User Manual/4. Sensor calibration

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This whole system is based on an amazing technology called MEMS. It stands for Micro Electro Mechanical Systems. What it means is that inside a chip that is usually smaller than 1mm2 the manufacturers are able to squash not only the logic ICs, but also microscopic moving parts. Otherwise they wouldn’t be able to detect physical properties like acceleration, wouldn’t they?

Inertial sensors based on this technology are inexpensive, and they deliver amazing results, but they have the drawback of having some inherent noise and offset on the delivered values. In addition, magnetometers are easily disturbed by the presence of metallic masses on the surroundings generating large distortions of the perceived magnetic field.

The good news is that most of these problems can be resolved with a proper calibration. On top of this, all that it takes to do it is your hands, some patience and a special cardboard box that you can download from our website. This calibration should only be done before the first time you use the system, or when you move to a drastically different place in terms of global latitude and electromagnetic distortion.

This article is work in progress, you might find that part of the information contained here is incomplete.

Build the cardboard calibration box[edit]

Start by downloading the printable unfolded calibration box, print it in A3 paper, glue it to some piece of cardboard, cut it following the solid lines, fold it following the dotted lines. The result should be something like this:

KC calibration box.jpg

If you have access to a laser cutter, you can download the laser-cuttable unfolded calibration box, that can be used to cut directly on cardboard.

the cardboard box is just a visual guide to be sure you position the sensors in all possible directions during the magnetometer procedure. It has no actual influence on the calibration itself. If this is the first time you do it, it’s strongly advised to use the box. Once you get more comfortable with the procedure, you can skip this step. Just imagine the box around the K-Ceptor and move it accordingly .

Calibration procedure[edit]

In order to perform the calibration, you will need the hub and SBC to be connected and working (as described in §2 of this manual).
You also need to have access to the remote console (as described in §2.1.3 of this manual).

Insert one K-Ceptor in the socket formed by the cardboard on the inside, make sure the K-Ceptor stays fixed in a stable position. Plug the K-Ceptor to the Hub, and from the remote console click on Calibrate K-Ceptor.

This calibration has to be done before one starts using the suit. It’s better to do it in an environment as free of electromagnetic distortion or big metallic masses as possible. Sources of electromagnetic distortion can be: big antennas, electric motors from refrigerators or other electro domestics, loudspeakers, etc.

Buildings have often a metallic structure. If you can try to do it in a place without metallic structure, on the outside, or at least in the most peripheric part of the building (for example: basements are usually not the best places to do it), that will surely lead to better results.

On the other hand, if you know you will be using the suit only in one place, or in some specific occasion, you need to have some extra accuracy: it’s a good idea to perform this sensor calibration on the exact same place where the capture will take place.

Step A[edit]

You will be asked to place the calibration box on flat surface facing upwards. This means that the big number 1 should be upwards. Be sure to let the box completely still (sometimes the tension of the wire might move it a little) and click the Go button.

In this step the program gathers a few samples from the accelerometer and gyroscope from which it then finds the inherent offsets that these sensors deliver.

Step B[edit]

Then you will be asked to perform the magnetometer calibration procedure. This procedure consists of grabbing the box with your hands and repeating the following steps for each face:

  • Hold the box in front of you and make sure to keep the current face as parallel to the ground as possible.
  • Put the letter “a” of the current face on the same direction you are looking to.
  • While holding the box parallel to the ground, turn it 90º in order to put the letter “b” on the direction you are looking to. Repeat for all 4 letters of the face.
  • Go to the next face, and repeat these steps.

Once you are done with the last face, place the box on the table and click on the Go button. The program will take some seconds to process the information, just wait and press Go when you are asked to. That’s it, now you have a calibrated and much more accurate sensor. You can keep using it like this for some time.

Magnetometer calibration.gif

In this step, the program gathers lots of samples, and performs a somehow complex procedure to calculate errors on the measurements. If you want to learn some more about this procedure you can visit this guide.