Fastrak/PiMgr digitization questions

Discussions
1

Hi brainstorm experts,

We have questions regarding if the position of the Polhemus Fastrak receiver and stylus are all within the same frame of the transmitter. We are trying to re-use an old Fastrak system for digitization. First, we couldn't get brainstorm to recognize the correct digitized points, so we then tried to verify the coordinates using PiMgr program and also compute the stylus-reference translation/rotation matrix. We did a test of digitizing some points on a paper (see the photo and the position of points output in PiMgr window below, sensor 1 is stylus, sensor 4 receiver).

6 points were digitized: The first 2 points are the center of the receiver (the black dots marked on receiver). The rest of the points are as follows: 2 inches from the receiver in both direction on z-axis, and 2 inches in both direction on y-axis. In the output below, the first 4 lines indicates the positions of sensor 1 and 4 when we digitized the center of receiver. We thought the x, y, z of sensor 1 and 4 should be very similar, but the number in z-axis (4th column) are ~ 9 inches away, like the receiver is referenced to a complete different coordinate frame.

Can anyone please help us understand why? I have tested with different receivers and stylus, and they are all like this. Our next action is to try using brainstorm again but we want to understand first if this is a sensor configuration issue before we test it using brainstorm digitization interface.

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Thank you!
Yuhan

2

@Marc.Lalancette, any suggestions?

3

This is more a question for Polhemus support, or looking through their documentation.
I'd have to do some tests with PiMgr to figure out what these coordinates represent and if we're seeing the same thing here. It does look odd though as you point out. Could it be that it's not configured to expect the stylus but some other receiver in channel 1? The stylus has multiple receivers inside and it's calibrated to return the position of the tip. Maybe what you're seeing is not that tip position but one of the internal receivers? Unfortunately, I don't have the time to look further into this now. Don't hesitate to post whatever you find here to help others that may visit in the future. If you haven't already, I also recommend double checking you don't have any system errors. They are accessible through PiMgr.