I love browsing old 19th and 20th century engineering
textbooks. It’s great fun seeing the ingenuity that our technical
predecessors used to take measurements that today we consider routine or
mundane. The vertical force magnetometer here is one example.
This figure and theory of operation was published by the
Royal Society of Edinburgh in 1848
and is available to anyone today, courtesy of Google Books.
163 years later, we have technologies for taking magnetic measurements based
on any number of physical phenomena and targeted at almost any range of
measurement needed. With names like AMR, GMR, TMR, Squid, fluxgate,
magneto-inductive, SERF and Caesium vapor, it’s easy to get confused.
If you are working in the consumer area, you can probably rule out most of
It’s only recently that price points have reached the level where we
can expect to start seeing magnetometers in low-cost consumer devices.
The ones that get my attention are those based on magnetoresistive technology
(AMR, GMR and TMR) and magneto-inductive.
Magneto-inductive sensors, like those offered by
PNI Sensor Corp., have an advantage in that they have no need to reset magnetic domains of
the material. I own a PNI game controller and use it often. One downside is
space, Z-axis devices of this type may not fit in tight environments
like cell phones. Magneto-resistive technologies such as those offered by us
and other vendors share a common technology base with your average CMOS
integrated circuit. X, Y and Z sensors are easily integrated into a single,
physically small, package.
Since sensors for all three axes can be combined into a single die using
standard IC manufacturing processes, cross-axis errors are minimized. The
MAG3110 goes one step further and includes a second die which provides signal
conditioning, analog-to-digital conversion and digital host interface.
TMR Magnetometer Principles of Operations
utilizes an effect called tunneling magneto-resistance (TMR) which is based
upon the magnetic tunnel junction (MTJ) illustrated below. The device includes
two electric contacts, two magnetic layers and one barrier layer. In the
figure, the tunneling current I is controlled by the relative
orientation of the magnetic fields in the free and pinned magnetic layers. The
current “I” is maximized when the fields in both layers are
aligned with one another. It is minimized when the two fields have opposite
polarity. Essentially, the MTJ acts as a resistor which is dependent upon the
Physically, each MTJ device can be optimized to sense magnetic fields in a
specific direction. The magnetic sensor is formed by arranging arrays of
these devices into three separate Wheatstone bridges (each sensitive in
a different direction) whose output voltage can be converted to digital format
by the ADC.
We’ve come a long way from that vertical force magnetometer shown in
the Royal Society transactions. The MAG3110 shown next to the thumb tack above
is 2mm x 2mm x 0.85 mm, provides measurements for all three dimensions and
consumes as little as 24 micro-amps at 1.25 Hertz sample rate. If your
microcontroller has an I2C port, you have all the circuitry you
need to take magnetic measurements.
I’ve obviously glossed over a lot of details. So if you’re
interested in learning more, take a look at some of the references I’ve
listed below. They do a far better and more exhaustive job at explaining your
magnetic measurement options than I can possibly provide in this space. And be
sure to visit the
A New Perspective on Magnetic Field Sensing, Michael J. Caruso, Tamara Bratland, Dr. Carl H. Smith and Robert
Induction coil sensors a review, Slawomir Tumanski, Measurement Science and Technology,
Institute of Physics Publishing, 2007
Magnetic Sensors and Their Applications, James Lenz and Alan S. Edelstein, IEEE Sensors Journal, Vol. 6, No. 3,
PNI White Paper, Magneto-Inductive Technology Overview, Andrew Leuzinger and Andrew Taylor, Feb. 2010
Transactions – The Royal Society of Edinburgh, Volume 18, 1844
Magnetoresistive Sensors for Nondestructive Evaluation, Albrecht Jander, Carl Smith and Robert Schneider, 10th SPIE
International Symposium, Nondestructive Evaluation for Health Monitoring and
Magnetometer Wikipedia Article