1. PLC, IPC, PC-Based PLC
With the rapid development of PC technology, IPCs (Industrial Control Computers) and IPC-based application technologies have also experienced rapid advancements. Simultaneously, with the application of Internet technology and the integration and development of all production and control information processes, and the ability to browse manufacturing processes, operate and monitor intelligent field devices via the Internet/Intranet, IPCs are increasingly undertaking human-machine interaction control tasks for SCADA and coordinating control tasks with lower-level small controllers or intelligent field devices. Overall, IPCs remain the most suitable for automation control platforms. However, PLCs, as the traditional mainstream controller, possess advantages such as high stability, high reliability, and strong logical sequence control capabilities, giving them irreplaceable advantages in the field of automation control. But a major drawback is their closed architecture and closed system (requiring developers to have their own or OEM-developed CPUs, chipsets, BIOS, operating systems, and ladder diagram programming software), resulting in poor openness and creating barriers to application. This also increases the difficulty of user maintenance and integration costs. Some predict that in the near future, PC-based controllers will gradually replace PLCs and become the mainstream control equipment. To improve this situation, traditional PLC manufacturers are gradually PC-izing the functions of PLCs (such as Siemens' WinAC), while IPC manufacturers are gradually PLC-izing the logic control functions of IPCs, making PLCs and IPCs increasingly similar in terms of functions and specifications. This has led to the emergence of intermediate controllers based on PLC and IPC technologies: PC-Based PLCs.
PC-Based PLCs, also known as embedded controllers, are not like IPCs, which consist of a chassis and motherboard as the main structure, along with functional I/O boards such as A/D, D/A, and DI/DO. Instead, they are independent, dedicated systems based on embedded PC technology, suitable for small SCADA systems. For example, the ICP DARPA I-8000 series uses a 40MHz 80188 CPU and runs on the DOS-compatible MiniOS 7 operating system. Its programming environment is based on standard C language programs for PCs, and the program development process is very similar to that of a PLC: first, a resident task program is written on the PC, compiled, and then transferred to the Flash memory in the host computer for offline execution. Furthermore, to leverage the advantages of PLCs, PC-Based PLCs can also be programmed using ladder logic, such as ICP DAY's ISaGRAF (with the I-8417/8817 main unit). Compared to PLCs, PC-Based PLCs offer the powerful computing, data processing, and communication capabilities of IPCs. On the software side, PC-Based PLCs support five international standard languages and soft logic logic based on IEC-61131-3 (LD, SFC, FBD, IL, ST). Due to these characteristics, PC-Based PLCs will be more open and standardized, capable of adapting to more complex integrated control and management information needs.
In general, IPCs are open architectures and open systems, while PLCs are closed architectures and closed systems. PC-based PLCs fall somewhere in between, being open architectures and closed systems. Strictly speaking, IPCs generally undertake management and control tasks and coordinate the control of lower-level small controllers or intelligent field devices, while PLCs are generally used as local controllers. Due to the alternating development of PC technology, information technology, and communication technology, investment in the research and development of PC-based PLCs has relatively decreased, and more manufacturers will jointly promote the development of PC-based PLCs. Therefore, PC-based PLCs have a very good development prospect, but this does not mean that PC-based PLCs will replace PLCs in the short term. PLCs and PC-based PLCs will gradually converge in the process of competitive development [1, 2].
2. Application Techniques for PC-Based PLC Architecture Systems
2.1 AI Module
The number of AI (Analog Inputs) significantly impacts the real-time performance and stability of a system, especially when there are many AI modules. This is primarily because the I-8000 module's CPU is merely an 80188 controller with a clock speed of only 40MHz. Its data processing capabilities and storage space are limited, resulting in slower computation, logic processing, and event response compared to an IPC. Since the CPU must perform a series of processes—sampling, holding, synchronization, conversion, storage, processing, and computation—to complete an A/D conversion, it is time-consuming. Therefore, a large number of AI channels will inevitably affect the real-time performance of sampling and the stability of the system. Generally, it is best not to use more than two AI modules, such as those from the I-8017H series, in an I-8000 module.
