How do MEMS sensors work?

GERMANY: Engineers create sensors containing the finest silicon structures. As the casing moves, these structures shift a fraction of a thousandth of a millimeter - and their electrical properties change in the process.

These properties can be measured and converted into a data stream that is capable of showing, say, a cell phone what position it is in. Bosch engineers work with incredibly small dimensions; while a human hair has a diameter of 70 thousandths of a millimeter (70 micrometers), some components measure only 4 micrometers - that is 17 times smaller than the diameter of a single human hair.

Since the micromechanical sensor produces only weak electrical signals, the developers built in another electronic component - sometimes in the casing beside the sensor, sometimes even directly on the same chip. This second component processes, amplifies, and converts the weak signal into digital data. In this way, MEMS (micro-electro-mechanical systems) sensors can supply control units directly with readings.

Soon, these tiny sensors will be able to do much more: they can be embedded in clothes in order to monitor heartbeat, serve as mobile weather stations measuring CO2 levels in the air, or register the typical movements people make when they put their phone in their pocket - so that the display is turned off automatically.

Possible wireless connections
There are several international standards, such as Wi-Fi, which devices use to exchange data via wireless technologies. This is how PCs send and receive data through the router next to the phone or cable outlet, for example, or how smartphones and MP3 players establish connections with PCs and routers.

Bluetooth is another wireless standard, though it bridges only short distances, for instance from headphones to a cell phone or from an MP3 player to the car radio. ZigBee is an industry standard used to control maintenance-free sensors in hard-to-reach areas. Depending on the desired application, MEMS sensors can be equipped with different wireless connections.

Energy supply
Bosch strives to build MEMS sensors that are as small as possible. Internet-enabled sensors need to be able to operate on the minimum amount of power if they are not to throw this size advantage away. Since the sensors themselves require little energy, they can function with very small batteries.

More power, however, is required for the transmission of data via wireless technologies. That's why clever mechanisms ensure that data is only sent if it is absolutely necessary. Moreover, the technology is advancing; in future, new batteries will last for months or even years.

The goal is to achieve a long-lasting, maintenance-free wireless sensor; for this reason, engineers are also focusing on what is called energy harvesting, which sees energy gathered from the environment. There are several possible ways of doing this.

Antennae can draw energy from radio waves present in the area. Acoustic noise also supplies energy that can be converted into electricity - as do vibrations, pressure, shock, and heat. While the quantity of energy recovered in each case is tiny, taken together they provide enough power for many applications.