developing a hybrid programmable logic controller platform for a flexible manufacturing system

Kevin J. McDermott

Wenlong Albert Yao

New Jersey Institute of Technology, Newark, NJ 07102

ABSTRACT

In this paper, we present the design and implementation 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 process 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 from the PC can be distributed through a company's local area network or web using client-server technologies. Nowadays with the convergence of underlying microprocessor technology and software programming technique, many users find PLCs to be a cost-effective solution to real-time control in small- to medium-sized process plants, especially when combined with supervisory PCs using hybrid systems. The major work of this paper demonstrates that PLCs are responsive to rapid and repetitious control tasks, using PCs that present the flow of information automation and accept operator instructions, therefore providing the user a tool to modify and monitor the process as the requirements change.

1.

Introduction

In a variety of product manufacturing industries, the most automated form of production is a flexible manufacturing system (FMS). These were first introduced in 1970s. Since the FMSs can provide a high potential for productivity improvement in batch manufacturing, the number of FMSs is growing substantially (Groover, 1984). The acceleration throughout the world is due to increased global competition, reduced manufacturing cycle times, and cuts in production costs.

Generally, an FMS consists of a group of machines or other automated workstations, which form into modular subsystems, such as CNC machines, robots, vision systems, and process station. These are interconnected by a materials handling system and usually driven by a computer (Maleki, 1991). 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 controller. 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 it can produce, the order in which operations 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). The minicomputers are best suited for complex, large-scale, continuous, regulatory control application. 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, and to operate in an industrial environment, to be easily modified by plant engineers and maintenance personnel, and to be maintained by plant electricians. Nowadays with the convergence of underlying microprocessor technology and software programming, many users find PLCs to be a cost-effective solution to real-time control in small- to medium- sized process plants, especially when combined with supervisory PCs using hybrid systems.

The purpose of this paper 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 machine interface (MMI) and data acquisition system (DAS) are presented. This paper is organized as follows. Section 2 begins with the description of the FMS on the factory floor of the Center for Manufacturing Systems, at the New Jersey Institute of Technology (N.J.I.T.). Section 3 shows the operational description of the FMS. Sections 4 and 5 present the applications of PLC-controlled and PC-based MMI/DAS for the FMS at N.J.I.T. It ends in section 6 with a summary of the advantages of this PLC-controlled and PC-based MMI/DAS for FMS application.

2. Description of the FMS

A view of the developed FMS is shown in Figure 1. It consists of the following components: (McDermott and Kamisetty, 1991, McDermott, 1987).

Figure 1. Flexible manufacturing system.

SI Handling Conveyor System This consists of four carts, 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.

NASA II CNC Milling Machine The milling machine accepts rectangular solid blanks and machines each part of different types according to its computer controller.

GE P50 Robot A 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 computer 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 Station This station includes a gravity-chute, which supplies rectangular solid blanks as raw materials. This station also contains two bins type, one each for accepted parts and rejected parts.

Computer Vision System The vision system provides for the visual automated inspection of the parts. It is a menu-driven, sixty-four level gray scale, edge detection system.

Drilling Machine An IBM 7535 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 conveyor system including transportation carts and positioning limit switches, and a vision system, which are supervised by a GE - Series Six PLC and monitored by a PC-based visual MMI/DAS.

3. Operational description

The 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 them from the parts presentation station.

2. The GE robot loads four parts, one by one, onto the four carts on the conveyor system. The carts move clockwise as they are being loaded.

The above steps (1) and (2) constitute the "loading state" for the FMS.

3. Once the four parts of different types are loaded, the positions acquired by the four carts are shown in Figure 2.

Figure 2. Loading state of the conveyor system

4. The 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, the 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.

8. The robot returns the finished part 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 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.

Steps (3) to (13) constitute the "working state" of the FMS.

4. Control of an FMS with a PLC

The significant features of the FMS control system are as follows :

· The system is easy to configure and to modify to accommodate changes and updates, because of the ladder logic capability of the system.

· In a similar manner, the system is easy to program and document.

· The system can be easily maintained and troubleshooting is minimized because on-line diagnostics are provided to pinpoint problems and minimize maintenance.

· 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 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:

· Good reliability.

· Less space required and operates in an industrial environment.

· Easier to maintain by plant engineer or technician.

· Can be reprogrammed if control requirements change.

· 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 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.

Table 1 summarizes the FMS devices and equipment which are monitored and controlled by the PLC system.

Table 1. Input/Output ports of FMS.

Input Monitoring Ports

Output Control Ports

Limit switches status

Electric solenoids

Pushbuttons and selectors switches and emergency stops

Electric motors

Robot output status

Robot commands

CNC status

Conveyor control

System status

Computer vision commands

Switches are wired as either normally open (NO) or normally closed (NC) in the same method one would use in a relay system. Start switches are wired as NO, stop switches as NC, and selector switches are wired to provide a closed contact in the selected position. This wiring convention follows normal plant methods to ensure safety and make troubleshooting more logical.

The FMS control program ladder logic is developed and documented on the GE Workmaster system using an industrial personal computer and the GE proprietary software package Logicmaster 6 system. The Logicmaster 6 system can be used to develop ladder logic off-line or connected to a PLC on-line to provide continuously updated displays of reference tables and program logic. The logic display features symbolic power flow through the rungs, so that program execution can be traced. Figure 3 illustrates one rung of the FMS program printout. There are 117 rungs in the ladder logic program.

Figure 3. Typical rung of the FMS ladder logic.

