Design of quality traceability system for a kind of electromechanical products based on OPC

Lu-1u ZHOU（School of Computer，Chongqing College of Electronic Engineering，Chongqing 401331，China）

Design of quality traceability system for a kind of electromechanical products based on OPC

Lu-1u ZHOU*
（School of Computer，Chongqing College of Electronic Engineering，Chongqing 401331，China）

Aimed at the puzzle of being difficult to carry through the quality traceability for the unqualified products resulted in a great variety of electromechanical products，the paper designed a kind of quality traceability system of electromechanical products based on OPC.In the paper，it pointed out the complexity and technology difficulty of product quality tracing，discussed on the technical specification，the essence of technology，the principle of data exchange and the data access interface，researched on the framework of product quality traceability system，and designed a quality traceability system of electromechanical products based on OPC.Taken the interconnection of monitoring software system as an example，it built the system which demonstrated that the seamless integration of heterogeneous system based on OPC could overcome the puzzle of information islands resulted in device diversity and driver incompatibility.The practice shows that it has a broad prospect of application to the quality traceability system based on OPC.

Quality traceability，Information islands，Heterogeneous system，OPC，System integration

Hydromechatronics Engineering

http：//jdy.qks.cqut.edu.cn

E-mail：jdygcyw@126.com

1 Introduction

With the fast development of economy and technology，the traceability system based on the technology of bar code and RFID etc of product quality has obtained considerable progress in all aspects［1-4］.Traceability means tracking and tracing，its meaning is an event sequence of events tracing analysis by means of a clue（phenomena，such as the ID），and from which，it can find out the root cause of the incident.The tracking and tracing of products have an extremely wide coverage from raw materials to the assembly line until the finished product off-line storage，it covers lots of other links such as tracking monitoring and real-time data acquisition and so on，it deals with the puzzle of how to integrate a large number of heterogeneous software system especially in the large-scale industrial automation production，and therefore，it is not easy to the construction of product quality traceability system［5-8］.The traceability of products quality is of great significance to help enterprises solve the analysis of product aspects such as quality defects，distinguishing responsibility，production improvement，retrospective recall，inventory management，process control，quality management and false proof and so on，it is directly related to product quality，service quality and promotion of enterprise image.From the perspective of building the traceability system，the following briefly discusses the integration of heterogeneous software system based on OPC.

2 Complexity and technical puzzle

2.1 Complexity of traceability system

The traceability belongs to post-action control，and the aim is that it can find out where is the problem ofquality defects through the feedback after the event. With the help of technology such as barcode，RFID and so on，it can getall the key information of traceability by means of integrating the key data of production process，however，traceability is very complex. The traceability is divided into the forward and reverse traceability，in which，the forward traceability refers to the traceability from top to bottom up to product serial number，and it traces the product composition and the information related to the production process.The reverse traceability means that it traces the all related to the batch of components or raw materials products from the top down for components or raw materials used in the product batch，and the aim is to reduce the range of processing unqualified products or recall. In order to investigate responsibility，the traceability can be divided into horizontal and longitudinal traceability.The horizontal traceability refers to the structure on the composition from the traceability parts of products and the raw materials constitution.The longitudinal traceability is focused on product production process，the aim is to trace the product after what station，what processing，and who operate machining，operation time experience，debugging and rework process and so on，and it is directly to the quality control services.In product quality control，it needs a variety of data acquisition in the process of production，Including process control，feed check，nonconforming product control，including technology control，feeding capacity check，nonconforming product control，job sequence control，operation quality alarm，packing specification control and so on.Although the above are very complex and belongs to things in the control，but the data has its special significance for product quality traceability and avoiding quality defects in the process of production.

2.2 Technical puzzle

The key of traceability system construction is how to overcome the complexity of data exchange.It not only deals with the real time data control information of control system such as DCS、PLC and SCADA etc in bottom layer control network，but also includes non real time data management information which reflects the management status in a variety of high-level management and assistant decision system.Due to the device diversity and the driver incompatibility，it is different in structure and function for each subsystem. The communication protocol is not necessarily uniform，the data format has a great difference，and it is bound to generate a large number of the information isolated island.And therefore，it would result in being difficult to make the system integration among each subsystem of the forward and reverse traceability system as well as the horizontal and longitudinal traceability system，and to realize the exchange of the data information among them.It is just about the complexity of traceability system which has brought great difficulties to the construction and implementation of traceability system.

