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content/notes/10-orientation-sensors.md
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@ -32,6 +32,7 @@ Accelerometers
![two phones|300](https://i.imgur.com/s45a4TF.png)
Compass / Magnetometer
- Measures the strength of earth’s magnetic field
- Strength is expressed in tesla [T]
@ -41,11 +42,42 @@ Compass / Magnetometer
![Error around metal/magnetic objects](https://i.imgur.com/YIJAcGS.png)
Compass / Magnetometer
- Advantage:
- Absolute orientation measurement
- Disadvantage:
- Usually slow update
- Sensitive to errors
- Local discontinuities in magnetic field
- Ferromagnetic materials
- Power sources
![gryroscope|300](https://i.imgur.com/VnL5seY.png)
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](https://i.imgur.com/q1LjfPD.png)
![gyro electronic|300](https://i.imgur.com/5Ti80sO.png)
- 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](https://i.imgur.com/dzTVCpE.png)
![gyro chip](https://i.imgur.com/zVoQ3wM.png)
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](https://i.imgur.com/CZKk1M7.png)
- We cannot take a perfectly continuous integral -> take the sum of a finite number of samples taken at a constant interval Ts
![eq2|100](https://i.imgur.com/vL0gRbV.png)
- 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