Key points in the design of PLC control systems

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In modern industrial production equipment, there are numerous digital and analog control devices, such as those used for starting and stopping motors, opening and closing solenoid valves, counting products, as well as setting and controlling temperature, pressure, and flow rates. For these automatic control tasks in industrial environments, using a programmable logic controller (PLC) has become one of the most effective solutions. This article discusses the issues that should be taken into consideration when designing PLC control systems. When selecting hardware, there are numerous PLC products available on the market. In addition to domestic brands, there are foreign brands such as OMRON, MITSUBISHI, FUJIKO, and ANASONIC from Japan; SIEMENS from Germany; and LG from South Korea. In recent years, the price of PLC products has dropped significantly, resulting in an increasingly favorable cost-performance ratio, which is a key reason why many technicians choose PLCs. So, how should one choose a PLC product? 1. System scale: First, it is necessary to determine whether the system will be controlled by a single PLC or through a network of PLCs, and based on this, calculate the number of input and output points for the PLC. Number, and when selecting a PLC, it is necessary to leave a certain margin (10%) based on the actual required number. 2. Determine the load type: Based on whether the load connected to the PLC output is of DC or AC type, whether it involves high or low current levels, and the frequency at which the PLC output points operate, it is determined whether to use relay output, transistor output, or thyristor output for the output terminal. Choosing the appropriate output method for different loads is crucial for the stable operation of the system. 3. Storage capacity and speed: Although PLC products from various foreign manufacturers are generally similar, there are still some differences. So far, no products that are fully compatible across different companies have been found. The software developed by different companies varies, and the storage capacity of user programs as well as the execution speed of instructions are two important metrics. Generally, the larger the storage capacity and the faster the speed of a PLC, the higher its price; however, the PLC product should be selected appropriately based on the size of the system. 4. Selection of a programmer: PLC programming can be carried out in three ways: using a regular handheld programmer, which can only be used to program with statements from the statement list specified by the manufacturer. This method is inefficient, but it is suitable for products with low system capacity and low usage. It is also compact, easy to debug on-site, and has a lower cost. It is programmed using a graphic programmer that employs ladder diagram programming, which is easy and intuitive; ordinary electrical technicians can master it quickly. However, this programmer is relatively expensive. It is programmed using an IBM personal computer along with PLC software packages, and this is the most efficient method. However, the PLC development software packages used by most companies are expensive, and this approach is not easy for on-site debugging. Therefore, PLC products should be selected appropriately based on the size and complexity of the system, the length of the development cycle, and financial considerations. 5. Try to choose products from large companies, as their quality is guaranteed, they offer good technical support, and their after-sales service is usually excellent as well; this also facilitates the expansion of your products and software upgrades. I. Design of the input circuit 1. Power supply circuit: The power supply for PLCs is generally AC85–240V (or DC24V), offering a wide range of compatible voltage levels. However, to reduce interference, power purification components such as power filters and 1:1 isolation transformers should be used. 2. Use of the DC24V power supply on PLCs: PLC products from various manufacturers generally come equipped with a DC24V power supply, but its capacity is low, ranging from several dozen milliamps to a few hundred milliamps. When using this power supply to drive loads, it is important to take into account its capacity, and measures should also be taken to prevent short circuits (as both overloading and short circuits of this power supply can affect the operation of the PLC). 3. External DC24V power supply: If there are proximity switches, photoelectric switches, etc. that rely on DC24V power in the input circuit, and the DC24V power capacity available on the PLC is insufficient, an external DC24V power supply must be provided ； However, the “—” terminal of this power supply should not be connected to the “—” terminal and the “COM” terminal of the PLC’s DC24V supply, as this will affect the operation of the PLC. 4. Input sensitivity: Different manufacturers have specified values for the voltage and current at the input terminals of PLCs. For example, the F7n series PLCs produced by Mitsubishi in Japan require an input voltage of DC24V and a current of 7mA; the starting current is 4.5mA, while the current when the device is turned off is less than 1.5mA. Therefore, if diodes or resistors are connected in series in the input circuit (which prevents full activation), or if there are parallel resistors or leakage currents (which prevent complete shutdown), errors may occur and the sensitivity will decrease. Measures should be taken to address this issue. On the other hand, when the input current of the input device exceeds the maximum input current of the PLC, this can also cause malfunctioning; therefore, input devices with low current levels should be used, and PLCs with a common-drain input configuration should be chosen. In such configurations, the potential at the common point of the