0112Micromo Robotics

8/2/2019 0112Micromo Robotics

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Lets step through the basics, taking a look first atcontrollers.

ControllersController coordinates all the movements of the

robots mechanical actuators. They also receive input

from the immediate environment through varioussensors. Machine vision, for example, gives robotseyes to see objects, patterns, and whether an objectis properly orientated for the next step in assembly.These days, controllers all contain a digital micro-processor linked to inputs and outputs, includingmonitoring devices.

Commands issued by controllers activate motion-control devices consisting of various sub-controllers,amplifiers, and actuators. Actuator are motors orvalves that converts power into movement of therobot. Movements are initiated by a series of instruc-tions or program stored in the controllers memory.

Controllers usually have three levels of hierarchi-cal control. In a hierarchical-control scheme, levelsof organization are assigned to various sub-control-lers. Each level sends control signals to the level be-low while getting feedback and instructions from thelevel above. Levels become more elemental as they moves toward the actuator.

The common three control levels are:Level I: Actuator Control: Heres where separatemovements of the robot along various planes, suchas the X, Y, and Z axes, are generated.

Level II: Path Control: At this level, separatemovements along the planes (determined in Level I)are combined into desired trajectories or paths.

Level III: Main Control: At this level, writteninstructions from a human programmer definingthe tasks required are interpreted and reconstructed

so that Level II controllers can understand them.In other words, the instructions are combined with various sensory signals and translated into the moreelementary instructions that Level II can understandand carry out.

Robots can be further classified in a number of ways, depending on their size, tasks, industry, anduse. For engineers and designers, robots are usu-ally broken down into three classifications: types of control, types of drives, and the shape of the work envelope.

Type of control

Robots traditionally use one of two control sys-tems: non-servo and servo. The earliest robots werenon-servo, which are considered non-intelligentrobots. Servo robots, however, are classified as eitherintelligent or highly intelligent, with the main differ-ence between intelligent and highly intelligent ro-bots being the level of awareness its sensors give it.

Non-Servo robots, the simplest robots, are oftenreferred to as limited sequence, pick-and-place,or fixed-stop robots. They operate in open-loopsystems where there is no feedback that lets the ro-

There are all types of industrial robots, and most can be broken down into a handfulof basic components. One of the most basic are the drives and the controls. Thedrive provides power and can be electric, hydraulic, or pneumatic, while thecontroller determines how that power is used to move arms and actuators which

carry effectors or tools to the workpiece.

Robotic motions rely on electrical, pneumatic

or hydraulic power, and digitl controllers.

The basics ofrobotic motion

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ity. In a sense, a servomechanism detects acorrects for errors.

Servo robots are: Relatively expensive to purchase, opera

and maintain. Use a sophisticated, closed-lo

controller. Have a wide range of capabilities.

Can transfer objects from one point another, as well as along a controlled, continous path.

Can interpret and use sophisticateprogramming.

Use a manipulator arm programmed tavoid obstructions within the work envelope

Actuator drive typesAnother common way of classifying r

bots is by the type of drive used by its actutors. Most robot use ether electric, pneumaor hydraulic actuators.

Electric drives encompass three kindsmotors: ac servo motors, dc servo motoand stepper motors. Many newer robots uservo motors rather than hydraulic or pneumatic ones. Small and medium-size robocommonly use dc servo motors. Larger rbots rely on ac servos for their high torqcapabilities,

Stepper motors are incrementally cotrolled dc synchronous motors. They are rareused in commercial industrial robots, but acommonly found in educational robots.

Robots that use electric drives take up lfloor space, and their energy source, electrity, is readily available compared to hydralic and pneumatic power. However, convetionally geared drives suffer from backlafriction, compliance, and wear. These prolems reduce accuracy, torque control, and dnamic response, while increasing the need fregular maintenance. They also limit the tospeed on longer moves. And heavy enouloads will stall (stop) the motor, which ccause damage.

Electric motors have relatively poor po

er-to-wight ratios, compared to hydraluland pneumatic drives. This means a biggheavier motor must be mounted on the roboarm when a large amount of torque is needeeven if its needed for just a small portionthe total movement.

