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Interactive Visualisation of Sequence Logic and Physical Machine Components within an Integrated Design and Control Environment
Copyright C IFAC Intelli gent Manu facruring Systems. Seoul , Korea. 1997
INTERACTIVE VISUALISA nON OF SEQUENCE LOGIC AND PHYSICAL MACHINE COMPONENTS WITHIN AN INTEGRATED DESIGN AND CONTROL ENVIRONMENT
R. Harrisoo, A.A. West, P. Hopkiosoo and C.D. Wrigbt
Manufacturing Systems InlegratiJJn (MSI ) Research Institute. Loughborough University, LDughborough, United Kingdom. LE] I 3TU.
Abstract: An Integrated Machine Design and Cmtrol (lMDC) envirooment fa the visual represeotatioo. and integratioo. r::J. the physical machine canpoo.ents and cootrollogic is discussed in this paper. The approach taken is unique in that (a) the cootrollogic and pbysical models of the elements can be investigated individually fa carectness and canpleteness, (b) the cootrollogic can be easily integrated with the solid models to animate the model eX the pbysical machin.e and (c) rec:oo.figuratioo. enables the same cootrollogic to be applied to real wald physical machjne elements. At aoy stage during the machine design and imp1ementatioo. prcx:ess. the user cl the enviroomeot can pause and questioo. the validity eX certain 0perations and control system parameters. Keywords: Machine. Control Logic. Design. Disuibuted. Objects. Modelling. Pe ui-nets.
vanced COOlputer technology throughout the machine life ~e fran requirements defin.itioo.. through the design and build stages to maiDtenance and recm· figuration.
l.INmODUcnON The design and implementatioo. d. manufacturing machines is under increasing time and financial pres· sures as custoolers demand inaeased product variety and quality at reduced product cost (Yoong. 1995). Increased canpetitioo. and governmental pressure to focus 00. enviroomeotal issues has faced modem machine OOilders and users to coo.sider the require· ments fa the next generatioo. cl machines that allow the recoo.figuratioo. cl both the coouol sd.tware and physical hardware (Rahkoo.en. 1995). Machines will be required to be developed in the minimum amoont d time and canprise (a) venda independent hardware canpooents. (b) sophisticated cootrol algorithms. (c) intelligent sensas and acruatcrs and (d) user friendly interlaces. In addition. open systems issues coo.c.erning the ease of integratioo.. interoper· ability d the software and hardware canpcnents and available standards must be addressed (Crowacit. 1995) to ensure that reuse aod recoo.figuratioo. can be achieved. The inherent canplexity inevitably necessitates the increased applicatioo. c:1 machine modelling software toolkits (i.e . fa cc:nuollogic and physi· cal machine element design and analysis) and ad·
End users. machine designers and machine builders require technical and operatiCDal knowledge fran disparate disciplines and specialised danain experts at various stages throughout the machine requirements. design, build. instaIlatioo.. set-up. maintenance and rt(;(Ilfi.guratioo. life ~e (Carrou. et al.. 1997). Ccmmoo. frames cl reference are vital to im,. prCNe the cmununicatioos between the above stakeholders in the machine design and OOild process. In particular it is impcrtant that discussims are focused aroond b
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Ma. I. Illustration or the Visual Interactive Simulation Process. ality is included. The develq>ment cl canplex manufacturing logic fa PLCs is a specialised activily. and it is difficult for a noo expert to appreciate the operatim cl the canpJete ~tem (Venk.ateSb. et 01.1994), Ladder logic diagrams and sequential fuoc· bOO charts (David and Alla. 1992) provide grapbical representatim cl sequential operatim but the design cl flexible. reusable and maintainable sdtware is nevenbeless di.£ficuJt to achieve.
virmment (an integrated software eDvirmment and toolset canprising third party and "in house" tools) has been developed that seeks to suppttt the Wttk d. COluol system engineers and mechanical designers lhrooghout the design and development life C)Cle d manufacturing machines. Visibility cl the pbysical machine (using the IMDC Machine Modeller developed around the AOS solid modelling kemel (Murry and Yue. 1993) and CQluot logic software (using the Synecr. modelling tool produced by Hopkio.soo COOlpuring Ltd. (Anoo. 1995» is a cere requirement in the IMDC system and is discussed in this paper.
There has been widespread use of visual representatiro of manufacturing products iD terms of surface and solid models (Hoffmann, 1997). Canpurer aided
design package usage e.g. AutoCAD and Unigraphlcs et 01 .. 1996) enables design engineers to visualise physical canpooents and layrulS prier to canmissiClliog. In c:ertain cases, q,eratiooal logic has been included into the solid model to enable a dy· nam..ic animatioo cl the moclel1ed system elements to be observed and optimised (HcBmann, 1997). A map limitatiCll with this approacb has been the fact that in ocder to ttansfet the results of the modelling exercise to a real system. the logic must be reimple-meated ootside c:i the solid mcdel and the ~ wry fCl' erTa'S and sub optimal implementatioo behavioor proliferate (Wright and Case. 1995). There is a requirement fa' an environment in which (a) the CQluol. logic: and physical models c:i the elements can be investigated individually fa' Ca'TeCmess and C(ID.pleteness. (b) the CCIluol logic: can be easily integrated with the solid models to animate the machine soI.id mcdel and (c) recoo.6guratioo enables the same COluot logic to be applied to real W(W"ld physical c:oo.uol elements.
