Customized application of computer-aided data acquisition and processing

Journal of Materials Processing Technology, 28 ( 1991 ) 201-210 Elsevier

CUSTOMIZED APPLICATION OF ACQUISITION AND PROCESSING

COMPUTER-AIDED

DATA

H.W. Wagener and K.J. Pahl Metal Forming Laboratory, Kassel University, Germany Abstract The developmentof customized integrated acquisition and on-line processing programs which put a non-specialist on the spot to solve even complex testing tasks is explained. This feature is achieved by extensive automation of data processing and by easy handling. The objective is to use this type of programs in production or inspection and get complete results directly after the actual test is conducted. Two examples for user-specific applications are presented, one that deduces the stiffness characteristics concerning the table die-cushion of mechanical and hydraulic presses and another that monitors a cold-extrusion tool with five stages and includes an error diagnosis option. Basic concept and modular structure of the flexible program system, the implemented calculation methods and the self-explanatory user guidance are discussed. 1. INTRODUCTION Personal computers are moving into industrial applications at an increasing rate. The capability of modern micro-computers combined with the performance of plug-in data acquisition cards allow their use as a universal device for process control and product testing. Meanwhile processing and interpretation of acquired data concerning complex tasks remains a problem. Already existing software can be categorized into two types. Tag. CH 1 Time 10 ,us

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202 Simple programs, often included in the hardware delivery, can only emulate a device like a digital oscilloscope and visualize acquired data as a function of time (Fig. 1). Program packages for data analysis, mostly off-line applications, offer numerous means of processing and display functions which on the other hand restrict their use to highly qualified personnel only (Fig. 2). It has been found that in most cases a mere stripchart gives insufficient information while otherwise the result of complex numerical treatment often consists of a single process characteristic or of a single machine characteristic. 2. DATA ACQUISITION AND ON-LINE PROCESSING One main objective in developing custom-made software is to keep a certain flexibility while keeping the program handling as easy as possible. In this case this is achieved by completely avoiding extensive pull-down menus and reducing the display of all processing options to one screen. In addition one screen manages the configuration of the installed acquisition hardware. This part of the menu is used only once and then locked, so that the staff member who actually carries out the tests has all the available options visible on his monitor screen. In Fig. 3 the layout of the acquisition screen is presented. I Ti~le ] The necessary adjustCO~ I~d~TION $ IQV~L ments include markings for input chan~ p l e period: 5.00 s 8.11118 O F 90~N Samples: 5888 8.888 O T lO=18ne nels used, scaling 8.0~ O $1 90=2m information, duration Dlspla9 buffe~: 56238 8.~ 0 82. 9U=Z,m 8.01~ V ~3 9A/=2am of acquisition, numFeequencg: 1888 Hz 8.~ 0 S4 W:2am ber of samples per ~-tlvs Channels: ? 8.888 0 SS 9U=2~ File size: 78.8 Id~rte 8.8(18 V B8 channel and a namefl.flm 0 89 Trigger: external prefix for the mana8 . 8 ~ I/ tO 8.8~ O ff.81~ 0 11 gement of data back12 8,888 0 up. The menu also 13 Pre£ix: [7.~1 8.fll~ 0 14 incorporates a sensor 8.868 O t5 Node : PROCESS 8.888 0 16 calibration option. Fig. 4 shows the actual processing screen in principle. Figure 3. Acquisition screen It consists of a graphics display area, a comment or result area, and two selection menus. On the right hand side a moving bar determines the objective of the mathematical treatment and another selects between two options for this treatment, e.g. choosing between two ordinates. In addition, several means of handling the gained results such as drawing up a hardcopy or saving numerical results can be activated by function keys. Data acquisition also is executed by pressing a function key without leaving the processing screen menu. A further option is an additional mathematical or statistical treatment of the computed graph, e.g. deduction of maxima or linear regression of a selected part of the curve.

