Robotics 2 E Part 11 potx

290 Functional Systems and Mechanisms FIGURE 8.11 Example of continuous chain of rivets for more effective assembly. simpler than feeding separate rivets; therefore, assembly will be more reliable. After the rivet is put into the appropriate opening, it is cut from the chain at neck 1. In Chapter 7 (see Figure 7.20) we also considered the idea of transforming essen- tially separate units into continuous form, for example, details used in electronic circuit assembly. Sometimes it is worthwhile to expend some effort in making this transfor- mation (e.g., gathering resistors into a paper or plastic bond) to increase the effec- tiveness of automatic assembly. Principle IV Design the component for convenient assembly. This principle is actually a par- ticular case of a more general principle which reads: Design the product so it is con- venient for automatic production. We have already met one relevant example in Figure 8.3. One of the most important features required in components one intends to assem- ble is convenience for automatic feeding and orientation. And here two recommen- dations must be made: • Design parts so as to avoid unnecessary hindrances; • Design parts so as to simplify orientation problems: with fewer possible distinct positions or emphasized features such as asymmetry in form or mass distribution. Some examples follow. Figure 8.12a) shows a spring that is not convenient for auto- matic handling. Its open ends cause tangling when the springs are placed in bulk in a feeder. The design shown in Figure 8.12b) is much better (even better is the solution discussed earlier and shown in Figure 8.10). Tangling also occurs with details such as those shown in Figure 8.13. Rings made of thin material and afterwards handled auto- matically must be designed with a crooked slit to prevent tangling. Analogously, thin flat details with a narrow slot, as illustrated in Figure 8.14, should be designed so that A < 5. This condition obviously protects these details from tangling when in bulk. An additional example appears in Figure 8.15, where a bayonet joint is used for a gasket- FIGURE 8.12 a) Spring design not recommended for automatic handling; b) Design of a spring more suitable for automatic handling. 8.2 Automatic Assembling 291 FIGURE 8.13 Ring-like parts: a) Tangling possible; b) Tangling almost impossible during automatic handling. FIGURE 8.14 To prevent tangling of these details, keep A<£. FIGURE 8.15 To avoid tangling, design b) is better than design a). like detail. Case a), with open horns, is dangerous from the point of view of automatic handling. Obviously, these horns cause tangling, they may be bent, and so on. The alternative shown in case b) is much more reliable. The behavior of details shaped as in Figure 8.16a) is clearly much worse than those in case b). The screws with cylindri- cally shaped heads behave more consistently on the tray than those with conical heads. The latter override one another, where the cylindrical screws stay in order. Reducing the number of stable positions on the orientation tray will simplify the orientation process and increase its reliability. For example, the part presented in case a) of Figure 8.17 is preferred over that in case b) because of the symmetry around the y-axis. This is true even if the design of the product requires only two openings (as in case b)). (Of course, the cost of making two additional openings must be taken into 292 Functional Systems and Mechanisms FIGURE 8.16 Details with the shape shown in case a) are less reliable on the feeding tray than those in case b). FIGURE 8.17 Orientation conditions of the part in case b) are worse than for those in case a), and those in case c) are best of all. consideration, in addition to the concurrent simplification of orientation and assem- bly.) We should also consider the dimensions b and h. As one can see, in cases a) and b), the difference between b and h is rather small. It is worthwhile to redesign the part so that b = h (see case c)) or, on the contrary, to increase their difference. In the first case (b = h) we obtain four indistinguishable positions of the detail on the tray, thus considerably simplifying the requirements for orientation. In the second case, making the dimensions b and h very different, for instance b « h, also facilitates orientation. The same idea, of exaggerating the difference in some feature of the part is useful in cases where a shift in the center of mass is used in orientation. Figure 8.18 illus- trates this for a stepwise-shaped roller. In cases a) and b) the difference A between the center of mass (m.c.) and the geometrical center (g.c.) of the detail is insignificant and difficult to detect and exploit reliably. To make this detail more suitable for automatic handling and assembling, use either cases c) or d), where the design is symmetrical, or case e), where the asymmetry is emphasized to make the difference A large enough for convenient and reliable orientation. For convenient assembly the details must be designed so