A Flexible Real-Time Adaptive Control System for Turning

Th. Lundholm - Submitted by B. Lindstrom (1) Received on January 15,1991

The paper deals with :he organizaticn of a iew adaptive control svstern for turmng and an mtegration framework for subsystems in tre adap tive control system Different voritorivg, adaptive Central constraint and adaptive cmtrol optimization functiors are assigred to five sLbsys- tems Monitcring and control of mac+mng operatiors is tased on measwemeits of macnirirg c’7arac:erwcs and the subsys:ems include a number of advanced sensor systems wnich are brierly descrioed The stochastic nature of the macnining process calls for ‘eai-time contrcl of machining ccnditions The subsystems are 3esgneo to ooerate in a reiativeiy irdependent manner Consequently computer ‘echrolcgies for real-time ana distributed control have been emoloved :o realize a flexible modular system arcnitecture for !he adaptive control system The iayout of :he system as well as the adaptive feedrate control srategy are presented

Key words Optimization :urning adaptive control

Introduction

The main economical argument for industrial automation is to increase the utilization of capital intensive production equipment 131. Several investiga- tions show that in the metal-cutting industry only a small pCRiOn-an Often caed figure is 5ve percent -of the theoretical available manufacturing time is machining time. Of course. the smaller the batches. the mcre evident this fad becomes. The predominant part of the !ime is spent in transporting and storing workpieces. Under such conditions. it is not easy to vindicate the necessQ of optimizing machining operations. Let us. however, look a little bit ahead. The aim is to reduce non-produdive time, thus leveling the mad for increasing the time available for machining. As the time pOrt /On for machinrng increases. the need for optimmng machmmg costs becomes more obvious. According to Kals and van Houten 12: the InVeStmentS as- sociated with the use of expensive CNC machine tools cause the machine- cost curve and the tool-cost curve, as a function of production speed. to be steeper than with the use of conventional machine tools. Consequently. to select appropriate machrning conditions becomes more and more Sig- nificant. Due to the stochastic nature of the metal-cutting process, the Op- timal machining conditions varies continuously. Theoretically, machining under optimal conditions can be achieved by using adaptive controlled machine tools. In practice, adaptive control (AC) undergoes a rapid growrh in importance as a way for improvements regarding optimrzation of machin- ing costs.

Classification of Machining Conditions

The following classification of machining conditions is useful when discuss- ing optimization of metal cutting operations Technical limitations can be critical during a cutting operation owing to lack of power or torque, exces- sive force, bad surface finish or feedlspeed range constraints Machining conditions that bring on acceptable results and meet the specified techni- cal limitations are referred to as technical machining conditions (TMC). Most industrial machining conditions are obtained from recommendations in standards or company files, recommended machining conditions (RMC). To prevent tool breakage, such machining conditions are uSua//y rather conservative or safeanduneconomical. Machining conditions that are com- puted, taking only the economical situation into account and disregarding technical limitations. are termed global economical optimal machining con- ditions (GEOMC). GEOMC, in many situations. are unreasonably high If both the economic optimum as well as the technical limitations of the specific machining situation are considered, the optimal machining conditions (OMC) are selected. OMC are, in most cases from the productivity point of view, more favorable than RMC (Figure 1).

Feedrate --- Technical limitations

I Cutting Speed

Figure 1 C/assrf/cabon of machrnrng conditions.

The IMTiKTH Adaptive Control Plan

The Online System

Adaptive control involves continuous charges in machining conditions Running an AC system oased on one objective might for instance cause an infraction of a limit in another respect An adaptive chatter control sys- tem that increases the feedrate as a reaction upon an indication of develop ing chatter, might cause an infracticn of force constraints as in Figure 2 11- lustrates this At the Department of Production Engineering. Royal Institute of Technology (IMT,KTH) in Stockholm we have recognized the necessity of combining contrcl modules based on different control objectives. respec- tively in order to cover a broad range of machining situations Furthermore, using multiple senscrs of different kinds and combining the measured data, can in some cases extend the possibilities to interpret :he obtained infor- mation appropriately The reverse case I e two or more control modules utilizing the same sensor for different purposes is conceivable as well

The AC turning center that is under development at IMTlKTH is primarily in- tended for roughing operations The aim of this online AC system is to real- ize control of basic machining conditions in such a way that machining

