CS 415 – A.I.
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CS 415 – A.I. Slide Set 3. Representation and Search. Representational System – function is to capture the essential features of a problem domain and make the information accessible Abstraction – being able to efficiently store the features of the problem domain

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CS 415 – A.I.

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Cs 415 a i

CS 415 – A.I.

Slide Set 3

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Representation and Search

  • Representational System – function is to capture the essential features of a problem domain and make the information accessible

    • Abstraction – being able to efficiently store the features of the problem domain

      • Note: the features will undoubtedly change

    • Balance trade-offs between efficiency and expressiveness

Representation in mobile robotics

Representation in Mobile Robotics

  • Kinematics – basic study of how mechanisms move

    • Basic goal: given all the angles and movement, what is you point in space at this time

    • 2 Frames of Reference

      • Global Frame of Reference

        • Robot gets through layers of representations (maps, etc)‏

      • Local Frame of Reference

        • Don't know what the world looks like

        • Remember how far I've traveled

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  • Given global frame of reference

    • If the robot moves how do we keep up with where we are in space

    • Kinematic Equations

      • A system of equations that determines our x,y position and our rotation (angle Θ) after k control steps

      • See second page of ARL paper

        • Synchro Drive Robots

        • Also exist for Differential Drive Robots

      • Store as a matrix system

        • Perform matrix operations to transform and solve

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  • Regardless, everyone should work in the same frame of reference

    • Homogenous Transform

      • Do matrix operations to transform from one frame of reference to another

How far has the robot moved

How far has the robot moved?

  • Apply power, robot moves, right?

    • Power does not relate well to speed

  • So, other options:

    • Check the particular motor velocity (left/right)‏

    • Some visual cue for how fast you're going

    • Encode inside motor

      • How many 'ticks' has the motor made as it rotated

      • Use PID, proportional integral derivative

        • Integral and derivative → smooths error/gives result



  • How do we abstract a map?

  • Efficiency vs Expressiveness

    • What are the tradeoffs

  • Dealing with Errors

    • Examples:

      • Synchro Drive

      • Differential Drive

Accurate relative localization using odometry

Accurate Relative Localization Using Odometry

  • Drawing Maps

    • Depends on relative localization

      • Can't escape the use of odometry

    • Have error built in

  • Overcoming Error

    • Odometry error modeling

    • Error Parameters estimation

    • Covariance matrix estimation

    • 1 and 2 – systematic errors

    • 3 – non-systematic errors

Systematic errors

Systematic Errors

  • Define these for the robot based on the appropriate error model for the drive type

    • Differential Drive → given in the literature

      • Borenstein paper

    • Synchro Drive → given in this paper

      • Major source: wheel misalignment

        • Major source of distortion for theta (angular velocity): drag AND rotate the robot

        • Provable by geometric analysis of kinematic equation

Non systematic errors

Non-systematic Errors

  • PC (POSTECH CMU)-method

    • 1st get the error model

    • 2nd use PC-method to generate error parameters and covariance matrix

  • Based on sensor-based navigation through the Generalized Voronoi Graph (GVG)‏

    • Voronoi extensively covered in literature

    • Creates a well-understood path based on obstacles

    • Robot drives it forward (FOP) and backward (BOP)‏

      • 2 diff odom paths, same real-world path

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  • Give an initial CFOP and CBOP based on error model and initial error parameters guess

  • Then, find the error parameters that minimize error between CFOP and CBOP

    • Steepest descent method

  • Now, build an error covariance matrix based on 3 assumptions and worst-case analysis

A little error uncorrected tends to flourish

A little error, uncorrected, tends to flourish

  • See Fig 8

    • Note: one possible approach, reset the odometry before the error gets too bad

  • See Fig 9

  • See Fig 12

  • See Fig 13

Representation search considerations

Representation/Search - Considerations

  • Real-time Systems

    • Is it schedulable

      • Has a lot to do with efficiency/expressiveness

    • How are we storing things (fast to slow)‏

      • I/O

      • Memory

      • Registers

      • Cache

    • What Language are we using (fast to slow)‏

      • Assembly

      • C

      • C++

      • Java

      • Python

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Other Forms of Representation

  • Example: A robot might be stacking elements from a table on top of one another

  • Might give the following predicates (state facts about our domain):









  • Might also define a set of rules which relate to these predicates

    For all X if there does not exist any Y where on(Y,X) than this implies clear(X)‏

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Using Predicate Calculus

  • Predicates can also be more advanced







  • Predicates are not functions in the sense of higher-level languages, nor should you think of them in terms of programming

    • There is no set of predicate functions

    • Any predicate can be defined

      • They are strictly useful for representing knowledge in conjunction with rules

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  • What are the possible moves?

    • The computer knows because of the knowledge representation

      • All moves are either stored or can be inferred from the stored knowledge and set of rules.

  • What is the best move?

    • This is the domain of search

    • Example: Tic Tac Toe

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Limitations of State-Space Search

  • Not sufficient to automate intelligent behavior

    • How big is the state-space for chess?

      • 10120 different board configurations

        • Larger than # molecules in the universe

        • Larger than the number of seconds since the “big bang”

    • How big is the state-space for human language?

