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Integrated Machine and Control Designfor Reconfigurable Machine ToolsD. M. Tilbury and S. KotaEngineering Research Center for Reconfigurable Machining SystemsDepartment of Mechanical Engineering and Applied MechanicsThe University of MichiganAnn Arbor, MI 48109-2125tilbury,kotaAbstract In this paper, we describe a systematic designprocedure for reconfigurable machine tools and their asso-ciated control systems. The starting point for the design isa set of operations that must be performed on a given partor part family. These operations are decomposed into a setof functions that the machine must perform, and the func-tions are mapped to machine modules, each of which hasan associated machine control module.Once the machineis constructed from a set of modules, the machine controlmodules are connected. An operation sequence control mod-ule, user interface control module, and mode-switching logiccomplete the control design. The integration of the machineand control design and the reconfigurability of the resultingmachine tool are described in detail.I. IntroductionIn todays competitive markets, manufacturing systemsmust quickly respond to changing customer demands anddiminishing product life cycles. Traditional transfer linesare designed for high volume production, operate in a fixedautomation paradigm, and therefore cannot readily accom-modate changes in the product design. On the other hand,conventional CNC-based “flexible” manufacturing systemoffer generalized flexibility but are generally slow and ex-pensive since they are not optimized for any particularproduct or a family of products.An effort at the University of Michigan aims to developthe theory and enabling technology for reconfigurable ma-chining systems 3, 4. Instead of building a machiningsystem from scratch each time a new part is needed, an ex-isting system can be reconfigured to produce the new part.In this paper, we describe how an integrated machine andcontrol design strategy can result in machine tools whichcan be quickly and easily configured and reconfigured.In order to provide exactly the functionality and capac-ity needed to process a family of parts, RMTs are designedaround a given family of parts. Given a set of operationsto be performed, RMTs can be configured by assemblingappropriate machine modules. Each active module in thelibrary has a control module associated with it. As the me-chanical modules are assembled, the control modules willbe connected and the machine will be ready to operate.Extensive and time-consuming specialized control systemdesign will not be required. Section II describes how themachine is designed from a set of basic machine modules,This research was supported in part by the NSF-ERC under grantnumber EEC95-92125.connected in a well-defined fashion, and Section III de-scribes how the control is similarly assembled from a li-brary of control modules.This modular construction ofthe machine and control allows for many levels of reconfig-urability as described in Section IV. The paper concludeswith a description of future work in Section V.II. Machine DesignOngoing work on manufacturing system configuration atthe University of Michigan addresses the problem of start-ing from a part (or part family) description and extractingthe machining operations necessary to produce the part(s)7. The operations are grouped according to tolerance, or-der of execution, and desired cycle time of the system, withthe intention that each operation “cluster” can be producedon a single machine tool. The operation cluster consideredhere is to drill a set of holes for the cam tower caps on V6and V8 cylinder heads shown in Figure 1. The input to thereconfigurable machine tool design procedure is the cutterlocation data generated by a process planner for this oper-ation cluster. Figure 2 shows sample data which includespositioning and drilling information.The RMT design procedure consists of three main stages:task clarification, module selection, and evaluation 14.After a brief literature review, these three stages will beoutlined in this section.A. Related researchSince reconfigurability is a relatively new concept in ma-chining systems, there is little, if any, published literatureon the design of reconfigurable machine tools. However,modular machine tools have been on the market for sev-eral years, and some of the published articles on modularrobots, modular machines and assembly do have some rel-evance to the design of reconfigurable machine tools. Forexample, Shinno and Ito 17, 18, 19, 20 proposed amethodology for generating the structural configuration ofmachine tools. They decomposed the machine tool struc-tures into simple geometric forms: e.g. boxes, cylinders,etc. Yan and Chen 21, 1 extended this work to the ma-chining center structural design. Ouyang et al. 12 adaptedItos method for modular machine tool synthesis and de-veloped a method for enumerating machine tool modules.Paradis and Khosla 15 determined the modular assemblyconfiguration which is optimally suited to perform a specific(a) V6 cylinder head(b) V8 cylinder headFig. 