2.2 Relay Output Module
The relay output module has the greatest impact on the entire system. Improper handling can lead to system crashes, frequent shutdowns, and motherboard burnout. Since the I-8000 module's power supply is typically 10-30VDC, with a total input power of 20W, unlike the 250W input power of an IPC, if too many relay output modules, especially high-power ones, are installed, insufficient system power will cause abnormal outputs, frequent control system malfunctions, system crashes, shutdowns, and even motherboard burnout, posing significant risks to system stability, safety, and reliability. Generally, it is recommended to use no more than two of modules such as I-8060, I-8058, I-8063, I-8064, I-8065, I-8066, I-8068, and I-8069, especially high-power modules like I-8060, I-8063, I-8064, I-8065, and I-8069, ideally only one. If the system needs to control a large number of power relays, a common opto-isolated switch input/output module such as I-8042 can be used to connect them using the principle of multi-stage amplification.
2.3 Communication Processing
In PC-based PLC control systems, the most crucial aspect is the real-time data communication with the host computer. This process often constrains the system's real-time performance and stability, making it prone to data bottlenecks. The host computer typically runs Windows, and its applications usually have a human-machine interface (HMI) and a real-time display interface. These HMIs and displays are often designed as foreground windows, while data communication, analysis, and storage are designed to run in the background. However, Windows is not designed as a real-time operating system; it's a preemptive, multitasking, message-passing-based system. Message scheduling alone is clearly insufficient for real-time systems, making it difficult to guarantee accurate and real-time completion of foreground and background control tasks. Therefore, in a Windows environment, employing multithreading technology can effectively utilize Windows' latency, accelerate program response, and improve execution efficiency. One thread manages computer data communication, while another handles data processing, analysis, and storage. This approach enhances the real-time performance of system event response and communication control while ensuring continuous data acquisition.
PC-Based PLCs and host computers typically use RS-485, CAN, ModBus, or Ethernet. If RS-485, CAN, or ModBus is used, the communication ports must be allocated appropriately. Generally, RS-485, CAN, and ModBus communication adapters have two ports. Therefore, if the control system has two I-8000 modules, the host computer can use one communication port to communicate with the two lower-level controllers. However, if there are four, six, or more, it is best to divide them into two groups, and the host computer should use two communication ports to communicate with them separately. The host computer uses two threads to write the communication program. The configuration diagram is shown in Figure 1.
2.4 Power Supply Configuration
If a control system has multiple I-8000 modules, considering system economy and safety, it's best to share a single switching or linear power supply between every two I-8000 modules. Considering the power supply's own power consumption, the power output must be greater than 60W. Each power supply module should be connected to a separate ~220VAC or ~380VAC power supply; never connect them in series. When selecting a switching power supply, choose one with a system power factor greater than 0.99 , a ripple voltage Vrms ≤ 1.0%, a ripple coefficient ≤ 0.2%, high power density, good electromagnetic compatibility, and low ripple. Furthermore, separating the power supply for the controller I/O channels and other devices using their own isolation transformers helps improve the control system's anti-interference capability.
2.5 Signal Ground Processing
Proper and good grounding can introduce interference signals mixed into the power supply and I/O circuits to the ground, eliminating or reducing the impact of interference. This is an important means of safety protection and noise suppression, and is extremely important for improving the stability and reliability of the I-8000 system. To minimize the impact of electromagnetic noise, the power supply circuit and control circuit should have separate grounding electrodes. In control systems, power devices such as frequency converters are inevitable. Ensure that the frequency converter heat sink, power supply neutral line, frequency converter housing and neutral terminal, and motor housing and Y-connection neutral terminal are reliably connected to the power supply circuit grounding electrode. All grounding wires must not form grounding loops. The lower the frequency converter grounding resistance, the better. The cross-sectional area of the grounding conductor should not be less than 4mm², and the length should be controlled within 20m. The shielding layer and digital signal ground are connected to the control circuit grounding electrode. To prevent loop formation, the shielding layer should be grounded at one end. The controller's grounding wire should be separate from the power supply line and power line. The I-8000 is best grounded separately, but it can also share a common ground with other equipment; however, it is strictly forbidden to connect it in series with other equipment for grounding.
