Chapter.2

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					                   Input Devices:
Trackers, Navigation and Gesture Interfaces




         Rutgers University - Virtual Reality Technology
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)




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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
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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).




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Input Devices




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Input Devices



   Tracker characteristics:
   Measurement rate – Readings/sec;
   Sensing latency;
   Sensor noise and drift;
   Measurement accuracy (errors);
   Measurement repeatability;
   Tethered or wireless;
   Work envelope;
   Sensing degradation .



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Input Devices



   Tracker characteristics:
                                         Accuracy




                                                 Resolution

           Real object position
                                              Tracker position
                                              measurements


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Input Devices

   Tracker characteristics:

   Tracker data

Real object fixed
position

                                                                      Signal noise




                                                                          Time


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Input Devices

  Tracker characteristics:

   Tracker data

Real object fixed                                                     Sensor drift
position




                                                                              Time


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Input Devices

  Tracker characteristics:
                                         Tracker data



Real object
position




                      Sensor latency

                                                                  Time


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Input Devices

  Tracker characteristics:
  Tracker Update Rate




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Input Devices



  Mechanical Trackers
  Definition: A mechanical tracker consists of a serial or
  parallel kinematic structure composed of links interconnected
  by sensorized joints.




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Input Devices



  Mechanical Trackers
   Use sensors imbedded in exoskeletons to measure position;
   Have extremely low latencies;
   Are immune to interference from magnetic fields;
   But limit the user’s freedom of motion;
   Can be heavy is worn on the body




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Input Devices




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Input Devices



     Exoskeleton
     structure                                                       Interface
                                                                     With
                                                                     computer




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Input Devices



  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




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Input Devices



  Magnetic Trackers
  Use low-frequency magnetic fields to measure position;
  Fields are produced by a fixed source;
  Size of source grows with the tracker work envelope;
  The receiver is attached to the tracked object and
   has three perpendicular antennas;
  Distance is inferred from the voltages induced in the
   antennas – needs calibration…




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Input Devices
                Magnetic tracker with Data Glove




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Input Devices Fastrack magnetic tracker system
                         Electronic interface




                                                                    Source




Stylus                                          Receiver

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Input Devices

                Long Ranger source for the tracker system




                                                                       Source




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  Receivers
                                                                    Fastrack
                                                                    magnetic
                                                                    tracker
                                                                    electronics




                                                                  Source




Input Devices
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Input Devices Polhemus Long Ranger tracking errors (Rutgers)




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                                                     Magnitude of Error Vector / Moving Tripod

                                     35.0
Magnitude of Error Vector (inches)




                                     30.0

                                     25.0
                                                                                                                     Err (54)
                                     20.0                                                                            Err (60)
                                                                                                                     Err (66)
                                     15.0                                                                            Err (72)

                                     10.0                                                                            Err (80)


                                      5.0

                                      0.0
                                            0   10        20       30       40        50       60          70   80
                                                       Transmitter-Receiver Distance (inches)


                                                       Tracking error as a function of tripod height


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Input Devices




                DC Magnetic Tracker Block Diagram
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Input Devices Flock of Birds magnetic tracker
              (Ascension Co.)




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Input Devices




Motion Star wireless tracker (courtesy of Ascension Technology)

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Input Devices Wireless suit (Ascension Technology)
Sensors: 20/suit

100 updates/sec
3 meters range
from base unit
Resolution<2 mm                                                      Electronic unit
and <.2 degrees                                                      (2 hours battery life)




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Input Devices




                      Motion Star block diagram
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Input Devices



  Magnetic Tracker Calibration
  Use mechanical measurements to reduce errors;
  Sensor noise – variation in measurement with no
  real object motion – solved by over-sampling;
  Size of errors grow from source outwards;
  Errors both in position and orientation.




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Input Devices

                Magnetic tracker accuracy degradation




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Input Devices



  Magnetic Tracker Errors
   due to ambient noise:

     e ambient = Kn (d transmitter-receiver)4

   due to metal:

             Kr (d transmitter-receiver)4
  e metal = ---------------------------------
             (d transmitter-metal)3 x (d metal-receiver)3


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Input Devices



  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




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Input Devices




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Input Devices




  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.




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Input Devices



  Ultrasonic Trackers
  Use low-frequency ultrasound to measure position;
  Sound produced by a fixed triangular source (speakers);
  Number of sources grows with the tracker work envelope;
  The receiver is triangular and attached to the tracked
   object and has three microphones;
  Distance is inferred from the sound time of flight;
  Sensitive to air temperature and other noise sources;
  Requires “direct line of sight”;
  Slower than magnetic trackers (max 50 updates/sec).


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Input Devices   Ultrasonic tracker (Logitech)




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Input Devices




           Large-volume ultrasonic tracker (Logitech)
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Input Devices




  Optical Trackers
  Definition: A non-contact position measurement
  device that uses optical sensing to determine the
  real-time position/orientation of an object




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Input Devices




 Optical trackers: a) outside-looking-in; b) inside-looking-out
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Input Devices
                   Inside-out optical tracker advantages




The best accuracy is close to the
work envelope.
Very large tracking surface and
resistance to visual occlusions
(line of sight).




