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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

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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 • Evaluate subsumption and pfield architectures in terms of: support for modularity, niche targetability, ease of portability to other domains, robustness • 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” 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. 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

  7. 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

  8. follow-corridor 2 wander 1 runaway 0 RUN AWAY PS MS PS MS HALT COLLIDE Level 0: Runaway Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  9. 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

  10. 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 Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  11. 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

  12. 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 Chapter 4: The Reactive Paradigm

  13. Subsumption Review • What is the Reactive Paradigm in terms of primitives? • What is the Reactive Paradigm in terms of sensing? • Does the Reactive Paradigm solve the Open World problem? • How does the Reactive Paradigm eliminate the frame problem? • What is the difference between a behavior and a level of competence? • What is the difference between suppression and inhibition in subsumption? Review Organization -SA -beh. specific Subsumption -Philosophy -Level 0 -Level 1 -Level 2 Summary Chapter 4: The Reactive Paradigm

  14. 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

  15. 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

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

  17. 5 Primitive Potential Fields Chapter 4: The Reactive Paradigm

  18. 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

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

  20. Magnitude profiles • Constant magnitude • linear drop off • exponential drop off Chapter 4: The Reactive Paradigm

  21. 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

  22. 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

  23. 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

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

  25. 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

  26. 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

  27. 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

  28. 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

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

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

  31. 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) • Box canyon problem • Jerky motion Chapter 4: The Reactive Paradigm

  32. 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 Chapter 4: The Reactive Paradigm

  33. Docking Behavior Video Chapter 4: The Reactive Paradigm

  34. Comparison of Architectures • Similar in philosophy and results; essentially equivalent • Support for modularity • Both decompose task into behaviors • Subsumption favors hardware, pfields pure software • could do with just rules but lose modularity, design discipline • Niche targetability • High; philosophy is to fit an ecological niche! • Ease of portability to other domains • Only to ones that can be done with reflexive behaviors • Subsumption not as easy with upper levels • Robustness • Subsumption has implicit graceful degradation Chapter 4: The Reactive Paradigm

  35. 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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