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A model of data flow in lower CIM levels
Journal of Materials Processing Technology 157–158 (2004) 123–130
A model of data flow in lower CIM levels I. Drstvensek∗ , I. Pahole, J. Balic Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia
Abstract After years of work in fields of computer-integrated manufacturing (CIM), flexible manufacturing systems (FMS), and evolutionary optimisation techniques, several models of production automation were developed in our laboratories. The last model pools the discoveries that proved their effectiveness in the past models. It is based on the idea of five levels CIM hierarchy where the technological database (TDB) represents a backbone of the system. Further on the idea of work operation determination by an analyse of the production system is taken out of a model for FMS control system, and finally the approach to the optimisation of production is supported by the results of evolutionary based techniques such as genetic algorithms and genetic programming. © 2004 Elsevier B.V. All rights reserved. Keywords: NC machine tools; Virtual manufacturing; Production automation; Total productive maintenance
1. Introduction The main objective of the model is to provide a common environment where the evaluation of a given general order and later composition of work orders, and designation of production resources could be done automatically under the operator’s supervision. The model is functionally included into the five level computer-integrated manufacturing (CIM) hierarchy. Functions of the model are plugged into the lover four levels by means of technological database management system that supports main functions of information flow, which flows through the backbone of our CIM system. The structure of the backbone is vertical with the beginning in the technological database (TDB) and the end in the production planning and control module (PPC). Between the two ends there are three levels of CIM: enterprise (CAD, CAE, CABSdesign, engineering, business system), system (CAQ, CAP, CAST-quality, planning, storage and transport), and cell level where the PPC module reside. The last two levels of CIM are connected to the backbone over the PPC module. These are the workstation level (CAM) and the hardware level (SFDCshop floor data collection and acquisition). Data flow among levels is vertical via the local area network (LAN) and sup∗
Corresponding author. Tel.: +386 2 220 7593; fax: +386 2 220 7990. E-mail address: [email protected] (I. Drstvensek).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.09.010
ported by the database management system (DBMS). Modules use an extended DBMS in the form of different interfaces for horizontal data flow over levels. The database is physically distributed but logically centralised. It means that files are stored locally in the level to which they functionally belong. At the same time all the files are controlled via the same DBMS, which allows any module at any level to use any file in the CIM system. In this way an overall informational interconnection is assured what at the same time shortens the response time in a case of design and construction changes forced by lover CIM levels. The last researches showed the necessity for new, integrated understanding of CAM and SFDC levels. The problems of equipment maintenance, online machine tool designation and control over the production flow forced the integration of tasks that were formerly distributed between the two levels. The fact is that shop floor data emerge from the production process so they need to be treated at the same time and in the same place as the production data. To deal with this fact in an effective way the DNC concept was broadened to meet the requirements of machine tool and production data acquisition as well as to fulfil some present needs of production environments [1,2]. These are mainly caused by the more and more important role of maintenance tasks, which are nowadays already incorporated into production plans. In this manner the DNC concept was not only broaden—it got the completely new meaning,
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Fig. 1. The DNC concept of the third generation.
so we can talk about a new DNC concept, a DNC of the third generation. The basic idea of this “next generation approach” is to upgrade the usual CNC control with a part of the technological database (TDB), and to connect the CNC control with an DNC computer via the database management system (DBMS) of the TDB as it is depicted in Fig. 1.
2. A structure of the model Since the model of the data flow deals with lower four levels it does not concern the design of subjects of production. This is already a fact in this stage, so from here on means and methods for their production have to be found. To this purpose a structure of the technological database plays a main role [3]. It stores data of all production parameters as well as data about their momentary availability. This is enabled by the idea of database’s key information. The key information of the technological database is a work operation. The work operation is a basic entity of the TDB. It pools together data from databases tables that form information about the momentary state of production resources. Such a structure enables the TDB to act as work operation’s magazine, e.g., all possible work operations in a production system are stored in the magazine from which they can be assigned to different work orders. A work operation once assigned, becomes unavailable to any other order until it is completed at the actual one. The described principle is the basic tool for production planning, and represents a unique way of queue scheduling. The key information/work operation is composed out of the TDB’s data that have to be structured in the proper way. The data of production resources are pooled into five groups: • machine tools, • tools, • operators,
• auxiliary means, and • maintenance means. Each group can consist of different subgroups/data depending on the type of work operation that it describes, e.g., the tool can furthermore consist of a handle, and a cutting plate. The further division is not so important any more since it only gives a detailed picture of a work operation. To understand a selection of the five groups of production resources, the Fig. 2 should be described in more details. A production process always starts with an order that describes the subject of the production, and consequently defines work operations. To complete, and control a work operation some production/information resources are needed, e.g., a machine where a manufacture will take place, an operator to control and serve the machine, a tool to machine with, and some auxiliary devices to hold and manipulate a work piece. All the resources involved will eventually need some maintenance and corresponding means. The activity of resources associated into the work operation influences the new value of the finished order/work part. Therefore, is the combination of these resources’ data key information of the production process that is mirrored in the technological database. In our research are work operations usually described by use of DIN 8589 notions as it is depicted in Fig. 3 [4].
Fig. 2. The composition of the key information.
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• stoppage, defect occur on the planned machine tool, • free working capacities on similar machine tools and reduction or even removal of bottle-neck is desired, • changes of the priority of orders (urgent order), • unavailability of proper working devices or tools specifically connected with a certain machine tool.
Fig. 3. Work operations are composed according to the notions in DIN 8589.
