This paper summarizes commonly used programmable logic controller (PLC) programming methods such as combinational logic function method, function transfer diagram method, and Petri net analysis method, and proposes an object-oriented PLC programming method. Following the principles of object-oriented technology, it elaborates on how to construct the underlying basic control object classes and their higher-level object model encapsulation classes in PLC programming. The definition of external signals is given, and the interaction between the object model and external signals is illustrated. Finally, the superior performance of the object-oriented method is analyzed in depth. 1 Introduction Programmable logic controllers (PLCs) are widely used due to their simple operation and reliable performance. Various types of PLCs flood the market. For PLC system designers, different designs are required for different PLC models. The overall system design and hardware design usually only require minor modifications to the controller model, or even no modifications at all. However, for the programming part, the changes are often drastic. Sometimes it is better to redesign according to the new controller model. Although programmable logic controller (PLC) programs are much simpler than various large-scale computer application software, the problems encountered in application software design also exist in PLC program design, such as communication problems, constantly changing requirements, and software reuse. Object-oriented technology is a solution to these problems. This paper first summarizes several commonly used design methods, and then proposes an object-oriented PLC program design method. 2 Introduction to Common Methods 1. Combinational Logic Function Method PLCs were born with the development of traditional relay logic. There is a one-to-one correspondence between electrical control circuits and Boolean algebra. In specific design, the Boolean algebra expression is first listed according to the control requirements, then simplified, and finally the ladder diagram is drawn based on the simplified Boolean algebra expression to obtain the program. This method is simple and intuitive. The simplified program is very concise. For designers familiar with electrical control circuits, this design method is easy to accept. However, from another perspective, the redundancy and security of the system are not well reflected after program simplification. The concise program makes it difficult to determine the exact location of the problem during debugging, and small changes in the program often involve changes to the entire system. Furthermore, Boolean algebra is only suitable for designing system switching quantities; analog quantity requirements necessitate other methods. However, the design of switching and analog quantities in a system is often closely integrated and inseparable. 2. Functional Transfer Diagram Method: This is a graphical representation of sequential control systems. It is suitable for handling sequential, random, and other types of problems. This method is like a manufacturing plant's assembly line, where one process must be completed before the next can begin, until the entire process is finished. During system design, functional blocks are first divided according to control requirements, then these blocks are sorted according to the process flow, and finally integrated into a system that meets user needs. Clearly, in programs developed using this method, each functional block only interfaces with the preceding and following functional blocks, and there is only a functional transfer relationship between them. For flexible manufacturing systems, its constituent elements need to operate in parallel, coordinate actions, and compete for resources. The first two methods cannot meet these requirements. 3. Petri Net Analysis Method: Systems described by Petri nets share a common characteristic: the dynamic behavior of the system manifests as the flow of resources (material resources and information resources). The control logic of the programmable controller fully embodies this characteristic, that is, whoever meets the condition is energized, and only the energized one can act. The specific design steps of this method are as follows: (1) Each execution element, position detection element, start signal, etc. are used as Petri state elements. (2) Each switching command signal is represented by a conversion. (3) Set the start state and list each state flag. (4) Design each subroutine of the Petri net. (5) For the coordinated control system, constrain the command signals of each unit to be coordinated. (6) For the competitive control system and the concurrent system, use parallel programs; for the cyclic system, use a method similar to single-sequence programs. (7) Allocate addresses, list the logic equations, and program. See full text: Object-oriented programmable controller programming method