Robotics 2 E Part 11 potx

FIGURE 8.16 Details with the shape shown in case a are less reliable on thefeeding tray than those in case b.. For instance, as shown in Figure 8.19, it is much more difficult toassemble

tially separate units into continuous form, for example, details used in electronic circuitassembly 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 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 Figure8.3 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:

par-• Design parts so as to avoid unnecessary hindrances;

• Design parts so as to simplify orientation problems: with fewer possible distinctpositions 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 afeeder The design shown in Figure 8.12b) is much better (even better is the solutiondiscussed earlier and shown in Figure 8.10) Tangling also occurs with details such asthose 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, thinflat 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 thesedetails, 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 automatichandling Obviously, these horns cause tangling, they may be bent, and so on Thealternative 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 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

cylindri-Reducing the number of stable positions on the orientation tray will simplify theorientation process and increase its reliability For example, the part presented in casea) of Figure 8.17 is preferred over that in case b) because of the symmetry around they-axis This is true even if the design of the product requires only two openings (as incase b)) (Of course, the cost of making two additional openings must be taken into

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 incase 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 trates this for a stepwise-shaped roller In cases a) and b) the difference A between thecenter of mass (m.c.) and the geometrical center (g.c.) of the detail is insignificant anddifficult to detect and exploit reliably To make this detail more suitable for automatichandling and assembling, use either cases c) or d), where the design is symmetrical,

illus-or case e), where the asymmetry is emphasized to make the difference A large enoughfor convenient and reliable orientation

For convenient assembly the details must be designed so as to decrease the ments for accuracy For instance, as shown in Figure 8.19, it is much more difficult toassemble the design shown in case b) than that in case a), where the right-hand openinghas an oblong shape The latter design provides the same relative location between

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 taneously to openings A and B during assembly, while in case b) the proper choice ofvalue L2 makes the assembly process sequential: first the pin is fitted to opening B andthen guided by this opening toward completion of assembly, i.e., penetration of thethicker part of the pin into opening A

simul-FIGURE 8.19 Use design a) for automatic (and even for manual) assembly; avoid

the situation shown in b)

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 examplebased on automatic screwing of an accurate screw (Figure 8.21) is obvious Case a) isnormal, while in cases b) and c) the slot or the head is not concentric on the body ofthe screw Cases d) and e) show defective screws: the first not slotted, the second notthreaded 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 anassembly 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 beconsidered 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 ofoperations, the overall reliability is defined by Expression (8.2) The more componentsthe whole assembly includes, the higher will be the number of failed assemblies andthe 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

FIGURE 8.23 Diagram of high-reliabilityassembly machine.

must put together two components, A and B However, each of these components isfed twice: A at both positions A: and A2, and B at positions B: and B2 Thus, if feedingfails at positions A: or B: the inspection devices placed at positions It and I3 give acommand to operate the feeding devices at positions A2 and B2, respectively Theconcept 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 I2 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 I4 directs

8.3 Special Means for Assembly 297

correctly assembled products into collector C and wrongly made products into lector W

col-We mentioned above that reliable assembly requires high accuracy in handlingcomponents There is a method based on vibration that can increase the reliability ofassembly To explain the principal idea of this method, let us consider the followingmodel 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 puttogether Bushing 1 is kept in pocket 3 while the pin is guided by part 4 The matingdiameters of the bushing and pin are D: and D2, 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 <50 is within certain limits [<50] To increase the chance of achieving satisfactoryalignment, 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 bythe 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

FIGURE 8.25 Skew phenomenon appearingduring 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 andthe 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 ofthe 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 beless 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, RigaPolitechnic, USSR.)