3 Data exchange based on OPC

3.1 Technical specification and essence of OPC

It is necessary to establish links between application software and device drivers for traceability system construction.According to the traditional method，if there are n devices and m applications，then the data exchange links among then they need the n×m pathways，and that is to say that it must develop n×m drivers.It deals with a wide range of not open and mutually incompatible driver of the product，due to the diversity of field equipment，and therefore，the link driven approach is not desirable.Aimed at the data exchange between the data source and client，The specification of OPC（OLE for process control）formulates interface standard between hardware and application software of system.It provides a unified data interface standard for field device and application software，and it can facilitate traceability system to realize openly the seamless link［9-10］.As shown in Fig.1，using client/server system and by means of OPC technology，it only needs to develop n+m driver programs that can realize the interface with OPC.OPC has the following technical characteristics.①It can realize remote communication with the aid of the DCOM technology.②It provides a standard access interface.③Based on the client/server structure，the process is transparent to the client and server，and the underlying communication is supported by COM/DCOM.④It can realize the high-speed and reliable data communication.Its advantage is that it can be independently designed，edited and debugged in separateness，and it has obvious characteristics such as easy maintenance and upgrading as well as openness.

The technical essence of OPC is that it first makes the decomposition of application functions according to the principle of decomposition and coordination，it will be divided into several independent modules those are respectively the component object model and the distributed component object model，namely the component of COM（Component Object Model）and DCOM（Distributed Component Object Model），and through the coordination of each independent module，they work together to complete the application software tasks in co-operation.The COM specification defines the communication mechanism between components，and it has the independent of language and is not dependent on any specific operating system.Each of the sub components of COM has the independent software function of oneself，and under certain conditions，the connection among them can realize the relevant functions.The COM component has client and server，the COM database provides the core services for standard implementation，and the client can achieve access for the server by means of COM interface.Because of it based on client/server model，it is strong in expansion capability，good in stability，and therefore，it can improve the interoperability and the reusability of code reusability.On COM based，the DCOM is a sort of specification and service.It can realize the communication and cooperation between different processes under the distributed computing environment，and the aim of the DCOM is that it enables COM components to take part into the transparent network protocol of network environment.The interactive entity is the COM object，and only through the interface，the clients can get the service of COM object.

Fig.1 Data exchange interface of OPC based on C/S structure

3.2 Data exchange interface of OPC

There are two sorts of data exchange interface，namely the Custom Interface and the Automation Interface.The former is a set of standard interface function of COM interface，and it is conducive to the development of the server end.As long as the standard interface function does not change，and based on this interface，it can increase the number of function to extend its interface function.With the help of development program of C/C++etc，it can make the accessing to the COM interface.The latter is actually further package on the former，and it is the standard interface defined by script based programming language.It shields the virtual function table of the former interface，and transforms the custom COM interface into the OLE interface of automation.Each one of two access interface of OPC specification has itself advantages，and the former is powerful in functions，but the developer must be proficient in COM technology.The latter has the advantage of easy development，but it is limited by the interface itself functions.Server end development of OPC specification specifies that the OPC server must provide custom interface，and also it selectivelyprovidesautomationinterface.Itcan choose any one interface to make development and access to the client program.

The object of OPC server is a sort of hierarchical structure，and it contains all the information of the server object.The relationship between layer and layer is that the Server is as a Group container，and it provides the function of creating and operating the OPC set object for the OPC client.Group is the container of Item，it provides the Item mechanism of organization and management，and it has all information of the set of objects.The Item represents a physical connection from the OPC server to the data source，and the client program is not directly accessible.It is a specific process variable and a minimum unit that reads and writes the data，and only through the set object access，it can achieve the access to data Item.The data structure contains three variables，Value，Quality and Time stamp.Although the OPC defines the COM interface，specifies the data interaction standards with the server program and customer，but it does not give the concrete implementation method of OPC.Interface can be divided into optional and required，the later contains the basic function that makes the data exchange between Server and Client program，for the former，the user can choose itself，and it only provides some additional advanced functions.According to their respective hardware characteristics in system development，the OPC service provider can realize the member function of its interface.If the server program implemented an optional interface function，then it can be judged through the query in the client end.

4 Frame of traceability system

4.1 Structural module

The traceability system adopts modularity structure，and the system consists of modules such as system settings，file maintenance，configuration settings，label printing，process management and query statistics etc. The function of each module configuration is as follows.

①System settings.The server setting module is used for setting the database name of database server and IP address，the user management is used for users to add，modify and delete operation user as well as setting up their respective permission，the data backup can make the backup database by timing automatic or manual manner，and the data recovery is used for the backup data recovery.

