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The basics of robotic motionRobotic motions rely on electrical, pneumaticor hydraulic power, and digital controllers.T    ...
A six-axis articulated-arm              bot compare pro-         ity. In a sense, a servomechanism detects and            ...
eliminate some of these limitations and prob-lems. These high-torque motors drive arms              The market for robotsd...
while others are irregular, and some robots                                                                     may have s...
smooth. This letssuch robots movedirectly to specificpoints instead offollowing trajecto-ries parallel to eachaxis.    One...
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  1. 1. The basics of robotic motionRobotic motions rely on electrical, pneumaticor hydraulic power, and digital controllers.T here are all types of industrial robots, and most can be broken down into a handful of basic components. One of the most basic are the drives and the controls. The drive provides power and can be electric, hydraulic, or pneumatic, while the controller determines how that power is used to move arms and actuators whichcarry effectors or tools to the workpiece. Let’s step through the basics, taking a look first at Level II: Path Control: At this level, separatecontrollers. movements along the planes (determined in Level I) are combined into desired trajectories or paths.Controllers Level III: Main Control: At this level, written Controller coordinates all the movements of the instructions from a human programmer definingrobot’s mechanical actuators. They also receive input the tasks required are interpreted and reconstructedfrom the immediate environment through various so that Level II controllers can understand them.sensors. Machine vision, for example, gives robots In other words, the instructions are combined witheyes to “see” objects, patterns, and whether an object various sensory signals and translated into the moreis properly orientated for the next step in assembly. elementary instructions that Level II can understandThese days, controllers all contain a digital micro- and carry out.processor linked to inputs and outputs, including Robots can be further classified in a number ofmonitoring devices. ways, depending on their size, tasks, industry, and Commands issued by controllers activate motion- use. For engineers and designers, robots are usu-control devices consisting of various sub-controllers, ally broken down into three classifications: types ofamplifiers, and actuators. Actuator are motors or control, types of drives, and the shape of the workvalves that converts power into movement of the envelope.robot. Movements are initiated by a series of instruc-tions or program stored in the controller’s memory. Type of control Controllers usually have three levels of hierarchi- Robots traditionally use one of two control sys-cal control. In a hierarchical-control scheme, levels tems: non-servo and servo. The earliest robots wereof organization are assigned to various sub-control- non-servo, which are considered non-intelligent Presented bylers. Each level sends control signals to the level be- robots. Servo robots, however, are classified as eitherlow while getting feedback and instructions from the intelligent or highly intelligent, with the main differ-level above. Levels become more elemental as they ence between intelligent and highly intelligent ro-moves toward the actuator. bots being the level of awareness its sensors give it. T h e c om m on t h re e c ont rol l e ve l s are : Non-Servo robots, the simplest robots, are often Sponsored byLevel I: Actuator Control: Here’s where separate referred to as “limited sequence,” “pick-and-place,”movements of the robot along various planes, such or “fixed-stop” robots. They operate in open-loopas the X, Y, and Z axes, are generated. systems where there is no feedback that lets the ro- 1 january 2012
  2. 2. A six-axis articulated-arm bot compare pro- ity. In a sense, a servomechanism detects and robot from Epson uses a series grammed (desired) corrects for errors. of electric drives to move the various joints and positions to actual Servo robots are: give the robot its positions. • Relatively expensive to purchase, operate, dexterity. A good example and maintain. of an open-loop sys- • Us e a s ophisticated, clos ed-lo op tem is the operating controller. cycle of a car-wash machine. At the begin- • Have a wide range of capabilities. ning of the operation, the car is hooked • Can transfer objects from one point to up to a chain that moves the car through a another, as well as along a controlled, continu- tunnel of hoses, cleaning brushes, and soap ous path. spreaders. The machine stops when the car • Can interpret and use sophisticated pops out the other end of the tunnel. Such a programming. car wash is considered an open-loop system • Use a manipulator arm programmed to for two reasons: avoid