Measuring and testing – Speed – velocity – or acceleration – Temperature compensator
Reexamination Certificate
2000-01-14
2001-08-07
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Temperature compensator
C073S514370
Reexamination Certificate
active
06269696
ABSTRACT:
FIELD OF INVENTION
This invention relates to a temperature compensated oscillating accelerometer with force multiplier.
BACKGROUND OF INVENTION
Accelerometers require either a frequency or voltage reference. Voltage references hardened against radiation are not available so that frequency referenced accelerometers are preferred for strategic applications. Frequency based accelerometers include silicon micromachined devices and quartz devices. Because of limitations in making small quartz beams, the quartz accelerometers are large and made of several hand-assembled pieces, whose joints cause performance-limiting errors. Silicon micromachining offers smaller size and lower cost in more reliable monolithic accelerometers but suffer less than desired performance, thermal sensitivity and fabrication yield.
These accelerometers typically have a proof mass suspended above a substrate. A tuning fork is connected to the substrate through an anchor at one end and to the proof mass at the other end. The tuning fork has a certain mechanical resonance with no acceleration applied. A force current supplied to the tuning fork causes a sense current whose frequency is a function of that mechanical resonance. When an acceleration occurs the proof mass moves relative to the substrate causing the tuning fork to be stretched or compressed depending on the direction of acceleration along the input axis. Stretching increases the stiffness and thus the frequency of the sensed current. Compression decreases stiffness and the frequency. This change in frequency can be used to measure the acceleration. Often two tuning forks are used arranged so that when one is stretched the other is compressed. Frequency shift between the two improves the representative signal and suppresses common mode errors. One improvement on these accelerometers employs a force multiplier in the form of a lever and a second anchor to multiply the force of the acceleration applied to the tuning fork. In both approaches errors occur with variation in temperature due to two different error sources, Young's modulus and the coefficient of thermal expansion. An increase in temperature causes the elements to expand, and increase the tension and, hence, stiffness and thus the frequency of the tuning fork and sense current. But an increase in temperature causes Young's modulus to decrease in silicon and stiffness decreases with decreases in Young's modulus. A decrease in stiffness results in a drop in frequency of the sense current. The two effects have opposing effects on the accuracy of the tuning fork.
BRIEF SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved temperature compensated oscillating accelerometer with force multiplier.
It is a further object of this invention to provide such an improved temperature compensated oscillating accelerometer with force multiplier which balances variations in Young's modulus with tensions induced by thermal expansion.
It is a further object of this invention to provide an improved temperature compensated oscillating accelerometer with force multiplier which has a high scale factor and high axial resonant frequency.
It is a further object of this invention to provide an improved temperature compensated oscillating accelerometer with force multiplier which is small in size suitable for micromachining and silicon implementation.
The invention results from the realization that an oscillating accelerometer can be temperature compensated by placing the anchor associated with the force multiplier in approximately the same area along the input axis as the primary tuning fork anchor to offset the opposing effects of thermal expansion and stiffness with variations in temperature.
This invention features a temperature compensated oscillating accelerometer with force multiplier including a support substrate and a tuning fork suspended above the substrate. There is a primary anchor device interconnected between the tuning fork and substrate and a proof mass having an input axis. A force multiplier is interconnected between the proof mass and the tuning fork and a force multiplier anchor is connected to the substrate and disposed at approximately the same area along the input axis as the primary anchor for offsetting the opposing effects of thermal expansion and stiffness in response to variations in temperature.
In a preferred embodiment the support substrate may be glass and the tuning fork may be silicon. The tuning fork may include a pair of tuning fork masses connected in parallel between an upper and lower base beam. The tuning fork and primary anchor may suspend the proof mass from the substrate. The proof mass may be suspended from the substrate by at least one auxiliary anchor. The force multiplier may include a force multiplier beam and a thermal compensation beam interconnected between the force multiplier beam and the force multiplier anchor. The force multiplier may be laterally spaced from and adjacent the primary anchor.
REFERENCES:
patent: 5188983 (1993-02-01), Guckel et al.
patent: 5289719 (1994-03-01), Egley et al.
patent: 5996411 (1999-12-01), Leonardson et al.
Borenstein Jeffrey T.
Nokes David S.
St. Michel Nathan A.
Weinberg Marc S.
Iandiorio & Teska
Kwok Helen
The Charles Stark Draper Laboratory Inc.
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