I. Problem Statement
Programmable logic controller (PLC) technology is primarily used in automation control engineering. This article introduces a general method for constructing a PLC control system, which involves comprehensively applying previously learned knowledge and rationally combining it with actual engineering requirements.
II. Basic Steps in Programmable Logic Controller (PLC) Control System Design
1. Main contents of system design
(1) Formulate the technical conditions for the control system design. The technical conditions are generally determined in the form of a design task statement, which serves as the basis for the entire design;
(2) Select the electrical drive type and actuators such as motors and solenoid valves;
(3) Select the PLC model;
(4) Compile the PLC input/output allocation table or draw the input/output terminal wiring diagram;
(5) Write the software specification according to the system design requirements, and then use the corresponding programming language (ladder diagram is commonly used) to design the program;
(6) Understand and follow user cognitive psychology, attach importance to the design of human-computer interface, and enhance the friendly relationship between humans and machines;
(7) Design the control panel, electrical cabinet and non-standard electrical components;
(8) Prepare design specifications and user manuals;
The above content may be adjusted as appropriate depending on the specific task.
2. Basic Steps in System Design
The main steps in the design and debugging of programmable logic controller (PLC) application systems.
(1) Thoroughly understand and analyze the process conditions and control requirements of the controlled object.
a . The controlled object is the controlled machinery, electrical equipment, production line, or production process.
b . Control requirements mainly refer to the basic control methods, the actions to be performed, the composition of the automatic work cycle, and necessary protection and interlocking. For more complex control systems, the control task can be divided into several independent parts, which simplifies the process and facilitates programming and debugging.
(2) Determine the I/O devices
Based on the functional requirements of the controlled object for the PLC control system, determine the user input and output devices required by the system. Commonly used input devices include buttons, selector switches, limit switches, and sensors, while commonly used output devices include relays, contactors, indicator lights, and solenoid valves.
(3) Select the appropriate PLC type
Based on the identified user I/O devices, count the required input and output signal points, and select the appropriate PLC type, including the selection of the model, capacity, I/O modules, and power supply modules.
(4) Allocate I/O points
Assign input/output points to the PLC, creating an input/output allocation table or drawing a wiring diagram for the input/output terminals. Next, PLC programming can be performed, along with the design and on-site construction of the control cabinet or operator console.
(5) Design the ladder diagram program for the application system.
Based on the function chart or state flowchart, design the ladder diagram, which is then programmed. This step is the core and most difficult part of the entire application system design. To design a good ladder diagram, one must first be very familiar with the control requirements and also have some practical experience in electrical design.
(6) Input the program into the PLC
When inputting a program into a PLC using a simple programmer, the ladder diagram needs to be converted into instruction mnemonics before input. When programming on a computer using programmable logic controller (PLC) auxiliary programming software, the program can be downloaded to the PLC via a connection cable between the upper and lower computers.
(7) Conduct software testing
After the program is input into the PLC, testing should be performed first. This is because oversights are inevitable during program design. Therefore, software testing is necessary before connecting the PLC to field devices to eliminate errors in the program and to lay a solid foundation for overall debugging, thus shortening the overall debugging cycle.
(8) Overall debugging of application system
After the PLC hardware and software design, control cabinet, and on-site construction are completed, the entire system can be online and debugged. If the control system consists of several parts, partial debugging should be performed first, followed by overall debugging. If the control program has many steps, segmented debugging can be performed first, followed by overall debugging. Problems discovered during debugging should be eliminated one by one until successful debugging is achieved.
(9) Prepare technical documents
The system technical documents include instruction manuals, electrical schematic diagrams, electrical layout diagrams, electrical component lists, and PLC ladder diagrams.
III. PLC Hardware System Design
1. Selection of PLC Model
Before making a decision on a system control scheme, it is necessary to understand in detail the control requirements of the controlled object in order to decide whether to use a PLC for control.