1.Two sample parts. The operation to be performed is to drillthe positioning holes for the cam tower caps. On the V8 engine,there are two such positioning holes located in a line. On the V6engine, there are eight holes in an array.PARTNO/DOHC25M2?UNITS/MMPPRINT/OPERATION CATEGORY & TYPE: Hole Making Ream?PPRINT/OPERATION NUMBER & NAME: Operation-1?PPRINT/TOOL IDENTIFIER: Drill6mm?PPRINT/POST TOOL ID: 0?PPRINT/TOOL DESCRIPTION: ?PPRINT/TOOL STATION NUMBER: 1?MODE/MILLMULTAX/OFFLOADTL/0, IN, 1, LENGTH, 0.000000, OSETNO, 0CUTTER/6, 0.79375LINTOL/0.050000SPINDL/1910.000, RPM, CLWFEDRAT/0.127000, MMPRCOOLNT/FLOODCYCLE/DRILL, 20.500000, MMPR, 0.127000, 3.020144, RAPTO, 2.020144, DWELLGOTO/0.000000, 0.000000, 0.000000CYCLE/OFFCOOLNT/OFFPPRINT/OPERATION CATEGORY & TYPE: Hole Making Ream?PPRINT/OPERATION NUMBER & NAME: Operation-2?PPRINT/TOOL IDENTIFIER: Drill6mm?PPRINT/POST TOOL ID: 0?PPRINT/TOOL DESCRIPTION: ?PPRINT/TOOL STATION NUMBER: 1?MODE/MILLMULTAX/OFFLINTOL/0.050000SPINDL/1910.000, RPM, CLWCOOLNT/FLOODCYCLE/DRILL, 20.500000, MMPR, 0.127000, 3.020144, RAPTO, 2.020144, DWELLGOTO/0.000000, -43.000000, 0.000000Set-upToolPositionFig. 2.Sample sequence of operations (cutter location) data fordrilling holes in the parts shown in Figure 1.The CL file isgenerated from a CAD package (such as IDEAS) and includesthe locations of the holes to be drilled along with the spindlespeed, feedrate, and coolant information.task. Chen 2 addressed the problem of finding an optimalassembly configuration for specified tasks; his procedurewas based on the assembly incidence matrix and employeda genetic algorithm to solve the optimization problem. Onthe systems front, Rogers and Bottaci 16 discussed thesignificance of reconfigurable manufacturing systems, andOwen et al. 13 developed a modular reconfigurable man-ufacturing system synthesis program for educational pur-poses.In our work, traditional methods of motion representa-tion and topology (i.e. screw theory, graph theory, etc.) areemployed to capture the characteristics of RMTs. Thesemathematical schemes are used for topological synthesis,function-decomposition, and mapping procedures; detailscan be found in 9.B. Task clarificationThe design of an RMT begins with task clarification,which entails analyzing the cutter location data to deter-mine the set of functions which are necessary to accom-plish the desired kinematic motions. There are three steps.First, graphs are generated which abstractly representationPos i t i onFeedSpi ndl eCool antt1t2t3t4t5t6t7t8Fig. 3.High-level operation sequence, showing causal dependenciesand concurrencies. This abstract representation of the sequenceof operations is derived from the CL data shown in Figure 2, andwill be used to design the sequencing control.the motions. These graphs are then decomposed into func-tions, and finally the functions are mapped onto machinemodules which exist in the library.A graph representation of the machine tool structureallows for systematic enumeration of alternate configura-tions and also provides a method of identification of non-isomorphic graphs. The graph representation is also usedfor bookkeeping to assign machine modules to the graphelements. A graph consists of a set of vertices connectedtogether by edges. In using a graph as an abstract represen-tation of a machine tool structure, we define two differenttypes of vertices: type 0 and type 1. A vertex representsa physical object with two ports; each port represents thelocation on the object where it can be attached to a neigh-boring object. A type 0 vertex has input and output portsthat are in-line with respect to each other, whereas a type 1vertex has input and output ports that are perpendicularto each other. Machining tasks are also classified as type 0or type 1, depending on whether the tool is parallel or per-pendicular to the workpiece.Figure 4 shows a graph for a type 0 task. Four type 1vertices are combined with several type 0 vertices to createa machine structure in the form of a C. Because type 0vertices dont change the orientation, they can be used asspacers in various combinations.The root vertex repre-sents the base or bed of the machine tool. The choice ofthe root vertex is not unique; different choices will resultin distinct machine tool designs. Structural functions areassigned to the vertices of the graph; kinematic functions(where needed) are assigned to the edges.For instance,Figure 4 shows one example of how translational motions inthe X,Y and Z directions can be assigned to graph edges,representing relative motion between the physical objectsrepresented by the two vertices of the edge.The basic functionality of a machine tool is described bythe kinematic motion between the tool and the workpiece.These kinematic functions will be represented by a homoge-neous transformation matrix 11; the desired functionalityof the machine tool will be encoded in the matrix T. Themotions necessary to carry out a given machining task arederived from the sequence of operations. The process fileshown in Figure 2 contains tool positions and motions in aCartesian coordinate system. For example, the first motion101010100 Type 0TZ?TaskTY?TX0BEDFig. 