Computer Aided Manufacturing Part-5 Robotic Systems Industrial Engineering Department King Saud University
Contents•What is an industrial robot? •The robot joints•The basic components of a •Robot classification · robot •Physical classification •Power sources for robots •Control classification •Hydraulic drive •Robot reach •Electric drive •Robot motion analysis and •Pneumatic drive control •Robot sensors •Robot Programming and •The hand of a robot Languages (end-effector) •Robot Selection•Robot Movement and •Robot applications Precision •Robot Economic
What is an industrial robot?The word "robot" is derived from a satirical fantasy play,"Rossums Universal Robots," written by Karel Capek in1921. In his play, Capek used the word to mean, "forcedlabor." The Robotics Industries Association (RIA), formerlyknown as the Robotics Institute of America, defines robot inthe following way:An industrial robot is a programmable multi-functional manipulator designed to move materials, parts,tools, or special devices through variable programmedmotions for the performance of a variety of tasks.
An industrial robot consists of a numberof rigid links connected by joints ofdifferent types, controlled and monitoredby a computer. To a large extent, thephysical construction of a robotresembles a human arm. The linkassembly mentioned above is connectedto the body, which is usually mounted ona base. This link assembly is generallyreferred to as a robot arm. A wrist isattached to the arm. To facilitate grippingor handling, a hand is attached at the end Figure 1of the wrist. In robotics terminology, thishand is called an end-effector. Thecomplete motion of the end-effector isaccomplished through a series of motionsand positions of the links, joints, andwrist. A typical industrial robot with six-degrees of freedom is shown in.
The widespread use of CNC in manufacturing is ideal for the use of industrial robots toperform repetitive tasks. Such tasks may involve handling heavy and sometimes hazardousmaterials. Sophisticated CNC machining centers can contain palette changers and specialinterfaces that can easily accommodate industrial robots.
Specialized robots can assist in both assembly and inspection processes.
Material handling robots are used in many industries. It may besurprising to find such robots used even in the fast food industry.
This material handling robot is used in preparingpalettes for shipping. Repetitive tasks are idealto be performed by such machines.
Shown is a Fanuc M-16i Robotic Arm used in a precision grindingprocess on automotive parts.
Shown is a FanucRobot arm liftingthree heavy boxes atonce. In usingrobotics, humansafety factors in sucha task are completelyeliminated. This alsogreatly reduces therisks of repetitivestress injuries tofactory workers.
Handling of dangerous materials is an important task for Robots toperform. The size and weight of some automotive parts may be toocumbersome and hazardous for humans to manipulate in certainprocesses.
Shown is a robotic arm used in conjunction with a small punch press.Together these two machines could comprise a small manufacturingcell. The use of Robotics in such a setup can greatly reduce thechance of human error and injury.
It is now commonplace to find automotive manufacturers usingrobotics in many phases of the automotive assembly line. Here anautomotive spray booth utilizes a Fanuc Robot arm is used to preciselydeposit paint on this car body. The use of robotics can improve thequality of certain manufactured goods.
Here a Fanuc S-420W material handling robot is used in theelectronic appliances industry. You will note several others in thebackground used in other steps of the manufacturing process.
Another Fanuc A-510 robot arm used in the food industry. Improvedproductivity is an important factor in using robotic equipment isrepetitive production line operations. It can greatly reduce thehuman factors which can lead to errors and risk of injury.
Shown are two Fanuch Robot arms employed to perform precisionwelding tasks. This type of process would be extremely difficult toachieve by humans.
THE BASIC COMPONENTS OF A ROBOTThe basic components of a robot include the manipulator, thecontroller, and the power supply sources. The types andattributes of these components are discussed next.Power Sources for RobotsAn important element of a robot is the drive system. The drivesystem supplies the power, which enables the robot to move. Thedynamic performance of the robot is determined by the drivesystem adopted, which depends mainly on the type of applicationand the power requirements.The three types of drive systems are generally used forindustrial robots: 1.Hydraulic drive 2.Electric drive 3.Pneumatic drive
Hydraulic Drive•A hydraulic drive system gives a robot great speed andstrength.•These systems can be designed to actuate linear or rotationaljoints.•The main disadvantage of a hydraulic system is that it occupiesfloor space in addition to that required by the robot.•Problems of leaks, making the floor messy.•Because they provide high speed and strength, hydraulicsystems are adopted for large industrial robots.•Hydraulic robots are preferred in environments in which theuse of electric-drive robots may cause fire hazards, for example,in spray painting.