The envirmment is based aroond a disuibuted object oriented represeutatioo ci manufacturing machines as aggregatims of basic canpments (Joannis and Krieger. 1992): single axes. multi-axes. digital and analogue input I o.uput and dumb and intelligent senscl'$ (e.g. visioo systems and robots). Distributed object technology (DOl) (Orfall, " 01 .. 1996) p"ovide$: the CCI'e framewCI'k within which the tools and real system intercommunicate.
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2. VISUAL INTERACTIVE SIMULATION, The visual interactive. simulatim paradigm (also termed visual interactive modelling and visual interactive problem solving was originally applied to the disaete event simulatioo of job shop scheduliJlg problems in manufacruring (HurriCll. 1980) and has mainly been utilised in Oper.tiooa1 Research (Bell, 1995). Visual interactive simulatim is particularly appropriate in the manufacturing machine dOOlain (Sadashir. et al .. 1989) and can provide a COOl.OlCll frame of reference to facilitate human canmunica· tim d. ideas. In the manufacturing machine dcmain it is vital that bolb the pbysical machine and COltrol logic are available fa- scrutiny. Fig. 1 illustrates the process. A visual model (i.e. a model based upm DCIl textural and noo verbal elements to canmUllicate the
A majcr research theme at the Manufacturing Systems Integratioo (MSI) Research Institute at Loogbbttoogh University. UK is the realisatim d. the next geraeratim c:i machine systems (Carrou. 1996). An In
334
state et a system) is developed that provides an abstraction of reality.
(see sectiOll 4) . The autcmatic code generation tool produces code which is then canpUed with IMIX: runtime litraries enabling the applicatiOD. logic to ccmmunicate with the distributed macbi.oe canpo-
The application d. visual interactive simulation to industrial machine design and control projects using the IMDC envirooment has resulted in a number of obselvations and benefits: • h is impatant to ensure general intetactioo. and early involvement by developing an animated picture as soon as possible. • Interaction allows the end users and macb.ine builders to make canplex decisions with increased confidence due to their increased understanding ci the machjne operaticn and interac-tiOG. • The visual image is widely accepted and unexpected simaticns can be envisaged via what if? scenarios. • Of vital impmance is the integraticn d the controllogic with the visual simu1atioo. in the IMDC envirooment. This enables the verificatioo. (by the
DeIllS.
3.1 Logic Visualisation I Representation The designer c1 a sequential control applicatiCll can visualise the problem and present the solutioo io a number of ways. Three possible approaches are: • consider the applicatioo as consisting ci a set cl intc:rlo:ks. If the target envirooment is hard-wired logic a, as in the majaity ci current industrial autcmatioo. projects. a PLC programmed in ladder logic. the design solutioo. maps easily CIl to the implementation (MicheL 1990). • take a functioo.al view. One cl the mere popular structured methods used fa real-time applicatiClls h~ been the Ward-Mella variant cl the YrurdCll method (Wud and Mellcr. 1985). Cootrol transfCl'lDs are triggered by events and then react by sending signals to other traDsfmns. • take an object crieo.ted view. Object crientatiCll has grown frem a modelling paradigm and. it is claimed. leads to mere intuitive soluti
3. VISUAUSATION OF SEQUENCE LOGIC Manufacturing and process industry control system applications invariably include sequence logic. The ccmp1exiry cl the application logic typically involves the need to manage Sle'VeJal cono.ment activities. coadinating their behavirur to achieve the desired application goal
The canbinatioaal logic approach is potentially the preferred optioo. if the application logjc is...er)' simple a there is a stroog need fa the salutioo to require the minimum cl memay a execute as fast as possible. Otherwise. the solUtiCll can be mere difficult to verify. is not cooducive to diagnos.ing qJeratiooal mis-behavioor and is mac: difficult to modify withoot causing UDwanted side-effects.
In additiOD. to the standard scitware engineering problems ci defining how the proposed system is to wale. and expressing the design in a fcnn which is readily understood, control systems designers must ensure that the system's dynamics do not contAin design erras, such as deadlock, livelo:::k and undesirable modes ci operanoo.. Traditioo.al approaches have tended to be ad-hoc a have inadequately addressed the designer's needs, leaving such erTCI'S to be identified late in the I.ife<}de. resulting in costly modifications.
Functional approaches have the benefit cl suppating the coocept cl sequences. typic.ally by a bm ci state diagram. The functiCllal approach helps the designer to consider how the different ccmpoo.eots need to be co-
SynectTW provides a set cl sdtware tools (appticatioo edita. ccmpiler, logic engine. logic malitcr and code geoerata) which are integrated into the IMDC platftnD (Anm. 1995). Applicatioos an: described by means cl a graphical edita. Synca is based on the generation cl a Petri-Net model frem the applicatioo. descriptim (\'tIersoa. 1981). The evolving design can then be examined analytically fCl' structUtal and behavirural deficiencies e.g. checking fa em:n such as deadlo:ks and unwanted state canbinatiau. An executable logic model is created which can then drive a visual solid model cl the physical macb.ine
Synect provides a methodology which canbines the co-acdinatim d the f\mctiooal apprcach with the encapsulatioo of object orientation.
