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Process improvement in volumetric roll-stretch bending
J ~ a ~ of
Muterials essing
ELSEVIER
Journal ofMal~'ials ~ s s i n g Tedmology 58 (1996) 337-342
Tectmology
Process improvement in Volumetric Roll-Stretch Bending Roland G. Weipperta, Prof. Dr. J. Reissnerb
ainstitute for Forming Technologies, ETH, Technopark, Pfingstweidstrasse 30, 8005 ZOrich, Switzerland binstitute for Forming Technologies, ETH, Tannenstrasse 3, 8092 ZEtrich, Switzerland
Abstract
In order to ensure success in the rapidly changing market manufacturing must be rapid and free of errors, with the lowest possible manufacturing costs. In this article some possibilities for rapid further development of an existing bending technology will be shown, at the end of which should be found an adaptive-controlled, reproducible and economic bending process. By using various analysis procedures and special computer-aided simulations for different development phases, it is possible to introduce a co-ordination of the development at a very early stage. This permits a shortening of the process development time and a concentration on quality-enhancing measures.
Keywords: Aluminium Space Frame, Finite Difference Simulation, Adaptive Control, Technology Management.
1. Introduction For many years no further progress has been made in the development of process technologies for volumetric bending of extruded hollow aluminium profiles. Generally the parts to be bent had generous tolerances, which could easily be attained with simple machines. However within the last two years a product technology has developed in the form of light, high-strength "space frame" structures in automobile design. These cannot be satisfied in qualitative and quantitative aspects by conventional bending technologies, The areas of tolerance to be maintained are such that economical manufacturing of small series can only be achieved with a reproducible process. The sections of the aluminium profiles often ihclude openings, cross-pieces or closed chambers, the contours of which have sealing or closing functions. As a result wrinkles or deviations in radius must be avoided under all circumstances during the bending process. The profiles are to be bent with variable radii in the range 400 to 15'000mm in at least one plane. For certain profiles a
Technc Technology I ,, PI
Technology Fig.l: Development Process 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rightsreserved PI10924-0136 (95) 02205-1
torsion can be superimposed on the bending process in certain radii ranges. Currently the length of the profile varies in the range 150 to 2'000ram. The description of the procedure in this report is illustrated in Fig1. Here the chapters are allocated to the corresponding development phases. 2. Technology Analysis The existing process is first checked in terms of the individual functions and then for their interrelation. For this purpose a sultabie tool is provided by function analysis. This is normally used to improve product properties and cost structure. However the basis principle of thinking in terms of functions makes it also suitable for the checking of technologies and processes. Concentration on only the most essential points permits a minimisation of the process complexity. However the technological function analysis requires a fundamental understanding of the process.
335
R.G. fFe~et. ~ Rctssnee/ Journal of Matcrtals Processing Technology 38 (1996) 337-342
The course of the original bending process can be described as follows: • Clamping of the semi*finished profile section in the machine. • Application of a compressive force, which presses the semi-finished product into the form die of the tool. • Rotation of the tool segment along the pressure force element. • Simultaneoussupport of the hollow profile by a mandrel held stationary at the point of plastic deformation. • Unloading of the bend profile and removal from the machine, • Complete measurement of the bern p,~,~lle on a coordinate measuring machine to check for any required correction of the setting parameters at the beginning of a new manufacturing order. Removal of random parts during manufacturing and comparison with the reference geometry.
i ill
Co-ordination of the machine kinematics through permanent process monitoring using adaptive control and regulation principles. Fig.2 shows the essential functions determined on the basis of the process and function analysis for the improved bending process. The improvement potential of the existing bending process is illustrated in Fig.3:
~
Improvement potential
Actual condition • Intended condition C)
l ml
im
ii n
Clamping from Generate tensile
~
Support plastic region
compression force II ~ ,
-
_
~
,
.
Monitor process ,~--...--.-,..~, Fig,2: Technology Functions
Inadequacies of the process were apparent during the manufacturing procedure, These appeared particularly in the form of inaccurate bending radii, which vaded from part to part, As a result it was possible to define further functions, which are essential for the attainment of the required reproducibility, Hence the new process should proceed as follows in the form of a process specification: • Insertion of the semi-finished product in the machine and holding from both sides with form-locking gdps, • Application of a constant tensile force during the entire course of the process to prevent the formation of wrinkles and buckling, • Pressing the bending profile into the rigid form die and specific support of the plastic regions,
Fig,3: Improvement Potential
A further analysis shows that the grouping of individual functions into multiple functions is easy and appropriate from the point of view of the process sequence, In the new bending process two gripping arms take over the functions of "clamping from both sides", "generate tension" and "support plastic region", A cost determination of the functions of the improved bending technology can be generated based on this function analysis, This process value analysis ensures specific assurance of the correct functioning of the machine with minimum operating costs and optimum process application. A sensitivity analysis is performed for the newlyconceived bending process, in order to evaluate possible design-related sources of nonconformance and to take specific account of quality-influencing process characteristics. This is predominantly based on experience acquired during the course of manufacturing using the earlier bending process. The major influences are exerted by the selected semi-finished product, the technological characteristics of the tool and bending machine, the influence of the operator on the system, the organisational boundary conditions and the environmental factors. As an example Fig.4 illustrates this for the effects on the part-system process.