2.1. Finding a machine tool in a case of known technological process During planning of technology the technological process is determined, and the work operations required by the shape and geometry of the product are selected. The operations are mainly defined by describing (to be face milled, to be milled up to depth etc.). Such type of describing however makes the processing by known programme tools on computers difficult. From the known standards it is possible to conclude that the area of describing the work operations has been studied very precisely (Table 1). Such a manner of designating the work operations makes describing of operations considerably easier. Thus the technologist must use the standard designations. Graphic devices on computer can be of great assistance to him. We can obtain the following data resulting from the CAP module: • designation (serial number) of order for a certain period of planning, • unambiguous designation of order, • code or description of work piece (s) in order, • number of work piece (s), • required NC-programmes for certain operations on certain units, • description of standard operations according to DIN standard, • anticipated start time, • anticipated calculated time for order, defined in advance. When an order has already been introduced into production, it is often necessary to search for an alternative machine tool for certain operation or group of operations. Typical examples are: Table 1 Example of survey of feasible work operations for machine tool DIN 8589 notion
Description
3.2.2.1. 3.2.2.2. 3.2.2.3. ... 3.2.3.1. 3.2.3.2.
Group of operations of face sinking Group of drilling operations Group of thread-cutting operations ... Plane milling Milling of round shapes
Automated searching for an alternative machine tool during the production management process is certainly the biggest wish of production managers. We can quickly conclude that complete automation isn’t possible since it is almost impossible to describe a work piece when searching for solutions of the problem presented across several factors by which the final solution—if existing at all—is conditioned. During the stage of planning it is possible to work out several alternative procedures, which are optimised regarding the: • most favourable version with regard to time or, • economically most favourable version. If the number of these versions of working processes increases, such manner of planning usually becomes economically questionable. It can also happen that it cannot be applied because it is too time-consuming [6]. In parallel it is necessary to prepare the conditions for execution of certain order on several units of the production system. This applies particularly for orders of high priority. The appropriate software can be of great assistance to the technologist/operator for determination and/or selection of the alternative machine tool. In our model several interfaces included into the database management system ensures that suitable machine tool can be found. They compare the operations required for execution of the order with the spectrum of operations that the machine tools can perform. The result is a proposed scenario, which the production manager has to confirm before it is activated. The structure of the TDB is also used in our model for evaluation of the general order, prescription of work operations, and designation of production resources. The model has three stages from evaluation of the general order to composition of work orders: • technological evaluation of a general order, • composition of a work order that consists of several designated work operations, • composition of a production scenario based on work operations prescribed in the work order. Tasks of each stage take places in different CIM levels (Fig. 4). The CAP in the system level performs technical evaluation. It is done by an analyse of: • • • •
the subject of production, a production time frame, the order’s importance, special demands for production resources (accuracy, handling, etc.), and • costs produced by the subject’s manufacture [7,8].
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Fig. 4. The data flow is divided into the off- and on-line part.
The cell level (PPC) is responsible for the composition of work orders since it has instant access to production resources data.The work order composition requires data about: • production resources (the operator, the machine tool, tools, auxiliary means, and maintenance means), and • their availability (occupation time, release time). Work operations are composed, according to the production demands prescribed in the general order and production possibilities found in the TDB. The beauty of this approach is in the fact that the same work operation can be composed of different production resources, what is especially important in cases of disturbances and resources’ failures. The workstation level (CAM) and later on the hardware (SFDC) level takes care for the composition of production scenario. This task requires data about a momentary availability of production resources. The first scenario is composed upon the availability data in the moment of its launching. When the necessities for changes occur a new scenario is created for the reminder of the order upon the new availability data. Therefore, it is inevitably to have an on-line access to the availability data stored in the SFDC part of the TDB.
3. The data flow through the model’s structure After the analyse of the general order when it is already established which approaches are most suitable for its manufacture in the prescribed time-frame, the production resources need to be designated to different parts of general order thus defining partial work orders. This is done by inquiries in the technological database and assigning the chosen resources to the new work order. The search mechanism can be made in different ways, among which simple SQL queries are commonly used, but the genetic algorithm search is definitely more interesting and in long term also more promising. Namely, the SQL is inevitably bound to the use of relational databases, and requires more complicated automation techniques [9]. Although less known and far less investigated, the
genetic algorithm search is very robust and independent of the type of a databases structure. On the other hand the search speed when using GA, reduces dramatically with increased complexity of the search field and number of fitness functions [10]. After a decomposition of the general order into work operations and composition of work orders, priorities are assigned to the work operations, what will assure their position among other work orders that are already under treatment in the production system (CAM). The last step to do is a composition of production scenario for the general order. It means, that the succession of work operations has to be established to prescribe the proper material flow. The scenario depends of the momentary conditions in the production system therefore, an insight into it is needed. The SFDC module of CIM is responsible for the overview of the production circumstances. It handles it in the connection with the TDB, where the availability data are written in terms of occupation time frames and condition data (ready, broken, engaged, etc.). To this part all the work can be done off-line by more or less simple TDB inquiries, however from this point on an on-line access to the production data is needed. The scenario strongly depends of momentary production circumstances, and has to be changed when unplanned situations occur. Therefore, a mechanism for automatic scenario composition has been made. It uses a GA optimisation method to search among available production resources [11–13]. When a suitable combination is found a new work operation is composed to exchange the actual one in a case of failure [14].
Fig. 5. The role of the DNC terminal.
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although its structure must be similar enough to assure the optimal connectivity. Therefore, the database in the CNC is structured according to the same principle of key information as the TDB in higher levels (as described before). Nowadays DNC systems are usually a mixture of: • terminals to connect to the machine tool, • network or serial wires and • a computer that connects the whole bundle together.
Fig. 6. CNC upgraded with the TDB.
