Final Post

In summary, I wanted to create a magnet-based sensor for the shoulder and started off by creating a proof of concept. I created a styrofoam model of the shoulder and placed a magnet on it. When moving the model’s arm around, I could detect the changing magnetic field on the magnetometer. It worked well in the fact that moving the arm with the magnet on it would change the magnetic field read by the magnetometer. However, I did not rotate the whole model or move it around to see the effect of the outside environment. Later, when testing on a real person, we found that the outside environment changes significantly due to the proximity of metal objects, including phones, metal screws in tables, and doorknobs. Upon creating the proof of concept, I created a model for the magnet that was placed on the arm. I treated the magnet as a dipole and predicted where that dipole would need to be given a magnetometer’s reading. Lastly, I defined the position of the arm in terms of yaw, pitch, and roll coordinates, and given a magnetometer’s reading, I was able to limit the position of the shoulder to two spots in the yaw, pitch, and roll coordinates. This model did not account for the thickness of the arm, and the estimate for the position of the magnetometer was rough. The model also did not account for the motion of the magnetometer when the arm was being raised. This plays a significant effect on the reading because the magnetometer rotates with the arm when it is raised, so the change in the magnetic field is less significant than it should be.

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Modeling the Shoulder

Modeling the Shoulder

Our goal in modeling the magnet is to be able to find the arm’s position given a magnetometer’s reading. In order to do that, we need to fix the magnet’s position on the arm. This will limit the total possible positions in which the magnet can be located, and it will allow us to locate the position of the arm given a magnets position. First, we will model the magnet’s range of motion on the shoulder. Then, we will combine that with the magnetometer’s position and the magnetic field produced by the magnet to localize the magnet on the arm and to identify the position of the shoulder joint.

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Modeling a Magnetic Field

Modeling the Magnetic Field around a Magnet

This week, I modeled a magnetic field around a magnet. The strength and direction of a magnetic field can be modeled by the following polar equation where θ is the angle from the direction of the magnetic north pole, r is the distance from the magnet, and |m| is the strength of the rare earth magnet.Since the magnetic field is only dependent on r and,it can be modeled in two dimensions. This would mean that the graph could be rotated around the x axis, and the magnetic field would not change. Below is a model of the magnet in the x, y plane, where the magnetic North pole is pointed in the positive x direction. The arrows indicate the direction of the magnetic field at a certain distance from the magnet, and the strength of the magnetic field (in nanoteslas) is indicated by the color, with purple being a field strength of 30 nanoteslas and over.

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Creating a Proof of Concept for the Sensor

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For creating the motion sensor of the shoulder, I posed three questions: [Read more…]