IMDC integrates the Synect simulata with the 3D solid modeller (see fig. 2). As described in section 4. the modeller incapaates concepts such as timing infcnnatioo and sensor emulatioo into the solid modelling S
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iooral models c:i the external systems being repre-
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sented. The tool. can be used to specify
CCIllJXlleD{
states. q>eratiooal parameters, motioo. parameters
and locatims. Compcnents. parameters and CCIllplete sub-assembo.s can be stcnd in the IMDC database.
4. VISUALISATION OF MAClfiNE ELEMENTS A machine designer can utilise solid models in cxder to visualise the pbysical machine elements and their interactioo (Hoffinann. 1997). The cooveutiooal aIr proach to the solid modelljng of machines involves: 1. initial static visualisatioo c:l the machine ele-
The interface to co:npcnents c::i each speciJic type is exp
ments and
2. determinatioo of the system dynamics and machine element interactioo by embedding q>eraticnal logic within the modelling tool (Yoo.g et
al.• 1985). The limitatioo. of this approach is that the coding fa the real system operatim is ocrmally undertaken after the simulatioo phase and the sdtware frequently bears little re1atim to the mechanisms used to drive the simuIatioo. In additioo this approach to the mooeUing c:i the pbysical machine elements cannot be used throoghoot the life C)de and cannot be used interactively with Olher design tools.
5. ENVIRONMENT TO SUPPORT VISUAL INTERACTIVE SIMULATION: IMDC The IMDC envircnment ccmprises frur distinct areas of functiOttlllity: • user tools to enable, fa example. logic simulatiCll, motiCll design. machine modelling and tools associated with the CCIltrol and mooitaing d. the runtime system. The user tools can be progressively changed (I' extended via a set eX generic interfaces.
The approach adopted within the IMDC envirooment is fundamentally different and
•
systems tools to enable system admjnistratiCll. access to infcrmatim and security. Typical system canfiguratiCll infCl1ll8tim includes the logical and physical system la)OOt (machines and oetwaks),
user names, passwads. access rights. project and user environment informatioo.. • the IMDC object criented database (poET a proouct frOOl the POET Scltware CapaatiCll (Anoo. 19(4) which provides persistent stcxage fer the outputs frOOl life C)'Cle activities. system
Models eX pbysical machines can be graphically coos1I'UCted within the machine modeller frOOl ccmponent building blocks such as actuatcn, sen.sas, cooveyas. alarms and structural elements. The model ccmpcnents incapcl'8te both geanetric and behav-
336
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ccnfiguration. uaceability and versioo control. and holds the generated elements d. specific target control solutions the distributed runtime machine canposed d canmunicating sdtware ccmponents. a subset d which provide interfaces to external devices and third party cmtrol sdtware fa the mooitaing and cmtrol of the physical mac.bi.oe.
These functional elements all inttt-ccmmunicate via Ille underlying Object Request B,oW (ORB) atehitectwe as described in the following section.
5.1 lMDC System Architecture The adqrtioo d an objcct-ociented approach. particularly the use et distributed object technology. has been the key to providing flexibility in the choice d implementation technologies. Fig. 3 illustrates the principle d object distribution across heterogeneoo.s host and netwak architcctures. Identical client software located on different host platfams 1 and 3 canmunicate with the server object 00 host 2. Hosts 1 and 3 may be pbysically linked to host 2 by differ-
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6.1 Application Example The proci mconcept and effectiveness d the IMDC approach is being evaluated using representative industrial problems in packaging. assembly. transferline and PCB handling applications. Within Synect. applicatims are described in terms cl a hierarchy d camnunicating objects. Pan cl the object hierarchy f", an example PCB handling machine is illustrated in the right hand window cl fig . 4. The internal logic fIX each object is expressed in the fcnn et Slate transitions diagrams (STDs). A fragment d a typical SIn can be seen in the left band window of fig. 4.
ent oetwak types. A multi-schema architecture has been implemented scitware tools) and server (such as data repo:s.i~) applicatiCllS via an underlying integration infrastructure. The integratioo infrastructure has been built using distribtued object u.:hnology based upaI Ille Common Object Request BroIcet Atcbitecture (CORBA) specification fnm Ill. OMG (Anoo. 1991). The infrastructw'e acts as a system-wide broker fa object services and provides ab-stractioo frem low level device specific problems. Object ...-vices are d}1lamically ~ and deregistered with the infrastructure by processes which impment the services (object servers). Oient processes query the infrastructure fa available services and are given the Decessary connecboo infcnnatioo to access services.
The PCB handling machine cmsists cl a frllIlleWO'k which suppttts a board carriage system. a movable gantry and a robot arm which are used to populate PCBs with canpooents. To enable visual interactive simulatioo the solid model cl each d these canponent can be associated with real wald input and wtput contact points from the control logic (see fig. 2).
