1 Introduction
Modern industrial production equipment employs 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. PLC technology is the most effective and convenient tool for solving these problems, hence its widespread application in industrial control. The following discussion addresses issues in the design of PLC industrial control systems.
2. PLC System Equipment Selection
The primary purpose of a PLC is to control external systems. This system could be a single machine, a cluster of machines, or a production process. Different PLC models have different applicable ranges. Based on the production process requirements, analyze the complexity of the controlled object, compile statistics on the number and types of I/O points (digital, analog, etc.), and create a list. Estimate the appropriate memory capacity to determine a suitable model (small, medium, or large machine) with sufficient margin to avoid resource waste. Furthermore, consider market conditions, examine the PLC manufacturer's products, after-sales service, technical support, network communication, and other comprehensive factors to select a PLC model with a good price-performance ratio.
There are numerous PLC products on the market, including well-known international brands such as Siemens from Germany; OMRON, MITSUBISHI, FUJI, and Panasonic from Japan; GE from the United States; and LG from South Korea. Domestic brands include Advantech, Acer, and Helix. In recent years, the price of PLC products has decreased significantly, making them increasingly cost-effective. The selection of a PLC should be based on the following aspects.
2.1 Determine the scale of the PLC control system
The size of the system is determined based on the factory's production process and complexity. It can be divided into three sizes: large, medium, and small.
Small-scale PLC control systems are used for single-machine or small-scale production processes where the control process mainly involves conditional and sequential control, primarily using on/off signals, and with fewer than 128 I/O points. Micro PLCs, such as the SIEMENS S7-200, are typically selected.
Medium-scale PLC control systems: The production process involves complex logic control and closed-loop control, with 128-512 I/O points. PLCs with analog control and PID control functions, such as the SIEMENS S7-300, should be selected.
Large-scale PLC control system: The production process involves large-scale process control, DCS system, and factory automation network control, with more than 512 I/O points. A high-end PLC with communication networking, intelligent control, database, interrupt control, and function operation capabilities, such as the SIEMENS S7-400, should be selected and combined with an industrial fieldbus to achieve communication and control within the factory's industrial network.
2.2 Determine the type of PLC I/O points
Based on the production process requirements, analyze the complexity of the controlled object, and compile a list of I/O points and their types (digital, analog, etc.). Estimate the appropriate memory capacity and determine suitable machine models (small, medium, and large machines) that allow for sufficient hardware and software resource margins without wasting resources.
The choice between relay output, transistor output, or thyristor output depends on factors such as whether the load connected to the PLC output is DC or AC, whether it draws a large or small current, and the frequency of the PLC output point's operation. Selecting the appropriate output method for different loads is crucial for the stable operation of the system.
For the opening and closing of solenoid valves, equipment with large inductive loads and low operating frequency, the PLC output should use relay output or solid-state relay output; various indicator lights and the start/stop of frequency converters/digital DC speed controllers should use transistor output.
2.3 Determine the PLC programming tool
(1) General handheld programmer programming. Handheld programmers can only program using the statement list (STL) specified by the manufacturer. This method is inefficient, but it is more suitable for products with small system capacity and small usage, and has the advantages of small size, low price and easy on-site debugging. This is mainly used for programming micro PLCs.
(2) Graphical programmer programming. Graphical programmers use ladder diagram (LAD) programming, which is convenient and intuitive. General electrical personnel can use it freely in a short period of time. However, the price of this programmer is relatively high, and it is mainly used for micro PLCs and mid-range PLCs.
(3) Computer plus PLC software programming. This method is the most efficient, but most companies' PLC development software packages are expensive, and this method is not easy to debug on site. It is mainly used for hardware configuration and software programming of mid-to-high-end PLC systems.
3. Design of PLC Control System
PLC control system design includes hardware design and software design.
3.1 Hardware Design of PLC Control System
Hardware design is a crucial aspect of a PLC control system, as it directly impacts the system's reliability, safety, and stability. It primarily comprises two parts: input and output circuits.