Feedrate Computed iew

Force constraint Current

machining point

Cutting speed

Figure 2 An imagined AC situation rllustratrng a charter control actron that might cause an rnfraction of the force constratnt

operations are run on OMC (optimal machining conditions). Thus, the fol- towing monitoring and control functions, divided into different levels of sophistication, are considered to be significant:

advanced process monitoring, to protect from fatal events, with respect lo

-tool wear -chipping -tool breakage -collisions -vibrations -motor currents

- cutting-forces -chatter vibrations

-maximum productivity -maximum production rate -mixed function of productivity and production rate.

adaptive control constraint (ACC) with respect to

adaptive control optimization (ACO) with respect to:

It is not an overstatement to regard this iist of functions as most ambitious. presently bordering on the unattainable, since it Implies a tremendously complicated multi-input and multi-output, i. e. a multi-variable, control sys- tem. Such a system should be able to control at least two variable inputs. i. e. the feedrate and the cutting speed. subject to a complex performance index (PI) funcrion, involving the constraints and the chosen optimization

Annals of the CIRP, Vol. 40/1/1991 441

MN

DSC,M) V1slon Svstem

\ ~ ~ i ~ ~ ~ ~ Monitoring of Force ACC CfM

Kennameid 8GC196KE TMS32010 SMT CNC300 T;y;t20r Microcontroller Dgnal Slgnal

Vibration ACC

Numerical +It Processor Controller

Ul""

cO?trd Collislon Monitoring of Tool Breakage Excessive

Tool Wear Currents Missing Tool (Electronic

Sh Pin) -

CritBrion. Furthermore, apart from directly measured forces and currents, the outpu!s can only be measured and estimated indirectly, e. g the tool life. Nevecheless. the long-term objective is to manage this. Meanwhile, a simplified arrangement has been agreed upon:

0 advanced process monitoring in accordance with the long-term plan ACC of feedrate with respect to cutting-forces ACO of feedrate with respect to maximum productivity.

In the current state of develooment. the cueing speed and the depth of cut are optimized offline during preparation of machining operations and kept constant during actual machining In order to fulfill the AC functions. the sys- tem requires different kinds of measurea data. In addition to the armature current measurement faciiity provided In the SMT CNC 300 system (used internally in the Cutting Force Monitor, CFM), four indeoendent sensor sys- :ems are being developed at IMTiKTH The monitoring and control tasks are assigned tc five individual subsystems. which include and utilize !he sen- sor systems. Table 1 summarizes the organization of the subsystems and how the sensor systems are utilized. TPe bet-efits of this multi-control and multi-sensor approach to the realization of AC have been touched UGOn. On the crher hand. deveccping the subsystems requires the possibility of testing and running the subsystems individually. To meet these circumstan- ces, some kind of integration framework to host and manage the subsys- tems was considered as necessary. The solution to this was the design and realization of a flexible modular real-time computer network.

The Offline System

The online optimization of machining conddions by means of an AC system including an ACO module does not exclude. but rather emphasizes the im- portance of integration with an offline optimization system that can prcvide the AC system with reliable initial OMC. This. of course. implies the neces- sity of an ability to update the machining conditions data base in the 3ffline system. Such a dynamic data base system is under development at IMTiKTH ill.

The AC System Network

Even ?hough some network nodes in the adaptive control system are pre- sertly not ccmpleted and connected to the network, this and ?ext section are, for the sake of convenience and comprehensiveness, wrltfen in the pre- sent tense. i. e. no distinction between reality and plans ,s made

Force sensor system based on Kistler force transducers/ Kennametal TSZOOW

Force sensor system based on special lorce

sub- , Mondored ACC ACO 1 Sensor oystem(r) system 1 event(r) subtect j oblective 1 CNC I Excesswe , 1 Armature current

I

1 motor \ measurement system/ , currents I SMT CFM

KEN Worntool 1 Force sensor system based on Kistler force

! ChiPpng

breakage 1 Collision 1

Force sensor system based on Kisller force

OSF I ! i transducers I

VIB Cata- I Vibration sensor strophic 1 1 system

Table 1 Organization of subsystems in the IMTIKTH AC system

As have previously been indicated the AC system is realized by means Of a real-time computer system organized as a distributed control system. as apoears from Figure 3 The application software comprises the AC strategies The master node (MN) connects to the BITBUS (an Intel specification) distributed control network through a host interface module The node modules are based on the dedicated Intel 8044 microcontrcller, which 9as a serial interface unit incsrporated in :he silicon Although the