10 separate client (typically
7. CONCLUSIONS
m
The novelty the IMDC approach is that the control system scitware and physical machine canponents can be individually cmceptualiscd and interactively tested. Furthermtte IMDC enables model1ed canponents to be progressively replaced with real ~ ccmponents until the final soIutioo is attained A particularly attractive feature is that the same sequence logic is used to control both the modelled and real wc.-Id components.
6. IN1ERACIlVE USE OF PHYSICAL AND
CONTROL LOGIC MODEU.JNG TOOLS
The canme:cial benefits cl IMDC relate to both its impact thrwgb. imprcwed maoufacruring efficiency fa the end user and its potential to provide a stimulus to machine and cootrol system venden by:
Applicatioo logic is Det embedded in the modelling tool but ;, geouated by use eX !he Synect logic toolset. By functicning as a server, the modeller can be cootrolled by remote prooesses fa example, (a) the Synea logic simulata tool. er (b) task cootrol software nmning in the target control system. This bighlights an impataDt feature d. the run-time architecture. namely that the IMDC defined interfaces to the controlled elements cl these solid models are identical to the IMDC interfaces to the real control system. Hence. the control lC¥ic processes can drive either the modelled hardware elements (thus animating the mcxiei). the real wald hardware elements. "' a mixture cl the two. This permits incremental proving cl the cmtrol. logic in a hardware independent manner
•
reducing the development cost aod time cl highly autcmated applicatiCllS. and e:ncooraging reuse cl softwuei1wdware building blocks.
•
reducing the cost of eliminating design faults (fast identificatioo) and ease cl service and mainteDaDCe.
•
allowing mtte effective adaptatioo and alteratioo
d cmtrol systems. withoot re.sttting to the expensive services of a specific system supplier.
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,........
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.:
Fig. 4. Typical User Interface Screen for the Synut Editor showing an Object Hierarchy and SlD. Liang M .• Ahamed S. and vandenBerg B. (1996). A STEP Based Tool Path Generatioo System for Rough Machining cl Planar Surfaces, Comput· ers in Industry. 1996.32. No.2. pp.219-231. Micbel. G. (1990). Programmable Logic Controllers: Architectures and Applications. Wiley. UK. Murray J.L. and Yue Y. (1993). Autanatic Machining of 2.5D Canpooents with the A(JS Modeller. Int. JourTl(li of Computer Integrated Manufacturing. 6. No.I-2. pp.94-104. Oriali. R.. lWkey. D. and Edw...ds. J .• (1996). The Essential Distributed Objects Survival Guide Wiley and Sons. Chichester. • PeterSCll 1.L.. (1981). Petri Net Theory and Model· ling of Systems. Prentice HaiL Rahkooen. T.(l995). Distributed Industrial Cootrol Systems - A Critical Review Regarding Openness. Ctrl. Eng. Praq .. 3. No. 8. pp.1155-1162. Sadashll-. A.. (1989) Software Modelling of Manu· facturing Systems: The Case fa an Object Oriented Programming Approach. Annals of Op· erational Research. 17. pp. 363-378. Venkatesb. K.. Zbou. M.• and Caudill. Rl. (1994). Canparing Ladder Logic Diagrams and Perti Nets fa Sequence Cootroller Design Through a Discrete Manufacruring System. IEEE Trans . on Ind. Electronics. 1994. 41. No 6. pp. 611-619. Ward. T.W. and Mella. SJ. (1995). Structured De· velopmentfor Real-Time Systems. Vol. 1. Yourdon Press. Prenbce-Hall. . Wright. CD. and Case K. (1996) Emulatioo cl Modular Manufacturing Machines using CAD Modelling. MecMrronics. 4. No. 7. pp. 713-735. Young. SL.(l995). Technology. The Enabler fa Tan
ACKNOWLEDGEMENTS The authers gratefully acknowledge the EPSRC fa the provisioo cl research funding and Hopkinsm Computing Ltd. for their collaborative inpul
REFERENCES AnOD.. (1991). The Common Object Request Broker: Architecture and Specification. Object Management Group (OMG) DocumeDl No. 91.12.1. AnOll. (1994). POET Reference Manual, Version 2.1. POET Software Corporation. AnOD. Synect User Guide. (/995). Hopkinson Corn· puting. Ltd.. 29 Deepdale. Guisborough. UK. Bell. P.e. and O'Keefe, R (1995). Visual Interactive Simulatioo - Histocy. Recent Developments and Major Issues. Simulation, 49. No 3. pp. 109-116. Carrott. AJ .. Mocre. P.R. WestCll. RH.. and Harri500. R. (1996). The UMC Software Envirooment fa Machine Cootrol System Integratioo. Coofiguratico and Programming, IEEE Trans . on Industrial Electronics. 43. No 1, pp. 88-97. Carrott. A.l .. Wright. CD .. West. A.A. HarriS(ll R and Westoo. R.H. (1997). A Toolset fa tributed Real Time Macbi.ne Cootrol. SPlE Photonics East Procs .• Ma USA. 2913, pp.2-12. Crowc::roft. 1.(1995). Open DistribUled Systems. Boston: London. Anech House. David. R and Alia. H. (1992). Petri Nets and Graf· cet. Englewood Qiffs. NJ. Prentice Hall Hoffmann C. Ros~gnac J (1997). Special issue: Solid modelling. Comp.-Aided Des .. 29. No.2. p.87. Hurrioo. Rn.. (1980). An Interactive Visual Simulaboo System for industrial Management. Euro· pean journal of Operational Res.• S. pp. 86-91 loannis. R and Krieger M.. (1992). Object Oriented Approach to the Specificatioo of Manufacturing Systems. Computer Integrated Manufacturing Systems. 5. No.2. pp. 133-145.
