Autonomous Mobile Robots CpE 470670

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Autonomous Mobile RobotsCpE 470/670

• Lecture 2
• Instructor Monica Nicolescu

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Review

• Definitions
• Robots, robotics
• Robot components
• Sensors, actuators, control
• State, state space
• Representation
• Spectrum of robot control
• Reactive, deliberative

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Robot Control

• Robot control is the means by which the sensing
  and action of a robot are coordinated
• The infinitely many possible robot control
  programs all fall along a well-defined control
  spectrum
• The spectrum ranges from reacting to deliberating

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Spectrum of robot control
From Behavior-Based Robotics by R. Arkin, MIT
Press, 1998
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Robot control approaches

• Reactive Control
• Dont think, (re)act.
• Deliberative (Planner-based) Control
• Think hard, act later.
• Hybrid Control
• Think and act separately concurrently.
• Behavior-Based Control (BBC)
• Think the way you act.

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Thinking vs. Acting

• Thinking/Deliberating
• involves planning (looking into the future) to
  avoid bad solutions
• flexible for increasing complexity
• slow, speed decreases with complexity
• thinking too long may be dangerous
• requires (a lot of) accurate information
• Acting/Reaction
• fast, regardless of complexity
• innate/built-in or learned (from looking into the
  past)
• limited flexibility for increasing complexity

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How to Choose a Control Architecture?

• For any robot, task, or environment consider
• Is there a lot of sensor noise?
• Does the environment change or is static?
• Can the robot sense all that it needs?
• How quickly should the robot sense or act?
• Should the robot remember the past to get the job
  done?
• Should the robot look ahead to get the job done?
• Does the robot need to improve its behavior and
  be able to learn new things?

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Reactive Control Dont think, react!

• Technique for tightly coupling perception and
  action to provide fast responses to changing,
  unstructured environments
• Collection of stimulus-response rules
• Limitations
• No/minimal state
• No memory
• No internal representations
• of the world
• Unable to plan ahead
• Unable to learn

• Advantages
• Very fast and reactive
• Powerful method animals are largely reactive

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Deliberative Control Think hard, then act!

• In DC the robot uses all the available sensory
  information and stored internal knowledge to
  create a plan of action sense ? plan ? act (SPA)
  paradigm
• Limitations
• Planning requires search through potentially all
  possible plans ? these take a long time
• Requires a world model, which may become outdated
• Too slow for real-time response
• Advantages
• Capable of learning and prediction
• Finds strategic solutions

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Hybrid Control Think and act independently
concurrently!

• Combination of reactive and deliberative control
• Reactive layer (bottom) deals with immediate
  reaction
• Deliberative layer (top) creates plans
• Middle layer connects the two layers
• Usually called three-layer systems
• Major challenge design of the middle layer
• Reactive and deliberative layers operate on very
  different time-scales and representations
  (signals vs. symbols)
• These layers must operate concurrently
• Currently one of the two dominant control
  paradigms in robotics

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Behavior-Based Control Think the way you act!

• An alternative to hybrid control, inspired from
  biology
• Has the same capabilities as hybrid control
• Act reactively and deliberatively
• Also built from layers
• However, there is no intermediate layer
• Components have a uniform representation and
  time-scale
• Behaviors concurrent processes that take inputs
  from sensors and other behaviors and send outputs
  to a robots actuators or other behaviors to
  achieve some goals

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Behavior-Based Control Think the way you act!

• Thinking is performed through a network of
  behaviors
• Utilize distributed representations
• Respond in real-time
• are reactive
• Are not stateless
• not merely reactive
• Allow for a variety of behavior coordination
  mechanisms

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Fundamental Differences of Control

• Time-scale How fast do things happen?
• how quickly the robot has to respond to the
  environment, compared to how quickly it can sense
  and think
• Modularity What are the components of the
  control system?
• Refers to the way the control system is broken up
  into modules and how they interact with each
  other
• Representation What does the robot keep in its
  brain?
• The form in which information is stored or
  encoded in the robot

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A Brief History of Robotics

• Robotics grew out of the fields of control
  theory, cybernetics and AI
• Robotics, in the modern sense, can be considered
  to have started around the time of cybernetics
  (1940s)
• Early AI had a strong impact on how it evolved
  (1950s-1970s), emphasizing reasoning and
  abstraction, removal from direct situatedness and
  embodiment
• In the 1980s a new set of methods was introduced
  and robots were put back into the physical world

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Control Theory

• The mathematical study of the properties of
  automated control systems
• Helps understand the fundamental concepts
  governing all mechanical systems (steam engines,
  aeroplanes, etc.)
• Feedback measure state and take an action based
  on it
• Idea continuously feeding back the current state
  and comparing it to the desired state, then
  adjusting the current state to minimize the
  difference (negative feedback).
• The system is said to be self-regulating
• E.g. thermostats
• if too hot, turn down, if too cold, turn up

