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The Reactive Paradigm

The Reactive Paradigm. Describe the Reactive Paradigm in terms of the 3 robot primitives and its organization of sensing List the characteristics of a reactive robotic system, and discuss the connotations of surrounding the reactive paradigm

morgan-hogan
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The Reactive Paradigm

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  1. The Reactive Paradigm • Describe the Reactive Paradigm in terms of the 3 robot primitives and its organization of sensing • List the characteristics of a reactive robotic system, and discuss the connotations of surrounding the reactive paradigm • Describe the two dominant methods for combining behaviors in a reactive architecture: subsumption and potential field summation • Be able to program a behavior using pfields • Be able to construct a new potential field from primitive pfields and sum pfields to generate an emergent behavior Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  2. Review: Lessons from Biology • Programs should decompose complex actions into behaviors. Complexity emerges from concurrent behaviors acting independently • Agents should rely on straightforward activation mechanisms such as IRM • Perception filters sensing and considers only what is relevant to the task (action-oriented perception) • Behaviors are independent but the output may be used in many ways including: combined with others to produce a resultant output or to inhibit others Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  3. Hierarchical Organization is“Horizontal” Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  4. More Biological is “Vertical” Higher level behaviors reuse or inhibit more primitive behaviors Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  5. Behaviors can “share” perception without knowing it This is behavioral sensor fusion Sensing is Behavior-Specific or Local Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  6. Reactive Robots • Most apps are programmed with this paradigm • Biologically based: • Behaviors (independent processes), released by perceptual or internal events (state) • No world models or long term memory • Highly modular, generic • Overall behavior emerges RELEASER behavior SENSE ACT Overview History Reactive USAR Summary Chapter 4: The Reactive Paradigm

  7. www.friendlymachines.com Example 1: Robomow • Behaviors? • Random • Avoid • Avoid(bump=obstacle) • Avoid(wire=boundary) • Stop • Stop(tilt=ON) • All active Overview History Reactive USAR Summary Chapter 4: The Reactive Paradigm

  8. www.irobot.com Example 2: My Real Baby • Behaviors? • Touch-> Awake • Upside down & Awake-> Cry • Awake & Hungry -> Cry • Awake & Lonely -> Cry • Note can get crying from multiple behaviors • Note internal state (countdown timer on Lonely) Overview History Reactive USAR Summary Chapter 4: The Reactive Paradigm

  9. Reactive Behaviors • Connotations • Execute rapidly • Can be implemented in hardware • Have no memory • Characteristics of reactive architectures • Robots are situated agents operating in an ecological niche • Behaviors are basic building blocks for robotic actions, and overall behavior of robot emerges from their interaction • Independent, concurrent • Schema of a behavior may have a coordinated control program, but there is no external controller of all behaviors for a task • Architecture may use combination, suppression, or cancellation for interaction • Only local, behavior-specific sensing in permitted • No world model, representation from sensing is ego-centric • Modular • Tasks are broken down into behaviors; behaviors can be tested independently; new behaviors can be built from more primitive ones • Based on animal models of behavior Chapter 4: The Reactive Paradigm

  10. Reactive • Historically, there are two main styles of creating a reactive system • Subsumption architecture • Layers of behavioral competence • How to control relationships • Potential fields • Concurrent behaviors • How to navigate • They are equivalent in power • In practice, see a mixture of both layers and concurrency Chapter 4: The Reactive Paradigm

  11. Subsumption:Rodney Brooks Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary From http://www.spe.sony.com/classics/fastcheap/index.html Chapter 4: The Reactive Paradigm

  12. Subsumption Philosophy • Modules should be grouped into layers of competence • Modules in a higher lever can override or subsume behaviors in the next lower level • Suppression: substitute input going to a module • Inhibit: turn off output from a module • No internal state in the sense of a local, persistent representation similar to a world model. • Architecture should be taskable: accomplished by a higher level turning on/off lower layers Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  13. follow-corridor 2 wander 1 runaway 0 RUN AWAY PS MS PS MS HALT COLLIDE Level 0: runaway when obstacle comes near turn around, run away; when collision imminent, stop, turn around, run away direction magnitude Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary sensors behaviors motor actions Chapter 4: The Reactive Paradigm

