1 Introduction
DNC (Distributed Numerical Control) is called Distributed Numerical Control (DNC), is to realize the integration of CAD/CAM and computer-aided production management system of the link, is another form of machining automation.
At present, the research of DNC system still exists in the following technical problems to be solved: ① DNC system architecture is not strong openness. Most of the domestic DNC system is limited to a single supplier of manufacturing equipment, poor portability between platforms, interoperability of different applications to be improved, not conducive to system integration; ② DNC system communication structure is mostly point-to-point, or LAN plus point-to-point, can not be a good solution to the problem of communication competition; ③ DND system with the interface between the NCP and CAD is still very weak; ④ DNC system Control software reusability is not strong, object-oriented design and implementation is needed. This paper proposes a DNC system based on CORBA (Common Object Request Broker Architecture) in the shop floor control system, which gives a good answer to the above problems and realizes the preparation of software and on-line tuning.
2 control system system structure
DNC system is based on CORBA shop floor control system of a functional unit, now the enterprise is faced with a variable demand environment, and thus the shop floor control system to face the processing task is also variable. This change includes the production of parts of varieties, types, specifications, production and delivery and other factors such as changes in the processing process route with the different production tasks and so on. This requires a shop floor level control system architecture that is open in both time and space to run on heterogeneous computer systems with different hardware environments, while at the same time adapting to the development of new technologies and accommodating the addition of new equipment.
In the CORBA-based shop floor control system, the construction of shop floor information integration and *** enjoyment of the public **** platform is one of the core issues, we use a distributed control platform based on the client/server structure (e.g., Orbix), not only can transform the traditional recursive control structure into a more suitable for information integration of the distribution or control structure, but also can be adapted to the different product manufacturing processes ( Discrete manufacturing or continuous manufacturing) in the unified production management and organization requirements.
The overall structure of the shop floor control system is divided into three layers: the bottom layer is the system support layer, consisting of two sub-layers of distributed computing environment and heterogeneous network integration system, providing the bottom computer system, network system and data system and other system-level functions; the middle layer is the open distribution processing layer, providing unified integrated communication services, consisting of open distribution processing platform and application program interface. The uppermost layer is the information integration layer, which supports a multi-client/server distributed multi-database integration system to integrate existing applications and data information into the system. In order to realize the distribution of the control structure, the distribution of the database and the distribution of the system functions, the proposed control system software for the shop floor layer adopts the distributed object architecture based on the CORBA specification.
The main feature of the CORBA specification is the realization of the software bus structure. The function of the so-called software bus is to play a role similar to the role of computer system hardware bus, as long as the application module according to the bus specification made of soft plug-ins, inserted into the bus can be realized integrated operation. The realization of the core system of the software bus is called ORB (Object Request Broker), which not only supports the standard OMG object model, but also has a distributed process management and communication management functions. In addition, CORBA defines IDL (Interface Definition Language) language to describe the plugs on the software bus. IDL provides encapsulation of member systems and isolation between member systems, and any member system as an object can be connected to the ORB to provide services for other systems or make requests to other systems by defining and describing its interface parameters through IDL. services or make requests to other systems to achieve the plug-and-play effect.
The shop floor control system is divided into a number of independent functional units, each functional unit corresponds to an abstract object containing a functional interface definition and entity, the interface of each type of object consists of attributes and operations, defined by the IDL of other functional units can be transparently accessed by the service to call the object's private data, the implementation of the specific function is encapsulated in the entity. We divide each type of object into a number of sub-objects according to its function, and design them as object plug-ins that can be directly plugged into the CORBA software bus. These object plug-ins form the entire platform system according to the client/server structure of each layer. This structure can bring long-term benefits, not only can quickly increase the application of the new DBMS, add new user interfaces, but also upgrade to support a variety of new features.
3 DNC system status and functions
DNC system as a shop floor control system as a functional unit.
DNC system functions include ① NC program and data transfer to a certain communication protocol (such as Philip532, etc.) to achieve the communication function; ② machine tool status acquisition and reporting; ③ according to the process plan, the automatic distribution of NC program and data to the corresponding machine tool; ④ tool data distribution and transfer.