Electric Drives•Compared with a hydraulic system,•An electric system provides a robot with less speed and strength.•Electric drive systems are adopted for smaller robots.•Robots supported by electric drive systems are more accurate,exhibit better repeatability•Cleaner to use.
Pneumatic Drive•Pneumatic drive systems are generally used for smaller robots.•These robots, with fewer degrees of freedom, carry out simplepick-and-place material-handling operations, such as picking upan object at one location and placing it at another location. Theseoperations are generally simple and have short cycle times.•Pneumatic robots are less expensive than electric or hydraulicrobots.•Most pneumatic robots operate at mechanically fixed end pointsfor each axis.•A big advantage of such robots is their simple modularconstruction, using standard commercially available components.This makes it possible for a firm to build its own robots atsubstantial cost savings for simple tasks such as pick and place,machine loading and unloading, and so forth.
Robotic SensorsThe motion of a robot is obtained by precise movements at its joints and wrist. Whilethe movements are obtained, it is important to ensure that the motion is precise andsmooth. The drive systems should be controlled by proper means to regulate themotion of the robot. Along with controls, robots are required to sense somecharacteristics of their environment. These characteristics provide the feedback toenable the control systems to regulate the manipulator movements efficiently. Sensorsprovide feedback to the control systems and give the robots more flexibility.Sensors such as visual sensors are useful in the building of more accurate andintelligent robots. The sensors can be classified in many different ways based on theirutility. In this section we discuss a few typical sensors that are normally used inrobots: 1.Position sensors. They are used to monitor the position of joints. 2.Range sensors. Range sensors measure distances from a reference point to other points of importance. 3.Velocity sensors. Velocity sensors are used to estimate the speed with which a manipulator is moved 4.Proximity sensors. Proximity sensors are used to sense and indicate the presence of an object within a specified distance or space without any physical contact
The Hand of a Robot: End-EffectorThe end-effector (commonly known as robot hand) mounted onthe wrist enables the robot to perform specified tasks. Varioustypes of end-effectors are designed for the same robot to make itmore flexible and versatile.End-effectors are categorized into two major types:1. Grippers:2. Tools.
Grippers are generally used to grasp and hold an object andplace it at a desired location. Grippers can be classified as: •Mechanical grippers, •Vacuum or suction cups, •Magnetic grippers, •Adhesive grippers, •Hooks, •Scoops, •Others. Figure 2Grippers usually operate in jaw type fashion by having fingerswhich either attach to the gripper, or are part of theconstruction, open and close. The attached fingers can bereplaced with new or different fingers, allowing for flexibility,see Figure 2. Grippers can operate with two fingers or more.
End-effector - ToolsAt times, a robot is required to manipulate a tool to perform anoperation on a workpart. Spot-welding tools, arc-weldingtools, spray-painting nozzles, and rotating spindles fordrilling and grinding are typical examples of tools used asend-effectors.
Gripper designs:There are manyapproaches to gripperdesigns. These Figuresshows the variouslinkages which result inpivoting action forgripping.
Robot Movement and PrecisionSpeed of response and stability are two important characteristicsof robot movement.Speed defines how quickly the robot arm moves from one pointto another.Stability refers to robot motion with the least amount ofoscillation. A good robot is one that is fast enough but at thesame time has good stability.The precision of robot movement is defined by three basicfeatures: 1.Spatial resolution 2.Accuracy 3.Repeatability
1. Spatial ResolutionThe spatial resolution of a robot is the smallest increment ofmovement into which the robot can divide its work volume. Itdepends on : •the systems control resolution and •the robots mechanical inaccuracies.The control resolution is determined by the robots positioncontrol system and its feedback measurement system. Thecontroller divides the total range of movements for anyparticular joint into individual increments that can be addressedin the controller. The bit storage capacity of the control memorydefines this ability to divide the total range into increments. Fora particular axis, the number of separate increments is given by Number of increments = 2n where n is the number of bits in the control memory.
