| Author | Message | ||
     Polhemus Rating: N/A |
Hello there, I'm in the extremly (un)lucky position to have a Visette including polhemus tracker setup. My problem is, everything works fine with one tracker but if I use the Visette and I turn 90 degrees to the left or right elevation will become roll. If you go 90deg further elevation will simply be inverted (in regards to the original orientation) I do have another application where this is not the case, but the values coming from the polhemus give me troubles. Does anyone here have any expirience working with this devices and might be able to point me to the right direction? Please email me at polhemus@hackersquest.org Thanks in advance | ||
| BOPrey Rating: N/A |
What product do you have? Their Fastrak is the closest one that fits your description on their website. Anyway, what you discribed is consistant with tracking devices that use electro-magnetic technologies for tracking. Yes, I am saying the range is less than 180 degrees. From left to right, and from top to bottom. If you want a tracker with larger range, go with one that uses a gyro; it will cost you. | ||
| Alatar Rating: N/A |
The Fastrak, Isotrak etc don't really have only a 180 degree range, it's just that the coordinate systems work in such a way that it can't distinguish between one hemisphere and the opposite one, so it interprets all coordinates as being in a single hemisphere. So you are limited to a single hemisphere of operation, but you can choose which hemisphere you want to have work (e.g. the top but not the bottom, the left but not the right, etc). Post here if you need the initialization strings for setting/changing the operational hemiphere. | ||
| Alatar Rating: N/A |
The Fastrak, Isotrak etc don't really have only a 180 degree range, it's just that the coordinate systems work in such a way that it can't distinguish between one hemisphere and the opposite one, so it interprets all coordinates as being in a single hemisphere. Thus you are limited to a single hemisphere of operation, but you can choose which hemisphere you want to have work (e.g. the top but not the bottom, the left but not the right, etc). Post here if you need the initialization strings for setting/changing the operational hemiphere. | ||
| Alatar Rating: N/A |
Sorry 'bout that :-( | ||
| BOPrey Rating: N/A |
Alatar, Coordinate system not being able to distinguish the two hemispheres. Isn't this the same thing as not having a range of more than 180 degree? I hope I am wrong. | ||
| Alatar Rating: N/A |
Well, it's a question of semantics, and I agree that basically you're right. I was trying to point out that the tracker can actually track an object anywhere in the 360 degree sphere around it, but that it only half of it can be enabled at a time, and that the sphere can be sliced (dynamically) in any of three ways x two sides = six workable hemispheres (top, bottom, right, left, front, back). Some of them -- I don't remember whether the Fastrak has this feature -- allow a tilted hemisphere so you can slice the sphere into two halves at any angle you like. I have been told (by Ascension, not Polhemus) of people who have written dynamic tracking routines that change the active hemisphere on the fly: they watch where the sensor is and switch the active hemisphere if the sensor gets too near the edge of the active area. So yes, you are right, but it's not that there is no support at all for 360 degree coverage, it just takes a bit of work. Not that this is much use to people who aren't writing their own drivers. It is only of interest to end-users in that they might use an init-string modification to change the current working hemisphere if that seems useful. | ||
| Alatar Rating: N/A |
What I can tell you is that, properly programmed (which is very complex, as all the math is in quaternions) the Fastrak and all its cousins do *not* exhibit any unusual behaviour (yaw becomes roll etc), except that an object going out of the working hemisphere and into the non-working one causes the device to believe the sensor has jumped to the diametrically opposing point and continues from there. I am not explaining this very well, so I will try a 2D analogy. If the source is at the center of a clock, and the sensor is at the end of the hour hand, and the active hemisphere is set to be noon to six o'clock, then all hour-hand positions from 0:30 to 5:30 will be reported correctly, but 6:30 will be reported as 12:30, 8:00 will be reported as 2:00, and so on. This can be fixed by changing the active hemisphere to be from 3:00 to 9:00 when the hand reaches 5:00, for example. Note that this entire discussion is only for translation (i.e. movement). Rotational measurement, when properly programmed, works perfectly at all angles. I can vouch for this as we have been using lots of these devices for years, and rotation is not a problem for any of them. | ||
| BOPrey Rating: N/A |
OK. I understand now. You could acturally change the hemisphere orientation dynamically. That will do it. |