R, G, g{ee~ppe~'t, J, Reisst~e~ ~/ dota'nat ¢~fMote~'icds Puxycess~t Tec~motogyr 58 (t 996) 337=342
[-- Semi-finished~
Flexib~]i~
[-- ProcessSelectio_n~
__
[--Machine ~
--. StateofTechnology--[
[- Tool
¢ ~
339
ReproducibiH~y~
Modernisation Potential--]
Fig.5: Solid Model ADAMS Fig.4: Influence Diagram
In this development phase disturbing factors withirJ the framework of a design improvement can be minimised to a great extent. Later these individual factors in the overall process are collected to a multi-causal network, in which specific influence can be taken, possibly by adjustment, but otherwise only by control and regulation components.
3. Process Kinematics Subsequent to the function and influence factor analysis, it is necessary, on one I~and, to constructively modify the process with the determined functions and, on the other hand, to design the machine in accordance with the kin,~matic processes and the later possibilities of process monitoring. Based on the definition of the interrelation of the individual functions it is possible to generate a simple time-displacement sequence diagram of the process functions. The verification of the model and an exact investigation of possible collision points is now performed with a simulation programme which generates the machine-tool system on the basis of the CAD data of the tool and machine components. In this process it is possible to prescribe movements, accelerations, forces and moments of the individual part-functions. On this basis the simulation programme generates a solid model of the machine-tool configuration, as illustrated in Fig.5. The overall kinematics of the process are visualised in the form of a process animation. The displacement-time and force-time diagrams required for the design of the machine control system are generated independently. These process curves provide not only the basis for the programming of the machine control; the adaptive process control in the first manufacturing phase is also based on these simulations.
4. FD Simulation The newly designed process should be integrated in a flexible manufacturing segment, in which control is taken by self-controlled and self-responsible structures. For this reason the concept anticipates that the machine adjustment data are determined, administered and updated within the segment. This requires that the operators are provided with a working platform from which, aftGr reading in the CAD file of the profile section, they are able to provide additional information concerning materi~ls characteristic values, bending ranges and radii and hence calculate adjustment and correction parameters. The CAD file is read into the simulation package using a dxf interface. The dxf interface was selected as a consequence of its uniform design and the simple processing and editing which it permits. As a restriction, for standardlsation, the use of polylines and the separation of inner and outer contours of the section onto two layers has been prescribed. The rapid calculation of the machine presettings should not be made using Finite Element Methods (FEM). On one hand this is because explicit FEM calculations require a long computing time, as a result of the large deformation displacements in volumetric bending processes and the FEM procedure breaks down due to the non-singular contact conditions which arise periodically between the profile and the tool die. On the other hand the computing capability of a PC is then adequate. This represents a much lower level of investment than for the work station required for FEM calculations. For this reason the simulation is carried out using the finite difference method. Based on the elementary bending theory the assumptions are made that a constant bending moment appears as the only external loading and that the section remains planar. The material, solely the alloy AIMgSil/2, exhibits elastic, isotropic and isotropically workhardening behaviour. Under these assumptions the tool radii to be
340
leG. W¢Ippert,J, R¢i~s~r / Journal of ~terials ~e:~slng Technology 38 (1996) 337-342
set are calculated on the basis of the unloaded intended radii. Any radial forces which appear are taken into account by a displacement of the neutral axis. The constant superimposed tensile force is included by addition of the appropriate tensile stresses to the bending stresses. The torsion of individual bending areas must be limited to only a few degrees because otherwise it is no longer p ~ i b l e to release the deformed profile from the diei The unloaded bending radii calculated in this way agree extremely well with the values determined by testing, This is illustrated in Fig,6. The setting data provide further input values for machine control. These data provide a preliminary basic setting for the hydraulic cylinders of the geometry adjustment elements. For the first optimisation determination the adaptive control is also based on these data. 5. Adaptive Control Adaptive regulation and control sy~tenls (AC) are not yet widely used in the area of forming technology. This may be due to the extremely short process cycle times and the sensitivity of the sensors in comparison with the high process forces and temperatures. However, if the boundary conditions for the application of adaptive systems, are considered it is apparent that it is particularly appropriate to make use of AC in ~orming technology (MOller-Duysing lg93). This should be illustrated for the case of various influencing facto=s in the design of the roll stretch bending process.