The rotary motion of most electric drivmust be geared down so that they can pr vide the speed or torque needed by the aror effector. However, manufacturers are nooffering direct-drive motors on robots, whic

bot compare pro-grammed (desired)positions to actualpositions.

A good exampleof an open-loop sys-tem is the operating

cycle of a car-wash machine. At the begin-

ning of the operation, the car is hookedup to a chain that moves the car through a

tunnel of hoses, cleaning brushes, and soapspreaders. The machine stops when the carpops out the other end of the tunnel. Such acar wash is considered an open-loop systemfor two reasons:

Neither cars or anything else is ever ex -amined by sensors during the washing cycleto see if they are clean. In other words, there isno feedback.

And cycle length is preset. It is not ad - justed to compensate for the amount of dirtremaining on the car or exactly how large acar is going through the washer. The cycle andall it attributes are predetermined.

Non-servo robots have limited number of movements, usually established by a mechani-cal stop. These types of robots do well at repet-

itive tasks, such as material transfer.In general, non-servo robots are:

Relatively inexpensive com -pared to servo robots.

Simple to understand,program, operate, and

maintain. Precise and reliable. Capable of fairly high

speeds of operation. Limited to relatively

simple programs.Servo Robots use feed-

back so they are consid-ered closed-loop devices.

In closed-loop devices,feedback sent to theservo amplifier af-fects the output.

Servo amps trans-late signals fromcontrollers intomotor voltages and

current signals.Servo amplifi-ers in motion-

control applicationfor robots provideprecise control of position or veloc-

A six-axis articulated-armrobot from Epson uses a series

of electric drives to movethe various joints and

give the robot itsdexterity.

A SCARA (selective compliantarticulated robot arm), such as thiselectrically-driven one from Epson,uses a parallel-axis joint layout,making it slightly compliant in the X-Y direction but rigid in the Z direction.And the articlated arm lets it extendinto confined areas or retract or foldup out of the way. This can be anadvantage when moving parts fromplace to place and for loading orunloading in enclosed areas.



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The market for robotsIn a recent market-research study, Global Industry Analysts Inc. San Jose,

Cali ., reported that the worldwide market or industrial robotics will hit143,000 units by 2015 and that most o the growth in robots will come romexpanding application possibilities, technology developments, rising valuepropositions, demographic shi ts, and ensuing labor shortages.

In 2008 and 2009, a drop in manu acturing and industrial production

limited demand or new robots. This was particularly noticeable in the auto-motive, consumer goods, semi-conductor and electronics, and rubber andplastic industries. For example, there was a major drop in new vehicle salesin the automotive industry, one o the largest user o robotics.

But the precipitous drop in robotics growth in the auto sector turned outto be largely temporary. And the recession has not changed the economic

undamentals o robotics. In other words, robots still give manu acturers ad-vantages in terms o production and labor cost e ciency.

The report credits the quick resurgence in spending on robots to theaccumulation o postponed and de erred orders, as well as manu acturersincreasing their investment in plant renovation, modernization, and capac-ity expansions. The growth in high-volume toy manu acturing and medi-cal and healthcare should also build demand or industrial robots over thenext ew years as these sectors look to beneft rom increased automation.

And demographic trends, especially the aging populations in most westerncountries as well as Japan will spur growth in robots as companies strive tomaintain production with smaller work orces.

Assembly-line tasks represents the largest application market or robotsworldwide. Welding, however, remains a key contributor to volume sales orrobots in North America and Europe.

Technological advances in robotics in areas such as artifcial intelligence,machine vision, and distributed motion control will let robots per orm awider range o tasks independently. These advances will make industrial ro-bots use ul and economical, boosting demand or them.

