The programmable controller, commonly referred to as a PLC in English, is frequently abbreviated as PC. However, due to the potential confusion with personal computers, the term PLC is still widely used instead. It is essentially a digital computing operation electronic system device based on a microprocessor, specifically designed for industrial field applications. The PLC utilizes programmable memory to execute logical operations, sequential controls, timing, counting, and arithmetic operations. Through digital or analog input and output interfaces, it controls various types of machinery or production processes. Combining microcomputer technology with traditional relay contact control technology, the PLC overcomes the limitations of complex wiring, low reliability, high energy consumption, and poor flexibility inherent in relay contact control systems. By leveraging the advantages of microprocessors and catering to the skills and habits of on-site electrical operators, the PLC simplifies programming through simple instruction forms like relay ladder diagrams, making it intuitive and easy to learn. Users can quickly adapt to the PLC by making minor adjustments to wiring and writing simple programs after purchasing the required equipment.
Firstly, the structure of the PLC and the role of each component. Despite the wide variety of PLC models with different functions and command systems, their structures and working principles are similar. Typically, they consist of several key components: the main unit, input/output interface, power supply interface, and external device interface. The hardware system structure of the PLC is illustrated below:
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1. The Main Unit:
The main unit includes the Central Processing Unit (CPU), system program memory, and user program and data storage. The CPU serves as the core of the PLC, executing user programs, monitoring the status of the input/output interface, making logical judgments, and performing data processing. This involves reading input variables, completing user-specified operations, and sending results to the output while responding to requests from external devices like computers or printers.
2. Input/Output (I/O) Interface:
The I/O interface connects the PLC to input/output devices. Input interfaces accept control signals from devices such as buttons, sensors, or switches, while output interfaces drive output devices like contactors, solenoid valves, or indicator lights. Optocoupler circuits are typically used to minimize electromagnetic interference and enhance reliability. The number of I/O points, representing the number of input/output terminals, is a significant technical indicator of the PLC. Small machines may have dozens of points, medium-sized ones hundreds, and large ones over a thousand.
3. Power Supply:
This refers to the DC switching power supply configured for internal circuits like the CPU, memory, and I/O interface, supplying DC power to input devices.
4. Programming:
Programming enables the use of external devices to input, check, modify, debug, or monitor the PLC's operation. Using a dedicated PC/PPI cable to connect the PLC to a computer, specialized software facilitates programming and monitoring.
5. Input/Output Expansion Unit:
The I/O expansion interface connects expansion units to increase the number of external input/output terminals to the base unit (host).
6. External Device Interface:
This interface allows connecting external devices like printers, barcode scanners, or inverters to the host for specific operations.
The host model used in our experiment is the CPU224 (AC/DC/RELAY) from the Siemens S7-200 series, featuring 14 input points and 10 output points. The CPU226 variant offers 26 input points and 14 output points.
Secondly, the working principle of the PLC operates through "sequential scanning, continuous cycling." When running, the CPU executes a program stored in user memory according to the control requirements, performing periodic cyclic scanning based on the instruction step number (or address number). Without jump instructions, the scanning begins at the first instruction and proceeds sequentially until the end of the program, then loops back to the beginning for a new scan. During each cycle, input sampling and output refresh are completed.
A PLC scan cycle passes through three stages: input sampling, program execution, and output refresh. During the input sampling stage, the PLC reads the on/off states or input data of all input terminals, temporarily storing them in the input latch and writing them into corresponding input status registers. The input port is then closed, transitioning to the program execution phase. In this phase, each instruction is executed in the order stored in the user program memory. Results are written into the output status register, which is refreshed alongside the program execution. Finally, during the output refresh phase, the on/off states of the output status register are sent to the output latch and output through a specific mode (relay, transistor, or thyristor) to drive the corresponding output device.
Thirdly, PLC programming. The PLC achieves control requirements through software programming using various programming components, offering numerous moving and breaking contacts. These include input registers, output registers, bit memories, timers, counters, general-purpose registers, data registers, and special function memories. Inside the PLC, these memories function similarly to those in relay contact control systems, with "coils" and "contacts," but they are "soft" relays rather than hard relays. Their number ranges and functions are illustrated in the following table:
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Programming languages for PLCs primarily include ladder diagrams and instruction statement languages, often used together. A ladder diagram is a graphical language derived from relay contact control circuit diagrams, visually representing the logical relationships between PLC inputs and outputs using symbols similar to relays, break contacts, coils, and series and parallel connections. Common graphic symbols in ladder diagrams represent the moving and breaking contacts of PLC programming components, and their coils are indicated using parentheses. The types of programming elements are distinguished by graphical symbols and the letters or numbers of the labels. A separate circuit consisting of contacts and coils is called a network. The ladder diagram and statement list generated by programming software include network numbers, allowing annotations in network units.