It is interesting to mention here that an electromagnetic field is a powerful meansfor assembly A diagram of its effects is presented in Figure 8.28 The components wewant to put together are placed in an alternating magnetic field so that the vector ofinduction 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 methodcan 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 inthe shortest way by this electrodynamic method At the end of the process the threerings 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 circularmagnetic fields B1; B2, and B3 (part c) of the figure) The interactions between thesefields move the rings together in the manner shown in part d) of Figure 8.28 until theycome into the assembled state, as in part e) The proper choice of frequency of themagnetic 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 Vibratingassembly device

FIGURE 8.27 Rotating assembly device

FIGURE 8.28 Assembly of three ringlike metal parts in

an alternating magnetic field

with tension These kinds of methods are described in USSR patents 38008 (1972),

R K Kalnin and others; 434699 (1972), B Joffe and others; and 413724 (1972), B Joffeand R K Kalnin

8.4 Inspection Systems

Three kinds of inspection devices are widely used in automatic production Thepurpose of one kind is to check the manufacturing process at various stages as theproduct is made This kind of inspection must prevent idle strokes or operations oftools when a component, blank, or material is missing for some reason; it must savematerials and components (by not completing assembly if something is missing); and

it must warn the operator that the process is out of order

The second kind of inspection relates to the tools and the system Its purpose is to

be aware of the wear of tools (for example, cutting tools), thus being able to change,tune, or sharpen the tools (automatically or manually) This sort of inspection ensuresthe quality of the product and saves time losses due to unexpected damages and theirreappearances

8.4 Inspection Systems 301The third type of inspection is connected with sorting and checking the finishedproduct Its purpose is to separate damaged products from the bulk and (no less impor-tant) to sort the products into several groups according to parameters measured duringinspection This is a very powerful approach when selective assembly is to be carriedout later, using the sorted products This is the way ball-bearings are assembled Theexample of sorting bushings of roller chain (Chapter 6, Figure 6.21) also belongs to thiskind of inspection.

To carry out these inspection operations, appropriate sensors are used (see Chapter5) The types of inspections can be arranged in several groups according to their level

of complexity For instance, very often during assembly, the presence of the propercomponent at the correct place at the right moment is important (discussed in Chapter5) For this purpose, sensors of the "on-off" type can be used These are not too sophis-ticated, and prevent the production of incomplete products, which is especially dan-gerous when the outside of the product does not indicate this defect This solution isalmost the only possibility when nonmetallic details and products are being handled

It seems worthwhile to make a brief digression here to mention an electromagneticdevice that can reveal defective metallic assemblies among finished products Such adevice is diagrammed in Figure 8.29 Here, assemblies 1 are falling through an alter-nating magnetic field created by coil 2, which is fed by an alternating voltage of a certainfrequency The eddy currents induced in assemblies 1 are a function of the mass andshape of their components Thus, the energy absorbed by bodies 1 depends on theirperfection 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 mea-surement within a certain range of values This inspection level is useful in two cases:

• When the product (either some detail, part, or piece of material) must be sortedand, say, collected into separate groups according to its dimensions or otherparameter;

• When the dimension or other parameter measured during production reflectsthe state of the instrument, tool, or process, and serves as a feedback for cor-recting, retuning, or replacing the tool or process

Figure 8.30 shows a system belonging to the second case Here, grindstone 1processes rotating cylindrical part 2 These parts are automatically fed and turned by

FIGURE 8.29 Scheme of a device forchecking assembly completeness

FIGURE 8.30 Device for examining and correcting grindstone wear.

lathe 3 Pick-up 4 (which can be pneumatic) measures the gap between its tip and thecylindrical surface of the detail Its signal is processed in unit 5 and transmitted tomotor 6, which moves grindstone 1 appropriately, by means of gears 7 and lead screw

8 Other, partial examples of this inspection level are presented in Figure 6.13 and 6.20(see Chapter 6), where inspection is combined with transportation and is done on adiscontinuous basis

The next inspection level is concerned with more complicated handling of the sured results In general, the results must be remembered, compared, processed, etc.Some algorithm governing the sequence and logic of handling the data obtained bythe system must also reach a certain conclusion and control the action of the machine

mea-We discuss here one example of this kind of equipment: an automatic machine forsorting aneroids (Chapter 2, Section 2.1) according to their sensitivity, and for check-ing their linearity When the aneroids have been sorted in different groups, it helps toassemble them into blocks of four or five aneroids so as to obtain approximatelyuniform sensitivity of these pressure sensors even when the sensitivity of every singleaneroid may differ significantly However, the characteristics of each aneroid must besufficiently linear This means that the maximum deviation of the measured defor-mations of the aneroid resulting from pressure changes must not fall outside a certainrange of allowed values (see Figure 8.31) When the aneroid is subjected to changingpressure (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 P0 through

8.4 Inspection Systems 303

FIGURE 8.31 Deformation versus pressure for aneroids.

