A Brief Discussion on Network Testing Technology

1 Introduction

With the continuous improvement of the requirements for automated control of production processes, the shortcomings of traditional test systems are becoming more and more prominent.

① The matching problem between instruments and the measurement accuracy between instruments limit the improvement of the accuracy of the entire test system;

② The traditional test system consists of multiple measuring instruments, and the signal transmission speed is limited, which brings difficulties to the real-time analysis of the measured signal. In many cases, if there is a time delay, the measured signal is very different from the real-time signal, and automatic control is difficult to achieve;

③ The test system composed of multiple instruments is relatively scattered, large in size, and not easy to carry, which limits the on-site measurement. In this regard, the transformation of the traditional test system is imperative.

Looking at the development of test technology in recent years, in addition to the need to continuously improve sensitivity, accuracy and reliability, modern test technology is mainly developing in the direction of miniaturization, non-contact, multi-functionality (multi-parameter measurement), intelligence and networking. In recent years, the rapid development of computer technology, microelectronics technology, communication technology and network technology has provided technical support for the development of networked test technology based on PCs and workstations. The application of network technology in test systems has become a major trend in the development of modern test technology.

2 Characteristics of networked testing technology

The biggest feature of the network is that it can realize resource sharing and make full use of existing resources, thereby realizing collaborative testing and diagnosis of multiple systems and experts. It solves the limitation of the existing bus on the number of instruments, enables one machine to be used by more users, realizes the sharing of measurement information, realizes the high automation and intelligence of the entire test process, and reduces the hardware settings, effectively reducing the cost of the test system. In addition, the network is not restricted by the region, which determines that the networked test system can realize remote measurement and control, so that testers can obtain the required information anytime and anywhere without being restricted by time and space. At the same time, the networked test system can also realize remote testing and diagnosis of test equipment and improve test efficiency. It is these advantages of the networked test system that make networked testing technology attract much attention.