(1) Input circuit design of PLC control system. The power supply of PLC is generally AC85-240V, which has a wide range of power supply adaptability. However, in order to resist interference, power purification components (such as power filters, 1:1 isolation transformers, etc.) should be added. The isolation transformer can also adopt double isolation technology, that is, the shielding layer of the primary and secondary coils of the transformer is connected to the ground and the primary electrical neutral point, and the shielding layer of the secondary coil is connected to the ground of the PLC input circuit to reduce high and low frequency pulse interference.
The power supply for the PLC input circuit should generally be DC 24V. When it is under load, attention should be paid to its capacity and short-circuit protection measures should be taken. This is crucial for the safety of the system power supply and the PLC, because overload or short circuit of the power supply will affect the operation of the PLC. Generally, the capacity of the power supply should be twice the power of the input circuit. Appropriate fuses should be installed in the power supply branch of the PLC input circuit to prevent short circuits.
(2) Output circuit design of PLC control system. According to the production process requirements, the start and stop of various indicator lights and frequency converters/digital DC speed controllers should adopt transistor output, which is suitable for high-frequency operation and has a short response time; if the output frequency of the PLC system is less than 6 times per minute, relay output should be preferred. With this method, the output circuit design is simple, and the anti-interference and load-carrying capacity are strong.
If the PLC output is connected to an inductive load such as an electromagnetic coil, a surge current will be generated on 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 to effectively protect the PLC.
When the PLC scanning frequency is less than 10 times/min, either a relay output method can be used, or the PLC output can drive an intermediate relay or a solid-state relay (SSR) to drive the load.
For the two important outputs, it is recommended to not only interlock them internally within the PLC, but also to implement hardware interlocks externally to enhance the safety and reliability of the PLC system.
For common AC220V switch loads, such as AC contactors and solenoid valves, they should be driven by DC24V miniature intermediate relays to avoid direct driving by the PLC's DO contacts, even though the PLC manual states that it has the capability to drive AC220V switch loads.
(3) Anti-interference design of PLC control system. With the rapid development of industrial automation technology, thyristor-controlled rectifiers and variable frequency speed control devices are increasingly widely used. This has led to AC power grid pollution and many interference problems for the control system. Anti-interference is a problem that must be considered when designing a PLC control system. The following methods are generally adopted:
Isolation: Since high-frequency interference in the power grid is mainly caused by distributed capacitance coupling between the primary and secondary windings, it is recommended to use a 1:1 ultra-isolation transformer and ground the neutral point through a capacitor.
Shielding: Generally, a metal casing is used for shielding, with the PLC system built into a metal cabinet. The metal cabinet casing is reliably grounded, providing good electrostatic and magnetic field shielding and preventing spatial radiation interference.
Wiring: High-voltage power lines and low-voltage signal lines should be run separately with a certain distance between them; analog signal transmission lines should use twisted-pair shielded cables.
3.2 Software Design of PLC Control System
Software design can begin simultaneously with hardware design. The main task of software design is to convert the process flow diagram into a ladder diagram based on control requirements. This is the most critical issue in PLC applications, and program writing is the concrete manifestation of software design. In control engineering applications, sound software design principles are crucial; excellent software design facilitates understanding, mastery, system debugging, and routine system maintenance by engineering technicians.
(1) Programming principles of PLC control systems. Due to the varying complexity of production process control requirements, programs can be categorized into basic programs and modular programs based on their structural form.
Basic program: It can be used as an independent program to control a simple production process, or as a unit program in a combined modular structure; according to the design philosophy of computer programs, there are only three structural ways of basic program: sequential structure, conditional branching structure, and loop structure.
Modular programming: This involves dividing a general control objective program into multiple program modules with clearly defined subtasks, writing and debugging each module separately, and finally combining them into a complete program to accomplish the overall task. This method is called modular programming. We recommend frequently adopting this programming approach because each module is relatively independent, the interconnections are simple, and the program is easy to debug and modify. It is particularly suitable for production processes with complex control requirements.
