2 introduction to ai robotics (mit press)chapter 2: the hierarchical paradigm1 the hierarchical...
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Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 1
2 The Hierarchical Paradigm• Describe the Hierarchical Paradigm in terms of the 3 robot
primitives and its organization of sensing
• Name and evaluate one representative Hierarchical architecture in terms of: support for modularity, niche targetability, ease of portability to other domains, robustness
• Solve a simple navigation problem using STRIPS (hint: work through Sec. 2.2.2)
• Understand precondition, closed world assumption, open world, frame problem
• List two advantages and disadvantages of the Hierarchical Paradigm
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 2
2 Organization
PLANSENSE ACT
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
World model:1. A priori rep2. Sensed info3. Cognitive
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 3
2 Shakey
• First AI robot
• Built by SRI (Stanford Research Institute) for DARPA 1967-9
• Used Strips as main algorithm for controlling what to do
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 4
2 Strips: Means-ends analysis
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
INITIAL STATE: Tampa, Florida (0,0)
GOAL STATE: Stanford, California (1000,200)
Difference: 1020 miles
“Go to Stanford AI Lab”
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 5
2 Difference Table
d<=200 miles FLY
100<d<200 TRAIN
d<=100 DRIVE
Distance(difference)
mode of transportation(OPERATOR)
d<1 WALK
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
mode=difference_table(INITIAL STATE, GOAL STATE, difference)
1. Look up what to do: FLY2. Not at SAIL, so repeat3. Look up what to do: DRIVE
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 6
2 Preconditions
d<=200 miles FLY
100<d<200 TRAIN
d<=100 DRIVE (rental)
DRIVE (personal car)
difference OPERATOR
d<1 WALK
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
How do I know if I’m at the airport or at home?Now must keep up with the state of the world
at airport
at home
PRECONDITIONS
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 7
2 Maintaining State of the World:Add and Delete Lists
d<=200 miles
FLY
100<d<200
TRAIN
d<=100 DRIVE (rental)
at airport
DRIVE (personal)
at home
distance OPERATOR PRE-CONDITIONS
d<1 WALK
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
at city Yat airport
at city Yat train station
ADD-LIST
at city X
at city X
DELETE-LIST
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 8
2 Class Exercise
• Write down the world model, the operator applied, the change in world state, etc. to go from Tampa to Stanford
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
d<=200 miles
FLY
100<d<200
TRAIN
d<=100 DRIVE (rental)
at airport
DRIVE (personal)
at home
distance OPERATOR PRE-CONDITIONS
d<1 WALK
at city Yat airport
at city Yat train station
ADD-LIST
at city X
at city X
DELETE-LIST
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 9
2 Strips Summary
• Designer must set up– World model representation
– Difference table with operators, preconditions, add & delete lists
– Difference evaluator
• Strips assumes closed world– Closed world: world model contains everything needed for
robot (implication is that it doesn’t change)
– Open world: world is dynamic and world model may not be complete
• Strips suffers from frame problem– Frame problem: representation grows too large to reasonably
operate over
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 10
2 Architecture• provides a principled way of organizing a control system.
However, in addition to providing structure, it imposes constraints on the way the control problem can be solved [Mataric]
• describes a set of architectural components and how they interact [Dean & Wellman]
• Types of architectures [Levis, George Mason University]– operational architecture: describes what the systems does, not how
it does it
– systems architecture: describes how a system works in terms on major subsystems
– technical architecture: implementation details
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 11
2 Evaluating an Architecture
• support for modularity: does it show good software engineering principles?
•
• niche targetability: how well does it work for the intended application?
• ease of portability to other domains: how well would it work for other applications or other robots?
• robustness: where is the system vulnerable, and how does it try to reduce that vulnerability?
