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Performance Comparison: Optical and Magnetic Head Tracking
Article · May 2013

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International Journal of IT, Engineering and Applied Sciences Research (IJIEASR) ISSN: 2319-4413

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Volume 2, No. 3, March 2013

Performance Comparison: Optical and Magnetic Head Tracking
Harjot Singh, Student, M.E (Electronics & Communication Engineering), University Institute of Engineering & Technology (UIET), Panjab University, Chandigarh, India Vinod Karar, CSIR-Central Scientific Instruments Organisation, Chandigarh Naresh Kumar, Assistant Professor (ECE), University Institute of Engineering & Technology (UIET), Panjab University, Chandigarh, India Surender Singh Saini, CSIR-Central Scientific Instruments Organisation, Chandigarh

ABSTRACT
Tracking also called Position and Orientation Tracking or Position Tracking and mapping, is used in VEs where the orientation and the position of a real physical object is required. Specifying a point in 3-D requires the transition position, that is the Cartesian coordinates x, y, and z. However, many VE applications manipulate entire objects and this requires the orientation to be specified by three angles known as pitch (elevation), roll, and yaw (azimuth). Thus, six degrees of freedom (DOF) are the minimum required to fully describe the position of an object in 3-D. Head tracking is basically related to the head movements and is used for updating the head moves. They provide accurate provide information to the flight computer about the orientation of the head of the pilot with high degree of accuracy and extremely low impact on helmet mounted display (HMD) weight, size, and packaging. This paper compares the performance of head tracking utilizing optical and magnetic tracking techniques.
Keywords:
Head tracker, field of view (FOV), Optical tracker, Electro-magnetic tracker, helmet mounted display, smoother
METHODS AND MATERIALS
The head-tracking process of determining the user’s head position, relaying this position to the sensor, the sensor’s movement to the correct line-of-sight, the sensor’s acquisition of the scene, and transmitting and presenting the final imagery on the HMD takes time [2].
One potential problem is that the time required measuring the head movements with the head tracking system and update the display creates a temporal lag that impairs perception and places constraints on the gain of the head tracking system. It is therefore important to investigate the relationships among head-tracking accuracy, lag, and size of head movement to determine an acceptable set of parameters [3].

Head tracking system tracks and calculates the head movements and updates the head moves. Head tracker system measure 3 degree of freedom, six degree of freedom measurement of position (X, Y and Z coordinates) and orientation (azimuth, elevation and roll). The head tracking system measures head movements in the range of 180° for angular azimuth, 130° for elevation, and 120° for roll, with an accuracy of about 1-2mR on bore-sight and 2-6 mR at 10° eccentricity, and linear displacements in the vertical of order of 450mm, 400mm in horizontal and 540mm in fore/aft direction[1].
The earliest head tracking displays used infrared light reflected from the viewer’s head. In a Fresnel lens single viewer display [4], the head detector moves with the lens. Illumination of each side of the viewer’s head by two wavelength bands of infrared light has also been applied [5]. A further display use a large format convex lens to illuminate the left side of the viewer’s head with IR in the 830-870 nm range , and the right side of the head in 930970 nm band. The outputs of a pair of cameras with filters are used to control the illumination from a monochrome 2-D display without the use of any additional processing. This method is used in other display by the same researchers [6-7]. Real-time position and orientation tracking of viewer is an important for 3-D displays [89]. For some display or application, only orientation or position can be tracked. This imposes many limitations bur also simplifies the task significantly. There already exist small, cheap and accurate inertial sensors which can be attached to HMDs. The accuracies of static position and orientation and dynamic movement measurements are important. Also features like sample rate, number of targets tracked, range of tracking, latency, update rate, registration, and space requirements may be important. It is highly important that the viewer is tracked and the scene gets update very fast. Many head tracking methods are available including electromechanical, electromagnetic, acoustic, inertial and optical tracking. The characteristics of head trackers as resolution, accuracy and system responsiveness [10] are given below:
Resolution: Measures the exactness with which a system can locate a reported position. It is measured in the terms