EXAMPLEA robots control memory has 8-bit storage capacity. It hastwo rotational joints and one linear joint Determine thecontrol resolution for each joint, if the linear link can varyits length from as short as 0.2 m to as long as 1.2 m.SolutionControl memory = 8 bitFrom the equation above, number of increments = 28 = 256(a) Total range for rotational joints = 360Control resolution for each rotational joint = 360/256= 1.40625(b) Total range for linear joint = 1.2 - 0.2 = 1.0 mControl resolution for linear joint = 1/256 = 0.003906 m= 3.906 mm
2. AccuracyAccuracy can be defined as the ability of a robot to position itswrist end at a desired target point within its reach.In terms of control resolution, the accuracy can be defined as one-half of the control resolution.3. RepeatabilityRepeatability refers to the robots ability to position its end-effector at a point that had previously been taught to the robot.The repeatability error differs from accuracy as described below
Let point A be the target point as shown in Figure a. Because of thelimitations of spatial resolution and therefore accuracy, the programmedpoint becomes point B. The distance between points A and B is a result ofthe robots limited accuracy due to the spatial resolution. When the robot isinstructed to return to the programmed point B, it returns to point C instead.The distance between points B and C is the result of limitations on therobots repeatability. However, the robot does not always go to point Cevery time it is asked to return to the programmed point B. Instead, it formsa cluster of points. This gives rise to a random phenomenon of repeatabilityerrors. The repeat- ability errors are generally assumed to be normallydistributed. If the mean error is large, we say that the accuracy is poor.However, if the standard deviation of the error is low, we say that therepeatability is high.We pictorially represent the concept of low and high repeatability as well asaccuracy in Figure b, c, d, and e. Consider the center of the two concentriccircles as the desired target point. The diameter of the inner circlerepresents the limits up to which the robot end-effector can be positionedand considered to be of high accuracy. Any point outside the inner circle isconsidered to be of poor or low accuracy. A group of closely clusteredpoints implies high repeatability, whereas a sparsely distributed cluster ofpoints indicates low repeatability.
Figure (a) Accuracy and repeatability; (b), high accuracy and highrepeatability; (c) high accuracy and low repeatability; (d) lowaccuracy and high repeatability; (e) low accuracy and lowrepeatability.
THE ROBOTIC JOINTS A robot joint is a mechanism that permits relative movementbetween parts of a robot arm.The joints of a robot are designed to enable the robot to moveits end-effector along a path from one position to another asdesired.The basic movements required for the desired motion of mostindustrial robots are: •Rotational movement.- This enables the robot to place its arm in any direction on a horizontal plane. •Radial movement. This enables the robot to move its end- effector radially to reach distant points. •Vertical movement. This enables the robot to take its end- effector to different heights.
These degrees of freedom, independently or in combination withothers, define the complete motion of the end-effector. Thesemotions are accomplished by movements of individual joints ofthe robot aim. The joint movements are basically the same asrelative motion of adjoining links. Depending on the nature of thisrelative motion, the joints are classified as prismatic or revolute.Prismatic joints are also known as sliding as well as linear joints.They are called prismatic because the cross section of the joint isconsidered as a generalized prism. They permit links to move in alinear relationship.Revolute joints permit only angular motion between links.
The five joint types are:1. Linear joint (L). The relative movement between the input link and the output link is a linear sliding motion, with the axes of the two links being parallel.2. Orthogonal joint (O). This is also a linear sliding motion, but the input and output links are perpendicular to each other during the move.3. Rotational joint (R). This type provides a rotational relative motion of the joints, with the axis of rotation perpendicular to the axes of the input and output links.4. Twisting joint (T). This joint also involves a rotary motion, but the axis of rotation is parallel to the axes of the two links.5. Revolving joint (V). IN this joint type, the axis of the input link is parallel to the axis of rotation of the joint, and the axis of the output link is perpendicular to the axis of rotation.
(a) two forms of linear joint- type L;(b) two forms of orthogonal joint-type O;(c) rotational joint-type R;(d) twisting joint-type T;(e) revolving joint-type V.
A typical robot manipulator can be divided into two sections: •A body-and-arm assembly, and •A wrist assembly.There are usually 3 degrees of freedom associated with the body-and-arm, andeither 2 or 3 degrees of freedom usually associated with the wrist.At the end of the manipulators wrist is an object that is related to the task thatmust be accomplished by the robot. For example, the object might be a workpartthat is to be loaded into a machine, or a tool that is manipulated to perform someprocess. The body- and-arm of the robot is used to position the object and therobots wrist is used to orient the object.To establish the position of the object, the body-and-arm must be capable ofmoving the object in any of the following three directions: 1.Vertical motion (z-axis motion) 2.Radial motion (in-and-out or y-axis motion) 3.Right-to-left motion (x-axis motion or swivel about a vertical axis on the base)
To establish the orientation of the object, we can define 3 degrees of freedom forthe robots wrist. The following is one possible configuration for a 3 d.o.f. wristassembly: •Roll. This d.o.f. can be accomplished by a T-type joint to rotate the object about the arm axis. •Pitch. This involves the up-and-down rotation of the object, typically done by means of a type R joint. •Yaw. This involves right-to-left rotation of the object, also accomplished typically using an R-type joint.These definitions are illustrated in the following Typical configuration of a 3-degree- of-freedom wrist assembly showing roll, pitch, and Yaw yaw.
