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actuators, or effectors
1. Robotics
Books
Valentino Braitenberg (1986) Vehicles: Experiments
in Synthetic Psychology. MIT Press.
Simulation of Braitenberg Vehicles:
http://people.cs.uchicago.edu/~wiseman/vehicles/
Rodney Brooks (1999) Cambrian Intelligence: The
Early History of the New AI. MIT Press.
Rolf Pfeifer & Josh Bongard (2006) How the Body
Shapes the Way We Think: A New View of
Intelligence. MIT Press.
2. โข Assembly lines (1920s)
โข Business machines (1930s)
โข Computers (1940s)
โข Industrial robots (1950s)
first comercial product at Planet Corp. 1959;
Employment of the first robot at Ford 1961;
Unimation's PUMA 1975/78
programmable universal machine for assembly
โข Autonomous robots (?1960s)
Walter's turtle 1948/50/51; Shakey 1968
โข Artificial Life (1970s), Multi robot systems (1980s)
โข Bipedal humanoids (1990s)
โข Today: > 1000 Robot labs
> 900.000 industrial robots (2003)
Decades
4. Components
โข Sensory components: Acquisition of
Information
โข Information processing and control
โข Actuatory components: Realization of
actions and behavior
โข Communication, central executive, selfevaluation, batteries, interfaces
5. Sensory components
Exteroception: Perception of external stimuli or
objects
Propriozeption: Perception of self-movement and
internal states
Exproprioception: Perception of relations and
changes of relations between the body and the
environment
6. Knowledge component
โข Computer or brain-like,
(symbolic/subsymbolic/hybrid)
โข Preprocessing of sensory signals
โข Memory: semantic, episodic, declarative, logical
โข Working memory
โข Processor
โข Strategy, planning and evaluation
โข Actuator control
โข Adaptation rules for the knowledge components
7. Actuatory component
Actuator components (in analogy to the
sensory part)
โข relating to the environment
โข relating to the own body
โข relating to perception
โข relating to communication
8. Question
What is the difference between โinternalโ and
โexternalโ to a robot?
9. Effectors and Actuators
Key points:
โข Mechanisms for acting on the world
โข โDegrees of freedomโ
โข Methods of locomotion: wheels, legs and beyond
โข Methods of manipulation: arms and grippers
โข Methods of actuation and transmission
โข The problem: mapping between input signals to
actuators and the desired effect in the world
10. Effector: A device that affects the
physical environment
โข Wheels on a mobile robot
โ or legs, wings, finsโฆ
โ whole body might push objects
โข Grippers on an assembly robot
โ or welding gun, paint sprayer
โข Speaker, light, tracing-pen
11. E.g. Prescott & Ibbotson (1997)
replicating fossil paths with toilet roll
Control combines thigmotaxis (stay near previous
tracks & phobotaxis (avoid crossing previous tracks)
12. Effector: a device that affects the
physical environment
โข Choice of effectors sets upper limit on what
the robot can do
โข Usually categorised as locomotion (vehicle
moving itself) or manipulation (an arm
moving things)
โข In both cases consider the degrees of
freedom in the design
13. Degrees of freedom
โข General meaning: How many parameters
needed to specify something?
E.g. for an object in space have:
X,Y,Z position
Roll, pitch, yaw rotation
Total of 6 degrees of freedom
How many d.o.f. to specify a vehicle on a flat
plane?
14. Degrees of freedom
In relation to robots could consider:
โข How many joints/articulations/moving parts?
โข How many individually controlled moving
parts?
โข How many independent movements with
respect to a co-ordinate frame?
โข How many parameters to describe the position
of the whole robot or its end effector?
15. โข How many moving parts?
โข If parts are linked need fewer parameters to
specify them.
โข How many individually controlled moving
parts?
โข Need that many parameters to specify
robotโs configuration.
โข Often described as โcontrollable degrees of
freedomโ
โข But note may be redundant e.g. two
movements may be in the same axis
โข Alternatively called โdegrees of mobilityโ
16. โข How many degrees of mobility in the
human arm?
โข How many degrees of mobility in the arm
of an octopus?
โข Redundant manipulator
Degrees of mobility > degrees of freedom
โข Result is that have more than one way to get
the end effector to a specific position
17. โข How many independent movements with
respect to a co-ordinate frame?
โข Controlled degrees of freedom of the robot
โข May be less than degrees of mobility
โข How many parameters to describe the position
of the whole robot or its end effector?
