- Email: [email protected]
Recent Developments in Material Handling
RECENT DEVELOPMENTS IN MATERIAL HANDLING R. D. Schraft Fraunhofer-Institut fur Produktionstechnik und Aut omatisierung (IPA) , Stuttgart, Holzgart enstrasse 17, 7000 Stuttgart 1, F. R. C.
Abstract. Over the last few decades, the research and development in production engineering were concentrated on the technological processes in the manufacturing of parts. New manufacturing techniques were developed there, in particular to get shorter productive times. Compared with this, the workpiece manipulation in the manufacturing of parts and in assembly has so far largely been neglected. This paper deals with the automation of workpiece manipulation in the manufacturing of parts and in assembly. Moreover, an example of a flexible automatic assembly system will be shown. Keywords. Assembling; manipulation; man-machine systems; robots; sensors. 1. INTRODUCTION
Mechanical assembly facilities in the form of assembly machines and automatic assembly machines have found wide application in specific areas of industrial assembly. Particularly in the assembly of large batches and in mass production, a wide variety of technical solutions can be found on today's market. These solutions are promising, wherever there are favourable amortization conditions on the market, due to large quantities and a sufficiently long service life of the products involved. It is however a characteristic of th~se assembly machines and automatic assembly machines that they are principally only suitable for assembling those products for which they were developed and built. Automatic assembly appliances, therefore, can predominantly be considered as equipment within the field of custombuilt machine construction /1/.
nical-economical reasons. These assembly jobs are essentially characterized by the following features: - low production service life (less than two years), - similar products of a range of variants in terms of assembly, - sufficiently large quantity of the range of variants as a whole, - lot-by-lot assembly of the variants, - complex joining operations. In this country and abroad, the development of new automatic assembly systems for these assembly jobs not yet solved satisfactorily is in progress. Strictly speaking, flexible automatic assembly system operating on the shop floor on a wide range of products do virtually not exist as yet. An important starting point in the considerations to be made in a flexible assembly system is the demand for programmability of the technical equipment required for assembly, i.e. to adapt the assembly system's components to different assembly jobs through a known algorithm, if necessary, supported by simple readjusting operations. Since the
Apart from this area of assembly automation, there is a large number of assembly jobs which, although capable of being automated at an appropriately great technical expenditure - e.g. the quantity of a range of variants which is large in sum - but not suitable for handling by conventional automatic assembly machines for tech123
124
R. D. Sc h ra ft
introduction of industry robots in the area of parts manufacturing, experience has been available that can also be transferred to the field of assembling. Added to this is the favourable development in the field of electronic controls. This development is also of benefit to the complex sensors, used where no "arranged peripheral equipment" is present around the assembly system. This is particularly true in lot-by-lot manufacturing or in the manufacturing of small batches, where the magazining of the individual parts or an ordered passing on by means of workpiece carriers cannot be implemented for cost reasons /2/.
2. FLEXIBLE ARRANGEMENT The problem of flexible arrangement, that becomes increasingly interesting in the automation of small batches, has today only been solved in its rudimentary technical form. Thanks to his sensory powers, man is able to carry out the so-called "grip into the box" without any problems. If one wished to implement this technically, one will very soon be faced with limits today; nevertheless, there are some initial approaches, which however involve considerable technical expenditure or which are extremely restricted in their transferability, due to different application prerequisites. Today's state of the art permits several approaches: a. Once an arrangement of workpieces is reached, it is tried to maintain this arrangement. b. The problem of arrangement is to be reduced to the greatest possible extent by a design that lends itself for manipulation. c. If the parts are available in a bin in unarranged form, arrangement can proceed in steps. d. The "grip into the box" in parts presented in unarranged form. These various possible ways will be described in the following in detail. 2.1 DESCRIPTION OF THE VARIOUS APPROACHES IN THE SOLUTION TO PROBLEMS OF ARRANGEMENT Maintaining the arrangement achieved brings a whole number of advan-
tages. One does, for example not deal with any individual workpieces, but with a "flow material". This flow material is comparatively easy to handle. However, aids such as adhesive tape and shrink film are used, to make it possible to magazine difficult workpieces as "quasiflow material" (figure 1). Endeavours are being made for an extended period to strengthen the sensibility of design engineers regarding a design that lends itself for manipulation. This becomes increasingly necessary in the light of flexible, automatic arrangement. Some basic rules can, for example, be set forth to this effect: - designing parts either fully symmetrically or fully asymmetrically, - providing parts with unambiguous work characteristics, which can be interrogated during arrangement, - avoiding interlacing random parts, - making use of the material's properties for joining (e.g. the elasticity of plastic parts). Figures 2 to 4 show some examples to this effect. If there is a problem of arrangement that cannot be solved by conventional means of arrangement in a single operation, arrangement in steps constitutes a possible approach. This includes the gradual reduction of the work's translational and rotational degrees of freedom. Figure 5 illustrates this approach. "The grip into the box" mentioned at the beginning, though technically possible today in parts, cannot be realized economically. If one ignores a few approaches made in research laboratories, in which extreme restrictions have been made, it is still connected with too much technical expenditure to technically simulate man's sensory powers, as e.g. the coordination of the hand and the eye. The current endeabours being made in the field of flexible arrangement aim at the utilization of all constructive possibilities in the part for the time being, in order to make the task as simple as possible and, on the other hand, to design extisting mechanical means for arranging in a more flexible manner.
Recent deve l opme nt s i n ma t e r ia l ha ndli ng
2.2 EXAMPLES OF THE DEVELOPMENT OF FLEXIBLE MEANS OF ARRANGEMENT The following describes some of the developments made within the "Arbeitsgemeinschaft Handhabungssysteme I" (work group in manipulation systems I). The project was sponsored by the Federal ministry of research and technology under code 01 VC 044 - Z 13 - TAP 0002. 2.2.1 Systematizing studies As a basis of the analysis and development of feed equipment, a behaviour-orientated work classification was built up, which contained all the manipulation-relevant work parameters. Extracts of the classification can be seen from Figure 6. To be able to tell where the areas of application for feed equipment are, a workpiece statistics containing more than 3,000 workpieces out of 11 manufacturing plants were evaluated in addition, which supplied information about the frequency of occurrence of the work parameters illustrated in figure 6. To find out what feed equipment is available on the market , comprehensive market analyses were conducted. The result was compiled and published in a catalogue of commercially available feed equipment. This catalogue furnishes the user with an outline of possible functional principles and of the supply of feed equipment. By means of the systematic approach shown it also permits the solution to a manipulation problem and its specific implementation in a given workpiece range with the aid of the catalogue and register parts.
125
the scope of application of a vibratory spiral feeder can be considerably increased. This device e.g. permits the arranging of a workpiece range consisting of cylindrical parts of different characteristic forms; conversion to another workpiece takes 5 minutes max. ~!~~!~!~_~!~E~_~~~~~Y~~ (Figure 8) The analysis of different arrangement principles with regard to an increase in their flexibility led to the development of a flexible slope conveyor. It is characteristic of this device that it permits a wide and different workpiece range to be arranged by adjusting or replacing individual device components. By re-setting the bin geometry, the inclination of the discharge belt and the discharge strips, the belt speed and the discharge height as well as by replacing the discharge strips, the slope conveyor can be adjusted to different geometries, positions of gravity, masses, rolling and sliding properties of the workpieces to be arranged. ~~~2~~~~~!~_~~~~~_~!_~~~~~2~~~~~ (Figure 9)
The workpieces are put in a V-shaped groove in any position and will invariable assume one out of a few preferred positions by themselves. In the groove, the workpiece will be sensed by the sensors by optical transmission. A logic unit composes the sensor signals into a matrix image of the workpiece. The work image will be compared with reference images previously entered. If there is identity between the work matrix image and the reference image, the workpiece will be detected. If there is no agreement between the work image and the reference image, the workpiece will be sorted out.
From the large number of developments carried out by this work group (in about 40 instants the necessary feed equipment was developed and/or adapted and modified in the production testing of industrial robots), only a few developments are to be briefly presented in this paper by way of example.
The detected workpieces will be gripped by an industrial robot linked with the detection equipment and placed at a location appropriate to the detected position.
2.2.2 Equipment for the "arrangement function"
2.2.3 Equipment for the "assignment fUnction"
!!~~!~!~_~!~~~~~EY_~E!E~!_!~~~~~
The workpieces arranged in a flexible discharge or arrangement device will be made available to the industrial robot One by one by the flexible assignment equipment. For this purpose, the assignment plate is adjusted to the workpiece width involved by an adjusting screw.