obstructions within the work envelope. • Neither cars or anything else is ever ex- amined by sensors during the washing cycle Actuator drive types to see if they are clean. In other words, there is Another common way of classifying ro- no feedback. bots is by the type of drive used by its actua- • And cycle length is preset. It is not ad- tors. Most robot use ether electric, pneumatic justed to compensate for the amount of dirt or hydraulic actuators. remaining on the car or exactly how large a Electric drives encompass three kinds of car is going through the washer. The cycle and motors: ac servo motors, dc servo motors, all it attributes are predetermined. and stepper motors. Many newer robots use Non-servo robots have limited number of servo motors rather than hydraulic or pneu- movements, usually established by a mechani- matic ones. Small and medium-size robots cal stop. These types of robots do well at repet- commonly use dc servo motors. Larger ro- itive tasks, such as material transfer. bots rely on ac servos for their high torque In general, non-servo robots are: capabilities, • Relatively inexpensive com- Stepper motors are incrementally con- pared to servo robots. trolled dc synchronous motors. They are rarely • Simple to understand, used in commercial industrial robots, but are program, operate, and commonly found in educational robots. maintain. Robots that use electric drives take up less • Precise and reliable. floor space, and their energy source, electric- • Capable of fairly high ity, is readily available compared to hydrau- speeds of operation. lic and pneumatic power. However, conven- • Limited to relatively tionally geared drives suffer from backlash, simple programs. friction, compliance, and wear. These prob- Servo Robots use feed- lems reduce accuracy, torque control, and dy- back so they are consid- namic response, while increasing the need for ered closed-loop devices. regular maintenance. They also limit the top In closed-loop devices, speed on longer moves. And heavy enough feedback sent to the loads will stall (stop) the motor, which can servo amplifier af- cause damage. fects the output. Electric motors have relatively poor pow- Servo amps trans- er-to-wight ratios, compared to hydralulic A SCARA (selective compliant articulated robot arm), such as this late signals from and pneumatic drives. This means a bigger, electrically-driven one from Epson, controllers into heavier motor must be mounted on the robot uses a parallel-axis joint layout, motor voltages and arm when a large amount of torque is needed, making it slightly compliant in the X-Y current signals. even if it’s needed for just a small portion of direction but rigid in the Z direction. Servo amplifi- the total movement. And the articlated arm lets it extend into confined areas or retract or “fold ers in motion- The rotary motion of most electric drives up” out of the way. This can be an control application must be geared down so that they can pro- advantage when moving parts from for robots provide vide the speed or torque needed by the arm place to place and for loading or precise control of or effector. However, manufacturers are now unloading in enclosed areas. position or veloc- offering direct-drive motors on robots, 2 january 2012
  3. 3. eliminate some of these limitations and prob-lems. These high-torque motors drive arms The market for robotsdirectly and do not need reducer gears. In a recent market-research study, Global Industry Analysts Inc. San Jose, The basic construction of a direct-drive Calif., reported that the worldwide market for industrial robotics will hitmotor couples the motor with the arm seg- 143,000 units by 2015 and that most of the growth in robots will come fromment being moved, and this eliminates back- “expanding application possibilities, technology developments, rising valuelash, reduces friction, and increases the me- propositions, demographic shifts, and ensuing labor shortages.”chanical stiffness of the drive mechanism. In 2008 and 2009, a drop in manufacturing and industrial productionUsing direct-drive motors in robots lets limited demand for new robots. This was particularly noticeable in the auto- motive, consumer goods, semi-conductor and electronics, and rubber andengineers come up with more streamlined plastic industries. For example, there was a major drop in new vehicle salesdesigns. Maintenance is also reduced. Ro- in the automotive industry, one of the largest user of robotics.bots using direct-drive motors can operate at But the precipitous drop in robotics growth in the auto sector turned outhigher speeds and with greater accuracy than to be largely temporary. And the recession has not changed the economicconventional electric-drive motors. fundamentals of robotics. In other words, robots still give manufacturers ad- vantages in terms of production and labor cost efficiency.Hydraulic drives The report credits the quick resurgence in spending on robots