Dis-
Yoog. Y.F. et al (/985). Off-Line Programming of Robots. Handbook of Industrial Robotics. John Wiley. New York. pp 3~386.
338
INTERACTIVE VISUALISA nON OF SEQUENCE LOGIC AND PHYSICAL MACHINE COMPONENTS WITHIN AN INTEGRATED DESIGN AND CONTROL ENVIRONMENT
R. Harrisoo, A.A. West, P. Hopkiosoo and C.D. Wrigbt
Manufacturing Systems InlegratiJJn (MSI ) Research Institute. Loughborough University, LDughborough, United Kingdom. LE] I 3TU.
Abstract: An Integrated Machine Design and Cmtrol (lMDC) envirooment fa the visual represeotatioo. and integratioo. r::J. the physical machine canpoo.ents and cootrollogic is discussed in this paper. The approach taken is unique in that (a) the cootrollogic and pbysical models of the elements can be investigated individually fa carectness and canpleteness, (b) the cootrollogic can be easily integrated with the solid models to animate the model eX the pbysical machin.e and (c) rec:oo.figuratioo. enables the same cootrollogic to be applied to real wald physical machjne elements. At aoy stage during the machine design and imp1ementatioo. prcx:ess. the user cl the enviroomeot can pause and questioo. the validity eX certain 0perations and control system parameters. Keywords: Machine. Control Logic. Design. Disuibuted. Objects. Modelling. Pe ui-nets.
vanced COOlputer technology throughout the machine life ~e fran requirements defin.itioo.. through the design and build stages to maiDtenance and recm· figuration.
l.INmODUcnON The design and implementatioo. d. manufacturing machines is under increasing time and financial pres· sures as custoolers demand inaeased product variety and quality at reduced product cost (Yoong. 1995). Increased canpetitioo. and governmental pressure to focus 00. enviroomeotal issues has faced modem machine OOilders and users to coo.sider the require· ments fa the next generatioo. cl machines that allow the recoo.figuratioo. cl both the coouol sd.tware and physical hardware (Rahkoo.en. 1995). Machines will be required to be developed in the minimum amoont d time and canprise (a) venda independent hardware canpooents. (b) sophisticated cootrol algorithms. (c) intelligent sensas and acruatcrs and (d) user friendly interlaces. In addition. open systems issues coo.c.erning the ease of integratioo.. interoper· ability d the software and hardware canpcnents and available standards must be addressed (Crowacit. 1995) to ensure that reuse aod recoo.figuratioo. can be achieved. The inherent canplexity inevitably necessitates the increased applicatioo. c:1 machine modelling software toolkits (i.e . fa cc:nuollogic and physi· cal machine element design and analysis) and ad·
End users. machine designers and machine builders require technical and operatiCDal knowledge fran disparate disciplines and specialised danain experts at various stages throughout the machine requirements. design, build. instaIlatioo.. set-up. maintenance and rt(;(Ilfi.guratioo. life ~e (Carrou. et al.. 1997). Ccmmoo. frames cl reference are vital to im,. prCNe the cmununicatioos between the above stakeholders in the machine design and OOild process. In particular it is impcrtant that discussims are focused aroond b
333
• K
-
VisulIl
Interactive Simul.ation (Le. IInim.aliCln)
N
0 \I'
Vis,utl ModcUina:
l E D
l ~bPS I
G E
...., "/" ",
:::::.:.notllel
,
"'yoCaIlo)'OUt
....
Abstraction ....
Op.1UOMI ~led. . • M.... upuioacll ood ," Iatowlqe
:.
I.)omllln Experts
• • •• •
Rulity
Jadnoolu.! ~ """"'"' - . J ODd_pmaJ
"
, . .1o....... odooIJ .... pPolo
Ma. I. Illustration or the Visual Interactive Simulation Process. ality is included. The develq>ment cl canplex manufacturing logic fa PLCs is a specialised activily. and it is difficult for a noo expert to appreciate the operatim cl the canpJete ~tem (Venk.ateSb. et 01.1994), Ladder logic diagrams and sequential fuoc· bOO charts (David and Alla. 1992) provide grapbical representatim cl sequential operatim but the design cl flexible. reusable and maintainable sdtware is nevenbeless di.£ficuJt to achieve.
virmment (an integrated software eDvirmment and toolset canprising third party and "in house" tools) has been developed that seeks to suppttt the Wttk d. COluol system engineers and mechanical designers lhrooghout the design and development life C)Cle d manufacturing machines. Visibility cl the pbysical machine (using the IMDC Machine Modeller developed around the AOS solid modelling kemel (Murry and Yue. 1993) and CQluot logic software (using the Synecr. modelling tool produced by Hopkio.soo COOlpuring Ltd. (Anoo. 1995» is a cere requirement in the IMDC system and is discussed in this paper.
