Monday, September 18, 2017

Inertial Measurement Units (IMUs)- Some key issues - III

We have come a long way since the first mechanical spinning flywheel gyros were made at the beginning of the 19th century. There are three novel techniques available for sensing now

1) Coriolis effect based devices [a nice discussion here] -  These devices are now very popular in commercial cell phones. The most popular versions use Micro Electro-Mechanical Systems (MEMS), and the rotation and acceleration is sensed as a change in the capacitance of a microfabricated circuit. The demands put on fab side are significant although things are getting cheaper as the scale of deployment grows. As the entire device is made in a semiconductor fab, the overhead associated with creating noise control electronics is reduced, as the signal is basically electrical in nature, a number of existing design for low noise signal amplification can be leveraged to improve performance.  Though not terribly good in terms of precision these devices are cheap enough to be deployed on scale. It is possible to remove noise by using a magnetometer or other sensors, but its still pretty bad relative to existing peers. With commercial applications growing, a lot of people are working on ways to fuse the data from multiple sensors and use cloud based big data filtering tools to get intelligence from these devices, but that stuff is still IMHO in its infancy. It is fantastically easy to get hold of a piece of python code that hacks into one of these and gets data out. If you are looking for a place to start learning about these, I recommend playing with the Arduino backed versions of these. I did this with a high school student on the robotics team some years ago - and it was a fantastic pain in the rear but great as a learning tool.

2) Sagnac effect based devices [a good place to start] - These devices are popular in the aerospace side. This effect is used for Ring Laser Gyros and Fiber Optic Gyros (This effect is also used in Atom Optics based systems but that is discussed as a separate topic). One would naively think these systems are the most robust form of sensing possible but there are subtle issues that limit their capabilities and utility [see here]. The main limitation on these devices comes from the fact that to get a very high resolution, one needs a very large path length. Such a path length can only be achieved by incurring penalties in weight and size. The manufacture of these devices is non-trivial and would require significant investment.

3) Atom Optics [See paper by Mark Kasevich etal. in this link] - These devices were originally conceived as sensitive tests of gravitational physics in the atom optics boom years of the last century. These ideas lounged in unwieldy room sized setups in the basements of physics departments for several decades but sustained investment from the NI-24 program by the Navy and a passing interest from the IC enabled the construction of very robust variants of these devices*. Though initially considered too fragile to be used in real world applications, the quality of engineering has steadily improved and I think we may see these on real world aerospace platforms. These devices hold the promise of a significant reduction in noise over current systems and the general thought is that with some effort this will lead to a much better place in the long run. That said the manufacture of these devices is non-trivial, the demands made on associated instrumentation are significantly larger than mechanical devices.

If I were to rate these platforms qualitatively in the order of error (given that hard metrics on this are difficult to come by in context) - I would say that Coriolis systems have the worst noise issues, followed by RLGs and Fiber Optic devices. The Atom Optics systems have the best noise characteristics cited in public sources. I would not take any of these numbers too literally as they are not available for the kinds of application that have spiked public interest,  those numbers are a closely guarded secret for obvious reasons.

As a rule of thumb, if you have a lot of noise on a sensor - you have a large overhead in terms of associated algorithms (and related electronics and software) needed to clean up that mess. IMHO this really limits the ability to use commercial/off-the-shelf stuff in strategic or mission critical applications.

In the next post I will make a few remarks in passing about the way things might change in the future.  (cont'd in next post)

* one of the prime drivers of this effort was the retirement of highly qualified technicians that could make mechanical gyroscopes and gradiometers in the US. Faced with a forced technological regression the S&T guys in the USG gravitated towards the only hope they had at the time of rapid advances. Hence the interest in Atom Optics which had emerged as one of the major candidates for other high impact technologies like Quantum Computation.


Post a Comment

<< Home