How to Learn Programmable Logic Controllers (PLC)?

Learning Siemens PLC is not as difficult as imagined, summarized in one sentence: For beginners, the SR20 with over 600 meters is sufficient, If detailed, it involves the hardware composition of PLCs, programming software, system instructions, and program structure, Next, I will introduce the key points of learning PLC from the aspects of PLC selection, PLC entry, and PLC expansion modules and advanced topics.

1. Selection and Cost

Recommendation: SR20, Reason for recommendation:

This starts with the Siemens PLC product family, Siemens has been making PLCs for over 40 years, and the latest product family is the SIMATIC S7, Siemens has launched different product series for various market needs, such as the high-end S7-1500 series, mid-range S7-1200 series, and low-end S7-200 SMART (hereinafter referred to as SMART) series.

For beginners, starting with the SMART series is the most suitable, SMART is a new generation PLC developed by Siemens for the Chinese market, widely used in packaging, textiles, machine tools, food, subways, and many other fields, It can effectively improve equipment performance and reduce costs, offering excellent cost-performance ratio and reliability, SMART is mainly divided into two major series: ST and SR, The primary difference between the SR series and ST series is that the ST series can receive/output high-speed pulse signals, Therefore, the ST series can be applied in motion control, common packaging production lines, and currently popular robotic arms, etc. Generally, these functions are not used during the learning phase, Moreover, there is no difference in programming and usage between ST and SR, and communication between them is also very easy to achieve.

In the SR series, for beginners, I highly recommend SR20.

Reason 1: High cost-performance, available from Siemens dealers on major e-commerce platforms, priced at only over 600 meters;

Reason 2: Configuration software is free, low learning and migration costs;

The configuration software for the SMART series (commonly referred to as the upper computer) is STEP 7-Micro/WIN SMART (abbreviated as Micro/WIN), The design concept of Micro/WIN is consistent with the configuration software TIA Portal of the other two high-end PSC series, S7-1200 and S7-1500, and supports mutual communication with S7-1200 and S7-1500, If in the future you need to learn Siemens’ mid to high-end PLC product series, the learning and migration costs will be very low.

The true face of the SR20 PLC, Next, let’s take a look at the appearance of the SR20 PLC.

A: The blue wire is the network cable for communication between Micro/WIN and PLC;

B: The top row of terminals is the signal input terminals and power supply terminals;

C: The bottom row of terminals is the output terminals and power supply terminals for the expansion module;

D: The lower left corner is the RS485 interface;

E: There are some identification information on both sides, such as order number, FS number, etc.;

In summary, the cost of learning PLC:

1. Purchase SR20: 600 meters;

2. Configuration software is free;

3. SR2 can be powered directly by 220V power supply or can use 24V power supply. From a safety perspective, it is recommended to use the 24V power supply, I once used an SR20 with some external devices to create a fully automated small irrigation system, with a total cost of no more than 1000 meters, Interested friends can see the simple design scheme and wiring method later.

2. PLC Entry

PLC is a highly integrated system, in actual work, only some functions may be used, but during learning, it is necessary to comprehensively understand the functions of PLC, the SMART PLC is a very good entry series, as mentioned earlier, the operation tool for SMART PLC is Micro/WIN, We will also exchange experiences in learning SMART PLC internally, and the general feeling is that the learning cost of the SMART series is very low, When designing the functions of the SMART PLC, we prioritize ease of use, so even without programming experience, one can become familiar with most of the functions of PLC products in a short time.

High cost-performance, easy to use, and reliable, this is one of the reasons why the SMART series is so successful in China, There are many books available on what to learn about PLC, and I won’t elaborate here, while regarding the tutorial for operating Micro/WIN, the following websites also have introductory operation video courses, all of which are free, www.pxplc.com, www.uy123.net, Long-term exposure to PLC, I have a few efficient learning tips to share with everyone, These tips are easily overlooked, but they are very important for beginners.