(2) Key points of PLC control system programming. The I/O allocation of the PLC control system should be based on the production line from front to back, with the number of I/O points increasing from small to large; the I/O signals of a system, equipment or component should be centrally addressed as much as possible to facilitate maintenance. Timers and counters should be uniformly numbered and the same number should not be reused to ensure the reliability of PLC operation.
Internal relays or intermediate flag bits (not I/O bits) used extensively in the program should also be uniformly numbered and assigned.
After address allocation is completed, an I/O allocation table and an internal relay or intermediate flag bit allocation table should be listed.
The output addresses of related devices, such as the forward/reverse rotation of a motor, should be arranged consecutively, such as Q2.0/Q2.1, etc.
(3) PLC control system programming techniques. The principles of PLC programming are simple and clear logical relationships, easy programming input, low memory usage, and reduced scanning time. These are principles that must be followed in PLC programming. Several techniques are introduced below.
PLC contacts can be reused multiple times without the need for complex programs to reduce the number of times they are used.
Using the same relay coil twice in the same program is called dual-coil output. Dual-coil output is prone to causing malfunctions, so the reuse of coils should be avoided as much as possible in the program. If dual-coil output is necessary, set and reset operations can be used (e.g., SQ4.0 or RQ4.0 for S7-300).
To make multiple PLC outputs a fixed value of 1 (normally closed), a word transfer instruction can be used. For example, if Q2.0, Q2.3, Q2.5, and Q2.7 are all 1 at the same time, a single instruction can be used to directly transfer the hexadecimal data 0A9H to QW2.
For less critical equipment, multiple contacts can be connected in series in the hardware and then connected to the PLC input terminal, or the number of I/O points can be reduced through PLC programming to save resources. For example, if we use a button to control the start/stop of the equipment, we can use a frequency divider to achieve this.
Application of modular programming concept: We can encapsulate the forward and reverse self-locking interlocking program into a module, and the forward and reverse jogging program into a module. In the PLC program, we can call the module repeatedly, which not only reduces the amount of programming, but also reduces the memory usage, which is beneficial for the development of large PLC programs.
4. Debugging of PLC control system program
The debugging of PLC control system programs generally includes two parts: I/O terminal testing and system debugging. Good debugging procedures can help accelerate the final assembly and debugging process.
4.1 I/O Terminal Testing
Use a manual switch to temporarily replace the field input signal, and manually check and verify each PLC input terminal one by one. If the indicator light of the PLC input terminal lights up, it means that it is normal; otherwise, the wiring or I/O point should be checked.
We can write a small program to check if all the indicator lights on the PLC output terminals are lit, assuming a good power supply. If the indicator lights on the PLC input terminals are lit, it indicates normal operation. Otherwise, the wiring or I/O points should be checked for faults.
4.2 System Debugging
System debugging should begin by connecting the power supply, external circuits, and input/output terminals according to control requirements. Then, load the program into the PLC and run the PLC for debugging. Connect the PLC to the field devices. Before formal debugging, thoroughly inspect the entire PLC control system, including the power supply, grounding wire, device connection cables, and I/O connections. Power can be supplied once all hardware connections are confirmed to be correct.
Set the PLC control unit to "RUN" mode and start running. Repeatedly debug to eliminate any potential problems. During debugging, appropriate hardware modifications can be made to complement software debugging based on actual needs. Sufficient running time should be maintained to fully expose and correct problems. Most debugging issues are related to the control program. Generally, the process involves the following steps:
(1) Test each field signal and control variable individually;
(2) Check hardware/modify program;
(3) Conduct comprehensive testing of on-site signals and control quantities;
(4) Equipment debugging;
(5) Debugging is complete.
5. Conclusion
The design of a PLC control system is a systematic engineering project with orderly steps. To achieve proficiency, repeated design and practice are necessary. This article summarizes the design and practical experience of PLC control systems, demonstrating good results in practical applications.
