Robotics 13

8/8/2019 Robotics 13

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CS 491/691(X) - Lecture 8 1

EXPERT SYSTEMS AND SOLUTIONS

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[email protected]

Cell: 9952749533www.researchprojects.info

PAIYANOOR, OMR, CHENNAI

Call For Research Projects Final

year students of B.E in EEE, ECE, EI,

M.E (Power Systems), M.E (Applied

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Research labs. Experts will be guiding theprojects.


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Topics: Introduction toRobotics

CS 491/691(X)

Lecture 8

Instructor: Monica Nicolescu


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CS 491/691(X) - Lecture 8 3

Review

Control Architectures

Languages for robot control

Computability

Organizing principles

Architecture selection criteria

Reactive control


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CS 491/691(X) - Lecture 8 4

Reactive Control

Reactive control is based on tight (feedback) loops

connecting a robot's sensors with its effectors

Purely reactive systems do not use any internal

representations of the environment, and do notlook ahead

They work on a short time-scale and react to the current

sensory information

Reactive systems use minimal, if any, state

information


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CS 491/691(X) - Lecture 8 5

Collections of Rules

Reactive systems consist ofcollections of reactiverules that map specific situations to specific actions

Analog to stimulus-response, reflexes

Bypassing the brain allows reflexes to be very fast

Rules are running concurrently and in parallel

Situations

Are extracted directly from sensory input

Actions

Are the responses of the system (behaviors)


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CS 491/691(X) - Lecture 8 6

Mutually Exclusive Situations

If the set of situations is mutually exclusive:

only one situation can be met at a given time

only one action can be activated

Often is difficult to split up the situations this way

To have mutually exclusive situations the controller

must encode rules for all possible sensory

combinations, from all sensors This space grows exponentially with the number of

sensors


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CS 491/691(X) - Lecture 8 7

Complete Control Space

The entire state space of the robot consists of allpossible combinations of the internal and external

states

A complete mapping from these states to actions is

needed such that the robot can respond to all

possibilities

This is would be a tedious job and would result in a

very large look-up table that takes a long time tosearch

Reactive systems use parallel concurrent reactive

rules parallel architecture, multi-tasking


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CS 491/691(X) - Lecture 8 8

Incomplete Mappings

In general, complete mappings are not used in hand-

designed reactive systems

The most important situations trigger the

appropriate reactions Default responses are used to cover all other cases

E.g.: a reactive safe-navigation controller

If left whisker bent then turn right

If right whisker bent then turn left

Ifboth whiskers bent then back up and turn left

Otherwise, keep going


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CS 491/691(X) - Lecture 8 9

Action Selection

If the rules are not triggered by unique mutually-exclusive conditions, more than one rule can be

triggered at the same time

Two or more different commands are sent to the

actuators

Deciding which action to take is called action selection

Arbitration: decide among multiple actions or

behaviors Fusion: combine multiple actions to produce a single

command


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CS 491/691(X) - Lecture 8 10

Arbitration

There are many different types of arbitration

Arbitration can be done based on:

a fixed priority hierarchy

rules have pre-assigned priorities

a dynamic hierarchy

rules priorities change at run-time

learning rule priorities may be initialized and are learned at run-

time, once or continuously


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CS 491/691(X) - Lecture 8 11

Multi-Tasking

Arbitration decides which one action to execute To respond to any rule that might become triggered

all rules have to be monitored in parallel, andconcurrently

Ifno obstacle in front move forward

Ifobstacle in front stop and turn away

Wait for 30 seconds, then turn in a random direction

Monitoring sensors in sequence may lead to missing

important events, or failing to react in real time

Reactive systems must support parallelism

The underlying programming language must have multi-

tasking abilities


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CS 491/691(X) - Lecture 8 12

Designing Reactive Systems

How to can we put together multiple (large number)

of rules to produce effective, reliable and goal

directed behavior?

How do we organize a reactive controller in a

principled way?

The best known reactive architecture is the

Subsumption Architecture (Rod Brooks, MIT,

1985)


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CS 491/691(X) - Lecture 8 13

Vertical v. Horizontal Systems

Traditional (SPA):

sense plan act

Subsumption:


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CS 491/691(X) - Lecture 8 14

Biological Inspiration

The inspiration behind the Subsumption Architecture

is the evolutionary process:

New competencies are introduced based on existing ones

Complete creatures are not thrown out and new

ones created from scratch

Instead, solid, useful substrates are used to build up to

more complex capabilities


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CS 491/691(X) - Lecture 8 15

The Subsumption Architecture

Principles of design

systems are built from

the bottom up

components are task-achieving

actions/behaviors (avoid-obstacles, find-doors, visit-rooms)

all rules can be executed in parallel, not in a sequence

components are organized in layers, from the bottom up

lowest layers handle most basic tasks

newly added components and layers exploit the existing

ones


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CS 491/691(X) - Lecture 8 16

Subsumption System Design

What makes a Subsumption Layer, what should gowhere?

