UNC Chapel Hill D. Manocha COMP Robotics Sensors & Actuators Introduction to Kinematics

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 Sensors Sensors 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. sIntro.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 ° –wide-angle (fisheye) lenses may cover ° –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 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.plasma actuatorspneumatic actuatorselectric actuatorsmotorshydraulic cylinderslinear actuators Human - Muscles HumanMuscles Biology - Actuator domains found in P, F and V type ATPases Biology

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.engineering transducersmotion Electrical motorspneumatic actuatorshydraulicpistonsrelays comb drivepiezoelectricthermalDigital Micromirror Deviceselectroactive 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. Motorstransducer 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.virtual instrumentationsensors

UNC Chapel Hill D. Manocha Actuators Locomotion Manipulation

UNC Chapel Hill M. C. Lin Actuators Locomotion Manipulation

UNC Chapel Hill D. Manocha Locomotion Legs Wheels Other exotic means

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:

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 Actuators Locomotion Manipulation

UNC Chapel Hill M. C. Lin Actuators Locomotion Manipulation

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

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 Manipulator Joint Types 1 DOF Joint types Revolute Prismatic

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.