Upload
katell-potts
View
58
Download
0
Tags:
Embed Size (px)
DESCRIPTION
CS 547: Sensing and Planning in Robotics. Gaurav S. Sukhatme Computer Science Robotics Research Laboratory University of Southern California [email protected] http://robotics.usc.edu/~gaurav. Administrative Matters. Signup - please fill in the details on one of the sheets - PowerPoint PPT Presentation
Citation preview
CS 547: Sensing and Planning in Robotics
Gaurav S. Sukhatme
Computer Science
Robotics Research Laboratory
University of Southern California
http://robotics.usc.edu/~gaurav
Administrative Matters
• Signup - please fill in the details on one of the sheets
• Web page http://robotics.usc.edu/~gaurav/CS547
• Email list [email protected]
• Grading (3 quizzes 45%, class participation 5%, and project 50%)
• TA: Mike Rubenstein ([email protected])
• Note: First quiz on Sept 8, scores available on Sept 10 by 1pm
Project and Textbook
• Project– Team projects only (exceptions are hard to obtain)– Equipment (Pioneer/Segway robots with sensors,
Player/Stage/Gazebo software)
• Book– Probabilistic Robotics (Thrun, Burgard, Fox)– Available at the Paper Clip in University Village (North of
Jefferson Blvd.)
I expect you to
• come REGULARLY to class• visit the class web page FREQUENTLY• read email EVERY DAY• SPEAK UP when you have a question• START EARLY on your project
• If you don’t– the likelihood of learning anything is small– the likelihood of obtaining a decent grade is small
In this course you will
– learn how to address the fundamental problem of robotics i.e. how to combat uncertainty using the tools of probability theory
– We will explore the advantages and shortcomings of the probabilistic method
– Survey modern applications of robots– Read some cutting edge papers from the
literature
Syllabus and Class Schedule• 8/25 Introduction, math review• 9/1 Math review, Bayes filtering• 9/8 Math review, Quiz 1• 9/15 Quiz 1 discussion, Player/Stage/Gazebo tutorial• 9/22 Kalman and Particle filters• 9/29 Probabilistic Kinematics• 10/6 Sensor Models• 10/13 Localization papers• 10/20 No class, but Project Proposals due to the TA• 10/27 Cooperative localization papers and Quiz 2• 11/3 Quiz 2 discussion, Mapping• 11/10 SLAM• 11/17 Acting under uncertainty• 11/24 Quiz 3• 12/1 Final project demos
What is robotics/a robot ?
• Background– Term robot invented by Capek in to mean a
machine that would willing and ably do our dirty work for us
– The first use of robotics as a word appears in Asimovs science
• Definition (Brady): Robotics is the intelligent connection of perception of action
Trends in Robotics Research
Reactive Paradigm (mid-80’s)• no models• relies heavily on good sensing
Probabilistic Robotics (since mid-90’s)• seamless integration of models and sensing• inaccurate models, inaccurate sensors
Hybrids (since 90’s)• model-based at higher levels• reactive at lower levels
Classical Robotics (mid-70’s)• exact models• no sensing necessary
Robots are moving away from factory floors to Entertainment, Toys, Personal service. Medicine, Surgery, Industrial automation (mining, harvesting), Hazardous environments (space, underwater)
Tasks to be Solved by Robots Planning Perception Modeling Localization Interaction Acting Manipulation Cooperation ...
Uncertainty is Inherent/Fundamental
• Uncertainty arises from four major factors:– Environment is stochastic, unpredictable– Robots actions are stochastic– Sensors are limited and noisy– Models are inaccurate, incomplete
Probabilistic Robotics
Key idea: Explicit representation of uncertainty using the calculus of probability theory
• Perception = state estimation
• Action = utility optimization
Advantages and Pitfalls
• Can accommodate inaccurate models
• Can accommodate imperfect sensors
• Robust in real-world applications
• Best known approach to many hard robotics problems
• Computationally demanding
• False assumptions
• Approximate
Pr(A) denotes probability that proposition A is true.
•
•
•
Axioms of Probability Theory
1)Pr(0 A
1)Pr( True
)Pr()Pr()Pr()Pr( BABABA
0)Pr( False
Discrete Random Variables
• X denotes a random variable.