to the Many early robots were driven by hydrau- accumulation of postponed and deferred orders, as well as manufacturerslics. A conventional hydraulic drive consists of increasing their investment in plant renovation, modernization, and capac-a pump connected to a reservoir tank, control ity expansions. The growth in high-volume toy manufacturing and medi-valves, and a hydraulic actuator, as well as a cal and healthcare should also build demand for industrial robots over the next few years as these sectors look to benefit from increased automation.working fluid. Hydraulic drives can gener- And demographic trends, especially the aging populations in most westernate linear and rotary motion using much sim- countries as well as Japan will spur growth in robots as companies strive topler arrangements than conventional electric maintain production with smaller work forces.drives. One advantage of hydraulics over elec- Assembly-line tasks represents the largest application market for robotstric drives is that the storage tank, in effect, can worldwide. Welding, however, remains a key contributor to volume sales forsupply a large amount of instant power, which robots in North America and not available from electric drives. Technological advances in robotics in areas such as artificial intelligence, Other advantages include precise motion machine vision, and distributed motion control will let robots perform acontrol over a wide range of speeds and the wider range of tasks independently. These advances will make industrial ro-ability to handle heavier loads on the end of bots useful and economical, boosting demand for them.the manipulator arm. They can also be used According to GIA’s report, Asia-Pacific will be the fastest growing regional market, with sales of industrial robots growing at 9.6% annually for the nextaround explosive materials and are not easily five years. The markets in that area pushing demand will be countries suchdamaged when quickly stopped while carry- as South Korea and China, which host some of the leading electronics manu-ing a heavy load. However, they are expensive facturers in the purchase and maintain, and are not energy Increases in outsourcing of manufacturing to low-cost locations such asefficient. Hydraulic actuator drivers are also China and India will also build demand for robots in the region.noisier than electric drives and are not recom- Major players in the market include ABB Limited, Adept Technology,mended for clean-room environments due to Inc., American Robot Corp., Denso Wave Inc., Evolution Robotics, Inc., Fanucthe possibility of hydraulic fluid leaks. Corp., iRobot Corp., Kawasaki Robotics (USA) Inc, Kuka Roboter GmbH, Pana- sonic Welding Systems Co., Ltd., Nachi-Fujikoshi Corp.n, RoboGroup Tek Ltd.,Pneumatic drives Rockwell Automation Inc, ST Robotics, Staubli Corporation, and Yaskawa Pneumatic drives use of air-driven actua- Electric Corp.tors. And because air is a fluid, many of thesame principles that apply to hydraulic drivesapply to pneumatic drives. For example, atmospheres. However, since air is compress-pneumatic and hydraulic motors and cylin- ible, precise placement and positioning re-ders are very similar. quire additional components to achieve the Most industrial plants have compressed- smooth control possible with a hydraulicair pipes running throughout assembly areas, system. It is also difficult to keep the air asso compressed air is not only economical, it clean and dry as the control system readily available. This makes it easier and Robots that use pneumatic actuator drivesless costly to install robots that use pneu- are noisy and vibrate as the air cylinders andmatic actuator drives than hydraulic robots. motors stop. For lightweight pick-and-place Pneumatic actuator drives work at high applications that require both speed and ac-speeds and are most useful for small-to-me- curacy, a pneumatic robot is potentially adium loads. They are economical to operate good choice.and maintain and can be used in 3 january 2012
  4. 4. while others are irregular, and some robots may have several different work envelopes. The four major configurations that deter- mine work envelope shape are: revolute, Cartesian, cylindrical, and spherical. Here’s a look at each: Revolute configuration (articulated or jointed arm): This is the most common. These robots are often referred to as being anthropomorphic because their movements resemble those of a human arm and upper torso. Rigid segments take on the roles of the forearm and upper arm while various joints mimic actions of the wrist, elbow, and shoul- der. A joint referred to as the sweep repre- sents the waist. A revolute robot generally has an irregularly shaped work envelope. Revolute configurations can be further broken down into two formats: vertically articulated and horizontally articulated. Vertically articulated robots usually have Working envelope five rotary joints. Horizontally articulated