4. Communication in CAM level After the definition of all the methods and tools for automatic scenario composition we can focus on the data flow in the CAM level. As it’s been said in the beginning, the tasks of the SFDC level can be integrated into the CAM level thus giving it a new meaning and a definition. Integration of the two levels is only possible if we transfer the SFDC tools along with the tasks to the CAM level. Having the idea of TDB in mind a new solution occurred that newly defined the meaning of direct numerical control. . 4.1. The DNC concept of the third generation The key point of the new DNC concept is in the idea of database integration into the CNC control device. Since modern CNC machine tools are already equipped with powerful computers, which are capable of demanding graphical and numerical operations, we can also equip them with a technological database that will store data crucial to the machine tool and operations performed on it. This idea comprises with the concept of distributed database and gives some new possibilities to the production managers. As soon as we equip a machine tool with a database it becomes an independent entity of the production process in the informational sense. As such it gains a certain degree of autonomy and can therefore, act as an informational resource (object), and not only as a production mean. What does a production system gain from such an “upgraded” CNC machine tool? In the first place the interconnectivity among other machine tools and the conducting computer is simplified. Special connection terminals are not needed any more, and the data flow gains both the bandwidth and the speed of data flow. On the other hand the database in the CNC is always up-to-date, because every input to the database is also an input to the machine tool and can therefore, be verified by use of different sensors. It is clear that the database in the CNC won’t store the same data as the database in higher levels of CIM. Its content is much simpler
The terminal is a part of modern DNC systems and usually serves as a connection between the machine tool and the computer network (Fig. 5). It is connected to the CNC control via the RS 232 connection and to the DNC computer via the network cable. Network protocols in use range from special industrial types to the TCP/IP, which becomes more and more popular since it enables an intranet connection among the machine tools. There are also known DNC solutions that use PC’s as the terminals or even connect the machine tools directly to the DNC computer via the RS 232 connection. In the last case the terminal is not needed but the solution is very sensitive for any type of electromagnetic interference. Whatever the solution is it always uses a central computer to conduct the work of machine tools. It is usually equipped with some form of logbook and interfaces that serve for data transfer between the computer and the terminal on the machine tool. Data flow is usually limited to NC programs transfer to and from the machine tool and to acquisition of simple machine data. All other work from a supervision of tool stores to NC programs distribution is also handled in the conducting computer. The new approach described here as the DNC concept of the third generation goes far beyond this scope of DNC tasks. By incorporating the TDB into a CNC control the latter becomes enough data to perform some more sophisticated operations that can be included into the database management system in a form of special interfaces (Fig. 6). One of these tasks is a post procession of tool path files. It is usually done in the CAP level as a part of computer aided NC programming where postprocessors are ran in the frame of CAP software. But it’s easy to move the postprocessor to the CNC control if there is a possibility to create a simple interface for this purpose. Another and nowadays more and more important possibility is inclusion of some maintenance tasks into the CNC control. The fact is that the maintenance of equipment became very important with increased machine tool’s complexity and qualitative demands of the market. Nowadays, a modern company can’t afford longer production cut outs because of insufficient maintenance or unforeseen breakage of parts. Even more, maintenance became a production category that does not bring costs any more but even makes profit. In this manner several techniques were developed among which the total productive maintenance (TPM) is most widely used. By studying these techniques one can ascertain that for an effective preventive maintenance a good and up-to-date documentation is needed. Another conclusion, which one can
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make in practical production circumstances, is that even if the documentation is properly taken for it is stored in a totally inappropriate place. So, a logical place to store documentation is the machine tool, which is on the other hand physically inappropriate for several reasons. The only exception to this rule is the control device if it is equipped with the database. Another important feature of the database incorporated into the CNC control is the possibility of distributed tool storage system. In this case the tools are stored in machine tools’ work stores and therefore, distributed over the shop floor. Their data are stored in the TDB of each CNC machine tool and thus always present in the TDB of the DNC computer. The special tool store is not needed any more instead of it we rather put a small spare parts store that could also serve as a buffer zone in a case of redundant tool demands. 4.2. Data structure of the CNC database The database of the machine tool consists only of data that occur in it. It means that the data that concern certain machine tool won’t reach the database of any other machine tool. The machine tool data can be divided into: • constant data and • variable data. Constant data are those, which define the machine tool—a kind of identity card. The variable data, on the other hand are those, which define the work operation performed on the machine tool. Constant data: • • • •
name of the machine tool, type of the machine tool, size of the workspace, production restrictions. ◦ feed rate (min/max), ◦ spindle speed (min/max), ◦ amount of memory. . . • type of tool clamping device, • power of the main drive, • amount of the coolant. . . Variable data: • • • • • •
work order’s ID, work operation’s ID, NC programme, list of tools in the work store, type of the coolant, type of the clamping device. . .
These two groups of data also have three subgroups that serve to some special purposes yet not present as a part of the DNC. These are: • maintenance data, • work process data and, • logbook data.
As the name tells the maintenance data should enable fast and effective maintaining in a case of planned preventive maintenance as well as in a case of unforeseen corrective maintenance. Among these are: • • • • •
a list of check points, a list of lubrication places, a list of lubricants, a list of wearing parts, a list of maintenance intervals for machine parts.
This group is a subgroup of the constant data with a small exception. Namely, a list of checkpoints can optionally contain a variable data of momentary condition, which can change, according to the condition of the checkpoint. In this case of course the machine needs to be equipped by an appropriate set of sensors to on-line control the checkpoints. The next group is a subgroup of variable data. It consists of usual DNC data: • • • • •
working condition (on, off, tool change, . . .), start time, end time, disturbed (tool break down, failure, error. . .), etc.
The last group is a hybrid between the constant and the variable data group. It stores the history of past work and events done on the machine tool. These events are either the consequences of work operations or the maintenance operations carried out on the machine tool. They are therefore, further divided into two groups: • • • • • • • •
the history of work operations, the history of maintenance operations. The history of work operations is logged by following data: work order’s ID, work operations’ IDs, start time, end time, number of produced items. . .
