Robotics 2 E Part 2 pptx

20 Introduction: Brief Historical Review and Main Definitions location, it is necessary to have an automatically controlled computerized system to carry out the complete operation. This example (which does not pretend to be either the sole or the best concept of circuit assembly) permits us to derive some very significant conclusions: 1. Single manipulators working in concert facilitate the performance of simulta- neous operations, thus saving time; 2. Simple manipulators are faster because their masses are smaller (no compli- cated transmission or drives are necessary) and their stiffness is greater (fewer backlashes, smaller dimensions); 3. There are groups of devices which carry out universal tasks, e.g., hoppers, mag- azines, feeders, carriers, conveyors, and grippers; 4. There are devices or specialized manipulators which carry out some specific tasks, such as assembling, bending, and cutting. 1.4 Structure of Automatic Industrial Systems It is possible to describe a generalized layout of an automatic machine almost regardless of the level of control to which it belongs. We will thus show that the build- ing-block approach is an effective means of design of automatic machine tools. The following building blocks for devices may be used in the layout of automatic machines: • Feeding and loading of parts (or materials) blocks, • Functional blocks, • Inspection (or checking) blocks, • Discharge blocks, • Transporting (or removing) blocks. Blocks that are responsible for the feeding and loading of materials in the form of rods, wires, strips, powders, and liquids, or of parts such as bolts, washers, nuts, and special parts must also be able to handle processes such as orientation, measuring, and weighing. Functional devices are intended for processing, namely, assembling, cutting, plastic deformation, welding, soldering, pressing in, and gluing. Inspection or checking blocks ensure that the part being processed is the correct one and that the part is in the right position. These devices also check tools for readiness, wear, etc. The necessity for such devices for purposes of safety and efficiency is obvious. A discharge device is obviously used for releasing an item from a position and preparing the position for a new manufacturing cycle. Transporting devices provide for the displacement of semi-finished items and parts during the manufacturing process. These devices are responsible for ensuring that the parts are available in a certain sequence and that each part is in the correct place under the relevant tool, device, or arrangement at a certain time. There are different approaches to combining these building blocks in the design of an automatic machine. Let us consider some of the more widely used combina- tions. In Figure 1.20 we show a circular composition. Here, the feeding 1, functional 2, inspection 3, and discharge 4 devices are located around the transporting block 5. 1.4 Structure of Automatic Industrial Systems 21 FIGURE 1.20 Circular configuration for an automatic tool. Obviously, in many cases there can be several feeding, functional, and inspection blocks in one machine. The transporting block, which in this case is a rotating table driven by an indexing mechanism, moves in an interruptive manner. Its movement can be described by the speed-change form shown in Figure 1.21. Here, it can be clearly seen that the table moves periodically and each period consists of two components of time, ?!—the duration of movement, and t 2 —the duration of the pause. Obviously, both the functional devices and the loading and discharging blocks can act only when the rotat- ing 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 smaller are the time losses for nonproductive transportation. In the periodically acting systems some time is required for the idle and auxiliary strokes that the tools have to execute. For instance, a drill has to approach a part before the actual drilling operation, and it has to move away from the part after the drilling has been accomplished. These two actions may be described as auxiliary actions because no positive processing is carried out during their duration. On the other hand, no pos- itive processing can be carried out without these two actions. The time the device spends on these two strokes can, however, be reduced by decreasing the approaching FIGURE 1.21 Speed-versus-time diagram for an indexing mechanism. 