Spatial Tracking Basics

Transcription

1 COMS W4172 3D Tracking Technologies (Non-Optical) Steven Feiner Department of Computer Science Columbia University New York, NY April 24, Spatial Tracking Basics Usually a sensor (receiver) determines position/orientation relative to a signal it detects from a source (transmitter) Source or sensor is fixed in a known position/orientation Technologies: Precise: electromagnetic, ultrasonic, mechanical, optical/vision-based*, earth-relative, inertial, GNSS, hybrid Coarse: radio, RFID, IR T T R R T = Transmitter R = Receiver * Covered earlier 2

2 Electromagnetic (EM) Tracking Three orthogonal source coils emit electromagnetic signals, either in sequence (AC or DC) or simultaneously (AC at three different frequencies) Three orthogonal sensor coils simultaneously measure the signals from each of the three source coils Nine measurements provide position and orientation To host computer Can be wireless V. Kindratenko, A survey of electromagnetic tracker calibration techinques, Virtual Reality, 5(3), 2000 Sensor Polhemus [ Ascension [ Sixense [ Source (typically stationary) 3 EM Tracking Advantages and Disadvantages does not need visual line-of-sight from source to sensor relatively unencumbering can have relatively large errors, particularly dynamic short usable range (15' for longrange xmtr) sensitive to electromagnetic radiation and metal in environment Polhemus, Polhemus 1.8mm diameter micro sensors: alone and mounted on fingers Sixense STEM, 4

3 Ultrasonic Tracking An emitter emits an ultrasonic burst, and three non-collinear receivers pick up the burst The time between emission and reception by each receiver gives emitter receiver distance Can reconstruct 3D position of emitter if positions of receivers are known Three non-collinear emitters allow full 3D orientation and 3D position Sensor (receiver) measurement Source (emitter) 5 Ultrasonic Tracking Advantages and Disadvantages low cost easy to build requires clear audio line-of-sight from emitters to receivers potentially noisy, high dynamic distortion very short range must account for temperature, control air turbulence Simple designs are susceptible to echos and environmental noise (e.g., jangling keys) Logitech Red Baron Feiner, MacIntyre, & Seligmann, CACM 93 6

4 Mechanical Linkage Tracking Typically a jointed structure that measures the changing angle at each joint between rigid arms Using known measured angles and unchanging lengths of arms, compute relative positions/orientations of end trans(l1) rot(j2) trans(l2) trans(l3) Another approach: Sutherland s Sword of Damocles rot(j1) Base rot(j3) Universal joints at the top and bottom of a telescoping arm 7 Mechanical Linkage Tracking Advantages and Disadvantages very high quality tracking with low dynamic distortion can be integrated with haptic feedback can support heavy displays mature off-the-shelf technology restricted motion Animazoo Gypsy 7 Geomagic Touch haptic device (3DOF force exerted on stylus) 8

5 Sourceless Tracking 9 Earth-Relative Tracking magnetometer determines yaw from earth s magnetic field accelerometers determine roll and pitch from earth s gravitational field E.g., Precision Navigation [ 10

6 Earth-Relative Tracking Advantages and Disadvantages no source needed (besides the earth!) lightweight inexpensive susceptible to magnetic field anomalies (large metal objects) and changes over time [older systems often had angle limits caused by gimbals for accelerometer and/or magnetometer; current systems are strapdown, without gimbals] 11 IMU (Inertial Measurement Unit): Often includes magnetometers in addition to accelerometers and gyros! Inertial Tracking Linear accelerometers Angular rate gyroscopes unlimited range measure gravitation currently difficult/expensive to make accurate systems error accumulates over time when computing position or orientation Combines 3-axis gyro, accelerometer, compass mm Inertial tracker in Google Glass 12

7 Inertial Tracking High-end example Tokimec TISS fiberoptic gyros (FOG) 3 accelerometers Heading accuracy: 1 /hr Attitude accuracy: ±0.5 Update rate: 250Hz Latency: 2ms K. Sawada, M. Okihara, and S. Nakamura, A Wearable Attitude Measurement System Using a Fiberoptic Gyroscope, Presence, 11(2), April 2002, Global Navigation Satellite System (GNSS) Global constellations of satellites: US (GPS), Russian (GLONASS), European (Galileo); Chinese (BeiDou [was COMPASS], currently regional, will be global ~2020) Radio receiver(s) determine distance from multiple satellites ~ 10m accuracy May 1, 2000: Clinton turns off Selective Availability (government-speak for approach that modified broadcast satellite position data to degrade public position tracking capabilities for security purposes). Establishes long-time viability for commercial use of satellite tracking. Trimble [ Qualcomm SiRF [ /sirfstar-v-5e] 14

