First, the problem is raised.
Programmable controller technology is mainly used in automation control engineering. How to comprehensively apply the previously learned knowledge points and reasonably combine them into control systems according to actual engineering requirements. The general method of composing programmable controller control systems is introduced here.
Second, the basic steps of the programmable controller control system design
1 . Main content of system design
(1) Formulate the technical conditions for the design of the control system. Technical conditions are generally determined in the form of a design task book, which is the basis for the entire design;
(2) Selecting the electric drive form and the actuators such as the motor and solenoid valve;
(3) The model of the selected PLC;
(4) Prepare the PLC input/output distribution table or draw the input/output terminal wiring diagram;
(5) Write the software specification according to the requirements of the system design, and then use the corresponding programming language (common ladder diagram) for programming;
(6) Understand and follow user cognitive psychology, pay attention to the design of human-machine interface, and enhance the friendly relationship between people and machines;
(7) Design consoles, electrical cabinets and non-standard electrical components;
(8) Prepare design specifications and instructions for use;
The above content can be appropriately adjusted according to specific tasks.
2 . Basic steps in system design
(1) Deeply understand and analyze the process conditions and control requirements of the controlled object
a . The object being controlled is the controlled mechanical, electrical equipment, production line or production process.
B. Control requirements mainly refer to the basic mode of control, the actions that should be completed, the composition of the automatic work cycle, the necessary protection and interlocking. For more complex control systems, the control task can also be divided into several independent parts, which can be simplified and simplified, which is conducive to programming and debugging.
(2) Identify I/O devices
According to the controlled object to the PLC
The functional requirements of the control system determine the user input and output devices required by the system. Commonly used input devices include buttons, selector switches, travel switches, sensors, etc. Commonly used output devices include relays, contactors, indicator lights, solenoid valves, and the like.
(3) Select the appropriate PLC type
According to the determined user I/O device, count the required input signal and the number of output signal points, select the appropriate PLC type, including model selection, capacity selection, I/O
Module selection, power module selection, etc.
(4) Assigning I/O points
Assign the input and output points of the PLC, compile the input/output distribution table or draw the wiring diagram of the input/output terminals. The next nine can be used for PLC programming, and the design and on-site construction of the control cabinet or console can be performed.
(5) Design application system ladder program
The ladder diagram is programmed according to the work function chart or the state flow chart. This step is the core work of the entire application system design, and it is also a difficult step. To design a ladder diagram, first of all, you must be very familiar with the control requirements, and also have some practical experience in electrical design.
(6) Enter the program into the PLC
When using a simple programmer to input a program into the PLC, you need to convert the ladder diagram to an instruction mnemonic for input. When programming on a computer using the programmable programming software of the programmable controller, the program can be downloaded to the connection cable of the upper and lower computer.
Go in the PLC.
(7) Perform software testing
After the program is input into the PLC, the test should be performed first. Because in the process of programming, there will inevitably be omissions. Therefore, before connecting the PLC to the field device, software testing is necessary to eliminate errors in the program, and also lay the foundation for the overall debugging, shortening the overall debugging cycle.
(8) Overall debugging of the application system
After the PLC hardware and software design and control cabinet and on-site construction are completed, the entire system can be debugged online. If the control system is composed of several parts, it should be partially debugged before the overall debugging; if the control program If there are more steps, you can perform segmentation debugging first, and then connect them to the total adjustment. The problems found in the debugging should be ruled out one by one until the debugging is successful.
(9) Preparation of technical documents
System technical documents include instructions, electrical schematics, electrical layout drawings, electrical component schedules, and PLC ladder diagrams.
Third, PLC hardware system design
1 . PLC model selection
Before making a decision on the system control plan, it is necessary to know the control requirements of the controlled object in detail to decide whether to use the PLC for control.
The logic relationship of the control system is complex (requires a large number of intermediate relays, time relays, counters, etc.), process flow and product modification are frequent, and data processing and information management are required (data operation, analog control, PID adjustment, etc.) When the system requires high reliability and stability, and ready to achieve factory automation networking, it is necessary to use plc control.
At present, many domestic and foreign manufacturers provide a variety of PLCs with different functions.
The product makes the user dazzled and confused. Therefore, we must balance the pros and cons and rationally select the model to achieve economic and practical purposes. Generally, it is necessary to select the model to meet the needs of the system function. Do not blindly seek for corruption, so as to avoid waste of investment and equipment resources. The choice of models can be considered from the following aspects.
(1) Selection of input/output points
Blindly selecting a model with a large number of points will cause a waste.