(Figure 7) By combining a vibratory spiral feeder equipped with exchangeable and adjustable standard elements with a straightforward optoe1ectronic position-detecting device,
126
R. D. Schra f t
The workpieces are guided by means of adjustable rolling or sliding rails. Work clamping is carried out pneumatically by means of round diaphragm elements. Actuation of the clamping elements for gating is triggered by light barriers. Built into this assignment equipment is a tactile sensor plate which is capable of detecting two distinct workpiece positions during clamping. 2.2.4 Equipment for the "positioning function" An optically programmable two-axis positioning table has been developed for positioning. The 2-D-positioning table is used for pointby-point positioning of workpieces of any distance on a pallet with a medium positioning accuracy. The positioning table is controlled via a read head fastened on the table-top; it scans the signals on the program template located beneath. The program change is made by exchanging the program template. The program is recorded on a commercially available metal-clad carton by means of drawing ink. This includes the connection of the machining pOints (direction X) by horizontal and vertical (direction y) lines. These lines are supplemented by the symbols for rapid traverse ON, rapid traverse OFF, advance START and HALT. 2.2.5 Equipment for the "magazining function" ~!~~!~!~_~~~~!:~!~~~!~9_~~9~~!~~
A sheet-stacking magazine has been developed which easily converts to other sheet-metal contours and thicknesses for placing flat sheet metal parts in a press by means of an industrial robot. In this design, a slide is mounted on two horizontally clamping carriages in the directions X and Y on which the parts stack is placed and secured against displacement by vertical easy-to-adjust holders (six max.). A vertically adjustable plate on the slide compensates the different sheet-metal thicknesses. A possible displacement of the positioned part is prevented by a finely adjustable permanent magnet. ~!~!!!~_!~~~!_!!9~~!~~ (Figure 11)
In the search for magazine systems featuring versatile application, the drawing of plastic sheets has been one among several considerations. This technique is based on
the idea to place the workpiece to be magazined on a plastic sheet, to heat the sheet up and then to vacuum-form the sheet around the workpiece. The advantages of these single-purpose magazines are the simple making and the low manufacturing costs. The problem of flexibility was thus avoided by finding a way of obtaining extremely costfavourable magazines, which can be manufactured at short notice as required. Compared with other flexible magazines to be used in known parts ranges, this solution has the advantage that magazines can also be made without any great expenditure for such parts which are new in a parts range or have to be taken up in addition. Thus, this magazine solution is particularly suitable for those plants which manufacture independent of customer requests and whose workpiece range is subject to constant changing. 2.3 Over the past few years there has been an increase in the compelling demand for more flexibility in the means of arrangement. It has been shown that the deliberate utilization of the workpiece properties, starting with the design to the utilization of material properties on the one hand, and through the development of flexible means of arrangement for manipulating different workpieces on the other, make progress in this area possible. In the future, a combination of mechanical feed equipment of a certain degree of flexibility with simple optical or tactile sensors will bring a greater degree of flexibility in means of arrangement. The problem solutions thus being originated will then also lend themselves for economical use in industral practice. The set-up of a pilot work station for the automatic feeding of a drilling machine illustrated in Figure 12 may be an example of such a problem solution. Conversion of this automatic work station takes only a few minutes. On this experimental set-up, the functional evidence was furnished for the integration of an industrial robot, flexible grippers, flexible means of arrangement, for the plastic magazine described and for an optical sensor used for workpiece recognition.
Rece nt d eve l opme nt s
~n
ma t e r ia l hand li ng
127
3. SENSORS USED IN INDUSTRIAL ROBOTS
3.1 CONTACT-TYPE SENSORS
By sensors are meant those components of a measuring device which are directly exposed to the phenomenon to be measured or recorded. In the literature, this definition of the term is partially extended to a cons derable extent. Sensors are used for recording stochastic variables in the surroundings of the industrial robot, for measuring physical quantities as well as in pattern and position recognition /3/.
Analyses of work stations reveal that the applications of industrial robots could be considerably extended, if the functions of arranging and testing were taken over by suitable sensors.
Technical sensors process the signals coming from one or several pick-ups into the full information (e.g. orientation and position) of an industrial robot, with this information being capable of containing both simple Yes/No-decisions and analogous digital data. Hence, a sensor consists of the pick-up and an associated signal analyzer; it is however also possible to perform signal conversions and amplification within the sensor. (Figure 13). The information reception of sensors may either be by contact (tactile) or contactless. Contactless sensors operate for example by the following physical principles: by optical, acoustic, capacitive, electromagnetic, inductive or fluidic means. Contact-type and contactless sensors are primarily used in the different manipulation and machining operations on the various workpieces and tools. The aUdio sensors also shown in Figure 14 have so far only be used in production engineering for quality inspection and for calibration. Visual sensors are of particular importance in automation and these sensors have been the subject of research work for many years. Despite their importance in automation, visual sensors have so far hardly found any practical application in industry, which can partially be explained by their high price and their low reliability. Figure 15 is an outline of the main properties of the most important video sensors. Tactile sensors record forces and moments by contact. Also recorded are the distribution of force and moments, e.g. in machining operations or during assembly work. ~ 1& shows some of the typical types out of the many possible types of tactile sensors.
Conditioned by the positioning uncertainty of industrial robots, the assembly of workpieceswith narrow tolerances is often not possible. Here, tactile sensors are capable of recognizing the occurrence of any deviation in the mating parts to be joined and to permit a correction of the position or an orientation of the mating parts to be joined. In the assembly of complete modules, test operations have to be carried out additionally and forces or moments have to be limited, in order to prevent damage to the components. In the automation of assembly by means of industrial robots in the manufacturing of small and medium batches, the test functions performed by man visually or by touching must be taken over by sensors. Under more straightforward conditions, the sensors can monitor the programmed motion cycle and interrupt the cycle in the event of errors or, else, they must take over direct control of the assembly operation, e.g. by search movements or by position or pattern recognition. When operating with power-driven and non-driven tools guided by hand, the contours must be obtained visually or by touching. In deburring or in dressing of castings, operations have for example to be partially carried out by means of rotating grinding tools, which requires considerable physical strain. In the automation of such operations by means of an industrial robot, tool guiding must be supported by sensors, which condition the information on the forces acting on the workpiece in such a way that the industrial robot's control can be influenced accordingly. 3.2 CONTACTLESS SENSORS The main reason for which industrial robots are only used in relatively small numbers at the present time is deemed to be the problem of workpiece arrangement, with the difficulties mainly lying in the lack of flexibility of the equipment used so far. In addition, there are many work stations at which visual checks are made by man as quality inspections, which would also have to be made by industrial robots in an automated environment. In the manufacturing of small and medium batches, the transport of the workpieces between the production and
R. D. Sc hr a f t
128
or assembly facilities is often in non-arranged form. Therefore, these workpieces cannot be directly gripped by the industrial robot. Apart from the recording of the direct environment of a robot, e.g. by means of optical sensors for flow monitoring, the workpiece recognition and classification are the main task of contactless sensors. Such sensors must permit the positioning of an industrial robot, e.g. by optical patterns or the "grip into the box", to enable an industrial robot to be used in places where man was necessary because of his visual faculties. 4. COMPLEX SENSOR SYSTEMS In connection with the "grip into the box" there has been a series of developments throughout the world, of which the development of the American University of Rhode Island /5/ is to be described on behalf of others. In the experimental set-up, a 6-axis industrial robot is used, for whose three linear axes simple elements of machine-tool building are used and to whose projecting axis Y three further rotational manual axles are fitted. A diode-array camera is located between two lamps on the jib, with the camera "looking into" the box. A suction gripper with its hose guided in a coil spring is approached for gripping a workpiece out of the "hand", so that any workpiece position is permissible in the box for gripping of the workpiece. This gripping operation works in 60 to 80 cases out of 100 on the first trial. To detect the position in which the workpiece has been gripped, it is shown to another stationary camera. If the workpiece requires re-gripping, it will be put on a place of deposit also provided with a vacuum gripper. For loading the workpiece in a machine (not available and thus fictitious in the experimental set-up), an additional gripper can be flange-mounted, by means of which a simpler loading in a machine is made possible than in the current design of the industrial robot. In this experimental set-up which, to be honest, must however not be mistaken for a pilot application for industrial use, the taking up, recognition and the selective depositing of a workpiece still takes about 30 seconds; this is rather a matter of basic research, in which the apparatus set-up is only an
experimental unit which certainly lends itself for being further optimized. In spring of 1979, the Fraunhofer-Institut fur Informationsverarbeitung in Technik und Biologie (Fraunhofer institute of information processing in engineering and biology - IITB) demonstrated its application of a TV system for pattern recognition in the "grip into the box". In this system, an industrial robot equipped with a magnetic gripper grips in a box under program control. An inductive sensor accommodated in the gripper signals the presence of a part at the gripper, so that the industrial robot moves the object gripped into the back-lighted image field of the optical sensor, which recognizes the object and measures it. The industrial robot then removes the object from the sensor image field by carrying out a selective gripping movement and thus performs the actual arranging operation. The research results achieved over several years with respect to computer-controlled automatic assembly of small and/or changing batches have been elucidated by Charles Stark Draper Lab., USA, in an experimental set-up for the assembly of electric generators /6/. The assembly system consists of an industrial robot with four degrees of freedom and the associated peripheral equipment of altogether 12 means of arrangement. The industrial robot assembles the 17 parts of the generator with the aid of two different appliances from a single direction, with the change of the six tools or grippers required being made automatically. To avoid bolts getting jammed or wedged in drilled holes, which may occur both due to workpiece tolerances and also due to faulty positioning of the industrial robot, the robot is equipped with a flexible wrist capable of compensating tolerances or faulty positioning within limits, without the use of sensors and additional drives. The assembly of the generator, chosen as an object of study mainly because of its pluggability from one direction, takes 2 min. , 42 secs. The assembly time could be cut to 1 min. , 5 secs. by improving the tools and appliances and by a simple modification in the design of the product. In the present case, the assembly time can be reduced even more by organizational measures such as cyclic mounting, by first carrying out identical assembly part operations on several generators in
