1、附录 A 英文文献 USING PETRI NETS TO DEVELOP PROGRAMS FOR PLC SYSTEMS Abstract： In this project we examined ways in which systems may be implemented on Programmable Logic Controllers. PLC systems exhibit a number of unique features, our objective was to create a practical approach to modelling systems whic
2、h would lead towards implementation on PLCs. A.1 Introduction This paper discusses a method by which Programmable Logic Controller (PLC) programs may be developed, from specification through to code, paying particular attention to the safety aspects of PLC systems. The project is a partnership betwe
3、en Systec and Sheffield City Polytechnic. A.2 Programmable Logic Controllers The PLC is a programmable device which uses an instruction set of logical commands. It is divided into three sections. 1、 Digital inputs connected to specific internal addresses. Analogue inputs can only be accepted by prio
4、r conversion to digital. 2、 Internal memory comprising timers, counters, registers and discreet internal relays. 3、 Digital outputs made up of relays, transistors and triacs and analogue outputs handled by digital to analogue converters. No standard approach to programming has been developed due to
5、the varied nature of systems. There is no standard language so most PLC programmers develop programs using Ladder Logic with no standard methodology. Ladder Logic is an attempt to simplify the programming task across a range of different PLCs. The ladder logic models the hard wiring thus modelling t
6、he solution not the problem. A simple example is the control of a start stop relay for a motor shown in figure A1- figure (1). The equivalent ladder logic looks very similar, figure A1-figure(2). This circuit is based on the start switch being connected to input number 400, the stop switch to 401 an
7、d the motor output to 436. The ladder is a low level modellingtool, the PLC program is a translation of the ladder logic into mneumonics and number that vary from PLC to PLC. Figure A1 For a Mitsubishi PLC the corresponding set of instruction is given. The PLC first scans its internal memory to esta
8、blish the state of the system. It will then run the program to establish all logical connections between input and output before actioning the outputs. In a complex control situation with a very large number of logical inputs and logical combinations it is difficult to predict the results of illegal
9、 inputs such as damaged input switches or sensors. This lack of control causes normally stable systems to behave unpredictably causing damage to plant and violating safety. A.3 PLCs in flexible manufacturing PLCs are used extensively in flexible manufacturing systems. The cross effect of one output
10、on another is difficult and often impossible to detect, consequently programmers do not take into account all possible permutations. The following example comprises three machining centres. Machine 1 is a pillar drill operating on a single axis with sensors positioned at the top and bottom of the ma
11、chine slide. Machine 2 is a planer with vertical movement and a rotating machine head with three selectable heads. Machine 3 is a horizontal miller with movement on two axis and sensors positioned at the end of each stroke. The three conveyor belts convey the components from the starting bay to each
12、 of the machining centres. Each of the components is individually coded, the code being entered into the PLC when it enters the starting bay. The component is then tracked through the system. As the component passes each of the machining centres a sensor informs the PLC of its arrival, the machining
13、 centre can now perform the appropriate operation for the component. In a system such as this it is relatively easy to loose control which results in jumped sequences, deadlock and lost components. The projects objective was to model this system. Figure A2 A.4 The Entrance Door 1 To slightly further complicate the solution for the entrance door we assort that the door is a hinged door (not sliding) thus when it opens it moves over mat B. As a consequence any person stood on mat B when the door opens risks injury. Figure A3 shows the entrance door.