4.An graph representing a machine tool structure. Transla-tional motions (TX,TY,TZ) are assigned to edges of the graph;the vertices have structural functionality.Milling & Drilling?Spindle?Feeding?Positioning?Tool Rotation?Translation?XTranslation?YTranslation Z?Translation Z?Merge?Motor?Lead Screw?Motor?Lead Screw?Motor?Lead Screw?Motor?Power Train?Fig. 5.Function decomposition template.can be extracted as:P1=10000101000012500001F1=10000101000012000001where P1represents the position and orientation of the toolfor the positioning task, and F1represents the feed motion.From the transformation between any two adjacent posi-tions, the motion description can be extracted:M1=P11F1=10000100001500001The other motion descriptions are extracted similarly.Corresponding to each type of machining operation, atemplate is retrieved as a starting point in identifying vari-ous kinematic functions necessary to carry out the machin-ing task. For instance, the template for milling and drillingoperations show that kinematic functions are necessary forspindle revolution, tool feeding and tool positioning. Byusing this template, together with the exact feeding andthe positioning information given in the process plan, wecan derive the exact set of kinematic functions that are nec-essary such as tool rotation, translations X,Y , and Z forfeeding and translation Z for tool positioning as depictedin Figure 5.Each of the kinematic functions identified in the functiondecomposition stage is mapped to an edge of the graph asdescribed above. Assigning the functions to different edgesFig. 6.The structural graph of Figure 4 can be realized by manydifferent choices of modules.100001010011000001Fig. 7.Representation of a machine module. The CAD model ofa slide for a modular machine tool is shown on the left, and itstransformation matrix is shown on the right.can generate multiple solutions. Because purely transla-tional motions are commutative, their order in the graphcan be interchanged. In function mapping, the importantinformation is the topology of screw motions (includingpure rotational motions) and the topology of the bed.C. Module selectionCommercially available modules are selected from themodule library for each of the functions (structural as wellas kinematic) that were mapped to the graph in the taskclarification stage.The data stored for each module inthe library includes the homogenous transformation matrixrepresenting its kinematic or structural function, the twistvector supplemented by range of motion information, acompliance matrix representing the module stiffness, mod-ule connectivity information, and power requirements (foractive modules such as spindles and slides).The first step in module selection is to compare the ho-mogeneous transformation matrices of the modules withthe task requirement matrix such that when appropriatemodules are selected to meet the task requirements, theproduct of all module matrices should be equal to the de-sired task matrix: T = T1T2Tn. Again, there may bemany possible choices of modules for a given structuralconfiguration. Figure 6 shows how different slides, spin-dles, and structural elements can be assembled accordingto the graph of Figure 4.A slide module, with its CAD model and transforma-tion matrix, is shown in Figure 7.It is capable of onedirection of linear motion, indicated by the 1variable inits transformation matrix. Its database entry, shown in Ta-ble I, stores not only its transformation matrix but also themanufacturer name, model number, initial position, powerlevel, and motion data. The twist vector is augmented byinformation on the minimum, initial, and maximum dis-placement of the module.TABLE IDatabase information and documentation for the machinemodule shown in Figure 7.ManufacturerSUHNERModel NameUA 35-ACInitial Position100001000011000001Twist vector000010TRange of motion1550155Max. force5500NCompliance matrix,Etc.Power requirements,Connectivity information, .(a) V6 machine(b) V8 machineFig. 8.Reconfigurable machine tool designs for the two differentparts.D. EvaluationOnce a set of kinematically-feasible modules have beenselected, the resulting machine design must be evaluated.The criteria for evaluation of the reconfigurable machinetools synthesized by the above systematic procedure in-clude the work envelope, the number of degrees of freedom,the number of modules used, and the dynamic stiffness.The number of kinematic degrees of freedom of the ma-chine tool must be kept to a minimum required to meetthe requirements, both to reduce the actuation power andminimize the chain of errors. Each active Examples showthat the designs generated by this methodology have ex-actly the number of degrees of freedom necessary to per-form the required machining operations on the given part10. Machine tool designs which are generated using thismethodology for the example parts of Figure 1 are shownin Figure 8.The resulting designs must be evaluated with respect tothe expected accuracy. The stiffness of the entire machinetool, one of the most important factors in performance, isestimated based on the module compliance matrices andthe connection method.III. Control DesignAs the machine is built from modular elements, so isthe control.In this work, we focus on the logic controlX - AxisOperationSequenceUserInterfaceManualModeModeSwitcherAutoModeConflictCheckerSpindleY - AxisFig. 