Ultrasonic Calibration of a Magnetic Tracker
in a Virtual Reality Space
Morteza Ghazisaedy
David Adamczyk
Daniel J. Sandin
Robert V. Kenyon
Thomas A. DeFanti
Electronic Visualization Laboratory (EVL) Department of Electrical Engineering and Computer Science
and School of Art and Design
The University of Illinois at Chicago
851 S. Morgan, Room 1120 SEO (M/C 154)
Chicago, IL 60607-7053
(312)996-3002
Email: sandin@eecs.uic.edu
ABSTRACT
This paper describes a system for calibrating the position component of a 6-degree-of-freedom magnetic tracker by comparing the output with a custom-built ultrasonic measuring system. A look-up table, created from the collected difference data, is ud to interpolate for corrected values. The error of the resulting corrected magnetic tracker position is measured to be less than 5% over the calibrated range.
Keywords:Virtual reality, CAVE, magnetic tracker, ultrasonic tracker
I. INTRODUCTION
A. Purpo and Motivation
A goal of any virtual reality (VR) system is to make ur control of the environment as natural as possible. Accurate tracking is needed for VR systems to generate correctly sized and oriented perspective views, to allow ur picking of objects, and to facilitate navigation.B. Background
There are 3 major types of tracking devices that detect both position and angular orientation.
1 - Mechanical Linkages. Mechanical linkage systems u an arm-like structure compod of veral joints with one end fixed and the other end free to move with the ur. The devices measure the position and angular orientation of the free end by measuring the angles at each joint of the structure, factoring in the length of each gment. The BOOM by Fake Space Labs us such a linkage tup well.感谢有你作文
Advantages include low latency and the potential of high positional accuracy. Disadvantages derive from the limited extent of movement determined by the total length of the arm, and the inertia of the structure (especially with a BOOM monitor attached) [3]. In addition, using a cond mechanical linkage system to capture the ur's hand information is highly tangle-prone.
2 - Ultrasonic Systems. Ultrasonic systems have two major components, a transmitter generating an ultrasound signal and a receiver detecting the signal. The distance is calculated by measuring the ti
me-of-flight of the ultrasonic pul. Three transmitters and receivers are needed to calculate a full 3D position and orientation [2]. A major
disadvantage is that an unobscured path from the transmitter to the receiver needs to be maintained. Two systems that u ultrasonic tracking are the Power Glove manufactured by Mattel and the 3D Mou by Logitech [3].
3 - Orthogonal Electromagnetic Field. Orthogonal field systems u magnetic fields to determine position and orientation. A transmitter generates electromagnetic signals which are received by a nsor. The strength of the electromagnetic signals are ud to determine the absolute position and orientation of the receiver relative to the transmitter. The advantage is that this type of tracker allows arbitrary movement in a relatively large (8 ft. radius) space. On the other hand, such trackers exhibit substantial delay and incread inaccuracy with distance from the transmitter. Two well known versions are the Polhemus 3-Space and the Ascension Flock of Birds.
表示情绪的词语
II. CAVE VR SYSTEM
A. CAVE Overview
The CAVE is a Virtual Reality system developed at the Electronic Visualization Laboratory (EVL) at the University of Illinois at Chicago. The current CAVE is ten feet on a side [4] where two or three walls are rear-projected and the floor is projected down from above. The Ascension Extended Range Transmitter Flock of Birds (ERTFOB) magnetic tracker is ud to measure the position of the ur's head and a hand-held position device called the "wand."
B. Tracking System
The electromagnetic transmitter has 3 orthogonal coils which are puld in quence. The receiver also contains 3 orthogonal coils which measure the components of the electromagnetic signal. The strength of the 3 components of the received pul are compared to the strength of the transmitted pul to determine the position. The strength of the 3 received signals are compared to each other to determine the orientation (thus the receiver coil most parallel to the transmitter coil will give the highest value and the one most orthogonal will give the lowest). For each position, the transmitter nds three puls, one for each of its coils. The three receiver coils each get 3 puls, for a total of 9 signals. The range is claimed to be up to an 8 ft. radius from the transmitter. Unfortunately, the accuracy of the system decreas markedly as distance from the nsor to the transmitter increas [4].
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Metal structures near the tracker distort the magnetic field, so the CAVE screen frame is made of austenetic stainless steel which is non-magnetic and has a low conductivity. However, other components needed for the CAVE to function such as projectors and mirrors
1 foot
Figure 1. ERTFOB errors in one plane of the CAVE, viewed from the left side. Transmitter is one foot in front of this plane.
