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title tags
10-orientation-sensors
lecture
info305

[!INFO] Location sensors dont know

Accelerometers

  • Measures proper acceleration (acceleration it experiences relative to freefall. gravity), felt by people or objects
  • Units: m/s2 or g
  • Most smartphone accelerometers trade large value range for high precision
    • iPhone range: ±2g, precision 0.018g g forces|300

Accelerometers

  • Acceleration is measured on 3 axes
  • Orientation of sensor (and coordinate system) varies among different device |100

Accelerometers

  • Miniaturisation using a MEMS (Microelectromechanical systems)
  • Measuring flowing current over an differential capacitor indicates the acceleration |300

Accelerometers

  • Advantage:
    • Fast update rate
    • Relatively accurate
  • Disadvantage:
    • Can’t easily identify certain kind of acceleration (small value range)

two phones|300 Compass / Magnetometer

  • Measures the strength of earth’s magnetic field
  • Strength is expressed in tesla [T]
  • iPhone 4 magnetometer range: ±2mT
  • Pro tip: prolonged exposure to a fridge magnet decalibrates your phones’ magnetometer for at least a week ;-) field strength examples|300

Error around metal/magnetic objects

Compass / Magnetometer

  • Advantage:
    • Absolute orientation measurement
  • Disadvantage:
    • Usually slow update
    • Sensitive to errors
      • Local discontinuities in magnetic field
      • Ferromagnetic materials
      • Power sources

gryroscope|300 Gyroscope

  • Detects the current orientation of the device, or changes in the orientation

  • Precisely: orientation can be computed from the angular rate that is detected by the gyroscope, expressed in rad/s [deg/s] on 3 axis.

  • Traditional gyroscopes use the e!ect of angular momentum gyro deg of freedom|300 gyro electronic|300

  • MEMS (microelectromechanical system) gyro

  • Used in most smartphones or mobile/embedded devices

  • Use the displacement of vibrating proof mass to compute orientation (Coriolis effect) MEMS gyro|100 gyro chip

Problem:

  • The gyroscope gives us angular rate with a unit of rad/s [deg/s]
  • We can find the angular position at any given moment t with the following equation (assuming t=0 theta=0) eq1|100
  • We cannot take a perfectly continuous integral -> take the sum of a finite number of samples taken at a constant interval Ts eq2|100
  • Gyroscope data changes faster than the sampling frequency
    • We will not detect it, and the integral approximation will be incorrect
    • This error is called drift/bias as it increases in time, no return to 0o

Recap

  • Several improvements to traditional GPS

  • AGPS for improved startup time and improved localisation using WIFI

  • DGPS for improved localisation using reference stations with known error

  • RTK GPS for improve localisation using DGPS and phase analysis

  • Accelerometers for measuring gravity along multiple axis (typically 3)

  • Usually implemented with MEMS

  • Typically limited in value range (in mobile devices)