5. I. Production Strategy Two strategies are generally used in the area of manufacturing: If the strategy of cost leadership is used,
then the manufacturing processes must exhibit a high level of efficiency. As a result there must be a strong accent in the area of innovative process control with AC, For the strategy of performance differentiation, the main emphasis lies in the creation of product innovations. Nowadays these are generally coupled with innovative process technologies. The use of AC in a manufacturing process means that a certain minimum batch size is to be manufactured. Here the quality of the statistical regression determines the amount of data required. This requirement is more easily satisfied for the manufacture of large series of parts, as is more generally found for manufacturing of bent parts. The geometrical and physical complexity of the spectrum of parts directly determin.~s the cost of the proposed regulation system when using AC. In volumetric bending technology the maintenance of narrow tolerances is not only influenced by processes determining the geometry but the problem lies rather in the influence exerted by the alteration in the physical properties of the material during deformation to the achievable intended geometry. This variation cannot be precisely described. However this is exactly what constitutes a major driving force for the application of AC in bending technology: The impossibility of providing an exact process description or prediction using intelligent controllers. It is appropriate that a bending process controlled by AC should exhibit a minimum level of reproducibility. In industrial manufacturing processes AC constitutes the last economic possibility of maintaining the specified quality level. Within the framework of total quality management, the integral quality achievement reached here constitutes both an end point and also the basis for further Improvements to the process.
5000
/
4000
f
~, TEST
,,,4,,4
•
SIMULATION
3000 2000 ! !
1000
:
~p==F
.¢--=3,,4p i
0
500
t'000 Radius Profile
Fig,6: Comparison of Test and Simulation
5'000
10'000
(Values in mm)
R,G, }~'~",ippe~"t,J: Re~ssne~*/J]oun~! qf ~te~'~ats Precess#~g ~'~"ch~olog),58 (t 9,96) 337-342
5.2. Control Strategy The machine controt provides the basis fro a
reproducible process which is abWeto satisfy the quaiity
requirements. The selection of the adjustment values is of paramount importance for the success of the controt strategy. The results of the kinematic simulation provide valuable support. For most forming processes there are only two principle adjustment value variants; the process is influenced either by the effect of one or more forces or through alteration of the tool geometry, in roll stretch bending this occurs through geometry variation: The hydraulic cylinder permits rough setting and a flexible tool chuck provides fine adjustment. The controller characteristic, the mathematicaD model of the controller behaviour, can be determined from the mathematical procedure of the FD simulation. This requires provisior, of results from various measurement procedures. The characteristics first serve as reference, but during the course of the process they are also optimised for the specific batch through a feedback procedure (Shah and Dumont 1988).
34 t
phase and the ~ength of the access time. On one hand the measurement must determine significant data, and on the other hand it must be made as soon as possible so that the evaluation and adjustment of the setting values can be made within a useful period. The maximum number of possible measurements is given by the sampling frequency of the sensor multiplied by the measurement time. An optimum selection of the measurement location can be made on the basis of a factor analysis. Measurements in the immediate vicinity of the forming zone have the greatest significance and permit the most rapid determination of correction values.