8 Global Navigation Satellite System (GNSS) Differential GNSS uses GNSS receiver(s) at precisely known location(s) [stationary base stations] to compute correction signal sent by radio to mobile users [moving rovers] with GNSS receivers and separate corrections receivers ~ meter submeter accuracy Ground-Based Augmentation Systems (GBAS) Terrestrial broadcast of correction signal Satellite-Based Augmentation Systems (SBAS) Satellite broadcast of correction signal: US (WAAS, Wide Area Augmentation System), European (EGNOS, European Geostationary Navigation Overlay Service), 15 Global Navigation Satellite System (GNSS) RTK (Real-Time Kinematic) carrier-phase measurement GNSS ~ cm accuracy Improves on differential GNSS by comparing phases of carrier signals at base station with phases of carrier signals at rover New chips will support submeter accuracy in upcoming phones Use additional signals broadcast by newer GNSS satellites E.g., Broadcomm BCM Swift Navigation Piksi Multi Piksi Multi [ u-blox NEO-8MP [ Columbia U CAVIAR system w. RTK GNSS 16

9 GNSS Advantages and Disadvantages Very long range RTK GNSS coming down in price (once >> $10K, now << $1K) Courtesy of demand for use in drones!!! must be within line-of-sight of satellites (not indoors or in urban canyons ) But, sensitivity of new receivers is increasing! relatively poor altitude tracking governments own the satellites (encryption, selective availability, politics in general) relatively low update rate but new receivers can support 20Hz Naval Research Lab BARS 007/ _24 (See an earlier version at / 012_Springer_BARShistory.pdf 17 Hybrid Trackers Combine different technologies Benefit from complementary advantages Examples Inertial tracker (fast, but drifts) and magnetometer ( slower, +ground truth, +sourceless 3DoF orientation): InterSense InertiaCube ultrasonic tracker ( slower, +ground truth 6DoF position and orientation): InterSense IS-600, IS-900 optical tracker ( slower, +ground truth 6DoF position and orientation): InterSense IS-1500 Orientation tracker, GPS, and dead reckoning: Honeywell DRM 4000L GPS, magnetometers, gyros, accelerometers, barometric altimeter InertiaCube IS-600 IS Honeywell Dead Reckoning Module DRM 4000L 18

10 Hybrid Trackers Examples Wii Remote 3-axis accelerometer Analog Devices ADXL 330 Infrared optical tracker Tracks 2D position of 4 sources PixArt CMOS sensor Wii MotionPlus 3-axis gyroscope add-on (when gyros became cheap enough) Wii Remote Plus Wii Remote with built-in gyros Wii Sensor Bar Not a sensor at all, but just a pair of IR LEDs at a known offset from each other, enabling Wii Remote camera to determine distance and roll Wii Remote Plus 19 Hybrid Trackers Examples Sony PlayStation Move Camera tracks RGB LEDilluminated sphere Color of LEDs can be set based on background 3DOF position determined from 2D position + size of projected sphere in image 3-axis accelerometer, 3- axis gyro, magnetometer 3DOF orientation, with ground truth PlayStation Camera PlayStation Move 20

11 Hybrid Trackers Examples (optical + IMU) Oculus Rift Constellation Outside-in Stationary cameras view sets of time-multiplexed IR diodes on HWD and controllers HWD and controllers have integrated IMUs sampled at high frequency Sensors Sources Similar to the optical motion capture systems discussed earlier, but use IMUs to interpolate sensor readings Only a subset of sources are shown 21 Hybrid Trackers Examples (optical + IMU) HTC Vive Lighthouse Inside-out Valve Lighthouse precedents include B. Sorenson et al., The Minnesota Scanner, IEEE Trans. on Robotics, 5(4), 1989, and Nikon Metrology igps (originally ArcSecond, later Metris; e.g., see IR diode cluster flashes at start of each cycle Lighthouse: Time between cluster flash and light strike from spinning horizontal IR line laser establishes azimuth spinning vertical IR line laser establishes altitude Stationary base stations (IR diode cluster and two spinning IR line lasers) illuminate sets of diode sensors on HWD and controllers HWD and controllers have integrated IMUs sampled at high frequency Sources Sensors Lighthouse: Spinning horizontal/vertical lasers surround diode cluster Only a subset of sensors are shown 22