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TeamLRN TEAM LRN ROBOTICS Designing the Mechanisms for Automated Machinery Second Edition TEAM LRN This page intentionally left blank TEAM LRN ROBOTICS Designing the Mechanisms for Automated Machinery Second Edition Ben-Zion Sandier The Hy Greenhill Chair in Creative Machine and Product Design Ben-Gurion University of the Negev, Beersheva, Israel ® ACADEMIC PRESS San Diego London Boston NewYork Sydney Tokyo Toronto A Solomon Press Book TEAM LRN x Preface to the Second Edition a similar nature; it improves and shortens some mathematical deductions; and it con- tributes greatly to an understanding of the subject. For instance, one can find here: • Solutions of essentially nonlinear equations describing the behavior of a piston in pneumatic systems; • Equations describing the behavior of a body on a vibrating tray, widely used in, for example, vibrofeeding devices, which can be effectively solved by this com- putation tool (substituting boring traditional calculations); • Description of the behavior of a slider on its guides (a common structure in machinery) when dry friction exists in this pair, resulting in limited accuracy in the slider's displacement; • Equations (and an example of a solution) describing the free oscillations of a robot's arm when reaching the destination point. This is important for accuracy and productivity estimations; • Solutions of nonlinear equations describing the behavior of an electric drive equipped with an asynchronous motor, etc. The second edition is now more informative, more reliable, and more universal. I wish to express my deep gratitude and appreciation to my colleagues at the Mechanical Engineering Department of the Ben-Gurion University of the Negev for their spiritual support and cooperation in creating this book; to the Paul Ivanier, Pearl- stone Center for Aeronautical Engineering Studies, Department of Mechanical Engi- neering, Center for Robotics and Production Management Research; to Inez Murenik for editorial work on the manuscript; to Eve Brant for help in production and proof- reading; to Sidney Solomon and Raymond Solomon for sponsoring the book and for their skill in the production/design processes and project management. Finally, I thank my wonderful wife and family whose warmth, understanding and humor helped me throughout the preparation of this book. Ben-Zion Sandier December, 1998 TEAM LRN > Introduction: Brief Historical Review and Main Definitions 1.1 What Robots Are The word "robot" is of Slavic origin; for instance, in Russian, the word pa6oTa (rabota) means labor or work. Its present meaning was introduced by the Czechoslo- vakian dramatist Karel Capek (1890-1938) in the early twentieth century. In a play enti- tled R. U.R. (Rosum's Universal Robots), Capek created automated substitutes for human workers, having a human outlook and capable of "human" feelings. Historically, in fact, the concept "robot" appeared much later than the actual systems that are entitled to answer to that name. Our problem is that there is as yet no clear, efficient, and universally accepted def- inition of robots. If you ask ten people what the word "robot" means, nine will most likely reply that it means an automatic humanoid creature (something like that shown in Figure 1.1), or they will describe a device that may be more accurately denned as a manipulator or an automatic arm (Figure 1.2). Encyclopaedia Britannica [1] gives the following definition: "A robot device is an instrumented mechanism used in science or industry to take the place of a human being. It may or may not physically resemble a human or perform its tasks in a human way, and the line separating robot devices from merely automated machinery is not always easy to define. In general, the more sophisticated and individualized the machine, the more likely it is to be classed as a robot device." Other definitions have been proposed in "A Glossary of Terms for Robotics," pre- pared for the Air Force Materials Laboratory, Wright-Patterson AFB, by the (U.S.) National Bureau of Standards [2]. Some of these definitions are cited below. 1 TEAM LRN 2 Introduction: Brief Historical Review and Main Definitions FIGURE 1.1 Android-type robot. "Robot—A mechanical device which can be programmed to perform some task of manipulation or locomotion under automatic control." [Note: The meaning of the words "can be programmed" is not clarified. Programs can differ in their nature, and we will discuss this aspect later in greater detail.] "Industrial robot— A programmable, multi-function manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks." "Pick and place robot—A simple robot, often with only two or three degrees of freedom, which transfers items from place to place by means of point-to-point moves. Little or no trajectory control is available. Often referred to as a 'bangbang' robot." "Manipulator—A mechanism, usually consisting of a series of segments, jointed or sliding relative to one another, for the purpose of grasping and moving objects usually in several degrees of freedom. It may be remotely controlled by a computer or by a human." [Note: The words "remotely controlled .by a human" indicate that this device is not automatic.] "Intelligent robot—A robot which can be programmed to make performance choices contingent on sensory inputs." "Fixed-stop robot—A robot with stop point control but no trajectory control. That is, each of its axes has a fixed limit at each end of its stroke and cannot stop except at one or the other of these limits. Such a robot with AT degrees of freedom can therefore FIGURE 1.2 Manipulator or automatic arm. TEAM LRN 1.1 What Robots Are 3 stop at no more than 2Nlocations (where location includes position and orientation). Some controllers do offer the capability of program selection of one of several mechan- ical stops to be used. Often