2
Organization
-SPA
-global
Strips
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
-Shakey
Rep. Arch.
• Solve a simple navigation problem using STRIPS (hint: work through
Sec. 2.2.2)
-evaluation
-NHC
• Understand precondition, closed world assumption, open world, frame
problem
-RCA
Summary
• List two advantages and disadvantages of the Hierarchical Paradigm
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
1
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
2
2
Organization
Organization
SENSE
-SPA
PLAN
ACT
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
World model:
1. A priori rep
2. Sensed info
3. Cognitive
Summary
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
3
2
Organization
Shakey
• First AI robot
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
• Built by SRI (Stanford
Research Institute)
for DARPA 1967-9
• Used Strips as main
algorithm for
controlling what to
do
Summary
Introduction to AI Robotics (MIT Press)
Chapter 2: The Hierarchical Paradigm
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2
Strips: Means-ends analysis
“Go to Stanford AI Lab”
Organization
-SPA
-global
INITIAL STATE:
Tampa, Florida (0,0)
GOAL STATE:
Stanford, California (1000,200)
Strips
-Shakey
Difference:
1020 miles
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
5
2
Difference Table
Distance
(difference)
Organization
-SPA
mode of transportation
(OPERATOR)
d>=200 miles
FLY
100<d<200
TRAIN
d<=100
DRIVE
d<1
WALK
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
mode=difference_table(INITIAL STATE, GOAL STATE, difference)
Summary
Introduction to AI Robotics
(MIT Press)
1. Look up what to do: FLY
2. Not at SAIL, so repeat
3. Look up what to do: DRIVE
Chapter 2: The Hierarchical Paradigm
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2
Preconditions
difference
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
OPERATOR
PRECONDITIONS
d>=200 miles
FLY
100<d<200
TRAIN
d<=100
DRIVE (rental)
at airport
DRIVE (personal car)
at home
d<1
WALK
How do I know if I’m at the airport or at home?
Now must keep up with the state of the world
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
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2
Maintaining State of the World:
Add and Delete Lists
distance
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
d>=200
miles
100<d<2
00
d<=100
d<1
Introduction to AI Robotics
(MIT Press)
OPERATOR
PRECONDITIONS
FLY
ADD-LIST
DELETELIST
at city Y
at airport
at city X
at city Y
at train
station
TRAIN
DRIVE
(rental)
at airport
DRIVE
(personal)
at home
at city X
WALK
Chapter 2: The Hierarchical Paradigm
8
2
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
Class Exercise
distance
OPERATOR
PRECONDITIONS
d<=200
miles
FLY
100<d<2
00
TRAIN
d<=100
DRIVE
(rental)
at airport
DRIVE
(personal)
at home
d<1
ADD-LIST
DELETELIST
at city Y
at airport
at city X
at city Y
at train
station
at city X
WALK
• Write down the world model, the operator applied, the change
in world state, etc. to go from Tampa to Stanford
Introduction to AI Robotics (MIT Press)
Chapter 2: The Hierarchical Paradigm
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2
Strips Summary
•
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
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
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
10
2
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
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
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
11
2
Nested Hierarchical Controller
(Meystel)
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
12
2
NHC Planner
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
13
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
14
2
RCS (Albus)
• 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
15
2
Examples of RCS Apps
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
16
2
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
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?
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
17
2
Evaluating NHC & RCS
• support for modularity:
Organization
-SPA
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
-RCA
Summary
– 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:
– RCS simulates plans in advance, but not sure
what it would do with sensor or mechanical
failures, etc.
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
18
2
Advantages & Disadvantages
of Hierarchies
Advantage of Hierarchal Paradigm
Orders the relationship between Sensing, Planning and Acting
• Natural way to organize
• Not basically rigid (it’s sort of flexible)
• Not basically inefficient (sort of organized)
– not the same as centralized planning
– priorities and goals are clear, therefore efficient
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
19
2
Advantages & Disadvantages
of Hierarchies
Disadvantages of Hierarchal Paradigm
• Planning bottleneck by updating cycle of the world model
and the planning.