There has been widespread use of visual representatiro of manufacturing products iD terms of surface and solid models (Hoffmann, 1997). Canpurer aided
design package usage e.g. AutoCAD and Unigraphlcs et 01 .. 1996) enables design engineers to visualise physical canpooents and layrulS prier to canmissiClliog. In c:ertain cases, q,eratiooal logic has been included into the solid model to enable a dy· nam..ic animatioo cl the moclel1ed system elements to be observed and optimised (HcBmann, 1997). A map limitatiCll with this approacb has been the fact that in ocder to ttansfet the results of the modelling exercise to a real system. the logic must be reimple-meated ootside c:i the solid mcdel and the ~ wry fCl' erTa'S and sub optimal implementatioo behavioor proliferate (Wright and Case. 1995). There is a requirement fa' an environment in which (a) the CQluol. logic: and physical models c:i the elements can be investigated individually fa' Ca'TeCmess and C(ID.pleteness. (b) the CCIluol logic: can be easily integrated with the solid models to animate the machine soI.id mcdel and (c) recoo.6guratioo enables the same COluot logic to be applied to real W(W"ld physical c:oo.uol elements.
The envirmment is based aroond a disuibuted object oriented represeutatioo ci manufacturing machines as aggregatims of basic canpments (Joannis and Krieger. 1992): single axes. multi-axes. digital and analogue input I o.uput and dumb and intelligent senscl'$ (e.g. visioo systems and robots). Distributed object technology (DOl) (Orfall, " 01 .. 1996) p"ovide$: the CCI'e framewCI'k within which the tools and real system intercommunicate.
(L.iang
2. VISUAL INTERACTIVE SIMULATION, The visual interactive. simulatim paradigm (also termed visual interactive modelling and visual interactive problem solving was originally applied to the disaete event simulatioo of job shop scheduliJlg problems in manufacruring (HurriCll. 1980) and has mainly been utilised in Oper.tiooa1 Research (Bell, 1995). Visual interactive simulatim is particularly appropriate in the manufacturing machine dOOlain (Sadashir. et al .. 1989) and can provide a COOl.OlCll frame of reference to facilitate human canmunica· tim d. ideas. In the manufacturing machine dcmain it is vital that bolb the pbysical machine and COltrol logic are available fa- scrutiny. Fig. 1 illustrates the process. A visual model (i.e. a model based upm DCIl textural and noo verbal elements to canmUllicate the
A majcr research theme at the Manufacturing Systems Integratioo (MSI) Research Institute at Loogbbttoogh University. UK is the realisatim d. the next geraeratim c:i machine systems (Carrou. 1996). An In
334
state et a system) is developed that provides an abstraction of reality.
(see sectiOll 4) . The autcmatic code generation tool produces code which is then canpUed with IMIX: runtime litraries enabling the applicatiOD. logic to ccmmunicate with the distributed macbi.oe canpo-
The application d. visual interactive simulation to industrial machine design and control projects using the IMDC envirooment has resulted in a number of obselvations and benefits: • h is impatant to ensure general intetactioo. and early involvement by developing an animated picture as soon as possible. • Interaction allows the end users and macb.ine builders to make canplex decisions with increased confidence due to their increased understanding ci the machjne operaticn and interac-tiOG. • The visual image is widely accepted and unexpected simaticns can be envisaged via what if? scenarios. • Of vital impmance is the integraticn d the controllogic with the visual simu1atioo. in the IMDC envirooment. This enables the verificatioo. (by the
DeIllS.
3.1 Logic Visualisation I Representation The designer c1 a sequential control applicatiCll can visualise the problem and present the solutioo io a number of ways. Three possible approaches are: • consider the applicatioo as consisting ci a set cl intc:rlo:ks. If the target envirooment is hard-wired logic a, as in the majaity ci current industrial autcmatioo. projects. a PLC programmed in ladder logic. the design solutioo. maps easily CIl to the implementation (MicheL 1990). • take a functioo.al view. One cl the mere popular structured methods used fa real-time applicatiClls h~ been the Ward-Mella variant cl the YrurdCll method (Wud and Mellcr. 1985). Cootrol transfCl'lDs are triggered by events and then react by sending signals to other traDsfmns. • take an object crieo.ted view. Object crientatiCll has grown frem a modelling paradigm and. it is claimed. leads to mere intuitive soluti
3. VISUAUSATION OF SEQUENCE LOGIC Manufacturing and process industry control system applications invariably include sequence logic. The ccmp1exiry cl the application logic typically involves the need to manage Sle'VeJal cono.ment activities. coadinating their behavirur to achieve the desired application goal
The canbinatioaal logic approach is potentially the preferred optioo. if the application logjc is...er)' simple a there is a stroog need fa the salutioo to require the minimum cl memay a execute as fast as possible. Otherwise. the solUtiCll can be mere difficult to verify. is not cooducive to diagnos.ing qJeratiooal mis-behavioor and is mac: difficult to modify withoot causing UDwanted side-effects.
