Linear capacitance detection circuit

Details Linear capacitance detection circuit Linear capacitance detection circuit

2000-01-13

2002-09-24

Jackson, Stephen W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

With specific circuit breaker or control structure

C301S015000, C301S064600, C301S064600

Reexamination Certificate

active

06456477

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to capacitance detection circuits for capacitance transducers used to sense force, pressure, strain, vibration, acceleration, gravity, sound, mechanical displacement, electric charge, radiation, and fluid flow. Specifically, the present invention relates to precision, low-noise, capacitive measurement circuits with a linear response for large changes of capacitance.
BACKGROUND OF THE INVENTION
Capacitive transducers with a flexible sensing diaphragm convert an applied force, pressure, or physical displacement to a change in capacitance. This capacitive change is transduced by an electrical circuit to a corresponding change in electrical voltage, current, or frequency. Prior art capacitive transducers use substantially parallel-plate electrodes separated a small distance apart in vacuum or a fluid dielectric medium. The sensitivity, linearity, and dynamic range of capacitive transducers is limited by the disadvantages of such variable parallel-plate capacitors.
When fringe fields are ignored, the capacitance C between two, conducting parallel plates is substantially given by:
C=&egr;A/d,
where &egr; is the permittivity of the dielectric medium, A is the effective area of the capacitor plates, and d is the effective spacing between the capacitor plates. Capacitance-displacement sensitivity, the change in capacitance as a function of plate spacing, is given by:
&Dgr;C/&Dgr;d=−&egr;A/d
2
which has a dependency on d
2
which results in a non-linear increase in capacitance sensitivity with decreasing plate spacing.
The capacitance-displacement sensitivity of a transducer with substantially parallel plates also can be affected by:
1. the non-linear reduction in capacitive sensitivity due to bending stresses in the sensing plate when the ratio of plate deflection to plate thickness is substantially greater than 0.2;
2. the non-linear reduction in capacitive sensitivity due to tensile stresses arising from the stretching of a thin sensing plate or diaphragm; and,
3. the reduction in capacitive sensitivity and frequency response due to viscous damping when a fluid dielectric, such as air, is squeezed between the capacitor plates. U.S. Pat. No. 5,048165 issued Sep. 17, 1991, discloses a method to construct a capacitive transducer with a deformable plate located between two, rigid plates. Differential capacitance detection allows two different and oppositely-sensed non-linearities to cancel to extend the linear range of the transducer. The disadvantage of this method is the complexity of using an additional capacitor plate and the requirement to construct a three-plate capacitive structure with well known, closely maintained and matched mechanical, thermal, and electrical characteristics.
U.S. Pat. No. 4,996,627 issued Feb. 26, 1991, discloses a three-plate, capacitance transducer used with an electronic circuit disclosed in U.S. Pat. No. 5,019,783 issued May 28, 1991, to provide a linear electrical output for a transducer with intrinsic non-linear sensitivity. U.S. Pat. No. 4,584,885 issued Apr. 29, 1986, discloses another of the many electronic circuits devised to electrically linearize the outputs of capacitive transducers. The disadvantages of these approaches is the requirement to use a third capacitor plate and the complexity and cost of signal compensation electronics. A general disadvantage of using mechanical or electrical methods to linearize the response of a capacitive transducer with substantially parallel-plate electrodes is that the sensitivity and dynamic range of the transducer cannot be significantly increased.
Other disadvantages and limitations of prior-art capacitive transducers arise from low values of quiescent capacitance. The maximum quiescent capacitance of a capacitive transducer is determined by the minimum spacing that can be reliably maintained between parallel-plate electrodes. Plate spacing is limited by the dimensional tolerances and stability of precision components and support structure. Plate spacing can also be limited by the voltage applied across the electrodes.
For microphones and capacitance transducers with thin sensing diaphragms, electrode spacing is further restricted by the space required to accommodate diaphragm displacement. As an example, a Bruel & Kjaer Model 41444, one-inch diameter, research-grade, capacitive microphone maintains a nominal 20-micron gap between a thin, nickel diaphragm and a rigid back-plate electrode. This spacing limits microphone capacitance to typically 55 pF and requires the device to be constructed from thermally stable components with precision tolerances. A 20-micron, dielectric gap is 100 to 1000 times larger that the thickness of dielectric films, such as silicon dioxide and silicon nitride, that are used to construct integrated circuit devices.
Low values of quiescent capacitance C
0
in capacitive transducers can cause a loss in sensitivity due to parallel stray capacitance. The total stray capacitance C
s
of support structure, electrodes, conducting leads, and the inputs of electronic circuits that shunts the quiescent capacitance reduces sensitivity by a factor C
0
/(C
0
+C
s
). Stray capacitance is of particular concern for transducers constructed with small, micromachined components and thin material layers. Low-capacitance transducers are more susceptible to electromagnetic interference and to changes in stray capacitance compared to transducers with higher quiescent capacitance.
Another disadvantage of transducers with parallel-plate capacitors is the increased noise in electrical networks with small capacitors. It is well known that the mean-squared voltage fluctuation, &Dgr;V
2
of a system with a capacitor at thermal equilibrium equals kT/C where k is Boltzmann's constant and T is absolute temperature. This noise source limits the accuracy and dynamic range of a capacitive transducer when it exceeds fundamental noise resulting from thermally induced motion of the sensing diaphragm.
Yet another disadvantage of a parallel-plate capacitive transducer, with a thin sensing diaphragm, is the maximum voltage that can be safely applied across the capacitor plates. Large displacements of a thin diaphragm resulting from shock or over-pressure loads can cause the diaphragm to collapse against its counter electrode. This occurs when the diaphragm deflects to a position where electrostatic force overcomes the mechanical restoring force of the diaphragm.
Capacitive transducers used to measure acceleration frequently use electrostatic force-feedback to maintain a suspended proof mass in a substantially fixed location. This minimizes non-linear capacitance sensitivity with electrode spacing. However, feedback cannot increase capacitance sensitivity or overcome the disadvantages of small quiescent capacitance limited by practical electrode spacing.
A variable capacitor has linear response if the area of the capacitor plates are changed while the plate spacing remains fixed. This can be accomplished by moving or rotating multiple plates in parallel planes. This approach was used to capacitively tune early radios, but is difficult to implement in small transducers.
An article titled “A capacitor transducer using a thin dielectric and variable-area electrode” appearing in the IEE Proc., Vol. 127, Pt. A, No. 6, July 1980, by Basarab-Horwath et al., reports high values of capacitive sensitivity for a transducer with capacitor plates that increase in area with applied force. The disadvantage of this transducer is that the shape of the flexible electrode changes with both displacement and applied loading. Therefore, it is difficult to obtain, maintain, and control a precision capacitance relationship between the electrodes. This article does not teach or suggest the benefits of using a rigid electrode with a surface contour chosen to obtain an accurate, repeatable, and specific capacitive relationship between the electrodes of a variable capacitor or a capacitance transducer. The work by Basarab-Horwath et. al. is reported as an

Affiliated with

Also associated with

Rating

Linear capacitance detection circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Linear capacitance detection circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Linear capacitance detection circuit will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2824211