In situations where the control system has complex logic (requiring a large number of intermediate relays, time relays, counters, etc.), the process flow and product changes are frequent, data processing and information management are required (including data calculation, analog quantity control, PID regulation, etc.), the system requires high reliability and stability, and the goal is to achieve factory automation networking, the use of PLC control is essential.
Currently, numerous manufacturers both domestically and internationally offer a wide variety of PLC products with diverse functions, leaving users overwhelmed and confused. Therefore, a comprehensive weighing of pros and cons and a rational selection of the appropriate model are essential for achieving economical and practical results. Generally, the selection should prioritize meeting the system's functional requirements, avoiding blindly pursuing larger and more comprehensive models to prevent wasting investment and equipment resources. The following aspects can be considered when selecting a model.
(1) Selection of input/output points
Blindly choosing a model with more points can lead to some waste.
First, determine the total number of I/O points of the control system, and then reserve 15-20% of the actual required total number of points as a spare (to allow for system upgrades, etc.) before determining the required number of PLC points.
Additionally, it's important to note that some high-density input point modules have limitations on the number of input points that can be simultaneously activated; generally, the number of input points activated simultaneously should not exceed 60% of the total input points. The drive capability (A/point) of each PLC output point is also limited, and for some PLCs, the output current per point varies depending on the applied load voltage. Generally, the allowable output current of a PLC decreases as the ambient temperature increases. These issues should be considered when selecting a PLC.
PLC output points can be categorized into common-point, grouped, and isolated connection methods. Isolated output points can use different voltage types and levels, but the average price per point is higher for this type of PLC. If isolation between output signals is not required, a PLC with one of the first two output methods should be selected.
(2) Selection of storage capacity
User storage capacity can only be roughly estimated. In systems that only control switching signals, it can be estimated by multiplying the total number of input points by 10 words/point and the total number of output points by 5 words/point; counters/timers are estimated at (3-5) words/unit; when there is arithmetic processing, it is estimated at (5-10) words/quantity; in systems with analog inputs/outputs, it can be estimated by estimating approximately (80-100) words of storage capacity per input/(or output) analog signal; when there is communication processing, it is roughly estimated at more than 200 words per interface. Finally, a margin of 50-100% of the estimated capacity is generally allowed. For inexperienced designers, a larger margin should be allowed when selecting the capacity.
(3) Selection of I/O response time
The I/O response time of a PLC includes input circuit delay, output circuit delay, and time delay caused by the scanning operation mode (generally 2-3 scan cycles). For systems with digital signal control, the PLC and I/O response time generally meet the requirements of practical engineering, and I/O response issues do not need to be considered. However, for systems with analog signal control, especially closed-loop systems, this issue must be considered.
(4) Select the model according to the characteristics of the output load.
Different loads have different requirements for the output type of a PLC. For example, for inductive loads that frequently switch on and off, a transistor or thyristor output PLC should be selected, rather than a relay output PLC. However, relay output PLCs have many advantages, such as low on-state voltage drop, isolation function, relatively low price, strong ability to withstand instantaneous overvoltage and overcurrent, flexible load voltage (AC and DC) and a wide voltage range. Therefore, relay output PLCs can be selected for AC and DC loads with infrequent operation.
(5) Choice between online and offline programming
Offline programming refers to the PLC sharing a single CPU between the host computer and the programmer. The programming, monitoring, and operational states of the PLC are selected via a switch on the programmer. In programming mode, the CPU only serves the programmer and does not control the field. Dedicated programmer programming falls into this category. In-circuit programming involves the host computer and programmer each having their own CPU. The host computer's CPU handles field control and communicates with the programmer at the end of each scan cycle. The programmer sends the modified program to the host computer, which then controls the field according to the new program in the next scan cycle. Computer-aided programming can achieve both offline and in-circuit programming. In-circuit programming requires a computer and programming software. The choice of programming method depends on the specific needs.