In additiOD. to the standard scitware engineering problems ci defining how the proposed system is to wale. and expressing the design in a fcnn which is readily understood, control systems designers must ensure that the system's dynamics do not contAin design erras, such as deadlock, livelo:::k and undesirable modes ci operanoo.. Traditioo.al approaches have tended to be ad-hoc a have inadequately addressed the designer's needs, leaving such erTCI'S to be identified late in the I.ife<}de. resulting in costly modifications.
Functional approaches have the benefit cl suppating the coocept cl sequences. typic.ally by a bm ci state diagram. The functiCllal approach helps the designer to consider how the different ccmpoo.eots need to be co-
SynectTW provides a set cl sdtware tools (appticatioo edita. ccmpiler, logic engine. logic malitcr and code geoerata) which are integrated into the IMDC platftnD (Anm. 1995). Applicatioos an: described by means cl a graphical edita. Synca is based on the generation cl a Petri-Net model frem the applicatioo. descriptim (\'tIersoa. 1981). The evolving design can then be examined analytically fCl' structUtal and behavirural deficiencies e.g. checking fa em:n such as deadlo:ks and unwanted state canbinatiau. An executable logic model is created which can then drive a visual solid model cl the physical macb.ine
Synect provides a methodology which canbines the co-acdinatim d the f\mctiooal apprcach with the encapsulatioo of object orientation.
IMDC integrates the Synect simulata with the 3D solid modeller (see fig. 2). As described in section 4. the modeller incapaates concepts such as timing infcnnatioo and sensor emulatioo into the solid modelling S
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Fig. 2. illustration of IMDC Interaction between Synect. the Solid Modeller and the Distributed Real Machine. time emulatioo cS how the implemented S)'tem will
iooral models c:i the external systems being repre-
perform.
sented. The tool. can be used to specify
CCIllJXlleD{
states. q>eratiooal parameters, motioo. parameters
and locatims. Compcnents. parameters and CCIllplete sub-assembo.s can be stcnd in the IMDC database.
4. VISUALISATION OF MAClfiNE ELEMENTS A machine designer can utilise solid models in cxder to visualise the pbysical machine elements and their interactioo (Hoffinann. 1997). The cooveutiooal aIr proach to the solid modelljng of machines involves: 1. initial static visualisatioo c:l the machine ele-
The interface to co:npcnents c::i each speciJic type is exp
ments and
2. determinatioo of the system dynamics and machine element interactioo by embedding q>eraticnal logic within the modelling tool (Yoo.g et
al.• 1985). The limitatioo. of this approach is that the coding fa the real system operatim is ocrmally undertaken after the simulatioo phase and the sdtware frequently bears little re1atim to the mechanisms used to drive the simuIatioo. In additioo this approach to the mooeUing c:i the pbysical machine elements cannot be used throoghoot the life C)de and cannot be used interactively with Olher design tools.
5. ENVIRONMENT TO SUPPORT VISUAL INTERACTIVE SIMULATION: IMDC The IMDC envircnment ccmprises frur distinct areas of functiOttlllity: • user tools to enable, fa example. logic simulatiCll, motiCll design. machine modelling and tools associated with the CCIltrol and mooitaing d. the runtime system. The user tools can be progressively changed (I' extended via a set eX generic interfaces.
The approach adopted within the IMDC envirooment is fundamentally different and
•
systems tools to enable system admjnistratiCll. access to infcrmatim and security. Typical system canfiguratiCll infCl1ll8tim includes the logical and physical system la)OOt (machines and oetwaks),
user names, passwads. access rights. project and user environment informatioo.. • the IMDC object criented database (poET a proouct frOOl the POET Scltware CapaatiCll (Anoo. 19(4) which provides persistent stcxage fer the outputs frOOl life C)'Cle activities. system
Models eX pbysical machines can be graphically coos1I'UCted within the machine modeller frOOl ccmponent building blocks such as actuatcn, sen.sas, cooveyas. alarms and structural elements. The model ccmpcnents incapcl'8te both geanetric and behav-
336
•
-,
ccnfiguration. uaceability and versioo control. and holds the generated elements d. specific target control solutions the distributed runtime machine canposed d canmunicating sdtware ccmponents. a subset d which provide interfaces to external devices and third party cmtrol sdtware fa the mooitaing and cmtrol of the physical mac.bi.oe.
These functional elements all inttt-ccmmunicate via Ille underlying Object Request B,oW (ORB) atehitectwe as described in the following section.
5.1 lMDC System Architecture The adqrtioo d an objcct-ociented approach. particularly the use et distributed object technology. has been the key to providing flexibility in the choice d implementation technologies. Fig. 3 illustrates the principle d object distribution across heterogeneoo.s host and netwak architcctures. Identical client software located on different host platfams 1 and 3 canmunicate with the server object 00 host 2. Hosts 1 and 3 may be pbysically linked to host 2 by differ-
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Fig. 3. The Principle of Object Distribution Across Heterogeneous Host and NetwOrk Architectures.. and also allows hardware to be included into the system as and wbeu. it becomes Il\'ailable.