Recent deve l opmen t s in ma t eria l ha ndl i ng
steps, before changing a tool or gripper. The economy considerations which resulted from the trial of the experimental set-up showed that programmed assembly systems including one-arm robots should only be used for the assembly of small batches (less than 50,000 products per year), provided that the one-arm industrial robot is capable of mounting each individual part (as man, too) in about 3 to 5 secs. In larger annual batches, it is recommended that several industrial robots or multiarm robots should be used. In 1978, SRI International demonstrated a new flexible means of arrangement /7/ by means of which above all such three-dimensional workpieces can be presented to an industrial robots in the proper location and position, which, due to their size, cannot be arranged in a vibratory spiral feeder. The set-up includes a small vibrating conveyor, an optical recognition unit, an industrial robot of Messrs. Auto-Place and the actual means of arrangement. The latter includes three cylindrical chambers over which moves a slide with a circular recess, capable of moving the parts from one chamber into the other. The bottom of the first chamber can be lowered to a specific height of fall, which can be programmed for the workpieces. A workpiece falls into this chamber from the vibrating conveyor onto the bottom in its upper position. The slide then moves the part onto the back-lighted and turnable cover plate of the second chamber, where the position and orientation of the workpiece are detected by means of the optical recognition unit located above the chamber. The recognition unit consists of a diode-array camera, a microcomputer and an appropriate interface. If the part is recognized as desired, the slide will eject it into the third chamber. If the part is the right one, but in an incorrect position, the disk of the second chamber will turn the workpiece into a suitable position, from where the workpiece may drop onto the lowered bottom of the first chamber as defined. For this purpose, the second chamber moves the part to the edge of the first chamber, until the part drops. This alters the workpiece position, so that after this and upon another recognition operation on the rotary disk, the part can be presented to the industrial robot in the proper position. If several stable positions of the workpiece are possible,
129
this throwing back is to be repeated several times, as the case may be. This application, then, involves a combination of optical sensors and the utilization of the workpiece behaviour. Another experimental set-up of SRI International involves the assembly of a compressor housing, and it comprises a compressor, a cover and eight bolts. A gripper is attached to the arm of a Unimate 2000 B, of which only four axes are required; a linear potentiometer is placed on the pneumatic cylinder of the gripper for controlling the gripper force. The experimental set-up further includes a programmable X-Y table, a diode-array camera, peripheral microcomputers and and LSI-ll as a control computer of the Unimate. The peripheral equipment comprises a bolt magazine, a bolt driver and two appliances, one for the reception of the driver and the other for reception of the cover. It was also intended to demonstrate that one can manage with a minimum of appliances and mechanical apparatus in the peripheral equipment in the automatic assembly using optical sensors. After a one-time calibration of the camera and the cross-table, a housing is placed on the cross-table at random. The camera captures the image of the housing in the top view; the computer analyzes this image, computes the position coordinates and the orientation of the housing and instructs the cross-table to move the housing into the centre of the camera's field of vision. The Unimate will then grip the cover and puts it onto the housing, while the cross-table is unlocked, so as to be able to compensate any position inaccuracies during joining. The computer will then analyze the camera picture of the cover put on, it computes the position of the first tapped hole and instructs to move this again into the centre of the field of vision by the cross-table (with the unlocking again being disabled before). The industrial robot will then grip the driver with the same gripper, it takes up a bolts and places it on the tapped hole. The bolting operation itself is again carried out with the cross-table unlocked. Upon completion of bolting, the computer takes the camera picture to check whether the hole in the cover is closed, ie. to see whether the assembly operation has been successfully completed. The total assembly time for 8 bolts is about 160 seconds.
130
R . D. Schraf t
Developments for the coordinated movement of multi-arm industrial robots are of special importance in assembly jobs. One of the most interesting and probably also the most elaborate example so far of the present state of development is an experimental facility for automatic assembly of vacuum cleaners, which was set up in the research laboratories of Hitachi Ltd. in Japan /8/. The assemblies of which the vacuum cleaner consists are the filter, dust-case and motor. The filter itself consists of a plastic frame and a dust-bag, both connected with a rubber ring. These three assemblies are mounted by a two-arm industrial robot (figure 17). Each arm has eight degrees of freedom, three fingers and its grippers are equipped with approx. 30 tactile sensors. The arm shown in the figure on the left-hand side, the power arm, has a gripper which is designed in such a way that it can lift and firmly hold the parts to be mounted. A TV camera is integrated in the gripper area of the right-hand arm, the more sensitive sensor arm; the camera permits the objects to be viewed from all sides. The fingers of its gripper are capable of recognizing and taking pictures of the parts by sensing. The parts of the vacuum cleaner are located on a table in non-arranged form. By the interaction of two vertical TV cameras and one horizontal TV camera, the eye in the sensor arm and the tactile sensors, the location of the non-arranged parts is captured in 16.7 ms. The assembly operation itself is then checked by another four cameras (one vertical and three horizontal cameras), den sensor eye and the tactile sensors. The total assembly operation takes approx. 120 secs.; it is controlled by a type HIDIC-500 computer, with the movements and the tactile information process being supervised by the industrial robot's controller and a type HIDIC-150 minicomputer as subordinate units. In image processing, both binary blackand-white pictures and also halftone images are used in 256 brightness levels. The masks and templates produced by software are used for the recognition of image patterns known in other respects, similar as in optical correlators. Thus far the present trends in the development as can predominantly be observed in research laboratories all over the world. In contrast to this, the following describes an assembly system that can be used in the automatic assembly of small batches and
changing lots in industrial practice already today. 5. EXAMPLE OF A FLEXIBLE ASSEMBLY SYSTEM FOR SMALL BATCHES WITH INTEGRATED TACTILE AND OPTICAL SENSORS If one considers the few industrial robots for assembly offered on the market, one finds that the devices as such are relatively rapid and flexible; in the majority of cases, the manufacturers do not, however, offer any generally valid solutions regarding flexible peripheral equipment. Therefore, a flexible programmable assembly system has been developed at the IPA on the basis of an Olivetti type SIGMA assembly robot; this system can be used for automating the assembly of small and medium batches (Figure 18). To enable the programmable assembly system to carry out not only simpler assembly jobs, but also the mounting of more complex products, the gripper or the tool fitted to the arm of the manipulation device at a time must be capable of being changed automatically. Therefore , the end of the arm is provided with a changeflange which is operated by the controller of the manipulation device and which can take the gripper or tool out of a magazine via a standardized interface and place it back again upon completion of the assembly operation. A series of grippers, whose gripping principle is based on that of a simple tongs' gripper has proved particularly advantageous. In all, this gripper consists of the standardized flange, a basic body and two tongs' levers and it can be adapted to different workpiece geometries by special jaws. It is operated by a pneumatically moved rod acting on the tongs' levers. When changing the gripper, all that has to be done is to change a straightforward and inexpensive mechanical set-up, since the gripper actuator remains on the arm of the manipulation device. Tools, too, can be automatically taken up by the industrial robot via the standardized interface. A measure for cutting the down times involved in changing the gripper or tool is the distribution of the down times over several identical assembly operations, so that the amount of down time is reduced in all. For this purpose, partially-mounted assemblies must be fed to the industrial robot in a specific number and sequence, and must be finish-mounted
131
Recent developments in material handling
in steps, with the industrial robot cyclically carrying out each assembly operation on each of the assemblied made available in consecutive order. It is thus possible to distribute the down times involved in changing the gripper and tool over the mounting of several identical individual components.
b
a
A programmable assembly system can be set up as follows and adapted to new assembly tasks: a) by changing-over the program of the manipulation device and of the sensors, e.g. by calling new programs or entering new program parameters and/or b) by reconstructing the peripheral equipment, as e.g. the means of arrangement and the appliances. Generally, new programs can be entered very fast. As a rule, the adaptation of the mechanical and non-programmable components of the assembly system is more time-consuming. To enable a rapid conversion of this peripheral equipment as well, the means of arrangement, assembly appliances, parts magazines and the like are fastened on a common work plate. This plate can be moved on guide bars equipped with air-inflated elements into the work room on an air cushion, where it is positioned and clamped. Travelling out of the old work plate and travelling in of the new plate with the mechanical equipment for assembly of the next product, the connection of the equipment to the pneumatic and electric supply lines can be accomplished very rapidly. This is an important prerequisite for an economical and meaningful conversion in the assembly of small batches /2/.
Fig. 1: Magazining of workpl.c •• "quasi-flow aaterial"
$-
a.
3 thick
improved
poor improving the symmetry
Fig. 2: Individual parts of a design that lends itself for proper manipulation
Possible arrangement
,
correct position
Fig. 3: Design of a plastic cover that lends itself for proper manipulation.
R. D. Schraft
132
FAVOURABLE
UNFAVOURABLE Straight joining movement
Curved joining movement
.
~~ i
,JJJ~./)
.,~
:
~. ~ L
' "", ..P
v
-~
s .. / "
Workpieces in bulk material
.. U.,:,
~.