- Sensing and planning algorithm were extremely slow.
• Eliminating of stimulus response (urgent actions)
- sensing and acting is disconnect.
• Frame problem -> Strips
• For Particular applications -> NHC
• Never handled Uncertainty
- semantics, sensor noise, actuator errors, action completion.
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
20
2 Nodes are made from Modules
xd is from “above”
sensors transform into x*
uff applies rule
actions
G is feedback
(transition rules)
u=uff+G(xd-x*)
u is control action
x* is predicted world state
xd is desired world state
uff is the feedforward control plan
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
21
2 Nodes are made from Modules
xd is from “above”
sensors transform into x*
G is feedback
actions
uff applies rule
(transition rules)
if BALL, move toward centroid
if NOT BALL, turn clockwise
(feedback determines how fast)
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
22
2 Nodes are made from Modules
xd is from “above”
sensors transform into x*
G is feedback
actions
uff applies rule
(transition rules)
when to stop?
how far is far enough?
what about noise/fuzzy ball?
…
sensor noise, actuator error,
rigid models
if BALL, move toward centroid
if NOT BALL, turn clockwise
(feedback determines how fast)
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
23
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
24
2
RCS-4 Levels
7
6
Battalion
Platoon
2-24h
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
25
2
Each Level has a RCS Node
Engineering of Mind, Albus & Mystel, 2001
Sensory Processing, World Modeling,
Behavior Generation, Value Judgment
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
26
2
Implementation View:
Nodes are Recursive
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
27
Demo III Control Hierarchy
2
Section 10m
PLANNER
vehicle1 vehicle2
Vehicle 1m
VEHICLE PLANNER
communications plan AM plan RSTA plan
Subsystem 5s
COMMS PLANNER
message list
Primitive 500ms
Servo 50ms
AM PLANNER
Driver Plan
Gaze plan
DRIVER PLANNER
Velocity Plan
RSTA PLANNER
gaze plan
GAZE PLANNER
Stereo Gaze Plan LADAR Gaze Plan
VELOCITY PLANNER
F Wheels R Wheels F Steer R Steer
F Wheel
R Wheel
Introduction to AI Robotics
(MIT Press)
F Steer
F Steer
Chapter 2: The Hierarchical Paradigm
28
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
29
2
t=0.5
Primitive Level:
Driver Plan
extends to A2
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
30
2
t=1 s
Obstacle Detected
Primitive Level:
Driver Plan
new waypoints
Vehicle Level:
detects too large a
variation
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
31
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
32
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
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2
t=4.5s
Vehicle Level:
new AM Plan
skip A1, go to A2
Primitive Level:
new Driver Plan
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
34
2
t=6 s
Vehicle Level:
new AM Plan
skip old A2
Primitive Level:
new Driver Plan
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
35
Exercise: Adapt to Rescue Robots?
2
Section 10m
PLANNER
vehicle1 vehicle2
Vehicle 1m
VEHICLE PLANNER
communications plan AM plan RSTA plan
Subsystem 5s
COMMS PLANNER
message list
Primitive 500ms
Servo 50ms
AM PLANNER
Driver Plan
Gaze plan
DRIVER PLANNER
Velocity Plan
RSTA PLANNER
gaze plan
GAZE PLANNER
Stereo Gaze Plan LADAR Gaze Plan
VELOCITY PLANNER
F Wheels R Wheels F Steer R Steer
F Wheel
R Wheel
Introduction to AI Robotics
(MIT Press)
F Steer
F Steer
Chapter 2: The Hierarchical Paradigm
36
2
Organization
-SPA
Hierarchical Review
• Describe the hierarchical paradigm in terms of the three
robot primitives
• Describe sensing in the hierarchical paradigm
-global
Strips
-Shakey
Rep. Arch.
-evaluation
-NHC
•
•
•
•
What is STRIPS?
What is the closed world assumption?
What are preconditions?
What is the frame problem?
-RCA
Summary
• What are two representative architectures?
• What is the NHC decomposition?
Introduction to AI Robotics
(MIT Press)
Chapter 2: The Hierarchical Paradigm
37