Fellow Science Lovers,
Game update: Still no game updates. Sorry for the delay, but I need to give Scribes Emerge more editorial noogies before I plop it in the laps of my beta readers. And just now, autocorrect wanted me to change “noogies” to “boogies.” 
Inertial Sensors
In chapter 1 of Scribes’ Descent, Mallory installs an inertial sensor suite into a mini drone. I plan to spend at least 2 articles explaining what that is.
Our cell phones use inertial sensors to know when they’re being tilted, where they’re located on earth, and when your cat knocked them off the kitchen counter. These sensors are usually made up of 3 parts:
- gyroscope – measures changes in rotational speed
- accelerometer – measures changes in the object’s overall speed
- magnetometer – measures the object’s position in space against the earth’s magnetic field (kind of like a compass)
Each part is a topic unto itself, and in this article, I’ll focus on gyroscopes. You’ve probably heard of those, but may not know the science behind them. Strap in. This is about to get geeky.
Gyroscopes
Watch this 3.5 minute video to get a solid visual of how a wheel on a rope seems to defy gravity:
Notice how huge that wheel is? You’d never fit that in your phone. Instead, smartphones use tiny circuit chips that house MEMS gyroscopes. MEMS stands for Micro Electro-Mechanical Systems. As the word mechanical implies, this electronic component has tiny moving parts. Yes, your phone has moving parts, each around 1 micron, and the whole chip is between 20 microns and 1 millimeter.
Here’s what it looks like:
-CC BY 3.0 via Wikimedia Commons
This looks nothing like a bike tire on a rope, does it? Yet these create Coriolis forces all the same, they just use thin vibrating structures instead of a spinning wheel. In a MEMS gyroscope, we vibrate these structures by applying a voltage to a piezoelectric material. (You may wonder: what does pizza electric mean? We’ll save that for another newsletter. Now I’m hungry.)
When you rotate a moving object, a Coriolis force pushes it at 90 degrees to its initial direction. Using 2 right hand rules, we can find the direction of that force. If you don’t have a right hand, enlist the help of a friend who does. (A human friend, to be precise. This won’t work with a cat’s paw. I tried. My cats weren’t feeling scientific at the time. They also don’t have thumbs.)
Right Hand Rule 1:
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Let’s identify the 3 arrows in the above image:
- Angular Velocity – the axis about which the phone is being tilted.
- Velocity of Proof Mass (the vibrating object) – the direction the object is moving in at a given moment.
- Resulting Coriolis Force – measured by the sensor to see which direction the phone is being tilted. It’s actually applied in the opposite direction of the blue arrow shown above. Why? Because the formula to calculate Coriolis force has a negative sign on one side:
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Or maybe the scientists who discovered this stuff didn’t want to use their left hand, which would let us avoid the minus sign. Who knows?
Right Hand Rule 2:
-CC SA 4.0 via Wikimedia Commons
Curl your right hand with your fingers wrapping around the vertical axis of rotation. Your thumb should be pointing up. That’s the direction of the angular velocity, or the red arrow in both right hand rule drawings.
Find the Force, Luke
Now that we know the rules, let’s learn how to apply them. Here’s a simplified MEMS gyroscope:
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The green blocks are “proof masses”, or tiny oscillating objects. They vibrate in the direction of the green arrows. While these blocks are moving away from each other, which direction will Coriolis forces push them when the phone spins counterclockwise? Get your right hand warmed up and follow these steps to find out:
1. Use Right Hand Rule 2 to find the Angular Velocity (the red arrow in the drawing above). Keep your thumb pointing up.
2. Stick your right forefinger out like you’re forming a gun. Point that finger in the direction that the left green box is moving in (the green arrow).
3. Now stick your middle finger out at 90 degrees to your index finger. It should be pointing back at you. (Not at a family member–gotta keep this civil.) The induced Coriolis force goes in the opposite direction of your middle finger, which should line up with the left blue arrow in the drawing. You could repeat the steps for the right block, too, if you want.
These steps finds the direction, but to measure the size of the Coriolis force, the sensor measures the distance between the proof mass (the middle plate in the drawing below) and a fixed electrode:
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When a Coriolis force pushes down on the middle plate, it gets closer to the bottom electrode, increasing the capacitance of the circuit. (Capacitance is the ability of an object to store electric charge–another concept that needs its own article.) The sensor measures the change in capacitance and converts it into a measure of rotational speed. Of course, the above diagram is an oversimplification. The real sensor looks more like this:
-CC BY-NC-SA 4.0 via MIT OpenCourseWare
With many small teeth, the sensor is more sensitive to small changes, making it more accurate. If you want to see this up close, watch this video:
To detect rotations in all directions, the sensor needs 3 assemblies laid out across different axes. The video above shows that in a way that words can’t describe very well. (Like trying to teach someone to tie their shoes using only words.)
If your head is spinning by now, at least you know how to measure its velocity.
In summary, tiny gyroscopes know when you’ve tilted your phone, can keep drones flying upright, and a lot more I don’t have time to cover here. Gyroscopes are just one example an inertial sensor. Next time, I’ll talk about accelerometers.
I glossed over a lot of details for the sake of simplicity because this is a big topic. I didn’t talk about old-fashioned mechanical gyroscopes or optical ones or dive into the advanced math behind this. I also didn’t explain why an egg stands on its long axis when you spin it on a surface having ample friction. But hopefully I made you curious enough to go on a “Wikipedia crawl” of this subject.
More References
–https://www.getwidget.dev/blog/how-does-gyroscope-sensor-work-in-your-smartphone/
–https://www.youtube.com/watch?v=KuekQ-m9xpw
–https://phys420.phas.ubc.ca/p420_12/tony/Coriolis_Force/Home.html
Writing update: I’ve finished most of my Scribophile critique edits, except for the ones that just came in during the past week. And I’m halfway through my final editing checklist.
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