Miniature Vibrating Inertial Sensors VIA quartz vibrating beam accelerometer
The first work in this field at Onera goes back to the 1980's. The challenges were then to understand better the physical phenomena involved in this type of sensor and to achieve very high performance. Accelerometers at that time were made of different assembled parts, and only the resonator (the vibrating beam) was in quartz. Obtaining maximum performance thus depended very much on the quality of the assemblies, which were delicate to control. At the end of the 1980's, a challenge was met by the Onera team that decided to make this type of sensor monolithically, that is, to cut the entire sensor out of the same quartz substrate in order to avoid the assemblies that limited performance, and to make miniaturization easier. Technological studies were then begun on machining the quartz by wet process (chemical machining of wafers) along with studies of the concept of a monolithic sensor with flat structure, and these led to the VIA concept patented in 1995.
This concept makes it possible to decouple the vibrating beam vibrations very efficiently, with the decoupling frame that links the active part of the accelerometer to the attachment regions. This structure makes better use of the excellent mechanical properties of the quartz: The beam vibration quality coefficient, which plays an essential role in the frequency stability of the beam, is at the ultimate limits achievable (coefficient Q ≈ 15,000 and the beam resonance frequency is about 60 kHz). The temperature properties are also very good (low hysteresis of the sensor output at temperature) because the decoupling frame preserves the whole active part from stresses of thermal origin (mechanical stresses due to the differential dilation of the base and the quartz wafer).
VIA :
Monolithic Accelerometer
Concept: The active part of the accelerometer, which consists of the vibrating beam, the proof mass, and the two hinges allowing the proof mass to rotate under the effect of the acceleration (the sensitive axis is transverse to the plane of the wafer) is linked to the attachment regions by a decoupling frame. This frame very efficiently isolates the useful vibrations of the beam and thereby gets the most out of the excellent mechanical properties of the quartz. We thus get excellent beam frequency stability (the bias) and very good temperature properties for the sensor (low hysteresis). This configuration was covered by a patent, applied for in 1995.
The complete accelerometer consists of two VIA quartz wafers assembled in a casing in differential configuration. This differential configuration is needed to reduce the effect of the parasitic physical quantities acting in common mode on the two wafers, like the temperature and the internal pressure of the casing.
The VIA accelerometer performance characteristics are the following:
Measurement range: 100 g
Passband: 1000 Hz
Resolution: 10 µg/Hz 1/2
Precision: 300 µg in severe environment (including the bias stability and the thermal properties in the range from -50 to + 80 °C )
The complete VIA accelerometer
The complete accelerometer consists of two VIA transducers mounted in a casing in differential configuration. The accelerometer body operates in a vacuum in order to preserve the quality coefficients of the resonators. The differential arrangement is used to reduce the sensor's sensitivity to the parasitic physical quantities like the temperature and casing pressure, acting in common mode on the two beams.
This accelerometer has undergone an industrial transfer to the main French inertial equipment makers and is today integrated in military systems.
More recently, two VIA developments have been undertaken:
One concerns the construction of the accelerometer's differential structure on the same quartz substrate. This is the true monolithic accelerometer concept such as it had been studied at the beginning of the research, but which had been dropped because of the difficulty of obtaining two beams on the same substrate vibrating at frequencies very close to each other and perfectly decoupled from each other mechanically. This inadequate decoupling between the two beams has disastrous consequences on the accelerometer operation because it leads to a major nonlinearity of the frequency-acceleration relation in the vicinity of the beam frequency crossing, going as far as "locking" their frequencies. In the locking region, the beam frequencies no longer vary with the acceleration, which results in a blind measurement zone that has to be reduced to values of the order of the precision sought for the accelerometer (for an accelerometer of VIA precision, this blind zone must be less than 1 mg).
The VIA concept made it possible to reconsider this version of monolithic accelerometer by combining two VIA transducers in differential configuration, integral with the same attachment frame. The excellent decoupling of the beam vibrations at the level of each VIA transducer, combined with the special configuration of the attachment frame, reduces this blind zone to extremely low values that are largely compatible with the precision sought. This original configuration was patented in 2004 and the experimental results obtained on complete accelerometer prototypes have shown the validity of the concept with very small blind zone measurements of the order of 300 µg, which hints that this concept can be transferred to the interested industrial concerns in the near future.
The single-wafer VIA accelerometer
In the single-wafer variant, the complete accelerometer is monolithic in quartz. The special configuration of this differential arrangement sufficiently reduces the mechanical coupling between the two resonators and achieves an acceleration measurement blind zone compatible with the precision sought (300 µg).
The other development is a vibrating quartz accelerometer of higher performance (in the MAIAS project in cooperation with the Safran group).