22 Introduction: Brief Historical Review and Main Definitions and withdrawal distances and by increasing the speeds of approach and withdrawal. Similarly, the transportation of a part from a drilling position to, say, a threading posi- tion is an idle stroke. In principle, neither the threading process nor the drilling oper- ation requires this transportation component, which appears only as a result of the chosen design concept. Let us denote the idle and auxiliary time losses r, then where Tis the pure processing time. From Equation (1.1) we obtain: We can now introduce the concept of a processing efficiency coefficient rj l in the form A modification of the composition discussed above may also be used. In this mod- ification the blocks 1, 2, 3, and 4 are partly or completely placed inside the rotating table, as shown in Figure 1.22. This modification is more convenient because it facili- tates free approach of the items to the tools and to all the devices, while the devices do not obscure the working zones. However, the drives, the kinematics, and the main- tenance of this type of composition are more complicated. Another possibility is to build the transporting device 5 in a linear shape as a sort of a conveyer, as is shown in Figure 1.23. In this configuration the devices 1,2,3, and 4 are located on the same side of the conveyer (although there is no reason that they should not be located on both FIGURE 1.22 Circular configuration for an automatic tool with partial internal location of blocks. 1.4 Structure of Automatic Industrial Systems 23 FIGURE 1.23 A linear configuration for an automatic tool. sides of the block 5), thus facilitating maintenance from the other side. Obviously, Expression (1.1) is also valid in this case. The ideal situation occurs when ^ = 0, i.e., where there are no time losses during the process and the process is nonperiodic or continuous. Figure 1.24 presents an example of such a machine—the rotary printing machine (for newspaper printing). Here, 1 and 2 are the paper feeding blocks which incorporate a number of guiding rollers; 3 and 4 are the printing blocks which include the printing ink feeders and dis- tributors for both sides of the paper 2 and the impression cylinders 4 and 5 is the dis- charge block, which receives the completed product—folded and cut newspapers. The productivity of such a system is measured in speed units, say V = 5 m/sec of printed paper in a rotary printing machine, or 10 m/sec of wire for a drawing bench, or 15 m/sec for rolled stock produced on a mill. If the length of the paper needed for one newspaper (or any other piece-like product) is 1, then the productivity P is: Obviously, the time T c needed per produced unit is: This is the actual time spent for production, while the systems working periodically require, per product unit, a time interval T p . which includes nonproductive items t l and T. FIGURE 1.24 Rotary printing machine as an example of a continuous (nonperiodic) system. 24 Introduction: Brief Historical Review and Main Definitions The comparison of Expressions (1.6) and (1.7) proves that, for equal concepts (T c ~ T], the continuous process is about (1 + 77) lr\ times more effective. This fact makes the continuous approach very attractive, and a great deal of effort has been spent in introducing this approach for the manufacturing and production of piece-type objects. Sometimes it is even possible to design a continuously acting machine for com- plicated manufacturing processes. The main idea underlying this type of automative machine tool is represented in Figure 1.25. The machine consists of a number of rotors 1, each of which is responsible for a single manufacturing operation. The design of each rotor depends on the specific operation, and its diameter and number of posi- tions or radius depend on the time that specific operation requires; i.e., if the opera- tion n takes T n seconds, the radius r of rotor number n can be calculated from the following expression: where V = the peripheral velocity of the rotors, /„ = the length of the arc where the product is handled for the n-th rotor, r n = the radius of the «-th rotor, and (f) n - the 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 from one operation to another. The machine is also filled with a feeding device 3, where the blanks are introduced into the processing and with a discharging device 4, where the finished (or semi-finished) product is extruded from the machine. Thus, the main feature of such a continuously acting system is that the manufacturing operations take place during continuous transportation of the product. Therefore, there are no time losses for pure transportation. Let us now look at an example of this type of processing. Figure 1.26, which shows the layout of a continuous tablet production process, can serve as an example of a device for continuous manufacturing of noncontinuous products. Figure 1.26 presents a cross section through one of the rotors. The rotor consists of two systems of plungers, an upper system 1 and a lower series 2. The plungers fit cylinders 3 which are made into a rotating body 4 (this body is, in fact, the rotor). The device operates in the fol- FIGURE 1.25 Layout of a rotary machine for periodic manufacturing processes. 1.4 Structure of Automatic Industrial Systems 25 FIGURE 1.26 Layout of a continuous tablet manufacturing process. lowing way. At some point, one of the cylindrical openings 5 is filled with the required amount of powder for the production of one tablet. (This process is carried out by means of the movement of the rotor.) Then, the upper plungers begin to descend while the lower plungers create the bottom of the pressing die. When the pressing of the tablet 6 is finished, both plungers continue their downward movement to push the finished product out of the die in position 7. All these movements of plungers take place while the rotor is in motion. We can also imagine an intermediate case. This case is illustrated by the example of a drilling operation shown schematically in Figure 1.27. The rotor 1, which rotates with a speed V, is provided with pockets 2 in which the blanks 3 are automatically FIGURE 1.27 Layout of a pseudo-continuous drilling process. 