3 Practical Application Cases
In small oil companies, a large amount of fuel measurement work is required, such as light oil, 0# gasoline, and 90# gasoline. The measurement process often involves the truck fleet hauling fuel back to the company from the freight station, weighing the gross weight at the company's weighbridge, unloading, and then weighing the vehicle again before leaving the factory. The weighing process, procedures, and registration are extremely cumbersome, and errors and omissions are common, making management very difficult and posing significant challenges to statistical and measurement work. Weighing workers also experience high workloads, and long queues at the weighbridge are frequent, resulting in extremely low efficiency. To change this situation… This system, employing a PC-based PLC I-8411 embedded controller, is equipped with an analog signal input module I-8017H, an analog signal output module I-8024, an optically isolated digital input/output module I-8042, an I-8060 relay output module, and an RS232/RS485 converter I-7520. Utilizing computer control technology, it has developed a distributed oil metering and statistical management system for various oil types, including inbound and outbound metering and statistics. This system is time-saving and labor-saving, and has been well-received by users. The system architecture diagram is shown in Figure 2.
3.1 Functional Modules
1) The differential input of the I-8017H is used to collect the liquid level, liquid temperature, flow rate values of two LUGB series vortex flow transmitters (for calculation purposes, the average value of the two flow meters is taken as the actual flow rate value) of the transport vehicle's oil tank through 6 channels, and store the liquid level value of the oil tank to prevent liquid overflow, temperature, etc.
2) Utilize the D/A function of I-8024 to output a 0-10V DC signal as the frequency conversion control input signal for Siemens' MicroMaster general-purpose frequency converter, so that the frequency converter can perform V/F conversion and convert it into a 0-50Hz alternating signal to control the three-phase asynchronous motor in real time, thereby achieving the purpose of making the motor run at a variable frequency and promoting the constant speed flow of liquid.
3) Use the output signal of the I-8060 power relay to control the opening of various flow relays, flow control solenoid valves, and electrical contactors in real time;
4) Utilize the digital I/O of the I-8042 for the detection and control of various switches, and simultaneously monitor the closing status of flow relays, flow control solenoid valves, and electrical contactors in real time;
5) Use the I-7520 as an RS-232/RS-485 converter to enable the I-8411 to communicate with the host computer server via serial port.
3.2 Safety and Reliability Measures
1) Spike Pulse Handling: Because this system uses large thyristors, their opening and closing generate high-energy spike pulses. If these pulses enter the signal system, they can cause malfunctions in the control system, and even worse, burn out control equipment or lock up control signal input channels. This is particularly problematic for modules such as I-8017H, I-8024, and I-8042. To mitigate this impact, a resistor-capacitor (RC) protection circuit is added to the input or output terminals of each control module to absorb the spike pulses. Simultaneously, signal ground and power ground must be separated.
2) Handling Inverter Overvoltage: In this system, an inverter drives a traction motor with high inertia. Because the inverter outputs a relatively high speed, while the load decelerates slowly due to its own resistance, the speed of the motor driven by the load is higher than the speed corresponding to the inverter's output frequency. The motor is in a generating state, and since the inverter lacks an energy feedback unit, the DC circuit voltage of the inverter rises, exceeding the protection value, resulting in an overvoltage fault. Therefore, a regenerative braking unit must be added; otherwise, it will interfere with the SCADA system.
3.3 System Functions
1) Data display: For each type of oil, the real-time collected parameters such as flow rate, temperature, switch status, and motor speed are displayed in the form of numbers, bar charts, and curves;
2) It can calculate traffic and total volume, generate daily, monthly, and annual reports, and store historical records for many years;
3) Data repair and maintenance: It has parameter setting and data loss repair functions.
4) Exchange data with the company's MIS system in real time.
4. Conclusion
The development of PC-Based PLCs has benefited from the advancements in embedded CPUs, embedded operating systems, and IEC-61131-3 (LD, SFC, FBD, IL, ST) standardized programming languages. PC-Based PLCs possess the dual characteristics of IPCs and PLCs, exhibiting both the system architecture of a PLC and the open architecture of an IPC. Currently, the industrial control field is in an era of coexistence of IPCs, PLCs, and PC-Based PLCs, and also an era of gradual convergence among the three. With the development of embedded CPUs, embedded operating systems, and development tools conforming to the IEC-61131-3 international standard languages, PC-Based PLCs or embedded controllers will become more open and standardized, with more powerful functions, stronger data communication capabilities, and faster data processing capabilities, making them better suited to more complex industrial control needs.
