The NG DIVA: A Navigation Grade Differential Inertial Vibrating Beam Accelerometer
Olivier Le Traon, Jean Guérard, Raphael Levy, Pierre Lavenus, Amandine Andrieux Ledier, DPhIEE, ONERA, Université Paris Saclay, France
Location: Big Sur
Twenty years ago, the VIA vibrating beam accelerometer was presented at the PLANS conference . The VIA is a non-differential quartz monolithic accelerometer -i.e. the complete accelerometer is made of 2 VIA chips in a push-pull arrangement - dedicated for tactical application: a high measurement range exceeding 100 g, a bias stability (all error combined) better than 300 µg, an excellent scale factor stability better than 10 ppm. Until now, the tactical grade VIA MEMS accelerometer has no equivalent produced in the world. This accelerometer has been transferred for some time to industry and is widely used in inertial systems.
More recently, even if it was ten years ago…, a complete monolithic quartz vibrating beam accelerometer called DIVA (i.e; Differential Inertial Vibrating Accelerometer) was proposed and also presented at the 2006 PLANS conference  as well as at the 2007 ION NTM symposium . The DIVA accelerometer is based on 2 VIA chips monolithically linked together by a frame. The DIVA monolithic structure reaches all of the 3 requirements needed for high accurate VBA: a high insulation of the 2 beams vibrations in order to preserve their intrinsic quality factors (bias stability), a high insulation against thermal stresses (bias stability without hysteresis under temperature change) and a high insulation between the 2 beams in order to reduce the locking zone when the beam frequencies get closer. The monolithic quartz DIVA accelerometer gives a slightly improved performance compared to the VIA but has the added advantage of a small single quartz cell (11.5 mm x 9.4mm) easier to package by wafer level packaging.
This paper presents the last generation of the DIVA called NG DIVA (i.e. New Generation/Navigation Grade DIVA accelerometer), aiming to propose a very compact, navigation grade (bias stability better than 50 µg all errors combined, measurement range above 50 g), robust to harsh environment (first eigenfrequency above than 3000 Hz) and easy to manufacture (collective micromachining by chemical etching). These performance improvements have required a hard work at different levels:
- Chemical etching of quartz wafer: a new manufacturing process with 2 steps of chemical etching has been has been developed. As a spectacular result, a 50 % reduction of the DIVA chip surface has been obtained at iso quartz mass (i.e. with the same proof masses for the differential accelerometer). As other result, through this very compact cell together with an optimization of the design, the eigenfrequency of the NG DIVA is now above 3000 Hz in comparison of 2200 Hz for the previous DIVA generation. Finally, a better control of the chemical etching process allowed also a reduction of etched surface roughness and an improvement of the differential effect.
- Improvement of the Factor Of Merit (FOM) QxSF/F, where Q is the quality factor of the beams, Sf the scale factor (Hz/g) and F the beam frequency (Hz): the NG DIVA FOM has been doubled compared to the previous DIVA design, mainly thanks to the use of an optimized beam (i.e; a beam with a non-constant section allowing a larger increase of the scale factor than the decrease in thermoelastic quality factor of the beam).
- A deeply revisited electronics: earlier electronics only included a simple feedback oscillator circuit to maintain the beam resonance, and the acceleration was measured from the difference between the two resonant frequencies of the differential pair. In the NG DIVA, the oscillator circuit has been replaced by a Phase Locked Loop (PLL) and a Direct Digital Synthesizer (DDS) to drive the beam resonance. Apart from removing analog components and reducing external circuitry, the advantage of this architecture is to deliver the frequency directly from the source (the synthesizer), not from measurement (like a counter), making the frequency difference a simple yet exact digital subtraction. Moreover, the synthesizer is also able to deliver the phase increment, which is related to the velocity increment, thus rigorously performing a first integration. The effect of thermal and long term drifts of parallel capacitance C0 of the resonators has been also drastically reduced by a dedicated arrangement (CO effect contributed for as much as 100 µg in the bias residual of the previous electronics).
The navigation grade quartz MEMS accelerometer NG DIVA is the result of twenty years of research on Vibrating Beam Accelerometer. Experimental results in terms of Allan Variance, bias and scale factor stability (aging and stability of temperature) will be presented.
The authors wish to thank to the Délégation Générale de l’Armement (DGA) of the French Ministry of Defense for its partial financial support.
 “A New Quartz Monolithic Differential Vibrating Beam Accelerometer”, O. Le Traon ; D. Janiaud ; M. Pernice ; S. Masson ; S. Muller ; J-Y. Tridera, Position, Location, And Navigation Symposium, 2006 IEEE/ION IEEE, April 25-27, Coronado CA
 “The VIA Vibrating Beam Accelerometer: Concept and Performances” , O. Le Traon, D. Janiaud, S. Muller, P. Bouniol, Position, Location, And Navigation Symposium, 1998, April 20-23, Palm Springs, CA
 “The DIVA Accelerometer and VIG Gyro: Two Quartz Inertial MEMS for Guidance and Navigation Applications”, O. Le Traon, D. Janiaud, S. Masson, S. Muller, and M. Pernice, 2007 National Technical Meeting of The Institute of Navigation, January 22-24, San Diego, CA