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of inch per inch of transmitter and receiver separation for position, and degrees for orientation.
Accuracy: the range with in which a reported position is correct. This is a function of the error involved in making measurements and often it is expressed in statistical error terminology as degrees root mean square (RMS) for orientation and inches RMS for position.
System responsiveness comprises: Sampling Rate-The rate at which sensors are checked for data, usually expressed as frequency.
Update Rate: The rate at which the system reports new position coordinates to the host computer, also usually given as frequency.
Data Rate: the number of computed position per second usually expressed as frequency.
Latency: It is also known as lag. The delay between the movements of the remotely sensed object and the report of new position. This is measured in milliseconds (ms). These characteristics provide some guidance for tracker performance. One of the most important is latency. Delays greater than 60 msec between head position and visual feedback impair adaptation and the illusion of presence [11]. Latencies of greater than 10 msec may contribute to simulator sickness Bryson [12] consider system with latency longer than 0.5 seconds not to be real-time interactive.
Head tracking system consists of four major assemblies: face detector, tracking mechanism, smoother and head position calculation. Face detector is used for detecting the face for all positions even if the head is tilted or turned slightly away from the camera with a fast and precise detection algorithm. The object tracker algorithm is used for object tracking and position called as camshift. The smoother calculates the current position by weighting average of the previous position and not the current one. For the head position calculation we need to know field of view of the camera. So head tracking is a special section of tracking. Since head have only one position and tracked only one position. This simplifies the task because enough focus on only one point [13]. A typical head tracking system process is given in figure 1.
Figure 1: Head Tracking System Process

OPTICAL HEAD TRACKING
Optical tracker employs inferred emitter on the helmet to measure the pilot head position. An optical head tracking system consists of three subsystems. The optical image system, the mechanical tracking platform and tracking computer.
Optical tracking imaging: It converts the light source into digital image. Depending upon the design it is very from simple standard digital camera to an astronomical telescope on the top of a mountain.
Mechanical tracking: It holds the optical imaging system and manipulating the optical imaging system in such a way it always point the target being tracked.
Tracking computer: It capturing the images from optical imaging system analysing the image to extract target position and controlling the mechanical tracking platform to follow the target. First the tracking computer has to be able to capture the image at a relatively high frame rate. This posts a requirement on the bandwidth of the image capturing hardware. The second challenge is that the image processing software has to be able to extract the target image from its background and calculate its position.
Disadvantages of optical system are sensitivity to sunlight and other heat source. MIG-29/AA-11 Archer system use optical tracking [20]. The optical head trackers require direct line-of-sight and large field of view (FOV) [1].Optical tracker have less temporal lag than magnetic tracker. The magnetic may be affected by metal object and magnetic radiation [14]. Optical head tracking system is compacter and lighter than magnetic .Optical tracker operate using remote measurement by camera of position in space of LED mounted on the helmet [16].
CHARACTERISTICS OF DIFFERENT TYPES OPTICAL HEAD TRACKER USED TILL NOW
Selcom AB, SELSPOT II: SELSPOT II is a commercial tracking system marketed by Selcom AB, a Swedish company. A camera registers light pulses from LEDs attached to the object being tracked. Located between the lens and electronics of the camera is the SELSPOT sensor, a patented Photo detector made by SiTek Laboratories and consisting of a flat semi-conductor disc. Each side of the diode has a light-sensitive coating to produce a high resolution, two-axis field. When a light pulse from one of the LED’s passes the lens system in the camera and strikes a point within this field, the electronics registers the x and y coordinates in the two axis field. Two or more cameras

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are required to analyze movements in three dimensions. Sampling rate is very high 10 KHz.
Optotrak 3020: The OPTOTRAK 3020 by Northern Digital Inc. is an infra-red (IR)-based, noncontact position and motion measurement system. Small IR LEDs (markers) attached to a subject are tracked by a number of custom designed sensors. The 3-D positions of the markers are determined in real-time or post hoc, up to 256 markers can be tracked. The position sensor consists of three 1-D charged coupled device (CCD) sensors paired with three lens cells and mounted in a 1.1m long stabilized bar. Within each of the three lens cells, light from the LED is directed onto a CCD and measured. All three measurements together determine the 3-D location of the marker, which is calculated and displayed in real time. Max. Data Rate 3500 Hz (raw), 600 Hz (real-time3D).
MacReflex Motion Measurement System: The MacReflex Motion Measurement System, by Qualisys, Inc. also is designed to measure the 3-D motion of subjects in real-time. The system is comprised: 1) One or more MacReflex position sensors (a 3-D system uses from two to seven position sensors). 2) Software to enable the user to set up and calibrate the field of view of the position sensors, and process the measured spatial coordinates of the target markers that are attached to the subject being tracked. 3) Passive reflective target markers, 4) A calibration frame for 3-D measurements, and 5) A Macintosh computer system. The position sensor has two components a CCD digital video camera, and a video processor. The camera views up to 20 markers in real-time. It then sends the video image to the video processor which determines the centroid of each marker and determines its x, y coordinates. A program converts the x, y coordinates to enable calculation of position, displacement, velocity, acceleration, angles, angular velocity, and angular acceleration. Sampling rate 50-200 Hz.
DynaSight: The Origin Instruments Corporation tracking product, DynaSight, is an electro-optical sensor with integrated signal processing that performs 3-D measurements of a passive, non-tethered target. A twocolor LED on the front of the sensor indicates the tracking status to the user. In a typical application, the sensor is mounted just above the viewable area of a real-time graphics display. The sensor’s field of view is a nominal 75 cone, and the sensor is pointed such that this field covers the comfortable range of head/eye positions for the user of the display. The sensor measures and reports on the 3-D movements of a tiny target that is referenced to the user’s forehead. The passive target itself can be mounted on eye glasses, stereoscopic goggles, or on the user’s forehead. Larger high-performance targets are