According to GIAs report, Asia-Pacifc will be the astest growing regionalmarket, with sales o industrial robots growing at 9.6% annually or the nextfve years. The markets in that area pushing demand will be countries suchas South Korea and China, which host some o the leading electronics manu-

acturers in the world.Increases in outsourcing o manu acturing to low-cost locations such as

China and India will also build demand or robots in the region.Major players in the market include ABB Limited, Adept Technology,

Inc., American Robot Corp., Denso Wave Inc., Evolution Robotics, Inc., FanucCorp., iRobot Corp., Kawasaki Robotics (USA) Inc, Kuka Roboter GmbH, Pana-sonic Welding Systems Co., Ltd., Nachi-Fujikoshi Corp.n, RoboGroup Tek Ltd.,Rockwell Automation Inc, ST Robotics, Staubli Corporation, and YaskawaElectric Corp.

atmospheres. However, since air is compress-ible, precise placement and positioning re-

quire additional components to achieve thesmooth control possible with a hydraulicsystem. It is also difficult to keep the air asclean and dry as the control system requires.Robots that use pneumatic actuator drivesare noisy and vibrate as the air cylinders andmotors stop. For lightweight pick-and-placeapplications that require both speed and ac-curacy, a pneumatic robot is potentially agood choice.

eliminate some of these limitations and prob-lems. These high-torque motors drive armsdirectly and do not need reducer gears.

The basic construction of a direct-drivemotor couples the motor with the arm seg-ment being moved, and this eliminates back-lash, reduces friction, and increases the me-chanical stiffness of the drive mechanism.

Using direct-drive motors in robots letsengineers come up with more streamlineddesigns. Maintenance is also reduced. Ro-bots using direct-drive motors can operate athigher speeds and with greater accuracy thanconventional electric-drive motors.

Hydraulic drivesMany early robots were driven by hydrau-

lics. A conventional hydraulic drive consists of a pump connected to a reservoir tank, controlvalves, and a hydraulic actuator, as well as aworking fluid. Hydraulic drives can gener-ate linear and rotary motion using much sim-pler arrangements than conventional electricdrives. One advantage of hydraulics over elec-tric drives is that the storage tank, in effect, cansupply a large amount of instant power, whichis not available from electric drives.

Other advantages include precise motioncontrol over a wide range of speeds and theability to handle heavier loads on the end of the manipulator arm. They can also be usedaround explosive materials and are not easily damaged when quickly stopped while carry-

ing a heavy load. However, they are expensiveto purchase and maintain, and are not energy efficient. Hydraulic actuator drivers are alsonoisier than electric drives and are not recom-mended for clean-room environments due tothe possibility of hydraulic fluid leaks.

Pneumatic drivesPneumatic drives use of air-driven actua -

tors. And because air is a fluid, many of thesame principles that apply to hydraulic drivesapply to pneumatic drives. For example,pneumatic and hydraulic motors and cylin-

ders are very similar.Most industrial plants have compressed-air pipes running throughout assembly areas,so compressed air is not only economical, itis readily available. This makes it easier andless costly to install robots that use pneu-matic actuator drives than hydraulic robots.

Pneumatic actuator drives work at highspeeds and are most useful for small-to-me-dium loads. They are economical to operateand maintain and can be used in explosive



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Working envelope

while others are irregular, and some robomay have several different work envelopThe four major configurations that detemine work envelope shape are: revoluCartesian, cylindrical, and spherical. Hera look at each:

Revolute configuration (articulated or jointed arm): This is the most commo

These robots are often referred to as beianthropomorphic because their movemenresemble those of a human arm and upptorso. Rigid segments take on the roles of tforearm and upper arm while various joinmimic actions of the wrist, elbow, and shouder. A joint referred to as the sweep reprsents the waist. A revolute robot generahas an irregularly shaped work envelope.

Revolute configurations can be furthbroken down into two formats: verticalarticulated and horizontally articulateVertically articu lated robots usually hafive rotary joints. Horizontally articulatconfiguration generally has one vertical linear joint and two rotary joints, and acommonly called SCARA (selective comance assembly robot arm) configurationSCARA robots are fairly yielding in horizotal motions, but rigid in vertical motionSCARA robots are well suited for operatioin which little vertical motion is needed, bsignificantly more horizontal motion is rquired. Such operations include assembwork where parts are taken from one loc

tion, perhaps a bin, and moved nearly horzontally to the product being assembled.The revolute configuration is far a

away the most flexible in terms of operatioand has the largest work envelope of all trditional configurations.

