FEATURE MEMS SENSORS
MEMS Sensors: The Next Level of Integration
determines only relative, not absolute, changes. This results in long-term deviation which increases with time. An acceleration sensor detects the home position of a device with which the software can compensate for the long-term deviation of the gyroscope. A magnetic field sensor can also act as an absolute reference.
The Bosch Sensortec BNO055 is the first
member of a new family of intelligent absolute orientation sensors with 9 Degrees of Freedom. As the first 9-axis sensor, it only contains technologies from a single manufacturer: a 3-axis 12-bit accelerometer, a 3-axis gyroscope and a 16-bit 3-axis geomagnetic sensor. The integrated 32-bit microcontroller runs on FusionLib BSX3.0 software, however there is no application available as yet. Being 5.2x3.8 mm², the System-in-Package (SiP) solution is significantly smaller than comparable discrete or system-on-board solutions. As a plug-and-play model it is designed
Martin Grimmer, Senior Marketing Manager Analogue & Sensors at Rutronik Elektronische Bauelemente, investigates the options
I
t all began in the automotive industry. Around 20 years ago the first MEMS sensors were used in airbags, ABS and ESP systems. The use in smartphones and gaming consoles have since ensured that the miniaturisation, energy efficiency and cost of these sensors has continued. Today, this development has reached a new level, providing completely new and interesting features for many applications. Wearables such as smart watches, bracelets, glasses or smart clothing would be unthinkable without MEMS sensors. But they also bring increasing added value to industrial, medical and gaming applications. This is especially true for current Sensor Hubs and Application Specific Sensor Nodes (ASSNs). They offer up to 9 Degrees of Freedom (DoF), i.e. a 3-axis accelerometer, a 3-axis gyroscope, a 3-axis magnetometer plus a microcontroller all within one housing. Other external sensors, such as pressure or
ambient light sensors, can achieve even higher than 10 Degrees of Freedom. This provides more accurate and robust sensor data than existing individual sensors, even under severe conditions. Due to the various types of sensors, each with its own drift and noise characteristics, they compensate for the shortcomings of individual sensors. Other plus points: they are much smaller
and more efficient than discrete solutions; the calibration of temperature as well as routine calibration by the user is not required; the majority of integration and development work is omitted, so that the time-to-market is shortened.
20 SPRING 2015 | MICROMATTERS
TOP TWO INTEGRATED SENSORS The BMX055 from Bosch Sensortec in a 3x4.5 mm² housing and the LSM9DS0 from STMicroelectronics in a 4x4 mm² housing are digital 9 DoF sensor modules, providing one of the industry's highest levels for integration of individual MEMS sensors. Both include a 3-axis accelerometer, a 3-axis gyroscope and a 3-axis magnetometer. In order to obtain the display values or for
further processing of raw data from the sensors, Bosch Sensortec and STMicroelectronics have developed their Sensor Fusion software. Magnetometers have a very sensitive
measuring range to detect the relatively weak geomagnetic field which in turn makes these very sensitive to external magnetic fields, e.g. from a speaker or a magnet. In the case of such errors, an electronic compass may not indicate the magnetic north. However, if a sensor hub is used, the software calculates the position of the device from the angular acceleration of the gyroscope and the linear acceleration of the accelerometer. Thus the compass will function by itself when the magnetometer is disturbed or rotated. The vulnerability of a gyroscope is also
compensated in this manner. It always measures against a reference motion, i.e. it
“To obtain the display
values or for further processing of raw data from the sensors, Bosch Sensortec and ST have developed their Sensor Fusion software ... ”
Figure 1: The BMX055 from Bosch
specifically for various applications, e.g. in the field of Augmented Reality, without having access to satellite-based navigation data, for the strikingly realistic world of gaming, health and fitness applications, indoor navigation and all applications that require ambient detection. Because of the broad spectrum of applications it will remain available though solely focused on the smartphone market model, as it is also suitable for industrial applications. The STMicroelectronics LIS331EB combines
a linear 3-axis accelerometer with a Cortex- M0 core containing up to 80MHz, 64KB Flash, 128KB of SRAM in a 3x3x1 mm LGA housing. It has freely selectable, graduated scales and can measure accelerations with an output data rate of up to 1.6kHz. Thanks to ultra-low-power operational modes it is extremely energy efficient and provides smart sleep-to-wake-up functions. With the iNEMO Engine software the
sensor hub functions for both internal as well as external sensors: via I
2 C it can retain the
data of the embedded accelerometer, as well as the additional external sensors (gyroscope, magnetometer or pressure sensor) and 9 or 10-axes bundles. For all three models: multiple
sensors provide, in combination with the Sensor Fusion software, more accurate and more robust values that are aligned to the respective
application - without custom software development. Matching hardware and software ensure a minimum level of
development and integration effort along with optimal performance.
Rutronik Elektronische
www.rutronik.com 01204 602 200
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