available that allow measurements at a sensor-to-target range of up to 20 feet. The Active Target Adapter enables tracking of up to four active targets tethered to the Adapter. Five DOF are achieved with two targets, while 6 DOF can be achieved by tracking three or four active targets. DynaSight is the first in a new line of 3-D measurement products. It is planned that future systems will offer 6 DOF for HMDs using passive sensors and multiple sensors for networked operations in large virtual volumes. Update Rate is 64 Hz and Latency 16-31 msec.
RK-447 Multiple Target Tracking System: The RK-447 Multiple Target Tracking System, by ISCAN, Inc., is a video tracking system which can track up to 64 facial points at 60 Hz with a latency of 16 msec. It is a real time digital image processor employing ISCAN's proprietary Simultaneous Multiple Area Recognition and Tracking (SMART) architecture. The ISCAN SMART processor computes the position and size of up to 256 areas that are within a particular range of intensity levels. Filtering the output of the SMART processor allows the complete system to specify targets of desired size, position, and intensity parameters from a field containing many potential targets. After positioning the imaging sensor to include the desired field of view, the image gray level corresponding to the target may be selected. The areas of the video image whose intensity is within the gray level threshold setting are presented on the monitor as a bright overlay, letting the operator see precisely the video information being processed. For each threshold area, size and position data are computed and stored in a data table which may be accessed by an external computer. The RK447 Multiple Target Tracking System divides the image signal into a 512 horizontal by 256 vertical picture element matrixes. As the targets’ position and size data are automatically determined over the monitor image area, the data within the azimuth and elevation coordinate table correspond to the horizontal and vertical coordinates within the video matrix. These coordinate data are updated every 16 msec and are available for input to a computer. Parametric information may be input to the RK-447 to automatically limit the data set to targets within a particular size or position range.
MAGNETIC HEAD TRACKING
Magnetic head trackers use set of coils in cockpit that produce magnetic field. Magnetic sensor (Receiver) is mounted onto the helmet which determines the strength and angle of fields [1]. The receiver senses the changes in the magnetic field caused by movement. These changes are recorded and processed by an algorithm that determines the position and orientation of the receiver in relation to the transmitter. This position and orientation data is then sent to the computer to update the virtual environment display [2]. Most military tracking systems are based on magnetic tracking and have

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dedicated processors. A small antenna behind the pilot’s head creates a multi-component electromagnetic field of well-defined shape and strength. A sensor on the pilot’s helmet measures the strength and direction of the different components of the field. The dedicated processor crunches these measurements to yield the position and orientation of the pilot’s head. Results are accurate and very nearly immediate.
Magnetic sensors used in magnetic head tracking are differ from most other detectors in that they do not directly measure the physical property of interest. Devices that monitor properties such as temperature, pressure, strain, or flow provide an output that directly reports the desired parameter (see Figure 2). Magnetic sensors, on the other hand, detect changes, or disturbances, in magnetic fields that have been created or modified, and from them derive information on properties such as direction, presence, rotation, angle, or electrical currents. The output signal of these sensors requires some signal processing for translation into the desired parameter. Although magnetic detectors are somewhat more difficult to use, they do provide accurate and reliable data without physical contact. They can measure these properties without actual contact to the medium being measured [19].

Insidetrak: it is smaller version of the fastrak sensor. Testing found that Insidetrak sensing data is much noisier than Fastrak [19]. Update rate 60 Hz divided by total no. of receiver. Latency is 12 msec with working range up to 5 feet.
Ultratrak: It is most expensive head tracker of Polhemus.Ultratrak consists of a 486-based Motion Capture Server unit which contains 4 to 8 motion capture boards (each board can support 2 receivers), a VGA controller, external synchronization board, and communications card. Ultratrak comes in a 60 Hz version and a 120 Hz version (Ultratrak 120). Both come with the Long Ranger transmitter (optional equipment for Fastrak and Insidetrak) that allows tracking and capturing a subject in an area in excess of 700 square feet. Update rate is 60 Hz up to 8 receivers and 30 Hz up to 16 receivers. Latency is 20 msec.
Flock of Birds: Flock of Birds is a 6 DOF tracking system by Ascension Technology Corporation. It is intended for tracking human motions in character animation, biomedics, and VE applications. In particular, Flock trackers are used for head tracking in flight simulators/trainers; head, hand, and body tracking in VE games; and full body tracking for character animation, performance animation, virtual walkthroughs, and sports analysis. Flock of Birds has full 360° coverage without blocking or echoing problems and a fast measurement rate-up to 144 position and orientation measurements per second. Update rate is up to 144 Hz.

