7 reactive and behavior-based paradigm.pptftang/courses/cs521/notes/reactive...
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Reactive & Behavior-Based Paradigm
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Objectives To understand two dominant methods for combining behaviors in a
reactive architecture: Subsumption Motor schemas (potential fields)
Evaluate subsumption and potential fields architectures in terms of: Support for modularity Niche targetability Ease of portability to other domains Robustness
Be able to program using potential fields
Be able to construct a new potential field from primitive potential fields, and sum potential fields to generate an emergent behavior
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Reactive Systems
A reactive robotic system tightly couples perception to action without the use of intervening abstract representations or time history The tight coupling of sensing and acting allows robots to
operate in real-time
Recall: reactive paradigm
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Attributes of Reactive Paradigm
All actions are accomplished through behaviors Behaviors are a direct mapping of sensory inputs to a pattern of
motor actions that are used to achieve a task We treat it as perceptual schema + motor schema
No PLAN component, all robotic activities emerge as the behaviors operating either in sequence or concurrently S-A doesn’t specify how the behaviors are coordinated; it is
addressed by architecture
Sensing is local to each behavior, or behavior specific One sensor, one behavior Multiple sensors, one perceptual schema, one behavior (sensor
fusion)
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Characteristics of Reactive Paradigm
Robots are situated agents operating in an ecological niche It is an integrated part of the world
Behaviors serve as basic building blocks for robotic actions Robotic activities emerge as the result of these behaviors
operating in sequence or concurrently
Use of explicit abstract representational knowledge is avoidedin the generation of a response Only local, behavior-specific sensing is allowed
Animal models of behavior often serve as a basis for these systems
These systems are inherently modular from a software design perspective
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Comparing Alternative Architectures
A reactive architecture provides mechanism for: triggering behaviors determining what happens when multiple behaviors are active in the
same time
Commonalities: Emphasis on the importance of coupling sensing and action tightly Avoidance of representational symbolic knowledge Decomposition into contextually meaningful units (behaviors)
Distinctions: Granularity of behavioral decomposition Basis for behavior specification (ethological, situated activity,
experimental) Response encoding method (discrete vs. continuous) Coordination methods (competitive vs. cooperative)
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Example Reactive/Behavior-Based Robots
Genghis, MIT
Subsumption
Callisto, GATech
Potential Field
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Evaluating Architectures
How do we evaluate an architecture’s suitability for a particular problem? Hardware targetability Support for modularity Robustness Timeliness in development Run time flexibility Performance effectiveness
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Subsumption Architecture Developed in mid-1980s by Rodney Brooks, MIT
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Tenets of the Subsumption Architecture
Complex behavior need not be the product of a complex control system
Intelligence is the eye of the observer The world is its own best model Simplicity is a virtue Robots should be cheap Robustness in the presence of noisy or failing sensors is a
design goal Planning is just a way of avoiding figuring out what to do
next All onboard computation is important Systems should be built incrementally No representation. No calibration. No complex computers. No
high-bandwidth communication
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Categorization of Subsumption Architecture
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Subsumption Robots Allen Tom and Jerry Genghis and Attila Squirt Toto Seymour Tito Polly Cog
“Veteran” robots of the MIT AI Lab using the subsumption architecture
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Some Aspects of Subsumption A behavior is a network of sensing and acting modules, which are
augmented finite state machines (AFSM)
Modules are grouped into layers of competence The layers reflect a hierarchy of intelligence Lower layers: basic survival functions Higher layers: more goal-directed actions
Modules in a higher layer can override, or subsume, the output from behaviors in the next lower layer Behavioral layers operate concurrently and independently
The use of internal state is avoided Some internal state is needed for releasing behaviors, but good behavioral
designs minimize this
A task is accomplished by activating the appropriate layer, which then activates the lower layers below it, and so on However, subsumption style systems are not easily taskable
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Coordination in Subsumption
“Subsumption” comes from coordination process used between layered behaviors of architecture
Complex actions subsume simpler behaviors Fixed priority hierarchy defines topology Lower levels of architecture have no “awareness”
of upper levels
Coordination has two mechanisms: Inhibition Suppression
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Inhibition and Suppression
Inhibition: the output of the subsuming module is connected to the
output of another module if the subsuming module is “on” or has any value, the
output of the subsumed module is blocked or turned “off” acts like a faucet
Suppression the output of the subsuming module is connected to the
input of another module if the output of the subsuming module is on, it replaces
the normal input to the subsumed module acts like a switch
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Foraging Example
Foraging: Robot moves away from home base looking for
attractor objects When detect attractor object, move toward it,
pick it up, and return it to home base Repeat until all attractors been collected
High-level behaviors: Wander Acquire (pick up) Retrieve (homing)
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Organization of Subsumption-Based Foraging Robot
I
I
I
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Example
Develop a robot that can follow the center of the corridor, while avoiding obstacles
This robot has multiple sonars (or other range sensors), each pointing in a different direction; and two actuators, one for driving forward, one for turning
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Example Perception: Polar Plot
if sensing is ego-centric, canoften eliminate need for memory, representation
Plot is ego-centric
Plot is distributed (available to whoever 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”)
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Level 0: Runawayrunaway 0
wander 1
follow-corridor 2
HALT
COLLIDE
PS MS
RUN AWAYPS MS
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Level 1: Wanderrunaway 0
wander 1
follow-corridor 2
encoders
AVOID
PS
MS
WANDER
PS MS
Note sharing ofPerception, fusion
What wouldthis do?
