Arduino: measuring the Earth’s magnetic field with the magnetometer HMC5883L

 

The HMC5883L magnetometer

HMC5883L compass main

This component (a small chip) HMC5883L, produced by Honeywell, bases its operation on  AMR (Anisotropic Magnetoresistive) technology and allows you to be able to measure both the direction and the magnitude of the earth’s magnetic field. This magnetometer HMC5883L has within 3 magneto-resistive sensors arranged on three perpendicular axes (the Cartesian axes x, y and z). Here you can find the component  datasheet.

 

HMC5883L_chip

Fig.1: The Honeywell HMC5883L chip

The magnetic field affects these sensors modifying in some way the current flowing through them. Applying a scale to this current, you know the magnetic force (in Gauss) exerted on each sensor.

The component HMC5883L communicates with Arduino through the I2C protocol, a protocol very simple to use.

For our example with Arduino, I have bought the breakout board HMC5883L, distributed by Sparkfun (see here), in which the integrated chip is visible in the center (see Fig.2).

Sparkfun-HMC5883L

Fig.2: The Sparkfun breakout board on which the HMC5883L is integrated

The market offers many other breakout board on which is integrated this chip. A classic example is the breakout board distributed by Adafruit (see here).

Adafruit-HMC5883L

Fig.3: The HMC5883L breakout board distribuited by Adafruit

The circuit diagram

  • Arduino GND -> HMC5883L GND
  • Arduino 3.3V -> HMC5883L Vcc
  • Arduino A4(SDA) -> HMC5883L SDA
  • Arduino A5(SCL) -> HMC5883L SCL

HMC5883L Fritzing

The HMC5884L library for Arduino

In the internet you can find many libraries programmed specifically for use of this sensor. For the examples discussed in this article I have chosen to use a library downloaded from the website of LoveElectronics some time ago (although now I can not find the site) called just HMC5884L.

Click here to download the library. Once you download the zip file, extract the contents inside the Program Files > Arduino > LIbraries (if you are on Windows), that is a folder named HMC5883L.

HMC5883L files library

Fig.4: Files contained within the HMC5883L library

Once you have extracted the contents you start the Arduino IDE. If everything is installed correctly you will find a new example loaded within the IDE (as shown in Fig.5).

HMC5883L library IDE

Fig.5: If the installation was successful, you will find the Simple example within HMC5883L

May I suggest a site where it presented an excellent tutorial with this library

The best way to learn how to use this library is to use it in simple examples, during which the commands and functions are explained gradually, their operation is that how to use them. For this purpose, in this article you will see two examples. First, you implement a digital compass, then we will use HMC5883L as a real magnetometer, collecting data and then use them in the representation of a vector field through MATLAB.

Let’s create a digital compass

All of you are familiar with this kind of objects. A small cylindrical box with a needle inside of ferrous material that affected by the Earth’s magnetic field, aligns itself according to field lines that run through the surface of the earth, thus indicating the north.

What we want to achieve with Arduino is the creation of a compass that will give us the value 0 ° when the sensor is pointed toward magnetic north, and 180 ° when in fact the sensor is pointing south.

The code that we are using is very common on the Internet and you can find it with endless small variations. I propose it because it is an example very useful to understand how to use this sensor, very simple and suitable as a whole to become familiar with commands of the HMC5883L library.

First you have to import the library HMC5883L.

Since the HMC5883L board communicates with Arduino through the I2C protocol, you must also import the Wire library (see here for more information).

After including all the required libraries, declare the instance HMC5883L as a global variable.

Now you can begin to define the content of the two setup() and loop() methods omnipresent in any Arduino sketch.

We begin, as logically, writing the setup() method. First we activate the I2C communication protocol between Arduino and the magnetometer.

Also, since you need to display the values of the magnetometer readings somewhere, you will use the serial communication between Arduino and a PC. Then, specify:

After all connections have been initialized, you can now also initialize the magnetometer by defining:

The next step will be to configure the sensor according to your needs. First you need to define the gain (response scale) on which to work with the magnetometer.

With these lines of code you have just set the gain at 1.3 Ga (end scale), that the measures should be between values of -1.3 and 1.3 Ga.

You can set only some end scale (gain) on the HMC5883L magnetometer, and these are: 0.88, 1.3, 1.9, 2.5, 4.0, 4.7, 5.6 and 8.1 gauss. Needless to say, if your measurements fall within a certain range, try to select a smaller scale can that contains it, as the precision of the measurement taken from your card will be higher.

Now you begin to define the content of the loop() function. The code written in this function will run continuously, and then each time it is performed, a measurement will be made by the sensor.

The ReadRawAxis() function returns the value obtained directly from the magnetometer. Thus the values obtained in this way have no reference to the true value of the magnetic field strength, but they are only proportional to it. So if you are interested in the true value of the magnetic field (in gauss) you should not consider this function, but replace it with the ReadScaledAxis() function.

In this case, however, you are interested in simulating the orientation of the magnetic needle of a compass, then the actual values do not interest us. So we can safely use the raw values of reading.

