Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-11-23
2001-11-06
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06311556
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the production of micro-accelerometers machined in silicon, in particular accelerometers for applications assisting with navigation in aircraft.
More precisely, the invention relates to a resonator accelerometer, in which a micromachined proof mass is connected by a vibrating beam, also micromachined, to a fixed frame forming a part of the framework of the accelerometer. The beam is mechanically tensioned by the weight of the proof mass, and the vibration of the beam is electrostatically excited by a tuned circuit comprising a moving-electrode capacitor, the position of the beam determining the position of the electrode. The electric field applied to the capacitor by the tuned circuit tends to move the beam, and the movement of the beam changes the value of the capacitor of the tuned circuit; the feedback of the tuned circuit is such that mechanical and electrical resonance occurs at a natural vibration frequency of the beam. The resonance frequency, that is to say the frequency at which the beam naturally enters into self-sustained vibration, depends on the mechanical tension longitudinally exerted on it, as is the situation, for example, with a musical instrument string. This mechanical tension itself depends on the acceleration to which the proof mass exerting the tension is subjected. A frequency measurement in the resonant circuit therefore represents an acceleration measurement.
Such a micro-accelerometer is therefore a combination of mechanical structure (proof mass, vibrating beam, other suspension arms, fixed framework) and electrical structure (capacitor electrodes, current-feed connections, and external circuitry forming a resonant circuit).
The characteristics expected of such an accelerometer are principally small size, good sensitivity along a well-identified axis of acceleration measurement, referred to as the sensitive axis, low sensitivity to accelerations along axes perpendicular to the sensitive axis, good linearity and good accuracy in the acceleration measurement, good mechanical strength both in the event of accelerations or impacts in the sense of the sensitive axis and in transverse directions, and lastly a low fabrication cost.
The cost is limited even further by the fact that batch fabrication can be used, which is why silicon-machining processes derived from integrated-circuit fabrication technologies have been envisaged.
2. Discussion of the Background
It has already been proposed, in particular, to produce both the proof mass and its suspension in silicon, the rest of the accelerometer being in quartz, which it is also known how to micromachine, electrical electrodes being deposited on the quartz such that they face towards the active silicon plate. Drawbacks have been observed with these hybrid structures, in particular because this makes it more difficult to produce the electrical parts of the resonator.
Silicon micro-accelerometer structures have also been proposed, in which the vibrating beam constitutes a deformable mechanical support carrying a strain gauge incorporated in a circuit capable of detecting resistance variations and amplifying them to inject them back into an electrostatic drive for the beam. This structure has the advantage of avoiding problems with electrical coupling between the elements which do not actually play a part in the measuring circuit, but fabrication is more complicated, especially because of the need for strain gauges and connections of the electrical circuit. In this case, only one of the plates is electrically active; the other plates are above all used as a closure cover. Errors are moreover induced by the use of heterogeneous materials.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a novel micro-accelerometer structure in silicon technology. This micro-accelerometer is characterized by the following principal arrangements: it has three micromachined-conducting silicon plates welded in superposition with the interposition of insulating layers, the central plate comprising a subassembly sensitive to accelerations, and a peripheral frame electrically insulated from the subassembly and surrounding the subassembly, this frame forming a spacer between a lower plate and an upper plate from which it is also electrically insulated. The subassembly comprises a base fixed on the lower plate and a cantilevered proof mass suspended from the base, an electrical connection being made between the lower plate and the base of the subassembly. The suspension of the proof mass comprises, on the one hand, a central vibrating beam which is connected to the proof mass and to the base and is placed substantially in the horizontal plane of an upper face of the proof mass, and, on the other hand, two short side suspension arms which are connected to the base and are placed on either side of the central beam but in a horizontal plane passing substantially through the center of gravity of the proof mass.
The expression horizontal plane is used here for convenience to denote the parallel plane of the three silicon plates, the accelerometer being assumed to be placed in such a way that the plates are horizontal (the measured accelerations then being vertical because the sensitive axis is perpendicular to the surface of the plates).
The base of the proof subassembly or the proof mass (or both) preferably has a U-shape seen on a horizontal plane, the inside of the U being turned towards the proof mass (or vice versa towards the base), the vibrating beam being connected to the center of the U and the side suspension arms being connected to the ends of the branches of the U. This U-shape makes it possible to produce suspension arms which are short compared with the length of the vibrating beam.
With the aim of avoiding bending deformation on the vibrating beam as far as possible, the point where this beam is fixed on the proof mass lies preferably substantially in line with a fictitious axis of articulation of the proof mass about the fixture represented by the suspension arms. In other words, if it is assumed that in the absence of any connection to the vibrating beam, the proof mass is connected to the base only by the side suspension arms, the suspension behaves somewhat like a horizontal axis of articulation about which the mass can turn, and arrangements are made for the point at which the vibrating beam is fixed on the proof mass to be substantially vertically in line with this axis. This limits the bending deformations of the vibrating beam as far as possible.
Provision may, however, also be made for it instead to be the middle of the vibrating beam which lies substantially in line with the axis of articulation, if it is preferred to promote the thermal stability of the sensor.
This accelerator is excited by connecting the upper plate, on the one hand, and the lower plate, on the other hand, to a resonant circuit, using connection contacts formed on the silicon of these two plates. The electrical voltage applied to the lower plate, via a connection contact on this plate, is transmitted by silicon conduction to the vibrating beam which forms one electrode (in conducting silicon) of a capacitor, opposite an upper-plate portion, also in conducting silicon, which forms the other electrode of the capacitor; this other electrode is electrically connected, once again by direct silicon conduction, to a connection contact on the upper plate. It is not necessary to provide conducting deposits on the silicon plates in order to form the electrodes of the capacitor.
The central plate is preferably connected, via an electrical connection contact on the silicon, to a fixed potential making it possible, by a capacitive screening effect, to avoid high-frequency capacitive current transmission between the upper plate and the lower plate.
This is possible, in particular, in a configuration in which the peripheral frame is physically separated completely from the proof subassembly by etching the silicon of the central plate through its full thicknes
Lefort Olivier
Thomas Isabelle
Chapman John E.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sextant Avionique
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