ROBOT CLASSIEFICATION AND ROBOT REACH Normally robots are classified on the basis of their physicalconfigurations. Robots are also classified on the basis of thecontrol systems adopted.Classification Based on Physical ConfigurationsFour basic configurations are identified: 1.Cartesian configuration; 2.Cylindrical configuration; 3.Polar configuration; 4.Jointed-arm configuration.
Cartesian ConfigurationRobots with Cartesian configurations, consist of links connectedby linear and orthogonal joints (L and O). The configuration ofthe robots arm can be designated as LOO. Because theconfiguration has three perpendicular slides, they are also calledrectilinear robots.
Cylindrical ConfigurationIn the cylindrical configuration, as shown in Figure 7, robotshave one twisting (T) joint at the base and linear (L) jointssucceed to connect the links. The robot arm in this configurationcan be designated as TLO. The space in which this robot operatesis cylindrical in shape, hence the name cylindrical configuration.
Polar ConfigurationPolar robots, as shown in Figure 8, have a work space of sphericalshape. Generally, the arm is connected to the base with a twisting (T)joint and rotatory (R) and/or linear (L) joints follow. The designationof the arm for this configuration can be TRL or TRR. Robots with thedesignation TRL are also called spherical robots. Those with thedesignation TRR are also called articulated robots.
Jointed-Arm ConfigurationThe jointed-arm configuration, is a combination of cylindrical andarticulated configurations. The arm of the robot is connected tothe base with a twisting joint. The links in the arm are connectedby rotatory joints.
Classification Based on Control SystemsBased on the control systems adopted, robots are classifiedinto the following categories: 1.Point-to-point (PTP) control robot 2.Continuous-path (CP) control robot 3.Controlled-path robot
Point-to-Point (PTP)The PTP robot is capable of moving from one point to another point. Thelocations are recorded in the control memory. PTP robots do not control thepath to get from one point to the next point. The programmer exercises somecontrol over the desired path to be followed by programming a series of pointsalong the path. Common applications include component insertion, spotwelding, hole drilling, machine loading and unloading, and crude assemblyoperations.Continuous-Path (CP)The CP robot is capable of performing movements along the controlled path.With CP control, the robot can stop at any specified point along the controlledpath. All the points along the path must be stored explicitly in the robotscontrol memory. Straight-line motion is the simplest example for this type ofrobot. Some continuous- path controlled robots also have the capability tofollow a smooth curve path that has been defined by the programmer. In suchcases the programmer manually moves the robot arm through the desired pathand the controller unit stores a large number of individual point locationsalong the path in memory. Typical applications include spray painting,finishing gluing, and arc welding operations.
Controlled-Path RobotIn controlled path robots, the control equipment can generatepaths of different geometry such as straight lines, circles, andinterpolated curves with a high degree of accuracy. Goodaccuracy can be obtained at any point along the specified path.Only the start and finish points and the path definition functionmust be stored in the robots control memory. It is important tomention that all controlled-path robots have a servo capability tocorrect their path.
Robot ReachRobot reach, also known as the work envelope or work volume, is the spaceof all points in the surrounding space that can be reached by the robot armor the mounting point for the end-effector or tool. The area reachable by theend-effector itself is not considered part of the work envelope. Reach is one ofthe most important character tics to be considered in selecting a suitable robotbecause the application space should not fall out of the selected robots reach.Robot reach for various robot configurations is shown in the following FigureFor a Cartesian configuration the reach is a rectangular-type space.For a cylindrical configuration the reach is a hollow cylindrical space.For a polar configuration it is part of a hollow spherical shape.For a jointed-arm configuration does not have a specific geometric shape.