When designing a ladder diagram, consider the following:
1. Layout from left to right and top to bottom, starting each logical line at the left bus, followed by strings of contacts, joined, and ending with the coil.
2. Each step flows through a "concept current," visually describing conditions for turning on the coil during program execution.
3. Input registers receive external signals and cannot be driven by other relays' contacts. Only input register contacts appear in the ladder diagram without its coil. Output registers output program results to external devices, and their coils are powered to produce output signals.
Instruction statement tables are mnemonic-based languages for programming PLCs, similar to assembly language but easier to understand. Programs composed of multiple instructions are instruction statement tables. An instruction consists of three parts: step sequence, instruction word, and action device number.
For example, the implementation of three-phase squirrel cage motor start/stop control using two PLC programming languages:
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Fourthly, the basic instructions of the PLC. For the S7-200 series, standard contact instructions include LD (moving contact command), LDN (break contact command), A (dynamic contact command), AN (non-break contact command), O (or moving contact command), and ON (non-break contact command). The data type of the variable in these commands is Boolean (BOOL). The LD and LDN commands connect contacts to the bus, while the A, AN, O, and ON commands can be repeated. For parallel connections of two or more contacts in series, the OLD instruction is used.
Parallel connection instruction OLD of series circuit block:
A circuit with two or more contacts connected in series is called a series circuit block. When these blocks are connected in parallel, the branch starts with LD and LDN instructions and ends with the OLD instruction. The OLD instruction and the subsequent ALD instruction are non-target component instructions, each occupying one program step.
Series connection command ALD for parallel circuits:
Two or more parallel circuits are called parallel circuit blocks. When connecting parallel circuit blocks in series with the previous circuit, the ALD command is used. The starting point of the branch is the LD and LDN instructions, and after completing the parallel circuit, the ALD instruction is used to connect it with the previous circuit. The ALD instruction is also referred to as a block instruction, occupying one program step.
Output instruction:
The output command directly connects the coil of the relay, timer, counter, etc., to the bus on the right side of the ladder diagram. The right side of the coil does not allow contacts. In programming, contacts can be reused without being limited by type or quantity.
Set and reset instructions S, R:
S is a set instruction that maintains the action; R is a reset instruction that resets the operation. Registers of N points starting from the specified position are all set or reset. If the timer bit or counter bit is specified to be reset, the current value of the timer or counter will be cleared.
Jump contact EU, ED:
When the positive transition contact detects a positive transition (input signal changes from 0 to 1), or the negative transition contact detects a negative transition (input signal changes from 1 to 0), the point is turned on for a scan cycle. The signs of positive/negative transitions are EU and ED, with no operands. The "P" and "N" in the contact symbol indicate positive and negative transitions, respectively.
Null operation instruction NOP:
The NOP instruction is a step-by-step instruction with no action and no target component. Replacing an already written instruction with the NOP instruction changes the circuit. Adding a NOP instruction reduces step number changes when modifying or appending the program.
Program end command END:
END is a step-by-step instruction with no target components. The PLC repeats input processing, program calculation, and output processing, and inserting an END instruction at the end of the program indicates the program ends, directly performing output processing. During program debugging, END instructions can be inserted in sections, expanding the inspection of each block motion. END instructions divide the program into segments, confirming the correctness of each circuit block before sequentially deleting the END instructions. Note that the watchdog clock is refreshed when the END instruction is executed.
Fifthly, the rules for programming ladder diagrams in programmable controllers. Several steps of programming include:
1. Determining the actions and order required by the system.
- Set the number of system inputs and outputs.
- Determine the control sequence, the corresponding relationship of each device, and the response.
2. Numbering of input and output devices.
3. Drawing a ladder diagram.
- Follow the left-to-right, top-to-bottom principle and the control path to the output coil.
- Place branches without contacts vertically for easy identification.
- When several series circuits are connected in parallel, place the series circuit with more contacts at the top.
- When several parallel circuits are connected in series, place the parallel circuit with the most contacts on the leftmost side.
- Contacts cannot be drawn to the right of the coil.