III. CALIBRATION METHOD
A. Overview
The goal of the calibration system is to correct for ERTFOB static position errors. For each position reported by the magnetic tracker, the physical position of the nsor is measured using our more accurate ultrasonic measuring device (UMD). A table is built containing positions of the magnetic nsor reported by the ERTFOB and their corresponding positions reported by the UMD. Using this
table, any point within the range can be corrected by interpolation between the corrected points in the calibrated area. The table must be rebuilt whenever the tracking system or the CAVE is moved.
Figure 1 shows the position errors for a plane placed one foot right of center, vertically oriented and perpendicular to the front wall of the CAVE. The positions as measured by the UMD are shown by the x's at the end of the lines who other ends are the positions as measured by the ERTFOB. Figures 2 and 3 show in 3D all the planes at once. Each figure has two stereograms: the left pair for cross-eyed viewing and the right pair for wall-eyed viewing.
鲁迅的故事
B. The Ultrasonic Measuring Device
The UMD generates an ultrasonic sound signal using a transducer and nds it toward an object. The sound reflected from the object, or echo, is also detected by the transducer. Distance is obtained by measuring the time interval between the moment the sound is transmitted and the echo is received. The elapd time between the transmission and echo signal is a linear function of the distance [5].
小恐龙简笔画
To measure position in all 3 dimensions, 4 Polaroid ultrasonic transducers are ud, one to measure the distance to each wall and the floor of the CAVE. The distance to the left and right walls is measured by two transducers and gives the X coordinate, the distance to the floor gives the Y coordinate, and the distance to the front wall gives the Z coordinate. Two transducers are ud redu
ndantly for the X coordinate to detect yaw error by checking that the sum of the two distances (left and right) are equal to the distance across the CAVE (10 ft.). If the sum is the greater than 10 ft., the left and right transducers are not perpendicular to
Right Eye
Left Eye Right Eye
Figure 2. Stereogram of ERTFOB errors viewed from back of CAVE.
Right Eye Left Eye Right Eye Figure 3. Stereogram of ERTFOB errors viewed from left side of CAVE.
the target [8].
贵重的反义词To calibrate the UMD, we u an optical bench and a target approximately 4 feet square. Using a velocity value of 1The speed of sound at 0° C is 331m/s. As the temperature increas so does the speed of sound. The relationship between temperature and speed of sound is given by: V = 20.034 273 + t where V is the speed of sound in meters/cond at a temperature t in centigrade [9]. Since the
temperature at EVL was 27º C we ud the value 347m/s for the speed of sound. If more accuracy is desired, one can measure the temperature and adjust the measured distances accordingly [11].
花菜烧.
Ultrasonic Measuring Device
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The advantages of this ultrasonic measurement system are good linearity over large distances, in
nsitivity to magnetic fields, more accuracy than the magnetic tracker, and its relatively low cost. Disadvantages include the lack of angular orientation information and the need to keep the transducers parallel to the walls and level (which prevents its u as a primary tracking mechanism, but works well for calibration purpos). In addition, the signal path must not be physically blocked.
Figure 6 shows a block diagram of the entire system [5]. The transducers are controlled by a circuit which us the Texas Instruments ultrasonic ranging module TI2728 [10]. This circuit, controlled by the PC, nds puls to each ultrasonic transducer in quence to eliminate fal echoes from the other transducers. It measures the elapd time between transmitted and received puls, and then transfers this data to the PC. The PC nds the data to the Onyx CAVE computer through an RS-232 line. The UMD circuitry itlf does not affect the magnetic field, becau it is located well outside the tracker range and the small UMD box upon which the ERTFOB sits is plastic and contains only the ultrasonic transducers. Empirical obrvations bear this out as well.
C. Calibration Procedure
The CAVE is first filled by a 3D stereo graphic image of 1-inch boxes on 1-foot intervals (Figure 8). A 1-inch cursor shows the position of the magnetic nsor which is placed atop the ultrasonic transduc
er housing. A person wearing 3D glass holds the UMD reasonably straight and moves it until the displayed cursor is inside of each box. The program records the position given by the ERTFOB and the Onyx nds a signal to the PC to get the position measured by the UMD. This procedure continues until all the boxes in the tracker range inside the CAVE are thus sampled. In practice less than 400 points are collected, esntially all points in the center of the CAVE. The collected points are not exactly at one foot intervals as measured by the ERTFOB, but lie somewhere inside the 1 inch box at that point, since trying to get the cursor on the exact point is nearly impossible. As we show below, this error is largely taken into account.
Figure 8: Display in the CAVE of boxes to collect.
Cursor (crosshairs) is moved inside boxes to collect point.