A Four-Jointed Robot in ThreeDimensions:Most robots possess a work volume withthree dimensions. Consider the fourdegree-of-freedom robot in Figure 7.18.Its configuration is TRL: R. Joint 1 (typeT) provides rotation about the z axis.Joint 2 (type R) provides rotation about ahorizontal axis whose direction isdetermined by joint 1. Joint 3 (type L) isa piston that allows linear motion in adirection determined by joints 1 and 2.And joint 4 (type R) provides rotationabout an axis that is parallel to the axis ofjoint 2.The values of the four jointsare, respectively, 1, 2, 3 and 4. Giventhese values, the forward transformationis given by: Figure 7.18 A four degree robot with configuration TRL:R.
ROBOT PROGRAMMING AND LANGUAGESThe primary objective of robot programming is to make the robotunderstand its work cycle. The program teaches the robot thefollowing:•The path it should take•The points it should reach precisely How to interpret the sensor data•How and when to actuate the end-effector•How to move parts from one location to another, and so forth
Programming of conventional robots normally takes one of two forms:(1) Teach-by-showing, which can be divided into: • Powered leadthrough or discrete point programming • Manual leadthrough or walk-through or continuous path programming(2) Textual language programmingIn teach-by-showing programming the programmer is required to move the robot arm through the desired motion path and the path is defined in the robot memory by the controller.Control systems for this method operate in either:1. teach mode : is used to program the robot2. run mode: is used to run or execute the program.
Powered leadthrough programming uses a teach pendant toinstruct a robot to move in the working space.A teach pendant is a small handled control box equipped withtoggle switches, dials, and buttons used to control the robotsmovements to and from the desired points in the space.These points are recorded in memory for subsequent playback. Forplayback robots, this is the most common programming methodused. However, it has its limitations:•It is largely limited to point-to-point motions rather thancontinuous movement, because of the difficulty in using a teachpendant to regulate complex geometric paths in space. In casessuch as machine loading and unloading, transfer tasks, and spotwelding, the movements of the manipulator are basically of apoint-to-point nature and hence this programming method issuitable.
Manual leadthrough programming is for continuous-pathplayback robots. In walk-through programming, the programmersimply moves the robot physically through the required motioncycle. The robot controller records the position and speed as theprogrammer leads the robot through the operation.If the robot is too big to handle physically, a replica of the robotthat has basically the same geometry is substituted for the actualrobot. It is easier to manipulate the replica during programming.A teach button connected to the wrist of the robot or replica actsas a special programming apparatus. When the button ispressed, the movements of the manipulator become part of theprogram. This permits the programmer to make moves of the armthat will not be part of the program. The programmer is able todefine movements that are not included in the final program withthe help of a special programming apparatus.
Teach-by-showing methods have their limitations:1. Teach-by-showing methods take time for programming.2. These methods are not suitable for certain complex functions, whereas with textual methods it is easy to accomplish the complex functions.3. Teach-by-showing methods are not suitable for ongoing developments such as computer-integrated manufacturing (CIM) systems.Thus, textual robot languages have found their way into robot technology.
Textual language programming methods use anEnglish-like language to establish the logical sequence of a workcycle. A cathode ray tube (CRT) computer terminal is used toinput the program instructions, and to augment this procedure ateach pendant might be used to define on line the location ofvarious points in the workplace.Off-line programming is used when a textual language program isentered without a teach pendant defining locations in theprogram.
Programming LanguagesDifferent languages can be used for robot programming, andtheir purpose is to instruct the robot in how to perform theseactions. Most robot languages implemented today are acombination of textual and teach-pendant programming.Some of the languages that have been developed are:WAVE VALAML RAILMCL TL- 10IRL PLAWSINGLA VAL II
VAL IIIt is one of the most commonly used and easily learned languages.It is a computer-based control system and language designed for theindustrial robots at Unimation, Inc.The VAL II instructions are clear, concise, and generally self explanatory.The language is easily learned.VAL II computes a continuous trajectory that permits complex motionsto be executed quickly, with efficient use of system memory and reductionin overall system complexity. The VAL if system continuously generates robot commands and cansimultaneously interact with a human operator, permitting on-lineprogram generation and modification.A convenient feature of VAL If is the ability to use libraries ofmanipulation routines. Thus, complex operations can be easily and quicklyprogrammed by combining predefined subtasks.
Programming With VAL II The first step in any robot programming exercise is the physical identification of location points using the teach pendant. We do not have to teach all the points that the robot is programmed to visit; only a few key points have to be shown (e.g., the comer of a pallet). Other points to which it can be directed can be referenced from these key points. The procedure is simple. First use the keys or button of the teach pendant to drive the robot physically to the desired location and then type the command HERE with the symbolic name for that location. For example,HERE P1This command will identify the present location as P1.