Process
[ Simulation
Parameter ~_ Parameter
!Simulation}
ConditionModel
._i Basic Setting
5.3. Measurement Strategy The subdivision of the process into sequential and parallel individual functions permits the determination of various factors which disturb the entire process. Control in the tightest tolerance ranges should no longer be disturbed by noncontrollable effects in the process environment. A determination of the starting values, performed before the process, can greatly simplify the adaptation of the process. In this way influences of material batches and even variations within one batch can be determined and provided to the AC as "feed forward element", However the filtering out of geometrical or material-specific deviations from the intended values before the beginning of the process is very time consuming and, for the determination of physical deviations, it is only possible using nonreproducible destructive material testing, The mutual interaction of tool, machine and process, in the form of elastic extensions, vibrations or electromagnetic fields is, as mentioned, to be reduced to a determined value already in the process development stage. In the bending process the loaded bending radii and torsion angle serve as measurement values. They are deduced directly from the setting values of the machine control and are compared in each case with an intended value calculated for the unloaded dimension. The measurement elements, i.e. the actually measured values, are the feed movements of the hydraulic cylinder of the tool. These measured values pass directly to the controller and are compared with the characteristic line of the controller. The time at which the measurement is made depends on the accessibility during the corresponding process
i
Neuron
Adjustment A d a p t i v e Value
i
~
Control [i
Network
Process
,~,,-
ii
(..............F .........~ Measurement . . . . . ~....................... i Value &
l ........................ ]
Output j
I
i Torsion endingAngle Radius Fig.7: Regulation Optimisation
The AC data are not only useful for the process control and regulation. It is appropriate to use the data measured by the sensors also for statistical process control (SPC), for rapid verification of conformance with quality requirements and for quality documentation of the process procedure. Within the framework of the quality control, the data of the SPC are provided for optimisation of the AC. Together with the adaptively modified control parameters, on the planing and controling of production side they
34~
g (3, W#~rt, d, ~ts~ncr /Journal of Materials Processing Technology 38 (1996) 33 7-342
provide the basis for the acquisition of the machine data. In a condensed form they are passed to the data acquisition system in the manufacturing segment. There they are used to optimise process control and are employed in the recalculation of the entire operation.
achieved with a 150 kHz sensor with 100-300 kHz bandpass filtering and 60 dB preamplification. The signal to noise ratio of individual transient events was poor under initial conditions, in the range of 1:1 to 6:1, however it was improved to ranges up to 15:1 by the completion of the testing.
5.4. Regulation Strategy For the AC system described, the regulation parameters determine, in accordance with the evaluated measurement values, vadable adjustment values which are fed continuously into the process. The determination of the deviations of the measurement results from the controller characteristic must be performed according to a fixed value regulation, in order to obtain an optimum regulation result. The statistical concentration of the measurement results is first carded out using multivariant regression. The condition monitor processes the measurement elements. Here an attempt should be made to achieve an elementary mathematical description of the process as basis for the regulation process. With the help of a superimposed neuron network, the difficult-to-predict data structures should be processed. Fig.7 should illustrate this, Not only the values from the measurement elements, should be used for this regulation optimisation in the framework of optimising the comparison of characteristic lines. Also the regulation and machine controller should be matched to the vadable process conditions.
5.5. Tensile Measurement and Acoustic Emission In the description of the measurement strategy, the aspect of the measurement cnodus was expressly excluded: On one side investigations with a tensile sensing system should show whether reliable results can be obtained, when measuring the total amount and the gradient of tensile stress and whether those measurement results are sufficiently meaningful to permit calculation of a correction of the tool geometry. On the other hand tests should be performed with acoustic measurements during tensile and simple bending tests, Aeoustte Emlss!on (AE) monitoring were made on alumtntum dogbone specimens tested in tension, The Intention of these tests was to capture as much emission against as low a noise background as could be obtained to show the acoustic difference between elastic and plastic forming. Hydraulic gdps were used to reduce mechanical contact. To reduce electronic noise, filtering techniques as well as a vadety of sensors were used to find optimal noise levels. Sensors placed directly on each grip could monitor the background coming directly through the tensile loading system. One mild steel specimen of similar ductile behaviour was tested for comparison (EMPA
Amp (dB)
55
ii
50
o
45
° •
,
t °
•
4o| 30
~--
0
-T-
50
o
i-
.
•
i-
°°
-~ ......
100
-~
150
~
200
....
J
lO
~
0
250
Time o Amp (dB)
- - Load (kN)
Fig.8: AE from a specimen opposite load curve Under these conditions, up to 100 (estimated) signals were seen as well as steady noise present before and after loading which interfered with the signal capture, as illustrated in Fi~.8. Signals clearly above the noise level at near or before ~ile yield point were on the order of 10 or less in all of the tests. Background noise of a steady nature was 20 I~Volts and reduced further to 10 I~Volts for the end tests. This is still rather high, when in non-mechanical test set.ups the Instrument can have noise floors well below t0 pVolts. The results show that AE can be used to create an AC for the roll.bending of Alumlnlum profiles. But the quality of the tensile measurement methods is e x i t e d to be much more precise. This should be verified in the next time during tests on a Stretch-Drawing machine. References
[ |] [2]
1994),
AE was noted near the linearly elastic limit of low amplitude. The maximum number of AE hits distinguishable against background from any piezoelectric sensors was
Load (kN)
[3]
M. M011er-Duysing, The Calculation and Adaptive Control of Three.Point Bending, VDI Series 2 No. 287, D0sseldorf, 1993. S.L. Shah, G. Dumont; Adaptive Control Strategies for Industrial Use; Proceedings of a workshop Kananaskis, Canada, 1988. EMPA Test Report Nr. 156'246; Generation of Acoustic Emission during Tensile Tosting of Aluminium Specimens, Duebendorf, 1994.