1. Set PG/PC Interface

When first installing Micro/WIN, you need to set the PG/PC interface, the SR/20 integrates a standard Ethernet port, allowing direct communication between the PC’s physical network card and the PLC, so there is no need to purchase expensive and low-speed PPI cables, What is PG/PC interface? Many students who are new to Siemens PLC have such doubts, PG/PC actually refers to the computer, and speaking of this, there is a small story about the PLC being “forced to change its name”.

In the 1970s, PLC was not called PLC, it was called PC, the original term was “programmable controller” meaning “programmable controller”; but later the personal computer (Personal Computer) became dominant and was also abbreviated as PC, leading to confusion, so the programmable controller had to be “forced” to change its name, adding the word “Logic” to “programmable controller”, becoming “Programmable Logic Controller”, abbreviated as PLC, while PG originally meant “Programmer”, which means “programmer”, actually referring to the computer.

Next, in the Access Point of the Application, select MWSMART, meaning we need to set a network interface for Micro/WIN to connect to the PLC, and select the wired Ethernet port from the list.

2. Connect and Set PLC

When connecting to the CPU, first select the communication interface, here select the physical network card, which serves the same purpose as the above PG/PC setting, click “Find CPU”, and the SR20 will respond to the command and send identification information to Micro/WIN.

Select SR20 and click OK, Micro/WIN and PLC will successfully establish a connection, and through “Edit” you can set the PLC’s identity information, and through “Blinking” the indicator light, you can make the PLC’s LED lights blink alternately, Here, it should be mentioned that for SMART, “CPU” and “PLC” refer to the same thing, so there is no need to be confused.

3. View and Set More Information

Once connected to the PLC, you can start operating the PLC, Many people do not know where to set the time, you can set the time by selecting “Set Clock” in the “PLC” menu.

The clock setting operation generally only needs to be performed when first connecting to the PLC, the PLC has an independent clock that can maintain for 7 days when powered off, and the PLC will also correct clock errors in real-time during operation, even after running for several years, the error will not exceed 2 seconds, in small PLCs this is a very high precision, In an automated system, one of the steps we often perform is to check whether the system is currently running normally, what the performance status is, and whether there are any alarm messages, All of these are in the “PLC” function block under the “PLC” menu, and during the debugging and operation of the PLC, we will often open this interface to check the operation status of the PLC.

4. Debugging and Simulating PLC

The debugging and simulation function is a very important feature of Micro/WIN, this function is very practical and easy to operate, it can conveniently simulate parameters that cannot be generated during actual operation, allowing operators to discover problems in programs and configurations in advance, and generate optimal parameters, effectively reducing system risks, However, often some experienced engineers do not know or cannot use this function, so I will elaborate on it here, Below is a simple ladder diagram for a start-stop circuit, and these instructions can be added through the “Instruction” directory in the left project tree.

The blue indicates that energy can flow, the gray indicates that energy cannot flow, from the figure, it can be seen that since the input terminal corresponding to I0.0 did not detect a high level, the value of this input impression area I0.0 is 0, the contact is open, so energy cannot flow to the back, and M0.0 is also not connected, remaining in an open state, If the input terminal corresponding to I0.0 is not connected to the electrical signal, then how to debug? Micro/WIN has a powerful tool, the “Status Chart”. We write to I0.0 and then click the monitoring button, we can see that the value of I0.0 is 0, then give it an input value of 1, and then click the “Force” button next to the monitor, and try again.

It can be seen that I0.0 has been forced to 1, the PLC no longer detects electrical signals from the input terminal, but reads the written value as I0.0, and forced operations can be executed on all accessible memory areas, which is very useful for debugging, Below is the state of the program after forcing:

M0.0 is closed, canceling the force on I0.0 and observing again, will see that M0.0 is still in a closed state, this is the function of the start-stop circuit.

It should be noted that when the user is in monitoring mode, system blocks, wizards, and other functions cannot be operated, if you click a button and Micro/WIN does not respond, it is mostly because you are currently in monitoring mode, As shown below, add two more instructions and then download them to the PLC.

