Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-09-20
2002-11-26
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06484578
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the detection and measurement of forces and more particularly to an improved accelerometer incorporating one or more vibrating force transducers for measuring the force applied to a proof mass. The present invention also relates to a method for manufacturing the accelerometer.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. In one such mechanical resonator, one or more elongate beams are coupled between an instrument frame and a proof mass suspended by a flexure. A force, which may be electrostatic, electromagnetic or piezoelectric, is applied to the beams to cause them to vibrate transversely at a resonant frequency. The mechanical resonator is designed so that force applied to the proof mass along a fixed axis will cause 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, vibratory force transducers 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 unmasked 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. Nos. 5,006,487, “Method of Making an Electrostatic Silicon Accelerometer” and 4,945,765 “Silicon Micromachined Accelerometer”, the complete disclosures of which are incorporated herein by reference.
The present invention is particularly concerned with Accelerometers having vibrating beams driven by electrostatic forces. In one method of fabricating such miniature accelerometers, a thin layer of silicon, on the order of about 20 micrometers thick, is epitaxially grown on a planar surface of a silicon substrate. The epitaxial layer is etched, preferably by reactive ion etching in a suitable plasma, to form the vibrating components of one or more vibratory force transducers (i.e., vibrating beams and electrodes). The opposite surface of the substrate is etched to form a proof mass suspended from a stationary frame by one or more flexure hinge(s). While the opposite surface of the substrate is being etched, the epitaxial layer is typically held at an electric potential to prevent undesirable etching of the epitaxial layer. During operation of the transducer, the beams and the electrodes are electrically isolated from the substrate by back biasing a diode junction between the epitaxial layer and the substrate. The transducer may then be coupled to a suitable electrical circuit to provide the electrical signals required for operation. In silicon vibrating beam accelerometers, for example, the beams are capacitively coupled to the electrodes, and then both the beams and electrodes are connected to an oscillator circuit.
The above described method of manufacturing force detection devices suffers from a number of drawbacks. One such drawback is that the beams and electrodes of the vibratory force transducer(s) are often not sufficiently electrically isolated from the underlying substrate. At high operating temperatures, for example, electric charge or current may leak across the diode junction between the substrate and the epitaxial layer, thereby degrading the performance of the transducer(s). Another drawback with this method is that it is difficult to etch the substrate without etching the epitaxial layer (even when the epitaxial layer is held at an electric potential). This undesirable etching of the epitaxial layer may reduce the accuracy of the transducer.
Another drawback with many existing force detection devices, such as accelerometers, is that they often have an asymmetrical design, which may make it more difficult to incorporate the accelerometer into a system, particularly in high performance applications. For example, the proof mass flexure hinge is typically etched on the opposite surface of the substrate as the transducers. This produces an asymmetrical device in which the input axis of the accelerometer is skewed relative to a direction normal to the surface of the silicon wafer.
Pendulous accelerometers, for example, vibrating beam accelerometers, capacitive accelerometers, capacitive rebalance accelerometers, and translational mass accelerometers comprise a reaction mass. Existing design and manufacturing techniques for these devices are described in U.S. Pat. Nos. 4,495,815 “Mass And Coil Arrangement For Use In An Accelerometer,” 5,396,798 “Mechanical Resonance, Silicon Accelerometer,” 4,766,768 “Accelerometer With Isolator For Common Mode Inputs,” 5,228,341 “Capacitive Acceleration Detector Having Reduced Mass Portion,” 5,350,189 “Capacitance Type Accelerometer For Air Bag System,” 4,335,611 “Accelerometer,” and 3,702,073 “Accelerometer” which are incorporated herein by reference. All practical pendulous accelerometers to date function on the principle of Neuton's law that force equals mass times acceleration. In many accelerometer applications high performance and small size are desirable. One problem with the design of small, high performance pendulous accelerometer sensors involves obtaining adequate reaction mass in a small space. A second problem with the design of small, high performance pendulous accelerometer sensors involves providing adequate isolation from the mounting structure such that mounting strains do not affect accelerometer performance. Typical accelerometer sensors include a pendulous reaction mass, often referred to as a proof mass, suspended from a stationary frame by, for example, a flexural suspension member of some other form of pivot mechanism. This pivot constrains the reaction mass to only one direction of motion; the reaction mass is free to move along this one direction of motion unless restrained to the null position. The pendulous reaction mass must be restrained under acceleration by an opposing force which may be the result of a position feedback circuit. Alternatively, the accelerometer may be an open-loop device in which the opposing force may be supplied a spring in the form of, for example, pivot stiffness. In a typical accelerometer sensor mechanism the pendulous reaction mass is suspended on a flexural suspension member inside an external support frame. Isolation is typically provided by mounting the supporting frame itself inside an isolation feature supported from a final exterior frame which provides mounting both to sensor covers and to the accelerometer housing. The above features as practiced in a typical vibrating beam accelerometer sensor are shown in
FIGS. 1 and 2
. The large exterior frame system is static and adds no mass to the active reaction mass. Additionally, any external strain couples through the exterior frame system directly across the length of the sensor mechanism. The resulting large frame dimensions tend to maximize the effect of error drivers, for example, thermal expansion mismatch, placing additional burden on the isolator function.
SUMMARY OF THE INVENTION
The present invention provides methods for detecting and measuring forces with mechanical resonators and improved methods of manufacturing these force detecting apparatus. These methods and apparatus are useful in a variety of applications, and they are particularly useful for measuring acceleration.
The present invention includes a substrate coupled to a thin active layer each comprising a semiconducting material. The substrate has a frame and a proof mass suspended from the frame by one or more flexures. The active layer includes one or more vibratory force transducers suitably coupled to the proof mass for detecting a force applied to the proof mass. According to the prese
Foote Steven A.
Woodruff James R.
Allied-Signal Inc.
Chapman John E.
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