Tracking devices are intrinsic components in any VR system. These devices communicate with the system's processing unit, telling it the orientation of a user's point of view. In systems that allow a user to move around within a physical space, trackers detect where the user is, the direction he is moving and his speed.
There are several different kinds of tracking systems used in VR systems, but all of them have a few things in common. They can detect six degrees of freedom (6-DOF) -- these are the object's position within the x, y and z coordinates of a space and the object's orientation. Orientation includes an object's yaw, pitch and roll.
From a user's perspective, this means that when you wear an HMD, the view shifts as you look up, down, left and right. It also changes if you tilt your head at an angle or move your head forward or backward without changing the angle of your gaze. The trackers on the HMD tell the CPU where you are looking, and the CPU sends the right images to your HMD's screens.
Every tracking system has a device that generates a signal, a sensor that detects the signal and a control unit that processes the signal and sends information to the CPU. Some systems require you to attach the sensor component to the user (or the user's equipment). In that kind of system, you place the signal emitters at fixed points in the environment. Some systems are the other way around, with the user wearing the emitters while surrounded by sensors attached to the environment.
The signals sent from emitters to sensors can take many forms, including electromagnetic signals, acoustic signals, optical signals and mechanical signals. Each technology has its own set of advantages and disadvantages.
- Electromagnetic tracking systems measure magnetic fields generated by running an electric current sequentially through three coiled wires arranged in a perpendicular orientation to one another. Each small coil becomes an electromagnet, and the system's sensors measure how its magnetic field affects the other coils. This measurement tells the system the direction and orientation of the emitter. A good electromagnetic tracking system is very responsive, with low levels of latency. One disadvantage of this system is that anything that can generate a magnetic field can interfere in the signals sent to the sensors.
- Acoustic tracking systems emit and sense ultrasonic sound waves to determine the position and orientation of a target. Most measure the time it takes for the ultrasonic sound to reach a sensor. Usually the sensors are stationary in the environment -- the user wears the ultrasonic emitters. The system calculates the position and orientation of the target based on the time it took for the sound to reach the sensors. Acoustic tracking systems have many disadvantages. Sound travels relatively slowly, so the rate of updates on a target's position is similarly slow. The environment can also adversely affect the system's efficiency because the speed of sound through air can change depending on the temperature, humidity or barometric pressure in the environment.
- Optical tracking devices use light to measure a target's position and orientation. The signal emitter in an optical device typically consists of a set of infrared LEDs. The sensors are cameras that can sense the emitted infrared light. The LEDs light up in sequential pulses. The cameras record the pulsed signals and send information to the system's processing unit. The unit can then extrapolate the data to determine the position and orientation of the target. Optical systems have a fast upload rate, meaning latency issues are minimized. The system's disadvantages are that the line of sight between a camera and an LED can be obscured, interfering with the tracking process. Ambient light or infrared radiation can also make a system less effective.
- Mechanical tracking systems rely on a physical connection between the target and a fixed reference point. A common example of a mechanical tracking system in the VR field is the BOOM display. A BOOM display is an HMD mounted on the end of a mechanical arm that has two points of articulation. The system detects the position and orientation through the arm. The update rate is very high with mechanical tracking systems, but the disadvantage is that they limit a user's range of motion.
To learn more about gear used in virtual reality systems, check out the links on the next page.