With these data, one can estimate the wear of tools, coolant and other important factors. The history of maintenance operations consists of: • • • • •
maintenance task (ID), start time, end time, used maintenance means, amount of means used.
These data enable effective estimation of maintenance costs, and also give a historic overview of the condition based preventive maintenance tasks as the basis for the proactive maintenance. Fig. 7 shows the types and subtypes of preventive maintenance, which are in a common use nowadays. In practice all these types are known as preventive maintenance and are all included into the philosophy of TPM, which is the leading maintenance idea in the modern industrial world
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4.3. The data flow in the DNC
Fig. 7. Diagram of preventive maintenance.
Fig. 8. The process matrix.
[15]. According this idea, some maintenance tasks are transferred from the maintenance experts to the operators at the machine tool. This transfer shortens the cut out times in a case of small machine errors and gives the operator greater responsibility and better overview of the production flow. The result is satisfied operator, higher quality of production and more competitive company. The common problems in the TPM approach are human errors caused by forgetfulness and other subjective errors. To overcome such obstacles, people invented checklists and similar documents that have to be filled out by the operator, but they bring more “pain than gain” because they mean another “unnecessary” task. By implementation of corresponding event-driven interface into the CNC control device the annoying checklists are moved from the operators sight into the control where the data can be promptly stored and retrieved either by the maintenance expert, operator or the production controller responsible for his or her shift.
The next important issue to explain is the dataflow between the DNC computer and CNC controls. The flow is of course bi-directional but not all data are travelling here and there all the time. A careful reader has already ascertained that the variable data will do the most of the traffic. They are subjects to changes in the production plan, so they have to change every time the scenario changes. If this change is significant it would cause many changes of variable data in TDBs in CNC controls. But the scenario changes according the momentary situation in the shop floor, which is reflected in the work process data, and, according the disposable resources–machine tools. The last are described by the constant data, so the DNC’s database management system (DBMS) has to acquire all these data every time the scenario change is required. The scenario changing process is based on the principle of process matrix, which is in fact a table with work operation in its cells. Every column of the matrix represent one stage of the production process and every row represents one of all possible scenarios (Fig. 8). Work operations are put into cells by means of the genetic algorithm, which becomes an overview of disposable production resources by checking the variable and constant data groups stored in the local, CNC technological databases (Fig. 9). Since the genetic algorithm runs constantly it always fills the matrix by the best momentary solutions, among which the production manager can chose one, according his momentary wishes. The necessity to change the scenario usually occurs in a case of serious disturbance, like machine break down or some other malfunction. In such a case, a process will have to terminate the operation where the malfunction occurred and to continue with an alternative scenario for all the units in production that reached the malfunctioned stage (Fig. 8).
5. Conclusion An effective approach to the production planning tasks is nowadays a crucial factor for a competitive company in
Fig. 9. Data flow in the DNC system.
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a modern global market [16]. Shop floor is a place where a new value is added to the raw material. Therefore, will quality and accuracy of the production process in the shop floor directly affect the costs and indirectly the profits made in the market. The described model not only gives a possibility for partial automation of time-consuming processes, but rather tends to introduce new methods and principles of process planning tasks. The presented idea of the technological database is unique in its structure, which gives a possibility of many different data manipulations that can be used for various automation purposes. It is suitable also to be incorporated into the CNC control device, what gives some new control possibilities to the operator and for the first time enables the production manager to gather the actual production data by a mean of simple queries incorporated into the database management system. The wealth of information gathered this way is far beyond the scope of DNC systems known in a nowadays market. The operator can combine different data to get any important conclusion he or she needs. Besides, the technological database is open and can be upgraded with different data sets to give a user a possibility to build his own information frame. The main goals that we are trying to reach by our research work are: • reduction of preparation times, and • speeding up the decision making process. The model rather tends to support the operator in the decision making process than to replace him or her. It is not a kind of so-called intelligent interfaces because the intelligence has to be brought into it by the operator. It is on the other hand a very powerful automation tool that prevents most of errors that usually occur as an outcome of sequential human errors. The structure of the model guarantees that all influencing production factors will be promptly taken into account thus assuring a precise planning and accurate scheduling, what brings an important competitive advantage in nowadays market. The presented DNC system represents a base for the development of a new generation of DNCs, which demands a new approach to the development of CNC control devices. It is still mainly focused to the machine tool movements and
to user friendliness. But the potential of the computerized numerical control are far beyond that. The incorporation of technological database into the control brings many important advantages, among which the possibility of on-line tool database is maybe the most transparent.
