Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific circuit breaker or control structure
Reexamination Certificate
2001-05-25
2002-12-17
Tso, Edward H. (Department: 2838)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific circuit breaker or control structure
Reexamination Certificate
active
06496348
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to force-balanced capacitive transducers including sensors and actuators. More specifically, it relates to a method to electrostatically force-balanced transducers that control the position of a surface, stylus, inertial mass, valve, electrical contact, electrical component; or an optical component such as a mirror, lens, grating, filter, or holographic element.
BACKGROUND OF THE INVENTION
Electrostatically, force-balanced, capacitive transducers maintain a compliant member, such as a beam, diaphragm, or bridge at a fixed position or predetermined gereratrix. One example is an accelerometer with capacitor electrodes that sense the displacement of an inertial mass suspended on a compliant member in relation to stationary support structure. A simple capacitive accelerometer measures a change in spacing between a capacitor electrode affixed to a surface of an inertial mass in close proximity to a cooperating capacitor electrode affixed to adjacent stationary structure. The two electrodes form a displacement sensing, parallel-plate capacitor with a capacitance value inversely proportional to the plate spacing. A force acting on the mass causes it to be displaced in proportion to the stiffness of the compliant member. When an unconstrained axis of the mass is orientated with the gravity gradient, it will deflect to an equilibrium or quiescent position where restoring forces due to the bending moment of the compliant member balance the force of acceleration acting on the mass. Motion of the support structure modulates the capacitor plate spacing due to the inertia of the mass. The corresponding change in capacitance is detected and transduced to provide a voltage proportional to the square root of a change in force acting on the mass.
The disadvantages of a simple capacitive accelerometer are: 1) a severely restricted dynamic measurement range due to a small capacitor gap, 2) a highly non-linear capacitance response with mass displacement, and 3) the requirement to dampen the response of the accelerometer at frequencies near the resonance of its spring-mass system. These disadvantages are avoided by operating a capacitive accelerometer in an electrostatic force-balanced feedback loop that maintains the inertial mass in a substantially fixed position under loading. An incremental displacement of the mass is capacitively detected, transduced, and amplified to provide a control voltage V across the capacitor plates to create an electrostatic force that restores the mass to its initial position. The electrostatic force F
E
applied to a electrode of a parallel-plate capacitor varies as V
2
/d, where d is the effective plate spacing. Because the force F
E
is independent of the polarity of the control voltage V, the compliant member of a simple capacitive transducer is required to be electrostatically biased or mechanically preloaded to allow the accelerometer to sense a bi-directional force. Accelerometers, actuators, and other capacitive transducers with one, force balancing capacitor are defined here as single-side, force-balanced transducers.
In a similar manner, a simple electrostatic actuator applies an electrostatic force to an electrode affixed to a compliant member and/or attached payload to control the position of the member or payload. The payload can include a surface, stylus, inertial mass, valve, electrical contact, electrical component; or an optical component such as a mirror, lens, grating, filter, or holographic element. This method is of controlling the position of a compliant actuator member is equivalent to applying a voltage bias to the electrodes of a capacitive accelerometer to move its inertial mass to a new equilibrium position. For most actuator applications, a static force such as gravity is generally small compared to the applied electrostatic force and the reaction force of the compliant member and payload. A general disadvantage of a single-side, capacitive transducers (e.g., sensors and actuators) is that the effective spring constant and elastic restoring force of the preloaded compliant member is influenced by ageing and physical effects such as temperature.
Precision capacitive transducers use at least a pair of differential parallel-plate capacitors to sense and force-balance an inertial mass or to sense and position of a compliant member of an actuator. Planar electrodes are affixed to opposing surfaces of the mass and/or a compliant member which are located in close proximity to cooperating planar electrodes affixed to adjacent stationary structure. The electrodes form one or more pairs of differential displacement sensing and force generating capacitors. When the mass of an accelerometer or payload of an actuator moves, the capacitance of one displacement sensing capacitor increases while that of a second capacitor decreases by substantially an equal amount. One advantage of a differential capacitive transducer is that the compliant member is not required to be preloaded for bi-directional force or position control. Another advantage is that an incremental capacitance change can be detected with a bridge circuit to minimize errors associated with unmatched electronic components and variations of supply and reference voltages.
Capacitance transducers with variable-gap, parallel-plate electrodes comprise a well-know and crowded art. The general disadvantage of prior-art capacitive transducers arise from the limitations imposed by parallel-plate capacitors: low-quiescent capacitance, low capacitance-load sensitivity, non-linear response, and the requirement to form and accurately maintain a precision structure with narrow electrode spacing. Variable-area capacitance transducers with a flexible electrode responsive to a physical effect are less known and less appreciated for their ability to provide an order of magnitude and greater increase in quiescence capacitance, capacitive-load sensitivity and linear dynamic range. Variable-area capacitors of U.S. Pat. No. 6,151,967 and those simply illustrated in
FIGS. 1 and 2
are constructed by sandwiching a thin dielectric layer between a flexible electrode and a cooperating rigid electrode with a curved surface. The dielectric layer maintains a region of fixed capacitance spacing between mutually opposed areas of the rigid electrode and flexible electrode. The region of fixed capacitive spacing increases in extent as the flexible electrode deflects in response to a physical effect. The flexible electrode can be of electrically conducting material, or it can be comprise an electrically conducting layer affixed to at least one surface of a compliant member of dielectric material.
A variable-area capacitor with a flexible electrode comprising a compliant cantilever beam was described by Carter et al., “Measurement of Displacement and Strain by Capacity Methods”,
Proc. J. Mech. Engr
. (152) 1945, pp. 215-221. The Carter transducer is generally of the design shown in FIG.
1
. The use of this transducer as a displacement sensor was described by Frank,
Electrical Measurement Analysis
, McGraw-Hill, NY, 1959. Variable-area capacitors of U.S. Pat. No. 6,151,967, can be fabricated with flexible electrodes that include circular and rectilinear diaphragms. Transducers of this general design are in
FIGS. 2 and 3
.
The electrostatic deflection of a cantilever beam is a well known art that was recently reviewed by Legtenberg, et al., “Electrostatic Curved Electrode Actuators,”
J. Micro Electro Mech Syst
. vol. 6, no. 3, 1997. The electrostatic deflection of circular and rectilinear diaphragms for a loudspeaker is an invention of Kyle, U.S. Pat. No. 1,644,387, Oct. 4, 1927. The Kyle invention is further discussed by Ford, et al., “The Kyle Condenser Loud Speaker,”
P. Inst. Radio Engr
., vol. 17, no. 7, 1929. Since this early work, a rich art has developed for electrostatically controlled actuators. None of the above cited prior-art references teach or suggest using a variable-area capacitor to develop an electrostatic force to position an actuator and the same capa
LandOfFree
Method to force-balance capacitive transducers does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method to force-balance capacitive transducers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method to force-balance capacitive transducers will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2922822