26 Introduction: Brief Historical Review and Main Definitions placed. The drilling head 4 (which can be considered as a two-degrees-of-freedom manipulator) carries out a complex movement. The horizontal component of this movement is equal to the rotor's speed Kon the section a-b-c-d. The vertical compo- nent is made up as follows: fast approach of the drill to the blank on the section a-b (the drill's auxiliary stroke), drilling speed on the section b-c (processing stroke), and high-speed extraction of the drill on the section c-d (second auxiliary stroke). As soon as the second auxiliary stroke has been completed, the opening in the blank has been processed, and the drill must return to the initial point a to meet the next blank and begin the processing style. The time of one cycle is: where L is the linear distance between the pockets. The time t that the drilling head follows the rotor can be calculated from the obvious expression: where / is the distance through which the drilling head follows the rotor. The time T remaining for the drilling head to return is: Thus, the returning speed (the horizontal component) of the drilling head V l is: This case combines the two main approaches in automatic machining. Irrespective of the nature of the conceptual design of the automatic machine, it consists (as was stated above) of feeding, transporting, inspecting, tooling, and discharging blocks. The design of these blocks and some relevant calculations will be the subject of our discussion in the following chapters. 1.5 Nonindustrial Representatives of the Robot Family In this section we will discuss in brief some robot systems that almost do not belong to the family of industrial robots, namely, 1. Mobile robots 2. Exoskeletons 3. Walking machines 4. Prostheses Each of these kinds of special robot can be classified in terms of certain criteria. We will thus describe the main features of these machines and the principal parameters and concepts characterizing them. 1. Mobile robots These devices have a wide range of applications which are often not of an indus- trial nature. We can classify the types of mobile robotic machines described here in terms of their means of propulsion, i.e., wheels or crawler tracks. 1.5 Nonindustrial Representatives of the Robot Family 27 A mobile robot may be controlled in one of the following ways: • Remotely controlled by wires, cable, or radio; • Automatically (autonomously) controlled or programmed; or • Guided by rails, or inductive or optic means. Mobile robots find application in the following situations: • In harmful or hostile environments, such as under water, in a vacuum, in a radioactive location, or in space; or • Handling explosive, poisonous, biologically dangerous, or other suspect objects. Let us consider here, in general, the industrial applications of mobile robots. In some factories mobile robots are used as a means of transportation of raw materials, intermediate and finished products, tools, and other objects. One of the problems arising here is that of navigation. One side of the problem is the technical and algo- rithmic solution to the creation of an automatic control system. (This solution will be described in greater detail in Chapter 9.1.) The other side of the problem is the choice of the strategy for designing a pathway for such a vehicle. As an aid in clarifying this specific problem, let us look at the layout given in Figure 1.28. In our illustration an automatic waiter must serve nine tables in a cafeteria. FIGURE 1.28 Layout of a cafeteria showing the possible courses of movement of an automatic waiter. 28 Introduction: Brief Historical Review and Main Definitions Analyzing Figure 1.28, we can offer the "waiter" several possible courses. For example, from the bar or counter it can move through the following points: 1. A1-A2-A3-A4-B4-B3-B2-B1-C1-C2-C3-C4-D4-D3-D2-D1, or, 2. A1-A2-A3-A4-B4-C4-D4-D3-D2-D1-C1-B1-B2-B3-C3-C2-C1, etc. The criteria we must satisfy here are: • The minimal service time (which includes the distribution of meals, the col- lection of dishes, the distribution of bills, and the collection of money); • The optimal number of dishes on the trays; • The minimal disturbance to the customers. If the mobile robot is propelled on tracks, then a separate drive is required for navi- gation of the tracks. If it is propelled by wheels, a steering wheel is required (3-wheel designs are conventional), or each wheel is equipped with an independent drive of a special kind (see Figure 9.55). In the case of the above-described cafeteria, control could be effected, for instance, by colored strips on the floor combined with a system capable of counting the number of times the robot crosses each strip. The memory of the "waiter's" computer is