• X can take on a finite number of values in {x1, x2, …, xn}.
• P(X=xi), or P(xi), is the probability that the random variable X takes on value xi.
• P( ) is called probability mass function.
• E.g.02.0,08.0,2.0,7.0)( RoomP
.
Continuous Random Variables
• X takes on values in the continuum.
• p(X=x), or p(x), is a probability density function.
• E.g.
b
a
dxxpbax )(]),[Pr(
x
p(x)
Joint and Conditional Probability
• P(X=x and Y=y) = P(x,y)
• If X and Y are independent then
P(x,y) = P(x) P(y)
• P(x | y) is the probability of x given y
P(x | y) = P(x,y) / P(y)
P(x,y) = P(x | y) P(y)
• If X and Y are independent then
P(x | y) = P(x)
Law of Total Probability
y
yxPxP ),()(
y
yPyxPxP )()|()(
x
xP 1)(
Discrete case
1)( dxxp
Continuous case
dyypyxpxp )()|()(
dyyxpxp ),()(
Reverend Thomas Bayes, FRS (1702-1761)
Clergyman and mathematician who first used probability inductively and established a mathematical basis for probability inference
Normalization
)()|(
1)(
)()|()(
)()|()(
1
xPxyPyP
xPxyPyP
xPxyPyxP
x
yx
xyx
yx
yxPx
xPxyPx
|
|
|
aux)|(:
aux
1
)()|(aux:
Algorithm:
Conditioning
• Total probability:
• Bayes rule and background knowledge:
)|(
)|(),|(),|(
zyP
zxPzxyPzyxP
dzyzPzyxPyxP )|(),|()(
Causal vs. Diagnostic Reasoning
• P(open|z) is diagnostic.
• P(z|open) is causal.
• Often causal knowledge is easier to obtain.
• Bayes rule allows us to use causal knowledge:
)()()|(
)|(zP
openPopenzPzopenP
count frequencies!
Example
• P(z|open) = 0.6 P(z|open) = 0.3• P(open) = P(open) = 0.5
67.03
2
5.03.05.06.0
5.06.0)|(
)()|()()|(
)()|()|(
zopenP
openpopenzPopenpopenzP
openPopenzPzopenP
• z raises the probability that the door is open.
Combining Evidence
• Suppose our robot obtains another observation z2.
• How can we integrate this new information?
• More generally, how can we estimateP(x| z1...zn )?
Recursive Bayesian Updating
),,|(
),,|(),,,|(),,|(
11
11111
nn
nnnn
zzzP
zzxPzzxzPzzxP
Markov assumption: zn is independent of z1,...,zn-1 if we know x.
)()|(
),,|()|(
),,|(
),,|()|(),,|(
...1...1
11
11
111
xPxzP
zzxPxzP
zzzP
zzxPxzPzzxP
ni
in
nn
nn
nnn
Example: Second Measurement
• P(z2|open) = 0.5 P(z2|open) = 0.6
• P(open|z1)=2/3
625.08
5
31
53
32
21
32
21
)|()|()|()|(
)|()|(),|(
1212
1212
zopenPopenzPzopenPopenzP
zopenPopenzPzzopenP
• z2 lowers the probability that the door is open.
Actions
• Often the world is dynamic since– actions carried out by the robot,– actions carried out by other agents,– or just the time passing by
change the world.
• How can we incorporate such actions?
Typical Actions
• The robot turns its wheels to move• The robot uses its manipulator to grasp an
object• Plants grow over time…
• Actions are never carried out with absolute certainty.
• In contrast to measurements, actions generally increase the uncertainty.
Modeling Actions
• To incorporate the outcome of an action u into the current “belief”, we use the conditional pdf
P(x|u,x’)
• This term specifies the pdf that executing u changes the state from x’ to x.
State Transitions
P(x|u,x’) for u = “close door”:
If the door is open, the action “close door” succeeds in 90% of all cases.
open closed0.1 1
0.9
0
Integrating the Outcome of Actions
')'()',|()|( dxxPxuxPuxP
)'()',|()|( xPxuxPuxP
Continuous case:
Discrete case: