Input Devices: Trackers, Navigation and Gesture Interfaces
Input Devices What is Virtual Reality? “A high-end user interface that involves real-time simulation and interaction through multiple sensorial channels.” (vision, sound, touch, smell, taste)
Input Devices Virtual objects have 6 degrees of freedom (D.O.Fs): - three translations; - three rotations. 3-D System of coordinates of a VR object
� Passive input: events triggered by the system’s monitoring of the user � Active input: events are triggered specifically by the user – Trackers – Navigation-object manipulation devices – Gesture recognition – Classical input devices – Special input devices & platforms – Speech, biosignals
Input Devices Trackers measure the motion of “objects” such as user’s wrist or his head vs. a fixed system of coordinates. Technologies to perform this task: � Magnetic trackers (prevalent); � Ultrasonic trackers (less used); � Mechanical trackers (special cases); � Inertial/ultrasonic trackers (new). � Optical trackers
Why use tracking � Render the correct view or sound � Manipulate objects � Control avatars � Give symbolic commands through an alphabet of predefined movements
Input Devices
Input Devices Tracker characteristics: Tracker characteristics: � Measurement rate – Readings/sec; � Sensing latency; � Sensor noise and drift; � Measurement accuracy (errors); � Measurement repeatability; � Tethered or wireless; � Work envelope; � Sensing degradation .
Input Devices Tracker characteristics: accuracy, resolution Tracker characteristics: accuracy, resolution Accuracy Resolution Real object position Tracker position measurements
Input Devices Tracker characteristics: Tracker characteristics: jitter jitter Tracker data Real object fixed position Signal noise Time
Input Devices Tracker characteristics: Tracker characteristics: drift drift Tracker data Sensor drift Real object fixed position Time
Input Devices Tracker characteristics: Tracker characteristics: latency latency Tracker data Real object position Sensor latency Considerable latency can lead to simulation sickness Time The total (system) latency is also of importance
Input Devices Tracker characteristics: Tracker characteristics: Tracker Update Rate
Input Devices Mechanical Trackers Mechanical Trackers Definition: A mechanical tracker consists of a serial or parallel kinematic structure composed of links interconnected by sensorized joints.
Input Devices Mechanical Trackers Mechanical Trackers � Use sensors embedded in exoskeletons to measure position; � Forward kinematics � Have extremely low latencies and low jitter; � Are immune to interference from magnetic fields; � No problems with occlusions. � But limit the user’s freedom of motion; � Motion interference from the tracker arms. � Limited work envelope. � Can be heavy if worn on the body
Input Devices
Input Devices Exoskeleton Interface structure With computer Angles measured by conductive potentiomet ers
Input Devices Magnetic Trackers Magnetic Trackers Definition: A magnetic tracker is a non-contact position measurement device that uses a magnetic field produced by a stationary TRANSMITTER to determine the real-time position of a moving RECEIVER element
Input Devices Magnetic Trackers Magnetic Trackers � Use low-frequency AC or DC magnetic fields to measure position; � Fields are produced by a fixed source; � Three orthogonal coils/antennas excited sequentially � Size of source grows with the tracker work envelope; � The receiver is attached to the tracked object and has three perpendicular antennas; � Distance/orientation is inferred from the voltages induced in the antennas � Nine voltages � needs calibration…
Input Devices Polhemus Isotrack AC Magnetic tracker with Data Glove
Input Devices Electronic interface 120meas/sec 30meas/sec for 4 receivers Source due to multiplexing Stylus Receiver Fastrack magnetic tracker system
Input Devices Source Long Ranger source for the tracker system
� AC trackers induce eddy currents to other metal objects which in turn produce small magnetic fields thus distorting the overall magnetic field.
Magnitude of Error Vector / Moving Tripod 35.0 Magnitude of Error Vector (inches) 30.0 25.0 Err (54) Err (60) 20.0 Err (66) 15.0 Err (72) Err (80) 10.0 5.0 0.0 0 10 20 30 40 50 60 70 80 Transmitter-Receiver Distance (inches) Tracking error as a function of distance from the floor and transmitter-receiver distance
Polhemus Long Ranger Input Devices
DC magnetic trackers � Emitter sends three orthogonal DC magnetic pulses using three antennas, one at a time � Time delay between excitation of source and sampling of receivers (eddy currents fade out) � Earth’s magnetic field is subtracted. � Calibration is needed. � Ascension: Flock of birds – 144 measurements/sec regardless of the number of receivers (parallel sampling of each receiver).