Rules for the location name are as follows:1. It is any string of letters, numbers, and periods.2. he first character must be alphabetic.3. There must be no intervening blank.4. Every location name must be unique.5. There may be a limit on the maximum number of characters that can be used.The following example illustrates the general command format for VAL II: 100 APPRO P1 15In this example, 100 is the label that refers to this instruction, APPRO is the instruction to the robot to approach the location named P1 by a distance of 15 mm.
In the following, we describe the most commonly used VAL IIcommands.MOVE P1 This causes the robot to move in joint interpolation motion from its present location to location P1.MOVES P1 Here, the suffix S stands for straight-line interpolation motion.MOVE P1 VIA This command instructs the robot to move from itsP2 present location to P1, passing through location P2.APPRO P1 10 This command instructs the robot to move near to the location P1 but offset from the location along the tool z-axis in the negative direction (above the part) by a distance of 10DEPART 15 Similar to APPRO, this instructs the robot to depart by a specified distance (15 mm) from its present position. The APPRO and DEPART commands can be modified to use straight-line interpolation by adding the suffix S.
DEFINE PATH 1= The first command (DEFTNE) defines a path that consistsPATH(P1,P2,P3,P5) of series of locations P1, P2, P3, and P5 (all previously defined). The second command (MOVE) instructs the robot to move through these points in joint interpolation. AMOVE PATH 1 MOVES command can be used to get straight-line interpolationABOVE & BELOW These commands instruct the elbow of the robot to point up and down, respectively.SPEED 50 IPS This indicates that the speed of the end- effector during program execution should be 50 inch per second (in./s).SPEED 75 This instructs the robot to operate at 75% of normal speed.OPEN Instructs end effector to open during the execution of the next motion.CLOSE Instructs the end-effector to close during the execution of the next motion.OPENI Causes the action to occur immediately.CLOSEI Causes the action to occur immediately
If a gripper is controlled using a servo-mechanism, the following commands may also be available.CLOSE 40 MM The width of finger opening should be 40 mm.CLOSE 3.0 LB This causes 3 lb of gripping force to be applied against the part..GRASP 10, 100 This statement causes the gripper to close immediately and checks whether the final opening is less than the specified amount of 10 mm. If it is, the program branches to statement 100 in the programSIGNAL 4 ON This allows the signal from output port 4 to be turned on at one point in the program andSIGNAL 4 OFF turned off at another point in the program.WAIT10 ON This command makes the robot wait to get the signal on line 10 so that the device is on there.
logarithmic, exponential, and similar functions. The followingrelational and logical operators are also available.EQ Equal toNE Not equal toGT Greater thanGE Greater than or equal toLT Less thanLE Less than or equal toAND Logical AND operatorOR Logical ORNOT Logical complement
TYPE "text“ This statement displays the message given in thequotation marks. The statement is also used to display outputinformation on the terminal.PROMPT "text", INDEX This statement displays the messagegiven in the quotation marks on the tenninal. Then the systemwaits for the input value, which is to be assigned to the variableINDEX.In most real-life problems, program sequence control is required.The following statements are used to control logic flow in theprogram.GOTO 10 This command causes an unconditional branch tostatement 10.
IF (Logical expression) If the logical expression is true, the groupTHEN of statements between THEN and ELSE is executed. If the logical expression is(Group of instructions) false, the group of statements between ELSE and END is executed. The programELSE continues after the END statement. The group of instructions after the DO(Group of instructions) statement makes a logical set whose variable value would affect the logicalEND expression with the UNTIL statement.DO After every execution of the group of instructions, the logical expression is(Group of instructions)UNTIL(Logical expression) valuated. If the result is false, the DO loop is executed again; if the result is true, the program continues.
SUBROUTINES can also be written and called in VAL IIprograms. Monitor mode commands are used for functions suchas entering locations and systems supervision, data processing,and communications. Some of the commonly used monitormode commands are as follows:EDIT (Program name) This makes it possible to edit theexisting program or to create a new program by the specifiedprogram name.
EXIT This command stores the program in controller memory and quitsthe edit mode.STORE (Program name) This allows the program to be stored on aspecified device.READ (Program name) Reads a file from storage memory to robotcontroller.LIST (Program name) Displays program on monitor.PRINT (Program name) Provides hard copy.DIRECTORY Provides a listing of the program names that are storedeither in the controller memory or on the disk.ERASE (Program name) Deletes the specified program from memory orstorage.EXECUTE (Program name) Makes the robot execute the specifiedprogram. It may be abbreviated as EX or EXEC.ABORT Stops the robot motion during execution.STOP The same as abort.