I
'
~
ic>C:~
~
!c>C3::~
Fig. 4: Set-up of products for ease of assembly
Hfupl!orm
I'yerHI(Jo' ul.Oefl1urq
"'''-
Lagedfr forf'l"
I'IabIH'lgS-
, re levAI'\I@i
3.Sltlle.
4.Slelle
b_Stelle
~. 51ell e
,<0
S~ncI-
[mpllndlich -
BnOfliXfr
sicherheit
Eljen -
'ulEbtne
sctw"en
""'
Schrage 1I.51elle
10.Sltll t
7 . 8.und9.
I finrac l'l 10 i d l l(l1tU"'l
Ofirltbl8:jf
O,O·CA.O(),4Q
'~m Kr eisform
3ot1r ull:l
OyC I.... · 1.49
~Mdtl.aund " I'lthrl~ c h n
h 1lflOerte lle I \Chief
!.S·CA'l,Q
Blodleilt
3.O"CA'3O
Il.Stelle
!~ , ' lhcMUnIj
Sldlf".~u l \ 1
;!
~I~I~'~(~M
met1r1Ach i1l1Jf~tllt
C-"' - 30
J).Slelle
I'-SI,II,
'" '"
S.u~tlh l
70~~~ "Il? Slihl
'~
u, r"lullt.
rolll lm ~re l S 'Kegel h31"ljtnICnt
h3r1t
;
"",m"i",:~;::,:
lIeh
8.JusUh l bh. roll! ner~us !Well e l 70 (p lmm l
Ausschn,n ' komb'n 'e rl
NU I,E ,nShch
rol ltn,cht
I GrA~ull
l
-
,,"
~""'moglichkei:
Sttlle
Sltlle
f 4Chltl le
.... trt.stolf
I abmnsun -
i
element
lund2
WERl:.:sr UC HlASStfIZtERUNG
I WerhtOa-
elemenlt
'orm-
Workpieces in the defi ned position and orientation
Fig. 5: Arrangement in steps
V(RHALTENSORIOH I ERTE
"""
~
~
~ !, ~-
!c> 'c:~
Vtrtwl1ens'
~
stehl.ul
obfrilachen-
e. nt'fS r llt
tmp/iodlieh
~Wll.ur
ilrueh " rmp/,ndl,eh
klftW"ig
!We,Stl'en Slthl.ul hOcMltr'ls
form -
hflij
emp/indlich
stehl'u l . lltnSt;le
"'.-
~'er S~len
IUnll'loJljntl I hanljliln elnlJchef Stlhl Sc!1ient
emp/lodl,eh
---.J Werts'udlelltns~ hdflen
. t~HlI1IN:.uf1Jstig t "s",..,ften
-
mil(;rill
Fig. 6: Extract from the behaviour-orientated work classification
Recent deve lopments in material handling
Fig. 7: Flexible vibratory spiral feeder (Bosch photo)
Fig. 9: Programmable arrangement equipment (IBP Pietzsch photo)
"
'JI
,
__.
•
Fig. 8: Flexible slope conveyor (IPA)
133
••
;--_ . • I ' ,
Fig. 10: Flexible assignment equipment (IPA)
Fig. 11: Plastic sheet magazine (IPA)
R. D. Schraft
134
Slope conveyor with - - - gate mechanism
Vibratory spiral feeder
~~==~
~----
-.....::::::~::::::::::::::::~~/ '\ \-0 )
Rotary table
//-'0/""\
COiJntrOller
TV system
Dilling machine with flexible appliance IL--.Jf?"~af='~
") I~I~~~
(v/ /
Plastic reshaping machine
I ndustrial robot Fig. 12: Components of a pilot work station
,
I
,
Sensor
Converter
Receiver
,
-
analog/ digital
-
linear
-
- digital/ analog
-
non-linear
I
Ana1yzer
Amplifier
- contact-type contactless
,
- measured result
- pulse-count Fig
. 13·
Constituent systems of a sensor Sensors
Evaluation
Type
Application
Visual Semiconducto~ Position recogsensors nition - point-form Quality test line-form as matrix Class associPicture tubes ation
-
Tactile
Sensors (pneumatic, electric, etc. )
Joining operations Assembly
Foil strain gauge
Tool supervision Position recognition
Load cell (piezo,cap.) Pressuresensitive plastics Wire matrix Audio
Microphone Acce1eration recorders
Quality inspection Adjustment
I
Fig. 14: Outline of sensors used /4/
Principle
Marginal conditions
Recording and processing of individual physical quantities (Signalling elements, feelers)
Signal pattern may occur in multiple arrangement or as a function of time
Recording and processing of n-dimensional patterns
Large amounts of information in visual sensors (TV picture: 500,000 pOints 30 m bits) Data reduction - Picture sections - Object arrangement in preferred positions - Optical aids Reduction in processing time - Multiple processors - Special hardware
Rece n t deve l opments in ma t erial ha ndli ng Size 01 rJu milcr 01 ligl1 t· sensi piclure live area elemenls
Sensor
a
I .='
el
elemenls
l) icttJre
,u,i
mm
' ,'.111
25
46
40.000
50x50
rr.ax. 7
~\ulli ·diode vidicone
16
25
\00. aeo· 1. COO. CCD
12x12
i.lJx •
Ph olo-diode ma lrix
3.2
5
50 x 50
100,Ieo
lUO x 18O
CCPD -malrix
~
CCD-malr ix
6 x6 3 x 11
.~
I'CO·2Z}
~ LO -l
GSO
12"0·2,:,}
0,5
2CO '! 100
15 If_' }
60,60
\0
2CO-! II.'}
w. IS l',)
m~x .
lOO x 100
30x~Q
7,3 x 9,7 5
512 x 320
30,·:0
3 x6
118 x 128
Ll5x::S
CID 'malrix
'" Fi g . 1 5 :
1)'.:
300- ECO
5
CC}
2 5~}
3
.::J -! eco
8 sr1
5
: :0- 1 \iJ'J
2 C':}
~ 00 ·1
5
c:
8
Pri ce
1197('1 Ca n:era
n~1
Plumbikon
1l
~
RCJ'jing Ire Usa~ dc lJency 01 S:,"clra l ele - ra nge 'me nls
Size 01 pic ture
135
Outli n e o f v id io s e nsor s / 4/ r------------L-_T~O~CI_II~'~S~en~I~0r~I__~r_
,.,\,
,1<'IHI IHI
I
I
or i nthvillUJ I
ql),!t\ \II II'\
~J~
1________-, 1I"lk l ll
-=.':L.=-----
]
(rorce, mOnlcnt, [JJlh ...
I
roil II" in
I
~"u~"
l' i('70 rr'coHJer5 Pf't'lI1.1 Jtic con-
1\'/f)' dir:I(,I ISlon,11
verters
CilP iH.i l ive re -
rr"s~ l iP' ~r.' nsjtiv('
cortlcrs Potentiomelers
=L[V?S ~
plastlcnl3trix
.:g
..i l '
'.'
Q
EmlJcd[J(>d graphite Induct ive lecll;r pins
threads
Q
Automatic joining anti assembly operations: used in ar ms and grippers of industrial
rolJots
Fi:ldin g p,s illon s in autol'l a lie manufactu ri ng (lnu
aSS cf:l I)ly facili lies
Fig 16:
Construction forms o f ta c tile sens o rs
q:J
: ', Y'
Fig. 17 :
E YE
Experimental set-up for the assembly of a vacuum cleaner (Hitachi photo)
R. D. Sc hraft
136
kinematischer Aufbau des
prQlrammie rbaren Ha nd habung sger3tes
;:-TV-Sensor
Mag az in 10 r Greiler und
takti ter Sensor
-----t--,
Wechselflansch fO r Greiler und Wer'
c;:"!=;.D~ ~~~=:::::~~===i~ .D'::=:
Werkzeuge
Fig . 1 8:
El e ments of a n ass emb l y s y s tem
6.
REFERENCES
/1/ Haaf, D.: "Montagegerate und Automaten: Stets nur Einzweck16sungen ? " (assembly devices and robots : single-purpose solutions only ? ). Maschinenmarkt 84 (1978), No. 11. /2/ Schraft, R.D.: "M6g1ichkeiten Haaf,D.: der automatischen Montage in der Kleinserienfertigung" (possible ways of automatic assembly in the manufacturing of small batches), Proceedings 8th ISIR, Stuttgart, 1978. /3/ Schweizer, M.: "Taktile Sensoren fur programmierbare Handhabungssysteme" (tactile sensors for programmable manipulation systems), thesis for a Dr.-Ing., Stuttgart University, 1978. /4/ Martin, R.: "Mittlere Technologie ist flexibel" (The medium technology is flexible) VDI-Nachrichten 32 (1978), No. 22.
/5/ Birk, J.; Kelley, R.et.al.: General Methods to Enable Robots with Vision to Acquire, Orient and Transport Workpieces. 4th report GRANT 7413935 , University of Rhode Island, Kingston, 1978. /6/ Nevins, J.L.; Whitney, D.E. : Computer-Controlled Assembly Scientific American 238 (1978) No. 2. /7/ Rosen, C.A.; Nitzan , D.et.al.: Machine Intelligence Research Applied to Industrial Automation. 8th report GRANT 7513074, SRI International , Menlo Park, 1978. /8/ Takeyasu, K. et.al.: An Approach to the Integrated Intelligent Robot with Multiple Sensory Feedbach : Construction and Control Functions, Proceeding 7th ISIR, Tokyo, 1977.
