Smart phones that don’t know where they are or where they’re going are seeming less smart by the minute. [That point is made in the February IEEE Spectrum news article, “A Compass in Every Smart Phone.”] Besides GPS, phones phones with electronic compass functions need accelerometers and, increasingly, digital gyroscopes.
“Cellphone companies continually demand smaller size, less power, and lower cost,” says Jay Esfandyari, MEMS product marketing manager at STMicroelectronics. But there have been some important limits.
Heretofore in gyroscopes, movement about the three axes was measured by three separate sensing structures—one for pitch, one for yaw, and another for roll. At most, two would be combined on a single die. The best you could do was, say, a 3-by-5-by-1-mm yaw sensor matched up with a 4-by-5-by-1-mm sensor that would detect pitch and roll. But now ST has managed to make a 4-by-4-by-1-millimeter MEMS gyroscope whose single sensing structure tracks all three angular motions. “The aim now,” says Esfandyari, “is to eventually shrink them down to 3 mm square, which is the average footprint of accelerometers inside smart phones.
The gyroscope comes preset with one of three sensitivity levels, which allow the device to trade speed for resolution. For gaming, it can capture movement as quick as 2000 degrees per second, but can only distinguish movements larger than 70 mllidegrees. The version for user interfaces—say a wand or a wearable mouse, which track smaller, more controlled shifts such as pointing and clicking on a computer screen—can pick up movements as fast as 500 degrees per second. It can distinguish movements of 18 millidegrees or more. The most sensitive version, which only picks up 250 degrees per second can sense the slightest movements, anything greater than 9 millidegrees.
The gyroscope also represents a high water mark in terms of energy consumption, according to ST. The new 3-axis device draws 6 milliamps; two years ago, ST’s single-axis gyroscopes drew 9 mA. The device gets more done with less energy because it operates in what’s called flip mode. The mechanical structure of the device is always on, but the sensing structure is off when a gadget’s direction-finding function is not in use. When the gyroscope is needed, the sensing structure can be flipped on and made ready to record movement readings in less than 40 milliseconds. ST aims to reduce the lag to roughly 15 ms within the next year or so. Esfandyari says this haste in turning the sensing structure on and off is critical because in applications such as dead reckoning, missing the initial movements will make it almost certain that every subsequent reading will be in error.