P l to P 2 , we obtain increases in S from S0 to values corresponding to points a, b, c and

A, B, C, respectively Point O is a floating one; thus, the value 50 for each aneroid differs

and we are forced to create a system of coordinates for measuring S from the floating

axis 0-0 By applying pressure to the aneroid, we obtain its characteristics in the form

of curves passing through the points mentioned above The S values corresponding to

points A, B, C, define the sensitivity of the aneroid and the group it belongs to Thewidth of the ranges AS may be made equal for each group, so that we have:

When, as a result of measuring, the value of the deformation S 2 corresponding to the

pressure P 2 falls within a certain range, the aneroid possesses a certain sensitivity and

belongs to a certain group All aneroids that have S 2 in this range must be collectedinto one box or compartment The first task of the machine is thus completed.The second task—checking the linearity of response—is based on measuring the

deformation S for some intermediate pressure P lt For ideally linear characteristics,these deformations are described by points on a straight line, as shown in Figure 8.31

In reality, there is usually some deviation of points a, b, c from the ideal locations Somerange of deviations is allowed, and the linearity check consists of examining whetherpoints a, b, c, lie in the allowed range of deviations, between Slfl, and S 2a , S lb , and

S 2b , S lc , and S 2c> , respectively

First we describe the mechanical layout of this automatic machine (see Figure 8.32).The aneroids are loaded into a magazine-type hopper 11, from which rotating index-ing table 12 brings them into test position 10 The table is driven by electric motor 16and wormgear speed-reducer 17 In the test position, the aneroid is closed in hermet-ically sealed chamber 20 (sealing is provided by the super-finished surfaces of housing

7, cover 8, and rings 21) Then electromagnet 5 raises pick-up 6 until the aneroid iscaught between the pick-up and upper support 9 Now the coils of inductance-typedisplacement sensor 22 come into action, and the distance between pick-up 6 and

upper support 9 creates the measured value S Three different pressures P0, P lt and P 2

FIGURE 8.32 Layout of an automatic machine for sorting and checking

the linearity of aneroids

are applied in turn in the chamber, by compressor 19 and vacuum pump 18 Control

valves 1 provide certain constant pressures in receivers 2 so that P0 = 800 mm of mercury column, P l = 350 mm, and P2 = 5 mm Valves 3 connect the receivers to the test chamber

in sequence Thus, when pressure P0 is applied to the chamber the value S0 is

mea-sured The values S l and S2 are similarly obtained for pressures P l and P 2 These S-values

are stored in the memory of the machine and processed, so that all aneroids that donot meet the linearity requirements are removed via tray 13 When the linearity test ispassed satisfactorily, the aneroids are sorted according to sensitivity The bottom oftray 13 is made of gates 14 actuated by magnets 15 The first gate at the upper end ofthe tray is used to remove the defective aneroids Only aneroids that pass this gate

reach the selection process Here, according to remembered value S 2 , the appropriate

gate opens and the aneroid falls into a box meant for this specific S2 The memory ofthe machine must hold the data until table 12 makes its next 90° rotation, bringing themeasured aneroid to the top of the tray (not shown in Figure 8.32), where it falls ontothe tray The machine is operated by master cam system 4 driven by main motor 16.Now we pass over to the "brains" of this machine, the layout of which is shown inFigure 8.33 We should note that the number of aneroids per year undergoing thisinspection and sorting is close to 1.5 million Thus, no special flexibility is needed forthe machine In addition, the response of an aneroid to pressure changes is relatively