unc chapel hill d. manocha comp790-058 robotics sensors & actuators introduction to kinematics
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UNC Chapel Hill D. Manocha
Sensors
Vision (Review)– Stereoscopic– Monoscopic
Sonar (see a later lecture)
Others (bump sensors, LIDAR, etc.)
UNC Chapel Hill D. Manocha
SensorsSensors are devices that are used to measure physical variables
like temperature, pH, velocity, rotational rate, flow rate, pressure and many others. Today, most sensors do not indicate a reading on an analog scale (like a thermometer), but, rather, they produce a voltage or a digital signal that is indicative of the physical variable they measure. Those signals are often imported into computer programs, stored in files, plotted on computers and analyzed to death.
http://newton.ex.ac.uk/teaching/CDHW/Sensors/
http://www.facstaff.bucknell.edu/mastascu/elessonshtml/Sensors/SensorsIntro.htm
UNC Chapel Hill D. Manocha
Cameras
Charge coupled devices (CCD’s) 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
UNC Chapel Hill D. Manocha
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
UNC Chapel Hill D. Manocha
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 textures+edges– 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 @ GHz at interactive frame rates
UNC Chapel Hill D. Manocha
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
UNC Chapel Hill D. Manocha
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
UNC Chapel Hill D. Manocha
Actuators
An actuator is a mechanical device for moving or controlling a mechanism or system.
Mechanics - plasma actuators, pneumatic actuators, electric actuators, motors, hydraulic cylinders, linear actuators, etc.
Human - Muscles Biology - Actuator domains found in P, F and V type ATPases
UNC Chapel Hill D. Manocha
Actuators In engineering, actuators are frequently used as mechanisms to introduce
motion, or to clamp an object so as to prevent motion. In electronic engineering, actuators ACTT, are a subdivision of transducers. They are devices which transform an input signal (mainly an electrical signal) into motion. Specific examples are Electrical motors, pneumatic actuators, hydraulic pistons, relays, comb drive, piezoelectric actuators, thermal bimorphs, Digital Micromirror Devices and electroactive polymers.
Motors are mostly used when circular motions are needed, but can also be used for linear applications by transforming circular to linear motion with a bolt and screw transducer. On the other hand, some actuators are intrinsically linear, such as piezoelectric actuators.
In virtual instrumentation actuators and sensors are the hardware complements of virtual instruments. Computer programs of virtual instruments use actuators to act upon real world objects.
UNC Chapel Hill D. Manocha
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
UNC Chapel Hill D. Manocha
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
UNC Chapel Hill M. C. Lin
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.
UNC Chapel Hill D. Manocha
Recent Trends
Humanoid Robots:
http://www.youtube.com/watch?v=cfaAiujrX_Y
http://www.youtube.com/watch?v=XfdsRUiOWUo&NR=1
UNC Chapel Hill M. C. Lin
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
UNC Chapel Hill M. C. Lin
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
UNC Chapel Hill D. Manocha
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 (SMA’s)– 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
UNC Chapel Hill M. C. Lin
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
UNC Chapel Hill M. C. Lin
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)
UNC Chapel Hill M. C. Lin
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.
UNC Chapel Hill M. C. Lin
More Joint Types
Many higher order joint types can be represented by combining 1-DOF joints by making axes intersect
UNC Chapel Hill M. C. Lin
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 notes for a simple 2D example.
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