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Level 2: Follow Corridorsrunaway 0
wander 1
follow-corridor 2
STAY-IN-MIDDLE
PS MS
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Class Exerciserunaway 0
wander 1
move2light 2
LIGHT move-to-light
S
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Genghis Subsumption Design
Behavioral layers implemented: Standup Simple walk Force balancing Leg lifting Whiskers Prowling Steered prowling
Two motors per leg:advance, which swings leg back and forth
balance, which lifts leg up and down
http://www.youtube.com/watch?v=BUxFfv9JimUhttp://www.youtube.com/watch?v=RKeBl0-msGQ
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Genghis AFSM Network
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Evaluation of Subsumption Strengths:
Hardware retargetability: subsumption can compile down directly onto programmable-array logic circuitry
Support for parallelism: each behavioral layer can run independently and asynchronously
Niche targetability: custom behaviors can be created for specific task-environment pairs
Null (not strength/not weakness): Robustness: can be successfully engineered into system but is often
hard-wired and hard to implement Timeliness for development: some support tools exist, but significant
learning curve exists
Weakness: Run time flexibility: priority-based coordination mechanism, ad hoc
aspect of behavior generation, and hard-wired aspects limit adaptation of system
Support for modularity: behavioral reuse is not widely done in practice
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Next …
Motor schemas
Potential fields
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Motor Schemas
Based upon schema theory: Explains motor behavior in terms of concurrent
control of many different activities Schema stores both how to react and the way
that reaction can be realized A distributed model of computation
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Motor Schemas Developed by Arkin in 1980s Based on biology’s schema theory Behavioral responses are all represented as vectors generated using
a potential fields approach Coordination is achieved by vector addition
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Categorization of Motor Schemas
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Differences of Motor Schemas v.s. Other Behavioral Approaches
Behavioral responses are all represented as vectors generated using a potential fields approach
Coordination is achieved by vector addition
No predefined hierarchy exists for coordination; instead, behaviors are configured at run-time
Pure arbitration is not used; each behavior can contribute in varying degrees to robot’s overall response
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Perception-Action Schema Relationships
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Schema-Based Robots
HARV George Ren and Stimpy Buzz Io, Callisto, Ganymede Mobile manipulator
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Output of Motor Schemas Defined as Vectors
Output Vector: consists of both orientation and magnitude components
Vmagnitude denotes magnitude of resultant response vector Vdirection denotes orientation Vector represents a force Typically drawn as an arrow
Potential field: array (or field) of vectors representing space
Length of arrow = m = magnitude
Angle of arrow = d = direction
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Potential Fields (con’t.) Vector space is 2D world, like bird’s eye view of map Map divided into squares, creating (x, y) grid Each element represents square of space Perceivable objects in world exert a force field on surrounding space
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Five Primitive Potential Fields
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Magnitude Profiles
Change in velocity in different parts of the field
Field closer to an attractor/repellor will be stronger
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Programming a Single Potential Field
Repulsive field with linear drop-off:
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Pseudocode for Single Repulsive Field
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Runaway Behavior
Assume we have a robot with a single sonar readSonar() is a perceptual schema that returns
a distance turn(dir) will turn the robot in the “dir” direction forward(mag) will set the speed of the robot
How do we use the repulsive potential field to program the runaway behavior?