Fig. 2 Conventional vs. magnetic sensing
CHARACTERISTICS OF DIFFERENT TYPES MAGNETIC HEAD TRACKER USED TILL NOW
Fastrak: The Polhemus developed the fastrak. Fastrak accept data from up to 4 receivers and up to 8 systems can be multiplexed with 32 receivers. Update rate is 120 Hz divided by total no. of receiver. Disadvantages of Fastrak are very high cost and less working range. Latency is 4 msec in Fastrak.

PC/BIRD: PC/BIRD is a new offering from Ascension Technology Corporation that uses the same patented Pulsed-DC magnetic technology employed in the other Ascension tracking products. Intended for use with PCs, this tracker is configured as an ISA-compatible board, a receiver that can be mounted on any nonmetallic object, and either a standard or extended range transmitter. With the standard range transmitter, PC/BIRD operates with a range of 4 feet; the extended range transmitter allows a range of up to 10 feet. Measurements are made at the rate of up to 144 per sec. update rate is same as flock of birds.
RESULTS
The comparative performance is given in table 1.

Isotrak II: It is lower cost Polhemus product which slightly reduced performance from fastrak. It consists of an electronic unit, a single transmitter and 1 or 2 receiver. Isotrak II with update rate 60 Hz divided by total no. of receiver. Latency is 20 msec.

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TABLE 1: COMPARATIVE PERFORMANCE OF OPTICAL
AND MAGNETIC HEAD TRACKING

REFERENCES

Optical Head Tracking Magnetic Head Tracking

It Employs infrared It Employs magnetic to create a

emitter on helmet to magnetic field; location sensor

measure the head mounted on helmet tracked

position.

through magnetic field [1].

Sensitivity to sunlight Requires precise magnetic

and other heat source; mapping of the cockpit to

Requires direct line-of- account for ferrous and

sight and large field of conductive material to reduce

view.

angular error in the

measurement [16].

Less temporal lag than Affected by metal object and

electro-magnetic tracker electro-magnetic radiation

[14].

Advantages:

High Advantages: Low cost; no drift;

resolution image of target no lighting conditions and

being tracked; High background or line of sight

availability; Can work constraints; Both wireless and

over large area; High wired models, real time

accuracy; No magnetic operations [17].

interference problem.

Disadvantages: High Disadvantages: High latencies

cost; Visible wavelengths due to filtering; Electromagnetic

are less optimal [15].

interference from radio;

Accuracy diminishes with

distance; Ferromagnetic/ metal

conductive surface cause field

distortion [18].

CONCLUSION
Head tracking system are basically used in helmetmounted displays for combat aircraft. The optical and magnetic head trackers have relative advantages and disadvantages. Optical head trackers needs direct line of sight and larger FOV as compared to magnetic tracker and hence magnetic head trackers are more suited for the military application. Magnetic tracking technology is relatively mature, has been militarized, and offer s best overall head tracking performance available at this time. It is likely to be predominant head tracking technique for the next generation of military head coupled system. In this given the characteristics performance of different type of trackers. The optical tracker are described in this paper are SELSPOT II, Optical 3020, Mac Reflex Motion, DynaSight and RK-447 multiple target tracking system and Magnetic tracker are PC/BIRD, Space Pad, flock of birds, Ultratrak, Insidertrak, Isotrak II and fastrak. Different types of tracker have their own advantages and disadvantages. At the end we give the comparison of optical and magnetic head tracking.

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296. Bibcode 1963SSRv....2...250V. doi:10.1007/B F00216781. http://en.wikipedia.org/wiki/Optical_motion_tracking [16] Air Power Australia. "Helmet Mounted Sights and Displays". Ausairpower.net. http://www.ausairpower.net/hmdtechnology.html. Retrieved 2010-08-20. [17] http://www.cadengineering.co.in/home6/products /3d-motion-trackers--capture-systems/electroma gnetic-3d-motion-trackers [18]http://www.hitl.washington.edu/scivw/scivwftp/publications/IDA-pdf/TRACK.PDF [19] J.E. Lenz, “A Review of Magnetic Sensors”, ProCeedings of the IEEE, vol. 78, no.6, (June 1990) 973989 [20] Garrett “Reduction of Latency in VE Applications”

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