References [1] T. Lenau, Integrating process planning with product design (design/CAPP integration), Manuf. Syst. 22 (1) (1993) 19–30. [2] I. Hunt, S. Roberts, R. Jones, The integration of design and manufacture, Integr. Manuf. Syst. 4 (2) (1993) 15–19. [3] I. Drstvensek, Model of the Technological Database in Procedures of Cutting Conditions Optimisation by the Use of Genetic Algorithm, Diss. Faculty of Mechanical Engineering, Maribor, Slovenia, 1998. [4] J. Balic, I. Pahole, A model of informational interface between flexible manufacturing system units, J. Mech. Eng. 3–4 (1997) 137– 142. [6] I. Pahole, J. Balic, Evaluations of flexibility of flexible manufacturing systems, in: B. Katalinic (Ed.), Proceedings of the 8th International DAAAM Symposium, DAAAM International, Vienna, 1997, pp. 67–77. [7] G. Halevi, R.D. Weill, Principles of process planning, in: A logical Approach, Chapman & Hall, London, 1995. [8] I. Drstvensek, J. Balic, Responding ability in dynamic production circumstances, J. Mater. Process. Technol. 133 (2003) 71–78. [9] G. Gardarin, P. Valduriez, Relational Databases and Knowledge Bases, Addison-Wesley Publishing Co., 1989. [10] P. Hajela, C.Y. Lin, Genetic search strategies in multicriterion optimal design, in: Structural Optimization, Springer-Verlag, 1992. [11] D.E. Goldberg, Genetic Algorithms in Search, Optimisation, and Machine Learning, Addison-Wesley Publishing, Massachusetts, 1989. [12] A.J. Joines, C.T. Culbreth, R.E. King, Manufacturing cell design using an integer-based genetic algorithm, in: Proceedings of the IFAC Symposium on Information Control Problems in Manufacturing, Beijing, 1995. [13] I. Drstvensek, I. Pahole, A genetic algorithm approach to the optimization of cutting parameters, in: B. Katalinic (Ed.), Proceedings of the 7th International DAAAM Symposium, Vienna, Austria, 1996, pp. 103–104. [14] I. Drstvensek, J. Balic, Technological database as an optimization environment,in: B. Katalinic (Ed.), Proceedings of the 8th International DAAAM Symposium, Dubrovnik, Croatia, 1997, pp. 67–77. [15] P. Willmott, TPM the Western Way, Butterworth-Heinemann, Oxford, 1994. [16] T.C. Chang, Expert Process Planning for Manufacturing, AddisonWesley Publishing, Massachusetts, USA, 1990.
A model of data flow in lower CIM levels I. Drstvensek∗ , I. Pahole, J. Balic Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia
Abstract After years of work in fields of computer-integrated manufacturing (CIM), flexible manufacturing systems (FMS), and evolutionary optimisation techniques, several models of production automation were developed in our laboratories. The last model pools the discoveries that proved their effectiveness in the past models. It is based on the idea of five levels CIM hierarchy where the technological database (TDB) represents a backbone of the system. Further on the idea of work operation determination by an analyse of the production system is taken out of a model for FMS control system, and finally the approach to the optimisation of production is supported by the results of evolutionary based techniques such as genetic algorithms and genetic programming. © 2004 Elsevier B.V. All rights reserved. Keywords: NC machine tools; Virtual manufacturing; Production automation; Total productive maintenance
1. Introduction The main objective of the model is to provide a common environment where the evaluation of a given general order and later composition of work orders, and designation of production resources could be done automatically under the operator’s supervision. The model is functionally included into the five level computer-integrated manufacturing (CIM) hierarchy. Functions of the model are plugged into the lover four levels by means of technological database management system that supports main functions of information flow, which flows through the backbone of our CIM system. The structure of the backbone is vertical with the beginning in the technological database (TDB) and the end in the production planning and control module (PPC). Between the two ends there are three levels of CIM: enterprise (CAD, CAE, CABSdesign, engineering, business system), system (CAQ, CAP, CAST-quality, planning, storage and transport), and cell level where the PPC module reside. The last two levels of CIM are connected to the backbone over the PPC module. These are the workstation level (CAM) and the hardware level (SFDCshop floor data collection and acquisition). Data flow among levels is vertical via the local area network (LAN) and sup∗
Corresponding author. Tel.: +386 2 220 7593; fax: +386 2 220 7990. E-mail address: [email protected] (I. Drstvensek).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.09.010
ported by the database management system (DBMS). Modules use an extended DBMS in the form of different interfaces for horizontal data flow over levels. The database is physically distributed but logically centralised. It means that files are stored locally in the level to which they functionally belong. At the same time all the files are controlled via the same DBMS, which allows any module at any level to use any file in the CIM system. In this way an overall informational interconnection is assured what at the same time shortens the response time in a case of design and construction changes forced by lover CIM levels. The last researches showed the necessity for new, integrated understanding of CAM and SFDC levels. The problems of equipment maintenance, online machine tool designation and control over the production flow forced the integration of tasks that were formerly distributed between the two levels. The fact is that shop floor data emerge from the production process so they need to be treated at the same time and in the same place as the production data. To deal with this fact in an effective way the DNC concept was broadened to meet the requirements of machine tool and production data acquisition as well as to fulfil some present needs of production environments [1,2]. These are mainly caused by the more and more important role of maintenance tasks, which are nowadays already incorporated into production plans. In this manner the DNC concept was not only broaden—it got the completely new meaning,
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Fig. 1. The DNC concept of the third generation.
so we can talk about a new DNC concept, a DNC of the third generation. The basic idea of this “next generation approach” is to upgrade the usual CNC control with a part of the technological database (TDB), and to connect the CNC control with an DNC computer via the database management system (DBMS) of the TDB as it is depicted in Fig. 1.
2. A structure of the model Since the model of the data flow deals with lower four levels it does not concern the design of subjects of production. This is already a fact in this stage, so from here on means and methods for their production have to be found. To this purpose a structure of the technological database plays a main role [3]. It stores data of all production parameters as well as data about their momentary availability. This is enabled by the idea of database’s key information. The key information of the technological database is a work operation. The work operation is a basic entity of the TDB. It pools together data from databases tables that form information about the momentary state of production resources. Such a structure enables the TDB to act as work operation’s magazine, e.g., all possible work operations in a production system are stored in the magazine from which they can be assigned to different work orders. A work operation once assigned, becomes unavailable to any other order until it is completed at the actual one. The described principle is the basic tool for production planning, and represents a unique way of queue scheduling. The key information/work operation is composed out of the TDB’s data that have to be structured in the proper way. The data of production resources are pooled into five groups: • machine tools, • tools, • operators,
• auxiliary means, and • maintenance means. Each group can consist of different subgroups/data depending on the type of work operation that it describes, e.g., the tool can furthermore consist of a handle, and a cutting plate. The further division is not so important any more since it only gives a detailed picture of a work operation. To understand a selection of the five groups of production resources, the Fig. 2 should be described in more details. A production process always starts with an order that describes the subject of the production, and consequently defines work operations. To complete, and control a work operation some production/information resources are needed, e.g., a machine where a manufacture will take place, an operator to control and serve the machine, a tool to machine with, and some auxiliary devices to hold and manipulate a work piece. All the resources involved will eventually need some maintenance and corresponding means. The activity of resources associated into the work operation influences the new value of the finished order/work part. Therefore, is the combination of these resources’ data key information of the production process that is mirrored in the technological database. In our research are work operations usually described by use of DIN 8589 notions as it is depicted in Fig. 3 [4].
