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1、Theoretical Computer Science 253 (2001) 6193Developing a Hybrid Programmable Logic Controller Platform for a Flexible Manufacturing SystemHenning Dierks 1University of Oldenburg, Fachbereich Informatik, Postfach 2503, 2900 Oldenburg, GermanyAbstract: In this article, we present the design and implem

2、entation of a flexible manufacturing system (FMS) control platform based on a programmable logic controller (PLC) and a personal computer (PC)-based visual man-machine interface (MMI) and data acquisition (DAS) unit. The key aspect of an FMS is its flexibility to adapt to changes in a demanding proc

3、ess operation. The PLC provides feasible solutions to FMS applications, using PC-based MMI/DAS, whereby PLCs are optimized for executing rapid sequential control strategies. PCs running MMI/DAS front-ends make intuitive operation interfaces, full of powerful graphics and reporting tools. Information

4、 from the PC can be distributed through a companys local area network or web using client-server technologies. Currently, with the convergence of underlying microprocessor technology and software program-ming techniques, many users find that PLCs provide a cost-effective solution to real-time contro

5、l in small- to medium-sized process plants, especially when combined with supervisory PCs using hybrid systems. The major work of this article demonstrates that PLCs are responsive to rapid and repetitious control tasks, using PCs that present the flow of information automation and accept operator i

6、nstructions, thereby providing the user a tool to modify and monitor the process as the requirements change.Key Words: PLC、FMS、PC.1IntroductionIn a variety of product manufacturing industries, the most automated form of production is a Flexible manufacturing system(FMS),first introduced in 1970s. Si

7、nce the FMSs can providea high potential for productivity improvement in batch manufacturing, the number of FMSs is growing substantially (Groover and Zimmers, 1984). The acceleration throughout the world is due to increased global competition, reduced manufacturing cycle times, and cuts in producti

8、on costs.Generally, an FMS consists of a group of machines or other automated work stations, which form into modular subsystems, such as CNC machines, robots, vision systems, and a process station. These are interconnected by a materials handling system and usually driven by a computer(Maleki,1991).

9、Each modular system requires an individual modular control system, with different components being controlled by individual controller units. All of the modular subsystems are controlled by computers as usual. These controllers perform their intended tasks under supervision of a higher level control

10、ler. To the system, both the control devices as well as the flow of information need to be automated. The key aspect of an FMS is its ability to adapt to changes in the control tasks. This flexibility includes the quantities and varieties of part types which it can produce, the order in which operat

11、ions may be performed, and its ability to reroute parts back into flow paths. In the end, the control platform should have the capability to automate the flow of information.Typically, there are three types of control platforms used in FMSs: minicomputers, microcomputers, and PLCs (Maleki, 1991). Th

12、e minicomputers are best suited for complex large-scale, continuous ,regulatory control applications . The PLCs are used for rapid and repetitious logic control. Personal computers (PCs) are suited for operator interface functions. Primarily, PLCs are designed to replace hardwiring relays, to operat

13、e in an industrial environment, to be easily modified by plant engineers and maintenance personnel, and to be maintained by plant electricians. Currently, with the convergence of underlying microprocessor technology and software programming, many users find that PLCs provide a cost-effective solutio

14、n to real-time control in small-to medium-sized process plants, especially when combined with supervisory PCs using hybrid systems.The purpose of this article is to address the state-of-the-art technology of FMSs. The design and construction of an FMS using PLC-controlled and PC-based visual man-mac

15、hine interface(MMI) and data acquisition system(DAS) are presented. It is organized as follows. Section 2 begins with the description of the FMS on the factory floor of the Center for Manufacturing System sat the NewJersey Institute of Technology(NJIT).Section 3 shows the operational description of

16、the FMS. Sections 4 and 5 present the applications of PLC-controlled and PC-based MMI/DAS for the FMS at NJIT. Section 6 contains a summary of the advantages of this PLC-controlled and PC-based MMI/DAS for FMS application.2. Description of the FMSSI handling conveyor systemThis consists of four cart

17、s, A, B, C, and D, with fixtures mounted on each, two transfer tables,TT1 and TT2 , and dual conveyors which transport materials to each workstation.Figure 1. Flexible manufacturing system.NASA II CNC milling machineThe milling machine accepts rectangular solid blanks and machines each part of diffe

18、rent types according to its computer controller.GE P50 robotA shared robot is used to load and unload the material between the CNC milling machine and the conveyor system, and between the parts presentation station and conveyor system. It contains five computer programs assignable by the PLC. The co

19、mputer programs direct the robot to load the material between the parts presentation station and the carts and between the CNC machine and the carts. The last two programs place the completed parts in the accept or reject area.Parts presentation stationThis station includes a gravity-chute, which su

20、pplies rectangular solid blanks as raw materials. This station also contains two bin types, one each for accepted parts and rejected parts.Computer vision systemThe vision system provides for the visual automated inspection of the parts. It is a menu-driven, 64-level gray scale, edge detection syste

