3.0 Positional Tracking
The Positional Tracking is a key part of the augmented reality system used to track the position and orientation of the Head Mounted Display, and selected body parts of the user in a three dimensional space. The Positional Tracking System operates by measuring the position and orientation of the HMD, and selected body part with Six Degree of Freedom (i.e. Translational motion and Rotational motion) with respect to a reference point. The three major parts of the Positional Tracking include: a marker, an infrared camera, and a digital signal processor. The marker emits signals (such as magnetic or light) and externally synchronizes with the infrared camera. The infrared camera detects the signal generated by the marker by capturing dots coming from the laser projector. The digital signal processor processes the signals received by the infrared camera, and determines the position and orientation detected by the infrared ray. The following sections discuss the applications of the Positional Tracking System in the Medical and Entertainment industry, and present the future advancements of the technology.
Figure 1: The Operational System of the Positional Tracking System, adapted from IEEE Xplore
3.1.1 Mechanical Tracking Device
3.1.2 Optical Tracking Device
3.1.3 Electromagnetic Tracking Device
This section would focus on the different types of positional tracking devices and how they are being applied in the medical and entertainment industry.
The Positional Tracking Device is very useful in the medical field, particularly in surgical platforms. It helps to detect the position and orientation of surgical instruments. There are different types of tracking devices which have been introduced to the image guided system. The types of tracking devices are: Mechanical tracking device, Optical tracking device, and Electromagnetic tracking device.
The Mechanical tracking device uses mechanical digitizers to measure the position and orientation of a surgical instrument placed at the end of a movable mechanical arm. The advantages of this tracking device are: high accuracy and resolution, lack of magnetic interference, and no line of sight necessary. However, because of the bulky nature of the digitizer, the mechanical tracking device is currently not used in Image-Guided Systems.
The Optical tracking device uses cameras to track the fiducial markers attached to a surgical instrument. Unlike the mechanical tracking device, the optical device is capable of tracking multiple surgical instruments. The advantages of this tracking device are: high frequency refresh rates, high accuracy, and large field of measurement. Nevertheless, this tracking device lacks the ability to track surgical instruments that are not in the line of sight.
The Electromagnetic tracking device uses electromagnetic field generators and small electromagnetic coils embedded in the surgical instruments to detect the position and orientations of the instrument. Like the optical tracking device, the electromagnetic tracking device also has the ability to track multiple surgical instruments. This tracking device does not require an object in the line of sight to operate. Nevertheless, surgical instruments made up of ferromagnetic materials cause magnetic interference.
The Positional Tracking Device is very important in the entertainment industry, particularly in gaming. It helps to detect the position and orientation of user in the physical environment. The different types of tracking devices highlighted in the preceding section are also introduced in the gaming industry.
The Mechanical tracking device uses a celling mounted hardware to track the users head motion. However, this tracking device is not used presently because of its bulky nature, and its inability to function outdoors.
The Optical tracking device uses infrared optical trackers to detect the positon and orientation of the target. The markers are either placed on fixed locations in the environment or directly on the target, and a set of calibrated cameras track the three-dimensional position and orientation. Despite the high accuracy of this tracking device, this device canâ€™t track the target if the markers are obstructed.
The Electromagnetic tracking device uses electromagnetic field to determine the position and orientation of the target. This tracking device operates by generating low frequency magnetic field vectors from a transmitter which are detected by a receiver. The signal received is computed as a mathematical algorithm which displays the position and orientation of the target. Like the optical tracking device, this tracking device has a high accuracy. Nevertheless, metallic interference can disrupt the device.
3.3 Future Advancement
In the medical and entertainment industry, the positional tracking device is seen as a very important technology. However, this technology still experiences major setbacks which can be improved. For this reason, engineers and medical professionals are currently searching for alternatives to improve the positional tracking device. In the medical application section, the applications of the different types of positional tracking devices were discussed. This section would focus on various engineering approaches to further advance the technology.
To further increase the accuracy of the positional tracking system in the medical field, medical researchers; Bernd Schwald and Helmut Seibert, proposed the use of a Hybrid tracking system. The Hybrid tracking system is a wireless device comprising of both the optical tracking and an electromagnetic tracking. The optical tracking is used to track the userâ€™s head, while the electromagnetic tracking is used to register the patient and track surgical instruments. The purpose of integrating the two tracking systems is to eliminate the problem of metallic interference in the surgical environment. This system also has a unique feature, a transparent display. This feature helps to improve the userâ€™s view of the patient, and does not require the use of a head mounted display.
To enhance the positional tracking system in the entertainment industry, a Wi-Fi based and Image based positioning device was proposed to improve accuracy in indoor navigation. This device is accompanied by a Head Mounted Device and sensors (gyroscope, accelerometer, and magnometer) which help with identifying the users position and orientation. This device also has a special attribute, a 3D point cloud localization which helps to improve indoor navigation.
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