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COMP 417 – Jan 12 th , 2006

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  1. COMP 417 – Jan 12th, 2006 Guest Lecturer: David Meger Topic: Camera Networks for Robot Localization

  2. Introduction • Who am I? • Overview, Camera Networks for Robot Localization • What • Where • Why • How (technical stuff)

  3. Introduction - Hardware

  4. Intro - What • Previously: Localization is a key task for a robot. It’s typically achieved using the robot’s sensors and a map. • Can “the environment” help with this?

  5. Typical Robot Localization

  6. Sensor Networks

  7. Sensor Networks

  8. Intro - Where • In cases where there is sensing already in the environment, we can invert the direction of sensing. • Where is this true? • Buildings with security systems • Public transportation areas (metro) • More and more large cities (scary but true)

  9. Intro – Why • Advantages: • In many cases sensors already exist • Many robots operating in the same place, can all share the same sensors • Computation can be done at a powerful central computer, saves robot computation • Interesting research problem

  10. Intro – How • As the robot appears in images, we can use 3-D vision techniques to determine its position relative to the cameras • What do we need to know about the cameras to make this work? • Can we assume we know where the cameras are? • Can we assume we know the camera properties?

  11. Problem Can we use images from arbitrary cameras placed in unknown positions in the environment to help a robot navigate?

  12. Proposed Method • Detect the robot • Measure the relative positions • Place the camera in the map • Move robot to the next camera • Repeat

  13. Detection – An algorithm to detect these robots?

  14. Detection (cont’d) • Computer Vision techniques attempt detection of (moving) objects • Background subtraction or image differencing • Image templates • Color matching • Feature matching • A robust algorithm for arbitrary robots is likely beyond current methods

  15. Detection – Our Method

  16. ARTag Markers

  17. Proposed Method • Detect the robot • Measure the relative positions • Place the camera in the map • Move robot to the next camera • Repeat

  18. Position Measurement • Question: Can we determine the 3-D position of an object relative to the camera from examining 2-D images? • Hint: start from the introduction to Computer Vision from last time

  19. Pinhole Camera Model

  20. Camera Calibration • An image depends on BOTH scene geometry and camera properties • For example, zooming in and out and moving the object closer and farther have essentially the same effect • Calibration means determining relevant camera properties (e.g. focal length f)

  21. Projective Calibration Equations

  22. Coordinate Transformation

  23. Calibration Equations • Matrix AT is a 3x4 and fully describes the geometry of image formation • Given known object points M, and image points m, it is possible to solve for both A and T • How many points are needed?

  24. Calibration Targets

  25. 3-Plane ARTag Target

  26. Position Measurement Conclusion • With enough image points whose 3-D location are known, measurement of coordinate transformation T is possible • The process is more complicated than traditional sensing, but luckily, we only need to do it once per camera

  27. Proposed Method • Detect the robot • Measure the relative positions • Place the camera in the map • Move robot to the next camera • Repeat

  28. Mapping Camera Locations • Given the robot’s position, a measurement of the relative position of the camera allows us to place it in our map • Question: What affects the accuracy of this type of relative measurement?

  29. Proposed Method • Detect the robot • Measure the relative positions • Place the camera in the map • Move robot to the next camera • Repeat

  30. Robot Motion • A robot moves by using electric motors to turn its wheels. There are numerous strategies here in each of the important aspects: • Physical Design • Control algorithms • Programming Interface • High-level software architecture

  31. Nomad Scout

  32. Differential Drive Kinematics

  33. Odometry Position Readings

  34. Robot Motion - Specifics • Robot control accomplished by using an in-house application – Robodaemon • Allows “point and shoot” motion, not continuous control • Graphical and programmatic interface to query robot odometry, send motion commands, collect sensor data

  35. Proposed Method • Detect the robot • Measure the relative positions • Place the camera in the map • Move robot to the next camera • Repeat Are we done?

  36. Challenges • In general, it’s impossible to know the robot or camera positions exactly. All measurements have error • What should the robot do if the cameras can’t see the whole environment? • I didn’t say anything about how the robot should decide where to go next • More?

  37. Mapping with Uncertainty • Given exact knowledge of the robot’s position, mapping is possible • Given a pre-built map, localization is possible • What if neither are present? Is it realistic to assume they will be? If so, when?

  38. Uncertainty in Robot Position • In general, kinematics equations do not exactly predict robot locations • Sources of error • Wheel slippage • Encoder quantization • Manufacturing artifacts • Uneven and terrain • Rough/slippery/wet terrain

  39. Typical Odometry Error

  40. Simultaneous Localization and Mapping (SLAM) • When both the robot and map features are uncertain, both must be estimated • Progress can be made by viewing measurements as probability densities instead of precise quantities

  41. SLAM Progress

  42. SLAM (cont’d) • A quantity of the work in robotics in the last 5-10 years has involved localization and SLAM, results are now very pleasing indoors with good sensing • These methods apply to our system • More on this later in the course, or after class today if you’re interested

  43. Motion Planning • The mapping framework described is dependant on the robot’s motion: • The robot must pass in front of a camera in order to collect any images • Numerous points are needed for each camera to perform calibration • SLAM accuracy affected by order of camera visitation

  44. Local and Global Planning • Local: how should the robot move while in front of one camera, to collect the set of calibration images? • Global: in which order should the cameras be visited?

  45. Local Planning • Modern calibration algorithms are quite good at estimating from noisy data, but there are some geometric considerations • Field of view • Detection accuracy • Singularities in calibration equations

  46. Local Planning • We must avoid configurations where all points collected lie in a linear sub-space of R3 • For example, a set of images of a single plane moved only through translation, gives all co-planar points

  47. Projective Calibration Equations

  48. Global Planning • Camera positions estimated by relative measurements from the robot • This information is only as accurate as our knowledge about the robot • “Re-localizing” is our only way to reduce error

  49. Distance / Accuracy Tradeoff • Returning to well-known cameras helps our position estimates but causes the robot to travel farther than necessary • An intelligent strategy is needed to manage this tradeoff • Some partial results so far, this is work in progress

  50. Review • Using sensors in the environment, we can localize a robot • In order to use previously un-calibrated and unmapped cameras, a robot can carry out exploration, and SLAM • This must only be done once, and then accurate localization is possible