Rung 20, when activated, allows the cart to move to the next station. The logic of rung 20 states that if input microswitch I0020 is closed or microswitch I0018 is closed and I0006 is not open then the output port O0014 is energized. The input microswitch I0020 indicates that the cart is in the correct position. The other input port I0018 allows the operator to override this condition, usually when maintenance of the system is being undertaken. Input I0006 inhibits the movement of the cart to the next station if another cart is present at the station. Output port O0014 sends a signal to energize solenoid 440 which releases the cart so that it can proceed to the next station.

5. PC-based visual operator interface unit

With the convergence of microprocessor technology and software techniques, the PC has become very useful in 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 company's local area network (LAN) or web using client-server technologies.

A PC-based visual 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 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 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 acquisition 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.

Figure 4 illustrates a typical application program for the FMS operator interface and data acquisition system. Here the DEV1 block denotes the PLC hardware. The DIN blocks are the digital input signals from the PLC which show the status of the microswitches of the FMS. The DOUT blocks are for digital outputs to the PLC. It allows operator to control the FMS from PC panel and keyboard. The AIN blocks stand for the analog inputs from the PC display screen to download the timer delay setpoints to the PLC registers from keyboard to meet the demanding of process change. AOUT receives the input value from the AIN block and then downloads the value to the PLC registers. The program is easy and quick to integrate under the PC windows environment. It looks like a group of block diagram linked together. With the enriched function blocks selection, including mathematics, statistics, and PID control, it provides outstanding flexibility for application strategy without any high programming skill. To create an application program is simply clicking on the desired function block icons, position them on the screen, and connect them.

Figure 4. Typical GENESIS application.

The other part of the MMI/DAS software is the enriched and user friendly graphic builder. The graphic builder is an object-oriented CAD-based tools. The graphic tools allow the user to generate intuitive and useful man-machine interface screens to display the dynamic status of the FMS. Unlike other pixel-based system, to create a display screen is simply by selecting a varying tools from the menu. The drawing tools provided include: lines, bars, boxes, circles, ellipses, arcs, area fill, and text. The created display screens can dynamically connected to the PLC register database. During runtime, the MMI/DAS provides intuitive operator displays, data logging, on-line historical replay, real-time trending, alarm management, operator events, as well report, recipe configuration and networking functions.

6. Conclusions

The particular 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 the nature of the application affects 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-sizes process plants. The system improvements are achieved in control system reliability, equipment maintainability, software maintainability, and system flexibility. With the automated DAS system, it has the capability to generate batch, shift, and day logs report, to report process and equipment alarms, and to refresh process data.

References

Bryan, L. A. and Bryan, E. A., "Programmable Controllers," Industrial Text Co., Chicago, IL, pp. 1-10, 1988.

Brown, J. et al., "Classification of Flexible Manufacturing System," The FMS Magazine, April 1982.

Buzacott, J. A., "The Fundamental Principles of Flexibility in Manufacturing System." 1st International conference on FMS, Brigton, 1982.

Groover, M. P., Zimmers, E. W., Jr., "CAD/CAM Computer-Aided Design and Manufacturing," Prentice Hall, Englewood Cliffs, NJ, Pp. 227-228, 1984.

Maleki, Reza A., "Flexible Manufacturing System," Prentice Hall, Englewood Cliffs, NJ, Pp. 90-100, 1991.

McDermott, K. and Kamisetty, K. V., "Development of an Industrial Engineering Based Flexible Manufacturing System (FMS)," Industrial Engineering, 1991.

McDermott, K., "An Expert System Based Flexible Manufacturing Cell," IBM Manufacturing Technology Digest, Vol. 5 No. 1. Pp71-75. 1987.

Studebaker, Paul, "PLC or PC?" Control Magzine, Vol. IX No.11, November, 1996, Pp24-30.

Yao, W. L., "Development of a Flexible Control and Automated Data Acquisition System for Wear Testing," Thesis, New Jersey Institute of Technology, NJ, 1990.

Biographies

Kevin J. McDermott is the Director of the CAD/CAM Robotics Consortium and an Associate Professor of Industrial and Manufacturing Engineering at New Jersey Institute of Technology. His research activities include the analysis of industrial robotics, flexible manufacturing systems, and expert and vision systems in computer aided design and manufacturing. He has published over 50 technical papers and made over 100 technical presentations for the IIE, IEEE, ASME, SME, ANS, ASEE, and IBM. Dr. McDermott is a senior member of the IEEE and SME and has developed NJIT Robot Training Laboratory, Robot Research Laboratory, and Flexible Manufacturing Laboratory which contain over $3,000,000 in CAD/CAM equipment.

He is active with the Society of Manufacturing Engineers and was selected as a Visiting Manufacturing Fellow by the IBM Corporation. in 1987 and the Beijing Polytechnic University in China in 1988. He has several years of technical experience with the Bell Telephone Laboratories, RCA Corp., Westinghouse Corporation., and was active in the NASA Apollo program before joining the staff at NJIT. He is a licensed Professional Engineer and a member of Tau Beta Pi and Alpha Pi Mu.

Wenlong Albert Yao is currently a Ph.D. student in the Department of Mechanical Engineering at New Jersey Institute of Technology. He holds an MS at NJIT in Mechanical Engineering. He worked as an Instrumentation and Control Engineer at Allied-Signal Aerospace Garrett Turbine Engine Division, and also at an industrial automation consulting firm. He has extensive PLC and MMI/DAS experience in industrial applications. His current research is in the areas of rapid prototyping and manufacturing. He is a member of ASME, SME, ISA and Sigma Xi.