There is no strict recipe, but some solutions arebetter than others, and most are derived empirically

How exactly layers are split up depends on thespecifics of the robot, the environment, and the task


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CS 491/691(X) - Lecture 8 17

Designing in Subsumption

Qualitatively specify the overall behavior neededfor the task

Decompose that into specific and independent

behaviors (layers) Determine behavior granularity

Ground low-level behaviors in the robots sensors

and effectors Incrementally build, test, and add


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CS 491/691(X) - Lecture 8 18

Subsumption Layers

First, we design, implement and debuglayer 0

Next, we design layer 1

When layer 1 is designed, layer 0 is

taken into consideration and utilized, itsexistence is subsumed (thus the name of

the architecture)

As layer 1 is added, layer 0 continues to

function

Continue designing layers, until the

desired task is achieved

level 2

level 1

level 0

sensors actuators


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CS 491/691(X) - Lecture 8 19

Suppression and Inhibition

Higher layers can disable the ones below

Avoid-obstacles can stop the robot from moving around

Layer 2 can either:

Inhibit the output of level 1 or Suppress the input of level 1

The process is continued all the way to the top level

AFSMinputs outputs

suppressor

inhibitor

I

s

level 2

level 1

level 0

sensors actuators


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CS 491/691(X) - Lecture 8 20

Subsumption Language and AFSMs

The original Subsumption Architecture was

implemented using the Subsumption Language

It was based on finite state machines (FSMs)

augmented with a very small amount of state (AFSMs) AFSMs were implemented in Lisp

AFSMinputs outputs

suppressor

inhibitor

Is


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CS 491/691(X) - Lecture 8 21

Subsumption Language and AFSMs

Each behavioris represented as an augmented finite statemachine (AFSMs)

Stimulus (input) or response

(output) can be inhibited or

suppressed by other active behaviors

An AFSM can be in one state at a time, can receive one or

more inputs, and send one or more outputs

AFSMs are connected communication wires, which pass

input and output messages between them; only the last

message is kept

AFSMs run asynchronously

AFSMinputs outputs

suppressor

inhibitor

I

s

collide

haltsonar


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CS 491/691(X) - Lecture 8 23

Wandering in Subsumption

Brooks 87


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CS 491/691(X) - Lecture 8 24

Layering in AFSMNetworks

Once a basic competence is achieved (e.g., moving aroundsafely), it can be labeled as layer 0

Layer 1 (e.g., one that looks for objects and collects them),

can then be added on top and communication is done through

wires

The use of layers is meant to modularize the reactive system,

so it is bad design to put a lot of behaviors within a single

layer

Also, it is bad design to put a large number of connections

between the layers, so that they are strongly coupled


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CS 491/691(X) - Lecture 8 25

Module Independence

Strong coupling implies dependence between

modules, which violates the modularity of the

system

If modules are interdependent, they are not asrobust to failure

In Subsumption, if higher layers fail, the lower ones

remain unaffected

Thus Subsumption has one-way independence

between layers

Two-way independence is not practical, why?


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CS 491/691(X) - Lecture 8 27

Using the World

How can you sequence activities in Subsumption?

Coupling between layers need not be through the

system itself (i.e., not through explicit

communication wires)

It could be through the world. How?


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CS 491/691(X) - Lecture 8 28

Collecting Soda Cans

Herbert collected empty soda cansand took them home

Herberts capabilities

Move around without running intoobstacles

Detect soda cans using a camera and a

laser

An arm that could: extend, sense if thereis a can in the gripper, close the gripper,

tuck the arm in


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CS 491/691(X) - Lecture 8 29

Herbert

Look for soda cans, when seeing one approach it

When close, extend the arm toward the soda can

If the gripper sensors detect something close the

gripper

If can is heavy, put it down, otherwise pick it up

If gripper was closed tuck the arm in and head home

The robot did not keep internal state about whatit had just done and what it should do next: it

just sensed!


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CS 491/691(X) - Lecture 8 30

Readings

M. Matari: Chapter 14

Robotics 13

Documents

Transcript of Robotics 13