Bp input elements is relatively negative, and the current flows out from the input terminal of the PLC. II. Design of the output circuit 1. Comparison of various output methods (1) Relay output: The advantage is that different AC and DC loads can be connected to different common points, and the voltages can also vary; the current that can be supplied to each load is up to 2A per point ； However, the relay output method is not suitable for loads that require high-frequency operation, which is determined by the relay’s lifespan. Its lifespan decreases as the load current increases; it is generally in the range of several hundred thousand to several million cycles, with some company products achieving over 10 million cycles, and the response time is 10 ms. (2) Thyristor output: The load-carrying capacity is 0.2A per point; it can only handle AC loads, is suitable for high-frequency operations, and has a response time of 1ms. (3) Transistor output: Its greatest advantage is its suitability for high-frequency operation, with a short response time of around 0.2 ms. However, it can only drive loads with a voltage of DC 5–30V; the maximum output current per terminal is 0.5 A, and this value shall not exceed 0.8 A for every 4 terminals. When your system’s output frequency is 6 times per minute or less, relay output should be the preferred choice, as it features a simple circuit design, strong resistance to interference, and the ability to handle loads. When the frequency is 10 times/min or less, either a relay output method can be used ； A PLC output can also be used to drive a Darlington transistor (5–10A), which in turn drives the load, thereby **reducing** the load. 2. Interference resistance and external interlocking: When a PLC’s output is connected to an inductive load, the disconnection of such a load can cause surge currents to affect the PLC’s output. To address this issue, a freewheeling diode should be connected in parallel with linear inductive loads, while a surge absorption circuit should be used for AC inductive loads; this helps to effectively protect the PLC. When the outputs of two physical quantities are already software-interlocked within the PLC, interlocking should also be implemented outside the PLC to enhance the reliability of the system. 3. For different PLC products with varying selections for the “GOM” point, the number of “COM” points varies; some have 1 COM point connected to 8 output points, some to 4 output points, while others have 2 or 1 output point. When there are many types of loads and high currents are involved, use a PLC model with one “COM” terminal and 1–2 output terminals ； When there is a large number of loads but few types, use a PLC product with one \"COM\" terminal and 4–8 output terminals. This brings a lot of convenience to circuit design: a fuse is added at each “COM” point; for 1–2 outputs, a 2A fuse is used, and for 4–8 outputs, a 5–10A fuse is used, as PLCs generally do not have fuses built in. 4. External drive circuit for PLC: When the PLC’s output cannot directly drive a load, an external drive circuit is necessary. This can be achieved using transistors, or solid-state relays or thyristor circuits. Protection circuits and surge absorption circuits should also be employed, with a display diode (LED) provided for each circuit to indicate its status. The printed circuit board should be designed to be plug-in type for easy maintenance. There are also certain requirements for the wiring of PLC inputs and outputs; please refer to the user manuals of each company. III. Selection of expansion modules For small systems, such as those with 80 points or fewer, expansion is generally not required ； When the system becomes large, it needs to be expanded. Products from different companies impose limits on the total number of system points and the quantity of expansion modules; when expansion still cannot meet the requirements, a network architecture can be used ； At the same time, some manufacturers’ products do not support expansion modules for certain commands, so this should be taken into account when developing software. When using simulation modules such as those for temperature, various manufacturers have their own regulations; please refer to the relevant technical manuals. Companies offer a wide variety of expansion modules, such as single-input modules, single-output modules, input-output modules, temperature modules, high-speed input modules, and more. The modular design of PLCs facilitates users’ product development. IV. PLC Network Design When designing a network using PLCs, the complexity is much greater than that of controlling a PLC individually. First, you should choose a model that you are familiar with, having a thorough understanding of its basic and functional commands, as well as being well aware of the command execution speed and the storage capacity for user programs. Otherwise, it will be unable to meet your real-time requirements, leading to system crashes. In addition, communication interfaces, communication protocols, data transfer speeds, etc., also need to be considered. Finally, it is also necessary to seek network design and software technical support as well as detailed technical documentation from the PLC supplier; as for the number of layers in the workstation, it depends on the size of your system. V. Software Development Before developing software, it is necessary to first familiarize oneself with the software manual of the selected PLC product, and only after gaining proficiency should programming begin. If programming is done using a graphic programmer or software package, it can be done directly; when programming with a handheld programmer, a ladder diagram should be drawn first before programming, which helps to reduce errors and increases speed. After programming is complete, run the air conditioning program first; once all operations are functioning properly, then proceed with debugging on the device. (