Converting ladder diagrams into programs:
Convert the relay ladder diagram into the code of the programmable controller. When the ladder diagram is completed, the next step is to compile its code into a program recognizable by the programmable controller. This programming language consists of a sequence number (address), an instruction (control statement), and a device number (data). The address is the location where the control statement and data are stored or placed. The instruction tells the programmable controller how to use the device to perform the corresponding action.
Entering the program with the keyboard in the programming mode.
Programming and designing control procedures.
Testing the error of the control program and modifying it.
Preserving the complete control program.
Sixthly, the function and working principle of common relay control circuits combined with corresponding PLCs. Below is a diagrammatic explanation: the functions and working principles of relay control circuits combined with corresponding PLCs are all part of the PLC basic knowledge.
Firstly, the jog circuit.
Features:
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Working Principle:
As the name suggests, the point moves when pressed, and stops when released.
Secondly, the automatic holding circuit with stop.
Features:
It is a basic form of maintaining the state of the circuit and is mainly used to maintain the state of the external signal.
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Working Principle:
Upon power-on, pressing the normally open button 0 energizes the relay coil J0, closing its main contact and powering the motor. Simultaneously, the auxiliary contact self-locks, keeping the motor running. Stopping requires pressing the normally closed button 1, de-energizing the coil J0 and opening its auxiliary contact, de-energizing the motor.
Circuit Application:
Below we compare the relay coil before and after power-on and power-off:
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Thirdly, the self-retaining interlock circuit.
Features:
One stop switch, two start switches, the signal that takes the first action first, the other signal is affected by the interlocking action, and does not act before the stop signal is not actuated.
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Working Principle:
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This circuit can be used for motor forward and reverse control.
Fourthly, the first action priority circuit.
Features:
Among the lines of the plurality of input signals, the signal that is the first action takes precedence. When the first input signal is not removed, the other signals cannot be operated.
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Working Principle:
Pressing any normally open button 0 to 3 energizes its corresponding relay coil, closing the normally open contact and self-locking, and the J4 relay also acts to disconnect the other 3 groups of power supply. As long as the first relay is not powered, the other relays will not operate.
Circuit Application:
This circuit can be used as a responder as long as a reset switch is added to the power input.
Fifthly, the post-action priority circuit.
Features:
Among the lines of the plurality of input signals, the signal of the last action takes precedence. The state determined by the previous action is released by itself.
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Working Principle:
Pressing any normally open button 0 to 3 in an energized circuit energizes the corresponding relay coil, opening its corresponding normally closed contact, and releasing the self-locking state of the other coils.
Circuit Application:
This circuit can be added with a reset normally closed switch at the input end of the power supply for program selection, production period control circuits, etc.
Sixthly, time relay (also known as time delay relay).
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Features:
When the input action signal is added (or removed), the output circuit must pass the specified precise time to generate a jump change (or contact action). Time relays are divided into on-delay, off-delay, and instantaneous delay.
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The following focuses on the application of time-delayed relays.
Figure 2-12 shows a simple on-delay application circuit. To facilitate the analysis principle, a light bulb is connected in series with the normally open contact of the time relay. Of course, other loads can also be connected in series, such as contactors or solid-state relays. Figure 2-13 shows the expression of the PLC ladder diagram, schematic diagram, and Figure 2-12 is the same. In order to analyze the action flow, add power to the on-delay application circuit as shown in Figure 2-14, at which point the delay relay does not work.
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Assume that the preset time of the delay relay is 10 seconds. Pressing the normally open button, the time relay coil is energized and starts to count. After 10 seconds, the normally open contact is closed, and the bulb is energized. See Figure 2-16 until released. When the normally open button is turned on, the time relay coil is de-energized, and the normally open contact returns to normal open. At this time, return to the state of Figure 2-14.
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Seventhly, the counter.
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Features:
The device is driven by the transmission mechanism to indicate the device to be measured (addition count) or inverse count (subtract count), and when the number reaches the preset value, the output is turned on or off.
The following describes the addition counting application.
Wiring instructions:
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In Figure 2-18, pins 1 and 4 are power input terminals, pins 2 and 3 are signal input terminals, and pins 5 and 6 are internal normally open contact output terminals.
Working Principle:
Pressing and releasing the normally open button once increments the counter display window by one. Assuming the preset value of the counter is 3. When the value of the normally open button is pressed and released three times, the internal normally open contact is closed.
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