  14. Example Perception: Polar Plot • Plot is ego-centric • Plot is distributed (available to whatever wants to use it) • Although it is a representation in the sense of being a data structure, there is no memory (contains latest information) and no reasoning (2-3 means a “wall”) Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary if sensing is ego-centric, can often eliminate need for memory, representation Chapter 4: The Reactive Paradigm

  15. follow-corridor 2 wander 1 runaway 0 WANDER AVOID PS MS MS encoders What would Inhibition do? PS Note sharing of Perception, fusion Level 1: Wander Avoid suppresses (replaces)output from runaway Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  16. move2light 2 wander 1 runaway 0 LIGHT PHOTO- TROPHISM S Class Exercise Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  17. follow-corridor 2 wander 1 runaway 0 STAY-IN-MIDDLE PS MS Level 2: Follow-Corridors Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Computed from shaft encoders Intended course Chapter 4: The Reactive Paradigm

  18. Class Exercise • Design the roomba with subsumption Chapter 4: The Reactive Paradigm

  19. Subsumption Review • What is the Reactive Paradigm in terms of primitives? • Sense, act • What is the Reactive Paradigm in terms of sensing? • Local to each behavior • Does the Reactive Paradigm solve the Open World problem? • Open world is non-monotonic; need truth maintenance mechanism • Reactive paradigm has no memory, no truth maintenance • How does the Reactive Paradigm eliminate the frame problem? • No world model, so no frame problem • What is the difference between a behavior and a level of competence? • Not schema theoretic; level of competence groups schema-like modules into abstract behaviors • What is the difference between suppression and inhibition in subsumption? • Suppression acts like a gate; inhibition like on/off switch Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  20. Potential Fields:Ron Arkin From http://www.cc.gatech.edu/aimosaic/faculty/arkin From http://www.cc.gatech.edu/aimosaic/robot-lab/MRLhome.html Chapter 4: The Reactive Paradigm

  21. Potential Fields Philosophy • The motor schema component of a behavior can be expressed with a potential fields methodology • A potential field can be a “primitive” or constructed from primitives which are summed together • The output of behaviors are combined using vector summation • From each behavior, the robot “feels” a vector or force • Magnitude = force, strength of stimulus, or velocity • Direction • But we visualize the “force” as a field, where every point in space represents the vector that it would feel if it were at that point Chapter 4: The Reactive Paradigm

  22. Example: Run Away via Repulsion Chapter 4: The Reactive Paradigm

  23. 5 Primitive Potential Fields perpendicular uniform tangential Chapter 4: The Reactive Paradigm

  24. Draw These Now!Common fields in behaviors • Uniform • Move in a particular direction, corridor following • Repulsion • Runaway (obstacle avoidance) • Attraction • Move to goal • Perpendicular • Corridor following • Tangential • Move through door, docking (in combination with other fields) • random • do you think this is a potential field? what would it look like? Chapter 4: The Reactive Paradigm

  25. Attractive Repulsive Class Exercise • Name the field you’d use for • Moving towards a light • Avoiding obstacles Chapter 4: The Reactive Paradigm

  26. Magnitude profiles • Constant magnitude • Linear drop off • Exponential drop off • P. 127 Chapter 4: The Reactive Paradigm

  27. Programming a potential field Example: repulsive field with linear dropoff, only one sensor • Vdirection = -180 degrees • Vmagnitude = (D-d)/D if d <= D where D is range of potential field 0 otherwise (magnitude controls velocity) Implementation in C on p. 129-130 With more than one sensor – fig. =4.18, p. 134 - 136 Chapter 4: The Reactive Paradigm

  28. Problems • Impact of update rates • if time between updates is too long • Path can be jerky • Can overshoot • Robots can’t change velocity and direction immediately • Fields may sum to 0 Chapter 4: The Reactive Paradigm

  29. goal goal obstacle obstacle Combining Fields forEmergent Behavior obstacle If robot were dropped anywhere on this grid, it would want to move to goal and avoid obstacle: Behavior 1: MOVE2GOAL Behavior 2: RUNAWAY The output of each independent behavior is a vector, the 2 vectors is summed to produce emergent behavior Chapter 4: The Reactive Paradigm