9.The overall structure of the modular control system.for sequencing and coordination of the machine modules;a discrete-event system formalism is used 6. There is onecontrol module associated with each active machine mod-ule; we refer to these as machine control modules. In themachine design, there are passive elements which connectthe active elements together. In the control design, theremust also be “glue” modules which connect the machinecontrol modules.The overall architecture of the controlsystem for an RMT is shown in Figure 9. The structure issimilar for either of the two machines shown in Figure 8;for the V8 machine, there is no Y -axis control module.As shown, the machine control modules are at the lowestlevel; these interact directly with the mechanical system.The user interface control module is at the highest level, in-teracting with the user through pushbuttons and a display.The operation sequence control module is defined based onthe high-level operation sequnce for the part as shown inFigure 3. Three modules handle the mode switching logic.In this section, we briefly describe each of these types ofcontrol modules as well as their interaction and coordina-tion.A. Machine control modulesEach machine control module has a well-defined interfacespecification: it accepts discrete-event commands from agiven set, and returns discrete-event responses from a givenset. Within the control module will be all of the continuous-variable control, such as servo control for axes. This con-tinuous control is designed using standard PID algorithmsand the axis parameters such as inertia, power, lead screwpitch, which come from the machine module definition. Inaddition, each machine control module will contain con-trols for any machine services associated with the machinemodule, such as lubrication or coolant.Thus, each ma-chine control module is a self-contained controller for themachine module it accompanies, and can be designed andtested independently of the rest of the machine.The design of a machine control module must be doneSlide ControllerMove to x CommandDoneStop CommandDoneJog in Positive x CommandDoneJog in Negative x CommandDoneCI1.MV(x)/ CO1.MV(X)RO1.DN /RI1.DN/ CO1.STRO1.DN /RI1.DNCI1.STCI1.JP/ CO1.JPRO1.DN /RI1.DNCI1.JN/ CO1.JNRO1.DN /RI1.DNFig. 10.Slide Controller. The slide controller includes (within theboxes) the servo controller for a slide. When the slide has reachedthe commanded position to within some tolerance, a “done” re-sponse is returned.only once for each machine module in the library. When-ever the machine module is used in a machine design, thecontrol module can be used in the associated control de-sign. The control module may be used independently, withits own processing power, I/O and a network connection tothe rest of the control system, or it may be used as a pieceof the overall machine controller which is implemented ina centralized fashion.An example of a machine control module for a slide isshown in Figure 10. There are four commands that themodule can accept: move to a position x, stop, jog in posi-tive x, and jog in negative x direction. When it has finishedthe desired operation, it returns the “done” response. Awatchdog timer is included (but not shown); if a prespec-ified amount of time elapses and a done response has notbeen issued, an “error” response will be returned.B. Operation sequenceThe operation sequence module is defined from the high-level sequence extracted from the cutter location datashown in Figure 3.The main structure of this controlmodule is a sequence of states representing the sequenceof operations that must be performed on the part; waitstates are included at the completion of each step. Fig-ure 11 showns the operation sequence module for the ma-chine of Figure 8(b) and the operation sequence of Figure 3.Simple error handling which merely passes the error up tothe user interface is incorporated in the design but is notshown in the figure for simplicity. If a “reset” commandis received, the spindle is turned offand the slide is resetto its home position. The operation sequence for the V6machine is similar, but has more operations because thereare two linear axes that need to be sequenced. As shown inthe overall structure of Figure 9, there are two ports to theoperation sequence control module: one connects to theAuto Mode control module, and another connects to theconflict checker. The interface to the operation sequencecontrol module is shown in Figure 12.proceed/?moveX(p1)doneX/?ccproceed/?spindle_ondoneS/?ccproceed/?spindle_feeddoneS/?ccproceed/?spindle_offdoneS/?ccproceed/?moveX(p2)doneX/?ccproceed/?spindle_ondoneS/?ccproceed/?spindle_feeddoneS/?ccproceed/?spindle_offdoneS/?part_completeIDLEreset/?moveX(p0)reset/?moveX(p0)reset/?spindle_offreset/?spindle_offreset/?spindle_offreset/?spindle_offreset/?spindle_offdoneS/?moveX(p0)doneX/?ccFig. 11.Operation sequence module, showing the overall sequenceof operations and events. The interface to the module is shownin Figure 12. The reset command can be received at any time;only some of the event traces are shown for simplicity. The errorevent traces are also omitted from the figure.cc (command_complete)?part_complete?errordoneS?errorS?doneX?errorXproceed?resetmoveX(p)?spindle_on?spindle_off?spindle_feed?Operation?SequenceFig. 