Then monitor the program running status, When I0.0 is forced again, Q0.0 immediately becomes 1, and you can hear the sound of relay Q0.0 closing, If a load, such as a motor or lamp, is connected to the PLC, then the device will start running immediately.

Here, let me explain how to wire:

Step 1: Connect the positive terminal of the 24V power supply to the output terminal of SR20, 1L;

Step 2: Connect the positive input of the load (such as a motor) to the output terminal like Q0.0;

Step 3: Connect the negative terminal of the 24V power supply to the negative input of the load;

If there are multiple loads, then connect from Q0.1 to Q0.7, each output terminal can carry a maximum current of 2A, so be careful not to overload, On the other hand, there are a total of 8 Q points, divided into two groups, Q0.0-Q0.3, all need to be connected to the 24V switch power supply through the input terminal 1L, If you need to connect more loads, then you need to power the input terminal 2L, connecting through Q0.4 to Q0.7 to the load.

3. Expansion Modules and Advanced Learning

Expansion Modules: The reason SMART has a high cost-performance ratio is that it integrates only core functions, for personalized automation projects/product functions, it is realized through expansion modules, So customers do not have to pay for unnecessary features, The directory of SMART’s expansion modules can be viewed in the online help document, The installation of expansion modules is also very simple, the rectangular plastic piece on the right side of the PLC can be pried off, and the position for inserting the module is behind it, After inserting the module, select the newly inserted expansion module in the “System Block” EM row to complete the configuration.

The most common module is AR02, the fully automated agricultural irrigation system I mentioned above was realized using the AR02 expansion module, Applying what has been learned, The AR02 expansion module is specially designed for temperature detection, It does not require additional power supply, the two terminals L+ and M under the 24VDC text on the lower right of the PLC can power it, Add a temperature sensor Pt100, connect it to the input terminals on AR02, After the wiring is complete, read real-time temperature through AIW16, and process differently based on temperature in the program, In this way, a fully automated agricultural irrigation system is completed.

The simplified design scheme for this fully automated agricultural irrigation system is as follows:

1. Hardware devices used in the simplified version:

A: One Pt100 to collect real-time temperature;

B: One AR02 expansion module to receive and process data from the temperature sensor;

C: One SR20 controller to process user programs;

D: One 24V2A water pump;

E: Water pipes and spray heads, purchase as needed;

F: PVC plastic pipe for fixing water pipes and spray heads (this is not mandatory);

G: One 24V5A power supply to power the controller and water pump;

H: Other items such as wires, brackets for fixing the PLC, etc., purchased as needed;

2. Wiring diagram: The detailed wiring method has been covered in the second part, the schematic diagram of the simplified version is as follows:

3. Expansion Training:

Expansion 1: Different plants often have inconsistent water needs, and can irrigate more plants by adding pumps and modifying programs;

Expansion 2: Connect heating equipment (such as a heater) instead of a water pump, automatically adjust indoor temperature based on air temperature;

Expansion 3: PLC can communicate with any device that supports networks (standard TCP/IP) through OUC, such as communicating with a PC, allowing machine learning or neural network programs (e.g., face recognition) to be executed on the PC, sending the actions to be executed to the PLC;

Returning to the original question, if you want to learn PLC better, one way is to apply what you have learned, everyone might as well try during the learning process, This fully automated agricultural irrigation system is just a starting point, PLC can support very complex functions, such as motion control, high-speed counters, Profinet, etc., all of which are under the “Wizard” directory in the left project tree of Micro/WIN, Once familiar with the functions under the Micro/WIN wizard, you can even use SR20 to build an automated production line.

4. Summary

1. It is recommended to choose SR20 as the entry-level PLC product, low cost, strong usability, free software, and strong expandability;

2. To learn PLC, you can use the free Micro/WIN software, there are online help manuals, Free tutorials【www.pxplc.com, https://www.uy123.net】, and the above text also introduces operations that are not commonly seen in regular tutorials;

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