as to decrease the require- ments for accuracy. For instance, as shown in Figure 8.19, it is much more difficult to assemble the design shown in case b) than that in case a), where the right-hand opening has an oblong shape. The latter design provides the same relative location between 8.2 Automatic Assembling 293 FIGURE 8.18 Effect of the relative location of the center of mass (m.c.) of a part with respect to its geometric center (g.c.). (See text for explanation.) parts 1 and 2 after assembly as case b) does; however, the effort in carrying out this assembly step is less for case a) because one can pay less attention to the accuracy of dimension /. The relations between the dimensions of components of an assembly are important in various ways. In addition to the previous example, Figure 8.20 illustrates the general subprinciple: do not try to fit two mounting surfaces simultaneously; do it in series: first one, then the other. The mounting surfaces in Figure 8.20 are denoted A and B. In case a) the pin (dimension IJ is designed so that it must be fitted simul- taneously to openings A and B during assembly, while in case b) the proper choice of value L 2 makes the assembly process sequential: first the pin is fitted to opening B and then guided by this opening toward completion of assembly, i.e., penetration of the thicker part of the pin into opening A. FIGURE 8.19 Use design a) for automatic (and even for manual) assembly; avoid the situation shown in b). 294 Functional Systems and Mechanisms FIGURE 8.20 Do not try simultaneous fitting of a pin into two openings. This kind of assembly must be done in series. Another subprinciple says: for automatic assembly the components must possess a certain degree of accuracy (which is correlated with their cost). A simple example based on automatic screwing of an accurate screw (Figure 8.21) is obvious. Case a) is normal, while in cases b) and c) the slot or the head is not concentric on the body of the screw. Cases d) and e) show defective screws: the first not slotted, the second not threaded. All the abnormal screw types should of course be prevented from arriving at the assembly position, or never be supplied in the first place. Even when all conditions are met, automatic assembly remains a serious problem, and its reliability influences the effectiveness of the whole manufacturing process. Reliability of Assembly Process Let us now suppose that some product consists of n components which are brought in sequence to the assembly positions, with the end result that a certain product is obtained (see Figure 8.22). Each position is characterized by reliability^, R 2 , R 3 , , R n FIGURE 8.21 a) Normal screwdriver and screw in position; b) and c) Eccentricity of the slot or screw head. Defective screws: d) Without slot; e) Without thread. 8.3 Special Means for Assembly 295 FIGURE 8.22 Simple model of an assembly process. of assembly. We define the values R t (where i - 1, , n] as the ratio between the number of successful assemblies N vi and the total number of attempts A/,; that is: The reliability of an automatic system R can be calculated as follows: For instance, if n = 4 and we have for R, The reasons for the appearance of defective assemblies have different sources: • Defective components, as shown in Figure 8.21, for example, • Defective operation of the assembly mechanism. Both types of reasons occur randomly. To increase the reliability special approaches can be taken, some of which will be considered in the following section. 8.3 Special Means of Assembly In this section we consider some possibilities for increasing the efficiency of auto- matic assembly. As a criterion for estimating the efficiency, we use the reliability R, which we defined above as Here N v =the number of successful assemblies, and Af=the number of assembly attempts. We also stated that, when an assembly or some other process requires a series of operations, the overall reliability is defined by Expression (8.2). The more components the whole assembly includes, the higher will be the number of failed assemblies and the smaller will be the estimated reliability. To improve this value we can propose dupli- cating some of the mechanisms comprising the assembly machine. A diagram of such an assembly machine of improved reliability is shown in Figure 8.23. This machine 296 Functional Systems and Mechanisms FIGURE 8.23 Diagram of high-reliability assembly machine. must put together two components, A and B. However, each of these components is fed twice: A at both positions A : and A 2 , and B at positions B : and B 2 . Thus, if feeding fails at positions A : or B : the inspection devices placed at positions I t and I 3 give a command to operate the feeding devices at positions A 2 and B 2 , respectively. The concept of failure includes: • Lack of a part in pocket 1 or 5, • A defective part, or • Defective orientation of a part. Let us compare the final reliability of this machine with one lacking duplicate feeding. Assuming that the reliability of each position in this machine