12. The block diagram of the operation sequence control module,showing the ports and shared events.Events received by themodule are in italics; events shared with the upper-level moduleare in bold face.C. Modular control structureThe user interface control module interacts with the userthrough a set of pushbuttons to turn the control systemon and off, switch between control modes, and single-stepthrough the operation sequence. Its main functions are topass the user commands through to the rest of the con-troller, and to display the current state of the machine tothe user.Machine tool controllers have several different modes. Inthe auto mode, the operation sequence executes continu-ally; another mode may execute the operation sequenceonly once. In step mode, a pushbutton command must beused to initiate every step of the operation sequence, andin manual mode, finer control is available through jog com-mands that move the active elements a small amount at atime. Instead of repeating the operation sequence for everycontrol mode, one representation of the sequence is used.The mode-switching logic determines the appropriate timesto send the “proceed” event to the operation sequence.The main function of the conflict checker control moduleis to pass the commands from the operation sequence andmanual mode modules to the appropriate machine controlmodule(s). It has access to the database of the machinemodule defintions, and can use those to check for illegalcommands which would result in mechanical interferences.Because of the well-defined interface to the low-level ma-chine control modules, the design of the conflict checkercan be done using high-level control commands. The de-tails of the physical I/O are handled in the machine controlmodules.As described above, each control module is representedby a finite state machine which accepts a certain language(sequences of events which are allowable). We have shownthat with some well-defined conditions on these languagesand the module connections, the overall control structurecan be guaranteed to be deadlock-free 8; enumerating allpossible sequences of the combined logic controller, whichwould be impractical, is not required for verification.IV. Reconfiguration PropertiesThe machine modules in the library can be used in manydifferent machine designs. The control module associatedwith each machine module will be incorporated into thecontrol design of the overall machine. This library of ma-chine and control modules can significantly reduce both thelead time of a new machining system, by shortening the de-sign cycle, and the ramp-up time, since each module canbe tested independently before they are connected.For some part changes (such as between the V6 andV8 cylinder heads shown in Figure 1), the machine toolwill need to be reconfigured, perhaps by adding an axis orchanging the spindle. When this type of reconfigurationoccurs, changes need to be made to the operation sequencecontrol module and the conflict checker (if new mechanicalinterferences are created).Because they posess a well-defined interface, each indi-vidual control module can be changed independently of theothers. As long as the redesigned control module has thesame discrete-event interface, the resulting system is guar-anteed to be deadlock free. For example, a friction com-pensation control algorithm may be added to one of theslide control modules. This would increase the performanceof that module, but the only changes necessary would bewithin the lowest-level module.V. Conclusions and Future WorkHistorically, machine tool design has been experience-based. In this research, we described a mathematical ba-sis for synthesis and evaluation of Reconfigurable MachineTools and their associated controllers. This research workhas addressed both the generation of machine tool configu-rations and modular control design. The systematic designprocess begins with the machining requirements.The presented methodology for synthesis of machinetools allows a library of machine modules to be pre-compiled and stored in a database, self-contained with con-trollers and ready to be used in any machine design. Themethodology also ensures that all kinematically viable anddistinctly different configurations are systematically enu-merated to reduce the chance of missing a good design.We have already developed a Java-based program whichautomates the machine design process; we are currentlyincorporating the control design procedure within the ex-isting framework.We are also expanding the currently-available machine and control module library and formal-izing the abstraction from the continuous-variable controlto the discrete-event domain.AcknowledgmentsThe authors would like to acknowledge the support andinvaluable feedback from the industrial members of theERC who have participated in this project. MEAM grad-uate students Eric Endsley, Morrison Lucas, and Yong-MoMoon contributed to the work described in this paper.References1F.-C. Chen and H.-S. Yan. Configuration synthesis of machiningcentres with tool change mechanisms. International Journal ofMachine Tools and Manufacture, 39(2):273295, February 1999.2I.-M. Chen. Theory and Application of Modular Reconf
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