Muterials essing
ELSEVIER
Journal ofMal~'ials ~ s s i n g Tedmology 58 (1996) 337-342
Tectmology
Process improvement in Volumetric Roll-Stretch Bending Roland G. Weipperta, Prof. Dr. J. Reissnerb
ainstitute for Forming Technologies, ETH, Technopark, Pfingstweidstrasse 30, 8005 ZOrich, Switzerland binstitute for Forming Technologies, ETH, Tannenstrasse 3, 8092 ZEtrich, Switzerland
Abstract
In order to ensure success in the rapidly changing market manufacturing must be rapid and free of errors, with the lowest possible manufacturing costs. In this article some possibilities for rapid further development of an existing bending technology will be shown, at the end of which should be found an adaptive-controlled, reproducible and economic bending process. By using various analysis procedures and special computer-aided simulations for different development phases, it is possible to introduce a co-ordination of the development at a very early stage. This permits a shortening of the process development time and a concentration on quality-enhancing measures.
Keywords: Aluminium Space Frame, Finite Difference Simulation, Adaptive Control, Technology Management.
1. Introduction For many years no further progress has been made in the development of process technologies for volumetric bending of extruded hollow aluminium profiles. Generally the parts to be bent had generous tolerances, which could easily be attained with simple machines. However within the last two years a product technology has developed in the form of light, high-strength "space frame" structures in automobile design. These cannot be satisfied in qualitative and quantitative aspects by conventional bending technologies, The areas of tolerance to be maintained are such that economical manufacturing of small series can only be achieved with a reproducible process. The sections of the aluminium profiles often ihclude openings, cross-pieces or closed chambers, the contours of which have sealing or closing functions. As a result wrinkles or deviations in radius must be avoided under all circumstances during the bending process. The profiles are to be bent with variable radii in the range 400 to 15'000mm in at least one plane. For certain profiles a
Technc Technology I ,, PI
Technology Fig.l: Development Process 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rightsreserved PI10924-0136 (95) 02205-1
torsion can be superimposed on the bending process in certain radii ranges. Currently the length of the profile varies in the range 150 to 2'000ram. The description of the procedure in this report is illustrated in Fig1. Here the chapters are allocated to the corresponding development phases. 2. Technology Analysis The existing process is first checked in terms of the individual functions and then for their interrelation. For this purpose a sultabie tool is provided by function analysis. This is normally used to improve product properties and cost structure. However the basis principle of thinking in terms of functions makes it also suitable for the checking of technologies and processes. Concentration on only the most essential points permits a minimisation of the process complexity. However the technological function analysis requires a fundamental understanding of the process.
335
R.G. fFe~et. ~ Rctssnee/ Journal of Matcrtals Processing Technology 38 (1996) 337-342
The course of the original bending process can be described as follows: • Clamping of the semi*finished profile section in the machine. • Application of a compressive force, which presses the semi-finished product into the form die of the tool. • Rotation of the tool segment along the pressure force element. • Simultaneoussupport of the hollow profile by a mandrel held stationary at the point of plastic deformation. • Unloading of the bend profile and removal from the machine, • Complete measurement of the bern p,~,~lle on a coordinate measuring machine to check for any required correction of the setting parameters at the beginning of a new manufacturing order. Removal of random parts during manufacturing and comparison with the reference geometry.
i ill
Co-ordination of the machine kinematics through permanent process monitoring using adaptive control and regulation principles. Fig.2 shows the essential functions determined on the basis of the process and function analysis for the improved bending process. The improvement potential of the existing bending process is illustrated in Fig.3:
~
Improvement potential
Actual condition • Intended condition C)
l ml
im
ii n
Clamping from Generate tensile
~
Support plastic region
compression force II ~ ,
-
_
~
,
.