very good repeatability can be obtained with a fixed-stop robot." "Android—A robot which resembles a human in physical appearance." "Sensory-controlled robot—A robot whose program sequence can be modified as a function of information sensed from its environment. Robot can be servoed or nonser- voed. (See Intelligent robot.)" "Open-loop robot—A robot which incorporates no feedback, i.e., no means of com- paring actual output to command input of position or rate." "Mobile robot—A robot mounted on a movable platform." "Limited-degree-of-freedom robot—A robot able to position and orient its end effec- tor in fewer than six degrees of freedom." We will not discuss here the problem of the possibility (or impossibility) of actu- ally creating a robot with a "human soul." The subject of our discussion will be limited mainly to industrial robots, including those which belong to the family of bangbang robots. The application of these robots in the modern world must meet the require- ments of industry, including functional and manufacturing demands and economic interests. Obviously, esthetics and environmental considerations are also involved. The mechanical component of the design of robotic systems constitutes the main focus of our consideration. Historically, the development of robot systems and devices may be considered as the merging of two distinct lines of creativity: 1) early automation and watchmaking, and 2) innovations and refinements in industrial machinery. A brief description of some of these devices will be useful for illustrating these two lines. As long ago as 400-350 B.C. Archytas of Tarentum, a Greek, built a wooden model of a pigeon actu- ated by a steam jet. In about the first century A.D., Hero of Alexandria designed a number of devices actuated by water, falling weights, and steam. In about 500 A.D. in Gaza the Byzantines erected a huge water-operated clock in which the figure of Her- cules struck the hour in an automatic manner. Roaring lions and singing birds were employed to impress foreigners by the Byzantine emperor Theophilus (829-842). Roger Bacon (1220-1292) created a talking head, and at approximately the same time Alber- tus Magnus (1200-1280) created an iron man. These two manmade creatures may be classified as "androids." A "magic fountain" was designed in 1242 by a Parisian gold- smith, Guillaume Boucher. The German astronomer and mathematician Johann Muller (1436-1476) built a flying iron eagle. In the Fifteenth century, a truly portable source of mechanical power was invented and applied—the coiled tempered-steel spring. This energy source stimulated the creation of a number of sophisticated mechanical automatons. In 1738, Jacques de Vancanson (1709-1782) built a "flute player" capable of playing a dozen songs. During the eighteenth century, another group of gifted men, Jacquet-Droz, his son Pierre, his grandson Henri-Louis, and Jean-Frederic Leshot, created several androids which wrote, drew, or played musical instruments. The list of automatically actuated animals, men, birds, and so forth is never-ending, and we do not need to discuss it in detail, but two important conclusions do emerge: 1. This line of technical creativity was intended for entertainment purposes, and nothing productive was supposed to be achieved by these devices. TEAM LRN 4 Introduction: Brief Historical Review and Main Definitions 2. A large body of technical skills and experience, and many innovations, were accu- mulated by the craftsmen engaged in the production of such automatons. This amal- gamation of knowledge, skills, and experience found application in the second line of development mentioned above—development of, and the drive for perfection in, industry. We have reason to consider the clepsydra (a type of water clock) as the earliest rep- resentative of robotic devices. Supposedly invented in 250 B.C., it was able to recycle itself automatically. The centrifugal-speed governor for steam engines invented in 1788 by James Watt, together with the system of automatically controlled valves, made the steam engine the first automatic device capable of keeping an almost constant rotat- ing speed of the fly wheel regardless of changes in the load. Analogously, the internal combustion engines invented in the nineteenth century serve as an example of another automatically recycling device realizing repeatedly the suction, compression, and igni- tion of the fuel mixture. The Industrial Revolution stimulated the creation of a number of automatically operated machines first in the textile industry and later in machine tools and other industrial operations. The most brilliant invention of this type was Jacquard's loom, which had a punched-paper-tape-controlled system for flexible fabric- pattern production. We will return to this example a number of times, but it is worth mentioning here that this machine, which was introduced into industry as long ago as 1801, was based on an idea which is applicable to almost every definition of a robot, i.e., the machine is programmable and is intended for the execution of a variety of fabric patterns. In 1797, Henry Mandslay designed and built a screw-cutting lathe. The main feature of this machine was that it had a lead screw for driving the carriage on which the cutter was fastened and which was geared to the spindle of the lathe. Actually, this is a kind of template or contour machining. By changing the gear ratio practically any thread pitch could be obtained, i.e., the lead screw controlled a changeable program. Obvi- ously, this is the precursor of the tracer techniques used widely in lathes and milling machines. The later tools are to some extent robotic systems. The further refinement of this machine tool led to the creation of automatic lathes of a purely mechanical nature for the mass production of parts such as bolts, screws, nuts, and washers. These machines