DC Magnetic Tracker Block Diagram Input Devices
Input Devices Motion Star wireless tracker (courtesy of Ascension Technology)
Input Devices Sensors: 20/suit 100 updates/sec 3 meters range from base unit Electronic unit Resolution<2 mm (2 hours battery life) and <.2 degrees Wireless suit (Ascension Technology)
Input Devices Magnetic tracker accuracy degradation
Input Devices Magnetic Tracker Errors Magnetic Tracker Errors � due to ambient noise: e ambient = K n (d transmitter-receiver ) 4 � due to metal: K r (d transmitter-receiver ) 4 e metal = --------------------------------- (d transmitter-metal ) 3 x (d metal-receiver ) 3
Input Devices Comparison of AC and DC magnetic trackers Comparison of AC and DC magnetic trackers � DC trackers are immune to non-ferromagnetic metals (brass, aluminum and stainless steel) � Both DC and AC trackers are affected by the presence of Ferromagnetic metals (mild steel and ferrite). � Both are affected by copper; � AC trackers have better resolution and accuracy. � AC trackers have slightly shorter range
Input Devices
Input Devices Ultrasonic Trackers Ultrasonic Trackers Definition: A non-contact position measurement device that uses an ultrasonic signal produced by a stationary transmitter to determine the real-time position/orientation of a moving receiver.
Input Devices Ultrasonic Trackers Ultrasonic Trackers � Use low-frequency ultrasound to measure position; � Sound produced by a fixed triangular source (speakers); � Each speaker is activated in cycle. � The receiver is triangular and attached to the tracked object and has three microphones;
Input Devices Ultrasonic Trackers Ultrasonic Trackers � Distance is inferred from the sound time of flight; � 9 distances are measured � Sensitive to air temperature and other noise sources; � Requires “direct line of sight”; � Up to four receivers can be tracked using time multiplexing at the cost of reduced sampling rate. � Slower than magnetic trackers (max 50 updates/sec). � Should wait for echoes to die out � Cheaper than magnetic trackers
Ultrasonic tracker (Logitech) Input Devices
� Typical operation range: 1.5m � For larger work envelopes multiple transmitters with overlapping tracking volumes can be used. – One transmitter is operational at each time instance
Input Devices 3D mouse Large-volume ultrasonic tracker (Logitech)
Input Devices Optical Trackers Optical Trackers Definition: A non-contact position measurement device that uses optical sensing to determine the real-time position/orientation of an object
Input Devices Optical trackers: a) outside-looking-in; b) inside-looking-out
Input Devices Inside- -out optical tracker advantages out optical tracker advantages Inside � Very large tracking surface and resistance to visual occlusions (line of sight).
Input Devices Inside-looking-out LaserBIRD optical tracker
Input Devices Inside-looking-out LaserBIRD optical tracker
HiBall 3000 wide area tracker 3000 wide area tracker HiBall The sensor advantages are: � High sampling rate (2000 Hz); � High accuracy (0.5 mm, 0.03°) and high resolution (0.2 mm, 0.03°) � Impervious to metallic or ultrasonic interference; � Very large tracking area (up to 40 ft x 40 ft), small weight (8 oz). HiBall Optical Sensor HiBall Optical Sensor HiBall Optical Sensor interior HiBall Optical Sensor interior 6 lateral- 6 lateral -effect photodiodes effect photodiodes 6 optical lenses 6 optical lenses Signal conditioning Signal conditioning electronics electronics (courtesy of 3rdTech Inc.)
HiBall 3000 tracker 3000 tracker HiBall on an HMD on an HMD Lateral effect photo diodes One LED is on at a time. A LED imaged by multiple sensors allows for position calculation
Types of VR Applications Types of VR Applications Beacon array modules Beacon array modules (6 strips with 8 LED/strip) (6 strips with 8 LED/strip)
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