I stra rate ales I

L ' 1 1 ( O d W

SNOPT SNKEN SNQSF SNVIB SNCNC , Fast serial interconnections in the

distributed control (BITBUS) network

Other communication link

Figure 3 Tne AC system network.

groups of "function boxes" in Figure 3 are not strictly surrounded by or- ganizational borders to distinguish between the different subsystems, they can certainly be identified. In some cases "function boxes" are shared by two or three subsystems. The naming convention IS quite straightforward. A XXX denotation refers to a role as a more-or-less Independent develop- ment subsystem. while a SNXXX denotation reiers to a slave node, includ- ing the XXX subsystem, acting as an integrated member of the AC system.

Node Activities and Information Protocol

The application sohware that has been developed for the nodes is briefly described in terms of "node activities".

MN - The master node is the commander-in-chief in the network The activities taking place in this node consequently include

0 initialization. control and synchronization of the activities in the slave no-

management of the communication within the network 0 reception of status and alarm messages from the slave nodes and dis-

0 acquisition of NC block numbers from SNCNC and suggestions for fe-

0 selection and transmission of optimal feedrate override values to the sla-

des

play of these on the system console

edrate override values from SNOSF and SNOPT

ve nodes

The initialization process includes

initialization of the real-time system configuration of the BITBUS network and establishment of node connec-

distribution of initial machining conditions and tool data

m e control and synchronization activities are closely associated with each other In the sense that they are basically determined by rhe state of the part program in the CNC 300. i. e. the current NC block number. Examples of control and synchronization points are:

an NC block number in which machining starts an NC block number in which a tool change takes place

tions

442

The machining start, for instance. signifies activation of s!ave node activities such as:

0 activation of monitoring functions in SNKEN and SNVlB and also reset

activation of force control in SNOSF 0 activaticn of the optimization function including a machinivg timer in

of charge amplifiers in SNKEN (SNOSF)

SNOPT

SNCNC - SNCNC is :he target for the AC activities rn the system. i. e. the destination for the COmpUted optimal feedrate override values. The BITBUS controller in this node - in conformity with other slave nodes - basically performs data packing and unpacking, data interoreting and data receiving and !ransmis- sion. The actiwties can be summarized as:

0 reception of initialization and control messages and optimal feedrate

0 feedrate override adjustments receprion and transmission of NC block numoers to MN.

The bi-directional communication between the BITBUS controller and the CNC 300 through the serial RS-232 link comprises:

feedrate (and -whenever applicable -cutting speed) override messages

current NC block number from the CNC 300 (upor, request from the BIT-

override values from MN

from the BITBUS controller to the CNC 3CO

BUS controller) to the BITBUS controller

Unfortunately. this communication link IS presently the bottleneck of the AC system, since the maximum message transmission rate IS limited to ap- proximately one message/2.5 s. Novak et al. have overcome this problem by developing a digital link for real-time communication ;4/. The SNKEN and SNVlB nodes are cascaded to :he emergency stop circuit of the CNC 300.

SNKEN

The activities taking place in SNKEN include.

0 force based monitoring of machining 0 reception of initialization and control messages from MN. 0 transmission of status and alarm messages to MN 0 emergency stop of CNC 300 in occasion of an alarm event.

m e BITBUS controller In the SNKEN node has opto-coupled digital I/O ter- minations for parallel communication with the Kennametal TS200W. The in- formation protocol for !his communication IS determined by the Kenname- tal system specifications and it covers, for example:

enabling and disabling of monitoring functions control of charge amplifiers tool changes

0 status and alarm messages from TSPOOW.

-

The activities in SNQSF involves.

0 force based ACC of feedrate reception of initialization and optimal feedrate override messages from

0 transmission of suggestions for feedrate override values to MN

In the list above !he concept "suggestions for feedrate override values" iS

introduced. This means that the control outputs from the control nodes- presently SNQSF and SNOPT- are regarded as suggestions to MN. which in turn is responsible for the selection of the "best" suggestion in every Single control period to be used as control input to SNCNC. The Control nodes continuously receive feedback information from MN about the selected Op- timal feedrate override values. This is a necessary condition for the proper performance of the control algorithms.

