Magnetic Field Sensors

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Introduction

One of the most highly requested sensors to have on the MetaWear board is a magnetometer. A magnetometer is a sensor that measures magnetic field strength, typically of the Earth’s magnetic field. They come in various shapes and sizes but are now small enough that they only occupy an area of ~1mm^2, perfect for placing in electronic devices.

There are several ways to measure magnetic field strength at integrated circuit sizes, however in this post, we will focus solely on how this is done using the Hall Effect.

How They Work

A Hall Effect sensor measures magnetic field strength via the Hall Effect, which is a potential difference produced across a conductor or semiconductor when a magnetic field is applied in a non-parallel direction to an electrical current. This potential difference, referred to as the Hall voltage, is directly proportional to the magnetic field strength though it is several orders of magnitude weaker. As such, amplifiers and voltage regulators are used to boost the output signal.

Block diagram of BMM150

BMM150 block diagram

MetaWear Magnetometer

The MetaWear comes with a Bosch® BMM150 geomagnetic sensor. Currently, this sensor is only available on the MetaWear CPRO boards. Below is a table highlighting some of the electrical and operation specifications.

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

Applications

Today, MEMS-based magneometers can be found in a plethora of electronics and is usually used in conjunction with other motion sensors. The Wii U WiiPad incorporates a magnetometer along with a gyroscrope and accelerometer to measure position and orientation using sensor fusion. In a similar vein, it can be used for in-door navigation if combined with a step counter. On its own, magnetometer data can be used to find magnetic north, turning any smartphone with one into a compass.

Additional Resources

As you may have noticed in the BMM150 block diagram, the BMM150 chip is actually a combination of a Hall Effect sensor and their proprietary “FlipCore” measurement system. You can read about how “FlipCore” works in the BMC050 press statement. Engineer’s Garage has a nice article covering other ways to sense magnetic field strength besides using a Hall Effect sensor.