Anti - Sound Wave Interference Thermal MEMS Motion Sensors

Xiang Zheng Tu

  

It has been reported that a research team of University of Michigan used a $5 speaker and precisely tuned acoustic tones to deceive 15 different models of accelerometers into registering movement that never occurred. The approach served as a backdoor into the devices - enabling the team to control other aspects of the system. This research calls into question the longstanding computer science belief that software can automatically trust hardware sensors, which feed autonomous systems with fundamental data they need to make decisions.

In this research the accelerometers are capacitive MEMS devices which are typically structured with a diaphragm acting as a mass that undergoes flexure in the presence of acceleration. As shown in the above figure two fixed plates sandwich the diaphragm, creating two capacitors, each with an individual fixed plate and each sharing the diaphragm as a movable plate. The flexure causes a capacitance shift by altering the distance between two parallel plates, the diaphragm itself being one of the plates. Under zero net force the two capacitors are equal but a change in force will cause the moveable plate to shift closer to one of the fixed plates, increasing the capacitance, and further away from the other fixed reducing that capacitance. This difference in capacitance is detected and amplified to produce a voltage proportional to the acceleration. The dimensions of the structure are of the order of microns.

It is not surprise that the diaphragm of the accelerometer is also sensitive to acoustic pressure and works like a capacitive MEMS microphone. A capacitive microphone is commonly formed by a movable membrane and a rigid back plate, forming a structure with a plate capacitor. The movable membrane responds and changes its position when the acoustic pressure hit its surface, producing a capacitance variation between the back plate and the membrane, which in turns produces a current flow proportional to the distance variation between the membrane and the back plate.

There is no essence difference between these two capacitive MEMS devices. It is true that said Kevin Fu, U-M associate professor of computer science and engineering, the fundamental physics of the hardware allowed us to trick capacitive accelerometers into delivering a false reality to the microprocessor. And their findings resonantly upend widely held assumptions about the security of the underlying hardware.

However POSIFA’s thermal MEMS motion sensors can make these things total different. Using the POSIFA’s thermal MEMS motion sensors sound waves are no longer allowed to hack everything from phones to fitness trackers. Reference to the above figure a POSIFA’s thermal motion sensor comprises a thermal isolated plate created in a silicon substrate, a resistive heater, and two thermopiles both are formed on the surface of the plate. The laws of physics teaches that the temperature field generated by a moving heat source is asymmetry and able to be measured. In steady state, the vertical cross-sectional temperature field is a sequence of symmetry concentric circles each representing an isotherm on the lateral plane. When the heat source moves the vertical cross-sectional temperature field will be skewed towards down motion direction. The skewed lateral cross-sectional temperature field consists of a contracted half plane and an expended half plane both are divided by a line perpendicular to the motion direction. Since two thermopiles sensors are placed on the plane around the heat source, all isotherms can be reconstructed. A lot of useful information including the direction and velocity of the moving heat source can be extracted form the reconstructed plane isotherms.

Acceleration is used to measure the change in velocity, or speed divided by time. For example, a car accelerating from a standstill to 60 mph in six seconds is determined to have an acceleration of 10 mph per second (60 divided by 6). So with several accelerometers on your smart phone, you can determine if the smart phone is moving uphill, whether it will fall over if it tilts any more, or whether it’s flying horizontally or angling downward. And you know how to tilt your smart phone it can rotate their display between portrait and landscape mode accordingly.

The thermopile flow sensors can replace capacitive accelerometers for measuring the speeds of any moving objects including smart phones. The working principle is based on the fact that a moving object experiences an apparent wind that is the wind in relation to the moving object. Suppose the object is a riding bicycle on a day when there is no wind. Although the wind speed is zero, the rider will feel a breeze on the bicycle due to the bicycle is moving through the air. This is the apparent wind. On the windless day, the measured apparent wind will always be directly in front and equal in speed to the speed of the bicycle. It is very clear that it is impossible for the thermal motion sensors to response sound wave because there is no sound wave sensing mechanism to take place.