21、m.Drilling machineAn IBM7535 industrial robot with an automated drill as an end-effector drills various holes in the parts as directed.In summary, the FMS has two robots, one CNC mill, a material transfer convey or system including transportation carts and positioning limit switches, and a vision sy

22、stem, which are supervised by a GE-Series Six PLC and monitored by a PC-based visual MMI/DAS.3. Operational descriptionThe working cycle for this FMS proceeds in the following manner:1.Initially, all four carts on the conveyor system are empty and available for the raw materials to be loaded onto th

23、em from the parts presentation station.2.The GE robot loads four parts, one by one, on to the four carts on the convey or system. The carts move clock wise as they are being loaded.3. Figure 2 shows the positions acquired by the four carts once the four parts of different types have been loaded.4. T

24、he IBM robot drills various holes on each blank part as the cart stops at the drilling machine.5. The GE robot moves to the conveyor, removes the part from the cart at position X1,and loads it into the fixture located on the CNC machine table.6. Once the part is loaded on the CNC milling machine, th

25、e robot retracts, and the milling machine mills the rectangular part as required.7. After the milling operation, the robot arm moves to the milling machine to remove the part that was machined from the holding fixture.Figure 2. Loading state of the conveyor system.8. The robot returns the finished p

26、art to the same cart on the conveyor.9. A signal is sent to the vision camera to inspect the part.10. The vision system analyzes the part and outputs a signal that directs the robot to accept or reject the part.11. The robot runs either an accept program to place the part in the accept bin or runs a

27、 reject program to place the part in the reject bin.12. The GE robot goes to the parts presentation station and loads a new blank part into the cart.13. The cart is released to the system and the next cycle is started.4. Control of an FMS with a PLCThe significant features of the FMS control system

28、are as follows:1.The system is easy to configure and to modify to accommodate changes and updates, because of the ladder logic capability of the system.2.In a similar manner, the system is easy to program and document.3.The system can be easily maintained, and troubleshooting is decreased because on

29、-line diagnostics are provided to pinpoint problems and decrease maintenance.4.Naturally, the system is readily interfaced with the computer.The PLC is a general purpose industrial computer which is widely used in industrial process control. It is capable of storing instructions to implement control

30、 functions such as sequencing, timing, counting, arithmetic, data manipulation, and communication to control industrial machines and processes. The PLC is chosen to perform an FMS control task based on the following features:1) good reliability;2) less space required and operates in an industrial en

31、vironment;3) easier to maintain by plant engineer or technician;4) can be reprogrammed if control requirements change;5) can communicate and network with other computers.In this application, a GE-Series Six PLC is equipped with a memory bank, and the I/O racks are loaded with the following input and

32、 output interfaces: 120 VAC input modules with 8 ports/module, 24 VDC input modules with 8 ports/module, and 120 VAC output modules with 8 ports/module.5. PC-based visual operator interface unitWith the convergence of microprocessor technology and software techniques, the PC has become very useful i

33、n operator interface applications. PCs running MMI/DAS front-ends make powerful, intuitive operation interfaces, full of useful graphics and reporting tools. Information from these PCs can be distributed through a companys local area network(LAN) or web using client-server technologies.A PC-based vi

34、sual MMI/DAS was developed to monitor the process and log data. The functions of the MMI are twofold. First, it opens a window between the operator and the process and then displays the process information on the CRT. It also allows the operator to modify the time delay constants or alarm setpoints

35、without changing the ladder logic, if the production requirements change. Second, it provides an automatic data logging device. It is capable of creating batch, shift, and day log reports. Information from the PC can be distributed through the local area network using client-server technologies. An

36、application program has been developed by using an off-the-shelf state-of-the-art GENESIS for Windows PC-based software to provide the data from the PLC through a RS232 interface. This approach allows the PC to combine the controller, the programming terminal, the operator interface, and the data ac

37、quisition system together in one unit. The PC-based MMI/DAS software provides an icon-based and mouse-driven open system for designing a real-time control strategy and dynamic operator displays. With the open architecture features, it provides support for user algorithms and LAN interfacing.The othe

38、r part of the MMI/DAS software is the enriched and user-friendly graphic builder. The graphic builder is an object-oriented CAD-based tool. The graphic tools allow the user to generate intuitive and useful man-machine interface screens to display the dynamic status of the FMS.6. ConclusionsThe parti

39、cular FMS example is fully automated by a hybrid control platform using a PLC controlled and PC-based supervisory operator interface unit and data acquisition system. The trend of flexible manufacturing demands more open system control and flexibility with affordable cost. Obviously, the size and th

40、e nature of the application affect the decision. This PLC and PC hybrid supervisory control platform provides a cost-effective solution to real-time control and automation of the flow of information for small- to medium sized process plants. The system improvements are achieved in control system rel

41、iability, equipment maintainability, software maintainability, and system flexibility. The automated DAS system has the capability to generate batch, shift, and day logs reports, to report process and equipment alarms, and to refresh process data.References1 R. Alur, C. Courcoubetis, D. Dill, Model-

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