Positioning systems will use positioning technology to determine the position and orientation of an object or person in a room, building or in the world.
Time of flight
Time of flight systems determine the distance by measuring the time of propagation of pulsed signals between a transmitter and receiver. When distances of at least three locations are known, a fourth position can be determined using trilateration.
Optical trackers, such as laser ranging trackers suffer from line of sight problems and their performance is adversely affected by ambient light and infrared radiation. On the other hand they do not suffer from distortion effects in the presence of metals and can have high update rates because of the speed of light.
Ultrasonic trackers have a more limited range because of the loss of energy with the distance traveled. Also they are sensitive to ultrasonic ambient noise and have a low update rate. But the main advantage is that they do not need line of sight.
A spatial scan system uses (optical) beacons and sensors. Two categories can be distinguished:
- Inside out systems where the beacon is placed at a fixed position in the environment and the sensor is on the object
- Outside in systems where the beacons are on the target and the sensors are at a fixed position in the environment
By aiming the sensor at the beacon the angle between them can be measured. With triangulation the position of the object can be determined.
The main advantage of an inertial sensing is that it does not require an external reference. Instead it measures rotation with a gyroscope or position with an accelerometer with respect to a known starting position and orientation. Because these systems measure relative positions instead of absolute positions they can suffer from accumulated errors and therefore are subject to drift. A periodic re-calibration of the system will provide more accuracy.
This type of tracking system uses mechanical linkages between the reference and the target. Two types of linkages have been used. One is an assembly of mechanical parts that can each rotate, providing the user with multiple rotation capabilities. The orientation of the linkages is computed from the various linkage angles measured with incremental encoders or potentiometers. Other types of mechanical linkages are wires that are rolled in coils. A spring system ensures that the wires are tensed in order to measure the distance accurately. The degrees of freedom sensed by mechanical linkage trackers are dependent upon the constitution of the tracker's mechanical structure. While six degrees of freedom are most often provided, typically only a limited range of motions is possible because of the kinematics of the joints and the length of each link. Also, the weight and the deformation of the structure increase with the distance of the target from the reference and impose a limit on the working volume.
Phase difference systems measure the shift in phase of an incoming signal from an emitter on a moving target compared to the phase of an incoming signal from a reference emitter. With this the relative motion of the emitter with respect to the receiver can be calculated Like inertial sensing systems, phase-difference systems can suffer from accumulated errors end therefore are subject to drift, but because the phase can be measured continuously they are able to generate high data rates.
Direct field sensing
Direct field sensing systems use a known field to derive orientation or position: A simple compass uses the Earth's magnetic field to know its orientation in two directions. An inclinometer uses the earth gravitational field to know its orientation in the remaining third direction. The field used for positioning does not need to originate from nature, however. A system of three electromagnets placed perpendicular to each other can define a spatial reference. On the receiver, three sensors measure the components of the field’s flux received as a consequence of magnetic coupling. Based on these measures, the system determines the position and orientation of the receiver with respect to the emitters' reference.
Because every technology has its pros and cons, most systems use more than one technology. A system based on relative position changes like the inertial system needs periodic calibration against a system with absolute position measurement. Systems combining two or more technologies are called hybrid positioning systems.
- Position trackers for Head Mounted Display systems: A survey, Devesh Kumar Bhatnagar, 29th of March, 1993
- A SURVEY OF TRACKING TECHNOLOGY FOR VIRTUAL ENVIRONMENTS, Jannick P. Rolland, Yohan Baillot, and Alexei A. Goon, Center for Research and Education in Optics and Lasers (CREOL), University of Central Florida, Orlando FL 32816