Fig. 2. The composition of the key information.
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• stoppage, defect occur on the planned machine tool, • free working capacities on similar machine tools and reduction or even removal of bottle-neck is desired, • changes of the priority of orders (urgent order), • unavailability of proper working devices or tools specifically connected with a certain machine tool.
Fig. 3. Work operations are composed according to the notions in DIN 8589.
2.1. Finding a machine tool in a case of known technological process During planning of technology the technological process is determined, and the work operations required by the shape and geometry of the product are selected. The operations are mainly defined by describing (to be face milled, to be milled up to depth etc.). Such type of describing however makes the processing by known programme tools on computers difficult. From the known standards it is possible to conclude that the area of describing the work operations has been studied very precisely (Table 1). Such a manner of designating the work operations makes describing of operations considerably easier. Thus the technologist must use the standard designations. Graphic devices on computer can be of great assistance to him. We can obtain the following data resulting from the CAP module: • designation (serial number) of order for a certain period of planning, • unambiguous designation of order, • code or description of work piece (s) in order, • number of work piece (s), • required NC-programmes for certain operations on certain units, • description of standard operations according to DIN standard, • anticipated start time, • anticipated calculated time for order, defined in advance. When an order has already been introduced into production, it is often necessary to search for an alternative machine tool for certain operation or group of operations. Typical examples are: Table 1 Example of survey of feasible work operations for machine tool DIN 8589 notion
Description
3.2.2.1. 3.2.2.2. 3.2.2.3. ... 3.2.3.1. 3.2.3.2.
Group of operations of face sinking Group of drilling operations Group of thread-cutting operations ... Plane milling Milling of round shapes
Automated searching for an alternative machine tool during the production management process is certainly the biggest wish of production managers. We can quickly conclude that complete automation isn’t possible since it is almost impossible to describe a work piece when searching for solutions of the problem presented across several factors by which the final solution—if existing at all—is conditioned. During the stage of planning it is possible to work out several alternative procedures, which are optimised regarding the: • most favourable version with regard to time or, • economically most favourable version. If the number of these versions of working processes increases, such manner of planning usually becomes economically questionable. It can also happen that it cannot be applied because it is too time-consuming [6]. In parallel it is necessary to prepare the conditions for execution of certain order on several units of the production system. This applies particularly for orders of high priority. The appropriate software can be of great assistance to the technologist/operator for determination and/or selection of the alternative machine tool. In our model several interfaces included into the database management system ensures that suitable machine tool can be found. They compare the operations required for execution of the order with the spectrum of operations that the machine tools can perform. The result is a proposed scenario, which the production manager has to confirm before it is activated. The structure of the TDB is also used in our model for evaluation of the general order, prescription of work operations, and designation of production resources. The model has three stages from evaluation of the general order to composition of work orders: • technological evaluation of a general order, • composition of a work order that consists of several designated work operations, • composition of a production scenario based on work operations prescribed in the work order. Tasks of each stage take places in different CIM levels (Fig. 4). The CAP in the system level performs technical evaluation. It is done by an analyse of: • • • •
the subject of production, a production time frame, the order’s importance, special demands for production resources (accuracy, handling, etc.), and • costs produced by the subject’s manufacture [7,8].
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Fig. 4. The data flow is divided into the off- and on-line part.
The cell level (PPC) is responsible for the composition of work orders since it has instant access to production resources data.The work order composition requires data about: • production resources (the operator, the machine tool, tools, auxiliary means, and maintenance means), and • their availability (occupation time, release time). Work operations are composed, according to the production demands prescribed in the general order and production possibilities found in the TDB. The beauty of this approach is in the fact that the same work operation can be composed of different production resources, what is especially important in cases of disturbances and resources’ failures. The workstation level (CAM) and later on the hardware (SFDC) level takes care for the composition of production scenario. This task requires data about a momentary availability of production resources. The first scenario is composed upon the availability data in the moment of its launching. When the necessities for changes occur a new scenario is created for the reminder of the order upon the new availability data. Therefore, it is inevitably to have an on-line access to the availability data stored in the SFDC part of the TDB.
3. The data flow through the model’s structure After the analyse of the general order when it is already established which approaches are most suitable for its manufacture in the prescribed time-frame, the production resources need to be designated to different parts of general order thus defining partial work orders. This is done by inquiries in the technological database and assigning the chosen resources to the new work order. The search mechanism can be made in different ways, among which simple SQL queries are commonly used, but the genetic algorithm search is definitely more interesting and in long term also more promising. Namely, the SQL is inevitably bound to the use of relational databases, and requires more complicated automation techniques [9]. Although less known and far less investigated, the
genetic algorithm search is very robust and independent of the type of a databases structure. On the other hand the search speed when using GA, reduces dramatically with increased complexity of the search field and number of fitness functions [10]. After a decomposition of the general order into work operations and composition of work orders, priorities are assigned to the work operations, what will assure their position among other work orders that are already under treatment in the production system (CAM). The last step to do is a composition of production scenario for the general order. It means, that the succession of work operations has to be established to prescribe the proper material flow. The scenario depends of the momentary conditions in the production system therefore, an insight into it is needed. The SFDC module of CIM is responsible for the overview of the production circumstances. It handles it in the connection with the TDB, where the availability data are written in terms of occupation time frames and condition data (ready, broken, engaged, etc.). To this part all the work can be done off-line by more or less simple TDB inquiries, however from this point on an on-line access to the production data is needed. The scenario strongly depends of momentary production circumstances, and has to be changed when unplanned situations occur. Therefore, a mechanism for automatic scenario composition has been made. It uses a GA optimisation method to search among available production resources [11–13]. When a suitable combination is found a new work operation is composed to exchange the actual one in a case of failure [14].
