UNC Chapel Hill D. Manocha COMP790-058 Robotics Sensors & Actuators Introduction to Kinematics

UNC Chapel Hill D. Manocha

COMP790-058Robotics

Sensors & Actuators

Introduction to Kinematics


Sensors

Vision (Review)– Stereoscopic– Monoscopic

Sonar (see a later lecture)

Others (bump sensors, LIDAR, etc.)


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


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


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


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


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


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


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


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.


Actuators

LocomotionManipulation

UNC Chapel Hill M. C. Lin

Actuators



Locomotion

LegsWheelsOther exotic means


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


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


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.


Recent Trends

Humanoid Robots:

http://www.youtube.com/watch?v=cfaAiujrX_Y

http://www.youtube.com/watch?v=XfdsRUiOWUo&NR=1


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


Actuators



Actuators



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


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


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


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)


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.


Manipulator Joint Types

1 DOF Joint typesRevolute

Prismatic


More Joint Types

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


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.

Documents

UNC Chapel Hill D. Manocha COMP790-058 Robotics Sensors & Actuators Introduction to Kinematics