Introduction to Virtual Instruments

Virtual Instruments (VI) technology is developing very rapidly. The main functions of all measuring and testing instruments can be composed of three parts: ① data acquisition ② data testing and analysis ③ result output display. Data analysis and result output can be completed by computer-based software systems. Therefore, as long as certain data acquisition hardware is provided, a computer-based measuring and testing instrument can be constructed. Computer-based digital measuring and testing instruments are called virtual instruments (VI). Note: The virtual instrument referred to here is completely different from the concept of virtual instruments in EDA simulation software. It can completely replace traditional desktop measuring and testing instruments. The virtual instruments in EDA simulation software are pure software and simulation.

Virtual instruments can use the same hardware system and realize various measurement and testing instruments with completely different functions through different software. That is, the software system is the core of the virtual instrument, and the software can be defined as various instruments. Therefore, it can be said that "software is the instrument."

Classification of virtual instruments:

The development of virtual instruments can be divided into five types with the development of microcomputers and the different bus methods adopted:

Category 1: PC bus - card-type virtual instrument

This method relies on the data acquisition card inserted into the computer and combined with dedicated software such as LabVIEW (Note: Labview of the American NI company is a graphical programming tool that can build various instruments by itself through various controls. Labview/cvi is a text-based programming programmer that provides efficient programming tools through three programming languages ​​Visual C++,

Visual Basic, Labviews/cvi constitute the test system, which makes full use of the convenience of the computer's bus, chassis, power supply and software. However, it is limited by the PC chassis and bus, and has the disadvantages of insufficient power supply, high noise level inside the chassis, a small number of slots, relatively small slot size, and no shielding inside the chassis. In addition, the virtual instrument of the ISA bus has been eliminated, and the price of the virtual instrument of the PCI bus is relatively expensive.

The second category: parallel port virtual instrument

The latest development of a series of test devices that can be connected to the computer parallel port, they integrate the instrument hardware in a collection box. The instrument software is installed on the computer, which can usually complete the functions of various measurement and test instruments, and can be composed of digital storage oscilloscopes, spectrum analyzers, logic analyzers, arbitrary waveform generators, frequency meters, digital multimeters, power meters, programmable voltage regulators, data recorders, and data collectors. The biggest advantage of the DSO-2XXX series of virtual instruments of LINK Company in the United States is that they can be connected to laptop computers for field operations, and can also be connected to desktop PCs to achieve desktop and portable dual-use, which is very convenient. Due to its low price and wide range of uses, it is particularly suitable for R&D departments and various teaching laboratories.

The third category: virtual instruments using GBIB bus

GPIB technology is the early development stage of virtual instruments of IEEE488 standard. Its emergence has enabled the development of electronic measurement from independent single manual operation to large-scale automatic test system. A typical GPIB system consists of a PC, a GPIB interface card and several BPIB instruments connected by GPIB cables. Under standard conditions, a GPIB interface can carry up to 14 instruments, and the cable length can reach 40 meters. GPIB technology can use computers to operate and control instruments, replacing traditional manual operation methods, and can easily combine multiple instruments to form an automatic measurement system. The structure and commands of the GPIB measurement system are simple, and it is mainly used in desktop instruments. It is suitable for applications with high accuracy requirements but no high-speed computer transmission conditions.

Category 4: VXI bus virtual instrument

VXI bus is an extension of high-speed computer bus VME bus in the VI field. It has stable power supply, strong cooling capacity and strict RFI/EMI shielding. Due to its open standards, compact structure, high data throughput, precise timing and synchronization, reusable modules, and support from many instrument manufacturers, it has quickly been widely used. After more than ten years of development, the establishment and use of VXI system has become more and more convenient, especially for the establishment of large and medium-sized automatic measurement systems and occasions with high speed and accuracy requirements. It has advantages that other instruments cannot match. However, the establishment of VXI bus requires a chassis, a zero-slot manager and an embedded controller, which is relatively expensive.

Category 5: PXI bus virtual instrument

The PXI bus mode is formed by adding mature technical specifications and requirements to the PCI bus core technology, adding the technical specifications and requirements of the multi-board synchronous trigger bus, and adding the multi-board trigger bus to make it a local bus for high-speed communication between adjacent modules. PXI is highly scalable. PXI has 8 expansion slots, while the desktop PCI system has only 3~4 expansion slots. By using the PCI-PCI bridge, it can be expanded to 256 expansion slots. The performance-price ratio of desktop PCs and the expansion advantages of the PCI bus in the instrument field will form the future virtual instrument platform.

The development process of virtual instruments has two lines: 1. GPIB → VSI → PXI bus mode (suitable for large-scale high-precision integrated systems)

GPIB was introduced in 1978, VXI in 1987, and PXI in 1997.

2. PC card → parallel port → serial port USB mode (suitable for popular and cheap systems, with broad application development prospects)

The PC card type came out in the early 1980s, the parallel port type came out in 1995, and the serial port USB type came out in 1999.

In summary, the development of virtual instruments depends on three important factors: ① Computer is the carrier, ② Software is the core, and ③ High-quality A/D acquisition card and conditioning amplifier are the key.

Regardless of the type of VI system, it is composed of hardware instruments (conditioning amplifiers, A/D cards) mounted on various computer platforms such as laptops, desktop PCs or workstations, plus application software, to achieve full digital acquisition, test and analysis using computers. Therefore, the development of VI is completely synchronized with the development of computers. Therefore, the flexibility and strong vitality of VI are demonstrated. The rise of virtual instruments is a "revolution" in test instrument technology and a new milestone in the field of instruments. The future VI can completely cover the entire field of computer-aided testing (CAT). It can almost replace all analog test equipment. Its significance lies in the fact that it is still a virgin land that needs to be explored by us, and it has many parts to be filled. The prospects for virtual instruments are very bright. Fully digital measurement and analysis based on computers is the future of acquisition, test and analysis.

However, it is worth pointing out that the word "VIRTUAL" can be translated into several meanings, such as ① "virtual", ② "virtual image", ③ "substantial", and ④ "effective". The virtual instruments we are currently developing should focus on "substantial" and "effective" instruments in terms of function, and not pursue "virtual image" one-sidedly, and simply pursue the similarity in appearance and spend a lot of effort to imitate the appearance of old instruments. This is a misunderstanding. We should vigorously advocate the pursuit of instrument functions and test accuracy and reliability to achieve a truly advanced instrument test effect, rather than going astray and pursuing similarity in appearance, wasting a lot of manpower and material resources.

The development of virtual instruments is a major area of ​​information technology. It has an immeasurable impact on the development of science and technology and the production of national defense, industry, and agriculture. It can be widely used in electronic measurement, vibration analysis, acoustic analysis, fault diagnosis, aerospace, military engineering, power engineering, mechanical engineering, construction engineering, railway transportation, geological exploration, biomedicine, teaching, scientific research, and many other fields.