Programmable Logic Controllers (PLCs) are a new type of industrial controller developed based on microprocessor technology and integrating computer technology and automatic control technology. They are widely used in various fields of industrial control. Modern industrial production equipment contains numerous digital and analog control devices, such as motor start/stop, solenoid valve opening/closing, product counting, and the setting and control of temperature, pressure, and flow. Using PLCs to solve automatic control problems in industrial settings has become one of the most effective methods. This article describes the considerations when designing a PLC control system.
I. Hardware Selection
Currently, there are numerous PLC products on the market, offering increasingly better value for money and a wider range of choices. Therefore, when purchasing a PLC, we should consider the following aspects comprehensively to make a reasonable selection:
1.1 System Scale First, it should be determined whether the system is controlled by a single PLC or by a PLC network, and the number of PLC input and output points should be calculated accordingly. When determining the number of PLC input and output points, a margin of 10% should generally be left based on the actual number of points required.
1.2 Load type: The output terminal should be determined based on factors such as whether the load connected to the PLC output terminal is DC or AC, whether it is a large current or a small current, and the frequency of the PLC output point operation. Different output methods should be selected for different loads. This is very important for the stable operation of the system.
1.3 Storage Capacity and Speed Generally speaking, the larger the storage capacity and the faster the speed of a PLC, the higher the price. When making a selection, we should not blindly pursue large capacity and high speed, but should choose PLC products reasonably according to the size of the system.
1.4 Working Environment The working environment is a crucial factor for PLC operation. Automatic control systems free people from busy work and harsh environments, requiring them to adapt to complex conditions such as temperature, humidity, noise, signal shielding, and operating voltage. Therefore, it is essential to select products that are suitable for the actual working environment.
1.5 For after-sales service, try to choose products from large companies, as their quality is guaranteed, their technical support is good, their after-sales service is generally better, and it is also conducive to your product expansion and software upgrades.
II. Input circuit design considerations:
2.1 Power supply circuit: The PLC power supply is generally AC85-240V (or DC24V), which has a wide range of power supply compatibility. However, in order to resist interference, power purification components (such as power filters, 1:1 isolation transformers, etc.) should be installed.
2.2 Use of DC24V power supply on PLC Most PLC products from various companies have a DC24V power supply, but the capacity of this power supply is small, ranging from tens of milliamps to hundreds of milliamps. When using it to drive a load, attention should be paid to the capacity, and short-circuit protection measures should be in place.
2.3 Use of external DC24V power supply If the input circuit has proximity switches, photoelectric switches, etc. powered by DC24V, and the DC24V power supply capacity on the PLC is insufficient, an external DC24V power supply must be provided. However, one end of this power supply should not be connected to the one end of the PLC's DC24V power supply or the COM terminal, otherwise it will affect the operation of the PLC.
2.4 Input Sensitivity: When the input voltage and current are less than the manufacturer's specifications for the PLC's input voltage and current, such as when there are diodes or resistors in series in the input circuit (preventing full startup), or when there are parallel resistors or leakage current (preventing complete disconnection), malfunctions will occur, causing a decrease in sensitivity. Measures should be taken to address this. When the input current of the input device exceeds the PLC's maximum input current, malfunctions will also occur. Low-current input devices should be used, and a PLC with common-drain inputs should be selected.
III. Considerations for Output Circuit Design:
3.1 Output Method Selection
(1) Relay output: The advantage is that different AC and DC loads can be connected between different common points, and the voltage can also be different. The load current can reach 2A. However, the relay output method is not suitable for high-frequency loads, which is determined by the life of the relay. (2) Thyristor output: The load capacity is 0.2A/point. It can only carry AC loads. It can adapt to high-frequency operation and the response time is 1ms. (3) Transistor output: The biggest advantage is that it is suitable for high-frequency operation and has a short response time, generally around 0.2ms. However, it can only carry DC5-30V loads. The maximum output load current is 0.5A/point, but it must not exceed 0.8A for every 4 points. When your system output frequency is less than 6 times per minute, the relay output should be preferred because its circuit design is simple, anti-interference and load capacity are strong. When the frequency is less than 10 times/min, the relay output method can be used, or the PLC output can be used to drive the Darlington transistor (5-10A) and then drive the load.
3.2 Anti-interference and external interlocking
When a PLC outputs an inductive load, a surge current will impact the PLC output when the load is de-energized. Therefore, a freewheeling diode should be connected in parallel next to the DC inductive load, and a surge absorption circuit should be connected in parallel for the AC inductive load. This can effectively protect the PLC. After the outputs of two physical quantities have been software interlocked inside the PLC, they should also be interlocked outside the PLC to enhance the reliability of the system.
3.3 Selection of COM Point
Different PLC products have different numbers of COM points. Some have 8 output points per COM point, some have 4 output points, and some have 2 or 1 output point. When there are many types of loads and the current is large, a PLC product with 1-2 output points per COM point is used. When there are many types of loads, a PLC product with 4-8 output points per COM point is used. This will bring a lot of convenience to the circuit design. Add a fuse at each COM point. Add a 2A fuse when there are 1-2 outputs, and add a 5-10A fuse when there are 4-8 outputs.
3.4 PLC External Drive Circuit
When a PLC output cannot directly drive a load, an external drive circuit must be used. This can be achieved using transistors, solid-state relays, or thyristor circuits. Protection circuits and surge absorption circuits should be included, and each channel should have an indicator LED. The printed circuit board should be pluggable for easy maintenance. There are also specific requirements for the PLC's input/output wiring; please refer to the respective company's instruction manual.
IV. Selection of Expansion Modules
For small systems, such as those with 80 or fewer points, expansion is generally not needed. However, expansion is necessary for larger systems. Different companies have limitations on the total number of system points and the number of expansion modules. When expansion still cannot meet the requirements, a network structure can be used. Some manufacturers' products do not support expansion modules for certain instructions; therefore, care must be taken when developing software. When using simulation modules such as temperature, each manufacturer has its own regulations; please refer to the relevant technical manuals. Various companies offer a wide variety of expansion modules, such as single-input modules, single-output modules, input/output modules, temperature modules, and high-speed input modules.
V. PLC Network Design
Designing a network using a PLC is significantly more complex than single-unit PLC control. First, you should choose a PLC model you are familiar with, have a thorough understanding of its basic and functional instructions, and carefully consider its execution speed and user program storage capacity. Otherwise, it may not meet your real-time requirements, causing system crashes. Additionally, you must consider the communication interface, communication protocol, and data transmission speed. Finally, you should seek network design and software technical support and detailed technical documentation from the PLC vendor. The number of workstation layers to choose depends on the size of your system.
VI. Software Development
Before developing the software, you should first familiarize yourself with the software manual of the selected PLC product. Once you are proficient, you can start programming. If you are using a graphical programmer or software package, you can program directly. If you are using a handheld programmer, you should first draw the ladder diagram and then program. This will reduce errors and speed up the process. After programming, first run the air conditioning program. After all actions are normal, you can then debug it on the equipment.