pro- grammed with the action it has to perform after each crossing point. The commands would be of the kind: • Go ahead; • Stop; • Turn left or right; or • Turn around. If such a vehicle were combined with a manipulator, we would obtain a very flexible device which would be able to handle, for example, a suspicious object such as a bomb. 2. Exoskeletons Let us imagine a person working in a "hostile environment," such as under water or in space. His safety suit must withstand high pressures (external or internal). Obvi- ously, the joints of such a suit, its weight, and its resistance to the environment will hamper the movement of the person. Thus, special means must be provided to com- pensate for these harmful forces. These means can comprise an external energy source linked to a type of amplifier which permits the person enclosed in the safety suit to act almost normally as a result of the fact that the real forces developed by the device are significantly larger than those developed by the working person. We can then "extrapolate" the situation of the hostile environment to normal circumstances to provide a person with a means of protection from the environment or with a means of handling heavy objects which are far beyond the limits of a normal person. What- ever the specific application of use of the device, it must: 1) free the skeleton of the person from physical overloads; 2) amplify the person's muscular efforts; and 3) provide feedback to enable the user to gauge the reaction of the object being manipulated. The first requirement described above indicates that the device has an auxiliary function to the human skeleton, hence the name, exoskeleton. One possible design comprises a double-layered structure. The internal layer, which includes the control mechanism, makes direct contact with the operator. The external cover follows the 1.5 Nonindustrial Representatives of the Robot Family 29 movements of the internal layer and is responsible for amplifying the forces. This kind of system was first used in about 1960 in the Cornell Aeronautical Laboratory, U.S.A. For instance, the American exoskeleton known as Hardiman enables its operator to lift weights up to 450 kg. It has about 30 degrees of freedom (arms, legs, and body) and permits the operator to move at about 1.5 km/hour. The power system is hydraulic. Figure 1.29 shows the basic structure of an exoskeleton. It consists of a frame 1 to which the links to moving parts of the body are connected: the thighs 2, shins 3, feet 4, shoul- ders 5, elbows 6, and hands 7. Hydrocylinders 8 are used to drive the links. The power supply is provided by the compressor station 9 fastened to the back of the exoskele- ton. The control of the cylinders shown in the figure, and those which are not shown (such as the rotation of the elbow around its longitudinal axis) is carried out by the person enveloped in the exoskeleton. By moving his limbs which are connected to cor- responding links of the mechanical device, the person activates a system of amplifiers which in turn actuates the corresponding cylinders. The principle of the operation of the hydraulic amplifier will be explained in Chapter 4. Means of exploiting the biocur- rents of human muscles for this purpose are now being investigated. 3. Walking machines The wheel was invented about 6,000 years ago. This invention, coupled to an animal as a source of driving power, increased the possibility of load displacement about ten times. However, this invention created the problem of providing roads. To circumvent this complication (since roads cannot cover every inch of countryside) caterpillar tracks were invented. (This solution reduces the pressure under the vehicle by about eight times.) Thereafter efforts were devoted to creating a walking machine able to simulate the propelling technique of animals in such a way that the machine could move over rough terrain. The idea of creating a walking vehicle is not new. We will take as an example the walking mechanism synthesized by the famous mathematician Cheby- shev (1821-1894). Figure 1.30 presents the kinematic layout of this mechanism, while the photographs in Figure 1.31 show its realization produced in the laboratory of the Department of Mechanical Engineering of the Ben-Gurion University of the Negev. This mechanism fulfills the main requirement of a properly designed walking device; i.e., in practice, the height of the mass center of the platform 1 (see Figure 1.30) does not change relative to the soil. This ingenious mechanism, however, is not able to change direction or move along a broken surface. (It is an excellent exercise for the reader to find a means of overcoming these two obstacles.) The link proportions shown in Figure 1.30 are obligatory for this walking machine. The walking technique is more effective than wheel- or track-based propulsion, not only because obstacles on the surface can easily be overcome (for instance, legs climb- ing stairs), but also because the nature of the contact between the leg and the surface is different from that between a wheel or tracks and a road. As can be seen from Figure 1.32, the rolling wheel is continuously climbing out of the pit it digs in front of itself. This process entails, in turn, the appearance of a resistance torque Tas a result of the force F acting on the lever /. On the other hand, any type of walking mechanism is a periodically acting system. At this stage we should remember that dynamic loads increase in direct proportion to the square of the speed. In addition the design of such a walking leg is much more complicated than that of a rotating wheel. Thus, we cannot [...]