โข For fixed robot, d.o.f. of end effector is determined
by d.o.f. of robot (max 6)
โข Mobile robot on plane can reach position described
by 3 d.o.f., but if robot has fewer d.o.f. then it
cannot do it directly โ it is non-holonomic
18. Alternative vehicle designs
โข โCarโ- steer and drive
โข Two drive wheels and castor 2DoF โ Non-H
โขThree wheels that
both steer and drive
โข Note latter may be
easier for path planning
mechanically more complex
19. Locomotion on uneven terrain
โข
โข
โข
โข
Use the world (ramps etc.)
Larger wheels
Suspension
Tracks
20.
21.
22.
23. Locomotion on uneven terrain
โข
โข
โข
โข
Use the world (ramps etc.)
Larger wheels
Suspension
Tracks
โข Alternative is to use legs
โ Note: wheels and variants are faster, for less
energy, and usually simpler to control)
27. Legged locomotion
Strategies:
โข Limit cycle in
dynamic phase
space e.g. โTekkenโ
(H. Kimura)
โข Cycle in joint
phase space +
forces that return to
cycle
32. E.g. RobotIII vs. Whegs
Roger Quinn et al. โ biorobots.cwru.edu
Realistic cockroach mechanics but uncontrollable (RobotIII),
vs. pragmatic (cricket?) kinematics, but controllable
33. Other forms of locomotion?
Swimming: e.g. robopike
project at MIT
Flight: e.g. Micromechanical
Flying Insect project at
Berkeley
35. Robot arms
โข Typically constructed with rigid links
between movable one d.o.f. joints
โข Joints typically
โ rotary (revolute) or prismatic (linear)
37. Robot arm end effectors
โข
โข
โข
โข
Simple push or sweep
Gripper โ different shape, size or strength
Vacuum cup, scoop, hook, magnetic
Tools for specific purposes (drills, welding
torch, spray head, scalpel,โฆ)
โข Hand for variety of purposes
38.
39. Actuation
What produces the forces to move the effectors?
Electrical:
โ DC motors (speed proportional to voltage โ voltage varied
by pulse width modulation)
โ Stepper motors (fixed move per pulse)
Pressurised โ Liquid: Hydraulics
โ Air: Pneumatics, air muscles
Connected via transmission: system gears, brakes,
valves, locks, springsโฆ
40. Issues in choosing actuators
โข
โข
โข
โข
โข
โข
โข
โข
โข
Load (e.g. torque to overcome own inertia)
Speed (fast enough but not too fast)
Accuracy (will it move to where you want?)
Resolution (can you specify exactly where?)
Repeatability (will it do this every time?)
Reliability (mean time between failures)
Power consumption (how to feed it)
Energy supply & its weight
Also have many possible trade-offs between
physical design and ability to control
41. The control problem
Goal
Motor
command
Outcome
Robot in
environment
โข For given motor commands, what is the
outcome?
= Forward model
โข For a desired outcome, what are the motor
commands? = Inverse model
โข From observing the outcome, how should we
adjust the motor commands to achieve a
goal?
= Feedback control
42. The control problem
Want to move robot hand through set of
positions in task space โ X(t)
X(t) depends on the joint angles in the arm A(t)
A(t) depends on the coupling forces C(t)
delivered by the transmission from the motor
torques T(t)
T(t) produced by the input voltages V(t)
V(t)
T(t)
C(t)
A(t)
X(t)
43. The control problem
V(t)
T(t)
C(t) A(t)
X(t)
Depends on:
โข geometry & kinematics: can
mathematically describe the relationship
between motions of motors and end
effector as transformation of co-ordinates
โข dynamics: actual motion also depends on
forces, such as inertia, friction, etcโฆ
44. The control problem
V(t)
T(t)
C(t) A(t)
X(t)
โข Forward kinematics is hard but usually
possible
โข Forward dynamics is very hard and at best
will be approximate
โข But what we actually need is backwards
kinematics and dynamics
This is a very difficult problem!
45. Summary
โข Some energy sources: electrical, hydraulic,
air, muscles, โฆ
โข A variety of effectors: wheels, legs, tracks,
fingers, tools, โฆ
โข Degrees of freedom and joints
โข Calculating control may be hard: Choose
either a sufficiently simple environment or
adapt to the environment by learning
46. Three laws of robotics (Asimov 1941/2)
1. A robot may not injure a human being or,
through inaction, allow a human being to
come to harm.
2. A robot must obey orders given to it by
human beings, except where such orders
would conflict with the First Law.
3. A robot must protect its own existence as
long as such protection does not conflict with
the First or Second Law.