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--- projects:robots:inertial_navigation_system [2024/04/26 15:41] – jhagstrand
+++ projects:robots:inertial_navigation_system [2024/04/29 04:51] (current) – [videos] jhagstrand
@@ Line 1: / Line 1: @@
 ====== Inertial Navigation System (INS) ======
+spreadsheet of data:\\
+https://docs.google.com/spreadsheets/d/1SMg0OuFvwoGWqhNJKjKyEFaudhpbxwleUFlkU9-4UDw/edit?usp=sharing
 INS Intertial Navigation System - Sensors, processors, and software to estimate position.
@@ Line 47: / Line 51: @@
 velocity vs acceleration
-linear velocity: meters per second
+per Steppe School youtubes:
-linear acceleration: meters per second squared
-angular velocity: radians per second
+bias, aka offset, must be removed from each measurement
-angular acceleration:
+  * accelerometer
+    * measures linear acceleration, not velocity
+    * unit: mg, 1g = 9.8 m/s2, 1mg = 1 thousandth of a g
+    * has bias, but not so critical as gyroscope and magnetometer
+    * gravity is an acceleration of 1g or 1000mg.  This acceleration is always acting on the sensor.
+    * the upright and at-rest measurement should be x:0, y:0, z:1000.
+    * as the sensor is rotated the acceleration due to gravity is distributed among the x,y,z axes.
+  * gyroscope
+    * gyroscope measures angular velocity, not acceleration
+    * gyroscope unit: degrees per second
+    * at rest, gyroscope returns the bias
+    * by subtracting the bias from a readingd, the at rest measurement is x:0, y:0, z:1000
+    * calibration is done at rest for some seconds, the average of readings is the bias
+    * 20948 chip has "offset" registers that we can set with the 3 bias values, each divided by 4
+  * magnetometer
+    * no registers, so we must keep variables and do the subtraction from each measurement
+    * calibration is done during 3D rotation for some seconds
+    * ((max - min) / 2) should be zero, so the actual value during calibration is the bias
+    * in addition to the bias, we also have an adjustment for declination, to convert magnetic north to true north
-rotation velocity: hertz, cycles per second
+localization = changing reference system from the body of the sensor to the earth
+  * sensor readings give data relative to the body, the vehicle
+  * ahrs values give data relative to the earth
-movement relative to the earth's magetic field
+units:
+  * linear velocity: meters per second
+  * linear acceleration: meters per second squared
+  * angular velocity: degrees per second, radians per second
+  * angular acceleration: ?
+  * rotation velocity: hertz, cycles per second
+  * 10**-6  one millionth, μ, micro
+  * 10**-3  one thousandth, m, milli
-o
+accelerometer and gyroscope measure motion
-Sensors
+we know that the z-axis is pointing straight up, but the x and y-axes are relative to some random or arbitrary starting point
+by adding a magnetometer, we can fix the x axis relative to north-south, and the y-axis relative to east-west
-motion sensors
+the magnetometer measures the strength of magnetic fields acting on the sensor
+the most significant magnetic field is that of the earth
-accelerometer
-gyroscope
-IMU - Inertial Measurement Unit -
+naming absurdities:
+  * IMU inertial measurement unit
+    * does not measure inertia. It's components measure accleration, velocity, and magnetic field strenth.
+    * is not a unit of measurement
+  * 6-axis imu or 9-axis imu.  There are only three axes: x,y,z.
+  * 6dof or 9dof.  A sensor does not have degrees of freedom.
@@ Line 149: / Line 180: @@
 Magnetometer calibration
+hold the sensor level and spin it horizontally
+the graph of the results should be a perfect circle, and the radius of the circle is the strength of the magnetic field
 https://youtu.be/MinV5V1ioWg?si=Bh-e9aDgxxzKsm14
-Scott Lobdell, IMU
+Scott Lobdell, on AHRS calculations, specifically the Madgwick Quaternion Update
+start with 9 measurements from the 3 sensors * 3 axes
+calibrate the magnetometer using a 3D bias to get corrected values of the 3 magnetometer measurements
+begin ahrs
+localize the acceleration vector from [x,y,z] to [north,east,down]
+convert the 9 measurements into a quaternion
+convert the quaternion into a 3x3 rotation matrix
+multiply the acceleration vector times the inverse of the rotation matrix, giving the localized acceleration vector
+remove the acceleration due to gravity [0,0,1] to [0,0,0]
+apply the declination offset
 https://youtu.be/T9jXoG0QYIA?si=HjldywVEFu-QSfJp
+===== videos =====
+Paul McWhorter: 9-axis IMUs with Arduino, youtube \\
+Adafruit Bosch BNO055 \\
+videos, in particular: \\
+#10: Making a tilt-compensated compass \\
+#21: Visualizing 3D rotations in VPython using Quaternions \\
+Scott Lobdell: How to implement an IMU \\
+no specific chip or code \\
+hand-drawn diagrams and verbal description of calibration and AHRS calculations
+MicWro Engr, Michael Wrona: Magnetometer Errors and Calibration \\
+Adafruit Precision NXP 9-DOF Breakout Board - FXOS8700 + FXAS21002 \\
+series of youtubes made while developing his custom "Hummingbird" flight controller
+Magneto calibration software \\
+[[Michael Wrona notes | notes]]
+Steppe School: STM32 and ICM20948 IMU part 1. accelerator and gyroscope
+https://youtu.be/Oen3HqUbctM?si=SeUm8s62af9PdqDz
+videos
 ===== Commercial Products =====
@@ Line 178: / Line 240: @@
 libraries:
   * https://github.com/adafruit/Adafruit_ICM20X
+  * https://wolles-elektronikkiste.de/en/icm-20948-9-axis-sensor-part-i
 === BNO055 ===
@@ Line 215: / Line 278: @@
   * expensive 160 USD
   * Redshift Labs appears to be out of business