Abstract. Over the last few decades, the research and development in production engineering were concentrated on the technological processes in the manufacturing of parts. New manufacturing techniques were developed there, in particular to get shorter productive times. Compared with this, the workpiece manipulation in the manufacturing of parts and in assembly has so far largely been neglected. This paper deals with the automation of workpiece manipulation in the manufacturing of parts and in assembly. Moreover, an example of a flexible automatic assembly system will be shown. Keywords. Assembling; manipulation; man-machine systems; robots; sensors. 1. INTRODUCTION
Mechanical assembly facilities in the form of assembly machines and automatic assembly machines have found wide application in specific areas of industrial assembly. Particularly in the assembly of large batches and in mass production, a wide variety of technical solutions can be found on today's market. These solutions are promising, wherever there are favourable amortization conditions on the market, due to large quantities and a sufficiently long service life of the products involved. It is however a characteristic of th~se assembly machines and automatic assembly machines that they are principally only suitable for assembling those products for which they were developed and built. Automatic assembly appliances, therefore, can predominantly be considered as equipment within the field of custombuilt machine construction /1/.
nical-economical reasons. These assembly jobs are essentially characterized by the following features: - low production service life (less than two years), - similar products of a range of variants in terms of assembly, - sufficiently large quantity of the range of variants as a whole, - lot-by-lot assembly of the variants, - complex joining operations. In this country and abroad, the development of new automatic assembly systems for these assembly jobs not yet solved satisfactorily is in progress. Strictly speaking, flexible automatic assembly system operating on the shop floor on a wide range of products do virtually not exist as yet. An important starting point in the considerations to be made in a flexible assembly system is the demand for programmability of the technical equipment required for assembly, i.e. to adapt the assembly system's components to different assembly jobs through a known algorithm, if necessary, supported by simple readjusting operations. Since the
Apart from this area of assembly automation, there is a large number of assembly jobs which, although capable of being automated at an appropriately great technical expenditure - e.g. the quantity of a range of variants which is large in sum - but not suitable for handling by conventional automatic assembly machines for tech123
124
R. D. Sc h ra ft
introduction of industry robots in the area of parts manufacturing, experience has been available that can also be transferred to the field of assembling. Added to this is the favourable development in the field of electronic controls. This development is also of benefit to the complex sensors, used where no "arranged peripheral equipment" is present around the assembly system. This is particularly true in lot-by-lot manufacturing or in the manufacturing of small batches, where the magazining of the individual parts or an ordered passing on by means of workpiece carriers cannot be implemented for cost reasons /2/.
2. FLEXIBLE ARRANGEMENT The problem of flexible arrangement, that becomes increasingly interesting in the automation of small batches, has today only been solved in its rudimentary technical form. Thanks to his sensory powers, man is able to carry out the so-called "grip into the box" without any problems. If one wished to implement this technically, one will very soon be faced with limits today; nevertheless, there are some initial approaches, which however involve considerable technical expenditure or which are extremely restricted in their transferability, due to different application prerequisites. Today's state of the art permits several approaches: a. Once an arrangement of workpieces is reached, it is tried to maintain this arrangement. b. The problem of arrangement is to be reduced to the greatest possible extent by a design that lends itself for manipulation. c. If the parts are available in a bin in unarranged form, arrangement can proceed in steps. d. The "grip into the box" in parts presented in unarranged form. These various possible ways will be described in the following in detail. 2.1 DESCRIPTION OF THE VARIOUS APPROACHES IN THE SOLUTION TO PROBLEMS OF ARRANGEMENT Maintaining the arrangement achieved brings a whole number of advan-
tages. One does, for example not deal with any individual workpieces, but with a "flow material". This flow material is comparatively easy to handle. However, aids such as adhesive tape and shrink film are used, to make it possible to magazine difficult workpieces as "quasiflow material" (figure 1). Endeavours are being made for an extended period to strengthen the sensibility of design engineers regarding a design that lends itself for manipulation. This becomes increasingly necessary in the light of flexible, automatic arrangement. Some basic rules can, for example, be set forth to this effect: - designing parts either fully symmetrically or fully asymmetrically, - providing parts with unambiguous work characteristics, which can be interrogated during arrangement, - avoiding interlacing random parts, - making use of the material's properties for joining (e.g. the elasticity of plastic parts). Figures 2 to 4 show some examples to this effect. If there is a problem of arrangement that cannot be solved by conventional means of arrangement in a single operation, arrangement in steps constitutes a possible approach. This includes the gradual reduction of the work's translational and rotational degrees of freedom. Figure 5 illustrates this approach. "The grip into the box" mentioned at the beginning, though technically possible today in parts, cannot be realized economically. If one ignores a few approaches made in research laboratories, in which extreme restrictions have been made, it is still connected with too much technical expenditure to technically simulate man's sensory powers, as e.g. the coordination of the hand and the eye. The current endeabours being made in the field of flexible arrangement aim at the utilization of all constructive possibilities in the part for the time being, in order to make the task as simple as possible and, on the other hand, to design extisting mechanical means for arranging in a more flexible manner.
Recent deve l opme nt s i n ma t e r ia l ha ndli ng
2.2 EXAMPLES OF THE DEVELOPMENT OF FLEXIBLE MEANS OF ARRANGEMENT The following describes some of the developments made within the "Arbeitsgemeinschaft Handhabungssysteme I" (work group in manipulation systems I). The project was sponsored by the Federal ministry of research and technology under code 01 VC 044 - Z 13 - TAP 0002. 2.2.1 Systematizing studies As a basis of the analysis and development of feed equipment, a behaviour-orientated work classification was built up, which contained all the manipulation-relevant work parameters. Extracts of the classification can be seen from Figure 6. To be able to tell where the areas of application for feed equipment are, a workpiece statistics containing more than 3,000 workpieces out of 11 manufacturing plants were evaluated in addition, which supplied information about the frequency of occurrence of the work parameters illustrated in figure 6. To find out what feed equipment is available on the market , comprehensive market analyses were conducted. The result was compiled and published in a catalogue of commercially available feed equipment. This catalogue furnishes the user with an outline of possible functional principles and of the supply of feed equipment. By means of the systematic approach shown it also permits the solution to a manipulation problem and its specific implementation in a given workpiece range with the aid of the catalogue and register parts.
125
the scope of application of a vibratory spiral feeder can be considerably increased. This device e.g. permits the arranging of a workpiece range consisting of cylindrical parts of different characteristic forms; conversion to another workpiece takes 5 minutes max. ~!~~!~!~_~!~E~_~~~~~Y~~ (Figure 8) The analysis of different arrangement principles with regard to an increase in their flexibility led to the development of a flexible slope conveyor. It is characteristic of this device that it permits a wide and different workpiece range to be arranged by adjusting or replacing individual device components. By re-setting the bin geometry, the inclination of the discharge belt and the discharge strips, the belt speed and the discharge height as well as by replacing the discharge strips, the slope conveyor can be adjusted to different geometries, positions of gravity, masses, rolling and sliding properties of the workpieces to be arranged. ~~~2~~~~~!~_~~~~~_~!_~~~~~2~~~~~ (Figure 9)
The workpieces are put in a V-shaped groove in any position and will invariable assume one out of a few preferred positions by themselves. In the groove, the workpiece will be sensed by the sensors by optical transmission. A logic unit composes the sensor signals into a matrix image of the workpiece. The work image will be compared with reference images previously entered. If there is identity between the work matrix image and the reference image, the workpiece will be detected. If there is no agreement between the work image and the reference image, the workpiece will be sorted out.
From the large number of developments carried out by this work group (in about 40 instants the necessary feed equipment was developed and/or adapted and modified in the production testing of industrial robots), only a few developments are to be briefly presented in this paper by way of example.
The detected workpieces will be gripped by an industrial robot linked with the detection equipment and placed at a location appropriate to the detected position.
2.2.2 Equipment for the "arrangement function"
2.2.3 Equipment for the "assignment fUnction"
!!~~!~!~_~!~~~~~EY_~E!E~!_!~~~~~
The workpieces arranged in a flexible discharge or arrangement device will be made available to the industrial robot One by one by the flexible assignment equipment. For this purpose, the assignment plate is adjusted to the workpiece width involved by an adjusting screw.
