COMP790-072 Robotics: An Introduction

1
COMP790-072Robotics An Introduction

• Sensors Actuators
• Introduction to Kinematics

2
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

4
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
5
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
  

6
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

7
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

8
Actuators

• Locomotion
• Manipulation

9
Actuators

• Locomotion
• Manipulation

10
Locomotion

• Legs
• Wheels
• Other exotic means

11
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

12
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

13
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.

14
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

15
Actuators

• Locomotion
• Manipulation

16
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

17
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

18
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

19
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)

20
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.

21
Manipulator Joint Types

• 1 DOF Joint types
• Revolute
• Prismatic

22
More Joint Types

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

23
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.