MN

SNVIB - Except for its basic monitoring task. the activities taking place in SNVIB are similar to those in SNKEN and SNQSF

0 vibration monitoring based on measurements of relative displacements

0 reception of initialization and control messages and optimal feedrate

transmission of alarm messages to MN 0 emergency stop of CNC 3M) in occasion of an alarm event

and dynamic forces

override values from MN

SNOPT differs from the other slave nodes in the sense that it Comprises a PClAT computer incorporating a BITBUS controller. It carries out the fol- lowing activities:

ACO of the feedrate based on online tool-wear estimations and oftline

reception of initialization and control messages and optimal feedrate

0 transmission of suggestions :or feedrate override values to MN

As may be seen in Figure 3 and also may be concluded from the activity list, SNOPT is seemingly a very diversified device handliq many activities. In fac:. it is a distributed control system in itself. In addition to the CPU n the 170s: PClAT. the PCSloO image processing sys:em comprises four Jis- trnguishable processing units. the positioning device for the CCD sersor is one Control unit and, finally, the special dynamic force sensor system forms one separate processing unit. The various activities are synchronized 3y the host PCiAT. Furthermore. the online cptimization algorithms, which produces the feedrate override suggestions are implemented on the PClAT.

In Table 2 the information protocol used in the network is summarized

Flexibility by Modularity

A guiding line for the development of the rntegration framework has oeen to provide a simple and flexible possibility to interconnect :he aavanced AC subsystem modules. This is accomplished by the proposed network s o b tion. The wiring is limited to standard serial and/or digital interfaces between node controllers and subsystems on the slave nodes. Twisted pair.wires or flat ribbon cables and standard 0-sub connectors are used for network con- nections. The electrical connections are insensitive to worksnop environ- mental disturbances. As a slave node is connected to the network. it is very easily configured. In its present shape of the AC system, re-configurations involve standardized changes in the application software, but !his may naturally be programmed to be carried out partly automatically andlor t m r - actively by the user.

flank-wear measurements

override messages from MN

From MN

To SNCNC:

CNC command 'AUTO' CNC command 'MANUAL' Request current NC Mock number Feedrafe override Cutting speed override

To SNKEN:

T o d number New job New tod Reset charge ampliliers Activate charge amplifiers Reset alarm

To SNVIB:

Nominal leedrate Nominal cutting speed Nominal depth of cul Current leedrate override (and stan) St3p vibration monitoring

To SNOSF:

Nomiral leedrate Current leedrate overrlde (and nan) Stop lorce ACC

To SNOPT

New job New tcd Nominal leedrate Nominal cutting speed Nominal depth of cut Localize fool (at new job or new t d Current feedrate overrkle (and stant Measure flank wear and optimlzatkn Stop ACO

Table 2 information protocol.

To PAN

From SNCNC

Acknowledge Current NC blmk number

From SNKEN:

Worn fool Tod breakageiworn tool alarm Missing tool alarm Cdlision alarm

From SNVIB:

Chaner alarm

From SNQSF

Acknowledge Suggestion for feedrate override

From SNOPT

Acknowledge Suggestion lor feedrate override Flank wear alarm T w l breakage alarm

443

The Adaptive Control Strategy

The employed AC strategy for the IMTiKTH system can be briefly ex- pressed:

The various monitoring and control functions affect the machining pro- cess in real-time and in priority order. Monitoring functions are assigned high priority. ACC functions medium and ACO is assigned relatively low priority. The lowest feedrate override value suggested prevails.

This strategy is realized as a real-time application system being executed in the MN computer The laboratory system is intended for single Cuts, but may, of course, easily be modified for use in consecutive cuts.

The Structure of me Appllcatlon System

The application system running in MN consists of the following three parts:

Intel RMX I Operating System Datem dDCM810R Network Manager (BITBUS support package) application scftware comprising the AC strategy.

The two commercial systems mentioned first form the environment for the application software and enable the real-time and distributed control specific activities in the application software to be performed (Figure 4).

Figure 5 shows the structure of the application software. The tasks are all assigned different priorities and the lower the priority number the higher the prioriy. The various objects are:

STARTTASK (priorty: 160), which is the initial task, creates all the other objects, but after doing this it Commits suicide by deleting itself.