DNC system software function model, in which the main function of the NC data management is to manage the NC data, the main NC data display, insert, modify, delete, update, lock (does not allow changes) and print and other operations; NC data to perform the main functions are: NC data in the computer and the machine tool transfer between, delete the machine tool on the NC data, start the machine tool on the NC program, at any time from the machine tool equipment. NC program, at any time from the machine tool equipment to obtain information on the state of work and stored in the database, as the basis for running the data acquisition module to evaluate the machining process; DNC communication interface through the DNC protocol and data link protocol to establish the unit control system and CNC connection.
4 DNC system software architecture
The realization platform of CORBA-based DNC system software is built on the basis of the shop floor control system platform. We divide the DNC system architecture into a three-tier client/server structure to clearly delineate the representation logic, business logic, and data processing logic. To this end, the representation layer is used to represent information and collect data, and here is a portable DNC human-computer interface realized by VB; the business layer responds to requests sent by users (or other business services) and performs certain business tasks, and here is the VC + + + to realize the DNC due program and the NC data management application program; the data layer includes the definition, maintenance, access and updating of data as well as the management and response to data requests from business services, and here is the data layer realized by VC + +. Data layer includes data definition, maintenance, access and update and management, and respond to business data requests, here by the IDL functional interface definition encapsulation of the NC local database (Access) server. These layers do not necessarily correspond to specific physical locations on the network; they are conceptual layers with which robust, component-based applications can be developed.
Using the model shown in Figure 3, the requirements of an application can be decomposed into clearly defined services. After the services are defined, specific physical components need to be further created to implement them. Depending on performance and maintenance requirements, workload, network bandwidth, and other factors, these components can be flexibly deployed on the network.
5 Data model of DNC system software
DNC system software involves data entities that contain four categories: ① data entities related to manufacturing equipment hardware (e.g., machine tools, etc.); ② data entities related to human-machine communication (e.g., communication protocol entities and serial communication entities); ③ numerical control data entities (e.g., NC program number, tool number, and process number); and ④ input operation instruction (iii) NC data entities (e.g. NC program number, tool number, process number); (iv) input operation instructions or work order entities. The above entities are abstracted into classes using object-oriented methods, and can be divided into capability unit class, NC machine class, NC controller class, communication protocol class, terminal server class, serial communication class, NC program class, etc.
The objects in a DNC application are inherited from these classes, and the methods of each object, i.e. the member functions of the object, are defined according to the corresponding functional requirements. The following is an example of NC Machine Class definitions:
‖ ncmach.h -NC Machine Class definations
‖ NC Machines are part processors.For this class, a part is loaded,
‖ a NC file is downloaded to the device,and the machine is started.
class MACHINE-TOOL｛
char* CurrentNCFile; ‖currently loaded NC file
int FixtureStatus; ‖fixture status
public:
MACHINE-TOOL();
char* getCurrentNCFile();
void setCurrentNCFile(char?F);
Int getFixtureStatus();
void setFixtureStatus(int S);
virtual int processPart(char? PartName);
virtual int downloadNCFile(char?File);
virtual int stopMachine();
virtual in graspPart();
virtual int releasePart();
｝;
6 Basic Architecture for Physical Configuration of a DNC System
The CORBA-based shop floor control system requires two levels of interconnection. The first level is to interconnect shop floor controllers, equipment controllers, etc., consisting of heterogeneous computers, using computer LAN technology and protocol software, and the second level is to build on this interconnection to realize information interaction between nodes and controlled heterogeneous manufacturing equipment (e.g., machining centers, robots, PLCs, etc.), which is realized through the Manufacturing Information Specification (MMS). As an important part of the control system at the shop floor level, the physical configuration of the DNC system in this paper is basically structured as shown in the figure below, where the main computer is connected to multiple CNC systems through the network medium (terminal servers with independent IP addresses) to realize the loading and unloading of NC programs, the transfer of tool data, the issuance of operation commands, and the feedback of status information, respectively. This is a communication structure connected through a LAN, which has a four-layer structure including physical layer, data link layer, transport layer and application layer, etc., in which the data link layer adopts the LSV2 communication protocol, and the transport layer adopts the DNC protocol (e.g. SINUMERIK or PHILIPS protocol).