I am getting hard to explain results when digitizing a head with Polhemus Patriot. The setup is quite standard: a source near the subject’s back, receiver taped to forehead, using stylus for collecting points on the scalp. Here is the strange part: when I keep the tip of the stylus FIXED on a single point on the scalp and change only the orientation of the stylus, the XYZ coordinates change (they shouldn’t, the tip if fixed)! In fact, when rotating the stylus in a circular manner while collecting data points, the points in the digitization image also form a circle, as if orientation is interpreted to mean location.
If you need more details about my hardware, software, environment etc. I will gladly provide them.
I have not idea what’s wrong. If you can give me any ideas in which direction to think, I’d appreciate it!
Hi.
I have found that the most accurate position measurement will be when the stylus is perpendicular to the surface, since the algorithm accounts for azimuth, elevation and roll. When you angle the stylus, the tip is no longer in the same position. How much distance do you see between points when you rotate the stylus? It should be quite small.
[QUOTE=ebock;7520]Hi.
I have found that the most accurate position measurement will be when the stylus is perpendicular to the surface, since the algorithm accounts for azimuth, elevation and roll. When you angle the stylus, the tip is no longer in the same position. How much distance do you see between points when you rotate the stylus? It should be quite small.
Beth[/QUOTE]
Hi Beth,
The distance is quite large (even several cm). I think I will write my own piece of code with just a few basic steps (measure stylus and reference coordinates, find difference, plot point) to see if the results are comparable to what I get with brainstorm.
Hi Maris,
That sounds good. Also, keep in mind that the environment has a lot to do with the accuracy of the collected points. I had to move my setup around the room, testing the accuracy, to find the best location. Be sure you are away from walls and other metal objects.
the closer the source to the stylus the more accurate will be the measure (if you tape the transmitter to the chair behind the patient, make sure the patient sits back in the chair)
it is very important that the receiver does not move, either in position but also in orientation (“receiver taped to forehead” : if you are not using the glasses, be sure the the receiver has a fixed position/orientation with respect with the patient’s head.
Finally, as Beth is mentioning the environment has a lot to do with the accuracy : in addition from being far from the walls, the operator should not be wearing metallic parts (watch, phone, etc.) and we obtained best results by placing the stylus’ wire around the operator’s neck during the digitization.