A Survey of Position Trackers

Kenneth Meyer A of Position Trackers and Survey Hugh L. Applewhite Piltdown Inc. 4470 SW Hall Blvd # 154. Beaverton, Oregon 97005 Abstract Frank A. Biocca University of North Carolina This paper is a survey of position-tracking technologies and their use in virtual reality Center for Research in (VR) applications. A framework is established to evaluate the suitability of a position- Journalism and Mass tracking implementation for virtual reality use. Mechanical, optical, magnetic, and acous- Communications tic implementations are discussed with examples of each. Also, the effect of position tracking on a virtual reality user is discussed, especially with regard to the position tracker's role as a cause of simulation sickness. A catalog of implementations and uses is included in an appendix. I Introduction Effective three-dimensional (3-D) position tracking is vital to virtual reality (VR) technology. To date, position trackers have been primarily designed and built for military applications. Virtual reality researchers have used these off-the-shelf trackers with mixed results. In the following, we survey the basic technologies used in existing position trackers. The principles of operation for each technology are discussed. A catalog of implementations and uses is included as an appendix. The earliest form of position tracking was the Plane Survey practised by the ancient Egyptians. Egyptian engineers used transits and plumb-lines to reestab- lish boundary markers washed away by the annual flooding of the Nile. Millen- nia later, European explorers used sextants and compasses to circumnavigate the globe. In recent decades, position tracking has been used by robotic engineers to enable intelligent machines to measure location. Most recently, position trackers have been used to control computer-generated images in virtual reality applications. In VR applications, the user interacts with the system through bodily movements; by moving a hand or the head, the user controls a computer-generated world. Since user movement is detected by the position tracker, position trackers are an integral part of a virtual reality system. In the mid-1960s, Sutherland first used a position tracker to dynamically de- termine the view needed for a 3-D computer-generated image (Sutherland, 1968). His initial system was mechanical, but later switched to an acoustic system. Since Sutherland's experiments, position tracking has been implemented using four different approaches: mechanical, optical, magnetic, and acoustic. Each technology offers inherent advantages and disadvantages that make it more or less suitable for VR use. None of the current implementations pro- vides ultimate performance. In the next section we will describe a framework Presence, Volume i. Number z Spring 1992 for evaluating position trackers for VR applications. o 1992 The Massachusetts institute of Technology Before embarking on the survey, we should describe this survey's scope. Our Meyer et al, 173 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/pres.1992.1.2.173 by guest on 27 September 2021 174 PRESENCE: VOLUME I, NUMBER 2 principal goal is to provide an overview of the field to 2.2 Responsiveness: Sample Rate, Data those who are using or integrating VR systems. The sur- Rate, Update Rate, and Lag vey does not include in-depth technical details of the Sample rate, data rate, update rate, and lag de- position-tracking systems described. We hope the result scribe system responsiveness. These terms have been will be informative for a technically sophisticated reader used to convey a variety of in the reference without inaccessible to a less technical reader. meanings being materials. We use these terms as follows: Also, since our goal is to survey position tracking as it The sample rate is the rate at which sensors are checked regards image control in VR applications, we will not for data. In most implementations, the sample rate will discuss any inertial implementations. We do not know of significandy exceed the data rate in order not to miss a any inertial system that has been used to date for control new sensed measurement. However, a of Likewise, we will not high sampling computer-generated graphics. rate of sensors without new measurements is cover technolo- meaning- eye-tracking technologies. Eye-tracking less. gies measure the direction the eyes are pointed out of the The data rate is the number of computed positions user's head by detecting movement of the fovea. Eye per second. (Some reference materials refer to data rate trackers do not directly measure head position or orien- as In most a data rate is as- tation. sample time.) systems, high sociated with low lag and high tolerance to environmen- tally induced errors. Consequently, high rates are desir- able. The maximum data rate achievable is a 2 Framework for Suitability characteristic of the implementation technology (Apple- white, 1991a). The suitability of a position tracker for a VR appli- Update rate is the rate at which the system reports new cation can be evaluated by a few key measures of perfor- coordinates to the host The usable mance. These measures are not intended to be applied as position computer. rate is not bound rates since some sys- dualistic criteria; rather, each is meant to suggest a con- update by display tems use intermediate for tinuum of quality. The need for quantitative suitability updates predictive modeling & Note that do measures is discussed in the last section of this survey. (Wang, Koved, Dukach, 1990). updates not reflect actual The of Position-tracking can be evaluated according to the always position. computation often extensive to offset the following key measures: (I) resolution and accuracy, (2) position requires filtering erroneous interferes responsiveness, (3) robustness, (4) registration, and (5) socia- inevitable data. This computation bility. with real time updates as time is required for both the data collection and filtering. Lag is the most meaningful of all the measures of re- 2.1 Resolution and Accuracy sponsiveness. It is the delay between the movement of the sensed and the of the new Resolution and accuracy measure the exactness remotely object report In as the between with which a system can locate a reported position. position. practice, lag appears delay the movement of the remote and the Resolution is the smallest the system can detect. object correspond- change received the host While low Changes in position and orientation that are smaller ing change by computer. lag on data and rates, neither than the system's resolution will not be detected. depends high update guaran- tees low 1991b). of the reference Accuracy is the range within which a reported position lag (Applewhite, Many materials refer to this measure as is correct. Accuracy can also be understood as a range of latency. error. For example, for a system with a rated accuracy of 0.1 inch, a reported position is within ±0.1 inch of the 2.3 Robustness actual position. Position trackers must accommodate the uncer- tainty and noise of the real world. Each type of position- Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/pres.1992.1.2.173 by guest on 27 September 2021 Meyer et al. 175 tracking technology is subject to different types of errors be that of two VR users shaking hands simultaneously in that make it more or less suitable for use in virtual reality both the physical and virtual worlds. applications. Robustness is a gauge of resistance to these The effect of misregistration is discussed in the "Ef- errors (Applewhite, 1991a). fects of Inaccurate Position Tracking" section of this survey. 2.4 Registration 2.5 Sociability: Range of Operation and is the between actual Registration correspondence Fitness for Tracking Multiple Objects position and orientation and reported position and orientation. A system with good registration accurately Sociability is contingent on a system's range of op- for users maps the remote object's movement throughout the eration and its fitness tracking multiple in the same work working volume. Good resolution and accuracy alone do space. is the volume in which a not ensure good registration, since poor registration can Range ofoperation position tracker this measure is also result from accumulated errors that cause the reported accurately reports position; referred to as volume. A of position to drift from the physical position. working system's range operation be bound intrinsic limitations such as a me- In some cases misregistration may not be significant; may by chanical or The of a this is the case with a conventional 2-D mouse. Mice linkage signal strength. ability system to monitor a for frequently lose their registration, yet users are able to sufficiently large space multiple users can be crucial in some mentally readjust the position and operate the mouse applications. Consequently, systems with volumes are considered to without further complication. Likewise, in some VR large working have better sociability. applications, a user of a six degrees-of-freedom (DOF) Thefitnessfor tracking multiple objects is determined position tracker can mentally adjust the translational po- both a system's architecture and its to col- sition as as the user does not encounter a by immunity long physical lateral effects. barrier such as a wall. System architectures take a variety of forms. In some However, unlike a 2-D mouse, a 6 DOF position systems the emitter is attached to the remote object; in tracker must report orientation. Registration must be other systems the sensor is attached to the remote object. because the tracker reports the orientation (as precise, In some systems a single emitter can support multiple well as the of the user's head. Errors in orienta- position) sensors; in others, multiple emitters are required for tion to be We the appear especially important.

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