②File maintenance.The inventory file is used for setting the name，inventory number and other relevant information，the product structure file is used for setting the product configuration information，the production order files is used for the evolution of number setting product information，the process files is used for the related information in assembly process，the shift of archive file is used for setting the shift categories，time，personnel and other information，and the worker information file is used for setting the name，number and contact information and so on of workers.

③Configuration settings.The print settings is used for attribute setting the barcode printer，and the barcode format setting is used for setting the data content of each part in barcode formation.

④Process management.The process management is used for each process involved in the system，and for configuring of the flow in production information. The scanning of process materials is used for scanning input operation to entry into the process material.The label printing of process product is used for printing labels to the product of each production process.

⑤Label printing.The label template editing is used for setting the label paper size as well as the composition and layout of each part in label content. The label printing is used for operating the user input part information，batch information，printing number and starting the print labels.

⑥Query and statistics.The product bar code traceability is used for querying the finished product，assembly personnel，schedules and other detailed information resulted in each process.The detail inquiry is used for selecting one or more query field needed to the operating user，and for setting the corresponding field value so as to query the detail data and statistical data of all meeting query conditions，then export or print.

4.2 Integration of traceability system based on OPC

The implementation architecture of traceability system can be shown as in Fig.2，and it is a typical distributed Internet application architecture for manufacturing（called as DNA-M）.From Fig.2，it can be seen that by means of OPC it can achieve the monitoring data of all control process including data acquisition system，PLC control system，industrial control network，and Windows CE equipment etc located in the control field.In the architecture of its application，all of them can be directly connected to the DNA-M，and therefore，each of them can conveniently exchange data each other.In the DNA-M system，the technology of OLE/COM and DCOM is the footstone of OPC，the OLE/COM and DCOM fully supports the distributed application，and therefore，it is the key to realize application system seamless integration under heterogeneous environment［11-12］

Fig.2 Position of OPC in the DNA-M system

The advantage of OPC based application is that it can make the traceability system design become more flexible，and the system developers have more choice of software and equipment.The device application is not necessary to rewrite the driver program for modifying or adding of the hardware characteristics，the user only needs to write driver interface program，and then it can connect the different devices.The manufacturer only provides a set of program set and driver program of meeting the specification in Server of the OPC，and the user can easily make the program development.In the traceability system，with the help of OPC universal interface，it can be conveniently linked to the signal that reflects the related development and application software platform，man-machine interface software HMI，upper monitoring software of SCADA system，and running state of the system equipment in industrial production field and so on.The relation among transmitting signals is shown as in Fig.3.

Fig.3 Link of fieId signaI based on OPC

The function of OPC located in the traceability system between control device and control software is that the aim is to establish the interface standard among application software program communication and to unify the access specification of data information.The interface specification can be used for different platform communication between applications and also for internal communication of a single computer，and more supports the network distributed communication between applications.The unified OPC standard is in charge of the effective link between the application program in data information and the field equipment，and based on OPC，it can make effective seamless integration for different levels of data information such as business decision making，production management，process monitoring and field control etc，and it is more conducive to the realization of traceability system.

5 Conclusions

The key of realizing the traceability system is the seamless integration of software application system. With the hardware update cycle becoming shorter and shorter and the software upgrading getting more and more fast，the engineering practice shows that the hardware and software products based on the OPC interface standard and the technical specification can realize the real-time data exchange among them in plug and play，because of the equipment having interoperability.Therefore，the OPC technology has wide application prospect in the traceability system.

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一类基于OPC的机电产品质量追溯系统设计

周璐璐*
（重庆电子工程职业学院计算机学院，重庆 401331）

针对机电产品种类繁多难于对不合格产品进行质量追溯，设计了一类基于OPC的机电产品质量追溯系统。指出了产品质量追溯的复杂性与技术难点，讨论了OPC的技术规范、技术本质、数据交换原理与数据访问接口，研究了产品质量追溯系统框架，设计了一个基于OPC的机电产品质量追溯系统。以监控软件系统互连为例，搭建系统验证了基于OPC的异构系统无缝集成可克服由于设备多样性与驱动程序不兼容性导致的信息孤岛难题。实践表明：基于OPC的质量追溯系统具有广泛的应用前景。

质量追溯；信息孤岛；异构系统；OPC；系统集成

10.3969/j.issn.1001-3881.2015.12.025Document code：A

TP273

13 December 2014；revised 16 January 2015；accepted 9 March 2015

*Corresponding author：Lu-lu ZHOU，Associate

professor.E-mail：178043838@qq.com