Recall that the magnetometer is able to detect simultaneously the measurements on the three Cartesian axes. To obtain these values, just call the following values belonging to the object variable MagnetometerRaw where they are stored. Here’s an example of how we can get the three values each for each axis (to not be included in the code).

Instead, if you wanted to use the actual values reported directly in scale (measures in Gauss). (to not be included in the code)

Now you can define the compass needle inside the loop() function.

since the angle you get from this formula is in radians, convert it to degrees format that is most familiar to you and very readable.

Now you just have to see the value displayed on your PC while it is connected with the Arduino board via serial communication.

I added delay() in order to make a measurement per second (1000 ms), but you may change this measure at your leisure.

Here’s the whole code:

Now connect the Arduino to the PC and open a serial communication with which you can control your digital compass. Rotate the HMC5883 board until you get the 0 value. If you get 0 you are pointing the north (magnetic).

HMC5883L compass serial

Fig.6: Serial monitor

The magnetic declination

The magnetic field that surrounds the terrestrial globe is neither perfect nor uniform, but is subject to continuous variations in both space and time. This effect is called magnetic declination. In this regard, there is a beautiful animated GIF that I want to propose that presents the variation of the magnetic field during the last four centuries (1590-1990).

As for the digital compass, you can take into account this effect by slightly changing the formula used in the previous example.

First we need to get the value of the magnetic declination of the position in which we are carrying out the measurement. There is a beautiful site in the internet that can provide real-time values of the magnetic declination (www.magnetic-declination.com). This site will give you immediately the magnetic declination of the position in which you are connected. Here’s my example below:

magnetic-declination-map-rome

Fig.7: the value of the magnetic declination in Rome at the time of writing this article.

From the box that appears at the center of your map, you will be interested mainly in:

Magnetic declination: +2° 38′ EAST

Now visit  WolframAlphaa beautiful site for the calculation and scientific informationonline. Thanks to it you will convert the value of the magnetic declination in radians. In the input field type

Wolframalpha_radians

Fig.8: WolframAlpha site allows you to do many calculations and convert units of measure online

 

Wolframalpha_radians-result

Fig.9: here is the conversion result in milliradians.

Thus add the following code within the loop() function :

This is because I have got a declination EAST, but if the declination is WEST you should write:

And finally…

Another example: let’s visualize the magnetic field on your table

Now we develop another example, in my opinion a bit more fun, though more laborious.

In this example we will take a series of measures on the table next to our computer to display the field lines that pass through it. In the second phase of the example we will introduce a magnet near the area in which we have carried out the previous measures and measures rieffettueremo again. It will be interesting to see the effect that it will introduce the magnet on the field lines that cross our desk.

For the example we are considering will consider only the vector components of the magnetic field on the X and Y axis, viewing the magnetic field that crosses the surface of our table.

First we look at the code for Arduino

As you can see from the code, I used raw measures, but if you want to make a more precise work I recommend you use the scaled measures. I also made sure that the value displayed on the PC serially, is the average of 500 readings. I made several attempts and I saw that this average produces fairly repetitive values. Increasing or decreasing the number of samples to average I get worse results (in your case you can do various tests).

Now we go to the actual measurement.

First I took a sheet of squared paper and I drew on it 35 squares each having the same size of the Sparkfun breakout board. I created a matrix of 5×7 squares, and then I numbered each of them in ascending order (see Fig 10).

HMC5883L matrix measurements

Fig.10: A squared paper is a good base to draw a matrix of measuring points.

Then I put the paper on the table, fixing it so that it can not move on it (with tape, or small weights). Once this is done I measured values of the magnetic field on each square, noting both the strength of the magnetic field of the X-axis that Y-axis (NB the values read very often fluctuate, try to see the value that occurs most often)

At the end you will get a table of values. I show you the table of values that I got on my table. They will be those shown in the example.

  X Y Z
1 -27 -348 -386
2 -17 -347 -388
3 14 -350 -389
4 14 -351 -390
5 -5 -360 -393
6 -20 -350 -377
7 -10 -345 -380
8 2 -350 -387
9 27 -353 -390
10 17 -356 -394
11 -46 -325 -368
12 -3 -334 -380
13 9 -340 -386
14 -26 -340 -388
15 -16 -347 -400
16 0 -318 -232
17 17 -340 -368
18 20 -347 -385
19 3 -340 -401
20 21 -340 -408
21 -27 -394 -308
22 -10 -350 -378
23 -5 -334 -398
24 -12 -338 -404
25 -48 -326 -423
26 -47 -350 none
27 1 -346 none
28 1 -343 none
29 38 -343 none
30 -35 -330 none
31 -80 -400 none
32 1 -346 none
33 -23 -342 none
34 15 -340 none
35 -107 -410 none

Once you have all the values, you have to open a session on MATLAB and begin to define a matrix SX and SY containing the measured values respectively for X and Y. These two matrices SX and SY have size 5×7 and the values are inserted into them in the same format that we have on the squared paper, so that the spatial distribution of the measures is respected.

also define the spatial arrangement X and Y of each square (measure).

and finally you obtain the visualization of the vectors of the magnetic field. The picture is inverted with respect to the picture of the table (the cell 1 is on the bottom left).

campo magnetico

Fig.11: The Earth’s magnetic field represented vectorially on the table top

Now add a magnet to the left of the sheet to about 7 cm from it and at the center of the sheet.

