7. Sensors

This tutorial section features information on different types of sensors available, how these sensors work, and what they are best used for.

You can reference our list of sensors available on each MetaSensor in order to choose the one that best fits your needs.

7.1. Accelerometer

Accelerometers are devices that measure acceleration, which is the rate of change of the velocity of an object. They measure in meters per second squared (m/s2) or in G-forces (g). A single G-force for us here on planet Earth is equivalent to 9.8 m/s2, but this does vary slightly with elevation (and will be a different value on different planets due to variations in gravitational pull). Accelerometers are useful for sensing vibrations in systems or for orientation applications.

The MetaSensors come with three accelerometers, the BOSCH BMI160, BOSCH BMA250, Freescale MMA8451Q.

The MMA8451Q is a smart, low-power, three-axis, capacitive, micromachined accelerometer with 14 bits of resolution. This accelerometer is packed with embedded functions with flexible user programmable options, configurable to two interrupt pins. Embedded interrupt functions allow for overall power savings relieving the host processor from continuously polling data. There is access to both low-pass filtered data as well as high-pass filtered data, which minimizes the data analysis required for jolt detection and faster transitions.

The BMA250E is an advanced, ultra-small, triaxial, low-g acceleration sensor with digital interfaces, aiming for low-power consumer electronics applications. Featuring 10 bit digital resolution, the BMA250E allows low-noise measurement of accelerations in 3 perpendicular axes.

The BMI160 is a small, low power, low noise 16-bit inertial measurement unit designed for use in mobile applications like augmented reality or indoor navigation which require highly accurate, real-time sensor data.

Parameter Bosch BMI160 Bosch BMA250A Freescale MMA8452Q
Temperature Range -40C - 85C -40C - 85C -40C - 85C
Noise Density 180ug/vHz 126ug/vHz 180ug/vHz
Supply Voltage (VDD) 1.71V - 3.6V 1.95V - 3.6V 1.62V to 3.6V
Measurement Range 2g, 4g, 8g, 16g 2g, 4g, 8g 2g, 4g, 8g, 16g
Digital Resolution 16 bit 12 bit 16 bit
Data Rates 0.78Hz - 3200Hz 1.56Hz - 800Hz 8Hz - 1000Hz
Active Current Consumption 180uA - 300uA (acc only) 6uA - 165uA 6 μA to 165 μA
Axis 6 3 3

The MetaWear libraries come fully equipped to help you configure and process data from the on board accelerometer.

Documentation on how to use the accelerometer functions is on the MbientLab site.

7.2. Gyroscope

Gyroscopes, or gyros, are devices that measure or maintain rotational motion. MEMS (microelectromechanical system) gyros are small, inexpensive sensors that measure angular velocity. The units of angular velocity are measured in degrees per second (°/s) or revolutions per second (RPS). Angular velocity is simply a measurement of speed of rotation.

Gyros, similar to the one above, can be used to determine orientation and are found in most autonomous navigation systems. For example, if you want to balance a robot, a gyroscope can be used to measure rotation from the balanced position and send corrections to a motor.

The MetaSensors come with the BOSCH BMI160.

The BMI160 is a small, low power, low noise 16-bit inertial measurement unit designed for use in mobile applications like augmented reality or indoor navigation which require highly accurate, real-time sensor data.

Specification Bosch BMI160
Temperature Range -40C - 85C
Noise Density @10Hz 0.007 degrees/s/vHz
Supply Voltage (VDD) 1.71V - 3.6V
Measurement Range 125, 250, 500, 1000, 2000 degrees/s
Digital Resolution 16 bit
Data Rates 25Hz - 3200Hz
Active Current Consumption 850uA - 900uA (gyro consumption only)
Axis 6

7.3. Magnetometer

The magnetometer sensor in your tablet or smartphone also utilizes the modern solid state technology to create a miniature Hall-effect sensor that detects the Earth’s magnetic field along three perpendicular axes X, Y and Z. The Hall-effect sensor produces voltage which is proportional to the strength and polarity of the magnetic field along the axis each sensor is directed. The sensed voltage is converted to digital signal representing the magnetic field intensity.

The magnetometer readings are reported in micro Tesla units (µT). When experimenting with this sensor, you can see the effect of device rotation relative to the magnetic north, or you can move a magnet near your device.

The MetaSensors come with the BOSCH BMM150.

The BMM150 is a low power and low noise 3-axis digital geomagnetic sensor to be used in compass applications. Applications like virtual reality or gaming on mobile devices such as mobile phones, tablet PCs or portable media players require 9-axis inertial sensing including magnetic heading information.

Parameter Bosch BMM150
Temperature Range -40C - 85C
Supply Voltage (VDD) 1.62V - 3.6V
Measurement Range 1300uT (xy), 2500uT (z)
Device Resolution ~0.3uT
Data Rates 10Hz, 20Hz
Active Current Consumption 0.17mA - 4.9mA

7.4. Sensor Fusion

Some of the MetaSensors that include an accelerometer, gyroscope, and magnetometer also run the Bosch Sensortec FusionLib.

