Many modern guidance and navigation systems use vibrating beam sensors to measure parameters used in controlling the flight path of aircraft, missile, or other flight vehicle. Vibrating beam sensors typically depend upon crystal beam oscillators to provide a frequency output that changes frequency as strain in the beam changes. As an example, in a typical accelerometer application, the beam is connected to a proof mass supported by flexures connected to another structure. When the proof mass is acted upon by acceleration, the proof mass deflects about the flexures, and stretches or compresses the crystal beam. In some applications, two crystal beams are used in such a way that one is compressed and the other is stretched as the proof mass deflects. The frequency of the beam in tension increases and that of the beam in compression decreases. In these types of accelerometers both frequencies are used to provide better performance.
An example of an accelerometer with two crystal beams is the Accelerex® RBA-500, made by Honeywell Inc., Redmond, Wash. In this accelerometer, the crystal beams are driven at one of their natural resonant frequencies and the oscillations generate nearly sinusoidal waveforms in closed loop electronics. The sinusoidal waveforms are internally, electronically converted to square wave output signals from the accelerometer.
The frequency output of a crystal beam accelerometer is dependent on the input acceleration. The frequency output is limited by the mechanical structure of the accelerometer as well as its internal electronics. Further, deflection of the proof mass is limited by physical stops. The stops are designed to allow the desired acceleration dynamic range for the accelerometer. Further, the stops limit the travel of the proof mass to keep from damaging the crystals and flexures from excessive strain. Since the proof mass deflection is limited, the strain in the crystal beams should be limited and the expected frequency change of the crystal beams should fall within an established frequency band. If the acceleration exceeds the magnitude at which the proof mass hits the stops, it is expected that the frequency output of the crystal beams would be limited to the values corresponding to the proof mass deflected at the stops. For example, the nominal output of an RBA-500 is two square waves with frequencies of 35 kHz. The frequencies vary with acceleration until the stops are contacted. When the stops are contacted, the frequency of one crystal is about 30 kHz and the frequency of the other crystal is about 40 kHz. These are only illustrative values and will vary for each accelerometer.
Typically, guidance and navigation systems determine the meaning of the output signals of the accelerometer with digital electronics. In some systems, the digital electronics count the number of rising or falling edges in a square wave signal output by the accelerometer. This provides a measure of the frequency of the output signal and, in turn, a measure of acceleration since the frequency of the output signal is related to the acceleration.
Unfortunately, the crystals of an accelerometer are known to output higher frequencies or lower frequencies than normal under high dynamic environments. This may be due to other resonant frequencies of the crystal beams or it may be due to transient strains on the crystal beams as a result of high velocity paddle impacts with the stops. The number of occurrences and the duration of the occurrences are unpredictable.
The anomalous output of higher or lower frequencies can lead to a greater or lesser number of counts than should be possible, leading to the types of errors already described. The effect of these higher or lower than expected counts is to cause the acceleration and velocity to be incorrectly computed, leading to an apparent velocity shift and a subsequent error in guidance or navigation.
Therefore, there is a need in the art for enhancing the accuracy of the output of a sensor.