1 Introduction
Programmable Logic Controllers (PLCs) are a new type of industrial control device based on microprocessor technology, integrating computer technology, automation technology, and communication technology. They are characterized by extremely high reliability, strong resistance to harsh environments, and ease of use, and are considered one of the three pillars of industrial production automation, alongside robotics and CAD/CAM. Developed in the 1960s, PLCs are considered the foremost of the "three pillars of advanced countries" in industrial automation and represent the ideal control device for industrial automation. They have experienced rapid development and widespread application in recent years, and are now widely used in various fields of automation.
2. Programmable Logic Controller (PLC)
PLC, short for Programming Logic Controller, is a technology first applied and developed in the automotive manufacturing industry. It replaces relays to automate the control of machines and equipment. Its most significant feature is its programmability; the program can be rewritten according to changes in control logic and requirements, unlike relay circuits which require replacement of components and rewiring. Today, PLCs integrate many advanced computing and communication functions, as well as modules for special control functions such as position control, speed control, and process control. They also have the ability to integrate with and network with computer systems. Since its invention, PLC has been widely used in industrial automation, traffic control, power transmission, building automation, and other fields.
3PLC Development History
Since the world's first programmable logic controller (PLC) was invented by Dec Corporation in the United States in 1969, PLCs have undergone more than 30 years of development. Looking back on its development, PLC technology can be divided into three stages:
(1) Traditional PLC stage. This is the initial stage of PLC and the foundation of modern PLC. Its structure is shown in Figure 1, and its working principle is shown in Figure 2.
As shown in Figure 2, the working principle of a PLC is as follows: First, read the state of the input contacts → then execute the program → then refresh the state of the output contacts based on the program execution result → then read the state of the input contacts again → read the state of the input contacts again, and so on in a loop.
As can be seen from the working principle of a PLC, from the change of the input signal state to the change of the output signal, a program (user program, system program) needs to be executed. Program execution takes time, and this time is unpredictable. This is unacceptable in some applications, such as position control, speed control, and control requiring high-speed response. This makes PLCs unusable in these situations or requires expensive dedicated modules. As shown in Figure 1, the core of a PLC system is the microprocessor (CPU). To prevent the system program from crashing and causing malfunctions, a series of hardware and software measures must be taken to overcome this problem. Furthermore, since PLCs use ladder logic, the system itself must have a powerful compiler, which makes PLC-based systems relatively expensive. Moreover, program crashes and infinite loops remain potential problems.
(2) OpenPLC stage. OpenPLC, also known as PCBasePLC or SoftPLC, is a concept proposed in recent years. It is a PLC based on an open PC platform and open development software, which can be easily integrated with other software and networks. Its structure is shown in Figure 3.
OpenPLC only introduces a new concept in its development environment: openness and standardization. While its operating principle offers a real-time multitasking mechanism compared to traditional PLCs, it still relies on program execution. Therefore, it doesn't fundamentally solve the problems inherent in traditional PLCs, and these problems persist in the systems implemented within it.
(3) Field Integration Stage. This is also known as the Hard PLC stage, a completely new term and the latest development trend of PLCs. It uses modern programmable logic devices CPLD/FPGA (complex programmable logic devices & field programming gate array) as the hardware platform and EDA (electronics design automation) development tools with hardware description language HDL (hardware description language) as the software platform. Like traditional PLCs, it is also programmable. Its structural diagram is shown in Figure 4.
Hard PLCs abandon the traditional PLC concept of "programs," implementing control functions through "hardware circuitry." Programming only alters the internal hardware connections of the chip, eliminating the need to run software programs. Therefore, they avoid issues like program crashes, power-on resets, and built-in language compilers. They perform the same functions as traditional PLCs, but at only one-tenth, or even less, the cost. During hardware circuitry operation, all signals run in parallel, and the signal paths are known and transmission times are predictable. This allows for precise control applications such as position control, speed control, signal processing, image processing, and high-speed machinery. Hard PLCs fundamentally address the shortcomings of traditional PLCs and represent the ultimate direction for their development.
