Sunday, July 30, 2017

MEMS Gyroscopes, Smartphones, and Ultrasound

This morning I was reading an article on Ars Technica (great tech website if you don't already read it) about the use of "sonic guns" to disrupt the operation of electronics and gadgets like drones. Here's a simple demonstration video of this happening.

As you can see the toy robot, which balances due to the gryoscopes in it, is quickly confused by the incident sound and ends up moving, then falling over. Why does this happen?

MEMS gyroscopes are Micro-Electrical-Mechanical Systems, essentially a very small structure that's often made by the same processes used to build computer chips, that has combined electrical and mechanical behaviors that are useful to us. In the case of a MEMS gyro, it leads to motion that creates an electrical signal which can be processed to determine rotation. These structures are small enough to be packaged into something smaller than your fingernail, and so can be fitted into compact spaces and consumer goods. Here's a video of a very simple MEMS gyro oscillating.

An image below from UC Davis MEMSLab shows how one small device can detect rotation in all three axes.

You can see that each sensing mode here has at least one "resonant frequency" at which the device naturally oscillates, which means it is very very sensitive to those frequencies. Like finding the right pitch for a glass, you can actually break them by causing them to vibrate at that frequency. Some work here, here, and here shows in more academic detail how the MEMS gyros can be rendered ineffective by ultrasound above around 100 dB. Sound is, after all, just a vibration at a particular frequency - match that sound to the resonant frequency of the gyro and it will play havoc with it.

Here's a drone showing the effect of ultrasound on its behaviour - notice that the transducers used to generate the ultrasound look like Murata devices, very similar in appearance to what uBeam look to be using in their transmitters. (I appreciate the safety precautions this researcher took!)

What this all seems to be pointing to is that high powered ultrasound in the environment can disrupt the activities of more and more of our devices such as drones - what happens if they fly through a high power ultrasound beam? Do they veer off and hit someone? And the smartphones we all use today? Those phones have multiple MEMS sensors in them, and the gyro is what allows you to play games just by tilting your phone. What happens when you direct ultrasound at levels far greater than 100 dB towards a smartphone? The manufacturers know, they spend a lot of time making sure that nothing in the phone vibrates at frequencies that disrupt their operation, but I'm not sure I've seen a study that's been made public.

Those may seem like simple examples, but there are safety considerations. What happens if a safety related system, such as positioning in a vehicle, is disrupted by high power ultrasound? Who is responsible for that? Cars and larger objects can usually shield the gyro to insulate the sound from it, but what about size and weight sensitive devices like smartphones?

If the videos above give an indication, then truly ubiquitous high power ultrasound in the environment is going to be disruptive in more ways than one. Just as well no-one is likely to try to put such loud ultrasound devices out there en-masse.

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