Fig. 5. The role of the DNC terminal.
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although its structure must be similar enough to assure the optimal connectivity. Therefore, the database in the CNC is structured according to the same principle of key information as the TDB in higher levels (as described before). Nowadays DNC systems are usually a mixture of: • terminals to connect to the machine tool, • network or serial wires and • a computer that connects the whole bundle together.
Fig. 6. CNC upgraded with the TDB.
4. Communication in CAM level After the definition of all the methods and tools for automatic scenario composition we can focus on the data flow in the CAM level. As it’s been said in the beginning, the tasks of the SFDC level can be integrated into the CAM level thus giving it a new meaning and a definition. Integration of the two levels is only possible if we transfer the SFDC tools along with the tasks to the CAM level. Having the idea of TDB in mind a new solution occurred that newly defined the meaning of direct numerical control. . 4.1. The DNC concept of the third generation The key point of the new DNC concept is in the idea of database integration into the CNC control device. Since modern CNC machine tools are already equipped with powerful computers, which are capable of demanding graphical and numerical operations, we can also equip them with a technological database that will store data crucial to the machine tool and operations performed on it. This idea comprises with the concept of distributed database and gives some new possibilities to the production managers. As soon as we equip a machine tool with a database it becomes an independent entity of the production process in the informational sense. As such it gains a certain degree of autonomy and can therefore, act as an informational resource (object), and not only as a production mean. What does a production system gain from such an “upgraded” CNC machine tool? In the first place the interconnectivity among other machine tools and the conducting computer is simplified. Special connection terminals are not needed any more, and the data flow gains both the bandwidth and the speed of data flow. On the other hand the database in the CNC is always up-to-date, because every input to the database is also an input to the machine tool and can therefore, be verified by use of different sensors. It is clear that the database in the CNC won’t store the same data as the database in higher levels of CIM. Its content is much simpler
The terminal is a part of modern DNC systems and usually serves as a connection between the machine tool and the computer network (Fig. 5). It is connected to the CNC control via the RS 232 connection and to the DNC computer via the network cable. Network protocols in use range from special industrial types to the TCP/IP, which becomes more and more popular since it enables an intranet connection among the machine tools. There are also known DNC solutions that use PC’s as the terminals or even connect the machine tools directly to the DNC computer via the RS 232 connection. In the last case the terminal is not needed but the solution is very sensitive for any type of electromagnetic interference. Whatever the solution is it always uses a central computer to conduct the work of machine tools. It is usually equipped with some form of logbook and interfaces that serve for data transfer between the computer and the terminal on the machine tool. Data flow is usually limited to NC programs transfer to and from the machine tool and to acquisition of simple machine data. All other work from a supervision of tool stores to NC programs distribution is also handled in the conducting computer. The new approach described here as the DNC concept of the third generation goes far beyond this scope of DNC tasks. By incorporating the TDB into a CNC control the latter becomes enough data to perform some more sophisticated operations that can be included into the database management system in a form of special interfaces (Fig. 6). One of these tasks is a post procession of tool path files. It is usually done in the CAP level as a part of computer aided NC programming where postprocessors are ran in the frame of CAP software. But it’s easy to move the postprocessor to the CNC control if there is a possibility to create a simple interface for this purpose. Another and nowadays more and more important possibility is inclusion of some maintenance tasks into the CNC control. The fact is that the maintenance of equipment became very important with increased machine tool’s complexity and qualitative demands of the market. Nowadays, a modern company can’t afford longer production cut outs because of insufficient maintenance or unforeseen breakage of parts. Even more, maintenance became a production category that does not bring costs any more but even makes profit. In this manner several techniques were developed among which the total productive maintenance (TPM) is most widely used. By studying these techniques one can ascertain that for an effective preventive maintenance a good and up-to-date documentation is needed. Another conclusion, which one can
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make in practical production circumstances, is that even if the documentation is properly taken for it is stored in a totally inappropriate place. So, a logical place to store documentation is the machine tool, which is on the other hand physically inappropriate for several reasons. The only exception to this rule is the control device if it is equipped with the database. Another important feature of the database incorporated into the CNC control is the possibility of distributed tool storage system. In this case the tools are stored in machine tools’ work stores and therefore, distributed over the shop floor. Their data are stored in the TDB of each CNC machine tool and thus always present in the TDB of the DNC computer. The special tool store is not needed any more instead of it we rather put a small spare parts store that could also serve as a buffer zone in a case of redundant tool demands. 4.2. Data structure of the CNC database The database of the machine tool consists only of data that occur in it. It means that the data that concern certain machine tool won’t reach the database of any other machine tool. The machine tool data can be divided into: • constant data and • variable data. Constant data are those, which define the machine tool—a kind of identity card. The variable data, on the other hand are those, which define the work operation performed on the machine tool. Constant data: • • • •
name of the machine tool, type of the machine tool, size of the workspace, production restrictions. ◦ feed rate (min/max), ◦ spindle speed (min/max), ◦ amount of memory. . . • type of tool clamping device, • power of the main drive, • amount of the coolant. . . Variable data: • • • • • •
work order’s ID, work operation’s ID, NC programme, list of tools in the work store, type of the coolant, type of the clamping device. . .
