Reactive Paradigm Overview Subsumption Architecture

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Reactive Paradigm Overview Subsumption
Architecture
By Ian Jonkers159.734 Studies in Machine
Learning Intelligent Robotics
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Reactive Paradigm Overview

• Two Representative Reactive Architectures
• Subsumption
• Potential Fields Summation
• Reactive Paradigm emerged late 1980s
• Reactive Paradigm still important for two
  reasons
• 1) Robotic systems in limited task domain still
  being constructed
• 2) Forms the basis for the Hybrid
  Reactive-Deliberative Paradigm
• Reactive Paradigm grew out of the dissatisfaction
  of the hierarchical paradigm and with the influx
  of ideas from biological intelligence.

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Horizontal Decomposition of Hierarchical Model

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Vertical Decomposition

• Instead, examination of ethological literature
  suggests that intelligence is layered in a
  vertical decomposition
• Agent starts with primitive survival behaviours.
• Evolve new layers of behaviour which either
• Reuse the lower, older behaviour
• Inhibit older behaviour
• Create parallel tracts of more advanced
  behaviours
• Parallel tracks can be thought of as vertically
  stacked layers
• Each layer has access to sensors and actuators
  independently of any other layer.
• If anything happens to the more advanced layer,
  lower levels should still operate. i.e. human
  brain (breathing) continue independently of
  higher order functions (counting, face
  recognition).

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Vertical Decomposition
Vertical decomposition of tasks into a S-A
Organisation, associated with the Reactive
Paradigm

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Attributes of Reactive Paradigm

• The fundamental attribute of reactive paradigm is
  all actions are accomplished through behaviours
• As in ethological systems, behaviours are a
  direct mapping of sensory inputs to a pattern of
  motor actions that are then used to achieve a
  task.

S-A organisation of the Reactive Paradigm into
multiple, concurrent behaviours
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Attributes of Reactive Paradigm (continued)

• From mathematical perspective, behaviours are
  simply a transfer function.
• The Reactive Paradigm essential threw away the
  PLAN component.
• The SENSE ACT are tightly coupled into
  behaviours sequential or concurrent robotic
  activities emerge.
• Sensing is local to each behaviour, but sensors
  may be shared and is immediately available to the
  behaviour's perceptual schema which can be
  computationally inexpensive.

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Characteristics of Reactive Behaviours

• Reactive robotic systems execute rapidly (tight
  coupling of senses permits real-time operation)
• Behaviours can be implemented directly in
  hardware circuits or low computational complexity
  algorithms (O(n)).
• Have no memory (limiting behaviours to
  stimulus-response reflexes)
• Main point Behaviours controlled by what is
  happening in the world, duplicating the spirit of
  the innate releasing mechanisms, rather than the
  program storing remembering what the robot last
  did
• Five Characteristics of reactive Paradigm are
• Situated Agent (integrated part of world) Robots
  are situated agents operating in an ecological
  niche.
• i.e. when a robot acts, it changes the world,
  and receives immediate feedback about the world
  through sensing.

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Characteristics of Reactive Behaviours (continued)

• Emergent Behaviours
• Behaviours serve as the basic building blocks
  for robotic action, and the overall behaviour of
  the robot is emergent.
• Behaviours are independent computational
  entities and operate concurrently, Hence there is
  no explicit controller module which determines
  what will be done, or which function call other
  functions.
• 3) Eco-centric
• Only local, behaviour specific sensing is
  permitted.
• i.e. does not matter that an obstacle is in the
  world at coordinates (x,y,z), only where it is
  relative to the robot.
• 4) Modular Behaviours
• These systems inherently follow good software
  design principles.
• The modularity of behaviours supports the
  decomposition of a task into component
  behaviours.
• 5) Biological Motivation
• Animal models of behaviour are often cited as a
  basis for these systems or a particular
  behaviour (unlike earlier AI days where a
  conscious effort not to mimic biological
  intelligence was made)

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Advantages of Programming by Behaviour

• Constructing a robotic system under Reactive
  Paradigm often referred to as programming by
  behaviour.
• Good Software Engineering since Behaviours are
  Modular.
• Robot becomes more intelligent by having more
  behaviours.
• Modules have Low Coupling
• ? can function independently of each other with
  minimal connections or interfaces ? promoting
  easy reuse.
• Modules have High Cohesion
• ? Data and operations contained by a module
  relate only to the purpose of that module.

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Reactive Paradigm Representative Architectures

• The overall action of the robot emerges from the
  multiple concurrent behaviours and the
  architecture must provide mechanisms for
• 1) Triggering behaviours
• 2) Conflict resolution when multiple behaviours
  are active at any one time.
• The two most well known Reactive Architectures
  are
• 1) Potential Fields Behaviours combined by
  summation of fields.
• 2) Subsumption Decomposition into layers of
  task achieving behaviours.