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203 The display area is auto-ranging for both axes in meaningful increments, e.g. 1, 2 and 5. In relation to the often used maximum-ranging this makes interpretation and comparison of results much easier. According to the type of hardware used the acquisition module of the program has to provide the functions of the acquisition screen (Fig. 3). The configuration parameters have to be sent to the acquisition device in form of system-specific commands. During the acquisition cycle the board, operating independently of the host computer's CPU, returns sampled data in the binary format. Advanced acquisition boards are generally supported by several programming languages which makes communication simple. The modular program concept used to realize the features presented is demonstrated in principle in Fig.5. The program system basically consists of four modules: the acquisition module, the processing module, the conditioning module and the display module which also handles the operation by the user. When started, the program prompts for a procedure file wherein the adjustments of the acquisition screen are stored and a file identification as an extension of the selected prefix is to be entered. The hardwareboard is then initiali~DmTIFICnT~ON zed by a subroutine r~,~tto° t which sends the configuration data to the board. Once acquisition is activated, r~,~t~o° 3 data is sent to the host computer meF~,~io, 4 mory and a complete back-up on a storage r~tio, s medium is perfor1 ~ 1 0,r,0~ med. After this is . . completed, the datapt itJ, ...... ' set undergoes a e,,,--.~ / ~u~t n.~ ~o. B conditioning routine, :Exi't F1 :~lmeit'ie 1~2:~,~-,~,,t ra :r~ot r4 :s,,~ which then hands over to the actual processing routine. Figure 4. Processing screen Via the selection menu subroutines are called which can comprise a complex numerical treatment of a chosen number of conditioned sample channels. This always results in two processed data channels for the X/Y-display in the graphics area of the screen. As mentioned, this display is designed to be autoranging. One additional advantage of the system described is the possibility of inclusion of stored data concerning the experimental set-up, e.g. dimensions or positions, and previously determined correction functions, e.g. transducer correction curves. By this feature a basic set-up with varying parameters or makes of sensors can be used without the necessity of any changes in the processing algorithms. Furthermore, a documentation of the utilized set-up conditions and sensor characteristics is obtained this way.

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Figure 5. Structure of program system Prior to the selection of the individual mathematical treatment by the user, a data conditioning in general takes place automatically. The first objective is to sample the exact timespan of the event under study. The application of external hardware triggers tends to complicate the set-up. Software triggers implemented in the on-board intelligence of acquisition systems have disadvantages too. Mostly only a starting function is supplied and samples are taken for a fixed time or number. It is found

205 convenient to start acquisition for a sufficient time manually and control the Time W i n d o w beginning and end of the f I +I0 actual event later on duTravel ~. . . . . ring transfer of data to the > host computer's memory. In Fig. 6 the automatic tcutting of a time window is demonstrated. The crite0 ria and conditions used are ¢/} a continuous increase of tthe force-signal for start Iand reaching of the first I t I I I I I I I I I I I I -10 force maximum for stop of Time Is] I ----.~ data transfer. It is advisable to use a I high sampling rate to sup1 i ~" ~ It Samples port a possible later close look at stored data-files as IIIIIIIIIIIIIIIIIIIII well as to avoid aliasing effects during D/A-converAutomatic - Cutting of Time Window sion. For reasons of comReduction of Data Set puting speed and monitor Scaling in Physical Units display resolution the number of samples in the time window to be considered is reduced automatically Figure 6. Automatic conditioning of acquired data (Fig. 6). The scaling information from the acquisition menu is then applied, i.e. the data channels are transformed to physical units and thus ready for specific interpretation. The objective of distinct separation of conditioning and processing tasks is to allow a simple adaption of the program to various applications. One simply has to replace the content of the processing routines or add new ones and correct the alphanumerical part of the processing screen. In some cases it might also be necessary to change the criteria for the cutting of the time window.

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3. APPLICATION EXAMPLE The main problem area that induced the development of customized acquisition and processing software was the need to increase the manufacturing accuracy in sheet metal forming. Problems arise with difficult drawn components involving off-centre loading of the tool and the slide. These parts react very sensitively on partial fit between blankholder and die. When using a die-cushion, effects of tilt and offset of slide and die-cushion interfere with the forming operation in a negative way due to the fact that the blankholder force is not induced into the workpiece uniformly (Fig. 7).

206 Possible consequences are either failure because of cracking or the formation of wrinkles. A decisive influence related to this phenomenon is the quality of the cushion guidance system, i.e. the resistance against tilting and displacement under eccentric loading. As an approach to solving these problems on the machine side research on the elastic behaviour of die-cushion designs was carried out with the objective of obtaining the stiffness characteristics of this important element of press design. In Fig. 7 the effects under study are demonstrated.