were, and still are, mechanically programmed, and after two to three hours the currently produced pattern can be exchanged for another. Many such machines were first produced between the years 1920 and 1930. In the 50s, after World War II, numerically controlled (NC) machine tools such as lathes and milling machines were first introduced into industry. These machines were, and still are, more flexible from the point of view of program changeability. At this level of refinement, the relative positioning between the tool and the blank had to be made by point-to-point programming of the displacements. When computerized numeri- cally controlled (CNC) machines replaced NC machines, the programming became more sophisticated—the trajectories were then computed by the computer of the machine. At this level of refinement the operator had to define both the kind of the trajectory (say, a straight line or an arc) and the actual parameters of the trajectory (say, the coordinates of the points connecting the straight line or the center coordi- nates and the radius of the arc, etc.). Other improvements were made in parallel, e.g., continuous measurement of the processed parts to fix the moment at which a tool TEAM LRN [...]... to pass the item to the manipulator 10 The manipulator 10 has two degrees of freedom, i.e., it rotates, and it has three possible positions at which it stops: at the first it obtains the item from the lever 9, at the second the leads of the electronic item are prepared for assembling by the device 11 , and at the third the actual assembling takes place For this latter operation, the lever of the manipulator... manipulator 10 drops so that the leads hit the corresponding openings in the X-Ftable which is already in position (The devices 6, 7 and 10 are of the cylindrical kind.) The device 11 cuts the leads to the proper length, and then bends them downwards to create the required pitch This action is shown schematically in Figure 1. 18 a and b, respectively After the items have been installed on the circuit plate, the. .. required Rotating the handle of the meat chopper, for example, the operator provides the device with the power needed for transporting the meat to the cutter, chopping it, and squeezing it through the device's openings The speed of feeding or meat transporting is coordinated with the chopping pace by the pitch of the snake and the dimensions and form of the cutter Analogously, when the key of the typewriter... Figure 1. 10 we show one way of transmitting the motion for only three (out of the total of seven) degrees of freedom The rotation relative to the X-X axis is achieved by the cylindrical pipe 1 which is placed in an immovable drum mounted in the partition The length of the pipe determines the distance between the operator and the servo-actuator The inside of the pipe serves as a means of communication for. .. seconds, the radius r of rotor number n can be calculated from the following expression: where V /„ rn (f)n = the = the = the - the peripheral velocity of the rotors, length of the arc where the product is handled for the n-th rotor, radius of the «-th rotor, and angle of the arc of the n-th rotor where the product is handled In addition, there are rotors 2 which provide for transmission of the product... serving arms When the distance between the arms is large, the deformations become significant; for example, for a distance of 1. 5-2 m, a force of 8 kg causes a linear deformation of 50-60 mm and angular deflections of 3-8° An additional problem occurs as a result of the mass of the mechanical "arms." To compensate for these weights, balance masses are used (in Figure 1. 10 they are fastened to the opposite... brings the corresponding sector 8 to the "pick-up" manipulator 7 The manipulator 7 has three degrees of freedom: it rotates, it moves vertically, and it has a gripper which moves radially along the lever 9 The barrel and the manipulator act in the following sequence The barrel 6 rotates to the required position, and then the gripper of the device 7 picks up the item For this purpose the lever 9 faces the. .. leads underneath the plate must be bent to fasten the items to the plate and to prepare them for soldering (which will provide proper reliable electric contact between the printed circuit and the electronic items) Figure 1. 19 presents a possible solution for bending of the leads Since there are different types of electronic items and the leads are different lengths and FIGURE 1. 18 Tool for cutting leads... ?! the duration of movement, and t2 the duration of the pause Obviously, both the functional devices and the loading and discharging blocks can act only when the rotating table is not in motion and the parts (or semifinished items) are in position under the tools that handle them Thus, the ratio becomes very important since it describes the efficiency of the transporting block The higher the ratio, the. .. r2 which is the domain in which, in TEAM LRN FIGURE 1. 11 Layout of a spherical manipulator 1. 3 Manipulators 15 principle, any point inside the sphere can be reached by a gripper fixed to the end of an arm In reality, there are certain restrictions imposed by the real dimensions of the links and the restraints of the joints which result in a dead zone in the middle of the sphere Sometimes the angle of . TeamLRN TEAM LRN ROBOTICS Designing the Mechanisms for Automated Machinery Second Edition TEAM LRN This page intentionally left blank TEAM LRN ROBOTICS Designing the Mechanisms for Automated Machinery Second . Figure 1. 9, in which the partition protects the operator sitting on the manual side of the device from the harmful environment of the working zone. The serving arm in the working. in the par- tition. The length of the pipe determines the distance between the operator and the servo-actuator. The inside of the pipe serves as a means of communication for