A Petri-net based distributed monitoring system using PIC microcontrollers

Abstract

This paper describes the development of a distributed system for process and condition monitoring. The developed system is based on the Petri-net concept, and uses PIC microcontroller technologies in its implementation. The resulting distributed system has data communication features such as Fieldbus and Internet connectivity thus enabling the deployment of a flexible and low-cost structure for monitoring purposes. The proposed system architecture is discussed in detail, considering both the hardware and software elements involved. The implementation of the Petri-net approach in a microcontroller environment is also presented. Finally, the benefits and limitations of such a system are considered, using some initial results obtained in an experimental implementation.

Introduction

Microcontrollers are being developed with increasing processing power and flexibility. This has encouraged their use in areas where computer based systems previously represented the best available choice [1], [2], [3], [4]. One such area is process and condition monitoring (PCM). This work outlines developments using microcontrollers that will offer low-cost alternatives to previously deployed PCM systems. It is hoped that the availability and functionality of these systems will stimulate their deployment in areas that have previously been reluctant to consider their use due to the high costs involved.

PCM will be increasingly used to meet demands for lower cost and more effective manufacturing [5]. The complexity represented by modern manufacturing systems calls out for the use of online and real-time monitoring technologies to enable critical parts of the process to be assessed and to ensure that adequate actions can be taken in a planned way [6], [7]. The use of Internet based (remote) monitoring strategies is also becoming a key factor when considering global manufacturing [8]. This relatively new scenario has led machine manufacturers to review the tasks related to the provision of support to their customers, forcing them to provide new machine developments with features that include, for example, the ability to undertake remote fault analysis. However, cost is still a factor that has prevented the use of PCM technology on a larger scale [9]. It is proposed here that Microcontrollers, supported by technologies such as industrial networks, can provide online and real-time capabilities at a much lower cost.

Many processes may be described as a logical sequence of events. This has led the authors, among others, to the development and use of Petri-nets as a tool for PCM [10], [11], [12], [13]. Details of the work previously undertaken by the authors can also be accessed via the IPMM research group web page [14].

The new approach presented in this work is centred upon the development and implementation of a Petri-net based monitoring structure employing PIC microcontrollers rather than the previously utilised PCs. The paper considers, in Section 2, the configuration of the PIC based system and the monitoring and connectivity aspects developed to support this new implementation. This section also introduces the concepts of the Petri-net based approach to process monitoring. An outline of the way in which the elements of the Petri-nets are utilised is described in Section 3. This section also considers the operation of the monitoring system using a CNC machining centre toolchanger as a Case Study. The paper finally considers, in Section 4, the advantages associated with these developments and the direction that future research may take.

The aim is to offer a low-cost alternative for process and machine monitoring thus making the technology more accessible. Additional benefits arise since these developments are based upon microcontroller technology that does not demand a large number of peripheral components and can be installed and operated close to the process.