MEMS accelerometers and gyroscopes are all the rage in portable design, putting the ‘smarts’ in smart phones and a new level of fun in gaming consoles. But exactly what are they, how are they made and how do they work?
By John Donovan, Low-Power Design
Despite the fact that MEMS accelerometers have been built into automotive airbags since the mid-90s, few people were aware of their existence until 2006 when the Nintendo Wii game consoles started taking over their living rooms. MEMS motion sensors are now widely used in automotive electronics, medical equipment, hard disk drives, and portable consumer electronics. Today a smart phone can hardly be called ‘smart’ if it doesn’t include a MEMS accelerometer, gyroscope and possibly a compass, too. A small niche product five years ago, MEMS sensors now constitute a multi-billion dollar industry.
So what exactly are MEMS motion sensors and how do they work?
MEMS Motion Sensors
Form follows function, and there are several different types of MEMS motion sensors, each with unique construction and best suited to a particular range of applications.
Single-axis accelerometers (Figure 1) detect a change in velocity in a given direction. They are almost universally used to inflate automotive airbags in the event of crashes. They are also used as vibration sensors to detect bearing wear in machinery, since vibration can be thought of as acceleration and deceleration happening quickly in a periodic manner. Analog Devices, Freescale and Bosch Sensortec all make single-axis MEMS accelerometers that are widely used in these applications.
Two-axis accelerometers add a second dimension, which can be as simple as detecting tilt by measuring the effect of gravity on the X-Y axis of the accelerometer. Accelerometers come in low-g and high-g sensing ranges, where low-g typically means less than 20x the force of gravity when the measuring body is at rest and high-g can range as high as 100x. Low-g MEMS accelerometers are used in handheld devices; high-g ones find a place in industrial, military and aerospace applications, where the g-forces are in excess of what humans could either generate or withstand.
Three-axis accelerometers can detect motion in three different directions. They widely used in mobile devices to incorporate tap, shake and orientation detection, all of which can result in different actions on the part of a cell phone.
The Freescale MMA7660FC 3-axis accelerometer (Figure 2) targets handsets by incorporating a range of user-programmable interrupts and sample rates in a small footprint (3 x 3 x 0.9 mm) DFN package. The MMA7660FC communicates 6-bit X-, Y- and Z-axis information with the processor over an I2C interface, eliminating the need for an A/D converter. The device draws as little as 47 µA in active mode at one sample per second; 2 µA in standby mode; and 0.4 µA in off mode.
Accelerometers measure linear motion, so they’re found in applications that measure acceleration, vibration, shock, and tilt. Gyroscopes on the other hand response to rotation, which is a measure of angular motion. They’re basically three-axis inertial sensors. Multi-axis MEMS gyroscopes are often embedded along with three-axis accelerometers in inertial measurement units (IMUs).
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