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Subsumption Architecture

• Rodney Brooks Subsumption Architecture most
  influential of the purely Reactive Paradigms.
• Many look like shoe-box sized insects (6 legs and
  antennae)
• Implementations quite often have the behaviours
  embedded directly in the hardware or small micro
  processors (unheard pre mid 1980s)
• Robots could now walk, avoid collisions and climb
  over obstacles without the move-think-move-think
  pauses of Shakey.
• A behaviour is a network of sensing and acting
  modules which accomplish a task.
• The modules are Augmented Finite State Machines
  (AFSM), or finite state machines which have
  registers, timers other enhancements to permit
  them to be interfaced with other modules.

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Subsumption Architecture (continued)

• Behaviours are released in a stimulus response
  way, without an external program explicitly
  coordinating and controlling them.
• Layers of Competence
• The layers reflect a hierarchy of intelligence
  or competence.
• ? lower layers encapsulate basic survival
  functions (collisions)
• ? higher levels create more goal directed
  actions (mapping)
• Each of the layers can be viewed as an abstract
  behaviour for a particular task.
• Subsumption of lower layers
• - Modules in a higher layer can override or
  subsume the output from behaviours in the next
  lower layers.
• - Behaviour layers operate concurrently and
  independently and hence need mechanism to handle
  potential conflicts ? winner always takes all
  (always the highest layer)

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Subsumption Architecture (continued)

• No World Model (Internal State)
• - No persistence representation of the world
  model since information comes directly from the
  world.
• - Dangerous to depend on internal state since
  may diverge from reality.
• - Some internal state needed for releasing
  behaviours (i.e. scared, hungry), but good design
  minimises this.
• Taskable
• - Tasks are accomplished by activating the
  appropriate layer, which then activate the lower
  layers.
• - In practice, Subsumption style systems are not
  easily taskable, that is, they cant be ordered
  to do another task without being reprogrammed.

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Obstacle Avoidance Example

• Sensor A SONAR module that gives the distance to
  the objects in polar coordinates.
• Internal Modules
• COLLIDE ? detects if front obstacle is too close
  (i.e. halt)
• FEELFORCE ? sensor reading acts as repulsive
  force field
• RUNAWAY ? provides direction to move
• Actuators
• TURN ? provides motor output to turn robot
• FORWARD ? switches forward motion on or off

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Sonar Module
? Sonar module reads the sonar ranges. ? Polar
plot range readings in polar coordinates (r,?)
surrounding the robot.

• robo-centric view of range readings

Unrolled into a plot
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Level 0 Architecture obstacle avoidance
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Level 1 Architecture wander
Wander - Cant pass directly onto Turn since
will sacrifice obstacle avoidance
How does Turn know which module to take heading
from?
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Suppression replaces all other inputs to the
module with input coming from the suppressing
module.
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Inhibition blocks output from the specified
module for the defined time interval.
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Level 2 Architecture follow corridor
Integrate - estimates how far robot has travelled
off course ? Supplies dangerous internal state
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Evaluating Subsumption Architecture

• Modularity
• ? behaviours are modular, but generally favour a
  hardware implementation
• Niche targetability
• ? high due to direct perception.
• Portability
• ? limited to tasks which can be accomplished
  with reflexive behaviours
• ? generally cant be transferred where planning
  is needed
• Robustness
• ? offers graceful degradation if anything should
  disable a higher level behaviour, then the lower
  level can be left intact.

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Summary

• Layers of abstract behaviours
• ? achieved by grouping schemas like modules into
  layers.
• Suppression and inhibition of lower layers
• ? Higher layers may subsume inhibit behaviours
  in lower layers, but the behaviours in lower
  layers never rewritten or replaced.
• ? Mimics biological evolution (i.e. frogs with
  large objects)
• Difficult to design
• ? More of a art form than a science.
• Behaviour Release Mechanisms
• ? Behaviours are released by the presence of
  stimulus in the environment.

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Summary (continued)

• Solution to the Frame Problem
• ? Solves the frame problem by eliminating the
  need to model the world.
• ? Behaviours dont remember the past.
• Direct Perception and Affordance
• ? The release for a behaviour is almost always
  the percept for guiding the motor schema.
• Perception is Eco-centric Distributed
• ? i.e. sensing objects relative to the robot and
  the sensors can be shared among modules.

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References

• Robin R. Murphy - Introduction to AI robotics -
  Cambridge, Mass. MIT Press, 2000
• Jonathan SIMPSON, Christian L. JACOBSEN and
  Matthew C. JADUD - Mobile Robot Control - The
  Subsumption Architecture and occam-pi -
  Communicating Process Architectures 2006
• http//www.jonsimpson.co.uk/weblog/2006-09-24/mob
  ile-robot-control-the-subsumption-architecture-and
  -occam-pi.html