Sorry for the slightly belated reply.
We spent some time looking at these systems for use in a scientific setting,
and I tested them for a couple of months (using VRPN to interface them under
Linux, along with some kernel modifications to reduce serial latency).
Here is generally what I found (may or may not be correct!):
Polhemus Fastrak (AC system):
- Not very susceptible to active interference (Monitors, mains power,...).
- Reliable. Talking to people they generally seemed to believe the Polhemus
system was more reliable than the Ascension system.
- The 120Hz update rate is shared between the sensors connected (max 4 to
one unit). Hence if 4 sensors are attached, each is sampled at 30Hz, and in
a round robin fashion.
- Distorted by fixed metal. Need to calibrate the environment to remove.
Ascension Flock of Birds (DC system):
- Higher update rate (Max 144Hz?)
- Sensors are measured synchronously.
- Not distorted by fixed metal as much (however, it depends on the type of
- Susceptible to active interference. In some environments there is a need
to use a reasonable amount of filtering, which causes higher latency (the
peak delay for the narrow notch filter reaches about 60ms).
It depends on your environment and requirements which system is better. For
our use, it was very important to have low latency and high accuracy. Hence
we went for a Polhemus Fastrak master/slave configuration with 8 sensors,
and are using a calibration frame to remove distortions from the
environment. Neither system was perfect, but for our use the Fastrak system
was the better option.
I can send you a selection of PDF's off list if you wish, looking at
latency, accuracy, and calibration of the two systems.
----- Original Message -----
From: "Foo Meng & Kay" <mengnkay@...>
To: "IHSAN DEVRIM GIN" <ihsangin@...>;
Sent: Saturday, March 22, 2003 1:10 AM
Subject: [vrjuggler-info] Polhemus or Ascension Trackers?
> I am interested to purchase electromagnetic trackers for my VR system. I
> just wondering if anyone has experience with Polhemus and Ascension
> Trackers....I was just wondering which is better in tracking body
> This email has been scanned for viruses by StarHub e.Scan.