Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-06-28
2001-08-07
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S862590
Reexamination Certificate
active
06269698
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to vibrating beams, including piezoelectric or silicon beams that may be piezoelectrically, electrostatically, electromagnetically, or thermally driven, and particularly to vibrating beams that are utilized as force sensors, for example, acceleration sensors or accelerometers. In particular, the present invention relates to a method and apparatus for reducing the forces transferred to the beam supporting structure to thereby improve the mechanical resonance amplification factor (Q) of the vibratory system.
A widely used technique for force detection and measurement in various mechanical resonators, including acceleration, and pressure sensors, employs one or more vibrating beams having a frequency of vibration which varies as a function of the force applied. An electrostatic, electromagnetic, piezoelectric or thermal force is applied to the beams to cause them to vibrate transversely or in various other modes at a resonant frequency. The resonant frequency of such a beam is raised when subjected to tension and lowered when subjected to compression. The mechanical resonator is designed so that the physical quantity to be measured results in tension or compression of the vibrating beam or beams, whereby the vibration frequency of the beam or beams is a measure of the amplitude of the quantity being measured. In one such mechanical resonator, one or more elongate vibrating beams are coupled between an instrument frame and a proof mass suspended by a flexure to measure acceleration. Acceleration force applied to the proof mass along a fixed axis causes tension or compression of the beams, which varies the frequency of the vibrating beams. The force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams.
Recently, mechanical resonators have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical resonator device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER and U.S. Pat. No. 4,945,765, SILICON MICROMACHINED ACCELEROMETER, the complete disclosures of which are incorporated herein by reference.
In electrostatically driven mechanical resonators, the elongate beam(s) are typically vibrated by a drive electrode(s) positioned adjacent to or near each beam. A drive voltage, e.g., alternating current, is applied to the drive electrode(s) in conjunction with a bias voltage to generate an electrostatic force that vibrates the beam(s) at a resonant frequency. Motion of the beam(s), in turn, generates a current between the electrode and the beam(s) to produce an electrical signal representing the vibration frequency of the beam. Typically, high bias voltages are considered desirable because the current signal from the charging capacitance is proportional to the bias voltage. Therefore, increasing the bias voltage increases the signal to noise ratio of the resonator such that less amplifier gain is required for the oscillator circuit.
Another important consideration in the manufacture of miniature vibratory force sensing mechanical resonators is to minimize variations in the frequency signal from the vibrating beams, except for frequency variations responsive to the applied force. To that end, manufacturers of these devices typically strive to maximize the resonance amplification factor (Q) of the vibrating beams, which generally represents the sharpness of the resonances. The resonance amplification factor, or Q, is typically maximized by partially or completely evacuating the chamber surrounding the mechanical resonator to reduce viscous damping of the resonator beams. Thus, mechanical resonators ideally operate in a vacuum to increase the Q and thereby increase the signal-to-noise ratio of the mechanical resonator.
Various transducers, including accelerometers, utilize one or more vibrating beams that vibrate laterally in the plane of the beams or in various other modes. The resonant frequency of such a beam or beams is raised when the beam is subject to tension and lowered when subjected to compression. The transducer is designed so that the physical quality to be measured results in application of tension or compression to the vibrating beam or beams so that the frequency of vibration of the beam or beams is a measure of the amplitude of the quantity being measured. The performance of a vibrating beam is also degraded if energy is transferred from the beam to other structures, for example, the beam supporting structure, through rotational and transverse forces at the ends of the beam. Such mechanical coupling between the beam and the supporting structure can lower the Q of the beam and cause undesirable frequency shifts. One prior art method used a double-ended tuning fork having multiple beams vibrating out of phase to cancel rotational and transverse forces to reduce the energy transfer from the beam. The double-ended tuning fork utilizes two or more beams located side-by-side vibrating in opposite directions to cancel the forces appearing at the ends of the beams. The out of phase vibrations of the double-ended tuning fork set up equal and opposite reaction forces in the supporting structure at the ends of the beams which cancel. Examples of multiple beam resonators used to reduce energy transfer to the supporting structure are disclosed in U.S. Pat. No. 4,215,570; U.S. Pat. No. 4,372,173; U.S. Pat. No. 4,415,827 and U.S. Pat. No. 4,901,586, the complete disclosures of which are incorporated herein by reference.
Another prior art approach used vibration isolators between the ends of the beams and the supporting structure to reduce the transfer of energy from the beam to the mounting structure. Such isolators usually have an isolation mass at each end of the vibrating beam and a resilient member between each isolation mass and the supporting structure. The resilient members permit the beam and the isolator masses to move relative to the supporting structure, whereby the amount of energy transferred from the vibrating beam to the supporting structure is reduced. The isolation systems are most effective when the isolator masses are large and the isolation springs are compliant. Such large isolator masses and compliance springs result in a low resonant frequency for the isolation system which is undesirable, particularly in accelerometer applications. In addition, isolation systems attenuate the reaction forces generated in the supporting structure, but cannot completely eliminate them.
U.S. Pat. No. 5,450,762, REACTIONLESS SINGLE BEAM VIBRATING FORCE SENSOR, the complete disclosure of which is incorporated herein by reference, provides yet another approach using a counter balance structure at the each end of the vibrating beam to cancel rotational and transverse forces appearing at the ends of the beam, whereby the transfer of energy from the beam to the mounting structure is reduced. The counter balances move in directions opposite to the ends of the beam in order to cancel both rotational moments and transverse forces normal to the longitudinal axis of the beam, i.e. moment and shear forces at the ends of the beam. The action of the counter balance generates equal and opposite reaction forces within the beam that cancel the moment and shear forces internally. Therefore, in contrast to the double-ended tuning fork, the counter-balanced beam transmits no energy into the supporting structure and no reaction force is developed within the supporting structure which must be cancelled by an equal and opposite force. The counter balances are configured relative to the beam to completely cancel only one of eit
Allied-Signal Inc.
Chapman John E.
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