Nonblocking Supervisory Control of Flexible Manufacturing Systems Based on State Tree Structures

Nonblocking Supervisory Control of Flexible Manufacturing Systems Based on State Tree Structures

Wujie Chao (Xi’an Jiaotong University, China), Yongmei Gan (Xi’an Jiaotong University, China), W. M. Wonham (University of Toronto, Canada) and Zhaoan Wang (Xi’an Jiaotong University, China)
Copyright: © 2013 |Pages: 19
DOI: 10.4018/978-1-4666-4034-4.ch001
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Much research has been addressed to nonblocking supervisory control of Discrete-Event Systems (DES) such as Flexible Manufacturing Systems (FMS), and a variety of approaches have been developed. One especially powerful approach, due to Chuan Ma, is based on DES representation by means of State Tree Structures (STS). Using STS, this chapter develops nonblocking supervisory control of a well-known benchmark FMS example taken from the literature, for which the description was given originally as a Petri net. The authors straightforwardly obtain the optimal (maximally permissive) and nonblocking supervisory control, and display the control logic for each (controllable) event transparently as a binary decision diagram.
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2. Background Of Flexible Manufacturing Systems

In the example of this chapter, our Flexible Manufacturing System (FMS), as shown in Figure 1, is built up from ten components. They are: four machine tools M1-M4, three robots R1-R3 and three Input/Output buffers B1-B3. Three different types of workpiece enter the FMS to be processed (Li, Zhou, and Wu 2008). Each machine tool can hold two workpieces at the same time, while each robot can hold one workpiece at a time. A workpiece of type i (i = 1,2,3) enters the system from its buffer Bi (as a Bi decrement) and leaves the system through its buffer Bi (as a Bi increment); the overall model is thus closed and cyclic. In the process diagrams of Figures 1 and 2, Bi is shown as Ii in its role as supplier of ‘raw’ workpieces of type i, or respectively as Oi in its role as receiver of ‘finished’ workpieces. The buffer sizes are: 3 for B1, 7 for B2, and 11 for B3. Each size is roughly matched to the respective holding capacity of the buffer’s downstream process, a choice that renders the problem more logically demanding. A workpiece of type i undergoes manufacturing process Pi, as detailed below.

Figure 1.

Logical layout of the FMS (Li, Zhou, and Wu 2008)

Figure 2.

Production processes for the workpieces

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