Monitor process ,~--...--.-,..~, Fig,2: Technology Functions
Inadequacies of the process were apparent during the manufacturing procedure, These appeared particularly in the form of inaccurate bending radii, which vaded from part to part, As a result it was possible to define further functions, which are essential for the attainment of the required reproducibility, Hence the new process should proceed as follows in the form of a process specification: • Insertion of the semi-finished product in the machine and holding from both sides with form-locking gdps, • Application of a constant tensile force during the entire course of the process to prevent the formation of wrinkles and buckling, • Pressing the bending profile into the rigid form die and specific support of the plastic regions,
Fig,3: Improvement Potential
A further analysis shows that the grouping of individual functions into multiple functions is easy and appropriate from the point of view of the process sequence, In the new bending process two gripping arms take over the functions of "clamping from both sides", "generate tension" and "support plastic region", A cost determination of the functions of the improved bending technology can be generated based on this function analysis, This process value analysis ensures specific assurance of the correct functioning of the machine with minimum operating costs and optimum process application. A sensitivity analysis is performed for the newlyconceived bending process, in order to evaluate possible design-related sources of nonconformance and to take specific account of quality-influencing process characteristics. This is predominantly based on experience acquired during the course of manufacturing using the earlier bending process. The major influences are exerted by the selected semi-finished product, the technological characteristics of the tool and bending machine, the influence of the operator on the system, the organisational boundary conditions and the environmental factors. As an example Fig.4 illustrates this for the effects on the part-system process.
R, G, g{ee~ppe~'t, J, Reisst~e~ ~/ dota'nat ¢~fMote~'icds Puxycess~t Tec~motogyr 58 (t 996) 337=342
[-- Semi-finished~
Flexib~]i~
[-- ProcessSelectio_n~
__
[--Machine ~
--. StateofTechnology--[
[- Tool
¢ ~
339
ReproducibiH~y~
Modernisation Potential--]
Fig.5: Solid Model ADAMS Fig.4: Influence Diagram
In this development phase disturbing factors withirJ the framework of a design improvement can be minimised to a great extent. Later these individual factors in the overall process are collected to a multi-causal network, in which specific influence can be taken, possibly by adjustment, but otherwise only by control and regulation components.
3. Process Kinematics Subsequent to the function and influence factor analysis, it is necessary, on one I~and, to constructively modify the process with the determined functions and, on the other hand, to design the machine in accordance with the kin,~matic processes and the later possibilities of process monitoring. Based on the definition of the interrelation of the individual functions it is possible to generate a simple time-displacement sequence diagram of the process functions. The verification of the model and an exact investigation of possible collision points is now performed with a simulation programme which generates the machine-tool system on the basis of the CAD data of the tool and machine components. In this process it is possible to prescribe movements, accelerations, forces and moments of the individual part-functions. On this basis the simulation programme generates a solid model of the machine-tool configuration, as illustrated in Fig.5. The overall kinematics of the process are visualised in the form of a process animation. The displacement-time and force-time diagrams required for the design of the machine control system are generated independently. These process curves provide not only the basis for the programming of the machine control; the adaptive process control in the first manufacturing phase is also based on these simulations.
4. FD Simulation The newly designed process should be integrated in a flexible manufacturing segment, in which control is taken by self-controlled and self-responsible structures. For this reason the concept anticipates that the machine adjustment data are determined, administered and updated within the segment. This requires that the operators are provided with a working platform from which, aftGr reading in the CAD file of the profile section, they are able to provide additional information concerning materi~ls characteristic values, bending ranges and radii and hence calculate adjustment and correction parameters. The CAD file is read into the simulation package using a dxf interface. The dxf interface was selected as a consequence of its uniform design and the simple processing and editing which it permits. As a restriction, for standardlsation, the use of polylines and the separation of inner and outer contours of the section onto two layers has been prescribed. The rapid calculation of the machine presettings should not be made using Finite Element Methods (FEM). On one hand this is because explicit FEM calculations require a long computing time, as a result of the large deformation displacements in volumetric bending processes and the FEM procedure breaks down due to the non-singular contact conditions which arise periodically between the profile and the tool die. On the other hand the computing capability of a PC is then adequate. This represents a much lower level of investment than for the work station required for FEM calculations. For this reason the simulation is carried out using the finite difference method. Based on the elementary bending theory the assumptions are made that a constant bending moment appears as the only external loading and that the section remains planar. The material, solely the alloy AIMgSil/2, exhibits elastic, isotropic and isotropically workhardening behaviour. Under these assumptions the tool radii to be
340
leG. W¢Ippert,J, R¢i~s~r / Journal of ~terials ~e:~slng Technology 38 (1996) 337-342
set are calculated on the basis of the unloaded intended radii. Any radial forces which appear are taken into account by a displacement of the neutral axis. The constant superimposed tensile force is included by addition of the appropriate tensile stresses to the bending stresses. The torsion of individual bending areas must be limited to only a few degrees because otherwise it is no longer p ~ i b l e to release the deformed profile from the diei The unloaded bending radii calculated in this way agree extremely well with the values determined by testing, This is illustrated in Fig,6. The setting data provide further input values for machine control. These data provide a preliminary basic setting for the hydraulic cylinders of the geometry adjustment elements. For the first optimisation determination the adaptive control is also based on these data. 5. Adaptive Control Adaptive regulation and control sy~tenls (AC) are not yet widely used in the area of forming technology. This may be due to the extremely short process cycle times and the sensitivity of the sensors in comparison with the high process forces and temperatures. However, if the boundary conditions for the application of adaptive systems, are considered it is apparent that it is particularly appropriate to make use of AC in ~orming technology (MOller-Duysing lg93). This should be illustrated for the case of various influencing facto=s in the design of the roll stretch bending process.
