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89 lines
3.3 KiB
Markdown
89 lines
3.3 KiB
Markdown
---
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title: "10-orientation-sensors"
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tags:
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- lecture
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- info305
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---
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Accelerometers
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- Measures proper acceleration (acceleration it experiences relative to freefall. gravity), felt by people or objects
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- Units: m/s2 or g
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- Most smartphone accelerometers trade large value range for high precision
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- iPhone range: ±2g, precision 0.018g
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Accelerometers
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- Acceleration is measured on 3 axes
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- Orientation of sensor (and coordinate system) varies among different device
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Accelerometers
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- Miniaturisation using a MEMS (Microelectromechanical systems)
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- Measuring flowing current over an differential capacitor indicates the acceleration
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Accelerometers
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- Advantage:
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- Fast update rate
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- Relatively accurate
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- Disadvantage:
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- Can’t easily identify certain kind of acceleration (small value range)
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Compass / Magnetometer
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- Measures the strength of earth’s magnetic field
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- Strength is expressed in tesla [T]
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- iPhone 4 magnetometer range: ±2mT
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- Pro tip: prolonged exposure to a fridge magnet decalibrates your phones’ magnetometer for at least a week ;-)
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Compass / Magnetometer
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- Advantage:
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- Absolute orientation measurement
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- Disadvantage:
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- Usually slow update
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- Sensitive to errors
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- Local discontinuities in magnetic field
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- Ferromagnetic materials
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- Power sources
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Gyroscope
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- Detects the current orientation of the device, or changes in the orientation
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- Precisely: orientation can be computed from the angular rate that is detected by the gyroscope, expressed in rad/s [deg/s] on 3 axis.
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- Traditional gyroscopes use the e!ect of angular momentum
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- MEMS (microelectromechanical system) gyro
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- Used in most smartphones or mobile/embedded devices
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- Use the displacement of vibrating proof mass to compute orientation (Coriolis effect)
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Problem:
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- The gyroscope gives us angular rate with a unit of rad/s [deg/s]
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- We can find the angular position at any given moment t with the following equation (assuming t=0 theta=0)
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- We cannot take a perfectly continuous integral -> take the sum of a finite number of samples taken at a constant interval Ts
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- Gyroscope data changes faster than the sampling frequency
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- We will not detect it, and the integral approximation will be incorrect
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- This error is called drift/bias as it increases in time, no return to 0o
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Recap
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- Several improvements to traditional GPS
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- AGPS for improved startup time and improved localisation using WIFI
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- DGPS for improved localisation using reference stations with known error
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- RTK GPS for improve localisation using DGPS and phase analysis
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- Accelerometers for measuring gravity along multiple axis (typically 3)
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- Usually implemented with MEMS
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- Typically limited in value range (in mobile devices) |