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Figure 7. Stiffness characteristics of the die-cushion in sheet-metal working The importance of elastic effects of machine elements of presses under load was established for the press-frame, the slide and the tool-guidance system [1-5]. The knowledge gained there can be applied to the behaviour of the die-cushion under eccentric load also. The motion of the die-cushion is considered with reference to a system of coordinates originating in the press center. The positive x-axis indicates the left to right direction with regard to the press front and the z-axis is congruent to the vertical press centerline. As presented in the diagrams in Fig. 7, angular and lateral offset plotted as a function of force and momentum both show a characteristic tendency. Followed by a linear increase of the effects a steep rise can be observed for smaller values of load. This is due to the closing of clearances and gaps in the guidance system (initial offse0. The linear section of the curve results from pure elastic deformation of the guidance system under load. The slope of this linear function incorporates the spring constant. If the tilting of a die-cushion under eccentric loading is to be described, the angular spring

207 constant is used to specify the angular stiffness of the system. It is derived from the linear part of the curve as the ratio of the value of momentum to the angular offset which describes the angle of tilt and of rotation respectively (Fig. 7). The lateral stiffness can be determined in the same way as the ratio of the value of force to the lateral offset. To obtain the mentioned machine characteristics for presses used in production (inspection) or for the test-run of new presses for acceptance, a universal testing set-up was designed and commissioned in experiments. The principle arrangement of the setup applied to a press, the alignment of sensors and the necessary processing tasks for signal interpretation are presented in Fig. 8.

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Figure 8. Experimental set-up and processing tasks In the experiments eccentric loading is applied to the die-cushion at an off-center distance r (eccentricity). The force is generated by the press slide via a load-cell and a universal joint which enables the cushion to adjust itself to the off-center loading. Five LVDTs are mounted on tubes which embrace two reference slabs on opposite sides of the die-cushion surface. In addition one LVDT determines the absolute travel in the negative direction of the z-axis.

208 This type of set-up combines two advantages: it monitors the complete downward stroke of the cushion while the resolution of the horizontal LVDTs can be kept very high. What makes evaluation complicated however is a certain tilting of the set-up itself due to elastic deformation of the die-cushion resulting from the small loading area. It was found that these effects can have a distinct influence on the evaluation and therefore are not to be neglected. Corresponding correction values for the tilting of the tubes and the slabs in all loading positions have to be determined only once in advance. The obtained data is stored in a data file (correction file) as well as the dimensions and specifics of the setup (arrangement file) -2 which are loaded automatically during processing (see Fig. OFFSET Y 8). Utilizing the corf ~Y [me/m] = F(H) rection values and J / LA'r]~P.L OFFSET X the set-up dimensions LX [me] : F(F) -1 / f the angular and ./ LATEI~L OFFSET lateral offset of the / LY [ m ) = F(F) ,./' die-cushion can be ROTATIOH Z / deduced from the BZ [me/m] : l'(H) conditioned sensor LOC~TIOH signals. 69 129 [kl~] 188 B Experiments were conducted on a hyP ~ t t t o . of load: X : 9 me ~ = +189 me Blankholdea,plarte draulic press with a ESC : E x i t FX : ~ : i f i c F2 :Measttremen't F"d :Plo% F4 :Same nominal force of 5,000 kN and 1,400 kN table die-cushion Figure 9. On-line processing of die-cushion tilting capacity. The dimensions of the die-cusTEST ZSSE~81 hion structure are OFFSET X 2,300 mm × 1,000 [me/n] = fCH) mm with a guidance N4GUI . A R OFFSET '/ length of 900 mm. / f AY[~m] = t'(H) Results obtained I ~ T E ~ L OFFSET X j l from tests on that LX [me] = f(F) press are presented in Figs. 9 and 10 as hardcopies of the ROTATIOll Z processing screen. In Fig. 9 the angu:U)m i ~LBO~~TOINim[i~mZ' ] = i'(N) lar offset in the e see ~ tiN] cushionplane is Characteristics : CI~I = 3145 (kM/m) A19 = - , ~ i (me) Bla~tholde~lana selected. The possible option ESC :Exi( ~ F2 : ~ l ~ :PIo~ F4 :Save 'Blankholderplane' takes into account the location - dependent Figure 10. Deduction of the lateral spring constant