These two groups of data also have three subgroups that serve to some special purposes yet not present as a part of the DNC. These are: • maintenance data, • work process data and, • logbook data.
As the name tells the maintenance data should enable fast and effective maintaining in a case of planned preventive maintenance as well as in a case of unforeseen corrective maintenance. Among these are: • • • • •
a list of check points, a list of lubrication places, a list of lubricants, a list of wearing parts, a list of maintenance intervals for machine parts.
This group is a subgroup of the constant data with a small exception. Namely, a list of checkpoints can optionally contain a variable data of momentary condition, which can change, according to the condition of the checkpoint. In this case of course the machine needs to be equipped by an appropriate set of sensors to on-line control the checkpoints. The next group is a subgroup of variable data. It consists of usual DNC data: • • • • •
working condition (on, off, tool change, . . .), start time, end time, disturbed (tool break down, failure, error. . .), etc.
The last group is a hybrid between the constant and the variable data group. It stores the history of past work and events done on the machine tool. These events are either the consequences of work operations or the maintenance operations carried out on the machine tool. They are therefore, further divided into two groups: • • • • • • • •
the history of work operations, the history of maintenance operations. The history of work operations is logged by following data: work order’s ID, work operations’ IDs, start time, end time, number of produced items. . .
With these data, one can estimate the wear of tools, coolant and other important factors. The history of maintenance operations consists of: • • • • •
maintenance task (ID), start time, end time, used maintenance means, amount of means used.
These data enable effective estimation of maintenance costs, and also give a historic overview of the condition based preventive maintenance tasks as the basis for the proactive maintenance. Fig. 7 shows the types and subtypes of preventive maintenance, which are in a common use nowadays. In practice all these types are known as preventive maintenance and are all included into the philosophy of TPM, which is the leading maintenance idea in the modern industrial world
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4.3. The data flow in the DNC
Fig. 7. Diagram of preventive maintenance.
Fig. 8. The process matrix.
[15]. According this idea, some maintenance tasks are transferred from the maintenance experts to the operators at the machine tool. This transfer shortens the cut out times in a case of small machine errors and gives the operator greater responsibility and better overview of the production flow. The result is satisfied operator, higher quality of production and more competitive company. The common problems in the TPM approach are human errors caused by forgetfulness and other subjective errors. To overcome such obstacles, people invented checklists and similar documents that have to be filled out by the operator, but they bring more “pain than gain” because they mean another “unnecessary” task. By implementation of corresponding event-driven interface into the CNC control device the annoying checklists are moved from the operators sight into the control where the data can be promptly stored and retrieved either by the maintenance expert, operator or the production controller responsible for his or her shift.
The next important issue to explain is the dataflow between the DNC computer and CNC controls. The flow is of course bi-directional but not all data are travelling here and there all the time. A careful reader has already ascertained that the variable data will do the most of the traffic. They are subjects to changes in the production plan, so they have to change every time the scenario changes. If this change is significant it would cause many changes of variable data in TDBs in CNC controls. But the scenario changes according the momentary situation in the shop floor, which is reflected in the work process data, and, according the disposable resources–machine tools. The last are described by the constant data, so the DNC’s database management system (DBMS) has to acquire all these data every time the scenario change is required. The scenario changing process is based on the principle of process matrix, which is in fact a table with work operation in its cells. Every column of the matrix represent one stage of the production process and every row represents one of all possible scenarios (Fig. 8). Work operations are put into cells by means of the genetic algorithm, which becomes an overview of disposable production resources by checking the variable and constant data groups stored in the local, CNC technological databases (Fig. 9). Since the genetic algorithm runs constantly it always fills the matrix by the best momentary solutions, among which the production manager can chose one, according his momentary wishes. The necessity to change the scenario usually occurs in a case of serious disturbance, like machine break down or some other malfunction. In such a case, a process will have to terminate the operation where the malfunction occurred and to continue with an alternative scenario for all the units in production that reached the malfunctioned stage (Fig. 8).
5. Conclusion An effective approach to the production planning tasks is nowadays a crucial factor for a competitive company in
Fig. 9. Data flow in the DNC system.
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a modern global market [16]. Shop floor is a place where a new value is added to the raw material. Therefore, will quality and accuracy of the production process in the shop floor directly affect the costs and indirectly the profits made in the market. The described model not only gives a possibility for partial automation of time-consuming processes, but rather tends to introduce new methods and principles of process planning tasks. The presented idea of the technological database is unique in its structure, which gives a possibility of many different data manipulations that can be used for various automation purposes. It is suitable also to be incorporated into the CNC control device, what gives some new control possibilities to the operator and for the first time enables the production manager to gather the actual production data by a mean of simple queries incorporated into the database management system. The wealth of information gathered this way is far beyond the scope of DNC systems known in a nowadays market. The operator can combine different data to get any important conclusion he or she needs. Besides, the technological database is open and can be upgraded with different data sets to give a user a possibility to build his own information frame. The main goals that we are trying to reach by our research work are: • reduction of preparation times, and • speeding up the decision making process. The model rather tends to support the operator in the decision making process than to replace him or her. It is not a kind of so-called intelligent interfaces because the intelligence has to be brought into it by the operator. It is on the other hand a very powerful automation tool that prevents most of errors that usually occur as an outcome of sequential human errors. The structure of the model guarantees that all influencing production factors will be promptly taken into account thus assuring a precise planning and accurate scheduling, what brings an important competitive advantage in nowadays market. The presented DNC system represents a base for the development of a new generation of DNCs, which demands a new approach to the development of CNC control devices. It is still mainly focused to the machine tool movements and
to user friendliness. But the potential of the computerized numerical control are far beyond that. The incorporation of technological database into the control brings many important advantages, among which the possibility of on-line tool database is maybe the most transparent.
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