(6) Selection based on whether network communication is required
If a PLC-controlled system needs to be connected to a factory automation network, the PLC must have communication and networking capabilities, meaning it must have interfaces to connect to other PLCs, host computers, and CRTs. Large and medium-sized PLCs typically have communication functions, and most small PLCs also currently have this feature.
(7) Selection of PLC structure
For the same functionality and I/O point data, an integrated PLC is cheaper than a modular one. However, modular PLCs offer advantages such as flexible function expansion, convenient maintenance (module replacement), and easier fault diagnosis. The PLC structure should be selected based on actual needs.
2. Assign input/output points
Generally, there is a one-to-one correspondence between input points and input signals, and between output points and output controls.
After allocation, each input and output signal is assigned a number according to the channel and contact number configured in the system.
In some cases, two signals may share a single input point. In such cases, the wires should be connected according to the logical relationship before connecting the input point (e.g., the two contacts should be connected in series or in parallel first), and then connected to the input point.
(1) Determine the I/O channel range
Different PLC models have different input/output channel ranges. You must consult the corresponding programming manual based on the selected PLC model; do not apply the wrong manual. The relevant operation manual must also be consulted.
(2) Auxiliary relays
Internal auxiliary relays do not output externally and cannot be directly connected to external devices. Instead, they are used for data storage or processing when controlling other relays, timers/counters.
Functionally, the internal auxiliary relay is equivalent to the intermediate relay in a traditional electrical control cabinet.
Input/output relay areas of unassigned modules and link relay areas when 1:1 connections are not used can all be used as internal auxiliary relays. The internal auxiliary relays of the PLC should be arranged reasonably according to the needs of the program design. The design specification should list in detail the purpose of each internal auxiliary relay in the program to avoid duplication. Refer to the relevant operation manual.
(3) Allocate timers/counters
The number of timers/counters for the PLC can be found in the relevant operation manual.
IV. PLC Software System Design Methods and Steps
1. Methods for PLC software system design
After understanding the PLC program structure, the next step is to write the program. There are many methods for writing PLC control programs; this section mainly introduces several typical programming methods.
(1) Graphical programming
The graphical method involves designing PLC programs by drawing diagrams. Common methods include ladder diagrams, logic flowcharts, timing flowcharts, and sequential control.
a . Ladder Diagram Method: The ladder diagram method uses ladder diagram language to program PLCs. This is a programming method that imitates relay control systems. Its graphics and even component names are very similar to relay control circuits. This method makes it easy to port the original relay control circuit to the PLC's ladder diagram language. This is the most convenient programming method for those familiar with relay control.
b . Logic Flowchart Method: The logic flowchart method uses a logic block diagram to represent the execution process of a PLC program, reflecting the relationship between inputs and outputs. It represents the system's technological flow using a logic block diagram. PLC control programs developed using this method have a clear logical structure, clearly defining the causal relationships between inputs and outputs and interlocking conditions. The logic flowchart makes the entire program structure clear, facilitating control program analysis, troubleshooting, and program debugging and maintenance. Sometimes, for a complex program, directly using statement lists and ladder diagrams may seem daunting. In such cases, a logic flowchart can be drawn first, and then the PLC application program can be developed for each part of the logic flowchart using statement lists and ladder diagrams.
c . Timing Flowchart Method: The timing flowchart method involves first drawing the timing diagram of the control system (i.e., the control timing diagram of which control should be performed at a certain time), then drawing the corresponding program flowchart of the control task based on the timing relationship, and finally writing the program flowchart into a PLC program. The timing flowchart method is well-suited for programming time-based control systems.
d . Step-by-Step Sequential Control Method: The step-by-step sequential control method involves designing complex control programs in conjunction with sequential control instructions. Generally, complex programs can be divided into several simpler program segments, each segment representing a step in the overall control process. From a holistic perspective, the control process of a complex system consists of these steps. The task of system control can be considered as controlling each step at different times or in different processes. Therefore, many PLC manufacturers have added step-by-step sequential control instructions to their PLCs. After drawing the state flow diagrams for each step, the control program can be easily written using these instructions.