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209 lateral offset [2]. The graphics display shows the offset curve versus the effective momentum which equals force times eccentricity. It demonstrates both the initial tilting and the linear increase of the angular offset thereafter. As an additional feature in this application, information about the loading position is displayed by utilizing a coding of the file identification. By assigning a certain digit of the file identification to a position the program selects appropriate values, inserts them in the comment line of the screen, and creates a corresponding pictogram. Besides the possibilities of saving the computed curve or dumping the screen to a plotter by one key stroke, the 'specific' function enables the user to deduce the stiffness characteristics (spring constant and initial offset). In Fig. 10 the lateral offset of the die-cushion during the test mentioned above is displayed as a funcion of the loading force. By pressing the Fl-key two cursor-movable vertical lines can be applied to the curve. For the selected (approximatly linear) part the lateral spring constant in direction of the y-axis Cly and the initial offset Aly are computed automatically by linear regression (dotted line) and displayed in the result area of the screen. Altogether the meaningful auto-scaling and the information supplied concerning the testing conditions make assessment of the machine behaviour under study fairly easy. Even semi-skilled personnel are enabled to carry out the rather complex task of determining the elastic properties of a machine element under loading. This distinguishes the customized application from general lab-type acquisition or analysing program systems. 4. P R O G R A M VARIATIONS The program system discussed in the previous chapters was adapted to several other types of projects concerning metal-forming processes. A slightly altered version is being used for testing of the accuracy performance of the press slide and the interaction of slide and tool guidance system [5]. Simi1880 7E~7 EOIEIIIZ3 lar to the die-cusREP. EBIEII001 hion, here the objective also is to determine angular and lateral offset and the SI~GB 2 correspondent spring FtkN] = t(t) constants. Results are STAGE 3 used to design optiI/ mized press slide and F[kN] = f(t) It tool guidance sySTA~E 4 // stems. F[ld~] = f(t) V A further applicaO 9 9.5 1.6 (z) 1.5 ST~E 5 tion (Fig. 11) was derived for monitoFoece m x l ~ : ?Z3 kN F[]CH] = tCt) ring a multistage :~1~ I 1 :l:, t t r, ,,t: I F2 :I~MtSUI'~II~a~ F3 :Plot F4 :Saue cold-extrusion tool which is equipped Figure 11. Monitoring of a multistage cold-extrusion tool with strain gages on

210 the punch mountings for the determination of punch stress and forming force. Here the objective is to connect a PC with the tool during production to check on the forming force vs. time characteristics of a selectable tool stage. These characteristics give indications concerning various process parameters. For example the wear of punch, mandrel, and die insert or fluctuations in material properties of the workpiece and slug tolerances can be related to certain specific changes of the forming force characteristic. The same applies to a non-optimal adjustment of press and tool. Monitoring of the forming force in the various tool stages presented in Fig. 11 is made convenient by two special features implemented in this program variant. One consists of a storable upper and lower force limit curve for each tool stage and the other is the option of loading a reference file that corresponds to a faultless operation. Using empirical experience the processing system can be expanded by a diagnosis option which displays the error type related to the deviations of the force graph. 5. CONCLUSIONS For the multitude of testing tasks and inspection procedures in production engineering, custom-made software solutions concerning data aquisition and processing offer a number of advantages such as easy handling and complete on-line evaluation. The program system described can be adapted to various tasks with the objective of creating a special measuring instrument for the phenomenon under study. In contrast to most of the existing data analysis software packages which support scientific work, the program system presented makes it possible to determine machine characteristics or process parameters without a time-consuming training of personnel. As mentioned, customization of the basic program system is simple. Only a few routines have to be changed or new ones added. Programming effort is kept minimal. In this way the objective of easy applicability to the varying types of problems in production engineering is met. 6. REFERENCES 1 2 3 4 5

H.W. Wagener, Werkstattstechnik 62 (1972) 358. E. Doege, Annals of the CIRP Vol. 29/1 (1980) 167. V. Hasek, wt Zeitschrift fiir industrielle Fertigung 66 (1976) 649. H. Schenk and K. Wiehl, Werkstatt und Betrieb No.10 (1978) 671. H.W. Wagener and C. Schlott, Journal of Mechanical Working Technology 20 (1989) 463.