(Figure 7) By combining a vibratory spiral feeder equipped with exchangeable and adjustable standard elements with a straightforward optoe1ectronic position-detecting device,
126
R. D. Schra f t
The workpieces are guided by means of adjustable rolling or sliding rails. Work clamping is carried out pneumatically by means of round diaphragm elements. Actuation of the clamping elements for gating is triggered by light barriers. Built into this assignment equipment is a tactile sensor plate which is capable of detecting two distinct workpiece positions during clamping. 2.2.4 Equipment for the "positioning function" An optically programmable two-axis positioning table has been developed for positioning. The 2-D-positioning table is used for pointby-point positioning of workpieces of any distance on a pallet with a medium positioning accuracy. The positioning table is controlled via a read head fastened on the table-top; it scans the signals on the program template located beneath. The program change is made by exchanging the program template. The program is recorded on a commercially available metal-clad carton by means of drawing ink. This includes the connection of the machining pOints (direction X) by horizontal and vertical (direction y) lines. These lines are supplemented by the symbols for rapid traverse ON, rapid traverse OFF, advance START and HALT. 2.2.5 Equipment for the "magazining function" ~!~~!~!~_~~~~!:~!~~~!~9_~~9~~!~~
A sheet-stacking magazine has been developed which easily converts to other sheet-metal contours and thicknesses for placing flat sheet metal parts in a press by means of an industrial robot. In this design, a slide is mounted on two horizontally clamping carriages in the directions X and Y on which the parts stack is placed and secured against displacement by vertical easy-to-adjust holders (six max.). A vertically adjustable plate on the slide compensates the different sheet-metal thicknesses. A possible displacement of the positioned part is prevented by a finely adjustable permanent magnet. ~!~!!!~_!~~~!_!!9~~!~~ (Figure 11)
In the search for magazine systems featuring versatile application, the drawing of plastic sheets has been one among several considerations. This technique is based on
the idea to place the workpiece to be magazined on a plastic sheet, to heat the sheet up and then to vacuum-form the sheet around the workpiece. The advantages of these single-purpose magazines are the simple making and the low manufacturing costs. The problem of flexibility was thus avoided by finding a way of obtaining extremely costfavourable magazines, which can be manufactured at short notice as required. Compared with other flexible magazines to be used in known parts ranges, this solution has the advantage that magazines can also be made without any great expenditure for such parts which are new in a parts range or have to be taken up in addition. Thus, this magazine solution is particularly suitable for those plants which manufacture independent of customer requests and whose workpiece range is subject to constant changing. 2.3 Over the past few years there has been an increase in the compelling demand for more flexibility in the means of arrangement. It has been shown that the deliberate utilization of the workpiece properties, starting with the design to the utilization of material properties on the one hand, and through the development of flexible means of arrangement for manipulating different workpieces on the other, make progress in this area possible. In the future, a combination of mechanical feed equipment of a certain degree of flexibility with simple optical or tactile sensors will bring a greater degree of flexibility in means of arrangement. The problem solutions thus being originated will then also lend themselves for economical use in industral practice. The set-up of a pilot work station for the automatic feeding of a drilling machine illustrated in Figure 12 may be an example of such a problem solution. Conversion of this automatic work station takes only a few minutes. On this experimental set-up, the functional evidence was furnished for the integration of an industrial robot, flexible grippers, flexible means of arrangement, for the plastic magazine described and for an optical sensor used for workpiece recognition.
Rece nt d eve l opme nt s
~n
ma t e r ia l hand li ng
127
3. SENSORS USED IN INDUSTRIAL ROBOTS
3.1 CONTACT-TYPE SENSORS
By sensors are meant those components of a measuring device which are directly exposed to the phenomenon to be measured or recorded. In the literature, this definition of the term is partially extended to a cons derable extent. Sensors are used for recording stochastic variables in the surroundings of the industrial robot, for measuring physical quantities as well as in pattern and position recognition /3/.
Analyses of work stations reveal that the applications of industrial robots could be considerably extended, if the functions of arranging and testing were taken over by suitable sensors.
Technical sensors process the signals coming from one or several pick-ups into the full information (e.g. orientation and position) of an industrial robot, with this information being capable of containing both simple Yes/No-decisions and analogous digital data. Hence, a sensor consists of the pick-up and an associated signal analyzer; it is however also possible to perform signal conversions and amplification within the sensor. (Figure 13). The information reception of sensors may either be by contact (tactile) or contactless. Contactless sensors operate for example by the following physical principles: by optical, acoustic, capacitive, electromagnetic, inductive or fluidic means. Contact-type and contactless sensors are primarily used in the different manipulation and machining operations on the various workpieces and tools. The aUdio sensors also shown in Figure 14 have so far only be used in production engineering for quality inspection and for calibration. Visual sensors are of particular importance in automation and these sensors have been the subject of research work for many years. Despite their importance in automation, visual sensors have so far hardly found any practical application in industry, which can partially be explained by their high price and their low reliability. Figure 15 is an outline of the main properties of the most important video sensors. Tactile sensors record forces and moments by contact. Also recorded are the distribution of force and moments, e.g. in machining operations or during assembly work. ~ 1& shows some of the typical types out of the many possible types of tactile sensors.
Conditioned by the positioning uncertainty of industrial robots, the assembly of workpieceswith narrow tolerances is often not possible. Here, tactile sensors are capable of recognizing the occurrence of any deviation in the mating parts to be joined and to permit a correction of the position or an orientation of the mating parts to be joined. In the assembly of complete modules, test operations have to be carried out additionally and forces or moments have to be limited, in order to prevent damage to the components. In the automation of assembly by means of industrial robots in the manufacturing of small and medium batches, the test functions performed by man visually or by touching must be taken over by sensors. Under more straightforward conditions, the sensors can monitor the programmed motion cycle and interrupt the cycle in the event of errors or, else, they must take over direct control of the assembly operation, e.g. by search movements or by position or pattern recognition. When operating with power-driven and non-driven tools guided by hand, the contours must be obtained visually or by touching. In deburring or in dressing of castings, operations have for example to be partially carried out by means of rotating grinding tools, which requires considerable physical strain. In the automation of such operations by means of an industrial robot, tool guiding must be supported by sensors, which condition the information on the forces acting on the workpiece in such a way that the industrial robot's control can be influenced accordingly. 3.2 CONTACTLESS SENSORS The main reason for which industrial robots are only used in relatively small numbers at the present time is deemed to be the problem of workpiece arrangement, with the difficulties mainly lying in the lack of flexibility of the equipment used so far. In addition, there are many work stations at which visual checks are made by man as quality inspections, which would also have to be made by industrial robots in an automated environment. In the manufacturing of small and medium batches, the transport of the workpieces between the production and
R. D. Sc hr a f t
128
or assembly facilities is often in non-arranged form. Therefore, these workpieces cannot be directly gripped by the industrial robot. Apart from the recording of the direct environment of a robot, e.g. by means of optical sensors for flow monitoring, the workpiece recognition and classification are the main task of contactless sensors. Such sensors must permit the positioning of an industrial robot, e.g. by optical patterns or the "grip into the box", to enable an industrial robot to be used in places where man was necessary because of his visual faculties. 4. COMPLEX SENSOR SYSTEMS In connection with the "grip into the box" there has been a series of developments throughout the world, of which the development of the American University of Rhode Island /5/ is to be described on behalf of others. In the experimental set-up, a 6-axis industrial robot is used, for whose three linear axes simple elements of machine-tool building are used and to whose projecting axis Y three further rotational manual axles are fitted. A diode-array camera is located between two lamps on the jib, with the camera "looking into" the box. A suction gripper with its hose guided in a coil spring is approached for gripping a workpiece out of the "hand", so that any workpiece position is permissible in the box for gripping of the workpiece. This gripping operation works in 60 to 80 cases out of 100 on the first trial. To detect the position in which the workpiece has been gripped, it is shown to another stationary camera. If the workpiece requires re-gripping, it will be put on a place of deposit also provided with a vacuum gripper. For loading the workpiece in a machine (not available and thus fictitious in the experimental set-up), an additional gripper can be flange-mounted, by means of which a simpler loading in a machine is made possible than in the current design of the industrial robot. In this experimental set-up which, to be honest, must however not be mistaken for a pilot application for industrial use, the taking up, recognition and the selective depositing of a workpiece still takes about 30 seconds; this is rather a matter of basic research, in which the apparatus set-up is only an
experimental unit which certainly lends itself for being further optimized. In spring of 1979, the Fraunhofer-Institut fur Informationsverarbeitung in Technik und Biologie (Fraunhofer institute of information processing in engineering and biology - IITB) demonstrated its application of a TV system for pattern recognition in the "grip into the box". In this system, an industrial robot equipped with a magnetic gripper grips in a box under program control. An inductive sensor accommodated in the gripper signals the presence of a part at the gripper, so that the industrial robot moves the object gripped into the back-lighted image field of the optical sensor, which recognizes the object and measures it. The industrial robot then removes the object from the sensor image field by carrying out a selective gripping movement and thus performs the actual arranging operation. The research results achieved over several years with respect to computer-controlled automatic assembly of small and/or changing batches have been elucidated by Charles Stark Draper Lab., USA, in an experimental set-up for the assembly of electric generators /6/. The assembly system consists of an industrial robot with four degrees of freedom and the associated peripheral equipment of altogether 12 means of arrangement. The industrial robot assembles the 17 parts of the generator with the aid of two different appliances from a single direction, with the change of the six tools or grippers required being made automatically. To avoid bolts getting jammed or wedged in drilled holes, which may occur both due to workpiece tolerances and also due to faulty positioning of the industrial robot, the robot is equipped with a flexible wrist capable of compensating tolerances or faulty positioning within limits, without the use of sensors and additional drives. The assembly of the generator, chosen as an object of study mainly because of its pluggability from one direction, takes 2 min. , 42 secs. The assembly time could be cut to 1 min. , 5 secs. by improving the tools and appliances and by a simple modification in the design of the product. In the present case, the assembly time can be reduced even more by organizational measures such as cyclic mounting, by first carrying out identical assembly part operations on several generators in
