Motion Tracking System

 

In a Virtual Reality environment interaction with the environment is very important. This interaction is often achieved by the use of a system that registers position and / or orientation of a user or a pointing device (3D mouse). For use in a CAVE a system with 6 Degrees of Freedom (DOF) is preferred. This implies that the system can register the position and rotation on 3 axes (x, y, z, yaw, pitch, roll). For motion tracking with 6DOF in a space of 3x3x3 meters there are several systems available. These systems can be distinguished on the basis of the technology used: magnetic, ultrasonic or optical.

Optical

Optical tracking systems make use of high resolution video cameras, and so-called markers. These markers are figures, or structures with spheres (see picture), which are recognized by the camera's. Underlying software analyzes the images of the cameras and determines the position and rotation of the markers. Optical tracking systems that use special expensive video cameras have a very high accuracy. The disadvantage of these systems is that the cameras must have the markers in sight. Between the camera and the marker there may be no obstruction. This makes such a system less suitable for use in a CAVE if used by several people at the same time. We have an optical tracking system of Optitrack with six cameras mounted on the edge of the Cave.

Ultrasonic

Ultrasonic tracking systems make use of ultrasonic sound, which is picked up by sensors. The collected signals are processed in order to determine a position accurately. The rotation of a sensor is determined by gyroscopes and accelerometers present in the sensor. Like optical systems, ultrasound systems require a free line between transmitter and receiver. By using multiple microphones per sensor, ultrasonic systems are less sensitive to disturbance of this line. In addition, these systems are immune to interference from equipment or metallic objects. A commonly used ultrasound system, the InterSense IS-900, has an accuracy average of 19 mm, with a maximum of 45 mm, within a space of 3x3x3 meters. A major disadvantage of the system is the price, which starts at about € 30,000 -. We do not have an ultrasound tracking system.

Magnetic

With electromagnetic tracking systems, an electromagnetic field is generated by a transmitter. Special sensors can determine where they are and what their rotation is, based on the properties of this field and the magnetic field of the earth. The most commonly used magnetic tracking system is the Flock Of Birds (FOB) of the Ascension Technology Corporation.

The advantage of this system is that no free line between transmitter and receiver is required. The downside is that it is not as accurate as the other systems. Accuracy is also affected negatively by sources of interference, such as metal objects or appliances that generate an electromagnetic field. A practical test, in which the IS-900 and the FOB were compared in a CAVE, showed the accuracy of an uncalibrated FOB to be on average 16 cm, with a maximum of 50 cm. After an extensive calibration the deviation could be pushed back to 3.3 and 9.2 cm, respectively, whereby the accuracy is highest at the center of the CAVE. A disadvantage compared to the IS-900 is that the sensors of the FOB are not wireless. The price of a FOB is a lot better, a system with one sensor and a range of 3x3x3m costs about € 8000, -. A system with two sensors costs approximately € 10,000, -. For the tracking in the Cave we have a FOB from ascension.

Wouter Wognum (2008)