外文翻译---基于Petri网的PLC开发程序.docx

附录a 英文文献using petri nets to develop programs for plc systemsabstract：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 which would lead towards implementation on plcs.a.1 introductionthis 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 between systec and sheffield city polytechnic.a.2 programmable logic controllersthe 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 prior 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 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 the 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 and the motor output to 436. the ladder is a low level modelling tool, the plc program is a translation of the ladder logic into mneumonics and number that vary from plc to plc. figure a1for a mitsubishi plc the corresponding set of instruction is given. the plc first scans its internal memory to establish 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 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 manufacturingplcs are used extensively in flexible manufacturing systems. the cross effect of one output 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 machine 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 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 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 a2a.4 the entrance door 1to 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. figure a3 figure a4figure a4 shows a simple ladder diagram that may have been produced by an inexperienced programmer.rungs 1 and 2 examine the environment and set internal flags, odf, open door flag, and cdf, close door flag.rungs 3 and 4 monitor internal flags and cause external action, in this case either the opening or closing of the door. simply standing on mat a causes the door to open and standing on mat b causes the door to close.a.5 modelling with petri netsthis really is a very basic solution. it shows that ladder logic is really a notation for plc programs in that is assumes the internal memory is set by external events. transitions a, d and g are not necessary for modelling of the ladder logic in that they model the environment of the door, not the door and mats themselves. observation of the net demonstrates the obvious problems with the logic. firstly the door may open or close when a person is stood on mat b, the so called hospital case. secondly the net allows for the door to be left open. the ladder logic, however would always detect a person on mat b and close the door. it is relatively easy to stop the door opening or closing when a person is on mat b. rungs 1 and 4 should be amended as shown below. this appears simple but would not work in practice. re-examine the new ladder logic and recall the operation of a plc, scan internal memory, establish logical connections, action outputs. the initial scan will detect on a, not on b, and set internal memory, odf, open door flag. a second scan will detect odfand the door will begin to open. however as soon as the door opens a fraction the person may step offmat a and start to pass through the door. a scan at this instant will detect neither a nor b, neither odf or cdf will be set and the door will halt part way open. secondly, assume that the person is patient and waits for the door to be fully open before stepping through onto mat b. a scan at this stage will set cdf, close door flag. nothing further will be effected until the person leaves mat b. subsequently at scans cdf will not be set, no one on b, rung 4 will never evaluate to true and the door will never close. the programmer