MEMS 2.0: IoT Pressure Sensors

Pressure sensors are rarely seen, but their influence is often felt. Pressure sensors have a couple different applications, the most obvious of which is weather monitoring. IoT pressure sensors are now quite common. You probably have one in your pocket right now! Yup, your phone has one. Your phone uses a pressure sensor to assist in tracking, getting your altitude, and aiding tracking calculations. Industrial applications of these sensors include controlled lab and manufacturing environments. Pressure sensors are great for IoT applications because they can easily be incorporated into any system due to their low cost, small power consumption, and small size.

How Pressure Sensors Work

A mems (micro-electro-mechanical systems) air pressure sensor works on two principles. The first is called the piezoelectric effect. Piezoelectric materials generate an electric current. When they're subject to force, they're deformed from their original shape, which allows for two different equations. The electric current can be used to calculate the deformation of the material, and the deformation of the material can be used to calculate the force, and thus the current can be used to calculate the force. These calculations can be performed in reverse order as well.

The second principle is that a stationary system has a net force of zero. Think of an inflated balloon. The size of the balloon is determined when the contracting force of the material equals the difference in pressure inside and outside of the balloon. Pressure is a force applied over an area so, in other words, the forces around the balloon are equal. You can force the balloon to be smaller by pushing it under water because there will be a larger outside pressure underwater. If you climbed a mountain, the balloon would get larger because the outside pressure would be less.

The mems pressure sensor is essentially a cavity of air with one side being a flexible balloon-like material. When the sensor is in an area with higher pressure (e.g. a low valley), the membrane is pushed into the cavity. When the sensor is in an area with low pressure (e.g. a mountain), the membrane bulges out of the cavity. This membrane has a piezoelectric material on it. The sensor reads the current generated in the material and then uses that reading to calculate the force applied to the membrane. Next, because the pressure inside the cavity is known, the sensor can calculate the outside pressure.  

Air Pressure Sensors in IoT

Mems air pressure sensors are accurate enough to be used in a variety of applications. One way to use them would be to measure your bike tire pressure and have them send a notification if the pressure gets to low. Another Applications—this time for farmers—is to spread mems air pressure sensors through a field in combination with humidity and temperature sensors. This sensor suite could be used to gather weather data that a higher order machine learning algorithm could leverage to predict local weather with extreme accuracy. Weather analysis has many applications beyond farming too. For example, weather influences air and sea travel. Knowing when extreme weather will happen can allow for preventive care of infrastructure.

Air pressure sensors are extremely useful in tracking. Leverege's Location Engine can take in data from an air pressure sensor to refine calculations. In addition to being able to distinguish between a building's floors based on relative air pressure, these sensors are responsive enough to enable a calculation of the z-velocity.

Many maps and tracking applications incorporate dead reckoning calculations when lacking a good GPS signal, for example, in a mall or a sports stadium. So, from knowing the last GPS position, dead reckoning is used to calculate a phone's position using the speed and heading determined by the sensors.

Air pressure sensors contribute a lot of information about the vertical movement to this calculation. Knowing the z velocity at any given time can simplify calculations to just a horizontal system. In addition to the z-axis information, if the sensor has enough accuracy it can be used for step detection.

Air pressure sensors are already incredibly useful in IoT applications. Their value stems from low cost, low power consumption, and small size. As an added value, air pressure sensors are easy to add to any IoT sensor suite. The main applications that affect the average person's daily life are their usefulness in weather and indoor asset tracking applications.