance response. However, feedback cannot significantly increase sensitivity or avoid the disadvantages of small quiescent capacitance.
The disadvantages of capacitance transducers with parallel electrodes (with or without an electret) are avoided by the variable-area capacitor embodiments of U.S. Pat. No. 6,151,967 and U.S. patent application Ser. Nos. 09/834,691 and 09/866,351.
A variable area capacitor (VAC) is referred to herein as a variable capacitor for which a substantial portion of a change in capacitance with electrode displacement is due to an increase in effective electrode area rather than to a change in electrode spacing. The capacitance of this type of VAC increases as an area of fixed capacitive spacing increases between cooperating electrodes while the approach of a movable electrode with respect to a stationary electrode remains small. This increases the effective area A contributing the majority of the capacitance between the electrodes and accommodates large displacements not limited by the dimensions of a narrow air gap. When a rising voltage is applied to a VAC, an electrostatic force of attraction continuously collapses a flexible electrode across a curved surface of a cooperating rigid electrode.
The large changes in capacitance of an electret VAC, up to 500% and more, can be linearly transduced by circuit inventions disclosed in U.S. patent application Ser. Nos. 09/794,198 and 09/816,551. An electret VAC with a thin flexible diaphragm can be operated as an electrostatic actuator or as an acoustic transmitter. When a variable voltage is applied across a VAC, acoustic or ultrasonic energy couples to the medium in which it is immersed.
SUMMARY OF THE INVENTION
The dynamic range of electret capacitive transducers can be extended several orders of magnitude using VAC's with the general construction of the embodiments of U.S. Pat. No. 6,151,967. Electret transducers of the present invention can be constructed in part by methods disclosed in U.S. patent application Ser. No. 09/834,691. The instant invention can be advantageously applied to VAC transducers constructed to detect physical effects including force, pressure, acceleration, and displacement. VAC transducers can be operated as electrostatic actuators, with and without, force-rebalanced feedback control. All aspects of the present invention are applicable to sensors and actuators with multiple VAC elements.
Accordingly, the principle object of the present invention is to provide electret sensors and actuators with the low-noise, high capacitive sensitivity, and wide linear dynamic range characteristic of VAC transducers. This object is realized by electrically polarizing a thin dielectric film placed between a flexible electrode and a curved rigid counter-electrode. Non-limiting examples of VAC transducer embodiments in which an electret can be used to provide a self-biased transducer with a predetermined response characteristic are disclosed in U.S. Pat. No. 6,151,967 and U.S. patent application Ser.
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