Recent deve l opmen t s in ma t eria l ha ndl i ng
steps, before changing a tool or gripper. The economy considerations which resulted from the trial of the experimental set-up showed that programmed assembly systems including one-arm robots should only be used for the assembly of small batches (less than 50,000 products per year), provided that the one-arm industrial robot is capable of mounting each individual part (as man, too) in about 3 to 5 secs. In larger annual batches, it is recommended that several industrial robots or multiarm robots should be used. In 1978, SRI International demonstrated a new flexible means of arrangement /7/ by means of which above all such three-dimensional workpieces can be presented to an industrial robots in the proper location and position, which, due to their size, cannot be arranged in a vibratory spiral feeder. The set-up includes a small vibrating conveyor, an optical recognition unit, an industrial robot of Messrs. Auto-Place and the actual means of arrangement. The latter includes three cylindrical chambers over which moves a slide with a circular recess, capable of moving the parts from one chamber into the other. The bottom of the first chamber can be lowered to a specific height of fall, which can be programmed for the workpieces. A workpiece falls into this chamber from the vibrating conveyor onto the bottom in its upper position. The slide then moves the part onto the back-lighted and turnable cover plate of the second chamber, where the position and orientation of the workpiece are detected by means of the optical recognition unit located above the chamber. The recognition unit consists of a diode-array camera, a microcomputer and an appropriate interface. If the part is recognized as desired, the slide will eject it into the third chamber. If the part is the right one, but in an incorrect position, the disk of the second chamber will turn the workpiece into a suitable position, from where the workpiece may drop onto the lowered bottom of the first chamber as defined. For this purpose, the second chamber moves the part to the edge of the first chamber, until the part drops. This alters the workpiece position, so that after this and upon another recognition operation on the rotary disk, the part can be presented to the industrial robot in the proper position. If several stable positions of the workpiece are possible,
129
this throwing back is to be repeated several times, as the case may be. This application, then, involves a combination of optical sensors and the utilization of the workpiece behaviour. Another experimental set-up of SRI International involves the assembly of a compressor housing, and it comprises a compressor, a cover and eight bolts. A gripper is attached to the arm of a Unimate 2000 B, of which only four axes are required; a linear potentiometer is placed on the pneumatic cylinder of the gripper for controlling the gripper force. The experimental set-up further includes a programmable X-Y table, a diode-array camera, peripheral microcomputers and and LSI-ll as a control computer of the Unimate. The peripheral equipment comprises a bolt magazine, a bolt driver and two appliances, one for the reception of the driver and the other for reception of the cover. It was also intended to demonstrate that one can manage with a minimum of appliances and mechanical apparatus in the peripheral equipment in the automatic assembly using optical sensors. After a one-time calibration of the camera and the cross-table, a housing is placed on the cross-table at random. The camera captures the image of the housing in the top view; the computer analyzes this image, computes the position coordinates and the orientation of the housing and instructs the cross-table to move the housing into the centre of the camera's field of vision. The Unimate will then grip the cover and puts it onto the housing, while the cross-table is unlocked, so as to be able to compensate any position inaccuracies during joining. The computer will then analyze the camera picture of the cover put on, it computes the position of the first tapped hole and instructs to move this again into the centre of the field of vision by the cross-table (with the unlocking again being disabled before). The industrial robot will then grip the driver with the same gripper, it takes up a bolts and places it on the tapped hole. The bolting operation itself is again carried out with the cross-table unlocked. Upon completion of bolting, the computer takes the camera picture to check whether the hole in the cover is closed, ie. to see whether the assembly operation has been successfully completed. The total assembly time for 8 bolts is about 160 seconds.
130
R . D. Schraf t
Developments for the coordinated movement of multi-arm industrial robots are of special importance in assembly jobs. One of the most interesting and probably also the most elaborate example so far of the present state of development is an experimental facility for automatic assembly of vacuum cleaners, which was set up in the research laboratories of Hitachi Ltd. in Japan /8/. The assemblies of which the vacuum cleaner consists are the filter, dust-case and motor. The filter itself consists of a plastic frame and a dust-bag, both connected with a rubber ring. These three assemblies are mounted by a two-arm industrial robot (figure 17). Each arm has eight degrees of freedom, three fingers and its grippers are equipped with approx. 30 tactile sensors. The arm shown in the figure on the left-hand side, the power arm, has a gripper which is designed in such a way that it can lift and firmly hold the parts to be mounted. A TV camera is integrated in the gripper area of the right-hand arm, the more sensitive sensor arm; the camera permits the objects to be viewed from all sides. The fingers of its gripper are capable of recognizing and taking pictures of the parts by sensing. The parts of the vacuum cleaner are located on a table in non-arranged form. By the interaction of two vertical TV cameras and one horizontal TV camera, the eye in the sensor arm and the tactile sensors, the location of the non-arranged parts is captured in 16.7 ms. The assembly operation itself is then checked by another four cameras (one vertical and three horizontal cameras), den sensor eye and the tactile sensors. The total assembly operation takes approx. 120 secs.; it is controlled by a type HIDIC-500 computer, with the movements and the tactile information process being supervised by the industrial robot's controller and a type HIDIC-150 minicomputer as subordinate units. In image processing, both binary blackand-white pictures and also halftone images are used in 256 brightness levels. The masks and templates produced by software are used for the recognition of image patterns known in other respects, similar as in optical correlators. Thus far the present trends in the development as can predominantly be observed in research laboratories all over the world. In contrast to this, the following describes an assembly system that can be used in the automatic assembly of small batches and
changing lots in industrial practice already today. 5. EXAMPLE OF A FLEXIBLE ASSEMBLY SYSTEM FOR SMALL BATCHES WITH INTEGRATED TACTILE AND OPTICAL SENSORS If one considers the few industrial robots for assembly offered on the market, one finds that the devices as such are relatively rapid and flexible; in the majority of cases, the manufacturers do not, however, offer any generally valid solutions regarding flexible peripheral equipment. Therefore, a flexible programmable assembly system has been developed at the IPA on the basis of an Olivetti type SIGMA assembly robot; this system can be used for automating the assembly of small and medium batches (Figure 18). To enable the programmable assembly system to carry out not only simpler assembly jobs, but also the mounting of more complex products, the gripper or the tool fitted to the arm of the manipulation device at a time must be capable of being changed automatically. Therefore , the end of the arm is provided with a changeflange which is operated by the controller of the manipulation device and which can take the gripper or tool out of a magazine via a standardized interface and place it back again upon completion of the assembly operation. A series of grippers, whose gripping principle is based on that of a simple tongs' gripper has proved particularly advantageous. In all, this gripper consists of the standardized flange, a basic body and two tongs' levers and it can be adapted to different workpiece geometries by special jaws. It is operated by a pneumatically moved rod acting on the tongs' levers. When changing the gripper, all that has to be done is to change a straightforward and inexpensive mechanical set-up, since the gripper actuator remains on the arm of the manipulation device. Tools, too, can be automatically taken up by the industrial robot via the standardized interface. A measure for cutting the down times involved in changing the gripper or tool is the distribution of the down times over several identical assembly operations, so that the amount of down time is reduced in all. For this purpose, partially-mounted assemblies must be fed to the industrial robot in a specific number and sequence, and must be finish-mounted
131
Recent developments in material handling
in steps, with the industrial robot cyclically carrying out each assembly operation on each of the assemblied made available in consecutive order. It is thus possible to distribute the down times involved in changing the gripper and tool over the mounting of several identical individual components.
b
a
A programmable assembly system can be set up as follows and adapted to new assembly tasks: a) by changing-over the program of the manipulation device and of the sensors, e.g. by calling new programs or entering new program parameters and/or b) by reconstructing the peripheral equipment, as e.g. the means of arrangement and the appliances. Generally, new programs can be entered very fast. As a rule, the adaptation of the mechanical and non-programmable components of the assembly system is more time-consuming. To enable a rapid conversion of this peripheral equipment as well, the means of arrangement, assembly appliances, parts magazines and the like are fastened on a common work plate. This plate can be moved on guide bars equipped with air-inflated elements into the work room on an air cushion, where it is positioned and clamped. Travelling out of the old work plate and travelling in of the new plate with the mechanical equipment for assembly of the next product, the connection of the equipment to the pneumatic and electric supply lines can be accomplished very rapidly. This is an important prerequisite for an economical and meaningful conversion in the assembly of small batches /2/.
Fig. 1: Magazining of workpl.c •• "quasi-flow aaterial"
$-
a.
3 thick
improved
poor improving the symmetry
Fig. 2: Individual parts of a design that lends itself for proper manipulation
Possible arrangement
,
correct position
Fig. 3: Design of a plastic cover that lends itself for proper manipulation.
R. D. Schraft
132
FAVOURABLE
UNFAVOURABLE Straight joining movement
Curved joining movement
.
~~ i
,JJJ~./)
.,~
:
~. ~ L
' "", ..P
v
-~
s .. / "
Workpieces in bulk material
.. U.,:,
~.
I
'
~
ic>C:~
~
!c>C3::~
Fig. 4: Set-up of products for ease of assembly
Hfupl!orm
I'yerHI(Jo' ul.Oefl1urq
"'''-
Lagedfr forf'l"
I'IabIH'lgS-
, re levAI'\I@i
3.Sltlle.
