COMP790072 Robotics: An Introduction

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COMP790-072Robotics An Introduction

• Sensors Actuators
• Introduction to Kinematics

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Sensors

• Vision (Review)
• Stereoscopic
• Monoscopic
• Sonar (see a later lecture)
• Others (bump sensors, LIDAR, etc.)

3
Cameras

• Charge coupled devices (CCDs) use arrays of
  photosensitive diodes to generate intensity maps
• grey-levels of color devices are available
• a range of image resolutions (pixels per image)
• 800 600 pixels is typical
• a range of frame rates (number of images per
  second)
• 30 Hz (frames per second) is typical
• The field of view can be changed
• high-resolution cameras typically view 45 - 60
• wide-angle (fisheye) lenses may cover 80 - 90
• curved mirrors increase field further without
  distortion

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Stereoscopic Vision

• Viewing the world with two cameras (eyes) allows
  a 3D representation to be formed
• unfortunately the signal is complex and noisy
• Each camera receives a slightly different view
• the distance between corresponding points in an
  image is known as the stereo disparity

disparity
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Stereo Ranging

• The amount of disparity is related to distance
• the difficulty lies in identifying corresponding
  points
• The general principle is
• left and right images are digitized
• raw images are rectified for distortion /
  misalignment
• rectified images are filtered to enhance
  texturesedges
• a stereo matching algorithm is applied
• modern techniques search along horizontal scan
  lines to find the best set of matching pixels
  (e.g. mean-squared-error)
• raw disparity map is filtered to remove noise
• This can now be done on modern computers
• e.g. Pentium P-4 _at_ GHz at interactive frame rates
  

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Monoscopic Vision

• Although stereo vision is popular, it has
  problems
• high hardware requirements, camera alignment,
  etc.
• consequently single camera input may be used also
• Monoscopic ranging
• optical flow
• the relative motion between the moving camera and
  viewed objects in the environment, seen over a
  sequence of images
• looming
• as an object gets close, it gets bigger!
• is simple to use this information to calculate
  distance
• but the object must have been identified and must
  be totally in view
• depth from focus
• depth-of-field of conventional lens systems can
  be used

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Object Recognition

• Much vision research on object recognition
• so easy for humans, but the problem not yet
  solved
• humans may use a combination of techniques and
  reasoning
• Edge detection
• fairly simple filter operations can detect clean
  edges
• e.g. the discrete Laplace filter
• reliable detection of all edges is much more
  difficult
• Area based techniques
• connected regions of similar color, texture or
  brightness probably belong to the same object

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Actuators

• Locomotion
• Manipulation

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Actuators

• Locomotion
• Manipulation

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Locomotion

• Legs
• Wheels
• Other exotic means

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Legs

• Two legs seems the most obvious configuration
• but in fact balance is an incredibly difficult
  problem
• e.g. the Honda Humanoid Project
• need knees, ankles and hips in order to move
  around
• two legs are inherently unstable difficult to
  stand still
• Six legs are much easier to balance and move
• stable when not moving
• can work with simple cams and rigid legs
• Brooks et al. (1989) evolved the walking Genghis
  robot

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Wheels

• Any number of wheels is possible
• there are many different configurations that are
  useful
• Two individually driven wheels on either side
• usually with one or more idler wheels for balance
• independently driven wheels allows zero turning
  radius
• one wheel drives forwards, one wheel drives
  backwards
• Rear wheel drive, with front wheel steering
• the vehicle will have a non-zero turning radius
• for two front wheels, turning geometry is complex
• rear wheels need a differential to prevent
  slippage
• 4WD is possible, but it is even more complex

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Exotic Wheels Tracks

• Tracks can be used in the same way as two wheels
• good for rough terrain (as compared to wheels)
• tracks must slip to enable turns (skid steering)
• In synchro drive, 3 wheels are coupled
• drive in same direction at same rate
• pivot in unison about their respective steering
  axes
• allows body of robot to remain in the same
  orientation
• Tri-star wheels are composed of 3 sub-wheels
• entire wheel assembly rolls over a large obstacle
• Many other exotic wheel configurations
• Multiple-degrees-of-freedom (MDOF)
• going side way, tight turns, etc.

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Mobility Considerations

• A number of issues impact selection of drive
• Maneuverability - ability to alter
  direction/speed
• Controllability - practical and not too complex
  traction sufficient to minimize slippage
• climbing ability - traversal of minor
  discontinuities, slope rate, surface type,
  terrain
• stability - must not fall over!
• efficiency - power consumption reasonable
• maintenance - easy to maintain, reliable
• environmental impact - does not do damage
• navigation - accuracy of dead-reckoning

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Actuators

• Locomotion
• Manipulation

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Manipulations

• Degrees of freedom
• independently controllable components of motion
• Arms
• convenient method to allow full movement in 3D
• more often used in fixed robots due to power
  weight
• even more difficult to control!
• due to extra degrees of freedom
• Grippers
• may be very simple (two rigid arms) to pick up
  objects
• may be complex device with fingers on end of an
  arm
• probably need feedback to control grip force

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Manipulation Actuator Types

• Electric
• DC motor is the most common type used in mobile
  robots
• stepper motors turn a certain amount / applied
  voltage
• Pneumatic
• operate by pumping compressed air through
  chambers
• Hydraulic
• pump pressurized oil usually too heavy, dirty
  and expensive to be used on mobile robots
• Shape memory alloys (SMAs)
• metallic alloys that deform under heat and then
  return to their previous shape used for
  artificial muscles
• see http//www.sma-inc.com/SMAPaper.html

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Measuring Motion Odometers

• If wheels are being used, then distance traveled
  can be calculated by measuring number of turns
• dead-reckoning or odometry is the name given to
  the direct measure of distance (for navigation)
• Motor speed and timing are very inaccurate
• measuring the number of wheel rotations is better
• shaft encoders, or rotation sensors, measure this
• Different types technologies of shaft encoder

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Motion Types

• holonomic the controllable degrees of freedom is
  equal to the total degrees of freedom, e.g.
  manipulator arm
• non-holonomic the controllable degrees of
  freedom is less than the total degrees of
  freedom, e.g. car (although it can move
  laterally, but no mechanism to control lateral
  movement)

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Introduction to Kinematics

• Kinematics study of motion independent of
  underlying forces
• Degrees of freedom (DoF) the number of
  independent position variables needed to specify
  motions
• State Vector vector space of all possible
  configurations of an articulated figure. In
  general, the dimensions of state vector is equal
  to the DoF of the articulated figure.

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Manipulator Joint Types

• 1 DOF Joint types
• Revolute
• Prismatic

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More Joint Types

• Many higher order joint types can be represented
  by combining 1-DOF joints by making axes intersect

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Forward vs. Inverse Kinematics

• Forward kinematics motion of all joints is
  explicitly specified
• Inverse kinematics given the position of the
  end effector, find the position and orientation
  of all joints in a hierarchy of linkages also
  called goal-directed motion.
• See handouts for a simple 2D example.