4PLC development trend
With the advancement and development of microprocessor technology, very large-scale integrated circuit technology, and digital communication technology, programmable logic controllers (PLCs) have also developed rapidly. Their functions have far exceeded the scope of their definition, and their concept has become increasingly blurred. The development trends of modern PLCs mainly include the following aspects:
(1) Use high-performance devices to minimize the gap with industrial control computers. For example, the German company Festo's IPC (industrialPC) consists of a series of modules that conform to industrial standards. It is compatible with microcomputers and has the functions of a PLC.
(2) Enrich the I/O modules to further improve and enhance the performance of the PLC in terms of real-time performance, accuracy, resolution, and human-computer interaction.
(3) Further enhance network functions to achieve automated information management. For example, IPC controllers have a variety of fieldbus interfaces, such as Festo bus, PROFIBUS, AS-I, CAN, etc., as well as various network connection modules, such as Novell, so that communication networks can be established between PLCs, between PLCs and PCs, and between PLCs and field devices.
(4) Multiple programming languages coexist and complement each other. In addition to ladder diagrams and instruction lists, IPC controllers can also be programmed using standardized languages for sequential control specified by IEC 1131, as well as computer languages such as C and BASIC.
(5) Hardware structure integration and redundancy. With the application of application-specific integrated circuits (ASICs) and surface-mount technology (SMT) in PLC hardware design, PLC products have fewer hardware components, higher integration, smaller size, and higher reliability. At the same time, in order to further improve the reliability of the system, PLC products also adopt hardware redundancy and fault-tolerant technology. Users can choose redundant configurations for CPU units, communication units, power supply units, or I/O units, or even the entire system, which further enhances the reliability of the entire PLC system.
5. Significance of Research on Programmable Controller Field Integration Technology
Current programmable logic controllers (PLCs) are all designed and manufactured by specialized companies. When users choose these dedicated controllers, they may only utilize a portion of their functions, leading to resource waste. Furthermore, dedicated controllers are expensive and uneconomical. Using modern programmable logic devices offers the following advantages:
(1) Users can design the functions of the controller as needed without causing too much waste of resources; and they do not need to bring their own dedicated compiler, which greatly reduces the price of the system.
(2) The user logic and interface can be integrated into the same device, thus allowing the interface and user logic to be more closely integrated; the system composed of FPGA/CPLD chips naturally avoids the disadvantages of CPU program crashes, infinite loops, unreliable resets, etc., and the system can have high reliability without taking too many measures.
(3) As the core of the controller, the FPGA's flexible field modifiability and reconfigurability make it very convenient for various system improvements. It can further enhance system performance without changing the hardware circuitry, thus achieving in-system hardware upgrades. In-system programming is a prominent feature of FPGAs/CPLDs. It eliminates the need to change the external I/O port connections, allowing direct programming of FPGAs/CPLDs within the user's own designed target system or circuit board. This breaks the convention of designing and assembling general digital devices and PLCs, allowing for assembly before programming. Furthermore, it can be repeatedly programmed after being used in the actual system, thus opening a new chapter in digital electronic system design technology. In addition, it can also be programmed online via infrared, ultrasonic, or through telephone lines or the internet. These functions have special applications in remote control or military fields.
(4) FPGA has a high performance-price ratio. The price of a controller implemented with it is almost one-tenth of the price of a commercially available programmable controller with the same input/output terminals. Moreover, its logic implementation works in parallel, and its speed is much greater than that of a PLC, which is a great advantage in real-time systems.
(5) It abandons the traditional PLC "program" concept and uses "hardware circuits" to realize control functions. When the hardware circuits are running, all signals run in parallel, and the signal path is known and the signal transmission time is predictable. Therefore, it can be used for precise control needs, such as position control, speed control, signal processing, image processing, high-speed machinery, etc.
From the advantages mentioned above, we can see that FPGA/CPLD-based Hard PLCs are more economical, stable, and convenient in meeting user needs, and their real-time performance and flexibility are far superior to traditional programmable controllers (PLCs). Therefore, field integration technology for programmable controllers has a wide range of applications and strong practical engineering value.