4.Slelle
b_Stelle
~. 51ell e
,<0
S~ncI-
[mpllndlich -
BnOfliXfr
sicherheit
Eljen -
'ulEbtne
sctw"en
""'
Schrage 1I.51elle
10.Sltll t
7 . 8.und9.
I finrac l'l 10 i d l l(l1tU"'l
Ofirltbl8:jf
O,O·CA.O(),4Q
'~m Kr eisform
3ot1r ull:l
OyC I.... · 1.49
~Mdtl.aund " I'lthrl~ c h n
h 1lflOerte lle I \Chief
!.S·CA'l,Q
Blodleilt
3.O"CA'3O
Il.Stelle
!~ , ' lhcMUnIj
Sldlf".~u l \ 1
;!
~I~I~'~(~M
met1r1Ach i1l1Jf~tllt
C-"' - 30
J).Slelle
I'-SI,II,
'" '"
S.u~tlh l
70~~~ "Il? Slihl
'~
u, r"lullt.
rolll lm ~re l S 'Kegel h31"ljtnICnt
h3r1t
;
"",m"i",:~;::,:
lIeh
8.JusUh l bh. roll! ner~us !Well e l 70 (p lmm l
Ausschn,n ' komb'n 'e rl
NU I,E ,nShch
rol ltn,cht
I GrA~ull
l
-
,,"
~""'moglichkei:
Sttlle
Sltlle
f 4Chltl le
.... trt.stolf
I abmnsun -
i
element
lund2
WERl:.:sr UC HlASStfIZtERUNG
I WerhtOa-
elemenlt
'orm-
Workpieces in the defi ned position and orientation
Fig. 5: Arrangement in steps
V(RHALTENSORIOH I ERTE
"""
~
~
~ !, ~-
!c> 'c:~
Vtrtwl1ens'
~
stehl.ul
obfrilachen-
e. nt'fS r llt
tmp/iodlieh
~Wll.ur
ilrueh " rmp/,ndl,eh
klftW"ig
!We,Stl'en Slthl.ul hOcMltr'ls
form -
hflij
emp/indlich
stehl'u l . lltnSt;le
"'.-
~'er S~len
IUnll'loJljntl I hanljliln elnlJchef Stlhl Sc!1ient
emp/lodl,eh
---.J Werts'udlelltns~ hdflen
. t~HlI1IN:.uf1Jstig t "s",..,ften
-
mil(;rill
Fig. 6: Extract from the behaviour-orientated work classification
Recent deve lopments in material handling
Fig. 7: Flexible vibratory spiral feeder (Bosch photo)
Fig. 9: Programmable arrangement equipment (IBP Pietzsch photo)
"
'JI
,
__.
•
Fig. 8: Flexible slope conveyor (IPA)
133
••
;--_ . • I ' ,
Fig. 10: Flexible assignment equipment (IPA)
Fig. 11: Plastic sheet magazine (IPA)
R. D. Schraft
134
Slope conveyor with - - - gate mechanism
Vibratory spiral feeder
~~==~
~----
-.....::::::~::::::::::::::::~~/ '\ \-0 )
Rotary table
//-'0/""\
COiJntrOller
TV system
Dilling machine with flexible appliance IL--.Jf?"~af='~
") I~I~~~
(v/ /
Plastic reshaping machine
I ndustrial robot Fig. 12: Components of a pilot work station
,
I
,
Sensor
Converter
Receiver
,
-
analog/ digital
-
linear
-
- digital/ analog
-
non-linear
I
Ana1yzer
Amplifier
- contact-type contactless
,
- measured result
- pulse-count Fig
. 13·
Constituent systems of a sensor Sensors
Evaluation
Type
Application
Visual Semiconducto~ Position recogsensors nition - point-form Quality test line-form as matrix Class associPicture tubes ation
-
Tactile
Sensors (pneumatic, electric, etc. )
Joining operations Assembly
Foil strain gauge
Tool supervision Position recognition
Load cell (piezo,cap.) Pressuresensitive plastics Wire matrix Audio
Microphone Acce1eration recorders
Quality inspection Adjustment
I
Fig. 14: Outline of sensors used /4/
Principle
Marginal conditions
Recording and processing of individual physical quantities (Signalling elements, feelers)
Signal pattern may occur in multiple arrangement or as a function of time
Recording and processing of n-dimensional patterns
Large amounts of information in visual sensors (TV picture: 500,000 pOints 30 m bits) Data reduction - Picture sections - Object arrangement in preferred positions - Optical aids Reduction in processing time - Multiple processors - Special hardware
Rece n t deve l opments in ma t erial ha ndli ng Size 01 rJu milcr 01 ligl1 t· sensi piclure live area elemenls
Sensor
a
I .='
el
elemenls
l) icttJre
,u,i
mm
' ,'.111
25
46
40.000
50x50
rr.ax. 7
~\ulli ·diode vidicone
16
25
\00. aeo· 1. COO. CCD
12x12
i.lJx •
Ph olo-diode ma lrix
3.2
5
50 x 50
100,Ieo
lUO x 18O
CCPD -malrix
~
CCD-malr ix
6 x6 3 x 11
.~
I'CO·2Z}
~ LO -l
GSO
12"0·2,:,}
0,5
2CO '! 100
15 If_' }
60,60
\0
2CO-! II.'}
w. IS l',)
m~x .
lOO x 100
30x~Q
7,3 x 9,7 5
512 x 320
30,·:0
3 x6
118 x 128
Ll5x::S
CID 'malrix
'" Fi g . 1 5 :
1)'.:
300- ECO
5
CC}
2 5~}
3
.::J -! eco
8 sr1
5
: :0- 1 \iJ'J
2 C':}
~ 00 ·1
5
c:
8
Pri ce
1197('1 Ca n:era
n~1
Plumbikon
1l
~
RCJ'jing Ire Usa~ dc lJency 01 S:,"clra l ele - ra nge 'me nls
Size 01 pic ture
135
Outli n e o f v id io s e nsor s / 4/ r------------L-_T~O~CI_II~'~S~en~I~0r~I__~r_
,.,\,
,1<'IHI IHI
I
I
or i nthvillUJ I
ql),!t\ \II II'\
~J~
1________-, 1I"lk l ll
-=.':L.=-----
]
(rorce, mOnlcnt, [JJlh ...
I
roil II" in
I
~"u~"
l' i('70 rr'coHJer5 Pf't'lI1.1 Jtic con-
1\'/f)' dir:I(,I ISlon,11
verters
CilP iH.i l ive re -
rr"s~ l iP' ~r.' nsjtiv('
cortlcrs Potentiomelers
=L[V?S ~
plastlcnl3trix
.:g
..i l '
'.'
Q
EmlJcd[J(>d graphite Induct ive lecll;r pins
threads
Q
Automatic joining anti assembly operations: used in ar ms and grippers of industrial
rolJots
Fi:ldin g p,s illon s in autol'l a lie manufactu ri ng (lnu
aSS cf:l I)ly facili lies
Fig 16:
Construction forms o f ta c tile sens o rs
q:J
: ', Y'
Fig. 17 :
E YE
Experimental set-up for the assembly of a vacuum cleaner (Hitachi photo)
R. D. Sc hraft
136
kinematischer Aufbau des
prQlrammie rbaren Ha nd habung sger3tes
;:-TV-Sensor
Mag az in 10 r Greiler und
takti ter Sensor
-----t--,
Wechselflansch fO r Greiler und Wer'
c;:"!=;.D~ ~~~=:::::~~===i~ .D'::=:
Werkzeuge
Fig . 1 8:
El e ments of a n ass emb l y s y s tem
6.
REFERENCES
/1/ Haaf, D.: "Montagegerate und Automaten: Stets nur Einzweck16sungen ? " (assembly devices and robots : single-purpose solutions only ? ). Maschinenmarkt 84 (1978), No. 11. /2/ Schraft, R.D.: "M6g1ichkeiten Haaf,D.: der automatischen Montage in der Kleinserienfertigung" (possible ways of automatic assembly in the manufacturing of small batches), Proceedings 8th ISIR, Stuttgart, 1978. /3/ Schweizer, M.: "Taktile Sensoren fur programmierbare Handhabungssysteme" (tactile sensors for programmable manipulation systems), thesis for a Dr.-Ing., Stuttgart University, 1978. /4/ Martin, R.: "Mittlere Technologie ist flexibel" (The medium technology is flexible) VDI-Nachrichten 32 (1978), No. 22.
/5/ Birk, J.; Kelley, R.et.al.: General Methods to Enable Robots with Vision to Acquire, Orient and Transport Workpieces. 4th report GRANT 7413935 , University of Rhode Island, Kingston, 1978. /6/ Nevins, J.L.; Whitney, D.E. : Computer-Controlled Assembly Scientific American 238 (1978) No. 2. /7/ Rosen, C.A.; Nitzan , D.et.al.: Machine Intelligence Research Applied to Industrial Automation. 8th report GRANT 7513074, SRI International , Menlo Park, 1978. /8/ Takeyasu, K. et.al.: An Approach to the Integrated Intelligent Robot with Multiple Sensory Feedbach : Construction and Control Functions, Proceeding 7th ISIR, Tokyo, 1977.