EXAMPLE 1:Develop a program in VAL II to command a PUMA robot to unload a cylindrical part of 10 mm diameter from machine 1 positioned at point P1 and load the part on machine 2 positioned at P2. The speed of robot motion is 40 in./s. However, because of safety precautions, the speed is reduced to 10 in./s while moving to a machine for an unloading or loading operation.
EXAMPLE 2:Suppose we want to drill 16 holes according to the pattern shown in the Figure. The pendant procedure can be used to teach the 16 locations, but this would be quite time-consuming and using the same program in different robot installations would require all points to be taught at each location. VAL II allows location adjustment under computer control.The program allows all holes to be drilled given just one location, called STA at the bottom right-hand corner of the diagram. Actually, two programs are required, since one will be a subroutine.
ROBOT SELECTIONThis phenomenal growth in the variety of robots has made the robot selection process difficult for applications engineers. Once the application is selected, which is the primary objective, a suitable robot should be chosen from the many commercial robots available in the market.The technical features are the prime considerations in the selection of a robot. These include features such as:(1) degrees of freedom,(2) control system to be adopted,(3) work volume,(4) load-carrying capacity, and(5) accuracy and repeatability.
The characteristics of robots generally considered in a selection process include :1. Size of class2. Degrees of freedom3. Velocity4. Actuator type5. Control mode6. Repeatability7. Lift capacity8. Right-Left-Traverse9. Up-down-traverse10. In-Out-Traverse11. Yaw12. Pitch13. Roll14. Weight of the robot
We elaborate on some of these characteristics.1. Size of class. The size of the robot is given by the maximum dimension (x) of the robot work envelope. Four different classes are identified: • Micro (x <=1M) • Small (1<x <=2 m) • Medium (2 <x<=5m) • Large (x >5m)2. Degrees of freedom. The degrees of freedom can be one, two, three, and so on. The cost of the robot increases with increasing number of degrees of freedom.
3. Velocity. Velocity considerations are affected by the robots arm structure. There are various types of arm structures. For example, the arm structure can be classified into the following categories: • Rectangular • Cylindrical • Spherical • Articulated horizontal • Articulated vertical4. Actuator types. Actuator types have been discussed in the earlier sections. They are: • Hydraulic • Electric • PneumaticSometimes, a combined electrical and hydraulic control system may be preferred.
5. Control modes. Possible control modes -include: •Nonservo •Servo point-to-point (PTP) •Servo continuous path (CP) •Combined PTP and CPCharacteristics such as lift capacity, weight, velocity, and repeatabilityare divided into ranges. Based on the ranges, the characteristics arecategorized in subclasses. For example, lift capacity can be categorizedas 0-5 kg, 5-20 kg, 20-40 kg, and so forth.A simple approach to selecting a robot is to identify all the requiredfeatures and the features that may be desirable.
The desirable features may play an important role in the selection of robots.These desirable features in an individual robot may be ranked on a scale of,say, 1 to 10 and the desirability of these features itself may be assignedweights. Finally, rank the available robots that have these features based oncost and quality considerations.
EXAMPLEA manufacturing company is planning to buy a robot. For the type ofapplication, the robot should have at least six required features. It will behelpful to have more features that would add some flexibility in its usagecapabilities. The company is looking at six more desirable features. Fiverobots are selected from the initial elimination process ba ed on requiredfeatures. The rating score matrix R is given as:The entry in position (i, j ) represents the score given to the ith robot modelbased on how well it satisfies the j th desirable feature. The score is givenon a scale of 0 to 10. These scores are assigned by the applicationsengineers based on their experience and practical requirements.Furthermore, if the importance of desirable features is given by thefollowing weight vector, determine the priority ranking of robots for thegiven application. W = (0.9 0.3 0.6 0.5 0.8 0.4 )
Robot ApplicationsThe common industrial applications of robots in manufacturing involve loading and unloading of parts. They include:• The robot unloading parts from die-casting machines• The robot loading a raw hot billet into a die, holding it during forging, and unloading it from the forging die• The robot loading sheet blanks into automatic presses, with the parts falling out of the back of the machine automatically after the press operation is performed• The robot unloading molded parts formed in injection molding machines• The robot loading raw blanks into NC machine tools and unloading the finished parts from the machinesSafety and relief from handling heavy loads are the key advantages of using robots for loading and unloading operations.