3 Technical support and networked instruments for networked testing

The actual testing work is extremely complex. The entire testing process involves test design, model theory, sensor design, signal processing, error processing, control engineering, system identification and parameter evaluation. Networked testing requires not only these technologies, but also complete computer technology, microelectronics technology, communication technology and network technology. The completion of these technologies provides strong material support for the generation and application of networked testing technology, and the generation of intelligent detection instruments has accelerated the process of networking of test systems. Various networked instruments that can be directly connected to the network have been applied in the actual measurement and control field. To meet the needs, the networked instruments currently on the market have standard Internet access specifications, such as the TCP/IP instrument protocol and interface conversion specifications issued by the VXI bus. This specification provides a method and standard for developing a universal interface to realize a networked automatic test system. The standard stipulates the conversion of this protocol to a specific interface. The formulation of the specification makes it possible to directly control the instrument through the network. As part of the test system, the networking of sensors is very important. Networked sensors are based on intelligent sensors, and the TCP/IP protocol is embedded in the ROM of the on-site intelligent sensors. Using local area networks and wide area networks, the networked sensors at the measurement and control points send the measurement and control parameter signals to the network after necessary processing. In addition, the specification of the control area network (CAN) fieldbus has also been formulated as the international standard ISO 11898, which has been widely used in discrete control systems. At present, there are also networked oscilloscopes and networked logic analyzers with good user interfaces that can realize real-time remote measurement and control on the market. The emergence of these networked instruments has accelerated the development of networked testing technology, and a new generation of networked testing instruments with stronger versatility and more complete functions will continue to emerge in the market. 4 Fieldbus technology in networked testing technology Fieldbus is both a new type of automation system and a low-bandwidth low-level control network. It can be connected to the Internet and the intranet, and is located at the lower level of production control and network structure, so it is called a low-level network. As a network system, its most notable feature is that it has an open and unified communication protocol, and is responsible for the unified task of measurement and control of the production operation line. Fieldbus technology has the following technical characteristics: ① Openness of the system: An open system means that the communication protocol is open, and the equipment of different manufacturers can be interconnected and information exchange can be realized. The fieldbus developer is committed to establishing an open system with a unified factory bottom network. The openness here refers to the consistency and openness of relevant standards, emphasizing the public knowledge and compliance with standards. An open system can be connected to any other equipment or system that complies with the same standards. A fieldbus network system with bus function must be open. The open system gives the right of system integration to users. Users can combine products from different suppliers into systems of any size according to their own needs and objects. ②Interoperability and interoperability: Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems, and point-to-point and point-to-multipoint digital communication can be implemented; while interoperability means that equipment with similar performance from different manufacturers can be interchangeable and interoperable. ③Intelligent and functional autonomy of field equipment: It disperses functions such as sensor measurement, compensation calculation, engineering quantity processing and control to field equipment, and the basic functions of automatic control can be completed by field equipment alone, and the operating status of the equipment can be diagnosed at any time. ④ High degree of decentralization of system structure: Since the field equipment itself can complete the basic functions of automatic control, the field bus has constituted a new fully distributed control system architecture, which fundamentally changed the existing DCS centralized and decentralized distributed control system system, simplified the system structure, and improved reliability. ⑤ Adaptability to the field environment: Working at the front end of the field equipment, as the field bus at the bottom of the factory network, it is designed to work in the field environment. It can support double-glued wire, coaxial cable, optical cable, radio frequency, infrared, power line, etc., has strong anti-interference ability, can use two-wire system to realize power transmission and communication, and can meet the requirements of intrinsic safety and explosion-proof, etc. Because of this, especially the simplification of the fieldbus system structure, the entire networked test system has shown great superiority from design, installation, commissioning to normal production operation and its overhaul and maintenance. Its superiority is mainly reflected in the following aspects: ① Saving the number of hardware and investment: Since the intelligent devices scattered at the front end of the equipment in the fieldbus system can directly perform a variety of sensing, control, alarm and calculation functions, the number of transmitters can be reduced, and separate controllers, calculation units, etc. are no longer required. There is no need for functional units such as signal processing, conversion, anti-corrosion technology of the DCS system and its complex wiring. Industrial PCs can also be used as operating stations, thus saving a large amount of hardware investment. Due to the reduction of control equipment, the area occupied by the control room can also be reduced. ② Saving installation costs: The wiring of the fieldbus system is very simple. Since a pair of double-glued wires or a cable can usually be connected to multiple devices, the amount of cables, terminals, trough boxes, and bridges is greatly reduced, and the workload of connection design and joint proofreading is also greatly reduced. When additional field control equipment is needed, there is no need to add new cables, but it can be connected to the existing cables nearby, which not only saves investment but also reduces the workload of design and installation. According to the budget data of typical test projects, the installation cost can be saved by more than 60%.



























③ Save maintenance costs: Since the field control equipment has the ability of self-diagnosis and simple fault handling, and sends the relevant diagnostic maintenance information to the control room through digital communication, users can query the operation, diagnosis and maintenance information of all equipment, so as to analyze the cause of the fault early and eliminate it quickly. Shorten the maintenance downtime, and reduce the maintenance workload due to the simplified system structure and simple wiring. [page]

④ Users have a high degree of initiative in system integration: Users can freely choose equipment provided by different manufacturers to integrate the system, avoid being "framed" by the selection of a certain brand of products, and will not be at a loss for incompatible protocols and interfaces in system integration, so that the initiative in the system integration process is completely in the hands of users.

⑤ Improve the accuracy and reliability of the system: Due to the intelligence and digitization of fieldbus equipment, compared with analog signals, it fundamentally improves the accuracy of measurement and control, reduces transmission errors, and at the same time, due to the simplified system structure, the reduction of equipment and wiring, and the strengthening of the internal functions of field instruments, the round-trip transmission of signals is reduced, and the working reliability of the system is improved. In addition, due to its standardized equipment and modular functions, it also has the advantages of simple design and easy reconstruction.

Fieldbus technology meets the requirements of networked test systems and has been widely used in them.

5 Overall design of networked test systems The

actual test process requires us to consider the test plan, maximize the required information, and express it in a more obvious form of information, use the most agile and reasonable method to obtain the most useful and expressive relevant information, and design and implement it with effective experimental methods. The composition of a general test system is shown in Figure 1.

