PowerPoint-Präsentation

PowerPoint-Präsentation

Computer Vision Group Prof. Daniel Cremers Autonomous Navigation for Flying Robots Lecture 3.2: Sensors Jrgen Sturm Technische Universitt Mnchen Sensors IMUs (inertial measurement units) Accelerometers Gyroscopes Range sensors (sonar) GPS

Cameras Jrgen Sturm Autonomous Navigation for Flying Robots 2 Accelerometers Measures all external forces acting upon them (including gravity) Acts like a spring-damper system To obtain inertial acceleration (due to motion alone), gravity must be subtracted http://en.wikipedia.org/wiki/File:Pendular_accel.svg

Jrgen Sturm Autonomous Navigation for Flying Robots 3 MEMS Accelerometers Micro Electro-Mechanical Systems (MEMS) Spring-like structure with a proof mass Damping results from residual gas

Implementations: capacitive, piezoelectric, mass spring applied acceleration Bernstein, J., An Overview of MEMS Inertial Sensing Technology, Sensors, Feb. 2003. Jrgen Sturm Autonomous Navigation for Flying Robots 4

Gyroscopes Measures orientation (standard gyro) or angular velocity (rate gyro, needs integration for angle) Spinning wheel mounted in a gimbal device (can move freely in 3 dimensions) Wheel keeps orientation due to angular momentum http://en.wikipedia.org/wiki/File:Gyroscope_operation.gif Jrgen Sturm http://en.wikipedia.org/wiki/File:Foucault%27s_gyroscope.jpg Autonomous Navigation for Flying Robots

5 MEMS Gyroscopes Vibrating structure gyroscope (MEMS) Based on Coriolis effect Vibration keeps its direction under rotation Implementations: Tuning fork, vibrating wheels, Bernstein, J., An Overview of MEMS Inertial Sensing Technology, Sensors, Feb. 2003. Jrgen Sturm Autonomous Navigation for Flying Robots 6

Inertial Measurement Units (IMUs) 3-axes MEMS gyroscope Provides angular velocity Integrate for angular position Problem: Drifts slowly over time (e.g., 1deg/hour), called the bias 3-axes MEMS accelerometer Provides accelerations (including gravity) Jrgen Sturm Autonomous Navigation for Flying Robots

7 IMU Strapdown Algorithm Accelerometers Gyroscopes Jrgen Sturm Gravity compensation Velocity integration

Attitude integration Gyro bias estimation Autonomous Navigation for Flying Robots Position integration 8 Example: AscTec Autopilot Board

Gyroscopes (3x single axis) Accelerometer (3-axes) http://www.asctec.de/downloads/manuals/AscTec-Autopilot-Manual-v10.pdf Jrgen Sturm Autonomous Navigation for Flying Robots 9 GPS Every satellite transmits its position and time Receiver measures time difference of satellite signals Calculate position by intersecting distances (pseudoranges)

Jrgen Sturm Autonomous Navigation for Flying Robots 10 GPS 24+ satellites, 12 hour orbit, 20.190 km height 6 orbital planes, 4+ satellites per orbit, 60deg distance Satellite transmits orbital location (almanach) + time 50bits/s, msg has 1500 bits 12.5 minutes http://en.wikipedia.org/wiki/File:ConstellationGPS.gif

Jrgen Sturm Autonomous Navigation for Flying Robots 11 GPS Position from pseudorange Requires measurements of 4 different satellites Low accuracy (3-15m) but absolute Position from pseudorange + phase shift (RTK/dGPS) Very precise (<1mm) Position is relative to a reference station Jrgen Sturm

Autonomous Navigation for Flying Robots 12 Ultrasound Range Sensors Emit signal to determine distance along a ray Make use of propagation speed of ultrasound Traveled distance is given by speed of sound (v=340m/s) http://en.wikipedia.org/wiki/File:Sonar_Principle_EN.svg Jrgen Sturm Autonomous Navigation for Flying Robots

13 Ultrasound Range Sensors Range between 12cm and 5m Opening angle around 20 to 40 degrees Problems: multi-path propagation, absorption Lightweight and cheap http://www.parallax.com/product/28015 Jrgen Sturm

Autonomous Navigation for Flying Robots 14 Cameras Pinhole Camera Lit scene emits light Film/sensor is light sensitive Jrgen Sturm Autonomous Navigation for Flying Robots 15

Lens Camera Lit scene emits light Film/sensor is light sensitive A lens focuses rays onto the film/sensor Jrgen Sturm Autonomous Navigation for Flying Robots 16 Lens Distortions Radial distortion of the image Caused by imperfect lenses Deviations are most noticeable for rays that pass through the edge of the lens

Jrgen Sturm Autonomous Navigation for Flying Robots 17 Lens Distortions Radial distortion of the image Caused by imperfect lenses Deviations are most noticeable for rays that pass through the edge of the lens Typically compensated with a low-order polynomial Jrgen Sturm

Autonomous Navigation for Flying Robots 18 Camera Calibration Pinhole model Distortion model How can we determine these parameters? Jrgen Sturm Autonomous Navigation for Flying Robots 19

Camera Calibration Collect corresponding observations between 3D points (3D location in world coordinates) 2D points (on the image plane) Typically using a calibration board Jrgen Sturm Autonomous Navigation for Flying Robots 20 Camera Calibration

Every observation Solve for Jrgen Sturm produces two constraints + distortion coefficients Autonomous Navigation for Flying Robots 21 Lessons Learned

Measure attitude, velocity and position with IMUs Measure distances with range sensors Measure position using GPS Camera calibration Jrgen Sturm Autonomous Navigation for Flying Robots 22

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