Accelerometers have many useful applications. For example, in the motor vehicle industry, accelerometers may be employed effectively to control systems, such as air-bag deployment, anti-skid braking and active suspension systems, which are designed to respond to changes in vehicle acceleration.
Deflection-based accelerometers have been developed to measure changes in acceleration. Such devices, which typically include a proof mass supported by an elastic member, measure the deflection of an object subjected to an acceleration force. The magnitude of the deflection depends on the size of the proof mass, the stiffness of the elastic member and various dimensions of both, in addition to the magnitude of the acceleration itself.
Accordingly, the sensitivity of such devices depends strongly on several typically small, process-variable dimensions. The need for accuracy in measuring such dimensions adds to the cost and complexity of their manufacture. In addition, the calibration factor for each accelerometer must be determined individually, further increasing the expense of manufacturing and deploying such devices.
Moreover, measurement of the deflection in such devices may be difficult. If the accelerometer employs a capacitor for this purpose, the thickness of the gap between the object and a fixed electrode, typically on the order of several microns, must be known. Other methods of measuring deflection have similar uncertainties. Hence the accuracy with which the electrical output signal of the device can be correlated to the deflection, and thus the acceleration, could advantageously be increased.
Accordingly, a need has persisted for an accelerometer sufficiently sensitive to changes in acceleration that is susceptible to batch production on an economical basis.
Frequency-type accelerometers have also been disclosed previously. For example, U.S. Pat. No. 4,805,456 describes a device with two vibrating members oppositely disposed with respect to a proof mass. Such devices compensate for temperature changes in the operating environment, as the frequency of vibration of the members changes oppositely in response to acceleration, but in tandem in response to changes in temperature.
However, because the proof mass of the disclosed device is a separate object, the frequency is determined both by the dimensions of the proof mass and by the dimensions of the vibrating members. Again, the small size of many of these dimensions makes measurement of the various elements and calibration of the device difficult. In addition, manufacture or assembly of devices comprising multiple elements may be difficult. Accordingly, the need has remained for an accelerometer susceptible to ease of manufacture, calibration and use.
A device for measuring acceleration based on the shift in resonant frequency of a vibrating cantilevered beam in response to an acceleration force having a component along its major axis is disclosed. Such accelerometers may be manufactured by micromachining in silicon, and comprise one or more beams, each provided with drive means for keeping the beam in vibration at the resonant frequency, and detection means for sensing changes in the resonant frequency. The sensitivity of accelerometers according to the invention is determined by the length of the cantilevered beam, a factor easily controlled during manufacture, and by the resonant frequency of the beam, which is continuously measured in use. Accelerometers constructed according to the invention are self-calibrating, self-testing and susceptible to large-scale batch manufacture.