  30. Note: in this example, robot can sense the goal from 10 meters away Note: In this example, repulsive field only extends for 2 meters; the robot runs away only if obstacle within 2 meters Fields and Their Combination Chapter 4: The Reactive Paradigm

  31. Path Taken Robot only feels vectors for this point when it (if) reaches that point • If robot started at this location, it would take the following path • It would only “feel”the vector for the location, then move accordingly, “feel” the next vector, move, etc. • Pfield visualization allows us to see the vectors at all points, but robot never computes the “field of vectors” just the local vector Chapter 4: The Reactive Paradigm

  32. Chapter 4: The Reactive Paradigm

  33. Discussion • Could you represent the Arctic Tern feeding behavior with potential fields? • what happens with multiple red dots? • can you inhibit with potential fields? Chapter 4: The Reactive Paradigm

  34. Example: follow-corridor or follow-sidewalk Perpendicular Uniform Note use of Magnitude profiles: Perpendicular decreases Combined Chapter 4: The Reactive Paradigm

  35. Just half of a follow-corridor, but… Class Exercise:Draw Fields for Wall-Following(assume that robot stands still if no wall) Chapter 4: The Reactive Paradigm

  36. But how does the robot see a wall without reasoning or intermediate representations? • Perceptual schema “connects the dots”, returns relative orientation MS: Perp. orientation PS: Find-wall S MS: Uniform Sonars Chapter 4: The Reactive Paradigm

  37. OK, But why isn’t that a representation of a wall? • It’s not really reasoning that it’s a wall, rather it is reacting to the stimulus which happens to be smoothed (common in neighboring neurons) Chapter 4: The Reactive Paradigm

  38. Note: multiple instances of a behavior vs. 1: Could just have 1 Instance of RUN AWAY, Which picks nearest reading; Doesn’t matter, but this Allows addition of another Sonar without changing the RUN AWAY behavior Level 0: Runaway Chapter 4: The Reactive Paradigm

  39. Level 1: Wander Wander is Uniform, but Changes direction aperiodically Chapter 4: The Reactive Paradigm

  40. Level 2: Follow Corridor Follow-corridor Should we Leave Run Away In? Do we Need it? Chapter 4: The Reactive Paradigm

  41. Pfields • Advantages • Easy to visualize • Easy to build up software libraries • Fields can be parameterized • Combination mechanism is fixed, tweaked with gains • Disadvantages • Local minima problem (sum to magnitude=0) • Jerky motion Chapter 4: The Reactive Paradigm

  42. Orientation, ratio of pixel countstangent vector Total countattraction vector • Arkin and Murphy, 1990, Questa, Grossmann, Sandini, 1995, Tse and Luo, 1998, Vandorpe, Xu, Van Brussel, 1995. Roth, Schilling, 1998, Santos-Victor, Sandini, 1997 Example: Docking Behavior Selective attraction field , width of +-45 degrees Chapter 4: The Reactive Paradigm

  43. Docking Behavior Video Chapter 4: The Reactive Paradigm

  44. Class Discussion:When Does a Field End? • Imagine the case of a “SodaPup” robot (MIT) • task: find and pick up a Coca-Cola can • environment: red cans are only red object in world Chapter 4: The Reactive Paradigm

  45. Pfield advantages • Easy to visualize • Easy to combine • Can be parameterized (different ranges, drop off, etc.) Chapter 4: The Reactive Paradigm

  46. disadvantages • Local minima • Solutions: • Add noise • Navigation templates • Express potential fields as harmonic functions – no local minima of 0, but computationally expensive Chapter 4: The Reactive Paradigm

  47. Pfields Summary • Reactive Paradigm: SA, sensing is local • Solves the Open World problem by emulating biology • Eliminates the frame problem by not using any global or persistent representation • Perception is direct, ego-centric, and distributed • Two architectural styles are: subsumption and pfields • Behaviors in pfield methodologies are a tight coupling of sensing to acting; modules are mapped to schemas conceptually • Potential fields and subsumption are logically equivalent but different implementations • Pfield problems include • local minima (ways around this) • jerky motion • bit of an art Chapter 4: The Reactive Paradigm

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