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Small single-package IMU

by Bryon Moyer

May 29, 2012 at 1:40 PM

Bosch-Sensortec recently announced a new integrated IMU, the BMI055.

Which, amongst other things, brings up the question: exactly what is an IMU? While researching this for a gyroscope article couple of years ago, I found that the term (which stands for “inertial measurement unit”) was used to refer generically to a class of sensors that use some type of inertia as a way of sensing motion. That inertia might be linear (using an accelerometer) or rotational (using a gyroscope).

The definition Bosch-Sensortec used differed from that, and as I look around now, I see other usage that is similar: an IMU is a combination of sensors – in particular, accelerometers and gyroscopes – for detecting motion. (Some so-called IMUs also include other sensors like magnetometers and possibly even a pressure sensor/barometer, for a so-called 10 degrees of freedom – 6 of which degenerate to 3). Whether this represents a change or simply varying definitions is unclear to me (it’s hard to recreate the internet of a couple years ago). Nonetheless, the term is, to some extent, overloaded; the combo definition seems to predominate now.

While we’re on definitions, you might think of a magnetometer, when used in a navigation application, as a compass (or eCompass) by analogy to an old-school needle compass, which is simply a magnetometer. But that’s not how the MEMS version is defined: a MEMS compass is the combination of an accelerometer and a magnetometer.

To be clear, Bosch-Sensortec announced what they claim to be the smallest combination accelerometer/gyroscope available. It is a multi-die integration (both with respect to the MEMS sensors and the accompanying ASICs); the size advantage comes from housing them in the same package.

As to whether those dice might ever merge, they said that it might happen, but that it’s more likely that the ASICs and MEMS chips will independently merge first, possibly followed by full MEMS/CMOS integration.

They’ve added a power-saving feature through this integration: the accelerometer can wake up the gyroscope. Gyros are notoriously power-hungry; you have to keep the proof mass moving (unlike an accelerometer). So the BMI055 allows the gyro to be turned off. Which isn’t a first, but they’ve sped up the wake-up time from a more typical 30 ms to 10 ms. This is intended to allow the gyro to be woken by the accelerometer without it taking so long that the gyro misses an event. The effect is to cut power in half.

The combined unit comes with free fusion software. There have been two ways of approaching fusion software: using either “tight” or “loose” coupling. Loose coupling means that the data from each sensor is independently processed to some degree before being presented for munging with the output of other sensors. Tight coupling performs the fusion with the raw data from the sensors.

Loose coupling is easier to do (and less reliant on the low-level data format of a sensor), but it’s less accurate. Tight coupling provides a more accurate result, but is more complex and needs to work at the lowest data level (which ties it more closely to the specific sensor).

Bosch-Sensortec uses both: where loose coupling provides sufficient accuracy, they use it, reverting to tight coupling when necessary. Where they make that cut is something they’re keeping to themselves.

You can find more information in their release

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MEMS and Smart Systems.

 
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