Design principle of PLC control system

(1) Practicability

Practicality is the basic principle of control system design. While studying the controlled object, engineers should also understand the use environment of the control system, so that the designed control system can meet all the requirements of users. The hardware should be as small and flexible as possible, and the software should be simple and convenient.

(2) Reliability

Reliability is an extremely important principle of control system. For some systems that may cause danger, it is necessary to ensure that the control system can operate stably, safely and reliably for a long time. Even if the control system itself has problems, it can at least ensure that there will be no significant loss of personnel and property. At the initial stage of system planning, the possible problems of the system shall be fully considered, different design schemes shall be proposed, and a very reliable and easy to implement scheme shall be selected; In hardware design, proper backup or redundancy shall be considered according to the importance of equipment; During software design, corresponding protective measures shall be taken, and online commissioning can be carried out after repeated tests to ensure no major omissions.

(3) Economy

This requires engineers to make the software and hardware configuration of the system as economical and affordable as possible on the premise of meeting the practicability and reliability, and not blindly pursue new technology and high performance. The selection of hardware shall be based on economy and sharing; Software should be balanced between development cycle and product function. It is also necessary to consider whether the products used can obtain complete technical data and after-sales service to reduce development costs.

(4) Scalability

This requires the engineer to take full account of the user’s needs for future production development and process improvement in the overall planning of the system, leave appropriate margins on the controller’s computing capacity and the number of I/O ports, and leave expansion interfaces for the external to meet the needs of system expansion and monitoring.

(5) Progressiveness

This requires engineers to give priority to the products with advanced technology and extensive application to form the control system in hardware design, so as to ensure that the system is progressiveness within a certain period of time and will not be eliminated by the market. This principle is considered together with economy, so that the control system has a high cost performance ratio.

Design flow of PLC control system

When designing the control system, a certain design process should be followed. Mastering the design process can increase the design efficiency and accuracy of the control system.

Analysis and description of controlled object

To analyze the controlled object is to analyze the process flow of the controlled object in detail and understand its working characteristics. At this stage, we must have in-depth communication with users to ensure that the analysis is comprehensive and accurate. In the design of control system, it is often necessary to meet some specific indicators and requirements, that is, to meet the actual application or customer needs. These indicators and requirements must be considered when analyzing the controlled object. After a comprehensive analysis, it is necessary to accurately describe the controlled object with engineering methods according to certain principles, so as to lay a good foundation for the control system design.

(1) System scale

The scale of the system can be determined according to the process flow, complexity and technical requirements of the customer of the controlled object, which can be divided into large, medium and small scales. Ensure that hardware resources have a certain margin without waste.

Small scale control system is applicable to single machine or small-scale production process. It mainly focuses on sequence control. The signals are mostly switching values, and the I/O points are less (less than 128 points). The requirements for precision and response time are not high. S7-200 is generally selected to meet the control requirements

Medium scale control system is applicable to the production process of complex logic and closed-loop control. There are many I/O points (between 128 points and 512 points), which need to complete some special functions, such as PID control. S7-300 is generally selected.

Large scale control system is applicable to large-scale process control, DCS system and factory automation network control. There are many I/O points (higher than 512 points), the process of controlled object is complex, and the requirements for precision and response time are high. High grade PLC with intelligent control, high-speed communication, database, function operation and other functions shall be selected, such as S7-400.

(2) Hardware configuration

Estimate the I/O points of the control system according to the system scale and the customer’s technology. Analyze the process of the controlled object, and count the system I/O points and I/O types. The location and function of each I/O point shall be defined according to the different equipment and production areas. A detailed list of I/O points shall be provided with 10%~20% of spare capacity.

(3) Software configuration

Select appropriate software according to the design requirements of the control system, including system platform software and programming software.

Selection of upper computer monitoring software. First, consider the limit of monitoring points; Whether there are alarm display, trend analysis, report printing and history recording functions.

(4) Control function

To correctly select the scale of the control system, we must first understand the characteristics of each controller, such as performance parameters, applications, industry solutions, reliability and versatility. How to select a control system generally follows the following points:

Whether the control system needs redundancy, whether the I/O signal module needs redundancy, and whether the communication needs redundancy.

The number of control points, including digital input and output points, analog input and output points.

Whether the process of the controlled object is complex and whether it needs to realize special functions, such as anti surge control.

Whether the load rate of the controller has sufficient working margin during normal operation of the system; Whether I/O signal points need a certain margin.

Whether relay isolation is required for digital signals; Consider the voltage and current level of the input signal; Whether solid state relay output is required for output signal.

Whether safety barriers are required for analog signals; Type of signal, voltage type or current type; Measuring range of voltage and current. Different I/O signal modules need to be selected for different signal types.