5. I. Production Strategy Two strategies are generally used in the area of manufacturing: If the strategy of cost leadership is used,
then the manufacturing processes must exhibit a high level of efficiency. As a result there must be a strong accent in the area of innovative process control with AC, For the strategy of performance differentiation, the main emphasis lies in the creation of product innovations. Nowadays these are generally coupled with innovative process technologies. The use of AC in a manufacturing process means that a certain minimum batch size is to be manufactured. Here the quality of the statistical regression determines the amount of data required. This requirement is more easily satisfied for the manufacture of large series of parts, as is more generally found for manufacturing of bent parts. The geometrical and physical complexity of the spectrum of parts directly determin.~s the cost of the proposed regulation system when using AC. In volumetric bending technology the maintenance of narrow tolerances is not only influenced by processes determining the geometry but the problem lies rather in the influence exerted by the alteration in the physical properties of the material during deformation to the achievable intended geometry. This variation cannot be precisely described. However this is exactly what constitutes a major driving force for the application of AC in bending technology: The impossibility of providing an exact process description or prediction using intelligent controllers. It is appropriate that a bending process controlled by AC should exhibit a minimum level of reproducibility. In industrial manufacturing processes AC constitutes the last economic possibility of maintaining the specified quality level. Within the framework of total quality management, the integral quality achievement reached here constitutes both an end point and also the basis for further Improvements to the process.
5000
/
4000
f
~, TEST
,,,4,,4
•
SIMULATION
3000 2000 ! !
1000
:
~p==F
.¢--=3,,4p i
0
500
t'000 Radius Profile
Fig,6: Comparison of Test and Simulation
5'000
10'000
(Values in mm)
R,G, }~'~",ippe~"t,J: Re~ssne~*/J]oun~! qf ~te~'~ats Precess#~g ~'~"ch~olog),58 (t 9,96) 337-342
5.2. Control Strategy The machine controt provides the basis fro a
reproducible process which is abWeto satisfy the quaiity
requirements. The selection of the adjustment values is of paramount importance for the success of the controt strategy. The results of the kinematic simulation provide valuable support. For most forming processes there are only two principle adjustment value variants; the process is influenced either by the effect of one or more forces or through alteration of the tool geometry, in roll stretch bending this occurs through geometry variation: The hydraulic cylinder permits rough setting and a flexible tool chuck provides fine adjustment. The controller characteristic, the mathematicaD model of the controller behaviour, can be determined from the mathematical procedure of the FD simulation. This requires provisior, of results from various measurement procedures. The characteristics first serve as reference, but during the course of the process they are also optimised for the specific batch through a feedback procedure (Shah and Dumont 1988).
34 t
phase and the ~ength of the access time. On one hand the measurement must determine significant data, and on the other hand it must be made as soon as possible so that the evaluation and adjustment of the setting values can be made within a useful period. The maximum number of possible measurements is given by the sampling frequency of the sensor multiplied by the measurement time. An optimum selection of the measurement location can be made on the basis of a factor analysis. Measurements in the immediate vicinity of the forming zone have the greatest significance and permit the most rapid determination of correction values.