(2) Empirical programming
The experience-based method involves using one's own or others' experience in design. Often, this begins by selecting procedures that closely match one's technological requirements, treating these procedures as "experimental procedures." These "experimental procedures" are then modified one by one to suit the specific project requirements. The experience gained here may come from one's own accumulated knowledge or from the design experience of others; it requires consistent accumulation and careful summarization.
(3) Computer-aided design programming
Computer-aided design (CAD) involves using PLC programming software to design programs on a computer, perform offline or online programming, offline simulation, and online debugging. Programming software makes it very convenient to program offline or online and debug online on a computer. It also makes it very convenient to access, encrypt, and create EXE executable files from programs on a computer.
2. Steps in PLC software system design
Having understood the program structure and programming methods, the next step is to actually write the PLC program. Writing a PLC program, like writing any other computer program, involves the following process.
(1) Divide system tasks into blocks
The purpose of chunking is to break down a complex project into several simpler smaller tasks. This reduces a large, complex problem into several smaller, simpler ones, making programming easier.
(2) Compile the logic diagram of the control system.
A logic diagram reveals the result of a given logical relationship and the actions that result lead to. This logical relationship can be based on the sequence of control activities or the time rhythm of the entire activity. The logic diagram reflects the control action and the activities of the controlled object during the control process, as well as the relationship between inputs and outputs.
(3) Draw various circuit diagrams
The purpose of drawing various circuits is to link the designed addresses and names of the system's inputs and outputs. This is a crucial step. When drawing the PLC input circuit, it's essential to consider not only whether the signal connection points match the naming conventions, but also whether the input voltage and current are appropriate, and the reliability and stability under special conditions. Particular attention must be paid to whether high voltage can be introduced to the PLC input terminals, as this can cause significant damage. When drawing the PLC output circuit, it's crucial to consider not only whether the output signal connection points match the naming conventions, but also the load-carrying capacity and voltage withstand capability of the PLC output modules. Furthermore, the power supply's output power and polarity must be considered. Throughout the entire circuit design process, design principles must be considered to strive for improved stability and reliability. Although PLC control is convenient and flexible, circuit design still requires caution and comprehensiveness. Therefore, when drawing circuit diagrams, meticulous consideration is needed; where to install buttons and switches requires careful attention.
(4) Compile PLC programs and perform simulation debugging.
After drawing the circuit diagram, you can begin writing the PLC program. Of course, you can use the method described above. When programming, in addition to ensuring the program is correct and reliable, you should also consider making it concise, time-saving, easy to read, and easy to modify. After writing a program block, conduct a simulation experiment. This helps in finding problems and making timely modifications. It's best not to perform a final check after the entire program is completed.
(5) Fabricate the control console and control cabinet
After designing the electrical components and writing the program, you can then build the control console and control cabinet. When time is tight, this work can be done concurrently with programming. When building the control console and cabinet, pay close attention to the quality of switches, buttons, relays, and other components; their specifications must meet requirements. The installation of the equipment must be safe and reliable. For example, issues such as shielding, grounding, and high-voltage isolation must be properly addressed.
(6) On-site commissioning
On-site commissioning is a crucial step in completing the entire control system. It's rare for any program to be usable without it. Only through on-site commissioning can we identify where control loops and programs fail to meet system requirements; only through on-site commissioning can we discover inconsistencies between control circuits and programs; and only through on-site commissioning can we finally conduct field tests and make final adjustments to the control circuits and programs to meet the requirements of the control system.
(7) Prepare technical documents and conduct on-site trial operation.
After on-site debugging, the control circuit and control program were basically finalized, and the hardware and software of the entire system were essentially problem-free. At this point, a comprehensive review of the technical documentation was required, including compiling circuit diagrams, PLC programs, user manuals, and help files. This essentially completed the work.
