Table of Contents

Magnetometers

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Summary

Introduction

Magnetometers are devices that measure the strength and/or direction of a magnetic field. Because magnetic fields are defined by containing both a strength and direction (vector fields), magnetometers that measure just the strength or direction are called scalar magnetometers, while those that measure both are called vector magnetometers. Today, both scalar and vector magnetometers are commonly found in consumer electronics, such as tablets and cellular devices. In most cases, magnetometers are used to obtain directional information in three dimensions by being paired with accelerometers and gyroscopes. This device is called an inertial measurement unit, or IMU.

Compass

A compass is a navigational instrument which is sensitive to the magnetic field of the earth. A typical compass has a magnetic strip which aligns itself with magnetic north, and from this, orientation can be determined. Thus, because a compass is used to determine just the direction of a magnetic field, they are considered scalar magnetometers.

A digital compass is not free to move the same way that an analogue compass is, as it is typically fixed to a circuit board. Consequently, an output corresponds to the orientation of a specific reference point of the compass.

The CMPS03 digital compass output data is acessible in two manners. First is as a digital query, conducted in the same manner as an EPROM query. To read the compass output this way, pins 2 and 3 can be connect to a micro- processor for a direct digital reading. The second manner is as a pulse width modulated (PWM) signal. If the PWM signal is to be used, pins 2 and 3 should be pulled to 5 volts with 47K pull up resistors. The purpose of a pull up resistor is to establish a default value on a pin if it is not to be used.

The PWM signal is output on pin 4. The bearing of the device in degrees corresponds to milliseconds the output is high, minus 1. The output thus will be high from 1ms (0 degrees) to 36.99 ms (359.9 degrees). Following this, the signal goes low unconditionally for 65ms.

The output of the PWM signal can be easily interpreted by the BASIC stamp microprocessor. The BASIC stamp contains a function pulsin in which returns the number of stamp time steps the input signal is high. The stamp time step is fixed, thus the value returned by pulsin may be scaled to determine the number of milliseconds the pulse is high. The time step for a 2SX basic stamp is 8 microseconds, fast enough to deliver the necessary time resolution. The digital compass is the final navigational electronic sensor. Together with the accelerometer and gyroscope a complete navigational sensing system can be constructed.

Other scalar magnetometers

Besides the compass, scalar magnetometers can exist in several other forms, using a variety of techniques to obtain information on a magnetic field.

Among these alternative methods include the proton magnetometer, which measures the resonance frequency of protons in a given magnetic field. This is achieved by moving an electric current through a solenoid submerged in a hydrocarbon liquid. This field has properties which can be specifically calibrated and measured. Therefore, the presence of an external magnetic field can detected by measuring the disruptions in the known field produced by the magnetometer. Proton magnetometers are generally used in geology and archeology studies.

Devices

Devantech CMPS03

Sources

Acroname US$ 51.00

Description: Robot Compass Module
Datasheet: http://www.robot-electronics.co.uk/htm/cmps3doc.shtml
Resources:
Notes: Uses Philips KMZ51 magnetic field sensor.
Variants:

Honeywell HMC6352

Sources

Digikey CAN$ 60.18

Description: 2-Axis Digital Integrated Compass On-A-Chip
Datasheet: HMC6352.pdf
Resources:
Notes:
Variants:

Precision Navigation Inc. Vector 2X

Sources

Jameco US$ 49.95

Description: Low-cost, 2-axis compass module
Datasheet: 126703.PDF
Resources:
Notes:
Variants:

Media

Images

Digital Compass board

External links & references

* Magnetoresistance

* Magnetometers

* Inertial Measurement Unit