configuration generally has one vertical or linear joint and two rotary joints, and are commonly called SCARA (selective compli- ance assembly robot arm) configurations. SCARA robots are fairly yielding in horizon- tal motions, but rigid in vertical motions. SCARA robots are well suited for operations in which little vertical motion is needed, but significantly more horizontal motion is re- quired. Such operations include assembly Here is an example of a working envelope work where parts are taken from one loca- for an articlated-arm robot. It shows the tion, perhaps a bin, and moved nearly hori- maximum vertical and horizontal reach zontally to the product being assembled. of the arm as well as areas the arm will sweep through. The revolute configuration is far and away the most flexible in terms of operations and has the largest work envelope of all tra- The work envelope ditional configurations. Another way to discriminate between However, revolute robots need sophis- robots is based on their work envelopes or ticated and more expensive controllers. the volume of space that can be reached Programming is also more complex. Other by the robot’s effector. In general, the en- considerations engineers must taken into velope shape and size is a function of the account is that the revolute robot’s posi- coordinate system used by the robot’s tional accuracy, load capacity, dynamics, arms and manipulators, and the arrange- and repeatability vary with the location in ment of joints and length of the manip- the work envelope. The robot can also be- ulator’s segments. Work envelopes also comes less stable as when the arm extends to vary from one manufacturer to another, its maximum reach. depending on type of manipulator or arm Cartesian configuration: Arm move- used. And combining different configu- ments of robots using the Cartesian configu- rations in a single robot can also create ration can be described by the three tradi- new working envelopes. Engineers should tional axis: X, Y, and Z, giving them a rectan- know the application and the exact work gular work envelope. envelope before choosing a specific robot Movements of the arm and its joints can configuration. start and stop simultaneously along all three Some work envelopes are geometric, axes, so motion at the tool tip or effector iswww. 4 january 2012
  5. 5. smooth. This letssuch robots movedirectly to specificpoints instead offollowing trajecto-ries parallel to eachaxis. One advantage ofrobots with a Carte-sian configurationis that their totallylinear movement This single-stageallows for simpler pneumatic robot fromcontrols, They also Max Robot was builthave a high degree to support injection molding. A special armof mechanical rigid- lets it remove runnersity, accuracy, and re- and sprues. It can carrypeatability. They can up to 4.5 lbcarry heavy loads,and this weight lift-ing capacity doesnot vary at different locations within the workenvelope. 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 outthe cylindrical work envelope, which is usuallylarger than the envelope of Cartesian robots. This hydrualicallyCylindrical robots are well suited for pick-and- lifting capabilities. Spherical robots are well driven servo-robotplace operations. suited to applications in which only a small has a rotary arm The downside of cylindrical robot includes amount of vertical movement is needed, such with two degrees of freedom.reduced mechanical rigidity due to the rotary as loading and unloading a punch press. Itsaxis needing to overcome inertia when rotat- disadvantages include reduced mechanical ri-ing. This gets amplified when the robots is car- gidity, limited vertical mobility, and the needrying a heavy load and the sliding arm is fully for more sophisticated control systems than ei-extended. Repeatability and accuracy is also ther the Cartesian or cylindrical robots. Thesereduced in the direction of rotary movement. robots also suffer form the same problems withCylindrical robots need more advanced con- inertia and accuracy cylindrical robots.trols than Cartesian robots. Many industrial robots are hybrids of these Spherical configuration (polar): These four basic types. Robot designers can alwaysrobots resemble turrets on military tanks. A make the base rotate or vertically collapse andpivot point gives the robot vertical movement, expand, or add additional joints and wristswhile a telescoping boom (the “gun barrel” of to make the robot more nimble, tailored to athe tank) provides variable reach by extend- specific task, or to the amount of space neededing or retracting the effector. Rotary motion to install a robot. And for even more flexibility,results from the turret or base turning. industrial robots can be mounted on walls and The spherical configuration generally pro- ceilings, as well as floors. MDvides a larger work envelope than Cartesian orcylindrical configurations. The spherical de-sign is also simple and provides good 5 january 2012