A Single-Machine Robotic Cell ApplicationConsider a machining center with input—output conveyors and a robot to load the parts onto the machine and unload the parts from the machine as shown in the Figure. A typical operation sequence consists of the following steps:• The incoming conveyor delivers the parts to a fixed position.• The robot picks up a part from the conveyor and moves to the machine.• The robot loads the part onto the machine.• The part is processed on the machine.• The robot unloads the part from the machine.• The robot puts the part on the outgoing conveyor.• The robot moves from the output conveyor to the input conveyor.This operation sequence of the robotic cell is accomplished by a cell controller. Production rate is one of the important performance measures of such cells. We provide an example of determining the cycle time and production rate of a robotic cell.
EXAMPLECompute the cycle time and production rate for a single-machine robotic cellfor an 8-h shift if the system availability is 90%. Also determine the percentutilization of machine and robot. On average, the machine takes 30 s to processa part. The other robot operation times are as follows:Robot picks up a part from the conveyor 3.0sRobot moves the part to the machine 1.3sRobot loads the part onto the machine l.0sRobot unloads the part from the machine 0.7sRobot moves to the conveyor 1.5sRobot puts the part on the outgoing conveyor 0.5sRobot moves from the output conveyor to the input conveyor 4.0s
A Single-Machine Cell with a Double-Gripper RobotA double-gripper robot has two gripping devices attached to the wrist. The two grip ping devices can be actuated independently. The double gripper can be used to handle a finished and an unfinished workpiece at the same time. This helps increase productivity, particularly in loading and unloading operations on machines. For example, with the use of a double-handed gripper, the following robot operations could be performed during the machine operation cycle time:1. Move to conveyor2. Deposit a part and pick up a new part3. Move to the machineHowever, it must be mentioned that this is possible only if the machine operation cycle time is more than the combined time for activities 1,2, and 3. Furthermore, there is no need to move the robot arm from the output conveyor to the input conveyor.
ECONOMIC JUSTIFICATION OF ROBOTSWe have seen in the previous section on robot applications that robots are being used in a variety of industrial and domestic environments. Some of these applications are justified on the basis that the type of work, such as welding or painting, is dangerous and unhealthy for humans. It is, however, equally important to study whether the robotization is also economically justified. A large number of models for economic evaluation exist (for details, refer to White et al, 1989). In this section we provide a simple treatment by considering the payback period as a measure of economic justification of robots.
Payback Period MethodThe primary idea behind the payback period method is to determine how long it takes toget back the money invested in a project. The payback period i can be determined fromthe following relation: net investment cost (NIC) of the robot system including accessories n= net annual cash flowsNet investment cost = total investment cost of robot and accessories -investment tax credits available from the government, if anyNet annual cash flows = annual anticipated revenues from robot installation including direct labor and material cost savings - annual operating costs including labor, material, and maintenance cost of the robot system
ExampleDetroit Plastics is planning to replace a manual paintingsystem by a robotic system. The system is priced at$160,000.00, which includes sensors, grippers, and otherrequired accessories. The annual maintenance andoperation cost of the robot system on a single-shift basisis $10,000.00. The company is eligible for a $20,000.00tax credit from the federal government under itstechnology investment program. The robot will replacetwo operators. The hourly rate of an operator is $2000including fringe benefits. There is no increase inproduction rate. Determine the payback period for one-and two-shift operations.
SolutionNet investment cost capital cost - tax credits = $160,000 - $20000.00 $=140000.00Annual labor cost operator rate ($20/hr) X number of operators (2) X days per year(250 d/yr) X single shift (8 h/d) = $80,000 (for a single shift)For double-shift operation, the annual labor cost is $160,000.00.For a single-shift operation:Annual savings = annual labor cost - annual robot maintenance and operating cost =$80,000.00 - $10,000.00= $70,000.00The payback period for single-shift operation is Sl40,000,00/$70,000.00= 2 yearsFor double-shift operation, Annual savings= $160,000.00 - $20,000.00= $140,000.00.Therefore, the payback period for double-shift operation is $140,000.00/ $140,000.00 =1.00 years.A payback period of 2 years or less is a very attractive investment. In this example we have not considered any production rate increase with the robot system installation. Typically, such a system results in 30 to 75% increase in productivity. Based on these figures. this is an attractive proposal.