An Inexpensive and Precise Gyroscope is Introduced for Tracking Autonomous Vehicles Without GPSApril 05, 2020 by Luke James
Researchers at the University of Michigan have developed a small and precise gyroscope for navigating and tracking autonomous vehicles and drones without the use of GPS.
As good as GPS is, it is not a system that can be fully relied on at all times. There are many situations when a GPS system could fail or not be available, such as when you are driving through a tunnel. Systems that track location typically use Internal Measurement Units (IMUs), which are a combination of accelerometers and gyroscopes, to estimate position by tracking changes in acceleration and rotation.
The accuracy of these systems depends on how accurate the sensors in those IMUs are. Naturally, the bigger and more expensive the gyroscope, the better they are at doing a good job of tracking over time.
Unfortunately, large and expensive gyroscopes are unsuitable for many systems, including some autonomous vehicles which are not only applicable to road use but also use in less forgiving environments such as underwater or in caves and caverns where space is limited.
Now, researchers at the University of Michigan, led by Khalil Najafi, have announced the development of an inexpensive and precise gyroscope for tracking autonomous vehicles without GPS. The team’s research was presented at the IEEE International Symposium on Inertial Sensors & Systems.
The new gyroscope with the vacuum package open, revealing the resonator inside. Image credited to Najafi Group
Achieving Precise Tracking and Long-Term Autonomy
According to the research paper, the team has developed a new type of gyroscope called a precision cell integrating (PSI) gyroscope. They claim that it is “10,000 times more accurate but… 10 times more expensive than gyroscopes used in your typical cell phone”.
It is also said to offer the same or better performance as larger gyroscopes at a fraction of the cost, theoretically meaning that highly precise location tracking and long-term autonomy may be achievable for not just autonomous vehicles but consumer-grade electronics.
The focus of the research is very much on autonomous vehicles though, with the team explaining that their PSI gyroscope is needed for unmanned autonomous vehicles that could go astray without a GPS signal. Should a GPS system fail, autonomous vehicles use a high-performance gyroscope in their backup navigation systems, but these are larger and very expensive.
A Unique Resonating Structure
To understand how the team’s PSI gyro is allegedly more accurate while being smaller and relatively inexpensive, we must look to the heart of it—a resonating structure made from ultra-pure metal-coated glass.
The way the vibrations move through the glass reveal how fast and by how much the gyroscope is spinning. To make this measure as accurate as possible, the research team began with a sheet of pure fused-silica glass. They then used a blowtorch to heat it and mold it into the wine-glass shape (a “birdbath resonator”) before adding a metallic coating to the shell and placing electrodes around it to initiate and measure vibrations.
This birdbath resonator is better at measuring rotation than the conventional tuning-fork-shaped resonators that are found in smartphones. This is because the axisymmetric shape means that the initial vibration and the ‘second vibration’ induced by the Coriolis effect happen at almost the same frequency. So, when the Coriolis effect transfers energy from the first to second mode, it is more efficient and you get more vibration per rotation, leading to higher sensitivity and a more accurate rotation figure.
The resonator also has a high ‘Q factor’, a dimensionless measurement of the ratio between energy stored in a resonator and how much energy is lost per oscillation. Your typical smartphone gyroscope has a Q factor of a few hundred and vibrates for less than a second because it quickly loses mechanical, acoustic, and thermal energy.
The PSI gyro’s resonator. Image credited to Najafi Group
PSI Gyro Resonator
In contrast, the PSI gyro’s resonator has a Q factor of 5.1 million, meaning that it can vibrate for over five minutes when packaged in a vacuum. A high Q factor is important because, for effective transfer of vibration from one mode to another, the resonator needs to be as clean as possible.
The resonator is attached to a glass substrate with electrodes around the rim that drive it and sense the vibration modes of the resonator. When packaged into its vacuum-sealed box, the entire gyroscope measures in at under two centimeters in size. Getting a high Q factor, mass, and stiffness in a resonator is much more difficult when scaled down to this size, which is what makes the PSI gyro unique.
Other characteristics of the birdbath resonator that attribute to its accuracy include a high mass and stiffness.
Accurate AV Tracking Without GPS
Together, all the birdbath resonator’s characteristics translate to what the researchers describe as 10,000 times better efficiency in contrast to a smartphone gyroscope. This means that it easily sensitive enough for use in an autonomous vehicle for accurate self-tracking without GPS.
As an example, an autonomous vehicle traveling at 50 m/h without GPS down a relatively flat road while equipped with a PSI gyro will exhibit just two millimeters of error. After five minutes, this will increase to only 30 centimeters. In contrast, a standard automotive gyroscope will exhibit a positional error of roughly seven meters after one minute and roughly 850 after five minutes.
This performance, the research team says, is comparable to those found on military-grade submarines despite being substantially cheaper.
Plans for commercial production of the PSI gyro are already in full swing, with the team behind it already having formed a start-up, Enertia Microsystems, to commercialize both the PSI gyro and a similar design called the Birdbath Resonating Gyro (BRG). The start-up will target autonomous vehicles, robotics, and consumer electronics.