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Control Theory through History

• Thought to have originated with the ancient
  Greeks
• Time measuring devices (water clocks), water
  systems
• Forgotten and rediscovered in Renaissance Europe
• Heat-regulated furnaces (Drebbel, Reaumur,
  Bonnemain)
• Windmills
• James Watts steam engine (the governor)

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Cybernetics

• Pioneered by Norbert Wiener in the 1940s
• Comes from the Greek word kibernts governor,
  steersman
• Combines principles of control theory,
  information science and biology
• Sought principles common to animals and machines,
  especially with regards to control and
  communication
• Studied the coupling between an organism and its
  environment

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W. Grey Walters Tortoise

• Machina Speculatrix (1953)
• 1 photocell, 1 bump sensor, 2 motor, 3 wheels, 1
  battery
• Behaviors
• seek light
• head toward moderate light
• back from bright light
• turn and push
• recharge battery
• Uses reactive control, with behavior
  prioritization

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Principles of Walters Tortoise

• Parsimony
• Simple is better
• Exploration or speculation
• Never stay still, except when feeding (i.e.,
  recharging)
• Attraction (positive tropism)
• Motivation to move toward some object (light
  source)
• Aversion (negative tropism)
• Avoidance of negative stimuli (heavy obstacles,
  slopes)
• Discernment
• Distinguish between productive/unproductive
  behavior (adaptation)

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Braitenberg Vehicles

• Valentino Braitenberg (1980)
• Thought experiments
• Use direct coupling between sensors and motors
• Simple robots (vehicles) produce complex
  behaviors that appear very animal, life-like
• Excitatory connection
• The stronger the sensory input, the stronger the
  motor output
• Light sensor ? wheel photophilic robot (loves
  the light)
• Inhibitory connection
• The stronger the sensory input, the weaker the
  motor output
• Light sensor ? wheel photophobic robot (afraid
  of the light)

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Example Vehicles

• Wide range of vehicles can be designed, by
  changing the connections and their strength
• Vehicle 1
• One motor, one sensor
• Vehicle 2
• Two motors, two sensors
• Excitatory connections
• Vehicle 3
• Two motors, two sensors
• Inhibitory connections

Vehicle 1
Being ALIVE
FEAR and AGGRESSION
Vehicle 2
LOVE
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Artificial Intelligence

• Officially born in 1955 at Dartmouth University
• Marvin Minsky, John McCarthy, Herbert Simon
• Intelligence in machines
• Internal models of the world
• Search through possible solutions
• Plan to solve problems
• Symbolic representation of information
• Hierarchical system organization
• Sequential program execution

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AI and Robotics

• AI influence to robotics
• Knowledge and knowledge representation are
  central to intelligence
• Perception and action are more central to
  robotics
• New solutions developed behavior-based systems
• Planning is just a way of avoiding figuring out
  what to do next (Rodney Brooks, 1987)
• Distributed AI (DAI)
• Society of Mind (Marvin Minsky, 1986) simple,
  multiple agents can generate highly complex
  intelligence
• First robots were mostly influenced by AI
  (deliberative)

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Shakey

• At Stanford Research Institute (late 1960s)
• A deliberative system
• Visual navigation in a very special world
• STRIPS planner
• Vision and contact sensors

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Early AI Robots HILARE

• Late 1970s
• At LAAS in Toulouse
• Video, ultrasound, laser rangefinder
• Was in use for almost 2 decades
• One of the earliest hybrid architectures
• Multi-level spatial representations

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Early Robots CART/Rover

• Hans Moravecs early robots
• Stanford Cart (1977) followed by CMU rover (1983)
• Sonar and vision

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Lessons Learned

• Move faster, more robustly
• Think in such a way as to allow this action
• New types of robot control
• Reactive, hybrid, behavior-based
• Control theory
• Continues to thrive in numerous applications
• Cybernetics
• Biologically inspired robot control
• AI
• Non-physical, disembodied thinking

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Challenges

• Perception
• Limited, noisy sensors
• Actuation
• Limited capabilities of robot effectors
• Thinking
• Time consuming in large state spaces
• Environments
• Dynamic, impose fast reaction times

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Key Issues of Behavior-Based Control

• Situatedness
• Robot is entirely situated in the real world
• Embodiment
• Robot has a physical body
• Emergence
• Intelligence from the interaction with the
  environment
• Grounding in reality
• Correlation of symbols with the reality
• Scalability
• Reaching high-level of intelligence

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Effectors Actuators

• Effector
• Any device robot that has an impact on the
  environment
• Effectors must match a robots task
• Controllers command the effectors to achieve the
  desired task
• Actuator
• A robot mechanism that enables the effector to
  execute an action
• Robot effectors are very different than
  biological ones
• Robots wheels, tracks, legs, grippers
• Robot actuators
• Motors of various types