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vector runaway() {double reading;reading = readSonar(); // psvoutput = repulsive(reading, MAX_DIST); // msreturn voutput;
}while (robot is on) {
vrunaway = runaway();turn(vrunaway.direction);forward(vrunaway.magnitude);
}
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Important Note
Entire field does not have to be computed
Only portion of field affecting robot is computed
Robot uses functions defining potential fields as its position to calculate component vector
Fine-tune update rate to generate a smooth path
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More Motor Schema Encodings Move-to-goal:
Vmagnitude = fixed gain value Vdirection = towards perceived goal Tolerance around the goal +/- 0.5m
Avoid-static-obstacle:
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More Motor Schema Encodings Stay-on-path:
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More Motor Schema Encodings
Move-ahead:
Noise:
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Combing Fields/Behaviors Compute each behavior’s potential field Sum vectors at robot’s position to get resultant
output vector
?
+
=
Issues: local minima: vectors may
sum to zero
R = Σ (Gi · Ri)
Solutions?
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Solutions for Dealing with Local Minima
Inject noise, randomness: “Bumps” robot out of minima
Include “avoid-past” behavior: Remember where robot has been and attracts the robot to
other places
Use “Navigation Templates”: The “avoid” behavior receives as input the vector summed
from other behaviors Gives “avoid” behavior a preferred direction
Insert tangential fields around obstacles
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Recall Our Previous Example
Develop a robot that can follow the center of the corridor, while avoiding obstacles
This robot has multiple sonars (or other range sensors), each pointing in a different direction; and two actuators, one for driving forward, one for turning
Sonsor
0
12
3
4
5 6
7
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Potential Field Solution (1)
Runaway behavior
while (robot is on) {vector.mag = vector.dir = 0; // initializationfor (i = 0; i <= numSonar; i++) {
vectorCurrent = Runaway(i);vectorOutput = VectorSum(Vector, vectorCurrent);
}turn(vector.dir);forward(vector.mag);
}
runaway 0
wander 1
follow-corridor 2
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Potential Field Solution (2)
Collide is treated as “panic” situations, triggering an emergency response outside the potential field framework
runaway 0
wander 1
follow-corridor 2
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Robot in a Box Canyon
0
12
3
4
5 6
7
What does the vector field look like?Can this robot escape this canyon?
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Alternative Implementation of Runaway
Runaway to process all 8 sonar readings
Can be used only with a robot that has 8 range sensors
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Potential Field Solution (3)
Wander behavior Follow corridor
runaway 0
wander 1
follow-corridor 2
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Motor Schemas Achieve Behavioral Fusion via Vector Summation
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S-R Diagram for Cooperating Behaviors in Foraging
Wander
Avoid-obstacle Σ
Σ
Deliver-home
Σ
no attractor
obstacle
robot
attractor
obstacle
robot
local minima
homebase
obstacle
robot
Sequencer
Avoid-obstacle
Avoid-obstacle
Avoid-obstacle
Avoid-obstacle
Avoid-obstacle
Noise
Noise
Goto-attractor
Wander
Acquire
Deliver
local minima
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Sequencing of Motor Schemas
Can sequence motor schemas if one activity needs to be completed before another
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Callisto, the Foraging Robot @Georgia Tech Winners in a robot competition at AAAI-94 http://www.cc.gatech.edu/ai/robot-lab/research/aaai94.html
Trash-collecting robots
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More Exercises
Construct a potential field for the following cases: move to a goal position with an obstacle in between move through a door
Please identify the potential fields that you use and draw the potential field
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Advantages and Disadvantages of the Potential Field Approach
Advantages: It is easier to visualize the robot’s overall
behavior It is easier to combine fields It can be parameterized It can be extended from 2D to 3D
Disadvantages: Local minima problem
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Design in Motor-Schema-Based System
Characterize problem in terms of motor behaviors needed to accomplish task
Decompose motor behaviors to most primitive level, using biological studies when feasible
Develop formulas to express robot’s reaction to perceived environmental events
Conduct simple simulation studies to assess desired behaviors’ approximate performance
Determine perceptual requirements needed to satisfy motor schema inputs
Design specific perceptual algorithms Integrate control system onto robot Test and Evaluate Iterate and expand as needed
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Evaluation of Reactive Architectures
Support for modularity Both decompose the actions and perceptions into
behaviors
Niche targetability Both high The use of direct perception emphasizes that reactive
robots are constructed to fill a niche
Portability to other domains Limited to applications that can be accomplished with
reflexive behaviors Potential fields approach performs better
Robustness No specific mechanisms to support
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Class So Far
History of robotics Robot software design strategies Robotics paradigms and architectures Behavior encoding, combinations, etc. Pros, cons, evaluation of various architectures Programming implementations/pseudocodes of
above