6.1 Application Example The proci mconcept and effectiveness d the IMDC approach is being evaluated using representative industrial problems in packaging. assembly. transferline and PCB handling applications. Within Synect. applicatims are described in terms cl a hierarchy d camnunicating objects. Pan cl the object hierarchy f", an example PCB handling machine is illustrated in the right hand window cl fig . 4. The internal logic fIX each object is expressed in the fcnn et Slate transitions diagrams (STDs). A fragment d a typical SIn can be seen in the left band window of fig. 4.
ent oetwak types. A multi-schema architecture has been implemented scitware tools) and server (such as data repo:s.i~) applicatiCllS via an underlying integration infrastructure. The integratioo infrastructure has been built using distribtued object u.:hnology based upaI Ille Common Object Request BroIcet Atcbitecture (CORBA) specification fnm Ill. OMG (Anoo. 1991). The infrastructw'e acts as a system-wide broker fa object services and provides ab-stractioo frem low level device specific problems. Object ...-vices are d}1lamically ~ and deregistered with the infrastructure by processes which impment the services (object servers). Oient processes query the infrastructure fa available services and are given the Decessary connecboo infcnnatioo to access services.
The PCB handling machine cmsists cl a frllIlleWO'k which suppttts a board carriage system. a movable gantry and a robot arm which are used to populate PCBs with canpooents. To enable visual interactive simulatioo the solid model cl each d these canponent can be associated with real wald input and wtput contact points from the control logic (see fig. 2).
10 separate client (typically
7. CONCLUSIONS
m
The novelty the IMDC approach is that the control system scitware and physical machine canponents can be individually cmceptualiscd and interactively tested. Furthermtte IMDC enables model1ed canponents to be progressively replaced with real ~ ccmponents until the final soIutioo is attained A particularly attractive feature is that the same sequence logic is used to control both the modelled and real wc.-Id components.
6. IN1ERACIlVE USE OF PHYSICAL AND
CONTROL LOGIC MODEU.JNG TOOLS
The canme:cial benefits cl IMDC relate to both its impact thrwgb. imprcwed maoufacruring efficiency fa the end user and its potential to provide a stimulus to machine and cootrol system venden by:
Applicatioo logic is Det embedded in the modelling tool but ;, geouated by use eX !he Synect logic toolset. By functicning as a server, the modeller can be cootrolled by remote prooesses fa example, (a) the Synea logic simulata tool. er (b) task cootrol software nmning in the target control system. This bighlights an impataDt feature d. the run-time architecture. namely that the IMDC defined interfaces to the controlled elements cl these solid models are identical to the IMDC interfaces to the real control system. Hence. the control lC¥ic processes can drive either the modelled hardware elements (thus animating the mcxiei). the real wald hardware elements. "' a mixture cl the two. This permits incremental proving cl the cmtrol. logic in a hardware independent manner
•
reducing the development cost aod time cl highly autcmated applicatiCllS. and e:ncooraging reuse cl softwuei1wdware building blocks.
•
reducing the cost of eliminating design faults (fast identificatioo) and ease cl service and mainteDaDCe.
•
allowing mtte effective adaptatioo and alteratioo
d cmtrol systems. withoot re.sttting to the expensive services of a specific system supplier.
337
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Fig. 4. Typical User Interface Screen for the Synut Editor showing an Object Hierarchy and SlD. Liang M .• Ahamed S. and vandenBerg B. (1996). A STEP Based Tool Path Generatioo System for Rough Machining cl Planar Surfaces, Comput· ers in Industry. 1996.32. No.2. pp.219-231. Micbel. G. (1990). Programmable Logic Controllers: Architectures and Applications. Wiley. UK. Murray J.L. and Yue Y. (1993). Autanatic Machining of 2.5D Canpooents with the A(JS Modeller. Int. JourTl(li of Computer Integrated Manufacturing. 6. No.I-2. pp.94-104. Oriali. R.. lWkey. D. and Edw...ds. J .• (1996). The Essential Distributed Objects Survival Guide Wiley and Sons. Chichester. • PeterSCll 1.L.. (1981). Petri Net Theory and Model· ling of Systems. Prentice HaiL Rahkooen. T.(l995). Distributed Industrial Cootrol Systems - A Critical Review Regarding Openness. Ctrl. Eng. Praq .. 3. No. 8. pp.1155-1162. Sadashll-. A.. (1989) Software Modelling of Manu· facturing Systems: The Case fa an Object Oriented Programming Approach. Annals of Op· erational Research. 17. pp. 363-378. Venkatesb. K.. Zbou. M.• and Caudill. Rl. (1994). Canparing Ladder Logic Diagrams and Perti Nets fa Sequence Cootroller Design Through a Discrete Manufacruring System. IEEE Trans . on Ind. Electronics. 1994. 41. No 6. pp. 611-619. Ward. T.W. and Mella. SJ. (1995). Structured De· velopmentfor Real-Time Systems. Vol. 1. Yourdon Press. Prenbce-Hall. . Wright. CD. and Case K. (1996) Emulatioo cl Modular Manufacturing Machines using CAD Modelling. MecMrronics. 4. No. 7. pp. 713-735. Young. SL.(l995). Technology. The Enabler fa Tan
ACKNOWLEDGEMENTS The authers gratefully acknowledge the EPSRC fa the provisioo cl research funding and Hopkinsm Computing Ltd. for their collaborative inpul
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Dis-
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