MAINTASK (priority: 161) is assigned the highest priority among the re- maining tasks. It is responsible for initialization and synchronization Of the rest of the appiication system. Furthermore, it handles most Of the system console ti0 functions i. e. data and CNC control command 110. It distributes initial data to other tasks and performs certain CNC Control functions. When a machining operation has started, MAINTASK Continu- ously selects optimal feedrate override vaiues and transmits selected va- lues to SNCNC. Finally, when a machining operahon has come to an end,

MAINTASK commands stop to the slave nodes.

Software Network Manager

Opera t ing System

SNKENTASK SNVIBTASK SNQSFTASK Priority: 162 Priority: 163 Priority: 164

t t Hardware

SNOPTASK Priority: 165

1 J

Figure 4 Schematic of the application system.

SNKENTASK. SNWBTASK, SNOSFTASK. and SNOPTTASK (priorities: 162 - 165) are, except from initialization, enabled to execute during machi- ning operations only. Either B SNXXXTASK may be in READY state pre- pared to execute in the CPU, or it may be in RUNNING state, e. g. col- lecting a message from its corresponding slave node, or it may alterna- tively be in WAIT state, waiting for receiving a message from its slave node.

SNTSKRDYSEMA semaphore is used for the purpose of synchronization during the initialization of the system.

KENDATSEG, VIBDATSEG, QSFDATSEGand OPTDATSEG segments are utilized to store information from the slave nodes.

CURCNCOATSEG segment contains nominal machining conditions, the current feedrate override value and the current NC block number.

NEWCNCDATSEG segment is used for intermediate storage of new Op- timal feedrate override values

Extensions of the AC Strategy

In a previous seaon the ambition to extend the set of functions in the IMT/KTH AC system was indicated The first step to be taken in rhis direc- tion. involves upgrading SNVlB to perform closed-loop control by adjusting the feedrate override. Except from the basic chatter control problem at the

SNTSKRDYSEMA

Priority: 161

C U RCNCDATSEG PI 1

Figure 5 Structure of the app/icat!on software

subsystem level it gives rise to an additional delicate decisional problem to be covered by the AC strategy How to react upon a suggestion from SNOSF to decrease the feedrate due to a critical cutting-force increase at the same time as a suggestion from SNVIB :o increase :he feedrate to avoid a predicted development of chatters Presently, we are not aole to give a clear-cut answer to this question. but the problem has to be investigated and solved An initial step could be to simply generate an alarm and an emergency stop as a reaction upon contradictory sdggestions for feedrate override In reference 51 Tonshoff et al give a valueable contribution in this matter by providing a survey of methods for monitoring and control Of machining processes

Laboratory Experience

The following node combinstions have been put into operation

SNCNC - MN - SNOSF SNCNC-MN-SNKEN SNCNC-MN-SNKEN-SNOSF MN - SNOPT (communication only)

Conrections of subsystems to slave node controllers as well as connec- tions of slave nodes to the BITBUS network have generally been carried out without trouble An illustrative example is the installation of the Kennametal TS2OOW Tool Sensor In addition to the preparatory sensor installations. the

programmed and configured in advance The actual installation (including connection to the CNC system) of the TS200W unit was completed in haif a day, the sensor calibration excluded The realization of a combination Of MN-SNCNC-SNKEN-SNOSF-SNOPT is approaching close at hand A further step is to include SNVIB, which is prepared in the AC strategy

References

AC system including the MN SNCNC and SNKEN node controller Were

Ahmadi. A., 1990, “The Design of a Database for Cutting Data Selection and Management in Adaptive Control and Computer Aided Process Plan- ning Systems”. Licentiate Thesis, Royal Institute of Technology, Stock- holm, ISRN KTHilMTilA--90/894, TRITA-IMT-2-894, ISSN 0284 0537

Kals, H. J.. van Houten. F. J. A. M.. 1982, “On Flexible Manufacture Based on a Production Information Management System (PIMS)”. Proceedings of the 14th CIRP International Seminar on Manufacturing Systems, Trondheim

Nicolin, C , 1987, “Economy of Production”, Remarks at World Produc- tivity Forum, Washington

Novak, A,. Ossbahr. G., Wikstrom. P, 1989. “A Digital Link for Adaptive Control and On-Line FMS-Applications”, Annals of the CIRP. Vol. 38/1/1989, pp. 447-450

Tonshofi H K Wulfsberg. J P , Kals, H J J Konig, W , van Luttervelt C A. 1988 “Developments and Trends in Monitoring and Control of Machining Processes’ Annals of the CIRP, Vol 37/2/1988 pp 611-621

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