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Input Devices
Outside-looking-in
LaserBIRD optical tracker




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Input Devices
Inside-looking-out
LaserBIRD optical tracker




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  HiBall 3000 wide area tracker
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 interior

                                              6 photodiodes




   6 optical lenses

                                                                                Signal conditioning
                                                                                   electronics

                                                                         (courtesy
                           Rutgers University - Virtual Reality Technology           of 3rdTech Inc.)
HiBall 3000 tracker
 on an HMD




                                 Lateral effect
                                 photo diodes




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Types of VR Applications

  Beacon array modules
  (6 strips with 8 LED/strip)




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Input Devices



  Hybrid Ultrasonic/Inertial Trackers

  No interference from metallic objects;
  No interference from magnetic fields;
  Large-volume tracking;
  “Source-less” orientation tracking;
  Full-room tracking;
  A newer technology.

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Input Devices

  But…

  Accelerometer  errors a lead to decreased
  accuracy since x= a t2
                       2
  Errors grow geometrically in time!
  Gyroscope errors compound position errors;
  Needs independent position estimation to reduce
  “drift”;
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Tracker components (InterSense Co.)                Base unit




                        Sonic Strips



                       I-cube



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Degrees of freedom: 6
Resolution: 1.5 mm RMS
Angular: 0.05o RMS
Update rate: 180 sets/s max
– one station
Down to 90 updates/sec
- for four stations.
Latency 4–10 ms
Max tracking area: 900 meters2
(300 strips, 24 hubs)




                  Tracker components (courtesy of Intersense Co.)

                            Rutgers University - Virtual Reality Technology
I-Cube
Accel./gyro

                                                                               Ultrasonic
                                                                               emitter




              InterSense Stereo Glasses tracker (courtesy of Intersense Co.)


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Accelerometer


                                                                           Ultrasonic
                                                                           emitter




           InterSense Stereo stylus tracker (courtesy of Intersense Co.)


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Input Devices




   IS 900 block diagram




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IS 900 software block diagram




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                    Link to VC 2.1 on book CD




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Input Devices




                Rutgers University - Virtual Reality Technology
Input Devices



       Navigation and Gesture Input Devices

        Navigation interfaces allow relative
       position control of virtual objects;
        Gesture interfaces allow dextrous
       control of virtual objects and interaction
       through gesture recognition.


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Input Devices



  Navigation Input Devices

   Are  the Cubic Mouse, the trackball and the
  3-D probe;
   Perform relative position/velocity
  control of virtual objects;
   Allow “fly-by” application by controlling
   a virtual camera.
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Input Devices




                                 The Cubic Mouse
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                    Link to VC 2.2 on book CD




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Input Devices




                                 Trackballs
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Input Devices




                      The Microscribe (Immersion Co.)
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Input Devices

         Gesture Input Devices
          Are sensing gloves such as:
          - Fakespace “Pinch Glove”
          - 5DT Data Glove;
         - The DidjiGlove
          - Immersion “CyberGlove”
          Have larger work envelope than
          trackballs/3-D probes;
          Need calibration for user’s hand.


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Input Devices
                Finger Degrees of Freedom




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Input Devices


                Hand work envelope vs. interface type




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The Pinch Glove (Fakespace Co.)
- no joint measures, but contact detection

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The Pinch Glove (Fakespace Co.)
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     One optical fiber/finger        5DT Data Glove




A)                                                   Two sensors/finger plus
            Roll/pitch sensing
                                                     abduction sensors

             100 datasets/sec, 12 bit A/D flexion resolution,
             wireless version transmits data at 30 m, needs calibration

The glove interface: a) five-sensor version; b) 16-sensor version
                                Rutgers University - Virtual Reality Technology
     5DT Data Glove




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Glove has less sensors        5DT Data Glove
than hand joints …
Needs to infer distal
joint flexion angle




     The coupling of intermediate and distal finger joints
                         Rutgers University - Virtual Reality Technology
Input Devices        5DT Data Glove




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Input Devices        5DT Data Glove


                Linear calibration method




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                     The Didgiglove



Inexpensive wired glove for computer animation;
Uses capacitive sensors (two per finger) and a 10-bit A/D
converter (1,024 points);
 Can do 70 hand configuration reads/sec.;
 Communicates with the host over an RS232 (19.2 k)




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                        The CyberGlove




 Uses 18-22 linear sensors – electrical strain gauges;
 Angles are obtained by measuring voltages on
  a Wheastone bridge;
 112 gestures/sec “filtered”.
 Sensor resolution 0.5 degrees, but errors accumulate
  to the fingertip (open kinematic chain);
 Sensor repeatability 1 degree
 Needs calibration when put on the hand;
 Is expensive (about $10,000)

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The CyberGlove (Vertex Co.)




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                    Link to VC 2.3 on book CD




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Rutgers University - Virtual Reality Technology

				
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posted:4/30/2011
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