For the signal module used for temperature measurement, consider whether it is thermal resistance or thermocouple.

Whether the signal module needs online hot plug replacement. If necessary, additional special backplane slots should be considered.

In case of system and external faults, such as signal short circuit or forging furnace, whether the signal module needs to automatically switch the input and output signals to the preset safety value. If required, fail safe controller and signal module shall be considered.

When it is necessary to communicate with third-party equipment, it is necessary to consider the length of communication distance and corresponding communication interface protocol, and select different communication modules.

For important interlocking signals in the system, whether special SOE modules are required to record the time sequence of signal changes.

Familiarity with the controlled object is the basis for designing control systems. Only by deeply understanding the controlled object and the controlled process can a reasonable and scientific control scheme be proposed.

(1) Analyze the controlled object. The process flow of the controlled object is analyzed in detail to understand its working characteristics. At this stage, we must have in-depth communication with users to ensure that the analysis is comprehensive and accurate.

(2) Draw the process flow chart. After the first step, we should have a deep understanding of the whole process flow of the controlled object. In order to be more intuitive and concise, we should draw a process flow chart to prepare for the subsequent system design.

(3) Analyze and define control tasks. According to the prepared process flow chart, engineers can convert the control requirements proposed by users into professional terms, decompose them one by one, and convert them into multiple control loops from the perspective of control. For process control system, PID diagram can be used to represent the control relationship.

Overall design of PLC control system

Before the control system design, it is necessary to demonstrate the system scheme. It is mainly to make a predictive estimate of the feasibility of the whole system. At this stage, various problems that will be encountered in the design and implementation of this system must be fully considered. If you have no experience in relevant projects, you should carefully investigate on the spot and demonstrate the feasibility of each step in the design of this system in detail. Especially in the hardware implementation phase, a little carelessness will cause great trouble, ranging from unsuccessful system to serious loss of personnel and property. The obstacles in the process of project implementation are often caused by insufficient effort in this step.

The overall design of the system is related to the overall architecture of the whole system, and every detail must be considered repeatedly. First of all, it should be able to meet the basic requirements of users; The second is to ensure the reliability of the system, which can not cause frequent failures, and even if failures occur, they will not cause large losses; Then consider the economy and other aspects.

In general, the following issues need to be considered in the overall design of the system:

(1) Determine whether the system is controlled by a single PLC or a PLC network; Determine whether the system uses remote I/O or local I/O. It is mainly selected according to the size of the system and the functions required by users. For the general small and medium-sized process control system, the single PLC control can basically meet the functional requirements. However, the concept of distributed control system can also be used for reference, that is, risk and control are decentralized, and management and monitoring are centralized. This can greatly improve the reliability of the system.

(2) Whether it is necessary to communicate with other parts. A complete control system will include at least three parts: controller, controlled object and monitoring system. Therefore, the controller should at least communicate with the monitoring system. Whether to communicate with other control units or departments depends on the user’s requirements. Generally speaking, if the user has no requirements, such communication interface will be reserved.

(3) What kind of communication mode is adopted. Generally speaking, PROFIBUS DP is used at the field control level; PROFINET is used for communication from the field control level to the monitoring system. However, sometimes they can also be common to each other, and the appropriate communication mode can be selected according to the specific situation.

(4) Whether redundant backup system is required. Select different methods according to the required security level of the system. During data archiving, OS server redundancy can be used to prevent archive data loss; In the Automation Station (AS), in order to prevent the system from downtime or unpredictable results due to failures, the controller redundancy backup system can be used. Choosing appropriate redundant backup can greatly improve the reliability of the system.

Before selecting the control system, first consider how to build the network structure of the system.

Determine the number and location of operation stations and process control stations of the system, and how they are connected with each other. Whether industrial Ethernet switch is required.

In general, the field control room, main control room and electrical control cabinet are respectively placed at two places with a long distance. To ensure the stability and reliability of signals, optical cables will be considered to connect their switches. At the same time, for the redundancy of communication lines, industrial Ethernet switch with honor management function will be considered to form an optical fiber ring network between the field operation station and the process control station. In this way, even if communication in one direction is disconnected, communication can continue in the other direction.

For the connection between the process control station and the field signal, the traditional connection method is to directly connect the field signal to the process control station through hardware. In this way, if the distance is too far, there will be loss in signal transmission, especially analog signals. In addition, when there are many signal points, the wiring is also complex and wastes materials. Therefore, it is generally necessary to install a distributed I/O slave station on the site (if the site is a hazardous area, an intrinsically safe distributed I/O slave station shall be selected), connect the field signal directly to the I/O slave station, and transmit the signal to the process control station through the field bus.