On systematic methods to remove redundant monitors from liveness-enforcing net supervisors

  • Authors:

  • ZhiWu Li;HeSuan Hu

  • Affiliations:

  • School of Electro-mechanical Engineering, Xidian University, No. 2 South Taibai Road, Xi'an, Shaanxi 710071, China;School of Electro-mechanical Engineering, Xidian University, No. 2 South Taibai Road, Xi'an, Shaanxi 710071, China

  • Venue:
  • Computers and Industrial Engineering
  • Year:
  • 2009

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Abstract

Petri nets based deadlock prevention for flexible manufacturing systems has received much attention over the past decade, primarily due to the seminal work of Ezpeleta et al. in 1995. A Petri net based deadlock prevention mechanism is usually implemented by adding monitors or control places to a plant Petri net model such that liveness can be enforced. The significance of this methodology lies in that both a plant model and its supervisor are in a same formalism-Petri nets. Due to the inherent complexity of Petri nets, in theory, the number of additional monitors that have to been added to achieve liveness-enforcement purpose for an uncontrolled plant model is exponential with respect to the size of the model. This paper first proposes a systematic method to minimize the number of additional monitors in a liveness-enforcing Petri net supervisor such that the resultant net system has the same permissive behavior while liveness can still be preserved. Furthermore, for the liveness-enforcing Petri net supervisors of flexible manufacturing systems, which have some particular property, an algorithm is developed such that more permissive liveness-enforcing Petri net supervisors can be obtained after liveness-restrictive monitor removal. Compared with the existing techniques of eliminating redundant monitors in the literature, the complete state enumeration of a supervisor is avoided, which implies the high computational efficiency of the methods in this paper. Flexible manufacturing examples are used to demonstrate the proposed approaches.