      • Untold possibilities

  • State-Space Representation and Search is an important tool only

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Exhaustive Search vs. Heuristic Search

  • Exhaustive Search

    • Brute force attempting all possible combinations till an optimized solution is found

  • Heuristic Search

    • Humans don’t use exhaustive search

    • Instead, we use rules of thumb based on what seems most “promising”

    • Heuristic – a strategy for selectively searching a state space ---- Examples?

Autonomous robotics

Autonomous Robotics

  • Key: operating in an unknown environment

    • Exploration: the act of moving through an unknown environment while building a map that can be used for subsequent navigation

      • The world is not made of right angles

    • Kinds of space: Open, Closed, Unknown

    • Frontiers: regions on the boundary between open and unknown space

The many faces of control

The Many Faces of Control

  • Deliberative Control

  • Reactive Control

  • Hybrid Control

  • Behavior-Based Control

    • Emergent Behavior

  • Note: some info taken from The Robotics Primer by Maja J. Mataric.

    • Highly recommend this book

Deliberative control

Deliberative Control

  • Think Hard, Act Later

    • Throw back to the early days of AI

      • Example: Chess

      • Useful When:

        • There's time to do it

        • Without strategy, things go bad

  • Planning

    • i.e. - Programming Assignment 1

      • But, done automatically

    • Search

      • (DFS, BFS, etc), Goal-Oriented, Forward-Oriented

Deliberative planning based architecture

Deliberative-planning based architecture

  • 3 Steps (done in order)‏

    • Sensing

    • Planning

    • Acting (executing the plan)‏

  • SPA Architectures

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  • The Good

    • Can expect to find many (all possible?) paths to the goal

    • Can cherry-pick the one that's best

      • Optimization

  • The Bad

    • All the time it takes isn't always necessary

    • Assumes you have stored knowledge

  • The Ugly

    • Not possible except in relatively small search spaces

      • Almost never possible in real-time

Additional drawbacks

Additional Drawbacks

  • Sensor data is coming all the time

  • Planning for all possible actions is memory intensive

    • Only need one action really

  • World model must always be accurate and updated

    • The real world is a dynamic place

    • Can't make it hold still while I act

Reactive control

Reactive Control

  • Don't think, react

    • Tight link between sensors and actuators

    • Doesn't have a world model

    • A set of rules is executed based on sensor states

  • Mutually-exclusive Conditions

    • One sensor state, one action

    • Keeps control system simple

      • But, how many sensor states are there? Many.

      • Giant lookup table, slow response

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  • Rules should be generated at Design-Time

    • Designer thinks, robot does not

    • Designer identifies important sensor states / situations

  • Actions can get stuck in loop

    • Use a little randomness

    • Keep a bit of history

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  • We want our robot to do multiple things (multi-tasking)‏

    • Action Selection

      • Command Arbitration (choose a command)‏

      • Command Fusion (combine the commands)‏

    • Example

      • Avoid Object

      • Follow-wall

  • Subsumption Architecture

    • More on this later

Hybrid control

Hybrid Control

  • Reactive – fast but inflexible

  • Deliberative – slow but smart

  • Hybrid – best of both worlds

    • 3 components

      • A reactive layer (on bottom)‏

      • A planner (on top)‏

      • A layer that links the above two together

        • (in the middle)‏

    • 3-layer architecture

Middle layer

Middle Layer

  • Has to:

    • Compensate for shortcomings of reactive and deliberative layers

    • Reconcile different time-scales

    • Deal with different representations

    • Reconcile any contradictory commands

  • Gopher bots (hospital deliveries): The What-ifs Problem

    • Need to get to room fast, but no plan

    • Taking optimal route, suddenly blocked

      • By priority personnel, because of outdated map

    • Always having to go to same room

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  • Dealing with changes

    • This is where the middle layer comes in

    • No one “correct” answer

      • Dynamic Replanning

      • Off-line Planning

      • On-line Planning

      • Building in domain knowledge

  • The middle-layer is hard

    • Best solutions are case-specific (so far)‏

Behavior based control

Behavior-based Control

  • What we know

    • Reactive is inflexible, no representation, or learning

    • Deliberative systems are slow

    • Hybrid systems are hard and complex

    • Biology (our model) has evolved from simple and consistent components

  • BBC → implemented as collections of behaviors

What are behaviors

What are behaviors

  • Many answers to this, but some commonalities

    • Achieve/maintain particular goals

    • Take time, are not instantaneous

      • More complex than simple actions

    • Take input from sensors and other behaviors

    • Send output to effectors and other behaviors

  • Behaviors can work at multiple levels of abstraction (levels of specificity)‏

How they are used

How they are used

  • Executed in parallel, controller can respond immediately when needed

  • Networks of behaviors store state and construct world models

  • Designed to work on the same time-scale

  • Internal behavior structure is not necessarily one-to-one with external behavior

    • Interesting behavior is the result of complex interaction between internal behavior structures

    • Example: Robot flocking

    • Emergent Behavior

  • Key challenge: distributing knowledge over the behavior structure

Example mapping

Example: Mapping

  • Build a plant-watering robot, also builds a map of plants as it goes

    • Distribute map data over behaviors

    • Link behaviors that are adjacent in the environment

  • Examples:

    • Toto: Behavior-based robot for mapping and navigation at MIT

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