overcomes this problem by latching,2 re-examine figures 1 and 2 where an output is reused as an input to latch the motor until the stop button is depressed, a note that some plcs have self latching relays. returning to the door. it is necessary to latch the open door flag until the 4 door is fully open. a door open sensor, dos, detects when the door is fully open, it resets when the door starts to close.figure a5 figure a6figure a7 shows that the open door flag is set when on a, not on b, door not fully open; and reset when the door reaches the fully open point. this net corresponds to the rung shown in figure a8. figure a7 figure a8these basic structures are then applied to the door to arrive at the net and ladder logic, below. figure a9 figure a10it is reletivelly easy to trace the operation of the door, figure a11. at normal switch on dos, odf, obf, cdf will be reset and des set. the system is safe, the door never opens when someone is stood on mat b.analysis of the net leads to a number of immediate obvious problem situations, for example, someone stepping on mat a and leaving without going through the door. in this case the door opens and stays open. there is no real problem if someone steps on mat b from the wrong side of the door.the plc programs on mat a but not through the door by using a timer. a timer is set when there is no one on mat a, no one on mat b, and the door is fully open, dos is set. the timer will reset if someone steps on mat a or mat b or not dos. if the timer is not reset and the time expires then cdf, close door flag, is set.rungs 6 and 4(iii), figure a11.figure a11we need to cater for all permutations of marking of mat a, mat b, door open sensor and door closed sensor since the setting of these parameters is outside the control of the plc program. a decision table was constructed to show the possible permutations which resulted in addition to the logic.finally we need to eater for malfunction of any sensor, mat a, mat b, dos and des. what happens if any of the sensors fail7 for example dos and des should never be set at the same time. a.6 conclusionsinitial conclusions indicate the use of petri nets as beneficial to the practical situation. software is now under development to commercialise the project.附录b 中文文献基于petri网的plc开发程序摘要：在这个项目中，我们探讨了如何在这种系统可以实施可编程逻辑控制器。plc系统展示了一些独特的功能，为我们的目标建立了一个切实可行的模拟系统。b.1 导言本文论述的是可编程逻辑控制器（ plc ）研发，不管从规格到代码，都要特别注意plc系统安全方面的问题。该项目的合作伙伴是思科和谢菲尔德城市理工学院。b.2 可编程逻辑控制器plc可编程逻辑控制器可划分为三部分：1、 数字输入连接到具体内部地址。 模拟输入可先转换再接受数字式。2、 内部记忆体，包括定时器，计数器，登记表和谨慎的内部继电器。3、 数字输出由继电器，触发三极管和双向模拟转换器模拟出的数字处理。 针对不同性质的系统设定了各种各样的编程方法。因为没有标准语言，所以大多数plc程序员开发程序使用梯形逻辑，没有标准的方法。梯形逻辑是为了简化plc编程任务。从而梯形逻辑模型的硬布线解决了模拟接线问题。一个简单的例子是控制继电器开始停止使电机转动，如图 b1-图（1）所示。与梯形逻辑非常相似，如图 b1-图（2）所示。这条电路是基于启动开关连接到输入号码400，停止开关，以401和电动机输出到436。梯子是一种低层次的建模工具， plc程序是一个记忆的梯形逻辑。 图 （1） 图 （2） 图 b1对于三菱plc相应的一套教学方法，第一次扫描plc内部的记忆体，把plc扫描到的内存建立在该系统。然后运行程序，建立逻辑连接时在所有输入和输出之前，将所有的逻辑连接在一个复杂的控制系统上，将非法输入、合理的逻辑组合在一起，是难以预料的结果，如非法的输入会损坏输入开关或传感器。这种缺乏控制会影响系统的稳定，会造成难以预测安全问题。b.3 plc弹性制造系统plc是广泛用于弹性制造系统。一个输出对另一个输出的影响是比较困难的，往往无法检测，因此程序员不可能考虑到所有可能的排列。下图b2包括三个加工中心。机1是唯一工作在轴的位置与传感器安置在机器幻灯片 图 b2的顶部和底部。机2是一个平面与垂直运动和旋转机械头三可选的头。机3是横向运动米勒的两个轴的传感器和温度传感器的位置，被安置在末端。这三个传送带将组分从启动到海湾的每一个加工中心。当组分进入到海湾时，每一个组分被单独地编码。组分通过系统被跟踪。因为组分通过每一个机械中心传感器通知plc它的到来，机械中心可为组分进行适当的操作。在这样的系统，因为这是比较容易的控制而导致跳序列，因而会锁死和失去部分。该项目的目标是塑造该系统模型。b.4 大门 1 稍进一步复杂化的解决方案是大门，我们分类的大门是一个铰链门（不滑动），当开启时移动的垫子b得任何人站在垫子乙时，门打开风险加大。如图b3显示了大门。 图 b3 图 b4图b4显示也许由一位无经验的程序员导致了的一张简单的梯子图。阶1和第2款审查环境和一套内部旗帜，odf格式，开放的旗帜，关闭门旗。 阶3和第4监测内部旗帜导致外在行动，在这种情况下，无论是开启或关闭上了门。只要站在垫子b的大门打开，并站在垫子a的大门关闭。b.5 模型与petri网这确实是一个非常基本的解决办法。图b5表明，梯形逻辑确实是一个符号的plc程序，是假设内部存储器是由