HMC5883L matrix measurements 2

Fig.8: the magnet is positioned to the right of the matrix

Make again all measures for each square as you did earlier. At the end you’ll get a table similar to the following.

  X Y Z
1 -109 -303 -383
2 -108 -284 -391
3 -130 -322 -394
4 -174 -188 -412
5 -244 -9 -446
6 -124 -290 -380
7 -158 -350 -380
8 -185 -267 -394
9 -329 -182 -412
10 -756 -147 -417
11 -125 -294 -363
12 -127 -323 -400
13 -199 -330 -388
14 -403 -275 -399
15 -975 -240 -475
16 -73 -321 -243
17 -105 -356 -365
18 -178 -398 -385
19 -406 -450 -387
20 -778 -820 -332
21 -98 -474 -343
22 -106 -370 -373
23 -154 -420 -390
24 -157 -517 -386
25 -390 -736 -290
26 -85 -373 -382
27 -93 -394 -385
28 -89 -430 -381
29 -121 -500 -380
30 -113 -650 -430
31 -74 -375 -380
32 -76 -465 -378
33 -38 -422 -381
34 -65 -500 -390
35 -160 -523 -400

 

Now, again with MATLAB, define two matrices 5×7 VX and VY containing the new measures, and finally re-enter MATLAB commands to obtain the graphical representation.

With the hold on command you overlay the new representation with the previous one in order to better visualize the changes in the earth’s magnetic field introduced by the approach of a magnet.

campo magnetico 2

Fig.12: The vector field without magnet (green) and the one with the magnet (blue)

As you can see the effect is quite significant in the vicinity of the right edge and is gradually decreasing with the move away from the magnet.

Conclusions

This small article is just a small introduction to the great potential of this sensor. It would be very interesting to extend the previous example to three dimensions considering the Z-axis. Another good idea would be to monitor the variation in time of the magnetic field.

But this is up to you! See you next article.

 

 

 

 

11 Comments:

  1. Complimenti Fabio, bell’articolo applicativo/sperimentale su una brekout board con magnetometro veramente interessante. La spiegazione dei passaggi e del codice utilizzato in questo tutorial è chiarissima e ineccepibile, come al solito.

  2. Ottimo articolo Fabio, professionale e competente come sempre.

  3. Bellissimo articolo!!!Mi chiedevo se è possibile impiegare (sempre che non si tratti di un osservazione fuori luogo dato che non ho nessuna competenza nel campo) il sensore descritto o, se esistono sensori in grado di misurare il campo magnetico sott’acqua. Ad esempio campo magnetico prodotto dalla corrente elettrica sott’acqua.

    • Ciao xnLibn,

      il sensore è un vero e proprio magnetometro a 3 assi (XYZ), quindi in linea teorica è in grado di misurare qualsiasi campo magnetico. Se si vuole misurare un campo magnetico sott’acqua dovrai utilizzare un’opportuna protezione del sensore, inoltre tale involucro, non dovrà in qualche modo schermare il campo magnetico.
      Poi per quanto riguarda l’intensità del campo magnetico da misurare, quello dipende dalla sensibilità del sensore. Inoltre si dovrà in qualche modo tenere conto del campo magnetico terrestre ed eliminarlo, per avere solo le misure del campo magnetico prodotto da un motore, dalla corrente elettrica, da una sorgente di qualche tipo, da un magnete, ecc..
      Spero di esserti stato di aiuto.

  4. Grazie mille sei stato gentilissimo!

  5. riccardopichi

    Buonasera secondo lei e’ possibile integrare in un solo sketch questa bussola con il sensore bmp180?

    • Ciao Riccardo,

      personalmente non l’ho ancora fatto, ma i device che comunicano con protocollo I2C possono lavorare tutti contemporaneamente sui canali A4 e A5 di Arduino. L’importante è che gli indirizzi siano diversi e noti, in modo da chiamarli correttamente durante la connessione tra Arduino e i corrispettivi device. Il sensore BMP180 sembra lavorare su indirizzo 0x77 che è diverso da quelli del magnetometro, quindi dovrebbe essere OK.
      Inutile dire che se funziona, ti invito a presentare un bell’articolo al riguardo. 😉 Stiamo cercando nuovi autori.

      • riccardopichi

        Ti ringrazio per la veloce risposta. Il protocollo i2c non dovrebbe dare problemi. Provo a fare qualcosa nei prossimi giorni.

      • riccardopichi

        Sono riuscito partendo dal tuo esempio e prendendo spunto da un altro esempio per il BMP180 GY-68 a mostrare su uno schermo Lcd I2C pressione, temperatura, altitudine e l’indicazione in gradi del nord magnetico. Ho un piccolo problema: non riesco a rappresentare il simbolo ° nel display lcd: hai qualche idea?

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