equals R { , we obtain the estimation of the probability P that the feeding of component A (or B) fails, from the following expression: here i = the number of the feeding positions A or B. And thus the reliability of the whole machine equals For example, for R t = 0.90 (for both A and B) we obtain For the same R t value, a machine without duplication has the following reliability: Inspection position I 2 serves to stop feeding B : if, despite the duplication, something is wrong with part A, and to remove defective part A from pocket 4. Position I 4 directs 8.3 Special Means for Assembly 297 correctly assembled products into collector C and wrongly made products into col- lector W. We mentioned above that reliable assembly requires high accuracy in handling components. There is a method based on vibration that can increase the reliability of assembly. To explain the principal idea of this method, let us consider the following model for assembling two components, as shown in Figure 8.24. Here, bushing 1 rep- resents one component and pin 2 the other component of the assembly being put together. Bushing 1 is kept in pocket 3 while the pin is guided by part 4. The mating diameters of the bushing and pin are D : and D 2 , respectively. Because of various kinds of deviations in these dimensions and in assembly-tool displacements, an error S 0 in alignment occurs. The assembling force P can complete the process as long as the value <5 0 is within certain limits [<5 0 ]. To increase the chance of achieving satisfactory alignment, relative vibration between the components in the plane perpendicular to the force P may be helpful. The real situation existing during the alignment process is, of course, more complicated than that shown in Figure 8.24. Bevels on both details create an inclination angle a at the contact point A between the two details (this is helpful), as shown in Figure 8.25. The skew between the axes, designated j in the figure (this is harmful), is an obstacle in assembling. When vibrating, say, guide 4 (Figure 8.24) relative to part 1, the chances of creating better conditions for the penetration of pin 2 into the hole of part 1 are improved. Of course, the amplitude of vibration, the speed of relative displacement between the two parts (in the horizontal plane), the force P, the deviation 8, and the dimensions of the bevels are mutually dependent. The value of the vibration amplitude A should be estimated from the following formula: This dependence is derived for the frequency 50 Hz (electromagnetic vibrators fed by the industrial AC supply). Here, 8 = manufacturing tolerance of the conjugate parts, m = mass of the parts including pin 1 moved by force J? r = radius of the bevels, both inner and outer. The rest of the symbols are clear from Figure 8.25. FIGURE 8.24 Model of assembling two components. 298 Functional Systems and Mechanisms FIGURE 8.25 Skew phenomenon appearing during assembly of a pin into a hole. Figure 8.26 shows a plan for a specific device for vibration-assisted assembly. Bushing 1 and pin 2 are in the assembly device. The bushings are fed into pocket 3 and the pins are placed in guide 4. Pusher 5 presses the pin against the bushing with force P. Guide 4 is vibrated by magnets 6 and springs 7. As is clear from the cross section A- A, the magnets are energized from the main supply by coils 8 and, due to rectifiers 9, they produce a 50-Hz force. This force actuates armature 10 of guide 4. Tray 11 serves to lead parts 2 from the feeder into the assembly device. Another idea for increasing the effectiveness of assembly is based on rotation of the pin relative to the bushing, as presented in Figure 8.27. Pin 1 is placed in rotating cylindrical guide 3 and pressed towards the hole in part 2 by pusher 4 with force P. The angle 7 between the device's axis of rotation and the pin's axis of symmetry must be less than 2°. (The use of vibration and rotation for improving assembly has been inves- tigated and recommended by K. J. Muceniek, B. A. Lobzov, and A. A. Stalidzan, Riga Politechnic, USSR.) It is interesting to mention here that an electromagnetic field is a powerful means for assembly. A diagram of its effects is presented in Figure 8.28. The components we want to put together are placed in an alternating magnetic field so that the vector of induction is directed along the assembly axis. Here, the components are three rings 1, 2, and 3 of different sizes. The rings can be scattered, in which case no other method can gather them together (part a) of the figure). This scattering may reach about 80-90% of the ring diameters. It is interesting to note that the gathering of the rings is done in the shortest way by this electrodynamic method. At the end of the process the three rings are assembled, as shown in line e) of Figure 8.28. This phenomenon has the fol- lowing explanation: the alternating magnetic field results in the appearance of alter- nating currents i lf i z , and i 3 in the rings (part b) of the figure). The latter induce circular magnetic fields B 1; B 2 , and B 3 (part c) of the figure). The interactions between these fields move the rings together in the manner shown in part d) of Figure 8.28 until they come into the assembled state, as in part e). The proper choice of frequency of the magnetic field can even heat one of the rings and thus help to carry out assembling 8.3 Special Means for Assembly 299 FIGURE 8.26 Vibrating assembly device. FIGURE 8.27 Rotating assembly device. [...]