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 12
2 Hierarchical Paradigm…
• Top-down: – Plan, plan, plan
• Control-theoretic: – must measure error in order to control device
• Planning means:– dependence on world models
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 13
2 Nested Hierarchical Controller(Meystel)
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 14
2 NHC Planner
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 15
2 RCS (Albus)
• the hierarchy
• how the hierarchy works for navigation
• how it is implemented– nodes and modules
– planning time periods
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 16
2 Examples of RCS Apps
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 17
2 RCS-4 Levels
7 Battalion 2-24h
6 Platoon 5m-2h
5 Section 1-10m
4 Individual Vehicle 5-50s
3 Subsystem Level 200-500ms
2 Primitive Level 50-500ms
1 Servo Level 5-50ms
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 18
2 Each Level has a RCS Node
Sensory Processing, World Modeling,Behavior Generation, Value Judgment
Sensory Processing, World Modeling,Behavior Generation, Value Judgment
Engineering of Mind, Albus & Mystel, 2001
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 19
2 Implementation View:Nodes are Recursive
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 20
2 Demo III XUV
http://museum.nist.gov/exhibits/timeline/item.cfm?itemId=38
Experimental Unmanned Vehicle in action at Ft. Indiantown Gap. Photo courtesy of the Army Research Labs. Nov. 2001
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 21
2 Demo III Control HierarchyPLANNER
vehicle1 vehicle2
VEHICLE PLANNERcommunications plan AM plan RSTA plan
AM PLANNERDriver Plan Gaze plan
COMMS PLANNERmessage list
RSTA PLANNERgaze plan
DRIVER PLANNERVelocity Plan
GAZE PLANNERStereo Gaze Plan LADAR Gaze Plan
VELOCITY PLANNERF Wheels R Wheels F Steer R Steer
F Wheel R Wheel F Steer F Steer
Servo 50ms
Primitive 500ms
Subsystem 5s
Vehicle 1m
Section 10m
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 22
2 RCS XUV Example
Vehicle Level:AM Plan(A1…A10)
Primitive Level:Driver Plan(D1…D10)
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 23
2 t=0.5
Primitive Level:Driver Planextends to A2
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 24
2 t=1 s
Obstacle Detected
Primitive Level:Driver Plannew waypoints
Vehicle Level:detects too large avariation
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 25
2 t=1 s Vehicle Level Planner Opt 1
Vehicle Level:new AM Plan
Primitive Level:new Driver Plan
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 26
2 t=3
More obstacle is seen…fail upwards again
Vehicle Level:new AM Plan
Primitive Level:new Driver Plan
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 27
2 t=4.5s
Vehicle Level:new AM Planskip A1, go to A2
Primitive Level:new Driver Plan
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 28
2 t=6 s
Vehicle Level:new AM Planskip old A2
Primitive Level:new Driver Plan
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 29
2 Exercise: Adapt to Rescue Robots?PLANNER
vehicle1 vehicle2
VEHICLE PLANNERcommunications plan AM plan RSTA plan
AM PLANNERDriver Plan Gaze plan
COMMS PLANNERmessage list
RSTA PLANNERgaze plan
DRIVER PLANNERVelocity Plan
GAZE PLANNERStereo Gaze Plan LADAR Gaze Plan
VELOCITY PLANNERF Wheels R Wheels F Steer R Steer
F Wheel R Wheel F Steer F Steer
Servo 50ms
Primitive 500ms
Subsystem 5s
Vehicle 1m
Section 10m
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 30
2 Nodes are made from Modules
uff applies rule(transition rules)
sensors actions
u=uff+G(xd-x*)u is control action
x* is predicted world statexd is desired world state
uff is the feedforward control plan
transform into x*G is feedbackxd is from “above”
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 31
2 Nodes are made from Modules
uff applies rule(transition rules)
sensors actions
if BALL, move toward centroidif NOT BALL, turn clockwise
(feedback determines how fast)
transform into x*G is feedbackxd is from “above”
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 32
2 Nodes are made from Modules
uff applies rule(transition rules)
sensors actions
if BALL, move toward centroidif NOT BALL, turn clockwise
(feedback determines how fast)
transform into x*G is feedbackxd is from “above”
when to stop?how far is far enough?what about noise/fuzzy ball?…sensor noise, actuator error,rigid models
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 33
2 Advantages of Hierarchies
Albus and Mystel 01:
• Natural way to organize• Not intrinsically rigid• Not intrinsically inefficient
– not the same as centralized planning
– priorities and goals are clear, therefore efficient
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 34
2 Summary RCS
• hierarchy with node structure at each level– have operator interface (in theory)
• nodes consist of– Sensory Processing– World Model– Behavior Generation– Value Judgment
• top-down, plan for a particular horizon– control theoretic
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 35
2 Evaluating the Two Architectures• support for modularity:
– decomposition by functionality
• niche targetability: – good, both have been used for apps like vehicle guidance,
mining equipment
• ease of portability to other domains: – unclear, not sure if code could be reused—lots of rewriting on
previous apps
• robustness:– RCA simulates plans in advance, but not sure what it would do
with sensor or mechanical failures, etc.
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary
Introduction to AI Robotics (MIT Press) Chapter 2: The Hierarchical Paradigm 36
2 Hierarchical Review• Describe the hierarchical paradigm in terms of the
three robot primitives• Describe sensing in the hierarchical paradigm
• What is STRIPS?
• What is the closed world assumption?• What are preconditions?• What is the frame problem?
• What are two representative architectures?• What is the NHC decomposition?
Organization-SPA-globalStrips-ShakeyRep. Arch.-evaluation-NHC-RCASummary