US 11,747,512 B1
Phononic comb enhanced MEMS gravity gradiometers
Randall L. Kubena, Oak Park, CA (US); and Logan D. Sorenson, Thousand Oaks, CA (US)
Assigned to HRL LABORATORIES, LLC, Malibu, CA (US)
Filed by HRL Laboratories, LLC, Malibu, CA (US)
Filed on Jul. 17, 2020, as Appl. No. 16/932,447.
Claims priority of provisional application 62/890,799, filed on Aug. 23, 2019.
Int. Cl. G01V 7/00 (2006.01); G01V 7/04 (2006.01)
CPC G01V 7/005 (2013.01) [G01V 7/04 (2013.01)] 19 Claims
OG exemplary drawing
 
1. A gravity gradiometer for detecting a change in an acceleration gradient δa(t), the gradiometer comprising:
a. first and second piezoelectric type electromechanical resonators, each of the first and second piezoelectric type electromechanical resonators having a piezoelectric structure that mechanically resonates in use and each of which has first and second electrodes disposed on its resonating piezoelectric structure;
b. a first proof mass mechanically coupled with the resonating piezoelectric structure of the first piezoelectric type electromechanical resonator and a second proof mass mechanically coupled with the resonating piezoelectric structure of the second piezoelectric type electromechanical resonator;
c. a first oscillator circuit coupled with the electrodes disposed on the resonating piezoelectric structure of the first piezoelectric type electromechanical resonator, the first piezoelectric type electromechanical resonator resonating at a frequency fD, and having an output providing a signal at said frequency fD:
d. the second piezoelectric type electromechanical resonator having a resonant mode at a frequency fΘ, frequency fΘ being different than said frequency fD:
e. first circuitry for applying the signal at said frequency fD to one of the electrodes disposed on the resonating piezoelectric structure of the second piezoelectric type electromechanical resonator for causing the resonating piezoelectric structure of the second piezoelectric type electromechanical resonator to resonate with a comb of frequencies each tooth of the comb being separated from each other by a frequency Δ(t) a change in which corresponds to the change in the acceleration gradient δa(t) in two gravity fields a1(t) and a2(t) respectively sensed by the first and second proof masses;
f. second circuitry for selecting a nth tooth output by the other one of the electrodes disposed on the resonating piezoelectric structure of the second piezoelectric type electromechanical resonator, where the frequency of the nth tooth of the comb is equal to fD+nΔ(t) and for detecting the change in frequency of the nth tooth using a PLL wherein a feedback error signal of the PLL is proportional to n times the change in the acceleration gradient.