... process Figure 8.30 shows a system belonging to the second case Here, grindstone 1 processes rotating cylindrical part 2 These parts are automatically fed and turned by FIGURE 8 .29 Scheme of a device for checking assembly completeness 3 02 Functional Systems and Mechanisms FIGURE 8.30 Device for examining and correcting grindstone wear lathe 3 Pick-up 4 (which can be pneumatic) measures the gap between... closed (This gate leads aneroids into the box for defective parts, as was stated earlier.) Thus, the aneroid continues its movement along the tray until it reaches open gate 2 or 3 (because contacts 5 to 9 belong to these two groups), depending on the specific one that the value S2 will indicate (See USSR Patent by B Sandier & A Strazdin, Automatic machine for pressure sensing elements linearity measurement... Figure 8. 32) The aneroids are loaded into a magazine-type hopper 11, from which rotating indexing table 12 brings them into test position 10 The table is driven by electric motor 16 and wormgear speed-reducer 17 In the test position, the aneroid is closed in hermetically sealed chamber 20 (sealing is provided by the super-finished surfaces of housing 7, cover 8, and rings 21 ) Then electromagnet 5 raises... are stored in the memory of the machine and processed, so that all aneroids that do not meet the linearity requirements are removed via tray 13 When the linearity test is passed satisfactorily, the aneroids are sorted according to sensitivity The bottom of tray 13 is made of gates 14 actuated by magnets 15 The first gate at the upper end of the tray is used to remove the defective aneroids Only aneroids... gate reach the selection process Here, according to remembered value S2, the appropriate gate opens and the aneroid falls into a box meant for this specific S2 The memory of the machine must hold the data until table 12 makes its next 90° rotation, bringing the measured aneroid to the top of the tray (not shown in Figure 8. 32) , where it falls onto the tray The machine is operated by master cam system... sufficiently linear This means that the maximum deviation of the measured deformations of the aneroid resulting from pressure changes must not fall outside a certain range of allowed values (see Figure 8.31) When the aneroid is subjected to changing pressure (in our example, the pressure changes from the atmosphere value P0 to zero), its thickness S in the center also changes By changing the pressure from... electromagnetic device that can reveal defective metallic assemblies among finished products Such a device is diagrammed in Figure 8 .29 Here, assemblies 1 are falling through an alternating magnetic field created by coil 2, which is fed by an alternating voltage of a certain frequency The eddy currents induced in assemblies 1 are a function of the mass and shape of their components Thus, the energy absorbed by... bodies 1 depends on their perfection and so does the current in the coil This results in a voltage drop t/output across the resistance R This voltage is used for sorting out defective assemblies Another level of inspection takes place when, for example, the dimensions of cut, ground, etc., parts are checked In this cases the sensors must provide continuous measurement within a certain range of values... inspection level is useful in two cases: • When the product (either some detail, part, or piece of material) must be sorted and, say, collected into separate groups according to its dimensions or other parameter; • When the dimension or other parameter measured during production reflects the state of the instrument, tool, or process, and serves as a feedback for correcting, retuning, or replacing the... pallets where they are placed in an oriented position in the order mentioned above, and brings them into the corresponding place of the "cradle," thus carrying out the assembly of the puzzle (This work was supervised by Dr V Lifshits in the CIM laboratory of the engineering faculty of the Ben-Gurion University of the Negev, Beersheva, Israel) In our case, at this stage of development, the puzzle parts . figure). The interactions between these fields move the rings together in the manner shown in part d) of Figure 8 .28 until they come into the assembled state, as in part e) . The . defined by Expression (8 .2) . The more components the whole assembly includes, the higher will be the number of failed assemblies and the smaller will be the estimated reliability. . serves to lead parts 2 from the feeder into the assembly device. Another idea for increasing the effectiveness of assembly is based on rotation of the pin relative to the