... laser beam, in which case it is a matter of indifference what material is selected (Because resistance welding depends on the specific electrical resistance of the materials being treated, bronze obviously has a lower resistance than steel The lower the specific resistance, the worse the conditions for resistance welding because of the smaller amounts of heat produced at the contact point Therefore resistance... sheet in order to correct defects due to the rollers which support the sheet during cooling Obviously, the size of the rollers and the intervals between them will influence the quality of the sheet: the smaller the rollers and the smaller the intervals between them, the smaller the deflection of the sheet From the idea of reducing the size of the support rollers was derived the concept of molecular supports,... corrugated membranes To make the description more specific we will define the diameter of the device D to be 60 mm (such barometers are useful, for instance, in meteorological probing of the upper layers of the atmosphere) The two membranes are sealed hermetically connected along the perimeter The sealing or connecting techniques can vary: soldering, welding, or gluing A vacuum is created in the inner volume... of the legs; 2 Control system providing the required sequence of leg movements; 3 Control of mechanical stability, especially for movement along a broken surface or an inclined plane One of the possible solutions is similar to that for an exoskeleton; i .e. , the driver moves his limbs and the vehicle repeats these movements Such a vehicle becomes more cumbersome as the number of legs is increased A... the pressure p outside the device changes, the interval h between the membranes changes too This dependence must stay linear within a certain range of pressure changes What happens in the case of bronze is that the straight line changes its location and inclination when the temperature of the aneroid barometer changes  42 Concepts and Layouts FIGURE 2. 5a) Aneroid barometer sensor 'h i 0 — P»- FIGURE... type of robot 1.6 Relationship between Robot "Intelligence" and the Product 35 Let us denote: T = the lifetime of the automatic equipment or its concept, i .e. , the time the machine or its concept is useful; T = the lifetime of the product produced by the automatic equipment, or the time the product keeps being sold on the market, or the market demand for the product is maintained; T! = the time needed... the cutters, about the work conditions as a function of the material under consideration, about dimensions, etc Moreover, there are different ways to generate relative movement between the cutter and the blank The designer need only compare the possibilities, compute the approximate costs and productivities and derive the conclusion; the menu of concepts is spread out before him For instance, when... 2. 5b) Pressure versus displacement characteristics of an aneroid barometer The advantage of steel membranes is that their elasticity modulus is practically independent of temperature changes and thus no thermocompensators are needed However, steel is not suitable for soldering, and the only way to join steel membranes is by welding Among possible welding techniques, it would seem that the most effective... effective one is so-called seam resistance welding This process requires much more sophisticated equipment than soldering does There are differences between the two materials with regard to stamping as well The stamping properties of steel alloys are poorer than those of bronze However, this difficulty can be overcome by using specially designed stamps Nowadays welding of membranes can be carried out... feeding separate parts, like the membranes in Example 3 Next come functional operations like cutting the wire, bending, soldering, welding, etc The last operation usually consists of extracting the ready product or part from the machine The total time needed for one cycle to be completed is at least the sum of operational times To this sum we have to add non-operational time intervals, which are needed: . process. These devices are responsible for ensuring that the parts are available in a certain sequence and that each part is in the correct place under the relevant tool, device, . cannot cover every inch of countryside) caterpillar tracks were invented. (This solution reduces the pressure under the vehicle by about eight times.) Thereafter efforts were devoted to . and the vehicle repeats these movements. Such a vehicle becomes more cumbersome as the number of legs is increased. A four-legged lorry built by General Electric, which weighs 1,500