Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1998-05-28
2001-05-15
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S497000, C073S514290
Reexamination Certificate
active
06230565
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to pressure-compensated transducers and force-sensing methods, and particularly to pressure-compensated and temperature-compensated accelerometers, and acceleration-sensing methods.
BACKGROUND OF THE INVENTION
Force transducers are often used as force-to-frequency converters in accelerometers and other instruments. One type of force transducer employs a vibratable assembly which can be used to sense acceleration. In one known arrangement, the transducers are used in push-pull pairs in which a given acceleration results in a compression force on one transducer, and a tension force on the other transducer. This mode of operation provides a high degree of compensation for many so-called common mode errors, i.e. errors that cause the frequencies of the transducer to shift by the same amount in the same direction, because the shifts cancel in the algorithms normally used to process the transducer outputs. Such errors include vibration rectification errors, errors induced by temperature change, aging errors, and measurement errors induced by a drift in the clock frequency.
Such force transducers can also be sensitive to density or pressure variations. The density or pressure sensitivity is primarily due to mass loading effects on the beams. Specifically, gas molecules near the beams tend to oscillate with the beams. Such effectively increases the mass of the beams, thereby affecting the frequencies at which the beam vibrates. When the pressure or density of the surrounding gas increases, the effective mass of the beams also increase which lowers the beams' vibration frequencies. Because the beams' vibration frequencies are employed as a measurement of the applied force, e.g. acceleration, the density-induced or pressure-induced variation can cause an unwanted error in the sensed acceleration output. Accordingly, it would be desirable to reduce, if not eliminate this density-induced or pressure-induced error.
Precision force transducers can be packaged in a vacuum to avoid errors resulting from the density-induced variation. However, the choice of internal materials used in precision sensor designs is severely constrained due to out-gassing concerns. Because gas density within the package directly drives density-induced variations, a heavy burden of hermeticity is place on the packaging. The material constraints in hermeticity requirements, in turn, increase the cost and limit performance. Accordingly, it would be desirable to provide an accelerometer with reduced pressure effects and simpler packaging constraints.
This invention arose out of concerns associated with improving forced-sensing transducer and accelerometer operations. In particular, the invention arose out of concerns associated with providing improved vibrating beam force transducers and methods.
SUMMARY OF THE INVENTION
Pressure-compensated transducers, pressure-compensated accelerometers, force-sensing methods, and acceleration-sensing methods are described.
According to one aspect of the invention, a pressure-compensated transducer is provided and includes a movable member and a vibratable assembly coupled with the movable member. The assembly is configured to vibrate at various frequencies responsive to movement of the movable member. The assembly has a frequency output which gives an indication of the various frequencies, with the various frequencies giving an indication of a force acting upon the movable member. A temperature sensor external to the vibratable assembly is configured to measure a temperature proximate the vibratable assembly and provide a temperature output. Preferably, a compensator is provided and includes a frequency input for receiving the frequency output of the vibratable assembly, and a temperature input for receiving the temperature output of the temperature sensor. The compensator is configured to compute, from the frequency output and the temperature output, a more accurate force measurement which compensates for a pressure condition proximate the vibratable assembly.
According to another aspect of the invention, a pressure-compensated accelerometer includes a deflectable proof mass and a vibrating beam assembly coupled with the proof mass. The assembly is configured to develop different vibratory frequencies responsive to an acceleration applied to the proof mass. A temperature sensor is configured to measure a temperature condition proximate the vibrating beam assembly. A compensation processor is operably coupled with the temperature sensor and configured to enable computation of a more accurate, pressure-compensated and temperature-compensated acceleration.
According to another aspect of the invention, a force-sensing method comprises using a pair of measured vibration frequencies and a measured temperature to compute a temperature-compensated, pressure-compensated force experienced by a vibrating beam force sensor.
According to another aspect of the invention, an acceleration-sensing method comprises providing a deflectable proof mass and a vibrating beam assembly coupled with the proof mass, the assembly and proof mass being collectively configured to develop different vibratory frequencies responsive to an acceleration applied to the proof mass. The different vibratory frequencies are determined and a temperature condition proximate the vibrating beam assembly is measured. Using the determined vibratory frequencies and the measured temperature condition, a pressure-compensated acceleration is derived.
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patent: 5511427 (1996-04-01), Burns
patent: 5546810 (1996-08-01), Arikawa et al.
Allied-Signal Inc.
Kwok Helen
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