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Passive Actuation

• Use potential energy and interaction with the
  environment
• E.g. gliding (flying squirrels)
• Robotics examples
• Tad McGeers passive walker
• Actuated by gravity

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Types of Actuators

• Electric motors
• Hydraulics
• Pneumatics
• Photo-reactive materials
• Chemically reactive materials
• Thermally reactive materials
• Piezoelectric materials

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DC Motors

• DC (direct current) motors
• Convert electrical energy into mechanical energy
• Small, cheap, reasonably efficient, easy to use
• How do they work?
• Electrical current through loops of wires mounted
  on a rotating shaft
• When current is flowing, loops of wire generate a
  magnetic field, which reacts against the magnetic
  fields of permanent magnets positioned around the
  wire loops
• These magnetic fields push against one another
  and the armature turns

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Motor Efficiency

• DC motors are not perfectly efficient
• Some limitations (mechanical friction)
• of motors
• Some energy is wasted as heat
• Industrial-grade motors (good quality) 90
• Toy motors (cheap) efficiencies of 50
• Electrostatic micro-motors for miniature robots
  ?50

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Operating Voltage

• Making the motor run requires electrical power in
  the right voltage range
• Most motors will run fine at lower voltages,
  though they will be less powerful
• Can operate at higher voltages at expense of
  operating life

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Operating/Stall Current

• When provided with a constant voltage, a DC motor
  draws current proportional to how much work it is
  doing Work Force Distance
• When there is no resistance to its motion, the
  motor draws the least amount of current
• Moving in free space ? less current
• Pushing against an obstacle (wall) ? drain more
  current
• If the resistance becomes very high the motor
  stalls and draws the maximum amount of current at
  its specified voltage (stall current)

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Torque

• Torque rotational force that a motor can deliver
  at a certain distance from the shaft
• Strength of magnetic field generated in loops of
  wire is directly proportional to amount of
  current flowing through them and thus the torque
  produced on motors shaft
• The more current through a motor, the more torque
  at the motors shaft

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Stall Torque

• Stall torque the amount of rotational force
  produced when the motor is stalled at its
  recommended operating voltage, drawing the
  maximal stall current at this voltage
• Typical torque units ounce-inches
• 5 oz.-in. torque means motor can pull weight of 5
  oz up through a pulley 1 inch away from the shaft

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Power of a Motor

• Power product of the output
• shafts rotational velocity and
• torque
• No load on the shaft
• Rotational velocity is at its highest, but the
  torque is zero
• The motor is spinning freely (it is not driving
  any mechanism)
• Motor is stalled
• It is producing its maximal torque
• Rotational velocity is zero

P0
A motor produces the most power in the middle of
its performance range.
P0
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How Fast do Motors Turn?

• Free spinning speeds (most motors)
• 3000-9000 RPM (revolutions per minute) 50-150
  RPS
• High-speed, low torque
• Drive light things that rotate very fast
• What about driving a heavy robot body or lifting
  a heavy manipulator?
• Need more torque and less speed
• How can we do this?

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Gearing

• Tradeoff high speed for more torque
• Seesaw physics
• Downward force is equal to weight
• times their distance from the fulcrum.
• Torque T F x r
• rotational force generated at the center of a
• gear is equal to the gears radius times the
• force applied tangential at the circumference

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Meshing Gears

• By combining gears with different ratios we can
  control the amount of force and torque generated
• Work force x distance
• Work torque x angular movement
• Example r2 3r1
• Gear 1 turns three times (1080 degrees)
• while gear 2 turns only once (360 degrees)
• Toutput x 360 Tinput x 1080
• Toutput 3 Tinput Tinput x r2/r1

Gear 1 with radius r1 turns an angular distance
of ?1 while Gear 2 with radius r2 turns an
angular distance of ?2.
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Torque Gearing Law

• Toutput Tinput x routput/rinput
• The torque generated at the output gear is
  proportional to the torque on the input gear and
  the ratio of the two gear's radii
• If the output gear is larger than the input gear
  (small gear driving a large gear) ? torque
  increases
• If the output gear is smaller than the input gear
  (large gear driving a small gear) ? torque
  decreases

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Gearing Effect on Speed

• Combining gears has a corresponding effect on
  speed
• A gear with a small radius has to turn faster to
  keep up with a larger gear
• If the circumference of gear 2 is three
• times that of gear 1, then gear 1 must
• turn three times for each full rotation
• of gear 2.
• Increasing the gear radius reduces the speed.
• Decreasing the gear radius increases the speed.

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Torque Speed Tradeoff

• When a small gear drives a large one, torque is
  increased and speed is decreased
• Analogously, when a large gear drives a small
  one, torque is decreased and speed is increased

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Readings

• F. Martin Sections 1.1, 2.4, 4.1
• M. Mataric Chapters 2, 4, 11