Shipboard gravimeter

Claim 1

Apparatus for removing short term frequency variations in the output signal from an accelerometer which is subjected to both steady and short term accelerations, which accelerometer is of the type which generates an electrical signal having an instantaneous frequency proportional to the acceleration to which it is subjected, comprising: phase detection means having one of two input circuits coupled to the output of said accelerometer and having an output circuit at which is generated a signal having an amplitude proportional to the phase difference between any pair of signals coupled to said input circuits; an oscillator whose frequency is initially set to the mean frequency of said accelerometer and is variable about said mean frequency in response to the amplitude of a control voltage applied to a control circuit thereof, the output of said oscillator being coupled to the other input circuit of said phase detection means; and means connected between the output circuit of said phase detection means and the control circuit of said oscillator for coupling only low frequency components of the signal generated by said phase detection means to said oscillator, whereby the frequency of said oscillator is proportional only to the steady accelerations to which said accelerometer is subjected.

Claim 5

A system for processing signals generated by a vibrating string accelerometer and for determining the strength of the gravitational field to which said accelerometer is subjected, comprising: a phase detector having one of its two inputs coupled to the output of said accelerometer so as to provide said detector with a first input signal and having an output at which is proviDed a signal whose amplitude represents the phase difference between any pair of signals coupled to said inputs; a voltage controlled oscillator for producing a pulsed output signal having a center frequency equal to N times the mean frequency of the output signal from said accelerometer where N is a predetermined positive integer and having a control circuit which varies the output frequency of said oscillator about said center frequency in response to the amplitude of a control voltage applied thereto; means coupled to the output of said oscillator for producing a signal at its output which has a frequency exactly equal to the frequency of said oscillator divided by N; means for coupling the output of said last-mentioned means to the other input of said phase detector so as to provide said detector with a second input signal; a low-pass filter for attenuating the high frequency components of the signal generated at the output of said phase detector; means for connecting said low-pass filter between the output of said detector and the control circuit of said oscillator whereby only low frequency components of the signal from said detector are coupled to said oscillator; and means for registering the number of output pulses generated by said oscillator during a predetermined time interval, whereby said number is proportional to the gravitational field to which said accelerometer is subjected.

Claim 6

A system for processing signals generated by a vibrating string accelerometer and for determining the strength of the gravitational field to which said accelerometer is subjected, comprising: a phase detector having one of its two inputs coupled to the output of said accelerometer so as to provide said detector with a first input signal and having an output at which is proviDed a signal whose amplitude represents the phase difference between any pair of signals coupled to said inputs; a voltage controlled oscillator for producing a pulsed output signal having a center frequency equal to N times the mean frequency of the output signal from said accelerometer where N is a predetermined positive integer and having a control circuit which varies the output frequency of said oscillator about said center frequency in response to the amplitude of a control voltage applied thereto; means coupled to the output of said oscillator for producing a signal at its output which has a frequency exactly equal to the frequency of said oscillator divided by N; means for coupling the output of said last-mentioned means to the other input of said phase detector so as to provide said detector with a second input signal; a low-pass filter for attenuating the high frequency components of the signal generated at the output of said phase detector; means for connecting said low-pass filter between the output of said detector and the control circuit of said oscillator whereby only low frequency components of the signal from said detector are coupled to said oscillator; and means for registering the number of output pulses generated by said oscillator during a predetermined time interval, whereby said number is proportional to the gravitational field to which said accelerometer is subjected.

Claim 8

A system for measuring local gravitational accelerations at sea comprising: a vibrating string accelerometer which generates a first signal having a frequency proportional to accelerations to which it is subjected; means for multiplying the frequency of said signal by a positive integer, M, so as to produce a second signal M times greater in frequency than said first signal; a phase detector having one of its two inputs coupled to the output of said multiplying means and having an output which provides a signal whose amplitude represents the phase difference between signals coupled to said inputs; a voltage controlled oscillator having a center frequency N times the mean frequency output of said multiplying means and having a control circuit which varies the output frequency of said oscillator about said center frequency in response to the amplitude of a control voltage applied thereto; a low-pass filter; means for connecting said low-pass filter between the output of said detector and the control circuit of said oscillator so as to complete a feedback loop to said oscillator which loop contains only low frequency components of the signal from said detector; divide-by-N means coupled between the output of said oscillator and the other input of said detector to provide said detector with a signal whose frequency corresponds to that component of the frequency of said second signal due only to gravitational accelerations; and means coupled to the output of said oscillator for registering the number of oscillations generated by said oscillator during a predetermined time interval.

Claim 9

A system for measuring local gravitational accelerations at sea comprising: a vibrating string accelerometer which generates a first signal having a frequency proportional to accelerations to which it is subjected; means for multiplying the frequency of said signal by a positive integer, M, so as to produce a second signal M times greater in frequency than said first signal; a phase detector having one of its two inputs coupled to the output of said multiplying means and having an output which provides a signal whose amplitude represents the phase difference between signals coupled to said inputs; a voltage controlled oscillator having a center frequency N times the mean frequency output of said multiplying means and having a control circuit which varies the output frequency of said oscillator about said center frequency in response to the amplitude of a control voltage applied thereto; a low-pass filter; means for connecting said low-pass filter between the output of said detector and the control circuit of said oscillator so as to complete a feedback loop to said oscillator which loop contains only low frequency components of the signal from said detector; divide-by-N means coupled between the output of said oscillator and the other input of said detector to provide said detector with a signal whose frequency corresponds to that component of the frequency of said second signal due only to gravitational accelerations; and means coupled to the output of said oscillator for registering the number of oscillations generated by said oscillator during a predetermined time interval.