FusionLib is a complete 9-axis fusion solution which combines the measurements from 3-axis gyroscope, 3-axis geomagnetic sensor and a 3-axis accelerometer, to provide a robust absolute orientation vector.

FusionLib provides orientation information in form of quaternion or Euler angles. The algorithm fuses the sensor raw data from 3-axis accelerometer, 3-axis geomagnetic sensor and 3-axis gyroscope in an intelligent way to improve each sensors output. This includes algorithms for offset calibration of each sensor, monitoring of the calibration status and kalman filter fusion to provide distortion-free and refined orientation vectors.

Since Bosch Sensortec 9-axis fusion software is developed together with the sensor hardware, optimized performance in terms of dynamics and immunity to distortion effects is achieved. Direct access to the Bosch sensor hardware allows the user to set use-case specific operation modes regarding data rates and noise thresholds. The solution provides a ready-to-use advanced 9-axis sensor fusion system which reduces the complexity for customers and helps in rapid development of advanced sensor applications.

Key features:

  • Advanced 9-axis sensor fusion algorithms
  • Fast in-use background calibration of all sensors and calibration monitor
  • Enhanced magnetic distortion detection and suppression
  • Real-time motion tracking due to built-in group delay compensation
  • Gyroscope drift cancellation
  • Supports 6-axis based applications (IMU)
  • Supports various power modes for switching on/off sensors and algorithm modules

Applications:

  • User experience in Mobile Platforms
  • Enhanced intuitive high speed gaming
  • Augmented reality and location based services
  • Motion-based user interfaces
  • Pointing devices, Remote controls
  • Toys, remote controlled flying toys
  • Robot navigation and position

7.5. Temperature

Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature. Negative Temperature Coefficient (NTC) thermistors exhibit a decrease in electrical resistance when subjected to an increase in body temperature and Positive Temperature Coefficient (PTC) thermistors exhibit an increase in electrical resistance when subjected to an increase in body temperature. Because of their very predictable characteristics and their excellent long term stability, thermistors are generally accepted to be the most advantageous sensor for many applications including temperature measurement and control.

MetaSensors use thermistor from several manufacturers including NXP which have an accuracy of ±5C.

7.6. Barometer/Pressure/Altimeter

Barometric pressure sensors measure the absolute pressure of the air around them. This pressure varies with both the weather and altitude. Depending on how you interpret the data, you can monitor changes in the weather, measure altitude, or any other tasks that require an accurate pressure reading.

The MetaSensors come with the BOSCH BMP280.

The BMP280 is an absolute barometric pressure sensor especially designed for mobile applications. The sensor module is housed in an extremely compact package. Its small dimensions and its low power consumption allow for the implementation in battery powered devices such as mobile phones, GPS modules or watches.

Parameter Bosch BMP280
Temperature Range -40C - 85C
Pressure Range 300...1100 hPa
Absolute Accuracy ±1 hPa
Relative Accuracy ±0.12 hPa (typical) equivalent to ±1 m
Resolution 0.01 hPa equivalent < 10 cm
Supply Voltage (VDD) 1.71V - 3.6V
Data Rates 1Hz - 20Hz
Sleep/Active Current Consumption 0.1 μA - 2.74 μA

7.7. Humidity

Humidity sensors (hygrometers) measure and report the air’s relative humidity. They are used in homes, wine cellars, and working environments that that are so humid that they negatively impact working conditions. Humidity sensors are also used in vehicles, industrial HVAC systems, and meteorological stations to help in the predicting and reporting of weather conditions.

The MetaSensors come with the BOSCH BME280.

The BME280 is an integrated environmental sensor developed specifically for mobile applications where size and low power consumption are key design constraints.

Parameter Bosch BME280
Temperature Range -40C - 85C
Pressure Range 300...1100 hPa
Accuracy ±3 % relative humidity
Supply Voltage (VDD) 1.71V - 3.6V
Data Rates 1Hz - 20Hz
Average Current Consumption 1.8 μA - 3.6 μA

7.8. Light

A light sensor is an electronic device used to detect light. There are several types of light sensors. A photocell or photo resistor is a small sensor which changes its resistance when light shines on it. A CCD (charged coupled device) transports electrically charged signals and can be found in digital cameras and night-vision devices. Photomultipliers detect light and multiply it.

The MetaSensors come with the LITEON LTR-329ALS-01.

The LTR-329ALS-01 is a low voltage I2C digital light sensor [ALS] in a low cost miniature chipled lead-free surface mount package. This sensor converts light intensity to a digital output signal capable of direct I2C interface. It provides a linear response over a wide dynamic range from 0.01 lux to 64k lux and is well suited to applications under high ambient brightness.

Features:

  • 2.4V to 3.6V supply voltage
  • Full dynamic range from 0.01 lux to 64k lux
  • 16-bit effective resolution
  • Current consumption is typically 220 μA
Parameter LITEON LTR-329ALS-01
Range 0.01 lux to 64k lux
Accuracy ±3 % relative humidity
Supply Voltage (VDD) 2.4V to 3.6V
Data Rates 1Hz - 20Hz
Average Current Consumption 1.8 μA - 3.6 μA