Figure 1. Block diagram of a general test system

The networked test system is faced with a test system with dispersed spatial locations, complex test tasks and large information exchange volume. The entire test system should have comprehensive information acquisition, information processing, storage, transmission and control, as well as data analysis functions. To build a powerful and complete networked test system, the following issues must be considered and focused on:

① In order to make full use of existing resources, the measurement device of the detection system can be redefined;
② Data acquisition is an important link in the networked test process, so the commonly used data acquisition hardware must be compatible;
③ Data analysis and signal processing are the last link in the test process and the most critical step in the entire test process, so the signal processing process of the test analysis system must be redefined;
④ For the entire test system, its basic functional unit must be intelligent, with a local microprocessor and memory, and a network interface.

The openness and interoperability of a fully functional networked test system are also very important. The system should also follow the principles of scalability, security and server distribution. Based on this, the basic block diagram of the networked test system is shown in Figure 2.

Figure 2 Networked test system structure diagram

The system can be divided into two basic functional structural units, namely the equipment test part and the data signal analysis and processing part. The test part mainly completes the detection of necessary information of the field equipment, converts the non-electric information of the system under test into the required electric information, and then sends it to the data processing center through the network to analyze and process the signal. The entire test system is based on PC technology, which has the characteristics of low cost, flexible control and strong versatility. Therefore, the networked test system developed based on high-performance PC technology has been widely used. In addition, for the selection of the software and hardware platform of the entire test system, while considering its versatility, it is also necessary to take into account the cost. The software and hardware development platform with relatively mature technology should be selected.

Since the network itself is flexible, in actual work, a networked test system can be established according to different needs: when the system under test is not very complex but relatively scattered, a single-user test system can be used; when the system under test is relatively complex and requires multi-party coordinated testing, a multi-user test system under a local area network can be used; when the system under test is complex and quite scattered, a remote networked test system under a wide area network can be used.

6 Difficulties in networked testing technology

Although networked testing technology has been developed to a considerable extent, it is not yet mature. There are still many difficult problems that need to be solved and need to be gradually improved in practice. The main points are as follows:

① Real-time problems and time determinism problems in the networked testing process

For some measurement processes, it is often necessary to control the measured object, measurement conditions, and measurement status based on the measurement results. This requires measuring and processing the measured signal while selecting and controlling the subsequent measurement process based on the processing results. However, the widely used network technology currently makes it difficult to achieve synchronous data transmission, which delays the test and causes unnecessary losses. Therefore, it is crucial to solve the real-time problems and time determinism problems in the networked testing process.

② The coordination problem between the cost and versatility of the networked test system
In the process of building a networked test system, both the cost and the versatility of the system should be considered. The lowest cost and the strongest versatility are the most ideal states we pursue. However, in practice, we can only take one of the two into account. If the versatility of the test system is sacrificed in order to save costs, the scope of use of the system will be reduced, and the constructed test system will not play its due role. On the contrary, it is not practical to pursue versatility without considering costs. Therefore, in the process of building a networked test system, the coordination problem between cost and versatility should be given high attention.

③ The security and stability of the networked test system

This is a problem that every tester is more concerned about. It is the guarantee for whether the test can proceed smoothly. Although modern network technology is quite mature, due to the limitations of the network itself, once a fault occurs during network operation, the entire test will be paralyzed. Therefore, the security and stability of the network are extremely important in networked testing.

④ Bus technology and standard issues in networked test systems

Network testing requires the field bus to be real-time and to achieve interoperability and on-site information processing. This requires careful selection of the field bus in actual testing. At present, there is no unified standard for fieldbus, and the existing fieldbus control systems have their own advantages. They have formed unique advantages in their respective fields. Therefore, it is necessary to select a fieldbus control system with openness, scalability and strong compatibility according to actual needs.

7 Conclusion

With the continuous improvement of the intelligence of test equipment, the level of modern testing has entered a new stage. As a new testing method, networked testing technology will also develop with the continuous development of computer technology, microelectronics technology, communication technology and network technology. However, due to the rapid development of these technologies, a new technology has just been put into use, and the equipment used in networked testing is relatively expensive. Therefore, it has become a trend to softwareize test equipment and upgrade the software to adapt to the smooth transition of technology to meet the requirements of updated networked test systems.