However, revolute robots need sophiticated and more expensive controllerProgramming is also more complex. Othconsiderations engineers must taken intaccount is that the revolute robots postional accuracy, load capacity, dynamicand repeatability vary with the location

the work envelope. The robot can also bcomes less stable as when the arm extendsits maximum reach.

Cartesian configuration: Arm moments of robots using the Cartesian configuration can be described by the three tradtional axis: X, Y, and Z, giving them a rectagular work envelope.

Movements of the arm and its joints cstart and stop simultaneously along all threaxes, so motion at the tool tip or effector

The work envelope

Another way to discriminate betweenrobots is based on their work envelopes orthe volume of space that can be reachedby the robots effector. In general, the envelope shape and size is a function of thecoordinate system used by the robotsarms and manipulators, and the arrange-

ment of joints and length of the manip-ulators segments. Work envelopes also vary from one manufacturer to another,depending on type of manipulator or armused. And combining different configu-rations in a single robot can also createnew working envelopes. Engineers shouldknow the application and the exact work envelope before choosing a specific robotconfiguration.

Some work envelopes are geometric,

Here is an example of a working envelopefor an articlated-arm robot. It shows themaximum vertical and horizontal reachof the arm as well as areas the arm willsweep through.

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lifting capabilities. Spherical robots are wellsuited to applications in which only a smallamount of vertical movement is needed, suchas loading and unloading a punch press. Itsdisadvantages include reduced mechanical ri-gidity, limited vertical mobility, and the needfor more sophisticated control systems than ei-ther the Cartesian or cylindrical robots. Theserobots also suffer form the same problems withinertia and accuracy cylindrical robots.

Many industrial robots are hybrids of these

four basic types. Robot designers can alwaysmake the base rotate or vertically collapse andexpand, or add additional joints and wriststo make the robot more nimble, tailored to aspecific task, or to the amount of space neededto install a robot. And for even more flexibility,industrial robots can be mounted on walls andceilings, as well as floors. MD

smooth. This letssuch robots movedirectly to specificpoints instead of following trajecto-ries parallel to eachaxis.

One advantage of

robots with a Carte-sian configurationis that their totally linear movementallows for simplercontrols, They alsohave a high degreeof mechanical rigid-ity, accuracy, and re-peatability. They cancarry heavy loads,and this weight lift-ing capacity doesnot vary at different locations within the work envelope. As to disadvantages, Cartesian ro-bots are generally limited in their movement toa small, rectangular work space.

Cylindrical configurations: Cylindricalrobots consists of a vertical post with a slidingarm mounted at 90, making it parallel to theground, The stationary post is often mountedso that it can rotate. The sliding arm robotmoves in and out, and can move up and downon the carriage that attaches it to the verticalpost. Movement along the three axes traces out

the cylindrical work envelope, which is usually larger than the envelope of Cartesian robots.Cylindrical robots are well suited for pick-and-place operations.

The downside of cylindrical robot includesreduced mechanical rigidity due to the rotary axis needing to overcome inertia when rotat-ing. This gets amplified when the robots is car-rying a heavy load and the sliding arm is fully extended. Repeatability and accuracy is alsoreduced in the direction of rotary movement.Cylindrical robots need more advanced con-trols than Cartesian robots.

Spherical configuration (polar): Theserobots resemble turrets on military tanks. Apivot point gives the robot vertical movement,while a telescoping boom (the gun barrel of the tank) provides variable reach by extend-ing or retracting the effector. Rotary motionresults from the turret or base turning.

The spherical configuration generally pro-vides a larger work envelope than Cartesian orcylindrical configurations. The spherical de-sign is also simple and provides good weight-

This single-stagepneumatic robot fromMax Robot was builtto support injectionmolding. A special armlets it remove runnersand sprues. It can carryup to 4.5 lb

This hydrualicallydriven servo-robothas a rotary armwith two degreesof freedom.


0112Micromo Robotics

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