DARPA programme seeks cheaper inertial navigation sensors
26 September 2012
New fabrication techniques for microscale inertial sensors could create enough accuracy to replace the large, expensive gyroscopes in use today.
Military missions of all types need extremely accurate navigation techniques to keep people and equipment on target. That is why the Military relies on GPS or, when GPS is unavailable, precise sensors for navigation. These sensors, such as gyroscopes that measure orientation, are bulky and expensive to fabricate.
For example, a single gyroscope designed as an inertial sensor accurate enough for a precision missile can take up to 1 month to be hand assembled and cost up to $1m. DARPA has made progress in developing less expensive fabrication methods for inertial sensors and is making them orders of magnitude smaller and less expensive.
The first phase of DARPA's Microscale Rate Integrating Gyroscope (MRIG) effort of the Micro-Technology for Positioning, Navigation and Timing (Micro-PNT) programme was recently completed. It focused on 3D microfabrication methods using non-traditional materials, such as bulk metallic glasses, diamond and ultra-low expansion glass. Small 3D structures such as toroids, hemispheres and wineglass-shaped structures were successfully fabricated, shifting away from the 2D paradigm of current state-of-the-art microgyroscopes.
These microscale inertial sensors work like Foucault pendulums. The swinging direction of the pendulum slowly changes as the Earth rotates. Instead of a swinging pendulum, microscale inertial sensors send out vibrations across the surface of a 3D structure. The precession of the standing wave is measured and any changes reflect a change in orientation. The new fabrication methods include:
Researchers developed fabrication methods that replicate traditional glass-blowing techniques at the microscale. The result is tiny 3D wineglass-shaped inertial sensors. These sensors are symmetrical enough to have a frequency split approaching 10Hz - a result never before achieved at this size and approaching levels of symmetry required for high-quality navigation devices. The frequency split is a measure to predict the symmetry - and thus the accuracy - of the device. It is a measure of the difference between two different axes of elasticity. The greater the difference, the more imperfection is present, resulting in a less accurate sensor.
Similar to glass blowing, quartz blowing can be used to make an even more symmetric structure. Researchers developed fabrication techniques needed to heat quartz to 1,700 degrees Celsius (a typical softening point for glass is about 800 degrees Celsius) and to then cool it rapidly. The fabrication can be performed in large quantity batches, producing hundreds of devices on a single wafer.
Atomic layering of diamond
Layering diamond over a blown structure or depositing CVD diamond in a micro-well on the substrate have been shown to be effective, promising methods for creating highly symmetric, accurate 3D inertial-sensor structures.
“These new fabrication methods were thought to be unrealistic just a few years ago,” said Andrei Shkel, the DARPA programme manager. “The first phase of MRIG has proven these new fabrication techniques and begun the process of validating the new structures and materials through testing. Phase 2 has kicked off, in which DARPA seeks to hone these methods to create and demonstrate operational devices.”
Phase 2 of this work will seek to make these devices even more accurate and reliable by reducing frequency split from 10Hz to 5Hz, increasing decay times from 10 seconds to 100 seconds, and decreasing volume from 20 mm3 to 10 mm3. The final goal of Phase 2 is to demonstrate a working, first-of-its-kind microrate integrating gyroscope.