It is necessary to first clear the total number of I/O points of the control system, and then reserve the spare amount according to the actual required total points of 15 to 20% (to leave room for the system transformation, etc.) and then determine the required PLC.
Points.
In addition, it should be noted that some modules with high-density input points have a limit on the number of input points that are simultaneously turned on. Generally, the input points that are simultaneously turned on must not exceed 60% of the total input points;
The driving capacity (A/point) of each output point is also limited. Some PLCs have different output currents depending on the applied load voltage. The allowable output current of PLC generally increases with the ambient temperature. Reduced and so on. Consider these issues when selecting a model.
PLC output points can be divided into co-point, group and isolated. Different voltage types and voltage levels can be used between the isolated sets of output points, but the average price per point of this PLC is higher. If there is no need for isolation between the output signals, the PLCs of the first two output modes should be selected.
(2) Choice of storage capacity
Only a rough estimate of the user's storage capacity can be made. In a system that only controls the amount of switching, you can multiply the total number of points by 10 words/point + the total number of points of the output by 5 words/
Point to estimate; counter/timer is estimated by (3~5) words/time; (5~10) words/quantity is estimated when there is arithmetic processing; in system with analog input/output, you can press per input/ (or output) One-way analog quantity is estimated to be approximately (80 ~ 100) words of storage capacity; when there is communication processing, it is roughly estimated by the number of more than 200 words per interface. Finally, there is typically a margin of 50 to 100% of the estimated capacity. For inexperienced designers, there is a margin of margin when choosing capacity.
(3) Choice of I/O response time
The PLC's I/O response time includes input circuit delay, output circuit delay, and time delay caused by the scan mode (typically 2 to 3 scan cycles). For systems with switching control, PLC and I/O response times generally meet the requirements of actual engineering, regardless of I/O response issues. However, this problem should be considered for systems with analog control, especially closed-loop systems.
(4) Selection according to the characteristics of the output load
Different loads have corresponding requirements for the output mode of the PLC. For example, inductive loads that are frequently switched on and off should be selected for transistor or thyristor output type, and should not be used for relay output type. However, the relay output type PLC has many advantages, such as small conduction voltage drop, isolation, relatively low price, strong ability to withstand transient overvoltage and overcurrent, and flexible load voltage (AC, DC) The voltage level range is large. Therefore, the relay output type PLC can be selected for the AC and DC loads with infrequent movements.
(5) Choice of online and offline programming
Offline programming means that the host and the programmer share a CPU, and the programmer selects the switch to select the programming, monitoring, and running status of the PLC. In the programming state, the CPU only serves the programmer and does not control the field. This is the case with dedicated programmer programming. Online programming means that the host and the programmer each have a CPU. The CPU of the host completes the control of the field. At the end of each scan cycle, it communicates with the programmer. The programmer sends the modified program to the host. The host will press the next scan cycle. The new program controls the site. Computer-aided programming enables both off-line programming and online programming. Online programming requires the purchase of a computer and configuration of programming software. Which programming method to use should be decided as needed.
(6) According to whether the network communication selection
If the PLC controlled system needs to be connected to the factory automation network, the PLC needs to have the communication networking function, that is, the PLC should have the connection to other PLCs, the host computer and
Interfaces such as CRT. Both large and medium-sized machines have communication functions, and most of the minicomputers currently have communication functions.
(7) Selection of PLC structure
In the case of the same function and the same I/O point data, the monolithic price is lower than the modular price. However, the modular type has the advantages of flexible function expansion, convenient maintenance (changing modules), easy judgment of faults, etc., and the structural form of the PLC should be selected according to actual needs.
2 . Assign input/output points
The general input point has a one-to-one correspondence with the input signal, the output point, and the output control.
After the allocation, the channel and the contact number configured by the system are assigned to each input signal and output signal, which is numbered.
In some cases, there are also two signals with one input point, so that the line should be connected logically before the input point is accessed (for example, two contacts are connected in series or in parallel) and then received.
(1) Determine the I/O channel range
Different types of PLCs, the range of their input / output channels is not the same, should refer to the corresponding programming manual according to the selected PLC model, and must not "Zhang Guan Li Dai". You must refer to the relevant operating manual.
(2) auxiliary relay
The internal auxiliary relay is not outputted externally. It cannot be directly connected to an external device, but is used for data storage or data processing when controlling other relays, timers/counters.
Functionally, the internal auxiliary relay is equivalent to the intermediate relay in the traditional electric control cabinet. The input/output relay area of ​​the unassigned module and the link relay area when the 1: 1 link is not used can be used as the internal auxiliary relay. According to the needs of the program design, the PLC's internal auxiliary relay should be arranged reasonably.
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