Process
[ Simulation
Parameter ~_ Parameter
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5.3. Measurement Strategy The subdivision of the process into sequential and parallel individual functions permits the determination of various factors which disturb the entire process. Control in the tightest tolerance ranges should no longer be disturbed by noncontrollable effects in the process environment. A determination of the starting values, performed before the process, can greatly simplify the adaptation of the process. In this way influences of material batches and even variations within one batch can be determined and provided to the AC as "feed forward element", However the filtering out of geometrical or material-specific deviations from the intended values before the beginning of the process is very time consuming and, for the determination of physical deviations, it is only possible using nonreproducible destructive material testing, The mutual interaction of tool, machine and process, in the form of elastic extensions, vibrations or electromagnetic fields is, as mentioned, to be reduced to a determined value already in the process development stage. In the bending process the loaded bending radii and torsion angle serve as measurement values. They are deduced directly from the setting values of the machine control and are compared in each case with an intended value calculated for the unloaded dimension. The measurement elements, i.e. the actually measured values, are the feed movements of the hydraulic cylinder of the tool. These measured values pass directly to the controller and are compared with the characteristic line of the controller. The time at which the measurement is made depends on the accessibility during the corresponding process
i
Neuron
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i
~
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Network
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ii
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i Torsion endingAngle Radius Fig.7: Regulation Optimisation
The AC data are not only useful for the process control and regulation. It is appropriate to use the data measured by the sensors also for statistical process control (SPC), for rapid verification of conformance with quality requirements and for quality documentation of the process procedure. Within the framework of the quality control, the data of the SPC are provided for optimisation of the AC. Together with the adaptively modified control parameters, on the planing and controling of production side they
34~
g (3, W#~rt, d, ~ts~ncr /Journal of Materials Processing Technology 38 (1996) 33 7-342
provide the basis for the acquisition of the machine data. In a condensed form they are passed to the data acquisition system in the manufacturing segment. There they are used to optimise process control and are employed in the recalculation of the entire operation.
achieved with a 150 kHz sensor with 100-300 kHz bandpass filtering and 60 dB preamplification. The signal to noise ratio of individual transient events was poor under initial conditions, in the range of 1:1 to 6:1, however it was improved to ranges up to 15:1 by the completion of the testing.
5.4. Regulation Strategy For the AC system described, the regulation parameters determine, in accordance with the evaluated measurement values, vadable adjustment values which are fed continuously into the process. The determination of the deviations of the measurement results from the controller characteristic must be performed according to a fixed value regulation, in order to obtain an optimum regulation result. The statistical concentration of the measurement results is first carded out using multivariant regression. The condition monitor processes the measurement elements. Here an attempt should be made to achieve an elementary mathematical description of the process as basis for the regulation process. With the help of a superimposed neuron network, the difficult-to-predict data structures should be processed. Fig.7 should illustrate this, Not only the values from the measurement elements, should be used for this regulation optimisation in the framework of optimising the comparison of characteristic lines. Also the regulation and machine controller should be matched to the vadable process conditions.
5.5. Tensile Measurement and Acoustic Emission In the description of the measurement strategy, the aspect of the measurement cnodus was expressly excluded: On one side investigations with a tensile sensing system should show whether reliable results can be obtained, when measuring the total amount and the gradient of tensile stress and whether those measurement results are sufficiently meaningful to permit calculation of a correction of the tool geometry. On the other hand tests should be performed with acoustic measurements during tensile and simple bending tests, Aeoustte Emlss!on (AE) monitoring were made on alumtntum dogbone specimens tested in tension, The Intention of these tests was to capture as much emission against as low a noise background as could be obtained to show the acoustic difference between elastic and plastic forming. Hydraulic gdps were used to reduce mechanical contact. To reduce electronic noise, filtering techniques as well as a vadety of sensors were used to find optimal noise levels. Sensors placed directly on each grip could monitor the background coming directly through the tensile loading system. One mild steel specimen of similar ductile behaviour was tested for comparison (EMPA
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Fig.8: AE from a specimen opposite load curve Under these conditions, up to 100 (estimated) signals were seen as well as steady noise present before and after loading which interfered with the signal capture, as illustrated in Fi~.8. Signals clearly above the noise level at near or before ~ile yield point were on the order of 10 or less in all of the tests. Background noise of a steady nature was 20 I~Volts and reduced further to 10 I~Volts for the end tests. This is still rather high, when in non-mechanical test set.ups the Instrument can have noise floors well below t0 pVolts. The results show that AE can be used to create an AC for the roll.bending of Alumlnlum profiles. But the quality of the tensile measurement methods is e x i t e d to be much more precise. This should be verified in the next time during tests on a Stretch-Drawing machine. References
[ |] [2]
1994),
AE was noted near the linearly elastic limit of low amplitude. The maximum number of AE hits distinguishable against background from any piezoelectric sensors was
Load (kN)
[3]
M. M011er-Duysing, The Calculation and Adaptive Control of Three.Point Bending, VDI Series 2 No. 287, D0sseldorf, 1993. S.L. Shah, G. Dumont; Adaptive Control Strategies for Industrial Use; Proceedings of a workshop Kananaskis, Canada, 1988. EMPA Test Report Nr. 156'246; Generation of Acoustic Emission during Tensile Tosting of Aluminium Specimens, Duebendorf, 1994.